ALBERT

Description: An algorithm for detecting Trichodesmium surface blooms in the South Western Tropical Pacific Biogeosciences Discussions, 8, 5653-5689, 2011 Author(s): C. Dupouy, D. Benielli-Gary, J. Neveux, Y. Dandonneau, and T. K. Westberry Trichodesmium , a major colonial cyanobacterial nitrogen fixer, forms large blooms in NO 3 -depleted tropical oceans and enhances CO 2 sequestration by the ocean due to its ability to fix dissolved dinitrogen. Thus, its importance in C and N cycles requires better estimates of its distribution at basin to global scales. However, existing algorithms to detect them from satellite have not yet been successful in the South Western Tropical Pacific (SWTP). Here, a novel approach based on radiance anomaly spectra (RAS) observed in SeaWiFS imagery is used to detect Trichodesmium during the austral summertime in the SWTP. Selected pixels are characterized by a restricted range of parameters quantifying RAS spectra quantitative parameters (e.g. slope, intercept, curvature). The fraction of valid pixels identified as Trichodesmium surface blooms in the region 5° S–25° S 160° E–190° E is low (between 0.01 and 0.2 %), but is about 100 times higher than suggested by previous algorithms. This represents a total surface area which varies from 1500 to 20 000 km 2 . A monthly distribution of Trichodesmium surface accumulations in the SWTP is presented which demonstrates that the number of selected pixels peaks in November–February each year, consistent with field observations. This approach was validated with in situ observations of Trichodesmium surface accumulations for the period 1998–2010.

Description: Rainfall patterns after fire differentially affect the recruitment of three Mediterranean shrubs Biogeosciences Discussions, 8, 5761-5786, 2011 Author(s): J. M. Moreno, E. Zuazua, B. Pérez, B. Luna, A. Velasco, and V. Resco de Dios In fire-prone environments, the "event-dependent hypothesis" states that plant population changes are driven by the unique set of conditions of a fire (e.g., fire season, climate). Climate variability, in particular changes in rainfall patterns, can be most important for seeder species, since they must regenerate after fire from seeds, and for Mediterranean shrublands, given the high yearly variability of rainfall in these ecosystems. Yet, the role of rainfall variability and its interaction with fire characteristics (e.g., fire season) on plant populations has received little attention. Here we investigated the changes in seedling emergence and recruitment of three seeder species ( Cistus ladanifer , Erica umbellata and Rosmarinus officinalis ) after fires lit during three different years and at two times during the fire season (early and late in the fire season) to account for potential changes in the soil seed-bank during the year. Three plots were burned at each season, for a total of 18 plots burned during the three years. After fire, emerged seedlings were tallied, tagged and monitored during three years (two the last burning year). Rainfall during the study period was rather variable, and in some years was well below average. Seedling emergence after fire varied by a factor of 3 to 10, depending on the species and on the burning year. The bulk of seedling emergence occurred in the first year after fire, and seedling recruitment at the end of the study period was tightly correlated with this early emergence. Seedling emergence in E umbellata and R officinalis , but not in C ladanifer , were correlated with precipitation in the fall and winter immediately after the fire, being E umbellata most sensitive to low rainfall. Fire season was generally not an important factor in controlling emergence and recruitment. We discuss how projected changes in rainfall patterns with global warming can alter the balance of species in this shrubland, and can drive some species to near local extinction.

Description: Phosphorus transformations as a function of pedogenesis: a synthesis of soil phosphorus data using Hedley fractionation method Biogeosciences Discussions, 8, 5907-5934, 2011 Author(s): X. Yang and W. M. Post In spite of the importance of phosphorus (P) as a limiting nutrient in terrestrial ecosystems, our understanding of terrestrial P dynamics and our ability to model P cycling are hampered by the lack of consistent measurements of soil P. The Hedley fractionation method provides a comprehensive assessment of soil P and has been widely used in recent decades. Here we expand an earlier study that summarized Hedley P data from the literature to create a larger Hedley P database and further investigate the relationships between distributions of different forms of P and the stages of soil development. Our expanded Hedley P database generally supports what the Walker and Syers (1976) conceptual model predicts: the gradual decrease and eventual depletion of primary mineral P (mainly apatite P); the continual increase and eventual dominance of occluded P; and the overall decrease of total P during soil development. However the analysis disagrees with Walker and Syers (1976) in that we found labile inorganic P(Pi) and secondary Pi (non-occluded P in Walker and Syers' model) to be a significant fraction of total P throughout all soil orders with different weathering stages. By analyzing the Hedley-labile P and vegetation P demand, we found that the amount of labile P is much greater than vegetation demand, even in highly weathered soils commonly considered P limited. We conclude that measured Hedley-labile P should not be defined as plant available P since most of this labile P likely ends up as immobilized by microbes. Our analysis of the database also shows that carbon (C) and nitrogen (N) in soil organic matter are closely linked in all soil orders, but P is decoupled from C and N in highly weathered soils with larger variations of nitrogen:organic P (N:Po) ratio and higher mean values of N:Po ratio, compared to slightly and intermediately weathered soils.

Description: Greenhouse gas flux measurements in a forestry-drained peatland indicate a large carbon sink Biogeosciences Discussions, 8, 5787-5825, 2011 Author(s): A. Lohila, K. Minkkinen, M. Aurela, J.-P. Tuovinen, T. Penttilä, and T. Laurila Drainage for forestry purposes changes the conditions in the peat and leads to increased growth of shrubs and trees. Concurrently, the production and uptake of the greenhouse gases carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) are likely to change: due to the accelerated decomposition of oxic peat, drained peatlands are generally considered to loose peat carbon (C). We measured CO 2 exchange with the eddy covariance (EC) method above a drained nutrient-poor peatland forest in Southern Finland for 16 months in 2004–2005. The site, classified as a dwarf-shrub pine bog, had been ditched about 35 years earlier. CH 4 and N 2 O fluxes were measured at 2–5 week intervals with the chamber technique. Drainage had resulted in a relatively little change in the water table level, being on average 40 cm below the ground in 2005. The annual net ecosystem exchange was −870 g CO 2 m −2 yr −1 in the calendar year 2005, varying from −810 to −900 g CO 2 m −2 yr −1 during the 16 month period under investigation. The site was a small sink of CH 4 (−0.12 g CH 4 m −2 yr −1 ) and a small source of N 2 O (0.10 g N 2 O m −2 yr −1 ). Photosynthesis was detected throughout the year when the air temperature exceeded −3 °C. As the annual accumulation of C in the above and below ground tree biomass (550 g CO 2 m −2 ) was significantly less than the net exchange of CO 2 , about 300 g CO 2 m −2 yr −1 (~80 g C m −2 ) was likely to have accumulated as organic matter into the peat soil. This is a higher average accumulation rate than previously reported for natural northern peatlands, and the first time C accumulation has been shown, by EC measurements, to occur in a drained peatland. Our results suggest that forestry-drainage may significantly increase the CO 2 uptake rate of nutrient-poor peatland ecosystems.

Description: Effect of mosaic representation of vegetation in land surface schemes on simulated energy and carbon balances Biogeosciences Discussions, 8, 5849-5879, 2011 Author(s): R. Li and V. K. Arora Energy and carbon balance implications of representing vegetation using a composite or mosaic approach in a land surface scheme are investigated. In the composite approach the attributes of different plant functional types (PFTs) present in a grid cell are aggregated in some fashion for energy and water balance calculations. The resulting physical environmental conditions (including net radiation, soil moisture and soil temperature) are common to all PFTs and affect their ecosystem processes. In the mosaic approach energy and water balance calculations are performed separately for each PFT tile using its own vegetation attributes, so each PFT "sees" different physical environmental conditions and its carbon balance evolves somewhat differently from that in the composite approach. Simulations are performed at selected boreal, temperate and tropical locations to illustrate the differences caused by using the composite versus the mosaic approaches of representing vegetation. Differences in grid averaged primary energy fluxes are generally less than 5 % between the two approaches. Grid-averaged carbon fluxes and pool sizes can, however, differ by as much as 46 %. Simulation results suggest that differences in carbon balance between the two approaches arise primarily through differences in net radiation which directly affects net primary productivity, and thus leaf area index and vegetation biomass.

Description: Contributions of agricultural plants and soils to N 2 O emission in a farmland Biogeosciences Discussions, 8, 5505-5535, 2011 Author(s): J. Li, X. Lee, Q. Yu, X. Tong, Z. Qin, and B. Macdonald The goal of this study was to quantify the roles of plants and soil in the N 2 O budget of a cropland in North China. Plant and soil N 2 O fluxes were measured with transparent and dark plant chambers and soil chambers, respectively, in three adjacent fields of fertilized cotton, fertilized maize and unfertilized soybean. During the observation period, the soil flux was 448 ± 89, 230 ± 74 and 90 ± 14 μg N 2 O m −2 h −1 in cotton, maize and soybean fields, respectively. The plant flux was 54 ± 43 and 16 ± 41 μg N 2 O m −2 h −1 , about 10 % and 26 % to the total ecosystem flux, for the cotton and the soybean field, respectively. Ignoring the contribution of plants would cause an obvious underestimation on the ecosystem N 2 O flux. The influence of sunlight on plant N 2 O flux was insignificant. However, in the cotton field, the responses of the plant N 2 O flux to air temperature and soil ammonium content were significant under sunlight but insignificant under darkness, suggesting that stomatal activity might influence the release process. In the cotton field, temperature sensitivity of plant N 2 O emission was 1.13, much lower than the value of soil flux (5.74). No relationship was found between plant N 2 O flux and soil nitrate content. It was implied that nitrate reduction in plants might not be the main source of plant N 2 O emission under field conditions. The seasonal patterns of the soil and plant N 2 O emissions were similarly affected by fertilization, indicating that plants might serve as a passive conduit transporting N 2 O produced in the soil.

Description: What controls biological productivity in coastal upwelling systems? Insights from a comparative modeling study Biogeosciences Discussions, 8, 5617-5652, 2011 Author(s): Z. Lachkar and N. Gruber The magnitude of the biological productivity in Eastern Boundary Upwelling Systems (EBUS) is traditionally viewed as directly reflecting the upwelling intensity. Yet, different EBUS show different sensitivities of productivity to upwelling-favorable winds (Carr and Kearns, 2003). Here, using a comparative modeling study of the California Current System (California CS) and Canary Current System (Canary CS), we show how physical and environmental factors, such as light, temperature and cross-shore circulation modulate the response of biological productivity to upwelling strength. To this end, we made a series of eddy-resolving simulations of the California CS and Canary CS using the Regional Ocean Modeling System (ROMS), coupled to a nitrogen based Nutrient-Phytoplankton-Zooplankton-Detritus (NPZD) ecosystem model. We find the nutrient content of the euphotic zone to be 20 % smaller in the Canary CS relative to the California CS. Yet, the biological productivity is 50 % smaller in the latter. This is due to: (1) a faster nutrient-replete growth in the Canary CS relative to the California CS, related to a more favorable light and temperature conditions in the Canary CS, and (2) the longer nearshore water residence times in the Canary CS which lead to larger buildup of biomass in the upwelling zone, thereby enhancing the productivity. The longer residence times in the Canary CS appear to be associated with the wider continental shelves and the lower eddy activity characterizing this upwelling system. This results in a weaker offshore export of nutrients and organic matter, thereby increasing local nutrient recycling and enhancing the coupling between new and export production in the Northwest African system. Our results suggest that climate change induced perturbations such as upwelling favorable wind intensification might lead to contrasting biological responses in the California CS and the Canary CS, with major implications for the biogeochemical cycles and fisheries in these two ecosystems.

Description: Impacts of UV radiation on plankton community metabolism along the Humboldt Current System Biogeosciences Discussions, 8, 5827-5848, 2011 Author(s): N. Godoy, A. Canepa, S. Lasternas, E. Mayol, S. Ruíz-Halpern, S. Agustí, J. C. Castilla, and C. M. Duarte The Humbolt Current System along the Chilean coast is one of the most productive regions in the world, where UV levels are particularly high due to stratospheric ozone depletion. Research has shown that phytoplankton photosynthesis can be severely inhibited by surface radiation and there are concerns that this will reduce not only algal carbon fixation, but also the carbon supply for higher trophic level. Experimental estimates of community metabolism (NCP, GPP and R) and the impacts of UV on community metabolism were assessed at 8 stations along the meridional track by the Humbold-2009 cruise (54.80° S–23.85° S) on board RV Hespérides from 5 to 15 March 2009. The results showed an increase UVB penetration towards the Equator, along the Humboldt Current System, suggesting a more important impact of UVB radiation towards the north. The metabolic rates observed were within average values reported for the Ocean Pacific and did not show the water mass investigated to be exceptionally productive at the time of the study. Experimental evaluation of the effect of UVB radiation on surface waters, those most strongly affected by UVB, showed that UVB radiation suppressed net community production, resulting in a dominance of heterotrophic communities in surface waters, compared to the prevalence of autotrophic communities inferred when materials, excluding UVB radiation, are used for incubation. These results show that UVB radiation, which has increased greatly in the study area, may have suppressed net community production of the plankton communities, possibly driving plankton communities in the Southwest Pacific towards CO 2 sources.

Description: Biomineralization of dolomite and magnesite discovered in tropical coralline algae: a biological solution to the geological dolomite problem Biogeosciences Discussions, 8, 5881-5906, 2011 Author(s): M. C. Nash, U. Troitzsch, B. N. Opdyke, J. M. Trafford, B. D. Russell, and D. I. Kline Dolomite is a magnesium-rich carbonate mineral abundant in fossil carbonate reef platforms but surprisingly rare in modern sedimentary environments, a conundrum known as the ''Dolomite Problem". Marine sedimentary dolomite has been interpreted to form by an unconfirmed, post-depositional diagenetic process, despite minimal experimental success at replicating this. Here we show that dolomite, accompanied by magnesite, forms within living crustose coralline alga, Hydrolithon onkodes , a prolific global tropical reef species. Chemical micro-analysis of the coralline skeleton reveals that not only are the cell walls calcitised, but that cell spaces are typically filled with magnesite, rimmed by dolomite, or both. Mineralogy was confirmed by X-ray diffraction. Thus there are at least three mineral phases present (magnesium calcite, dolomite and magnesite) rather than one or two (magnesium calcite and brucite) as previously thought. Our results are consistent with dolomite occurrences in coralline algae rich environments in fossil reefs. Instead of a theory of post-depositional dolomitisation, we present evidence revealing biomineralization that can account for the massive formations seen in the geologic record. Additionally, our findings imply that previously unrecognized dolomite and magnesite have formed throughout the Holocene. This discovery together with the scale of coralline algae dominance in past shallow carbonate environments raises the possibility that environmental factors driving this biological dolomitisation process have influenced the global marine magnesium/calcium cycle. Perhaps, most importantly, we reveal that what has been considered a geological process can be a biological process, having many implications for both disciplines.

Description: Transport and fate of hexachlorocyclohexanes in the oceanic air and surface seawater Biogeosciences Discussions, 8, 5537-5562, 2011 Author(s): Z. Xie, B. P. Koch, A. Möller, R. Sturm, and R. Ebinghaus Hexachlorocyclohexanes (HCHs) are ubiquitous organic pollutants derived from pesticide application. They are subject to long-range transport, persistent in the environment, and capable of accumulation in biota. Shipboard measurements of HCH isomers (α-, γ- and β-HCH) in surface seawater and boundary layer atmospheric samples were conducted in the Atlantic and the Southern Ocean in October to December of 2008. ΣHCHs concentrations (the sum of α-, γ- and β-HCH) in the lower atmosphere ranged from 11.8 to 36.9 pg m −3 (mean: 26.6 ± 11.0 pg m −3 ) in the Northern Hemisphere (NH), and from 1.5 to 4.0 pg m −3 (mean: 2.8 ± 1.1 pg m −3 ) in the Southern Hemisphere (SH), respectively. Water concentrations were: α-HCH 0.33–46.8 pg l −1 , γ-HCH 0.02–33.2 pg l −1 and β-HCH 0.11–2 pg l −1 . HCH concentrations decreased from the North Atlantic to the Southern Ocean, indicating historical use of HCHs in the NH. Spatial distribution showed increasing concentrations from the equator towards North and South latitudes illustrating the concept of cold condensation and less interhemispheric mixing process. In comparison to concentrations measured in 1987–1999/2000, gaseous HCHs were slightly lower, while dissolved HCHs decreased by factor of 2–3 orders of magnitude. Air-water exchange gradients suggested net deposition for α-HCH (mean: 3759 pg m −2 day −1 ) and γ-HCH (mean: 1987 pg m −2 day −1 ), whereas β-HCH varied between equilibrium (volatilization:

Description: Confocal Raman microscopy as a tool to describe different mineral and organic phases at high spatial resolution within marine biogenic carbonates: case study on Nerita undata (Gastropoda, Neritopsina) Biogeosciences Discussions, 8, 5563-5585, 2011 Author(s): G. Nehrke and J. Nouet Marine biogenic carbonates formed by invertebrates (e.g. corals and mollusk shells) represent complex composites of one or more mineral phases and organic molecules. This complexity ranges from the macroscopic structures observed with the naked eye down to sub micrometric structures only revealed by micro analytical techniques. Understanding to what extent and how organisms can control the formation of these structures requires that the mineral and organic phases can be identified and their spatial distribution related. Here we demonstrate the capability of confocal Raman microscopy applied to cross sections of a shell of Nerita undata to describe the distribution of calcite and aragonite including their crystallographic orientation with high lateral resolution (∼300 nm). Moreover, spatial distribution of functional groups of organic compounds can be simultaneously acquired, allowing to specifically relate them to the observed microstructures. The data presented in this case study highlights the possible new contributions of this method to the description of modalities of Nerita undata shell formation, and what could be expected of its application to other marine biogenic carbonates. Localization of areas of interest would also allow further investigations using more localized methods, such as TEM that would provide complementary information on the relation between organic molecules and crystallographic lattice.

Description: Exploring B/Ca as a pH proxy in bivalves: relationships between Mytilus californianus B/Ca and environmental data from the northeast Pacific Biogeosciences Discussions, 8, 5587-5616, 2011 Author(s): S. J. McCoy, L. F. Robinson, C. A. Pfister, J. T. Wootton, and N. Shimizu A distinct gap in our ability to understand changes in coastal biology that may be associated with recent ocean acidification is the paucity of directly measured ocean environmental parameters at coastal sites in recent decades. Thus, many researchers have turned to sclerochronological reconstructions of water chemistry to document the historical seawater environment. In this study, we explore the relationships between B/Ca and pH to test the feasibility of B/Ca measured on the ion probe as a pH proxy in the California mussel, Mytilus californianus . We compare the M. californianus B/Ca record to directly measured environmental data during mussel growth 1999–2009 to determine the correlation between B/Ca and seawater chemistry and discuss methods for assigning sample chronology when sampling an organism with variable growth rates.

Description: Monoterpene and sesquiterpene emissions from Quercus coccifera exhibit interacting responses to light and temperature Biogeosciences Discussions, 8, 5691-5728, 2011 Author(s): M. Staudt and L. Lhoutellier Light and temperature are known to be the most important environmental factors controlling biogenic volatile organic compound (BVOC) emissions from plants, but little is known about their interdependencies especially for BVOCs other than isoprene. We studied light responses at different temperatures and temperature responses at different light levels of foliar BVOC emissions, photosynthesis and chlorophyll fluorescence on Quercus coccifera , an evergreen oak widespread in Mediterranean shrublands. More than 50 BVOCs were detected in the emissions from Q. coccifera leaves most of them being isoprenoids plus a few green leaf volatiles (GLVs). Under standard conditions non-oxygenated monoterpenes (MT-hc) accounted for about 90 % of the total BVOC release (mean ± SD: 738 ± 378 ng m −2 projected leaf area s −1 or 13.1 ± 6.9 μg g −1 leaf dry weight h −1 ) and oxygenated monoterpenes (MT-ox) and sesquiterpenes (SQTs) accounted for the rest in about equal proportions. Except GLVs, emissions of all BVOCs responded positively to light and temperature. The light responses of MT and SQT emissions resembled that of CO 2 -assimilation and were little influenced by the assay temperature: at high assay temperature, MT-hc emissions saturated at lower light levels than at standard assay temperature and tended even to decrease in the highest light range. The emission responses to temperature showed mostly Arrhenius-type response curves, whose shapes in the high temperature range were clearly affected by the assay light level and were markedly different between isoprenoid classes: at non-saturating light, all isoprenoids showed a similar temperature optimum (~43 °C), but, at higher temperatures, MT-hc emissions decreased faster than MT-ox and SQT emissions. At saturating light, MT-hc emissions peaked already around 37 °C and rapidly dropped at higher temperatures, whereas MT-ox and SQT emissions strongly increased between 40 and 50 °C accompanied by a burst of GLVs. In all experiments, decreases of MT-hc emissions under high temperatures were correlated with decreases in CO 2 -assimilation and/or photosynthetic electron transport. We conclude that light and temperature can have interactive short-term effects on the quantity and quality of BVOC emissions from Q. coccifera through substrate limitations of MT biosynthesis occurring at temperatures supraoptimal for photosynthetic processes that are exacerbated by oxidative stress and membrane damages. Such interactive effects are likely to occur frequently during hot and dry summers and simulations made in this work showed that they may have important consequences for emission predictions.

Description: Stand age and tree species affect N 2 O and CH 4 exchange from afforested soils Biogeosciences Discussions, 8, 5729-5760, 2011 Author(s): J. R. Christiansen and P. Gundersen Afforestation of former agricultural land is a means to mitigate anthropogenic greenhouse gas emissions. The objectives of this study were to assess the effect of pedunculate oak and Norway Spruce of different stand ages (13–17 and 40 yr after afforestation, respectively) on N 2 O and CH 4 exchange and identify the environmental factors responsible for the differences in gas exchange between tree species of different ages. N 2 O and CH 4 fluxes (mean ± SE) were measured for two years at an afforested site. No species difference was documented for N 2 O emission (oak: 4.2 ± 0.7 μg N 2 O-N m −2 h −1 , spruce: 4.0 ± 1 μg N 2 O-N m −2 h −1 ) but the youngest stands (1.9 ± 0.3 μg N 2 O-N m −2 h −1 ) emitted significantly less N 2 O than older stands (6.3 ± 1.2 μg N 2 O-N m −2 h −1 ). CH 4 exchange did not differ significantly between tree species (oak: −8.9 ± 0.9, spruce: −7.7 ± 1) or stand age (young: −7.3 ± 0.9 μg CH 4 -C m −2 h −1 , old: −9.4 ± 1 μg CH 4 -C m −2 h −1 ) but interacted significantly; CH 4 oxidation increased with age in oak and decreased with age for Norway Spruce. We conclude that the exchange of N 2 O and CH 4 from the forest soil undergoes a quick and significant transition in the first four decades after planting in both oak and Norway Spruce related to physical changes in the top soil and availability of soil N.

Description: Species-specific trajectories of nitrogen isotopes in Indiana hardwood forests, USA Biogeosciences Discussions, 8, 5935-5954, 2011 Author(s): K. K. McLauchlan and J. M. Craine Humans have drastically altered the global nitrogen (N) cycle, and these alterations have begun to affect a variety of ecosystems. In North America, N deposition rates are highest in the Central US, yet there are few studies that examine whether N availability has been increasing to different tree species in the forests of the region. To determine the species-specific trajectories of N availability in secondary temperate forests experiencing high N deposition, we measured the N concentrations and composition of stable N isotopes in wood of four tree species from six hardwood forest remnants in Northern Indiana, USA. Annual nitrogen deposition rates averaged 5.8 kg ha −1 from 2000 to 2008 in this region. On average, wood δ 15 N values in Quercus alba have been increasing steadily over the past 100 yr. In contrast, wood δ 15 N values have been declining in three other hardwood species – Acer saccharum , Carya ovata , and Fagus grandifolia – over the same time period. The species-specific trends suggest a change in the partitioning of ammonium and nitrate among species, a change in nitrification rates, and/or offsetting changes in the dependence of species on mycorrhizal fungi. With no apparent net change in wood δ 15 N over the past century at the stand level, on average there appears to have been little change in N availability for these forests despite high levels of N deposition in the region.

Description: Decreased summer drought affects plant productivity and soil carbon dynamics in Mediterranean woodland Biogeosciences Discussions, 8, 5955-5990, 2011 Author(s): M. F. Cotrufo, G. Alberti, I. Inglima, H. Marjanović, D. LeCain, A. Zaldei, A. Peressotti, and F. Miglietta Precipitation patterns are expected to change in the Mediterranean region within the next decades, with projected decreases in total rainfall and increases in extreme events. We manipulated precipitation patterns in a Mediterranean woodland, dominated by Arbutus unedo L., to study the effects of changing precipitation regimes on above-ground net primary production (ANPP) and soil C dynamics, specifically plant-derived C input to soil and soil respiration (SR). Experimental plots were exposed to either a 20 % reduction of throughfall or to water addition targeted at maintaining soil water content above a minimum of 10 % v/v. Treatments were compared to control plots which received ambient precipitation. The throughfall manipulation experiment started in 2004 and we report data up to the 2009 growing season. Enhanced soil moisture during summer months highly stimulated annual stem primary production, litter fall, SR and net annual plant-derived C input to soil which on average increased by 130 %, 26 %, 50 % and 220 %, respectively, as compared to control. In contrast, the 20 % reduction in throughfall (equivalent to 10 % reduction of precipitation) did not significantly change soil moisture at the site, and therefore did not significantly affect ANPP or SR. We conclude that minor changes (around 10 % reduction) in precipitation amount are not likely to significantly affect ANPP or soil C dynamics in Mediterranean woodland. However, if summer rain increases, C cycling will significantly accelerate but soil C stocks are not likely to be changed in the short-term. More studies involving modelling of long term C dynamics are needed to predict if the estimated increases in soil C input under wet conditions is going to be sustained and if labile C is being substituted to stable C, with a negative effect on long term soil C stocks.

Description: Novel applications of carbon isotopes in atmospheric CO 2 : what can atmospheric measurements teach us about processes in the biosphere? Biogeosciences Discussions, 8, 4603-4631, 2011 Author(s): A. P. Ballantyne, J. B. Miller, I. T. Baker, P. P. Tans, and J. W. C. White Conventionally, measurements of carbon isotopes in atmospheric CO 2 (δ 13 CO 2 ) have been used to partition fluxes between terrestrial and ocean carbon pools. However, novel analytical approaches combined with an increase in the spatial extent and frequency of δ 13 CO 2 measurements allow us to conduct a global analysis of δ 13 CO 2 variability to infer the isotopic composition of source CO 2 to the atmosphere (δ s ). This global analysis yields coherent seasonal patterns of isotopic enrichment. Our results indicate that seasonal values of δ s are more highly correlated with vapor pressure deficit ( r =0.404) than relative humidity ( r =0.149). We then evaluate two widely used stomatal conductance models and determine that Leuning Model, which is primarily driven by vapor pressure deficit is more effective globally at predicting δ s (RMSE = 1.7 ‰) than the Ball-Berry model, which is driven by relative humidity (RMSE = 2.8) ‰. Thus stomatal conductance on a global scale may be more sensitive to changes in vapor pressure deficit than relative humidity. This approach highlights a new application of using δ 13 CO 2 measurements to test global models.

Description: Effects of cloudiness on carbon dioxide exchange over an irrigated maize cropland in northwestern China Biogeosciences Discussions, 8, 1669-1691, 2011 Author(s): B. C. Zhang, J. J. Cao, Y. F. Bai, S. J. Yang, L. Hu, and Z. G. Ning Clouds can strongly influence solar radiation and affects other microclimatic factors (such as air temperature and vapour pressure deficit), and those changed environmental conditions may exert strong effects on carbon exchange between terrestrial ecosystems and the atmosphere. In this study, we analyzed how canopy photosynthesis and ecosystem respiration respond to changes in cloudy conditions, based on two years of eddy-covariance and meteorological data from an irrigated maize cropland in Yingke oasis of northwestern China. The results showed that net carbon uptake was more negative under cloudy than under clear conditions, it indicates that net carbon uptake increased under cloudy days. The rate of ecosystem respiration (Re) decreased under cloudy conditions due to decreased air temperature. However, photosynthesis was suppressed by the decreasing air temperature and vapour pressure deficit (VPD) under cloudy skies. Thus, the enhancement of net carbon uptake under cloudy skies mainly contributed from increasing photosynthesis with diffuse radiation. Those results improve our understanding of the effects of cloud cover on carbon exchange process in maize (C4) cropland, and improve our understanding of the driver improving net carbon uptake under cloudy conditions.

Description: The effect of resource history on the functioning of soil microbial communities is maintained across time Biogeosciences Discussions, 8, 1643-1667, 2011 Author(s): A. D. Keiser, M. S. Strickland, N. Fierer, and M. A. Bradford Historical resource conditions appear to influence microbial community function. With time, historical influences might diminish as populations respond to the contemporary environment. Alternatively, they may persist given factors such as contrasting genetic potentials for adaptation to a new environment. Using experimental microcosms, we test competing hypotheses that function of distinct soil microbial communities in common environments (H1 a ) converge or (H1 b ) remain dissimilar over time. Using a 6 × 2 (soil community inoculum × litter environment) full-factorial design, we compare decomposition rates in experimental microcosms containing grass or hardwood litter environments. After 100 days, communities that develop are inoculated into fresh litters and decomposition followed for another 100 days. We repeat this for a third, 100-day period. In each successive, 100-day period, we find higher decomposition rates (i.e. functioning) suggesting communities function better when they have an experimental history of the contemporary environment. Despite these functional gains, differences in decomposition rates among initially distinct communities persist, supporting the hypothesis that dissimilarity is maintained across time. In contrast to function, community composition is more similar following a common, experimental history. We also find that "specialization" on one experimental environment incurs a cost, with loss of function in the alternate environment. For example, experimental history of a grass-litter environment reduced decomposition when communities were inoculated into a hardwood-litter environment. Our work demonstrates experimentally that despite expectations of fast growth rates, physiological flexibility and rapid evolution, initial functional differences between microbial communities are maintained across time. These findings question whether microbial dynamics can be omitted from models of ecosystem processes if we are to predict reliably global change effects on biogeochemical cycles.

Description: Impacts of land cover and climate data selection on understanding terrestrial carbon dynamics and the CO 2 airborne fraction Biogeosciences Discussions, 8, 1617-1642, 2011 Author(s): B. Poulter, D. C. Frank, E. L. Hodson, and N. E. Zimmermann Terrestrial and oceanic carbon cycle processes remove ~ 55% of global carbon emissions, with the remaining 45%, known as the "airborne fraction", accumulating in the atmosphere. The long-term dynamics of the component fluxes contributing to the airborne fraction are challenging to interpret, but important for informing fossil-fuel emission targets and for monitoring the trends of biospheric carbon fluxes. Climate and land-cover forcing data for terrestrial ecosystem models are a largely unexplored source of uncertainty in terms of their contribution to understanding airborne fraction dynamics. Here we present results using a single dynamic global vegetation model forced by an ensemble experiment of climate (CRU, ERA-Interim, NCEP-DOE II), and diagnostic land-cover datasets (GLC2000, GlobCover, MODIS). Forcing uncertainties resulted in a large range of simulated global carbon fluxes, up to 13% for net primary production (52.4 to 60.2 Pg C a −1 ) and 19% for soil respiration (44.2 to 54.8 Pg C a −1 ). The sensitivity of contemporary global terrestrial carbon fluxes to climate strongly depends on forcing data (1.2–5.9 Pg C K −1 or 0.5 to 2.7 ppmv CO 2 K −1 ), but weakening carbon sinks in sub-tropical regions and strengthening carbon sinks in northern latitudes are found to be robust. The climate and land-cover combination that best correlate to the inferred carbon sink, and with the lowest residuals, is from observational data (CRU) rather than reanalysis climate data and with land-cover categories that have more stringent criteria for forest cover (MODIS). Since 1998, an increasing positive trend in residual error from bottom-up accounting of global sinks and sources (from 0.03 (1989–2005) to 0.23 Pg C a −1 (1998–2005)) suggests that either modeled drought sensitivity of carbon fluxes is too high, or that the trend toward decreased net land-use fluxes (~ 0.5 Pg C) is overestimated.

Description: Diversity of cultivated and metabolically active aerobic anoxygenic phototrophic bacteria along an oligotrophic gradient in the Mediterranean Sea Biogeosciences Discussions, 8, 4421-4457, 2011 Author(s): C. Jeanthon, D. Boeuf, O. Dahan, F. Le Gall, L. Garczarek, E. M. Bendif, and A.-C. Lehours Aerobic anoxygenic phototrophic (AAP) bacteria play significant roles in the bacterioplankton productivity and biogeochemical cycles of the surface ocean. In this study, we applied both cultivation and mRNA-based molecular methods to explore the diversity of AAP bacteria along an oligotrophic gradient in the Mediterranean Sea in early summer 2008. Colony-forming units obtained on three different agar media were screened for the production of bacteriochlorophyll- a (BChl- a ), the light-harvesting pigment of AAP bacteria. BChl- a -containing colonies represented a low part of the cultivable fraction. In total, 52 AAP strains were isolated and the phylogenetic analyses based on their 16S rRNA and pufM genes showed that they were all affiliated to the Alphaproteobacteria . The most frequently isolated strains belonged to Citromicrobium bathyomarinum , and Erythrobacter and Roseovarius species. Most other isolates were related to species not reported to produce BChl- a and/or may represent novel taxa. Direct extraction of RNA from seawater samples enabled the analysis of the expression of pufM , the gene coding for the M subunit of the reaction centre complex of aerobic anoxygenic photosynthesis. Clone libraries of pufM gene transcripts revealed that most phylotypes were highly similar to sequences previously recovered from the Mediterranean Sea and a large majority (~94%) was affiliated with the Gammaproteobacteria . The most abundantly detected phylotypes occurred in the western and eastern Mediterranean basins. However, some were exclusively detected in the eastern basin, reflecting the highest diversity of pufM transcripts observed in this ultra-oligotrophic region. To our knowledge, this is the first study to document extensively the diversity of AAP isolates and to unveil the active AAP community in an oligotrophic marine environment. By pointing out the discrepancies between culture-based and molecular methods, this study highlights the existing gaps in the understanding of the AAP bacteria ecology, especially in the Mediterranean Sea and likely globally.

Description: Anthropogenic impact on biogenic Si pools in temperate soils Biogeosciences Discussions, 8, 4391-4419, 2011 Author(s): W. Clymans, E. Struyf, G. Govers, F. Vandevenne, and D. J. Conley Human land use changes directly affect silica (Si) mobilisation and Si storage in terrestrial ecosystems and influence Si export from the continents, although the magnitudes of the impact are unknown. Yet biogenic silica (BSi) in soils is an understudied aspect. We have quantified and compared total biogenic (PSi a ) and easily soluble (PSi e ) Si pools at four sites along a gradient of disturbance in southern Sweden. An estimate of the magnitude of change in temperate continental BSi pools due to human disturbance is provided. Land use clearly affects BSi pools and their distribution. Total PSi a and PSi e for a continuous forested site at Siggaboda Nature Reserve (66 900 ± 22 800 kg SiO 2 ha −1 and 952 ± 16 kg SiO 2 ha −1 ) are significantly higher than disturbed land use types from the Råshult Culture Reserve including arable land (28 800 ± 7200 kg SiO 2 ha −1 and 239 ± 91 kg SiO 2 ha −1 ), pasture sites (27 300 ± 5980 kg SiO 2 ha −1 and 370 ± 129 kg SiO 2 ha −1 ) and grazed forest (23 600 ± 6370 kg SiO 2 ha −1 and 346 ± 123 kg SiO 2 ha −1 ). Vertical PSi a and PSi e profiles show significant ( p

Description: Response of δ 13 C in plant and soil respiration to a water pulse Biogeosciences Discussions, 8, 4493-4527, 2011 Author(s): Y. Salmon, N. Buchmann, and R. L. Barnard Stable carbon isotopes have been used to assess the coupling between changes in environmental conditions and the response of soil or ecosystem respiration, usually by studying the time-lagged response of δ 13 C of respired CO 2 (δ 13 C R ) to changes in photosynthetic carbon isotope discrimination (Δ i ). However, the lack of a systematic response of δ 13 C R to environmental changes in field studies stresses the need to better understand the mechanisms to this response. We experimentally created a wide range of carbon allocation and respiration conditions in Fagus sylvatica mesocosms, by growing saplings under different temperatures and girdling combinations. After a period of drought, a water pulse was applied and the short-term responses of δ 13 C in soil CO 2 efflux (δ 13 C R soil ) and δ 13 C in aboveground plant respiration (δ 13 C R above ) were measured, as well as leaf gas exchange rates and soil microbial biomass δ 13 C responses. Both δ 13 C R soil and δ 13 C R above values of all the trees decreased immediately after the water pulse. These responses were not driven by changes in Δ i , but rather by a fast release of C stored in roots and shoots. Changes in δ 13 C R soil associated with the water pulse were significantly positively correlated with changes in stomatal conductance, showing a strong impact of the plant component on δ 13 C R soil . However, three days after the water pulse in girdled trees, changes in δ 13 C R soil were related to changes in microbial biomass δ 13 C, suggesting that changes in the carbon source respired by soil microorganisms also contributed to the response of δ 13 C R soil . Our study shows that improving our mechanistic understanding of the responses of δ 13 C R to changes in environmental conditions requires the understanding of not only the plant's physiological responses, but also the responses of soil microorganisms and of plant-microbial interactions.

Description: Paleofires and the dynamics of carbon cycling in Chinese Loess Plateau over the last two glacial cycles Biogeosciences Discussions, 8, 4459-4492, 2011 Author(s): X. Wang and Z. L. Ding The spatio-temporal changes in trace gas emissions and burnt biomass by paleofires in Chinese Loess Plateau over the last two glacial cycles have been reconstructed using vegetation (C3/C4) specific fire emission factors and black carbon records in three loess-paleosol sections. Results show that the average mass emission rate (AMER) of total trace gases (TTG) and burnt biomass by fires (BBF) in glacial periods are 1~2 times higher than in interglacial periods, and they display a clear southward decrease during both glacial and interglacial periods. This pattern reflects the combined control on paleofire emissions by climate-induced fire regimes and succession of vegetation types. The substantial increases in TTG-AMER and BBF during the late Holocene relative to the middle-to-early Holocene at Lingtai and Weinan support existing conclusion that increased anthropogenic fire activities have occurred in Middle and Southern Plateau during late Holocene. To assess the influence of paleofires on soil carbon stocks, the ratios of BBF to above-ground net primary productivity (ANPP), estimated by magnetic susceptibility-based paleorainfall and paleotemperature reconstruction, were calculated. In the Northern Plateau, the BBF/ANPP ratios during glacial periods are nearly 90%, about 2~3 times higher than during interglacial periods, suggesting paleofires may be the overwhelming force modulating the cycling of terrestrial organic carbon in the region. However, in Middle and Southern Plateau, the large decrease in BBF/ANPP ratios to around 20% and 10% respectively during glacial and interglacial periods suggests that paleofires have had a minor impact on carbon storage in these areas during both glacial and interglacial periods.

Description: Recent global CO 2 flux inferred from atmospheric COsub〉2 observations and its regional analyses Biogeosciences Discussions, 8, 3497-3536, 2011 Author(s): F. Deng and J. M. Chen The net surface exchange of CO 2 for the years 2002–2007 is inferred from 12 181 atmospheric CO 2 concentration data with a time-dependent Bayesian synthesis inversion scheme. Monthly CO 2 fluxes are optimized for 30 regions of the North America and 20 regions for the rest of the globe. Although there have been many previous multiyear inversion studies, the reliability of atmospheric inversion techniques is not yet been systematically evaluated for quantifying regional interannual variability in the carbon cycle. In this study, the global interannual variability of the CO 2 flux is found to be dominated by terrestrial ecosystems and is mostly caused by tropical land, and the variations of regional terrestrial carbon fluxes are closely related to climate variations. These interannual variations are mostly caused by abnormal meteorological conditions in a few months in the year or part of a growing season and cannot be well represented using annual means, suggesting that we should pay attention to monthly or submonthly climate variations in ecosystem modeling. We find that, excluding fossil fuel and biomass burning emissions, terrestrial ecosystems and oceans absorb an average of 3.63±0.49 and 1.94±0.41 Pg C/yr, respectively. The terrestrial uptake is mainly in northern land while the tropical and southern lands contribute 0.62±0.47, and 0.67±0.34 Pg C/yr to the sink, respectively. In North America, terrestrial ecosystems absorb 0.89±0.18 Pg C/yr on average with a strong flux density found in the south-east of the continent.

Description: Modelling post-fire vegetation recovery in Portugal Biogeosciences Discussions, 8, 4559-4601, 2011 Author(s): A. Bastos, C. Gouveia, C. C. DaCamara, and R. M. Trigo Wildfires in Mediterranean Europe have been increasing in number and extension over the last decades and constitute one of the major disturbances of these ecosystems. Portugal is the country with more burnt area in the last decade and the years of 2003 and 2005 were particularly devastating, the total burned areas of 425 000 and 338 000 ha being several times higher than the corresponding average. The year of 2005 further coincided with one of the most severe droughts since early 20th century. Due to different responses of vegetation to diverse fire regimes and to the complexity of landscape structures, fires have complex effects on vegetation recovery. Remote sensing has revealed to be a powerful tool in studying vegetation dynamics and in monitoring post-fire vegetation recovery, which is crucial to land-management and to prevent erosion. The main goals of the present work are (i) to assess the accuracy of a vegetation recovery model previously developed by the authors; (ii) to assess the model's performance, namely its sensitivity to initial conditions, to the temporal length of the input dataset and to missing data; (iii) to study vegetation recovery over two selected areas that were affected by two large wildfire events in the fire seasons of 2003 and 2005, respectively. The study relies on monthly values of NDVI over 11 yr (1998–2009), at 1 × 1 km spatial resolution, as obtained by the VEGETATION instrument. According to results from sensitivity analysis, the model is robust and able to provide good estimations of recovery times of vegetation when the regeneration process is regular, even when missing data is present. In what respect to the two selected burnt scars, results indicate that fire damage is a determinant factor of regeneration, as less damaged vegetation recovers more rapidly, which is mainly justified by the high coverage of Pinus Pinaster over the area, and by the fact that coniferous forests tend to recover slower than transitional woodland-shrub, which tend to dominate the areas following the fire event.

Description: Quantifying in-situ gas hydrates at active seep sites in the eastern Black Sea using pressure coring technique Biogeosciences Discussions, 8, 4529-4558, 2011 Author(s): K. Heeschen, M. Haeckel, I. Klaucke, M. K. Ivanov, and G. Bohrmann In the eastern Black Sea, we determined methane (CH 4 ) concentrations, gas hydrate volumes and their vertical distribution from combined gas and chloride (Cl − ) measurements within pressurized sediment cores. The total gas volume collected from the cores corresponds to concentrations of 1.2–1.4 mol of methane per kg porewater at in-situ pressure, which is equivalent to a gas hydrate saturation of 15–18% of pore volume and amongst the highest values detected in shallow seep sediments. At the central seep site, a high-resolution Cl − profile resolves the upper gas hydrate stability boundary and a continuous layer of hydrates in a sediment column of 120 cm thickness. Including this information, a more precise gas hydrate saturation of 22–24% pore volume can be calculated. This is higher in comparison to a saturation calculated from the Cl − profile alone, resulting in 14.4%. The likely explanation is an active gas hydrate formation from CH 4 gas ebullition. The hydrocarbons at Batumi Seep are of shallow biogenic origin (CH 4 〉 99.6%), at Pechori Mound they originate from deeper thermocatalytic processes as indicated by the lower ratios of C 1 to C 2 –C 3 and the presence of C 5 .

Description: Revisiting land cover observations to address the needs of the climate modelling community Biogeosciences Discussions, 8, 7713-7740, 2011 Author(s): S. Bontemps, M. Herold, L. Kooistra, A. van Groenestijn, A. Hartley, O. Arino, I. Moreau, and P. Defourny One of the relevant processes driven by political discussion under the United Framework Convention on Climate Change is the monitoring of Essential Climate Variables. Land Cover is one of those variables and efforts are therefore to be made to develop land cover observation approaches which meet the climate modelling community expectations. This paper aims at contributing to this necessity. First, consultation mechanisms were established with the climate modelling community to identify its specific requirements in terms of satellite-based global land cover products. This assessment highlighted specific needs in terms of land cover characterization and products accuracy, stability and consistency that are currently not met. Based on this outcome, the paper calls into question the current land cover representation and for revisiting global land cover mapping approaches. Increasing the flexibility of current classification systems and making the mapping techniques less sensitive to the period of observation are proposed as two key aspects to enhance the usability of global land cover dataset.

Description: Integration of remote sensing data and surface observations to estimate the impact of the russian wildfires over Europe and Asia during August 2010 Biogeosciences Discussions, 8, 7741-7790, 2011 Author(s): L. Mei, Y. Xue, G. de Leeuw, J. Guang, Y. Wang, Y. Li, H. Xu, L. Yang, T. Hou, X. He, C. Wu, J. Dong, and Z. Chen A series of wildfires broke out in western Russia starting in late July of 2010. Harmful particulates and gases released into the local Russian atmosphere have been reported, as have possible negative consequences for the global atmosphere. In this study, an extremely hazy area and its transport trajectory on Russian wildfires were analysed using aerosol optical depth (AOD) images retrieved via the synergy method from Moderate Resolution Imaging Spectroradiometer (MODIS) data. In addition, we used trace gases (NO 2 and SO 2 ) and CO 2 products measured using Ozone Monitoring Instrument (OMI) data, vertical distribution of AOD data retrieved from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data, the mass trajectory analyses, synoptic maps from a HYSPLIT model simulation and ground-based data, including AERONET (both AOD and Ångström exponent) data and PM 2.5 . First, an Optimal Smoothing (OS) scheme was used to develop more precise and reliable AOD data based on multiple competing predictions made using several AOD retrieval models; then, integrated AOD and PM 2.5 data were related using a chemical transport model (GEOS-Chem), and the integrated AOD and visibility data were related using a 6S model. The results show that the PM 2.5 concentration is 3–5 times the normal amount based on both satellite data and in situ values with peak daily mean concentrations of approximately 500 μg m −3 . Also, the visibility of many parts of Russia, even Moscow, was less than 100 m; in some areas, the visibility was less than 50 m. Additionally, the possible impact on neighbouring countries due to the long-transport effect was also analysed during 31 July and 15 August 2010. A comparison of the satellite aerosol products and ground observations from the neighbouring countries suggests that wildfires in western Russian have had little impact on most European and Asian countries, the exceptions being Finland, Estonia, Ukraine and Kyrgyzstan. However, a possible impact on the Arctic region was also identified; such an effect would have a serious influence on the polar atmospheric environment and on animals such as polar bears.

Description: Biogeochemical controls on the bacterial population in the eastern Atlantic Ocean Biogeosciences Discussions, 8, 7791-7821, 2011 Author(s): S. B. Neogi, B. P. Koch, P. Schmitt-Kopplin, C. Pohl, G. Kattner, S. Yamasaki, and R. J. Lara Little is known about bacterial dynamics in the oligotrophic ocean, particularly about its cultivable population. We examined the abundance of total and cultivable bacteria in relation to changes in biogeochemical conditions in the eastern Atlantic Ocean with special regard to Vibrio spp., a group of bacteria that can cause diseases in human and aquatic organisms. Surface, deep water and plankton samples ( 55 μm) were collected between 50° N and 24° S. Chlorophyll- a was very low (

Description: Characterization of the bio-optical anomaly and diurnal variability of the particulate matter, as seen from the scattering and backscattering coefficients, in ultra-oligotrophic eddies of the Mediterranean Sea Biogeosciences Discussions, 8, 7859-7919, 2011 Author(s): H. Loisel, V. Vantrepotte, K. Norkvist, X. Mériaux, M. Kheireddine, J. Ras, M. Pujo-Pay, Y. Combet, K. Leblanc, R. Mauriac, D. Dessailly, and T. Moutin The variability of the inherent optical properties is investigated in the ultra-oligotrophic waters of the Mediterranean Sea sampled during the BOUM experiment performed during the early summer 2008. Bio-optical relationships found for the ultra-oligotrophic waters of the three anticyclonic gyres sampled significantly depart from the mean standard relationships provided for the global ocean, confirming the particular character of these Mediterranean waters. These optical anomalies are diversely related to the specific biological and environmental conditions occurring in the studied ecosystem. Specifically, the surface specific phytoplankton absorption coefficient exhibits values lower than those expected from the general relationships mainly in relation with a high contribution of relatively large sized phytoplankton. Conversely, the particulate backscattering coefficient, b bp , values are much higher than the mean standard values for a given chlorophyll- a concentration, TChl- a . This feature can presumably be related to the relevant influence of highly refractive submicrometer particles of Saharan origin in the surface layer of the water column. The present measurements also show that the Mediterranean Sea is greener than TChl- a alone indicates, as already stressed in previous studies. This color anomaly is partly explained by the estimated colored dissolved organic matter and submicrometer particles absorption coefficients, and to a greater extent by the high b bp /TChl- a values assuming that these particles backscatter light similarly in the green and blue parts of the visible spectrum. The diel variation of both the particulate matter attenuation and backscattering coefficients were also investigated specifically. Despite some differences in the timing and the magnitude of the daily oscillations found for these optical parameters, potential for the backscattering coefficient daily oscillation to be used, similarly to that for the attenuation coefficient, as a proxy for estimating carbon community production budget has been highlighted for the first time. This result is particularly relevant for present and future geostationary spatial ocean color missions.

Description: Contrasting biogeochemistry of nitrogen in the Atlantic and Pacific oxygen minimum zones Biogeosciences Discussions, 8, 8001-8039, 2011 Author(s): E. Ryabenko, A. Kock, H. W. Bange, M. A. Altabet, and D. W. R. Wallace We present new data for the stable isotope ratio of inorganic nitrogen species from the contrasting oxygen minimum zones (OMZs) of the Eastern Tropical North Atlantic, south of Cape Verde, and the Eastern Tropical South Pacific off Peru. Differences in minimum oxygen concentration and corresponding N-cycle processes for the two OMZs are reflected in strongly contrasting δ 15 N distributions. Pacific surface waters are marked by strongly positive values for δ 15 N-NO 3 − reflecting fractionation associated with subsurface N-loss and partial NO 3 − utilization. This contrasts with negative values in NO 3 − depleted surface waters of the Atlantic which are lower than can be explained by N supply via N 2 fixation. We suggest the negative values reflect inputs of nitrate, possibly transient, associated with deposition of Saharan dust. Strong signals of N-loss processes in the subsurface Pacific OMZ are evident in the isotope and N 2 O data, both of which are compatible with a contribution of canonical denitrification to overall N-loss. However the apparent N isotope fractionation factor observed is relatively low (ε d = 11.4 ‰) suggesting an effect of influence from denitrification in sediments. Identical positive correlation of N 2 O vs. AOU for waters with oxygen concentrations [O 2 ] 〉 50 μmol l −1 in both regions reflect a nitrification source. Sharp decrease in N 2 O concentrations is observed in the Pacific OMZ due to denitrification under oxygen concentrations O 2 〈 5 μmol l −1 .

Description: Carbon dioxide emissions from an Acacia plantation on peatland in Sumatra, Indonesia Biogeosciences Discussions, 8, 8269-8302, 2011 Author(s): J. Jauhiainen, A. Hooijer, and S. E. Page Peat surface CO 2 emission, groundwater table depth and peat temperature were monitored for two years along transects in an Acacia plantation on thick tropical peat (〉4 m) in Sumatra, Indonesia. A total of 2300 emission measurements were taken at 144 locations. The autotrophic root respiration component of the CO 2 emission was separated from heterotrophic emissions caused by peat oxidation in three ways: (i) by comparing CO 2 emissions within and beyond the tree rooting zone, (ii) by comparing CO 2 emissions with and without peat trenching (i.e. cutting any roots remaining in the peat beyond the tree rooting zone), and (iii) by comparing CO 2 emissions before and after Acacia tree harvesting. On average, the contribution of root respiration to daytime CO 2 emission is 21 % along transects in mature tree stands. At locations 0.5 m from trees this is up to 80 % of the total emissions, but it is negligible at locations more than 1.3 m away. This means that CO 2 emission measurements well away from trees are free of any root respiration contribution and thus represent only peat oxidation emission. We find daytime mean annual CO 2 emission from peat oxidation alone of 94 t ha −1 yr −1 at a mean water table depth of 0.8 m, and a minimum emission value of 80 t ha −1 yr −1 after correction for the effect of diurnal temperature fluctuations, which resulted in a 14.5 % reduction of the daytime emission. There is a positive correlation between mean long-term water table depths and peat oxidation CO 2 emission. However, no such relation is found for instantaneous emission/water table depth within transects and it is clear that factors other than water table depth also affect peat oxidation and total CO 2 emissions. The increase in the temperature of the surface peat due to plantation development may explain over 50 % of peat oxidation emissions.

Description: A new concept for simulation of vegetated land surface dynamics – Part 1: The event driven phenology model Biogeosciences Discussions, 8, 5281-5333, 2011 Author(s): V. Kovalskyy and G. M. Henebry Phenologies of the vegetated land surface are being used increasingly for diagnosis and prognosis of climate change consequences. Current prospective and retrospective phenological models stand far apart in their approaches to the subject. We report on an exploratory attempt to implement a phenological model based on a new event driven concept which has both diagnostic and prognostic capabilities in the same modeling framework. This Event Driven Phenological Model (EDPM) is shown to simulate land surface phenologies and phenophase transition dates in agricultural landscapes based on assimilation of weather data and land surface observations from spaceborne sensors. The model enables growing season phenologies to develop in response to changing environmental conditions and disturbance events. It also has the ability to ingest remotely sensed data to adjust its output to improve representation of the modeled variable. We describe the model and report results of initial testing of the EDPM using Level 2 flux tower records from the Ameriflux sites at Mead, Nebraska, USA, and at Bondville, Illinois, USA. Simulating the dynamics of normalized difference vegetation index based on flux tower data, the predictions by the EDPM show good agreement (RMSE 〈 0.08; r 2 〉0.8) for maize and soybean during several growing seasons at different locations. This study presents the EDPM used in the companion paper (Kovalskyy and Henebry, 2011) in a coupling scheme to estimate daily actual evapotranspiration over multiple growing seasons.

Description: Chemical composition of modern and fossil Hippopotamid teeth and implications for paleoenvironmental reconstructions and enamel formation: 1. major and minor element variation Biogeosciences Discussions, 8, 5197-5250, 2011 Author(s): G. Brügmann, J. Krause, T. C. Brachert, O. Kullmer, F. Schrenk, I. Ssemmanda, and D. F. Mertz Bioapatite in mammalian teeth is readily preserved in continental sediments and represents a very important archive for reconstructions of environment and climate evolution. This project intends to provide a detailed data base of major, minor and trace element and isotope tracers for tooth apatite using a variety of microanalytical techniques. The aim is to identify specific sedimentary environments and to improve our understanding on the interaction between internal metabolic processes during tooth formation and external nutritional control and secondary alteration effects. Here, we use the electron microprobe, to determine the major and minor element contents of fossil and modern molar enamel, cement and dentin from hippopotamids. Most of the studied specimens are from different ecosystems in Eastern Africa, representing modern and fossil lakustrine (Lake Kikorongo, Lake Albert, and Lake Malawi) and modern fluvial environments of the Nile River system. Secondary alteration effects in particular FeO, MnO, SO 3 and F concentrations, which are 2 to 10 times higher in fossil than in modern enamel; secondary enrichments in fossil dentin and cement are even higher. In modern and fossil enamel, along sections perpendicular to the enamel-dentin junction (EDJ) or along cervix-apex profiles, P 2 O 5 and CaO contents and the CaO/P 2 O 5 ratios are very constant (StdDev ~1 %). Linear regression analysis reveals very tight control of the MgO ( R 2 ∼0.6), Na 2 O and Cl variation (for both R 2 〉0.84) along EDJ-outer enamel rim profiles, despite large concentration variations (40 % to 300 %) across the enamel. These minor elements show well defined distribution patterns in enamel, similar in all specimens regardless of their age and origin, as the concentration of MgO and Na 2 O decrease from the enamel-dentin junction (EDJ) towards the outer rim, whereas Cl displays the opposite variation. Fossil enamel from hippopotamids which lived in the saline Lake Kikorongo have a much higher MgO/Na 2 O ratio (∼1.11) than those from the Neogene fossils of Lake Albert (MgO/Na 2 O∼0.4), which was a large fresh water lake like those in the western Branch of the East African Rift System today. Similarly, the MgO/Na 2 O ratio in modern enamel from the White Nile River (∼0.36), which has a Precambrian catchment of dominantly granite and gneisses and passes through several saline zones, is higher than that from the Blue Nile River, whose catchment is the Neogene volcanic Ethiopian Highland (MgO/Na 2 O∼0.22). Thus, particularly MgO/Na 2 O might be a sensitive fingerprint for environments where river and lake water have suffered strong evaporation. Enamel formation in mammals takes place at successive mineralization fronts within a confined chamber where ion and molecule transport is controlled by the surrounding enamel organ. During the secretion and maturation phases the epithelium generates different fluid composition, which in principle, should determine the final composition of enamel apatite. This is supported by co-linear relationships between MgO, Cl and Na 2 O which can be interpreted as binary mixing lines. However, if maturation starts after secretion is completed the observed element distribution can only be explained by recrystallization of existing and addition of new apatite during maturation. Perhaps the initial enamel crystallites precipitating during secretion and the newly formed bioapatite crystals during maturation equilibrate with a continuously evolving fluid. During crystallization of bioapatite the enamel fluid becomes continuously depleted in MgO and Na 2 O, but enriched in Cl which results in the formation of MgO, and Na 2 O-rich, but Cl-poor bioapatite near the EDJ and MgO- and Na 2 O-poor, but Cl-rich bioapatite at the outer enamel rim. The linkage between lake and river water composition, bioavailability of elements for plants, animal nutrition and tooth formation is complex and multifaceted. The quality and limits of the MgO/Na 2 O and other proxies have to be established with systematic investigations relating chemical distribution patterns to sedimentary environment and to growth structures developing as secretion and maturation proceed during tooth formation.

Description: Distribution of typical denitrifying functional genes and diversity of the nirS -encoding bacterial community related to environmental characteristics of river sediments Biogeosciences Discussions, 8, 5251-5280, 2011 Author(s): S. Huang, C. Chen, Q. Wu, R. Zhang, and X. Yang Denitrification in river sediments leads to nitrate removal from the aquatic system; therefore, it is necessary to understand functional diversity of denitrifier communities in the system. Sediment samples (0–25 cm depth) were collected from three typical locations along the Pearl River. The real-time PCR approach was used to measure the abundance of nitrate ( narG ), nitrite ( nirS , nirK and nrfA ), and nitrous oxide ( nosZ ) reductase genes from the sediment samples. Assemblages of nirS , nirK and nosZ indicated that complete denitrification occurred in sediment cores, with the greatest number of gene copies from 5–15 cm depth. Dissimilatory nitrate reduction appeared to be important below 15 cm depth, based on increasing gene copies of narG and nrfA with sediment depth. There was a close match (78–94 %) between the nirS sequences recovered from Pearl River sediment and those detected in estuarine and marine sediments as well as active sludge, suggesting that domestic sewage inputs and irregular tides. Canonical correspondence analysis indicated that the spatial distribution of denitrifying bacteria was highly correlated with dissolved inorganic N (DIN: NH 4 + , NO 2

Description: Alternative methods to predict actual evapotranspiration illustrate the importance of accounting for phenology – Part 2: The event driven phenology model Biogeosciences Discussions, 8, 5335-5378, 2011 Author(s): V. Kovalskyy and G. M. Henebry Evapotranspiration ( ET ) flux constitutes a major component of both the water and energy balances at the land surface. Among the many factors that control evapotranspiration, phenology poses a major source of uncertainty in attempts to predict ET . Contemporary approaches to ET modeling and monitoring frequently summarize the complexity of the seasonal development of vegetation cover into static phenological trajectories (or climatologies) that lack sensitivity to changing environmental conditions. The Event Driven Phenology Model (EDPM) offers an alternative, interactive approach to representing phenology. This study presents the results of an experiment designed to illustrate the differences in ET arising from various techniques used to mimic phenology in models of land surface processes. The experiment compares and contrasts two realizations of static phenologies derived from long-term satellite observations of the Normalized Difference Vegetation Index (NDVI) against canopy trajectories produced by the interactive EDPM trained on flux tower observations. The assessment was carried out through validation of predicted ET against records collected by flux tower instruments. The VegET model (Senay, 2008) was used as a framework to estimate daily actual evapotranspiration and supplied with seasonal canopy trajectories produced by the EDPM and traditional techniques. The interactive approach presented the following advantages over phenology modeled with static climatologies: (a) lower prediction bias in crops; (b) smaller root mean square error in daily ET – 0.5 mm per day on average; (c) stable level of errors throughout the season similar among different land cover types and locations; and (d) better estimation of season duration and total seasonal ET .

Description: Free and protected soil organic carbon dynamics respond differently to abandonment of mountain grassland Biogeosciences Discussions, 8, 9943-9976, 2011 Author(s): S. Meyer, J. Leifeld, M. Bahn, and J. Fuhrer Land-use change (LUC) and management are among the major driving forces of soil carbon (C) storage. Abandonment of mountain grassland promotes accumulation of aboveground biomass and litter, but related responses of soil organic matter (SOM) dynamics are uncertain. To determine SOM-C turnover we sampled 0–10 cm of soils along land-use gradients (hay meadows, grazed pastures and abandoned grasslands) in the European Alps varying in management intensity at Stubai Valley (MAT: 3 °C, P: 1097 mm) in Austria and Matsch Valley (MAT: 6.6 °C, P: 527 mm) in Italy. We determined C input and decomposition rates of labile water-floatable and free particulate organic matter (wPOM, fPOM

Description: Apparent oxygen utilization rates calculated from tritium and helium-3 profiles at the Bermuda Atlantic Time-series Study site Biogeosciences Discussions, 8, 9977-10015, 2011 Author(s): R. H. R. Stanley, S. C. Doney, W. J. Jenkins, and D. E. Lott, III We present three years of Apparent Oxygen Utilization Rates (AOUR) estimated from oxygen and tracer data collected over the ocean thermocline at monthly resolution between 2003 and 2006 at the Bermuda Atlantic Time-series Study (BATS) site. We estimate water ages by calculating a transit time distribution from tritium and helium-3 data. The vertically integrated AOUR over the upper 500 m, which is a regional estimate of export, during the three years is 3.1 ± 0.5 mol O 2 m −2 yr −1 . This is comparable to previous AOUR-based estimates of export production at the BATS site but is several times larger than export estimates derived from sediment traps or 234 Th fluxes. We compare AOUR determined in this study to AOUR measured in the 1980s and show AOUR is significantly greater today than decades earlier because of changes in AOU, rather than changes in ventilation rates. The changes in AOU may be a methodological artefact associated with problems with early oxygen measurements.

Description: Effects of soil rewetting and thawing on soil gas fluxes: a review of current literature and suggestions for future research Biogeosciences Discussions, 8, 9847-9899, 2011 Author(s): D.-G. Kim, R. Vargas, B. Bond-Lamberty, and M. R. Turetsky The rewetting of dry soils and the thawing of frozen soils are short-term, transitional phenomena in terms of hydrology and the thermodynamics of soil systems. The impact of these short-term phenomena on larger scale ecosystem fluxes has only recently been fully appreciated, and a growing number of studies show that these events affect various biogeochemical processes including fluxes of soil gases such as carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), ammonia (NH 3 ) and nitric oxide (NO). Global climate models predict that future climatic change is likely to alter the frequency and intensity of drying-rewetting events and thawing of frozen soils, highlighting the importance of understanding how rewetting and thawing will influence soil gas fluxes. Here we summarize findings in a new database based on 338 studies conducted from 1956 to 2010, and highlight open research questions. The database revealed conflicting results following rewetting and thawing in various terrestrial ecosystems, ranging from large increases in gas fluxes to non-significant changes. An analysis of published field studies ( n = 142) showed that after rewetting or thawing, CO 2 , CH 4 , N 2 O, NO and NH 3 fluxes increase from pre-event fluxes following a power function, with no significant differenced among gases. We discuss possible mechanisms and controls that regulate flux responses, and note that a high temporal resolution of flux measurements is critical to capture rapid changes in gas fluxes after these soil perturbations. Finally, we propose that future studies should investigate the interactions between biological (i.e. microbial community and gas production) and physical (i.e. flux, diffusion, dissolution) changes in soil gas fluxes, and explore synergistic experimental and modelling approaches.

Description: Tracing the transport of colored dissolved organic matter in water masses of the Southern Beaufort Sea: relationship with hydrographic characteristics Biogeosciences Discussions, 8, 11003-11040, 2011 Author(s): A. Matsuoka, A. Bricaud, R. Benner, J. Para, R. Sempéré, L. Prieur, S. Bélanger, and M. Babin Light absorption by colored dissolved organic matter (CDOM) ( a CDOM (λ)) plays an important role in the heat budget of the Arctic Ocean, contributing to the recent decline in sea ice, as well as in biogeochemical processes. We investigated a CDOM (λ) in the Southern Beaufort Sea where a significant amount of CDOM is delivered by the Mackenzie River. In the surface layer, a CDOM (440) showed a strong and negative correlation with salinity, indicating strong river influence and conservative transport in the river plume. Below the mixed layer, a weak but positive correlation between a CDOM (440) and salinity was observed above the upper halocline, resulting from the effect of removal of CDOM due to brine rejection and lateral intrusion of Pacific summer waters into these layers. In contrast, the relationship was negative in the upper and the lower haloclines, suggesting these waters originated from Arctic coastal waters. DOC concentrations in the surface layer were strongly correlated with a CDOM (440) ( r 2 = 0.97), suggesting that this value can be estimated in this area, using a CDOM (440) that is retrieved using satellite ocean color data. Implications for estimation of DOC concentrations in surface waters using ocean color remote sensing are discussed.

Description: Landscape patterns of soil oxygen and atmospheric greenhouse gases in a northern hardwood forest landscape Biogeosciences Discussions, 8, 10859-10893, 2011 Author(s): S. F. Werner, C. T. Driscoll, P. M. Groffman, and J. B. Yavitt The production and consumption of the greenhouse gases, carbon dioxide (CO 2 ), nitrous oxide (N 2 O), and methane (CH 4 ), are controlled by redox reactions in soils. Together with oxygen (O 2 ), seasonal and spatial dynamics of these atmospheric gases can serve as robust indicators of soil redox status, respiration rates, and nitrogen cycling. We examined landscape patterns of soil oxygen and greenhouse gas dynamics in Watershed 3 at the Hubbard Brook Experimental Forest, NH, USA. We analyzed depth profiles of soil O 2 , CO 2 , N 2 O, and CH 4 approximately bimonthly for one year. Soil gas depth profiles were obtained from several different soil types encompassing a range of topographic positions, drainage classes, and organic matter content. Soil O 2 was a good predictor of greenhouse gas concentrations. Unsaturated soils always had O 2 concentrations 〉18 %, while saturated soils had O 2 ranging from 0 to 18 %. For unsaturated soils, changes in CO 2 were nearly stoichiometric with O 2 . High concentrations of CH 4 (〉10 μL L −1 ) were typically associated with saturated soils; CH 4 was typically below atmospheric concentrations ( 5000 nL L −1 ) were found only in well-aerated soils after summer rainfall events and in marginally-anoxic soils; N 2 O was consumed (

Description: Annual emissions of CH 4 and N 2 O, and ecosystem respiration, from eight organic soils in Western Denmark managed by agriculture Biogeosciences Discussions, 8, 10017-10067, 2011 Author(s): S. O. Petersen, C. C. Hoffmann, C.-M. Schäfer, G. Blicher-Mathiesen, L. Elsgaard, K. Kristensen, S. E. Larsen, S. B. Torp, and M. H. Greve The use of organic soils by agriculture involves drainage and tillage, and the resulting increase in C and N turnover can significantly affect their greenhouse gas balance. This study estimated annual fluxes of CH 4 and N 2 O, and ecosystem respiration ( R eco ), from eight organic soils managed by agriculture. The sites were located in three regions representing different landscape types and climatic conditions, and three land use categories (arable crops, AR, grass in rotation, RG, and permanent grass, PG) were covered. The normal management at each site was followed, except that no N inputs occurred during the monitoring period from August 2008 to October 2009. The stratified sampling strategy further included six sampling points in three blocks at each site. Environmental variables (precipitation, PAR, air and soil temperature, soil moisture, groundwater level) were monitored continuously and during sampling campaigns, where also groundwater samples were taken for analysis. Gaseous fluxes were monitored on a three-weekly basis, giving 51, 49 and 38 field campaigns for land use categories AR, PG and RG, respectively. Climatic conditions in each region during monitoring were representative based on 20-yr averages. Peat layers were shallow, typically 0.5 to 1 m, and with a pH of 4–5. At six sites annual emissions of N 2 O were in the range 3 to 24 kg N 2 O-N ha −1 , but at two arable sites (spring barley, potato) net emissions of 38 and 61 kg N 2 O-N ha −1 were recorded. Both were characterized by fluctuating groundwater with elevated SO 4 2− concentrations. Annual fluxes of CH 4 were generally small, as expected, ranging from –2 to 4 kg CH 4 ha −1 . However, two permanent grasslands had tussocks of Juncus effusus (soft rush) in sampling points that were consistent sources of CH 4 throughout the year. Emission factors for organic soils in rotation and permanent grass, respectively, were estimated to be 0.011 and 0.47 g m −2 for CH 4 , and 2.5 and 0.5 g m −2 for N 2 O. This first documentation of CH 4 and N 2 O emissions from managed organic soils in Denmark confirms the levels and wide ranges of emissions previously reported for this region. However, the factorial approach also identified links between gaseous emissions and site-specific conditions with respect to soil, groundwater and vegetation which point to areas of future research that may account for part of the variability and hence lead to improved emission factors or models.

Description: Localising the nitrogen imprint of the Paris food supply: the potential of organic farming and changes in human diet Biogeosciences Discussions, 8, 10979-11002, 2011 Author(s): G. Billen, J. Garnier, V. Thieu, M. Silvestre, S. Barles, and P. Chatzimpiros The Seine watershed has long been the food-supplying hinterland of Paris, providing most of the animal and vegetal protein consumed in the city. Nowadays, because of the land specialisation of agriculture made possible by the shift from manure-based to synthetic nitrogen fertilisation, the Seine watershed, although it exports 80% of its huge cereal production, still provides most of the cereal consumed by the Paris agglomeration. The meat and milk supply originate, however, mainly from regions in the North and West of France, specialised in animal farming and importing about 30% of their feed from South America. As it works today, this system is responsible for a severe nitrate contamination of surface groundwater resources. Herein two scenarios of re-localising Paris's food supply are explored, based on organic farming and local provision of animal feed. We show that for the Seine watershed it is technically possible to design an agricultural system able to provide all the plant- and animal-based food required by the population, to deliver sub-root water meeting the drinking water standards and still to export a significant proportion of its production to areas less suitable for cereal cultivation. Decreasing the share of animal products in the human diet has a strong impact on the nitrogen imprint of urban food supply.

Description: Processes controlling the Si-isotopic composition in the Southern Ocean and application for paleoceanography Biogeosciences Discussions, 8, 10155-10185, 2011 Author(s): F. Fripiat, A.-J. Cavagna, F. Dehairs, A. de Brauwere, L. André, and D. Cardinal Southern Ocean biogeochemical processes have an impact on global marine primary production and global elemental cycling, e.g. by likely controlling glacial-interglacial p CO 2 variation. The natural silicon isotopic composition (δ 30 Si) of sedimentary biogenic silica has been used to reconstruct past Si-consumption:supply ratio in the surface waters. We present a new dataset in the Southern Ocean which includes for the first time summer δ 30 Si signatures of suspended biogenic silica (i) for the whole water column at three stations and (ii) in the mixed layer at seven stations from the sub-tropical zone up to the Weddell Gyre. In general, the biogenic silica isotopic composition at depth reflected a mixed layer origin and seemed not affected by any diagenetic effect in the water column, even if in the northern part of the Weddell Gyre an effect of biogenic silica dissolution cannot be ruled out. We develop a mechanistic understanding of the processes involved in the modern Si-isotopic balance, by implementing a mixed layer model. We observe that the accumulated biogenic silica (sensu Rayleigh) should satisfactorily describe the δ 30 Si composition of biogenic silica exported out of the mixed layer, within the limit of the current analytical precision on the δ 30 Si. The failures of previous models (Rayleigh and steady state) become apparent especially at the end of the productive period in the mixed layer, when biogenic silica production is low. This results from: (1) a higher biogenic silica dissolution:production ratio imposing a lower net fractionation factor and (2) a higher Si-supply:Si-uptake ratio supplying light Si-isotopes into the mixed layer. The latter effect is especially expressed when the summer mixed layer becomes strongly Si-depleted together with a large vertical silicic acid gradient.

Description: A model study on the sensitivity of surface ocean CO 2 pressure with respect to the CO 2 gas exchange rate Biogeosciences Discussions, 8, 10797-10821, 2011 Author(s): P. Landschützer, J. F. Tjiputra, K. Assmann, and C. Heinze Rising CO 2 concentrations in the atmosphere and a changing climate are expected to alter the air-sea CO 2 flux through changes in the respective control factors for gas exchange. In this study we determine the sensitivity of the CO 2 fluxes on the gas transfer velocity using the MICOM-HAMOCC isopycnic carbon cycle model. The monthly generated MICOM-HAMOCC output data are suitable to investigate seasonal variabilities concerning the exchange of CO 2 . In a series of 3 sensitivity runs the wind dependent gas exchange rate is increased by 44%, both in the northern and southern westerly regions, as well as in the equatorial area to investigate the effect of regional variations of the gas transfer rate on the air-sea fluxes and the distribution of the ocean surface p CO 2 . For the period between 1948–2009, the results show that locally increasing gas transfer rates do not play an important role concerning the global uptake of carbon from the atmosphere. While effects on a global and annual scale are low, the regional and intra-annual variability shows remarkable variations in the gas fluxes and the surface p CO 2 . An accurate quantification of the variable gas transfer velocity therefore provides a potential source to enhance model predictions over small spatial and temporal scales and to successfully reconcile model results on surface p CO 2 and air-sea CO 2 fluxes with observations.

Description: Multiple-factor controls on terrestrial N 2 O flux over North America from 1979 through 2010 Biogeosciences Discussions, 8, 10935-10977, 2011 Author(s): X. F. Xu, H. Q. Tian, M. L. Liu, W. Ren, G. S. Chen, C. Q. Lu, and C. Zhang Nitrous oxide (N 2 O) is a potent greenhouse gas which also contributes to the depletion of stratospheric ozone (O 3 ). However, the magnitude and underlying mechanisms for the spatiotemporal variations in the terrestrial sources of N 2 O are still far from certain. Using a process-based ecosystem model (DLEM – the Dynamic Land Ecosystem Model) driven by multiple global change factors, including climate variability, nitrogen (N) deposition, rising atmospheric CO 2 , trophospheric O 3 pollution, N fertilizer application, and land conversion, the spatial and temporal variations in terrestrial N 2 O flux over North America were examined and attributed to various driving factors. From 1979 to 2010, the North America accumulatively emitted 55.1 ± 0.8 Tg N 2 O-N (1 Tg = 10 12 g), of which global change factors contributed 2.8 ± 1.0 Tg N 2 O-N, and baseline emission contributed 52.3 ± 0.6 Tg N 2 O-N. Climate variability, N deposition, O 3 pollution, N fertilizer application, and land conversion increased N 2 O emission by 0.3 ± 0.7 Tg N 2 O-N, 0.5 ± 0.1 Tg N 2 O-N, 0.11 ± 0.02 Tg N 2 O-N, 1.2 ± 0.1 Tg N 2 O-N, and 0.2 ± 0.02 Tg N 2 O-N, respectively. The elevated atmospheric CO 2 led to a decrease in terrestrial N 2 O emission by 0.5 ± 0.07 Tg N 2 O-N. The interactive effect among multiple factors enhanced N 2 O emission by 0.9 ± 0.3 Tg N 2 O-N over the 32 years. At country level, climate variability and elevated atmospheric CO 2 decreased, while all other single factors and multiple-factor interaction enhanced N 2 O emission in the United States of America (USA) over the study period. During the same time period, elevated atmospheric CO 2 and multiple-factor interaction decreased, while other factors enhanced N 2 O emission from Canada. Elevated atmospheric CO 2 and land conversion decreased while other factors enhanced N 2 O emission from Mexico. The interactive effects among climate variables play a predominant role in controlling climate -induced changes in N 2 O emission at both continental and country levels. Central and southeastern parts of the North America – including central Canada, central USA, southeastern USA, and all of Mexico – experienced increases in N 2 O emission from 1979 to 2010. The effects of climate variability and multiple-factor interaction dominating the inter-annual variations in terrestrial N 2 O emission at both continental and country levels indicate that projected changes in the global climate system during this century may substantially alter the regime of N 2 O emission from terrestrial ecosystems. They also imply that the interactive effect among global change factors may significantly affect N 2 O flux, needing more investigations through field experiments.

Description: Simulation of anthropogenic CO 2 uptake in the CCSM3.1 ocean circulation-biogeochemical model: comparison with data-based estimates Biogeosciences Discussions, 8, 10895-10933, 2011 Author(s): S. Wang, J. K. Moore, F. W. Primeau, and S. Khatiwala The global ocean has taken up a large fraction of the CO 2 released by human activities since the industrial revolution. Quantifying the oceanic anthropogenic carbon (C ant ) inventory and its variability is important for predicting the future global carbon cycle. The detailed comparison of data-based and model-based estimates is essential for the validation and continued improvement of our prediction capabilities. So far, three global estimates of oceanic C ant inventory that are "data-based" and independent of global ocean circulation models have been produced: one based on the ΔC * method, and two are based on reconstructions of the Green function for the surface-to-interior transport, the TTD method and the maximum entropy inversion method (KPH). The KPH method, in particular, is capable of reconstructing the history of C ant inventory through the industrial era. In the present study we use forward model simulations of the Community Climate System Model (CCSM3.1) to estimate the C ant inventory and compare the results with the data-based estimates. We also use the simulations to test several assumptions of the KPH method, including the assumption of constant climate and circulation, which is common to all the data-based estimates. Though the integrated estimates of global C ant inventories are consistent with each other, the regional estimates show discrepancies up to 50 %. The CCSM3 model underestimates the total C ant inventory, in part due to weak mixing and ventilation in the North Atlantic and Southern Ocean. Analyses of different simulation results suggest that key assumptions about ocean circulation and air-sea disequilibrium in the KPH method are generally valid on the global scale, but may introduce significant errors in C ant estimates on regional scales. The KPH method should also be used with caution when predicting future oceanic anthropogenic carbon uptake.

Description: Role of land surface processes and diffuse/direct radiation partitioning in simulating the European climate Biogeosciences Discussions, 8, 11601-11630, 2011 Author(s): E. L. Davin and S. I. Seneviratne The influence of land processes and in particular of diffuse/direct radiation partitioning on surface fluxes and associated regional-scale climate feedbacks is investigated. ERA-40 driven simulations over Europe are performed using the COSMO-CLM 2 Regional Climate Model (RCM). Two alternative Land Surface Models (LSMs), a 2nd generation LSM (TERRA_ML) and a more advanced 3rd generation LSM (Community Land Model version 3.5), and two versions of the atmospheric component are tested, as well as a revised coupling procedure allowing for variations in diffuse/direct light partitioning at the surface, and their accounting by the land surface component. Overall, the RCM performance for various variables (e.g., surface fluxes, temperature and precipitation) is improved when using the more advanced 3rd generation LSM. These improvements are of the same order of magnitude as those arising from a new version of the atmospheric component, demonstrating the benefit of using a realistic representation of land surface processes for regional climate simulations. Taking into account variability in diffuse/direct light partitioning at the surface further improves the model performance in terms of summer temperature variability at the monthly and daily time scales. Comparisons with observations show that the RCM realistically captures temporal variations in diffuse/direct light partitioning as well as the evapotranspiration sensitivity to these variations. Our results suggest that a modest but consistent fraction (up to 3 %) of the overall variability in summer temperature can be explained by variations in the diffuse to direct ratio.

Description: Rapid carbon cycling in the oligotrophic ocean Biogeosciences Discussions, 8, 11661-11687, 2011 Author(s): C. M. Duarte and S. Agustí The dynamics of organic carbon production, release and bacterial use was examined across a range of communities spanning from highly oligotrophic ones in the Subtropical Atlantic Ocean, mesotrophic ones in the Mediterranean Sea and productive ones in the Northern African upwelling and the Southern Ocean. A comparative analysis of experiments examining total and particulate organic carbon production across a range of time scales (15 min to 24 h) for 20 communities with contrasting phytoplankton cell status, as assessed by cell lysis rates, and the use of a simple inverse model was used to resolve patterns of carbon flow in the microbial food web. Communities in productive ocean waters accumulated organic carbon over hourly time scales, whereas only a small fraction of net primary production accumulated in communities from oligotrophic waters. These communities supported high phytoplankton cell lysis rates leading to a rapid flux of organic carbon to bacteria, which had high affinity for phytoplankton-derived carbon, much of which was rapidly respired. Conventional assessments of primary production in the oligotrophic ocean severely underestimate net phytoplankton production, as carbon flow in microbial communities from oligotrophic ocean waters occurs within short (minutes) time scales. This explains difficulties to reconcile estimates of primary production with independent estimates of carbon use by bacteria in oligotrophic marine ecosystems.

Description: Spatial variations of nitrogen trace gas emissions from tropical mountain forests in Nyungwe, Rwanda Biogeosciences Discussions, 8, 11631-11660, 2011 Author(s): N. Gharahi Ghehi, C. Werner, L. Cizungu Ntaboba, J. J. Mbonigaba Muhinda, E. Van Ranst, K. Butterbach-Bahl, R. Kiese, and P. Boeckx Globally, tropical forest soils represent the second largest source of N 2 O and NO. However, there is still considerable uncertainty on the spatial variability and soil properties controlling N trace gas emission. To investigate how soil properties affect N 2 O and NO emission, we carried out an incubation experiment with soils from 31 locations in the Nyungwe tropical mountain forest in southwestern Rwanda. All soils were incubated at three different moisture levels (50, 70 and 90% water filled pore space (WFPS)) at 17 °C. Nitrous oxide emission varied between 4.5 and 400 μg N m −2 h −1 , while NO emission varied from 6.6 to 265 μg N m −2 h −1 . Mean N 2 O emission at different moisture levels was 46.5 ± 11.1 (50% WFPS), 71.7 ± 11.5 (70% WFPS) and 98.8 ± 16.4 (90% WFPS) μg N m −2 h −1 , while mean NO emission was 69.3 ± 9.3 (50% WFPS), 47.1 ± 5.8 (70% WFPS) and 36.1 ± 4.2 (90% WFPS) μg N m −2 h −1 . The latter suggests that climate (i.e. dry vs. wet season) controls N 2 O and NO emissions. Positive correlations with soil carbon and nitrogen indicate a biological control over N 2 O and NO production. But interestingly N 2 O and NO emissions also showed a negative correlation (only N 2 O) with soil pH and a positive correlation with free iron. The latter suggest that chemo-denitrification might, at least for N 2 O, be an important production pathway. In conclusion improved understanding and process based modeling of N trace gas emission from tropical forests will not only benefit from better spatial explicit trace gas emission and basic soil property monitoring, but also by differentiating between biological and chemical pathways for N trace gas formation.

Description: Factors influencing the stable carbon isotopic composition of suspended and sinking organic matter in the coastal Antarctic sea ice environment Biogeosciences Discussions, 8, 11041-11088, 2011 Author(s): S. F. Henley, A. L. Annett, R. S. Ganeshram, D. S. Carson, K. Weston, X. Crosta, A. Tait, J. Dougans, A. E. Fallick, and A. Clarke A high resolution time-series analysis of stable carbon isotopic signatures in particulate organic carbon (δ 13 C POC ) and associated biogeochemical parameters in sea ice and surface waters provides an insight into the factors affecting δ 13 C POC in the coastal western Antarctic Peninsula (WAP) sea ice environment. The study covers two austral summer seasons in Ryder Bay, northern Marguerite Bay between 2004 and 2006. A shift in diatom species composition during the 2005/2006 summer bloom to near-complete biomass dominance of Proboscia inermis is strongly correlated with a large ~10‰ negative isotopic shift in δ 13 C POC that cannot be explained by a concurrent change in concentration or isotopic signature of CO 2 . We hypothesise that the δ 13 C POC shift may be driven by the contrasting biochemical mechanisms and utilisation of carbon-concentrating mechanisms in different diatom species. These short-lived yet pronounced negative δ 13 C POC excursions drive a 4‰ decrease in the seasonal average δ 13 C POC signal, which is transferred to sediment traps and core-top sediments and consequently has the potential for preservation in the sedimentary record. This 4‰ difference between seasons of contrasting sea ice conditions and upper water column stratification matches the full amplitude of glacial-interglacial Southern Ocean δ 13 C POC variability and, as such, we invoke phytoplankton species changes as a potentially important factor influencing sedimentary δ 13 C POC . We also find significantly higher δ 13 C POC in sea ice than surface waters, consistent with autotrophic carbon fixation in a semi-closed environment and possible contributions from post-production degradation, biological utilisation of HCO 3 - and production of exopolymeric substances (EPS). This study demonstrates the importance of surface water diatom speciation effects and isotopically heavy sea ice-derived material for δ 13 C POC in Antarctic coastal environments and underlying sediments, with consequences for the utility of diatom-based δ 13 C POC in the sedimentary record.

Description: Critical loads of nitrogen deposition and critical levels of atmospheric ammonia for mediterranean evergreen woodlands Biogeosciences Discussions, 8, 11139-11163, 2011 Author(s): P. Pinho, M. R. Theobald, T. Dias, Y. S. Tang, C. Cruz, M. A. Martins-Loução, C. Máguas, M. Sutton, and C. Branquinho Nitrogen (N) has emerged in recent years as a key factor associated with global changes, with impacts on biodiversity, ecosystems functioning and human health. In order to ameliorate the effects of excessive N, safety thresholds have been established, such as critical loads (deposition fluxes) and levels (concentrations). For Mediterranean ecosystems, few studies have been carried out to assess these parameters. Our objective was therefore to determine the critical loads of N deposition and long-term critical levels of atmospheric ammonia for Mediterranean evergreen woodlands. For that we have considered changes in epiphytic lichen communities, which have been shown to be one of the most sensitive to excessive N. Based on a classification of lichen species according to their tolerance to N we grouped species into response functional groups, which we used as a tool to determine the critical loads and levels. This was done under Mediterranean climate, in evergreen cork-oak woodlands, by sampling lichen functional diversity and annual atmospheric ammonia concentrations and modelling N deposition downwind from a reduced N source (a cattle barn). By modelling the highly significant relationship between lichen functional groups and N deposition, the critical load was estimated to be below 26 kg (N) ha −1 yr −1 , which is within the upper range established for other semi-natural ecosystems. By modelling the highly significant relationship of lichen functional groups with annual atmospheric ammonia concentration, the critical level was estimated to be below 1.9 μg m −3 , in agreement with recent studies for other ecosystems. Taking into account the high sensitivity of lichen communities to excessive N, these values should be taken into account in policies that aim at protecting Mediterranean woodlands from the initial effects of excessive N.

Description: A molecular perspective on the ageing of marine dissolved organic matter Biogeosciences Discussions, 8, 11453-11488, 2011 Author(s): R. Flerus, B. P. Koch, O. J. Lechtenfeld, S. L. McCallister, P. Schmitt-Kopplin, R. Benner, K. Kaiser, and G. Kattner Dissolved organic matter (DOM) was extracted with solid phase extraction (SPE) from 137 water samples from different climate zones and different depths along an Eastern Atlantic Ocean transect. The extracts were analyzed with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with electrospray ionization (ESI). Δ 14 C analyses were performed on subsamples of the SPE-DOM. In addition, the amount of dissolved organic carbon was determined for all water and SPE-DOM samples as well as the yield of amino sugars for selected samples. Linear correlations were observed between the magnitudes of 43 % of the FT-ICR mass peaks and the extract Δ 14 C values. Decreasing SPE-DOM Δ 14 C values went along with a shift in the molecular composition to higher average masses ( m/z ) and lower hydrogen/carbon (H/C) ratios. The correlation was used to model the SPE-DOM Δ 14 C distribution for all 137 samples. Based on single mass peaks a degradation index was developed to compare the degradation state of marine SPE-DOM samples analyzed with FT-ICR MS. A correlation between Δ 14 C, degradation index, DOC values and amino sugar yield supports that SPE-DOM analyzed with FT-ICR MS reflects trends of bulk DOM. A relative mass peak magnitude ratio was used to compare aged SPE-DOM and fresh SPE-DOM regarding single mass peaks. The magnitude ratios show a continuum of different reactivities for the single compounds. Only few of the compounds present in the FT-ICR mass spectra are expected to be highly degraded in the oldest water masses of the Pacific Ocean. All other compounds should persist partly thermohaline circulation. Prokaryotic (bacterial) production, transformation and accumulation of this very stable DOM occurs probably primarily in the upper ocean. This DOM is an important contribution to very old DOM, showing that production and degradation are dynamic processes.

Description: Greenhouse gas emissions from the grassy outdoor run of organic broilers Biogeosciences Discussions, 8, 11529-11575, 2011 Author(s): B. Meda, C. R. Flechard, K. Germain, P. Robin, M. Hassouna, and C. Walter Nitrous oxide (N 2 O), methane (CH 4 ) and carbon dioxide (CO 2 ) fluxes over the grassy outdoor run of organically grown broilers were monitored using static chambers over two production batches in contrasted seasons. Measured N 2 O and CH 4 fluxes were extremely variable in time and space for both batches, with fluxes ranging from a small uptake by soil to large emissions peaks, the latter of which always occurred in the chambers located closest to the broiler house. In general, fluxes decreased with increasing distance to the broiler house, demonstrating that the foraging of broilers and the amount of excreted nutrients (carbon, nitrogen) largely control the spatial variability of emissions. Spatial integration by kriging methods was carried out to provide representative fluxes on the outdoor run for each measurement day. Mechanistic relationships between plot-scale estimates and environmental conditions (soil temperature and water content) were calibrated in order to fill gaps between measurement days. Flux integration over the year 2010 showed that around 3 ± 1 kg N 2 O-N ha −1 were emitted on the outdoor run, equivalent to 0.9 % of outdoor N excretion and substantially lower than the IPCC default emission factor of 2 %. By contrast, the outdoor run was found to be a net CH 4 sink of about −0.56 kg CH 4 -C ha −1 , though this sink compensated less than 1.5 % (in CO 2 equivalents) of N 2 O emissions. The net greenhouse gas (GHG) budget of the outdoor run is explored, based on measured GHG fluxes and short-term (1.5 yr) variations in soil organic carbon.

Description: A hypothesis linking sub-optimal seawater p CO 2 conditions for cnidarian- Symbiodinium symbioses with the exceedence of the interglacial threshold (〉 260 ppmv) Biogeosciences Discussions, 8, 11215-11253, 2011 Author(s): S. A. Wooldridge Most scleractinian corals and many other cnidarians host intracellular photosynthetic dinoflagellate symbionts ("zooxanthellae"). The zooxanthellae contribute to host metabolism and skeletogenesis to such an extent that this symbiosis is well recognised for its contribution in creating the coral reef ecosystem. The stable functioning of cnidarian symbioses is however dependent upon the host's ability to maintain demographic control of its algal partner. In this review, I explain how the modern envelope of seawater conditions found within many coral reef ecosystems (characterised by elevated temperatures, rising p CO 2 , and enriched nutrient levels) are antagonistic toward the dominant host processes that restrict excessive symbiont proliferation. Moreover, I outline a new hypothesis and initial evidence base, which support the suggestion that the additional "excess" zooxanthellae fraction permitted by seawater p CO 2 levels beyond 260 ppmv significantly increases the propensity for symbiosis breakdown ("bleaching") in response to temperature and irradiance extremes. The relevance of this biological threshold is discussed in terms of historical reef extinction events, glacial-interglacial climate cycles and the modern decline of coral reef ecosystems.

Description: Interactions between uptake of amino acids and inorganic nitrogen in wheat plants Biogeosciences Discussions, 8, 11311-11335, 2011 Author(s): E. Gioseffi, A. de Neergaard, and J. K. Schjoerring Soil-borne amino acids may constitute a nitrogen (N) source for plants in various terrestrial ecosystems but their importance for total N nutrition is unclear, particularly in nutrient-rich arable soils. One reason for this uncertainty is lack of information on how the absorption of amino acids by plant roots is affected by the simultaneous presence of inorganic N forms. The objective of the present study was to study absorption of glycine (Gly) and glutamine (Gln) by wheat roots and their interactions with nitrate (NO 3 – ) and (NH 4 + ) during uptake. The underlying hypothesis was that amino acids, when present in nutrient solution together with inorganic N, may lead to down-regulation of the inorganic N uptake. Amino acids were enriched with double-labelled 15 N and 13 C, while NO 3 – and NH 4 + acquisition was determined by their rate of removal from the nutrient solution surrounding the roots. The uptake rates of NO 3 – and NH 4 + did not differ from each other and were about twice as high as the uptake rate of organic N when the different N forms were supplied separately in concentrations of 2 mM. Nevertheless, replacement of 50 % of the inorganic N with organic N was able to restore the N uptake to the same level as that in the presence of only inorganic N. Co-provision of NO 3 – did not affect glycine uptake, while the presence of glycine down-regulated NO 3 – uptake. The ratio between 13 C and 15 N were lower in shoots than in roots and also lower than the theoretical values, reflecting higher C losses via respiratory processes compared to N losses. It is concluded that organic N can constitute a significant N-source for wheat plants and that there is an interaction between the uptake of inorganic and organic nitrogen.

Description: Integrative analysis of the interactions between Geobacter spp. and sulfate-reducing bacteria during uranium bioremediation Biogeosciences Discussions, 8, 11337-11357, 2011 Author(s): M. Barlett, K. Zhuang, R. Mahadevan, and D. R. Lovley Enhancing microbial U(VI) reduction with the addition of organic electron donors is a promising strategy for immobilizing uranium in contaminated groundwaters, but has yet to be optimized because of a poor understanding of the factors controlling the growth of various microbial communities during bioremediation. In previous field trials in which acetate was added to the subsurface, there were two distinct phases: an initial phase in which acetate-oxidizing, U(VI)-reducing Geobacter predominated and U(VI) was effectively reduced and a second phase in which acetate-oxidizing sulfate reducing bacteria (SRB) predominated and U(VI) reduction was poor. The interaction of Geobacter and SRB was investigated both in sediment incubations that mimicked in situ bioremediation and with in silico metabolic modeling. In sediment incubations, Geobacter grew quickly but then declined in numbers as the microbially reducible Fe(III) was depleted whereas the SRB grow more slowly and reached dominance after 30–40 days. Modeling predicted a similar outcome. Additional modeling in which the relative initial percentages of the Geobacter and SRB were varied indicated that there was little to no competitive interaction between Geobacter and SRB when acetate was abundant. Further simulations suggested that the addition of Fe(III) would revive the Geobacter , but have little to no effect on the SRB. This result was confirmed experimentally. The results demonstrate that it is possible to predict the impact of amendments on important components of the subsurface microbial community during groundwater bioremediation. The finding that Fe(III) availability, rather than competition with SRB, is the key factor limiting the activity of Geobacter during in situ uranium bioremediation will aid in the design of improved uranium bioremediation strategies.

Description: A global compilation of over 13 000 dissolved iron measurements: focus on distributions and processes in the Southern Ocean Biogeosciences Discussions, 8, 11489-11527, 2011 Author(s): A. Tagliabue, T. Mtshali, O. Aumont, A. R. Bowie, M. B. Klunder, A. N. Roychoudhury, and S. Swart Due to its importance as a limiting nutrient for phytoplankton growth in large regions of the world's oceans, ocean water column observations of concentration of the trace-metal iron (Fe) have increased markedly over recent decades. Here we compile 〉 13 000 global measurements of dissolved Fe (dFe) and make this available to the community. We then conduct a synthesis study focussed on the Southern Ocean, where dFe plays a fundamental role in governing the carbon cycle, using four regions, six basins and five depth intervals as a framework. Our analysis reveals the importance of biological activity and dFe inputs in governing the inter-region and inter-basin differences in surface dFe, respectively. In deep waters, the major controls of inter-region and inter-basin dFe variability are ligand distributions and deep dFe inputs or water mass characteristics, respectively. We find that even in regions where many dFe measurements exist, the processes governing the seasonal evolution of dFe remain enigmatic, suggesting that, aside from broad sub-Antarctic-Antarctic trends, biological activity might not the major driver of dFe variability. Nevertheless, missing measurements during key seasonal transitions make it difficult to better quantify and understand surface water replenishment processes and the seasonal Fe cycle. Statistical differences exist in the measured dFe between measurements taken over the period 1989–2002 and 2003–2008, which may reflect progress in clean sampling and analysis techniques. Finally, we detail the degree of seasonal coverage by region, basin and depth. By synthesising prior measurements we suggest a role for different processes and highlight key gaps in understanding, which we hope can help structure future research efforts in the Southern Ocean.

Description: Process based model sheds light on climate signal of mediterranean tree rings Biogeosciences Discussions, 8, 11089-11105, 2011 Author(s): R. Touchan, V. V. Shishov, D. M. Meko, I. Nouiri, and A. Grachev We use the process-based VS (Vaganov-Shashkin) model to investigate whether a regional Pinus halapensis tree-ring chronology from Tunisia can be simulated as a function of climate alone by employing a biological model linking day length and daily temperature and precipitation (AD 1959–2004) from a climate station to ring-width variations. We use two periods to calibrate (1982–2004) and verify (1959–1981) the model. We have obtained highly significant positive correlation between the residual chronology and estimated growth curve ( r = 0.76 p 〈 0.001). The model shows that the average duration of the growing season is 191 days. On average, soil moisture limits tree-ring growth for 128 days and temperature for 63 days.

Description: Soil carbon dynamics during secondary succession in a semi-arid Mediterranean environment Biogeosciences Discussions, 8, 11107-11138, 2011 Author(s): A. Novara, L. Gristina, T. La Mantia, and J. Rühl Clarifying which factors cause an increase or decrease in soil organic carbon (SOC) after agricultural abandonment requires integration of data on the temporal dynamics of the plant community and SOC. A chronosequence of abandoned vineyards was studied on a volcanic island (Pantelleria, Italy). Vegetation in the abandoned fields was initially dominated by annual and perennial herbs, then by Hyparrhenia hirta (L.) Stapf, and finally by woody communities. As a consequence, the dominant photosynthetic pathway changed from C3 to C4 and then back to C3. Conversion of a plant community dominated by one photosynthetic pathway to another changes the 13 C/ 12 C ratio of inputs to soil organic carbon (SOC). Using the time since abandonment and the shift in belowground δ 13 C of SOC relative to the aboveground δ 13 C plant community, we estimated SOC turnover rate. SOC content (g kg −1 ) increased linearly ( R 2 = 0.79 and 0.73 for 0–15 and 15–30 cm soil depth) with the age of abandonment, increasing from 12 g kg −1 in cultivated vineyards to as high as 26 g kg −1 in the last stage of the succession. δ 13 C increased in the bulk soil and its three fractions during succession, but only for soil fractions the effects of soil depth and its interaction with succession age were significant. Polynomial curves described the change in δ 13 C over the chronosequence for both depths. δ 13 C in the bulk soil had increased from −28 to −24‰ by 30 yr after abandonment for both depths but then decreased to −26‰ at 60 yr after abandonment (corresponding with maturity of the woody plant community). Overall, the results indicate that abandoned vineyards on volcanic soil in a semi-arid environment are C sinks and that C storage in these soils is closely related to plant succession.

Description: The composition and flux of particulate and dissolved carbohydrates from the Rhône River into the Mediterranean Sea Biogeosciences Discussions, 8, 11165-11213, 2011 Author(s): C. Panagiotopoulos, R. Sempéré, J. Para, P. Raimbault, C. Rabouille, and B. Charrière Carbohydrates are important components of the carbon cycle and may be used as indicators of the origin and the diagenetic status of marine and terrestrial organic matter. Nevertheless, comprehensive studies of both particulate (PCHO) and dissolved (DCHO) carbohydrates in rivers are scarce, and the seasonal and interannual variability of these compounds in relationship to the bulk particulate (POM) and dissolved organic matter (DOM) is largely unknown. For the period 2007–2009, we sampled once per month POM and DOM and measured the total suspended matter (TSM), POM, DOM, PCHO, and DCHO for the Rhône River, which flows into the Mediterranean Sea. Using these measurements, we estimated for the above parameters annual fluxes for the period 2001–2010. The estimated carbohydrate fluxes averaged 0.061±0.043×10 10 moles Cy −1 for PCHO and 0.041±0.0062×10 10 moles Cy −1 DCHO, representing 8% and 7% of the annual flux of POC and DOC, respectively. During flood and low-water periods, POM variations were reflected into the PCHO pool, whereas this was not observed for DOC and DCHO, indicating a decoupling between particulate and dissolved organic matter. Our results also showed that flood and low-water periods may be differentiated using the ratios PCHO/DCHO and POC/DOC, which had a significant linear relationship. Based on the carbohydrate abundances in both the PCHO and DCHO pools, we conclude that this material mainly derives from allochthonous sources (vascular plants, bacteria and soils). Moreover, during flood events, an enrichment in mannose in POM was observed, probably reflecting an angiosperm source (leaves or grasses). By expanding our results to the northwestern Mediterranean Sea (Gulf of Lions), we found that the total organic carbon (TOC) fluxes of the Rhône River accounted for ~1% of the standing stock of seawater TOC. Considering that glucose is the most abundant carbohydrate in both particulate and dissolved organic matter pools (~33%), its annual flux in the northwestern Mediterranean Sea was estimated to 19.2×10 8 moles glucose-C.

Description: Temperature thresholds for Arctic plankton community metabolism: an experimental assessment Biogeosciences Discussions, 8, 11285-11309, 2011 Author(s): J. M. Holding, C. M. Duarte, J. M. Arrieta, A. Coello, P. Wassmann, and S. Agustí Climate warming is especially severe in the Arctic, where the average temperature is increasing 0.4 °C per decade, two to three times higher than the global average rate. Furthermore, the Arctic has lost more than half its summer ice extent since 1980 and predictions suggest that the Arctic will be ice free in the summer as early as 2050, which could increase rate of warming. Predictions based on the metabolic theory of ecology assume that temperature increase will enhance metabolic rates and thus both the rate of primary production and respiration will increase. However, these predictions do not consider the specific metabolic balance of the communities. We tested experimentally the response of Arctic plankton communities to seawater temperature spanning from 1 °C to 10 °C. Two types of communities were tested, open-ocean Arctic communities from water collected in the Barents Sea and Atlantic influenced fjord communities from water collected in the Svalbard fjord system. Metabolic rates did indeed increase as suggested by metabolic theory, however these results suggest a temperature threshold of 5 °C, beyond which the metabolism of plankton communities shifts from autotrophic to heterotrophic. Barents Sea communities showed a much clearer threshold response to temperature manipulations than fjord communities.

Description: Sea-to-air and diapycnal nitrous oxide fluxes in the eastern tropical North Atlantic Ocean Biogeosciences Discussions, 8, 10229-10246, 2011 Author(s): A. Kock, J. Schafstall, M. Dengler, P. Brandt, and H. W. Bange Sea-to-air and diapycnal fluxes of nitrous oxide (N 2 O) into the mixed layer were determined during three cruises to the upwelling region off Mauritania. Both fluxes were elevated close to the shelf break, but elevated sea-to-air fluxes reached further offshore as a result of the offshore transport of upwelled water masses. To calculate a mixed layer budget for N 2 O we compared the regionally averaged sea-to-air and diapycnal fluxes and estimated the potential contribution of other processes, such as vertical advection and biological N 2 O production in the mixed layer. Using common parameterizations for the gas transfer velocity, the comparison of the average sea-to-air and diapycnal N 2 O fluxes indicated that the mean sea-to-air flux is about three to four times larger than the diapycnal flux. Vertical and horizontal advection or biological production were found not sufficient to close the mixed layer budget. Instead, the sea-to-air flux, calculated using a parameterization that takes into account the attenuating effect of surfactants on gas exchange, is in the same range as the diapycnal flux. This indicates that common parameterizations for the gas transfer velocity might overestimate the air-sea gas exchange within highly productive upwelling zones.

Description: The metabolic response of pteropods to ocean acidification reflects natural CO 2 -exposure in oxygen minimum zones Biogeosciences Discussions, 8, 10295-10316, 2011 Author(s): A. E. Maas, K. F. Wishner, and B. A. Seibel Shelled pteropods (Thecosomata) are a group of holoplanktonic mollusks that are believed to be especially sensitive to ocean acidification because their aragonitic shells are highly soluble. Despite this concern, there is very little known about the physiological response of these animals to conditions of elevated carbon dioxide. This study examines the oxygen consumption and ammonia excretion of five pteropod species, collected from tropical regions of the Pacific Ocean, to elevated levels of carbon dioxide (0.10%, 1000 ppm). Our results show that pteropods that naturally migrate into oxygen minimum zones, such as Hyalocylis striata , Clio pyramidata , Cavolinia longirostris and Creseis virgula , were not affected by carbon dioxide at the levels and duration tested. Diacria quadridentata , which does not migrate, responds to high carbon dioxide conditions with reduced oxygen consumption and ammonia excretion. This indicates that the natural chemical environment of individual species influences their resilience to ocean acidification.

Description: Reply to Nicholson's comment on "Consistent calculation of aquatic gross production from oxygen triple isotope measurements" by Kaiser (2011) Biogeosciences Discussions, 8, 10517-10541, 2011 Author(s): J. Kaiser and O. Abe The comment by Nicholson (2011a) questions the "consistency" of the "definition" of the "biological end-member" used by Kaiser (2011a) in the calculation of oxygen gross production. "Biological end-member" refers to the relative oxygen isotope ratio difference between photosynthetic oxygen and Air-O 2 (abbreviated 17 δ P and 18 δ P for 17 O/ 16 O and 18 O/ 16 O, respectively). This comment has no merit for the following reasons: (a) the isotopic composition of photosynthetic oxygen cannot be "defined", it can only be measured, modelled or calculated based on other data; (b) the isotopic composition of photosynthetic oxygen was not "defined" in Kaiser (2011a), but derived from published measurements; (c) the published measurements themselves were inconsistent and no single result could be identified as best; (d) since no best value could be identified, a hypothetical base case was constructed in a way that was consistent with previous publications; (e) the values of 17 δ P =−11.646‰ and 18 δ P =−22.835‰ assumed for the base case are compatible with the experimental evidence published before the paper of Kaiser (2011a); (f) even if the "biological end-member" was based on a~definition, there could be no argument about the "consistency" of this definition – as per its nature, a definition is arbitrary. The qualification of base case gross production values as being "30 % too high" must therefore also be rejected. Even though recently revised measurements of the relative 17 O/ 16 O isotope ratio difference between VSMOW and Air-O 2 , 17 δ VSMOW (Barkan and Luz, 2011), do support lower estimates of gross production, our own measurements disagree with these revised 17 δ VSMOW values. If scaled for differences in 18 δ VSMOW , they are actually in good agreement with the original data (Barkan and Luz, 2005). Moreover, species-dependent differences in photosynthetic isotope fractionation (Eisenstadt et al., 2010) correspond to an uncertainty of at least 15 % around the central estimate for the inferred gross production. Nicholson (2011a) also suggests that approximated calculations of gross production should be performed with a triple isotope excess defined as 17 Δ # ≡ln(1+ 17 δ)−λln(1+ 18 δ), with λ=θ R =ln(1+ 17 ϵ R )/ln(1+ 18 ϵ R ). However, this only improves the approximation for certain 17 δ P and 18 δ P values, for certain net to gross production ratios ( f ) and for certain ratios of gross production to gross Air-O 2 invasion ( g ). In other cases, the approximated calculation based on 17 Δ † ≡ 17 δ−κ 18 δ with κ=γ R = 17 ϵ R / 18 ϵ R gives better results.

Description: Impact of rapid sea-ice reduction in the Arctic Ocean on the rate of ocean acidification Biogeosciences Discussions, 8, 10617-10644, 2011 Author(s): A. Yamamoto, M. Kawamiya, A. Ishida, Y. Yamanaka, and S. Watanabe The largest pH decline and widespread undersaturation with respect to aragonite in this century due to uptake of anthropogenic carbon dioxide in the Arctic Ocean have been projected. The reductions in pH and aragonite saturation state have been caused primarily by an increase in the concentration of atmospheric carbon dioxide. However, in a previous study, simulations with and without warming showed that these reductions in the Arctic Ocean also advances due to the melting of sea ice caused by global warming. Therefore, future projections of pH and aragonite saturation in the Arctic Ocean will be affected by how rapidly the reduction in sea ice occurs. In this study, the impact of sea-ice reduction rate on projected pH and aragonite saturation state in the Arctic surface waters was investigated. Reductions in pH and aragonite saturation were calculated from the outputs of two versions of an earth system model (ESM) with different sea-ice reduction rates under similar CO 2 emission scenarios. The newer model version projects that Arctic summer ice-free condition will be achieved by the year 2040, and the older version predicts ice-free condition by 2090. The Arctic surface water was projected to be undersaturated with respect to aragonite in the annual mean when atmospheric CO 2 concentration reached 480 (550) ppm in year 2040 (2048) in new (old) version. At an atmospheric CO 2 concentration of 520 ppm, the maximum differences in pH and aragonite saturation state between the two versions were 0.08 and 0.15, respectively. The analysis showed that the decreases in pH and aragonite saturation state due to rapid sea-ice reduction were caused by increases in both CO 2 uptake and freshwater input. Thus, the reductions in pH and aragonite saturation state in the Arctic surface waters are significantly affected by the difference in future projections for sea-ice reduction rate. The critical CO 2 concentration, at which the Arctic surface waters become undersaturated with respect to aragonite on annual mean bias, would be lower by 70 ppm in the version with the rapid sea-ice reduction.

Description: Daily CO 2 partial pressure and CO 2 outgassing in the upper Yangtze River basin: a case study of Longchuanjiang, China Biogeosciences Discussions, 8, 10645-10676, 2011 Author(s): S. Y. Li, X. X. Lu, M. He, Y. Zhou, L. Li, and A. D. Ziegler Rivers have been under sampled to establish them as sinks or sources of the atmospheric carbon oxide (CO 2 ). Such poor coverage is well known for tropical and sub-tropical, particularly monsoon driven rivers. An unprecedented high-temporal-resolution (daily) sampling during July 2008–August 2009 were conducted from the Longchuanjiang River of the upper Yangtze basin, a subtropical monsoon river in China to reveal the daily-to-seasonal dynamics of the partial pressure of CO 2 ( p CO 2 ) and CO 2 degassing flux from the river. The p CO 2 levels were supersaturated in CO 2 with respect to atmospheric equilibrium (380 μatm) during the entire survey period with obvious daily and seasonal variations, ranging from 450 to 63 000 μatm with an average of 3900 μatm. p CO 2 values in the surface water in the wet season were relatively low, except flooding period in November, due to a dilution effect by heavy rainfall. However, both daily and monthly minimal and maximal p CO 2 also occurred in this period. In contrast, the p CO 2 levels in the dry season were much higher, mainly resulted from lower pH by anthropogenic activities. Net CO 2 flux and p CO 2 were strongly correlated with pH, but weakly with water temperature, dissolved inorganic carbon and water discharge, and uncorrelated with particulate nutrients and biogenic elements. The estimated water-to-air CO 2 degassing flux in the Longchuanjiang River was about 110 mol m −2 yr −1 , with the upper limit of 460 mol m −2 yr −1 . Our study also indicated that among the total organic carbon remobilized through soil erosion, around 17% (11 400 t C yr −1 ) of was emitted to the atmosphere, 52% (35 000 t C yr −1 ) deposited in the river-reservoirs system and 31% (21 000 t C yr −1 ) exported further downstream. High spatial and temporal resolution of estimates of CO 2 emission from the world large rivers is required due to that catchment characteristics and anthropogenic activities are extremely heterogeneous in space and time.

Description: Oxygen exchange and ice melt measured at the ice-water interface by eddy correlation Biogeosciences Discussions, 8, 11255-11284, 2011 Author(s): M. H. Long, D. Koopmans, P. Berg, S. Rysgaard, R. N. Glud, and D. H. Søgaard This study uses the eddy correlation technique to examine fluxes across the ice-water interface. Temperature eddy correlation systems were used to determine rates of ice melting and freezing, and O 2 eddy correlation systems were used to examine O 2 exchange rates as driven by biological and physical processes. The research was conducted below 0.7 m thick sea ice in mid March 2010 in a southwest Greenland fjord and revealed low average rates of ice melt amounting to a maximum of 0.80 ± 0.09 mm d −1 (SE, n =31). The corresponding calculated O 2 flux associated with release of O 2 depleted melt water was less than 13 % of the average daily O 2 respiration rate. Ice melt and insufficient vertical turbulent mixing due to low current velocities caused periodic stratification immediately below the ice. This prevented the determination of fluxes during certain time periods, amounting to 66 % of total deployment time. The identification of these conditions was evaluated by examining the velocity and the linearity and stability of the cumulative flux. The examination of unstratified conditions through velocity and O 2 spectra and their cospectra revealed characteristic fingerprints of well-developed turbulence. From the observed O 2 fluxes, a photosynthesis/irradiance curve was established by least-squares fitting. This relation showed that light limitation of net photosynthesis began at 4.2 μmol photons m −2 s −1 , and that the algal communities were well-adapted to low-light conditions as they were light saturated for 75 % of the day during this early spring period. However, the sea ice associated microbial and algal community was net heterotrophic with a daily gross primary production of 0.69 ± 0.02 mmol O 2 m −2 d −1 (SE, n =4) and a respiration rate of −2.13 mmol O 2 m −2 d −1 (no SE, see text for details) leading to a net primary production of −1.45 ± 0.02 mmol O 2 m −2 d −1 (SE, n =4). Modeling the observed fluxes allowed for the calculation of fluxes during time periods when no O 2 fluxes were extracted. This application of the eddy correlation technique produced high temporal resolution O 2 fluxes and ice melt rates that were measured without disturbing the environmental conditions while integrating over a large area of approximately 50 m 2 which encompassed the highly variable activity and spatial distributions of sea ice algal communities.

Description: The effect of meter-scale lateral oxygen gradients at the sediment-water interface on selected organic matter based alteration, productivity and temperature proxies Biogeosciences Discussions, 8, 11359-11403, 2011 Author(s): K. A. Bogus, K. A. F. Zonneveld, D. Fischer, S. Kasten, G. Bohrmann, and G. J. M. Versteegh A valid assessment of selective aerobic degradation on organic matter (OM) and its impact on OM-based proxies is vital to produce accurate environmental reconstructions. However, most studies investigating these effects suffer from inherent environmental heterogeneities. This includes differences in the initial OM composition, as a result of variable upper water column conditions, or from those induced by selective aerobic degradation. In this study, we used surface samples collected along two meter-scale transects and one longer transect in the northeastern Arabian Sea to constrain initial OM heterogeneity, in order to evaluate selective aerobic degradation on temperature, productivity and alteration indices at the sediment-water interface. All of the alteration indices, the higher plant alkane index, alcohol preservation index, and diol oxidation index, demonstrated that they are sensitive indicators for changes in oxygen content at the sediment-water interface. The export production indices, a cholesterol-based stanol/stenol and dinoflagellate lipid- and cyst-based ratios, showed significant (more than 20%) change over the lateral oxygen gradients. Therefore, they do not exclusively reflect surface water productivity, but can be altered after deposition with varying oxygen content at the sediment-water interface. Two of the investigated proxies, the glycerol dibiphytanyl glycerol tetraethers (GDGT) based TEX 86 sea surface temperature indices and a productivity index based on phytol, phytane and pristane, did not show any trends related to oxygen concentration at the sediment-water interface. Nevertheless, unrealistic sea surface temperatures were obtained after application of the TEX 86 , TEX 86 L , and TEX 86 H proxies. The phytol-based ratios were likely modified by the sedimentary production of pristane. Our results demonstrate the rapid and selective impact of aerobic organic matter degradation on the lipid and palynomorph composition of surface sediments on a small spatial scale and suggests useful tracers of changing redox conditions along the sediment-water interface.

Description: Size distribution of particles and zooplankton across the shelf-basin system in Southeast Beaufort Sea: combined results from an Underwater Vision Profiler and vertical net tows Biogeosciences Discussions, 8, 11405-11452, 2011 Author(s): A. Forest, L. Stemmann, M. Picheral, L. Burdorf, D. Robert, L. Fortier, and M. Babin The size distribution and mean spatial trends of large particles (〉100 μm, in equivalent spherical diameter, ESD) and mesozooplankton were investigated across the Mackenzie Shelf (Southeast Beaufort Sea, Arctic Ocean) in July–August 2009. Our main objective was to combine results from an Underwater Vision Profiler 5 (UVP5) and traditional net tows (200 μm mesh size) to characterize the structural diversity and functioning of the Arctic shelf-basin ecosystem and to assess the large-scale correspondence between the two methodological approaches. The core dataset comprised 154 UVP5 profiles and 29 net tows conducted in the shelf ( 1000 m) regions of the study area. The mean abundance of total particles and zooplankton in the upper water column ( 50 %) when progressing offshore and as the ESD of particles was increasing. Both the UVP5 and net tows determined that copepods dominated the zooplankton community (~78–94 % by numbers) and that appendicularians were generally the second most abundant group (~1–11 %). The vertical distribution patterns of copepods and appendicularians indicated a close association between primary production and the main grazers. Manual taxonomic counts and ZooScan image analyses shed further light on the size-structure and composition of the copepod community – which was dominated at ~95 % by a guild of 10 typical taxa. The size distributions of copepods, as evaluated with the 3 methods (manual counts, ZooScan and UVP5), showed consistent patterns co-varying in the same order of magnitude over the upper size range (〉1 mm ESD). Copepods 〈 1 mm were not well quantified by the UVP5, which estimated that only ~13–25 % of the assemblage was composed of copepods 〈 1 mm ESD compared with ~77–89 % from the net tow estimates. However, the biovolume of copepods was overwhelmingly dominated (~93–97 %) by copepods 〉1 mm ESD. Our results illustrate that the combination of traditional sampling methods and automated imaging techniques is a powerful approach that enabled us to conclude on the prevalence of a relatively high productivity regime and dominant herbivorous food web over the shelf when compared with the low-productive recycling system detected offshore.

Description: The moisture response of soil heterotrophic respiration: interaction with soil properties Biogeosciences Discussions, 8, 11577-11599, 2011 Author(s): F. E. Moyano, N. Vasilyeva, L. Bouckaert, F. Cook, J. Craine, J. Curiel Yuste, A. Don, D. Epron, P. Formanek, A. Franzluebbers, U. Ilstedt, T. Kätterer, V. Orchard, M. Reichstein, A. Rey, L. Ruamps, J.-A. Subke, I. K. Thomsen, and C. Chenu Soil moisture is of primary importance for predicting the evolution of soil carbon stocks and fluxes, both because it strongly controls organic matter decomposition and because it is predicted to change at global scales in the following decades. However, the soil functions used to model the heterotrophic respiration response to moisture have limited empirical support and introduce an uncertainty of at least 4 % in global soil carbon stock predictions by 2100. The necessity of improving the representation of this relationship in models has been highlighted in recent studies. Here we present a data-driven analysis of soil moisture-respiration relations based on 90 soils. With the use of linear models we show how the relationship between soil heterotrophic respiration and different measures of soil moisture is consistently affected by soil properties. The empirical models derived include main and moisture interaction effects of soil texture, organic carbon content and bulk density. When compared to other functions currently used in different soil biogeochemical models, we observe that our results can correct biases and reconcile differences within and between such functions. Ultimately, accurate predictions of the response of soil carbon to future climate scenarios will require the integration of soil-dependent moisture-respiration functions coupled with realistic representations of soil water dynamics.

Description: Inferring Amazon leaf demography from satellite observations of leaf area index Biogeosciences Discussions, 8, 10389-10421, 2011 Author(s): S. Caldararu, P. I. Palmer, and D. W. Purves Seasonal and year-to-year variations in leaf cover imprint significant spatial and temporal variability on biogeochemical cycles, and affect land-surface properties related to climate. We develop a demographic model of leaf phenology based on the hypothesis that trees seek an optimal Leaf Area Index (LAI) as a function of available light and soil water, and fitted it to spaceborne observations of LAI over the Amazon Basin, 2001–2005. We find the model reproduces the spatial and temporal LAI distribution whilst also predicting geographic variation in leaf age from the basin center (2.1 ± 0.2 yr), through to the lowest values over the deciduous Eastern Amazon (6 ± 2 months). The model explains the observed increase in LAI during the dry season as a net addition of leaves in response to increased solar radiation. We anticipate our work to be a starting point from which to develop better descriptions of leaf phenology to incorporate into more sophisticated earth system models.

Description: As different as day and night: evidence from root lifespan Biogeosciences Discussions, 8, 10459-10486, 2011 Author(s): W. Bai, J. Xia, S. Wan, W. Zhang, and L. Li Roots are key components of terrestrial ecosystem C cycling and play an important role in regulation of the response of terrestrial ecosystem to global climate warming, which occurs with greater warming magnitudes at night than during daytime across different regions on the Earth. However, there has been no detailed study to examine the effect of asymmetrical warming on root dynamics at the level of terrestrial ecosystem. To understand the effects of day and night warming on root lifespan in the semiarid temperate steppe in Northern China, a field study with a full factorial design including control, day warming, night warming and diurnal warming was conducted. The responses of root survivorship to day and night warming with modified rhizotron technique were monitored during the growing seasons of 2007–2009. We demonstrate, for the first time, that longevity of roots born in spring, summer and autumn showed different response to day, night and diurnal warming, and that day warming significantly prolonged the overall lifespan for the roots born in the growing seasons of 2007–2009 pooled as a whole, while night warning had no effect on the overall lifespan in the semi-arid grassland in Northern China. The differential response of root lifespan to day and night warming may be accounted for by the photoassimilate allocation as evidenced by that day and night warming had different effect on root non-structural carbohydrate content. This finding differed from other processes associated with ecosystem C cycle such as total ecosystem productivity, net ecosystem productivity and soil respiration. Thus our findings highlight that it is essential to incorporate the differential effects of day and night warming into the simulating and predicting the responses and feedbacks of terrestrial ecosystem C cycling to global warming.

Description: Chemodiversity in terpene emissions at a boreal Scots pine stand Biogeosciences Discussions, 8, 10577-10615, 2011 Author(s): J. Bäck, J. Aalto, M. Henriksson, H. Hakola, Q. He, and M. Boy Atmospheric chemistry in background areas is strongly influenced by natural vegetation. Coniferous forests are known to produce large quantities of volatile vapors, especially terpenes to the surrounding air. These compounds are reactive in the atmosphere, and contribute to the formation and growth of atmospheric new particles. Our aim was to analyze the variability of mono- and sesquiterpene emissions between Scots pine trees, in order to clarify the potential errors caused by using emission data obtained from only a few trees in atmospheric chemistry models. We also aimed at testing if stand history and seed origin has an influence on the chemotypic diversity. The inherited, chemotypic variability in mono- and sesquiterpene emission was studied in a seemingly homogeneous 47-yr-old stand in Southern Finland, where two areas differing in their stand regeneration history could be distinguished. Sampling was conducted in August 2009. Terpene concentrations in the air had been measured at the same site for seven years prior to branch sampling for chemotypes. Two main compounds, α-pinene and Δ 3 -carene formed together 40–97% of the monoterpene proportions in both the branch emissions and in the air concentrations. The data showed a bimodal distribution in emission composition, in particular in Δ 3 -carene emission within the studied population. 10% of the trees emitted mainly α-pinene and no Δ 3 -carene at all, whereas 20% of the trees where characterized as high Δ 3 -carene emitters (Δ 3 -carene forming 〉80% of total emitted monoterpene spectrum). An intermediate group of trees emitted equal amounts of both α-pinene and Δ 3 -carene. The emission pattern of trees at the area established using seeding as the artificial regeneration method differed from the naturally regenerated or planted trees, being mainly high Δ 3 -carene emitters. Some differences were also seen in e.g. camphene and limonene emissions between chemotypes, but sesquiterpene emissions did not differ significantly between trees. The atmospheric concentrations at the site were found to reflect the species and/or chemodiversity rather than the emissions measured from any single tree, and were strongly dominated by α-pinene. We also tested the effect of chemodiversity on modeled monoterpene concentrations at the site and found out that since it significantly influences the distributions and hence the chemical reactions in the atmosphere, it should be taken into account in atmospheric modeling.

Description: Indications for a ubiquitous dissolved pigment degradation product in subsurface waters of the global ocean Biogeosciences Discussions, 8, 10697-10724, 2011 Author(s): R. Röttgers and B. P. Koch Measurements of light absorption by chromophoric dissolved organic matter (CDOM) from sub-surface waters of the tropical Atlantic and Pacific showed a distinct absorption shoulder at 410–415 nm, indicating an underlying absorption of a pigment. A similar absorption maximum at ~410 nm was also found in the particulate fraction and is usually attributed to absorption by respiratory pigments of heterotrophic unicellular organisms. The CDOM absorption shoulder was described earlier in the Indian Ocean at 600 m depth and was related to a "deep red fluorescence" found in the same depth, i.e. in the oxygen minimum zone (Breves et al., 2003; Broenkow et al., 1983). In our study, fluorescence measurements of pre-concentrated DOM samples confirmed that the absorption at ~410 nm was related to a specific fluorescence at 650 nm. The absorption characteristic of this specific fluorophor was examined by fluorescence emission/excitation analysis and this showed a clear excitation maximum at 415 nm (in methanol) that can explain the absorption shoulder in the CDOM spectra. The spectral characteristics of the substance found in the dissolved and particulate fraction did not match with those of chlorophyll a degradation products (as found in a sample from the sea surface) but can be explained by the occurrence of respiratory pigments from heterotrophs. Combining the observations of the "deep red fluorescence" and the 410 nm-absorption shoulder suggests that there are high concentrations of a pigment degradation product (cytochrome c ) in DOM of all major oceans. Most pronouncedly we found this signal in the deep chlorophyll maximum and the oxygen minimum zone of tropical regions. The origin, chemical nature, turn-over rate, and fate of this molecule is so far unknown.

Description: Characterization of turbulence and validation of fine-scale parametrization in the Mediterranean Sea during BOUM experiment Biogeosciences Discussions, 8, 8961-8998, 2011 Author(s): Y. Cuypers, P. Bouruet-Aubertot, C. Marec, and J.-L. Fuda One main purpose of BOUM experiment was to give evidence of the possible impact of submesoscale dynamics on biogeochemical cycles. To this aim physical as well as biogeochemical data were collected along a zonal transect through the western and eastern basins. Along this transect 3 day fixed point stations were performed within anticyclonic eddies during which microstructure measurements were collected over the first 100 m. We focus here on the characterization of turbulent mixing induced by internal wave breaking. The analysis of microstructure measurements revealed a high level of turbulence in the seasonal pycnocline and a moderate level below with energy dissipation mean values of the order of 10 −6 W kg −1 and 10 −8 W kg −1 , respectively. Fine-scale parameterizations developed to mimic energy dissipation produced by internal wavebreaking were then tested against these direct measurements. Once validated a parameterization has been applied to infer energy dissipation and mixing over the whole data set, thus providing an overview over a latitudinal section of the Mediterranean sea. The results evidence a significant increase of dissipation at the top and base of eddies associated with strong near inertial waves. Vertical turbulent diffusivity is increased both in these regions and in the weakly stratified eddy core.

Description: An approach to the investigation of CO 2 uptake by soil microorganisms Biogeosciences Discussions, 8, 9235-9281, 2011 Author(s): K. M. Hart, B. W. Moran, C. C. R. Allen, V. Kouloumbos, S. F. Oppenheimer, L. Barron, A. J. Simpson, L. A. Kulakov, and B. P. Kelleher Sequestration of CO 2 via biological sinks is a matter of great scientific importance due to their potential to lower atmospheric CO 2 levels. In this study a custom built incubation chamber was used to cultivate a soil microbial community, under ideal conditions, to investigate soil chemoautotrophy. The internal atmospheric CO 2 concentrations were monitored and used to estimate the mass of CO 2 uptake. It was found after careful background corrections that 256.4 μg CO 2 kg −1 dry soil was removed from the chamber atmosphere over 16 h. Comparisons were made to photosynthetic controls (i.e. grass and soil algae) whereupon it was observed that the chemoautotrophic fraction sequestered 2.6 and 5.4 % of that taken up by grass and soil algae respectively. Using isotopically labelled 13 CO 2 and GCMS-IRMS it was also possible to extract and identify labelled fatty acids after a short incubation time, hence confirming the CO 2 uptake potential of the soil slurry. Provided with favourable conditions, chemoautotrophic soil bacteria have the potential to make a significant impact on inorganic carbon sequestration within the environment. The results of this in vivo study have provided ground work for future studies intending to mimic the in situ environment by providing a reliable method for investigating CO 2 uptake by soil microorganisms.

Description: Nitrogen balance and fate in a heavily impacted watershed (Oglio River, Northern Italy): in quest of the missing sources and sinks Biogeosciences Discussions, 8, 9201-9234, 2011 Author(s): M. Bartoli, E. Racchetti, C. A. Delconte, E. Sacchi, E. Soana, A. Laini, D. Longhi, and P. Viaroli We present data from a comprehensive investigation carried out from 2007 to 2010, focussing on nitrogen pollution in the lower Oglio River basin (3800 km 2 , Po plain, Northern Italy). Nitrogen mass balances, computed for the whole basin with 2000 and 2008 data, suggest a large N surplus in this area, over 40 000 t N yr −1 , and increasing between 2000 and 2008. Calculations indicate a very large impact of animal husbandry and agricultural activities in this watershed, with livestock manure and synthetic fertilizers contributing 85 % of total N inputs (about 100 000 t N yr −1 ) and largely exceeding crop uptake and other N losses (about 60 000 t N yr −1 ). Nitrogen from domestic and industrial origin is estimated as about 5800 and 7200 t N yr −1 , respectively, although these loads are overestimated, as denitrification in treatment plants is not considered; nonetheless, they represent a minor term of the N budget. Annual export of nitrogen from the basin, calculated from flow data and water chemistry at the mouth of the Oglio River, is estimated at 13 000 t N yr −1 , and represents a relatively small fraction of N inputs and surplus (~12 % and 34 %, respectively). After considering N sinks in crop uptake, soil denitrification and volatilization, a large excess remains unaccounted (~26 000 t N yr −1 ) in unknown temporary or permanent N sinks. Nitrogen removal via denitrification was evaluated in the Oglio riverbed with stable isotope techniques (δ 15 N and δ 18 O in nitrate). The downstream final segment of the river displays an enriched nitrate stable isotope composition but calculations suggest a N removal corresponding to at most 12 % of the unaccounted for N amount. Denitrification was also evaluated in riverine wetlands with the isotope pairing technique. Areal rates are elevated but overall N removal is low (about 1 % of the missing N amount), due to small wetland surfaces and limited lateral connectivity. The secondary drainage channel network has a much higher potential for nitrogen removal via denitrification, due to its great linear development, estimated in over 12 500 km, and its capillary distribution in the watershed. In particular, we estimated a maximum N loss up to 8500 t N yr −1 , which represents up to 33 % of the unaccounted for N amount in the basin. Overall, denitrification in surface aquatic habitats within this basin can be responsible for the permanent removal of about 12 000 t of N per year; but the fate of some 14 000 t remains unknown. We provide evidences that an important N sink in this basin could be represented by groundwater. In the short term, the aquifers can store nitrogen and available data on nitrate concentration in wells support this hypothesis. In the mid-long term, part of the accumulated nitrate can be substantially recycled via springs and can pollute surface waters via river-groundwater interactions. This probably explains the ten fold increase of nitrate concentration in a reach of the lower Oglio River where no point pollutions sources are present.

Description: Subsidence and carbon loss in drained tropical peatlands: reducing uncertainty and implications for CO 2 emission reduction options Biogeosciences Discussions, 8, 9311-9356, 2011 Author(s): A. Hooijer, S. Page, J. Jauhiainen, W. A. Lee, X. X. Lu, A. Idris, and G. Anshari Conversion of tropical peatlands to agriculture leads to a release of carbon from previously stable, long-term storage, resulting in land subsidence that can be a surrogate measure of CO 2 emissions to the atmosphere. We present an analysis of recent large-scale subsidence monitoring studies in Acacia and oil palm plantations on peatland in SE Asia, and compare the findings with previous studies. Subsidence in the first 5 years after drainage was found to be 142 cm, of which 75 cm occurred in the first year. After 5 years, the subsidence rate in both plantation types, at average water table depths of 0.7 m, remained constant at around 5 cm yr −1 . Bulk density profiles indicate that consolidation contributes only 7 % to total subsidence, in the first year after drainage, and that the role of compaction is also reduced quickly and becomes negligible after 5 years. Over 18 years after drainage, 92 % of cumulative subsidence was caused by peat oxidation. The average rate of carbon loss over the first 5 years was 178 t ha −1 yr −1 CO 2eq , which reduced to 73 t ha −1 yr −1 CO 2eq over subsequent years, resulting in an average loss of 100 t ha −1 yr −1 CO 2eq annualized over 25 years. Part of the observed range in subsidence and carbon loss values is explained by differences in water table depth, but vegetation cover and addition of fertilizers also influence peat oxidation. A relationship with groundwater table depth shows that subsidence and carbon loss are still considerable even at the highest water table levels theoretically possible in plantations. This implies that improved water management will reduce these impacts by only 20 % at most, relative to current conditions, and that high rates of carbon loss and land subsidence should be accepted as inevitable consequences of conversion of forested tropical peatlands to other land uses.

Description: Methanotrophic activity and diversity in different Sphagnum magellanicum dominated habitats in the southernmost peat bogs of Patagonia Biogeosciences Discussions, 8, 9357-9380, 2011 Author(s): N. Kip, C. Fritz, E.S. Langelaan, Y. Pan, L. Bodrossy, V. Pancotto, M. S. M. Jetten, A. J. P. Smolders, and H. J. M. Op den Camp Sphagnum peatlands are important ecosystems in the methane cycle. Methanotrophs living in and on the Sphagnum mosses are able to act as a methane filter and thereby reduce methane emissions. We investigated in situ methane concentrations and the corresponding activity and diversity of methanotrophs in different Sphagnum dominated bog microhabitats. In contrast to the Northern Hemisphere peat ecosystems the temperate South American peat bogs are dominated by one moss species; Sphagnum magellanicum . This permitted a species-independent comparison of the different bog microhabitats. Potential methane oxidizing activity was found in all Sphagnum mosses sampled and a positive correlation was found between activity and in situ methane concentrations. Substantial methane oxidation activity (23 μmol CH 4 gDW −1 day −1 ) was found in pool mosses and could be correlated with higher in situ methane concentrations (〉35 μmol CH 4 l −1 pore water). Little methanotrophic activity ( 4 μmol CH 4 gDW −1 day −1 ) in Sphagnum litter situated at depths around the water levels and rich in methane. The total bacterial community was studied using 16S rRNA gene sequencing and the methanotrophic communities were studied using a pmo A microarray and a complementary pmo A clone library. The methanotrophic diversity was similar in the different habitats of this study and surprisingly comparable to the methanotrophic diversity found in peat mosses from the Northern Hemisphere. The pmo A microarray data indicated that both alpha- and gammaproteobacterial methanotrophs were present in all Sphagnum mosses, even in those mosses with a low initial methane oxidation activity. Prolonged incubation of Sphagnum mosses from lawn and hummock with methane revealed that the methanotrophic community present was viable and showed an increased activity within 15 days. The high abundance of methanotrophic Methylocystis species in the most active mosses suggests that these might be responsible for the bulk of methane oxidation.

Description: High nitrate to phosphorus ratio attenuates negative effects of rising p CO 2 on net population carbon accumulation Biogeosciences Discussions, 8, 6833-6857, 2011 Author(s): S. A. Krug, S. L. Eggers, and B. Matthiessen The ongoing rise in atmospheric p CO 2 and the consequent increase in ocean acidification have direct effects on marine calcifying phytoplankton which potentially translates into altered carbon export. To date it remains unclear first, how nutrient ratio, in particular from coccolithophores preferred phosphate limitation, interacts with p CO 2 on particulate carbon accumulation. Second, how direct physiological responses on the cellular level translate into a net population response. In this study cultures of Emiliania huxleyi were full-factorially exposed to two different N:P ratios (Redfield and high N:P) and three different p CO 2 levels. Effects on net population particulate inorganic and organic carbon (PIC, POC) were measured after E. huxleyi cultures reached stationary phase. Thereby cell sizes and total cell abundance were taken into account. Corresponding to literature results show a significant negative cellular PIC and POC response which, however, was strongest under high N:P ratio. In contrast, net population PIC and POC accumulation was significantly attenuated under high N:P ratio. We suggest that less cellular nutrient accumulation allowed for higher cell abundances which compensated for the strong negative cellular PIC and POC response to p CO 2 on the population level. Moreover, the design of this study also allowed following natural alteration of carbon chemistry through changing DIC and alkalinity. Our results suggest that at high initial p CO 2 natural alteration of p CO 2 during the experimental runtime was regulated by algal biomass. In contrast, at low initial p CO 2 the PIC/POC ratio was responsible for changes in p CO 2 . Our results point to the fact that the physiological (i.e. cellular) PIC and POC response to ocean acidification cannot be linearly extrapolated to total population response and thus carbon export. It is therefore recommended to consider effects of nutrient limitation on cell physiology and translate these to net population carbon accumulation when predicting the influence of coccolithophores on both, the atmospheric p CO 2 feedback and their function in carbon export mechanisms.

Description: North American CO 2 exchange: intercomparison of modeled estimates with results from a fine-scale atmospheric inversion Biogeosciences Discussions, 8, 6775-6832, 2011 Author(s): S. M. Gourdji, K. L. Mueller, V. Yadav, D. N. Huntzinger, A. E. Andrews, M. Trudeau, G. Petron, T. Nehrkorn, J. Eluszkiewicz, J. Henderson, D. Wen, J. Lin, M. Fischer, C. Sweeney, and A. M. Michalak Robust estimates of regional-scale terrestrial CO 2 exchange are needed to support carbon management policies and to improve the predictive ability of models representing carbon-climate feedbacks. Large discrepancies remain, however, both among and between CO 2 flux estimates from atmospheric inverse models and terrestrial biosphere models. Improved atmospheric inverse models that provide robust estimates at sufficiently fine spatial scales could prove especially useful for monitoring efforts, while also serving as a validation tool for process-based assumptions in terrestrial biosphere models. A growing network of continental sites collecting continuous CO 2 measurements provides the information needed to drive such models. This study presents results from a regional geostatistical inversion over North America for 2004, taking advantage of continuous data from the nine sites operational in that year, as well as available flask and aircraft observations. The approach does not require explicit prior flux estimates, resolves fluxes at finer spatiotemporal scales than previous North American inversion studies, and uses a Lagrangian transport model coupled with high-resolution winds (i.e. WRF-STILT) to resolve near-field influences around measurement locations. The estimated fluxes are used in an inter-comparison with other inversion studies and a suite of terrestrial biosphere model estimates collected through the North American Carbon Program Regional and Continental Interim Synthesis. Differences among inversions are found to be smallest in areas of the continent best-constrained by the atmospheric data, pointing to the value of an expanded measurement network. Aggregation errors in previous coarser-scale inversion studies are likely to explain a portion of the remaining spread. The spatial patterns from a geostatistical inversion that includes auxiliary environmental variables from the North American Regional Reanalysis were similar to those from the median of the biospheric model estimates during the growing season, but diverged more strongly in the dormant season. This could be due to a lack of sensitivity in the inversion during the dormant season, but may also point to a lack of skill in the biospheric models outside of the growing season, particularly in agricultural areas. For the annual continental budget, the boundary conditions used as an input into the inversions were seen to have a substantial impact on the estimated net flux, with a difference of ~0.8 PgC yr −1 associated with results using two different plausible sets of boundary conditions.

Description: The significance of nitrous oxide emission from biofuel crops on arable land: a Swedish perspective Biogeosciences Discussions, 8, 6743-6774, 2011 Author(s): Å. Kasimir Klemedtsson and K. A. Smith The current regulations governing biofuel production in the European Union require that they have to mitigate climate change, by producing 〉35 % less greenhouse gases (GHG) than fossil fuels. There is a risk that this may not be achievable, since land use for crop production inevitably emits the strong GHG nitrous oxide (N 2 O), due to nitrogen fertilisation and cycling in the environment. We conclude that efficient agricultural crop production resulting in a good harvest and low N 2 O emission can fulfill the EU standard, and is possible under certain conditions for the Swedish agricultural and refinery production systems. However, in years having low crop yields total GHG emissions can be even higher than those released by burning of fossil fuels. In general, the N 2 O emission size in Sweden and northern Europe is such that there is a 〉50 % chance that the 35 % saving requirement will not be met. Thus ecosystem N 2 O emissions have to be convincingly assessed. Here we compare Swedish emission data with values estimated by means of statistical models and by a global, top-down, procedure; the measurements and the predictions often show higher values that would fail to meet the EU standard and thus prevent biofuel production development.

Description: Pteropods from the Caribbean Sea: dissolution as an indicator of past ocean acidification Biogeosciences Discussions, 8, 6901-6917, 2011 Author(s): D. Wall-Palmer, M. B. Hart, C. W. Smart, R. S. J. Sparks, A. Le Friant, G. Boudon, C. Deplus, and J. C. Komorowski The aragonite shell–bearing thecosome pteropods are an important component of the oceanic plankton. However, with increasing p CO 2 and the associated reduction in oceanic pH (ocean acidification), thecosome pteropods are thought to be particularly vulnerable to shell dissolution. The distribution and preservation of pteropods over the last 250,000 years have been investigated in marine sediment cores from the Caribbean Sea close to the island of Montserrat. Using the Limacina Dissolution Index (LDX), fluctuations in pteropod dissolution through the most recent glacial/interglacial cycles is documented. By comparison to the oxygen isotope record (global sea ice volume), we show that pteropod dissolution is closely linked to global changes in p CO 2 and pH and is, therefore, a global signal. These data are in agreement with the findings of experiments upon living pteropods, which show that variations in pH can greatly affect aragonitic shells. The results of this study provide information which may be useful in the prediction of future changes to the pteropod assemblage caused by ocean acidification.

Description: Predicting and partitioning ozone fluxes to maize crops from sowing to harvest: the Surfatm-O 3 model Biogeosciences Discussions, 8, 6701-6741, 2011 Author(s): P. Stella, E. Personne, B. Loubet, E. Lamaud, E. Ceschia, P. Béziat, J. M. Bonnefond, M. Irvine, P. Keravec, N. Mascher, and P. Cellier Terrestrial ecosystems represent a major sink for ozone (O 3 ) and also a critical control of tropospheric O 3 budget. However, due to its deleterious effects, plant functioning is affected by the ozone absorbed. It is thus necessary to both predict total ozone deposition to ecosystems and partition the fluxes in stomatal and non-stomatal pathways. The Surfatm-O 3 model was developed to predict ozone deposition to agroecosystems from sowing to harvest, taking into account each deposition pathways during bare soil, growth, maturity, and senescence periods. An additional sink was added during senescence: stomatal deposition for yellow leaves, not able to photosynthesise but transpiring. The model was confronted to measurements performed over three maize crops in different regions of France. Modelled and measured fluxes agreed well for one dataset for any phenological stage, with only 3 % difference over the whole cropping season. A larger discrepancy was found for the two other sites, 16 % and 19 % over the entire study period, especially during bare soil, early growth and senescence. This was attributed to site-specific soil resistance to ozone and possible chemical reactions between ozone and volatile organic compounds emitted during late senescence. Considering both night-time and daytime conditions, non-stomatal deposition was the major ozone sink, from 100 % during bare soil period to 70–80 % on average during maturity. However, considering only daytime conditions, especially under optimal climatic conditions for plant functioning, stomatal flux could represent 75 % of total ozone flux. This model could improve estimates of crop yield losses and projections of tropospheric ozone budget.

Description: Dynamics of phytoplankton community structure in the South China Sea in response to the East Asian aerosol input Biogeosciences Discussions, 8, 6637-6680, 2011 Author(s): C. Guo, J. Yu, T.-Y. Ho, B. Chen, L. Wang, S. Song, L. Kong, and H. Liu Recent studies have regarded atmospheric deposition as an increasingly important source of nutrients to the ocean. The South China Sea (SCS), where aerosol loading is among the highest in the world, however, is poorly studied particularly on the in situ changes of phytoplankton community structures in response to atmospheric deposition. By conducting a series of microcosm bioassays at different hydrographical locations and simulating different aerosol event scales, we observed both positive and negative responses induced by the input of East Asia (EA) aerosol with high nitrogen (N) and trace metal contents, in terms of both community structure and physiological characteristics of phytoplankton. High levels of aerosol loading profoundly relieved phytoplankton nitrogen and trace metal limitations in SCS, and thus increased total phytoplankton biomass, enhanced their physiological indicators (e.g. photosynthetic efficiency) and shifted phytoplankton assemblages from being dominated by picoplankton to microphytoplanton, especially diatoms. However, under low levels of aerosol loading, the composition shift and biomass accumulation were not apparent, suggesting that the stimulation effects might be counterbalanced by enhanced grazing mortality indicated by increased abundance of protist grazers. Trace metal toxicity of the aerosols was also an important negative factor to phytoplankton growth, especially picocyanobacteria, implicated by the high copper (Cu) concentration in the microcosm that surpassed the toxicity threshold of marine cyanobacteria. Moreover, the magnitude and duration of the deposition event, as well as the hydrographical and trophic conditions of receiving waters are also important factors when predicting the influence of an aerosol deposition event. Our results demonstrated that the EA aerosol deposition events could profoundly change nutrient and phytoplankton dynamics in SCS and highlighted the need for achieving an accurate comprehension of atmospheric nutrient on the biogeochemical cycles of the oceans.

Description: Soil moisture control over autumn season methane flux, Arctic Coastal Plain of Alaska Biogeosciences Discussions, 8, 6519-6554, 2011 Author(s): C. S. Sturtevant, W. C. Oechel, D. Zona, and C. E. Emerson Two shortfalls in estimating current and future seasonal budgets of methane efflux in Arctic regions are the paucity of non-summer measurements and an incomplete understanding of the sensitivity of methane emissions to changes in tundra moisture. A recent study in one Arctic region highlighted the former by observing a previously unknown large methane pulse during the onset of autumn soil freeze. This study addresses these research gaps by presenting an analysis of eddy covariance measurements of methane efflux and supporting environmental variables during the autumn season of 2009 and associated soil freeze-in period at our large-scale water manipulation site near Barrow, Alaska (the Biocomplexity Experiment). We found that methane emissions during the autumn were closely tied to liquid soil moisture in the top 30 cm of soil. Declines in soil moisture between manipulated wet, intermediate, and dry conditions as well as through time during the soil freeze-in period led to corresponding declines in methane efflux. During the period of soil freeze-in (from 23 September to 28 October), we estimate that our wet section emitted 623 mg CH 4 m −2 while the dry section emitted only 253 mg CH 4 m −2 , the average of which represents 18 % of net emissions from the typically measured growing season. We did not find evidence for a pulse in methane emissions during soil freeze at this site. Results from this study imply that future changes in tundra moisture will have a large effect on methane emissions in this region, and changes which span the saturation point are likely to have the largest effect. We speculate that changes in autumn soil moisture are also likely to affect winter emissions via the insulative effects of ice on winter soil temperature and liquid soil moisture availability after bulk soil freeze. Further research should expand the use of eddy covariance methane flux measurements to investigate ecosystem-level effects of tundra moisture on autumn and winter methane emissions in this and other Arctic regions.

Description: Calcification in the planktonic foraminifera Globigerina bulloides linked to phosphate concentrations in surface waters of the North Atlantic Ocean Biogeosciences Discussions, 8, 6447-6472, 2011 Author(s): D. Aldridge, C. J. Beer, and D. A. Purdie Marine calcifiers, such as planktonic foraminifera, form a major component of the global carbon cycle, acting as both a source and sink of CO 2 . Understanding factors that affect calcification in these organisms is therefore critical in predicting how the oceans will respond to increased CO 2 concentrations in the atmosphere. Here, size-normalised weights (SNWs) of the planktonic foraminifera Globigerina bulloides , collected from the surface waters of the North Atlantic, are compared with in situ carbonate ion concentrations ([CO 3 2− ]), optimum growth conditions (implied from G. bulloides abundances), and nutrient concentrations. Significant positive relationships suggest that phosphate concentration ([PO 4 3− ]) has the greatest effect on G. bulloides SNWs, with reduced test masses at higher concentrations (range: 0.04–0.31 μM). [CO 3 2− ] appears to have a minor effect over the range of values examined (148–181 μmol kg −1 ), and no evidence was found for increased SNWs under apparent optimum growth conditions. These findings point to the potential importance of phosphate concentration in determining calcification rates in foraminifera, a factor which has been overlooked by previous studies on these organisms. The confirmation of these results via carefully controlled culture studies is recommended in the future.

Description: A Holocene record of mercury accumulation in a pristine lake in Southernmost South America (53° S) – climatic and environmental drivers Biogeosciences Discussions, 8, 6555-6588, 2011 Author(s): Y.-M. Hermanns and H. Biester Mercury accumulation in lake sediments has been extensively studied with regard to anthropogenic signals, but natural processes controlling mercury accumulation in the past and the influence of environmental and climatic changes are still poorly understood. In this study we present a 17 300 years old record of mercury accumulation in lake sediments in combination with a multi-proxy investigation of climatic and environmental changes in Southernmost Patagonia (53° S). Mercury accumulation in the remote pristine lake has been highly variable (up to a factor of 16), clearly exceeding the anthropogenic forcing of atmospheric mercury fluxes (factor of 3–5). Statistical data evaluation based on Principle Component Analysis reveals that the dominant driver of variations in mercury accumulation along with other organically bound elements (e.g. copper and yttrium) has been leaching of soluble organic complexes from catchment soils. This indicates that changes in precipitation rates and soils leaching are the most important climatic driver for short term variations in mercury in the lake's sediments. In the long term, however, mercury accumulation rates remain relatively constant (29 μg m −2 yr −1 ). This reflects that mercury accumulation in the catchment soils through atmospheric deposition and bedrock weathering has been fairly constant, too. As carbon storage in the soils and transport into the lake have increased, Hg/C ratios in the soil OM decreased. Fluxes of Hg into the lake are therefore mainly determined by the amount of OM leached into the lake and its Hg/C ratio.

Description: Volatile diterpene emission from dominant conifers in Japan Biogeosciences Discussions, 8, 6681-6700, 2011 Author(s): S. N. Matsunaga, S. Chatani, S. Nakatsuka, D. Kusumoto, K. Kubota, Y. Utsumi, T. Enoki, A. Tani, and T. Hiura Reactive volatile organic compounds (VOCs) are known to affect atmospheric chemistry, especially biogenic VOCs (BVOCs) which have a significant impact on regional air quality due to their large emission rates and high reactivities. Diterpenes (Kaur-16-ene and a diterpene likely to be Hibaene) were detected in all of the 205 enclosure air samples collected over multiple seasons at two different sites from Cryptomeria japonica and Chamaecyparis obtusa trees, the dominant coniferous trees in Japan. The emission rate of Kaur-16-ene, was determined to be from 0.01 to 7.1 μg dwg −1 h −1 (average: 0.61 μg dwg −1 h −1 ) by branch enclosure measurements and solid phase – liquid extraction techniques. The emission rate was an order of magnitude higher than that of monoterpenes which are known to comprise a major portion of total BVOC emissions. In addition, total emission of Kaur-16-ene at 30 °C was estimated to be comparable to that of total anthropogenic VOC emissions. To our knowledge, this is the first report of volatile diterpene emissions from trees.

Description: Sensitivity analysis of the GEMS soil organic carbon model to land cover land use classification uncertainties under different climate scenarios in Senegal Biogeosciences Discussions, 8, 6589-6635, 2011 Author(s): A. M. Dieye, D. P. Roy, N. P. Hanan, S. Liu, M. Hansen, and A. Touré Spatially explicit land cover land use (LCLU) change information is needed to drive biogeochemical models that simulate soil organic carbon (SOC) dynamics. Such information is increasingly being mapped using remotely sensed satellite data with classification schemes and uncertainties constrained by the sensing system, classification algorithms and land cover schemes. In this study, automated LCLU classification of multi-temporal Landsat satellite data were used to assess the sensitivity of SOC modeled by the Global Ensemble Biogeochemical Modeling System (GEMS). The GEMS was run for an area of 1560 km 2 in Senegal under three climate change scenarios with LCLU maps generated using different Landsat classification approaches. This research provides a method to estimate the variability of SOC, specifically the SOC uncertainty due to satellite classification errors, which we show is dependent not only on the LCLU classification errors but also on where the LCLU classes occur relative to the other GEMS model inputs.

Description: Can Mg isotopes be used to trace cyanobacteria-mediated magnesium carbonate precipitation in alkaline lakes? Biogeosciences Discussions, 8, 6473-6517, 2011 Author(s): L. S. Shirokova, V. Mavromatis, I. Bundeleva, O. S. Pokrovsky, P. Bénézeth, C. Pearce, E. Gérard, S. Balor, and E. H. Oelkers The fractionation of Mg isotopes was determined during the cyanobacterial mediated precipitation of hydrous magnesium carbonate precipitation in both natural environments and in the laboratory. Natural samples were obtained from Lake Salda (SE Turkey), one of the few modern environments on the Earth's surface where hydrous Mg-carbonates are the dominant precipitating minerals. This precipitation was associated with cyanobacterial stromatolites which were abundant in this aquatic ecosystem. Mg isotope analyses were performed on samples of incoming streams, groundwaters, lake waters, stromatolites, and hydromagnesite-rich sediments. Laboratory Mg carbonate precipitation experiments were conducted in the presence of purified Synechococcus sp cyanobacteria that were isolated from the lake water and stromatolites. The hydrous magnesium carbonates nesquehonite (MgCO 3 ·3H 2 O) and dypingite (Mg 5 (CO 3 ) 4 (OH) 2 5(H 2 O)) were precipitated in these batch reactor experiments from aqueous solutions containing either synthetic NaHCO 3 /MgCl 2 mixtures or natural Lake Salda water, in the presence and absence of live photosynthesizing Synechococcus sp. Bulk precipitation rates were not to affected by the presence of bacteria when air was bubbled through the system. In the stirred non-bubbled reactors, conditions similar to natural settings, bacterial photosynthesis provoked nesquehonite precipitation, whilst no precipitation occurred in bacteria-free systems in the absence of air bubbling, despite the fluids achieving a similar or higher degree of supersaturation. The extent of Mg isotope fractionation (Δ 26 Mg solid-solution ) between the mineral and solution in the abiotic experiments was found to be identical, within uncertainty, to that measured in cyanobacteria-bearing experiments, and ranges from −1.4 to −0.7 ‰. This similarity refutes the use of Mg isotopes to validate microbial mediated precipitation of hydrous Mg carbonates.

Description: A model study of the seasonal and long term North Atlantic surface p CO 2 variability Biogeosciences Discussions, 8, 10187-10227, 2011 Author(s): J. F. Tjiputra, A. Olsen, K. Assmann, B. Pfeil, and C. Heinze A coupled biogeochemical-physical ocean model is used to study the long term variations of surface p CO 2 in the North Atlantic Ocean. The model agrees well with recent underway p CO 2 observations from the Surface Ocean CO 2 Atlas (SOCAT) database in various locations in the North Atlantic. The distinct seasonal cycles observed at different parts of the North Atlantic are well reproduced by the model. In most regions except the subpolar domain, the recent observed trends in p CO 2 and air–sea carbon fluxes are also simulated by the model. Over a long period between 1960–2008, the primary mode of surface p CO 2 variability is dominated by the increasing trend associated with the invasion of anthropogenic CO 2 into the ocean. We show that, to first order, the ocean surface circulation and air–sea heat flux patterns can explain the spatial variability of this dominant increasing trend. Regions with strong surface mass transport and negative air–sea heat flux have the tendency to maintain lower surface p CO 2 . Regions of surface convergence and mean positive air–sea heat flux such as the subtropical gyre and the western subpolar gyre have faster increase in p CO 2 over a long term period. The North Atlantic Oscillation (NAO) plays a major role in controlling the variability occurring at interannual to decadal time scales. The NAO predominantly influences surface p CO 2 in the North Atlantic by changing the physical properties of the North Atlantic water masses, particularly by perturbing the temperature and dissolved inorganic carbon in the surface ocean. We show that present underway observations are valuable for both calibrating the model, as well as for improving our understanding of the regionally heterogeneous variability of surface p CO 2 . In addition, they can be important for detecting any long term change in the regional carbon cycle due to ongoing climate change.

Description: Denitrification and inference of nitrogen sources in the karstic Floridan Aquifer Biogeosciences Discussions, 8, 10247-10294, 2011 Author(s): J. B. Heffernan, A. R. Albertin, M. L. Fork, B. G. Katz, and M. J. Cohen Aquifer denitrification is among the most poorly constrained fluxes in global and regional nitrogen budgets. The few direct measurements of denitrification in groundwaters provide limited information about its spatial and temporal variability, particularly at the scale of whole aquifers. Uncertainty in estimates of denitrification may also lead to underestimates of its effect on isotopic signatures of inorganic N, and thereby confound the inference of N source from these data. In this study, our objectives are to quantify the magnitude and variability of denitrification in the Upper Floridan Aquifer (UFA) and evaluate its effect on N isotopic signatures at the regional scale. Using dual noble gas tracers (Ne, Ar) to generate physical predictions of N 2 gas concentrations for 112 observations from 61 UFA springs, we show that excess (i.e. denitrification-derived) N 2 is highly variable in space and inversely correlated with dissolved oxygen (O 2 ). Negative relationship between O 2 and δ 15 N NO 3 across a larger dataset of 113 springs, well-constrained isotopic fractionation coefficients, and strong 15 N : 18 O covariation further support inferences of denitrification in this uniquely organic-matter-poor system. Despite relatively low average rates, denitrification accounted for 32% of estimated aquifer N inputs across all sampled UFA springs. Back-calculations of source δ 15 N NO 3 based on denitrification progression suggest that isotopically-enriched nitrate (NO 3 − ) in many springs of the UFA reflects groundwater denitrification rather than urban- or animal-derived inputs.

Description: Microbial nitrogen cycling on the Greenland Ice Sheet Biogeosciences Discussions, 8, 10423-10457, 2011 Author(s): J. Telling, M. Stibal, A. M. Anesio, M. Tranter, I. Nias, J. Cook, G. Lis, J. L. Wadham, A. Sole, P. Nienow, and A. Hodson Microbial nitrogen cycling was investigated along a 79 km transect into the Greenland Ice Sheet (GrIS) in early August 2010. The depletion of dissolved nitrate and production of ammonium (relative to icemelt) in cryoconite holes within 7.5 km of the ice sheet margin suggested microbial uptake and ammonification respectively. Nitrogen fixation (

Description: Analyzing precipitationsheds to understand the vulnerability of rainfall dependent regions Biogeosciences Discussions, 8, 10487-10516, 2011 Author(s): P. W. Keys, R. J. van der Ent, L. J. Gordon, H. Hoff, R. Nikoli, and H. H. G. Savenije It is well known that rivers connect upstream and downstream ecosystems within watersheds. Here we describe the concept of precipitationsheds to show how upwind terrestrial evaporation source areas contribute moisture for precipitation in downwind sink regions. We illustrate the importance of upwind land cover in precipitationsheds to sustain precipitation in critically water stressed downwind areas, i.e. dryland agricultural areas. We first identify seven regions where rainfed agriculture is particularly vulnerable to reductions in precipitation, and then map their precipitationsheds. We further develop a framework for qualitatively assessing the vulnerability of precipitation for these seven agricultural regions. We illustrate that the sink regions have varying degrees of vulnerability to changes in upwind evaporation rates depending on the extent of the precipitationshed, source region land use intensity and expected land cover changes in the source region.

Description: Carbon dynamics in the western Arctic Ocean: insights from full-depth carbon isotope profiles of DIC, DOC, and POC Biogeosciences Discussions, 8, 10677-10696, 2011 Author(s): D. R. Griffith, A. P. McNichol, L. Xu, F. A. McLaughlin, R. W. Macdonald, K. A. Brown, and T. I. Eglinton Arctic warming is projected to continue throughout the coming century. Yet, our currently limited understanding of the Arctic Ocean carbon cycle hinders our ability to predict how changing conditions will affect local Arctic ecosystems, regional carbon budgets, and global climate. We present here the first set of concurrent, full-depth, dual-isotope profiles for dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and suspended particulate organic carbon (POCsusp) at two sites in the Canada Basin of the Arctic Ocean. The carbon isotope composition of sinking and suspended POC in the Arctic contrasts strongly with open ocean Atlantic and Pacific sites, pointing to a combination of inputs to Arctic POCsusp at depth, including surface-derived organic carbon (OC), sorbed/advected OC, and OC derived from in situ DIC fixation. The latter process appears to be particularly important at intermediate depths, where mass balance results suggest that OC derived from in situ DIC fixation contributes up to 22% of POCsusp. As in other oceans, surface-derived OC is still a dominant source to Arctic POCsusp. Yet, we suggest that significantly smaller vertical POC fluxes in the Canada Basin make it possible to see evidence of DIC fixation in the POCsusp pool even at the bulk isotope level.

Description: Macrobenthic assemblage structure and organismal stoichiometry control faunal processing of particulate organic carbon and nitrogen in oxygen minimum zone sediments Biogeosciences Discussions, 8, 10725-10760, 2011 Author(s): W. R. Hunter, L. A. Levin, H. Kitazato, and U. Witte The Arabian Sea oxygen minimum zone (OMZ) impinges on the western Indian continental margin between 150 and 1500 m, causing gradients in oxygen availability and sediment geochemistry at the sea floor. Oxygen availability and sediment geochemistry are important factors structuring macrofaunal assemblages in marine sediments. However, relationships between macrofaunal assemblage structure and sea-floor carbon and nitrogen cycling are poorly understood. We conducted in situ 13 C: 15 N tracer experiments in the OMZ core (540 m [O 2 ] = 0.35 μmol l −1 ) and lower OMZ boundary (800–1100 m, [O 2 ] = 2.2–15.0 μmol l −1 ) to investigate how macrofaunal assemblage structure, affected by different oxygen levels, and C:N coupling influence the fate of particulate organic matter. No fauna were present in the OMZ core. Within the OMZ boundary, relatively high abundance and biomass resulted in the highest macrofaunal assimilation of particulate organic carbon (POC) and nitrogen (PON) at the lower oxygen 800 m stations ([O 2 ] = 2.2–2.65 μmol l −1 ). At these stations the numerically dominant cirratulid polychaetes exhibited greatest POC and PON uptake. By contrast, at the higher oxygen 1100 m station ([O 2 ] = 15.0 μmol l −1 ) macrofaunal C and N assimilation was lower, with POC assimilation dominated by one large solitary ascidian. Macrofaunal POC and PON assimilation were influenced by changes in oxygen availability, and significantly correlated to differences in macrofaunal assemblage structure between stations. POC and PON assimilation was characterised by carbon accumulation within the macrofauna, suggesting the importance of anaerobic metabolism at all stations. However, macrofaunal feeding responses were ultimately characterised by preferential organic nitrogen assimilation, relative to their internal C:N budgets.

Description: Investigating the stomatal, cuticular and soil ammonia fluxes over a growing tritical crop under high acidic loads Biogeosciences Discussions, 8, 10317-10350, 2011 Author(s): B. Loubet, C. Decuq, E. Personne, R. S. Massad, C. Flechard, O. Fanucci, N. Mascher, J.-C. Gueudet, S. Masson, B. Durand, S. Génermont, Y. Fauvel, and P. Cellier Ammonia concentration and fluxes were measured above a growing triticale field for two months during May and June 2010 at the NitroEurope crop site in Grignon (Fr-Gri) near Paris, France. The measurement campaign started 15 days following a 40 kg N ha −1 application of an ammonium nitrate solution. A new mini-WEDD (Wet Effluent Denuder) flow injection analyser with three channels (ROSAA, RObust and Sensitive Ammonia Analyser) was used to measure NH 3 fluxes using the aerodynamic gradient method. The measured ammonia concentrations varied from 0.01 to 39 μg NH 3 m −3 and were largely influenced by advection from the nearby farm. The ammonia fluxes ranged from −560 to 220 ng NH 3 m −2 s −1 and averaged −29 ng NH 3 m −2 s −1 . During some periods the large deposition fluxes could only be explained by a very small surface resistance, which may be due to the high concentrations in certain acid gases (HNO 3 and SO 2 ) observed in this suburban area. Ammonia emissions were also measured occasionally. The canopy compensation point C c was around 1.5 μg NH 3 m −3 on average. The canopy emission potential Γ c ( C c normalised for the temperature response of the Henry equilibrium) decreased over the course of the measurement campaign from Γ c = 2200 to Γ c = 450, the latter value being close to the median stomatal emission potential (Γ s ) for managed ecosystems reported in the literature. The temporal dynamics of the measured NH 3 flux compared well with the Surfatm- NH 3 model using fitted parameters. The subjectivity of the model fitting is discussed based on a sensitivity analysis.