ALBERT

Description: Linking diverse nutrient patterns to different water masses within anticyclonic eddies in the upwelling system off Peru Yonss Saranga José, Heiner Dietze, and Andreas Oschlies Biogeosciences, 14, 1349-1364, doi:10.5194/bg-14-1349-2017, 2017 This study aims to investigate the diverse subsurface nutrient patterns observed within anticyclonic eddies in the upwelling system off Peru. Two simulated anticyclonic eddies with opposing subsurface nitrate concentrations were analysed. The results show that diverse nutrient patterns within anticyclonic eddies are related to the presence of water mass from different origins at different depths, responding to variations in depth of the circulation strength at the edge of the eddy.

Description: Gas chromatography vs. quantum cascade laser-based N 2 O flux measurements using a novel chamber design Christian Brümmer, Bjarne Lyshede, Dirk Lempio, Jean-Pierre Delorme, Jeremy J. Rüffer, Roland Fuß, Antje M. Moffat, Miriam Hurkuck, Andreas Ibrom, Per Ambus, Heinz Flessa, and Werner L. Kutsch Biogeosciences, 14, 1365-1381, doi:10.5194/bg-14-1365-2017, 2017 We present a novel chamber design for measuring soil–atmosphere N 2 O fluxes and compare the performance of a commonly applied gas chromatography (GC) setup with laser-based (QCL) concentration detection. While GC was still a useful method for longer-term investigations, we found that closure times of 10 min and sampling every 5 s is sufficient when using a QCL system. Further, extremely low standard errors (

Description: Drivers of multi-century trends in the atmospheric CO 2 mean annual cycle in a prognostic ESM Jessica Liptak, Gretchen Keppel-Aleks, and Keith Lindsay Biogeosciences, 14, 1383-1401, doi:10.5194/bg-14-1383-2017, 2017 We analyzed the evolution of the atmospheric CO 2 mean annual cycle simulated during 1950–2300 under three scenarios designed to separate the effects of climate change, CO 2 fertilization, and land use change. CO 2 fertilization in boreal and temperate ecosystems drove mean annual cycle amplification over the NH midlatitudes during 1950–2300. Boreal and Arctic climate change drove high-latitude amplification before 2200, after which CO 2 fertilization contributed nearly equally to amplification.

Description: Viable cold-tolerant iron-reducing microorganisms in geographically diverse subglacial environments Sophie L. Nixon, Jon P. Telling, Jemma L. Wadham, and Charles S. Cockell Biogeosciences, 14, 1445-1455, doi:10.5194/bg-14-1445-2017, 2017 Despite their permanently cold and dark characteristics, subglacial environments (glacier ice–sediment interface) are known to harbour active microbial communities. However, the role of microbial iron cycling in these environments is poorly understood. Here we show that subglacial sediments harbour active iron-reducing microorganisms, and they appear to be cold-adapted. These results may have important implications for global biogeochemical iron cycling and export to marine ecosystems.

Description: Can land degradation drive differences in the C exchange of two similar semiarid ecosystems? Ana López-Ballesteros, Cecilio Oyonarte, Andrew S. Kowalski, Penélope Serrano-Ortiz, Enrique P. Sánchez-Cañete, M. Rosario Moya, and Francisco Domingo Biogeosciences Discuss., doi:10.5194/bg-2017-77,2017 Manuscript under review for BG (discussion: open, 0 comments) The concept of land degradation stems from the loss of an ecosystem's biological productivity, which in turn relies on several degradation processes, such as long-term loss of natural vegetation, depletion of soil nutrients, soil compaction or water and wind erosion, to which drylands are especially vulnerable. Currently, drylands occupy more than one third of the global terrestrial surface and will probably expand under future climate change scenarios. Drylands' key role in the global C balance has been recently demonstrated, but the effects of land degradation on C sequestration by these ecosystems needs further research. In the present study, we compare net carbon exchange, together with satellite data and meteorological, ambient and vadose zone (CO 2 , water content and temperature) variables, between two nearby (~23 km) experimental sites representing natural (i.e. site of reference) and degraded grazed semiarid grasslands located in SE Spain, via eddy covariance measurements over 6 years, with highly variable precipitation magnitude and distribution. Results show a striking difference in the annual C balances with an average release of 196 ± 40 and −23 ± 20 g C m −2 yr −1 for the degraded and natural sites, respectively. At the seasonal scale, differing patterns in net CO 2 fluxes were detected over both growing and dry seasons. As expected, during the growing seasons, greater net C uptake over longer periods was observed in the natural site, however, much greater net C release was measured in the degraded site during drought periods. We tested differences in all monitored meteorological and soil variables and found it most relevant that CO 2 at 1.50 m belowground was around 1000 ppm higher in the degraded site. Thus, we believe that subterranean ventilation of this vadose zone CO 2 , previously observed at both sites, largely drives the differences in C dynamics between them, especially during the dry season maybe due to enhanced subsoil-atmosphere interconnectivity in the degraded site. Overall, the 12 site-years of data allow direct exploration of the roles of climate and land degradation in the biological and non-biological processes that ultimately control the C sequestration capacity of semiarid ecosystems.

Description: Quantifying nutrient fluxes with a new hyporheic passive flux meter (HPFM) Julia Vanessa Kunz, Michael D. Annable, Jaehyun Cho, Wolf von Tümpling, Kirk Hatfield, Suresh Rao, Dietrich Borchardt, and Michael Rode Biogeosciences, 14, 631-649, doi:10.5194/bg-14-631-2017, 2017 The hyporheic zone, the subsurface region of streams, is a key compartment for in-stream nutrient retention. Knowledge on actual hyporheic processing rates is still limited due to methodological restrictions which are mainly related to the high local and temporal variability of subsurface flow patterns and nutrient transformation processes. We present a new device which allows quantitative assessment of hyporheic nutrient fluxes and demonstrate its advantages in an exemplary field testing.

Description: Simulating precipitation decline under a Mediterranean deciduous Oak forest: effects on isoprene seasonal emissions and predictions under climatic scenarios Anne-Cyrielle Genard-Zielinski, Christophe Boissard, Elena Ormeño, Juliette Lathière, Ilja M. Reiter, Henri Wortham, Jean-Philippe Orts, Brice Temine-Roussel, Bertrand Guenet, Svenja Bartsch, Thierry Gauquelin, and Catherine Fernandez Biogeosciences Discuss., doi:10.5194/bg-2017-17,2017 Manuscript under review for BG (discussion: open, 0 comments) From seasonal in situ observations on how a Mediterranean ecosystem responds to drought, a specific isoprene emission (ER) algorithm was developed, upon which 2100 projections (IPCC RCP2.6 and RCP8.5 scenarios) were made. ER were found to be mainly sensitive to soil water content, and poorly represented by current empirical emission model. Drought was found to aggravate thermal stress on ER, increase ER intensity and their frequency response to environmental conditions (present and future).

Description: Evaluating environmental drivers of spatial variability in free-living nematode assemblages along the Portuguese margin Lidia Lins, Frederik Leliaert, Torben Riehl, Sofia Pinto Ramalho, Eliana Alfaro Cordova, André Morgado Esteves, and Ann Vanreusel Biogeosciences, 14, 651-669, doi:10.5194/bg-14-651-2017, 2017 Anthropogenic impacts in the deep sea are regarded as high disturbers of the deep-sea bottom. This study focus in trying to understand the impact of disturbance in the ecosystem and how important functions, such as hydrodynamics and surface primary productivity, affect benthic organisms. Our results showed that high sediment variability due to hydrodynamics, and consequently disturbance, combined with high resource availability are probably the main factors promoting deep-sea species coexistence

Description: Small-scale variability in geomorphological settings influences mangrove-derived organic matter export in a tropical bay Geraldina Signa, Antonio Mazzola, James Kairo, and Salvatrice Vizzini Biogeosciences, 14, 617-629, doi:10.5194/bg-14-617-2017, 2017 Using a combined analytical approach (fatty acid and isotopic markers), we found that the different settings of the creeks flowing into Gazi Bay influenced the mangrove export to the adjacent seagrasses and coral reef. Given the major influence that organic matter exchange between habitats has on organic matter availability and consumption by fauna, the small-scale variability in mangrove export may have far-reaching consequences for local food webs and for the functioning of the whole ecosystem.

Description: No abstract is available for this article.

Description: Amplification of global warming through pH-dependence of DMS-production simulated with a fully coupled Earth system model Jörg Schwinger, Jerry Tjiputra, Nadine Goris, Katharina Six, Alf Kirkevåg, Øyvind Seland, Christoph Heinze, and Tatiana Ilyina Biogeosciences Discuss., doi:10.5194/bg-2017-33,2017 Manuscript under review for BG (discussion: open, 0 comments) Transient global warming under the high emission scenario RCP8.5 is amplified by up to 6 % if a pH-dependency of marine DMS-production is assumed. Importantly, this additional warming is not spatially homogeneous but shows a pronounced north-south gradient. Over the Antarctic continent, the additional warming is almost twice the global average. In the Southern Ocean we find a small DMS-climate feedback that counteracts the original reduction of DMS-production due to ocean acidification.

Description: Isoprene emission and photosynthesis during heat waves and drought in black locust Ines Bamberger, Nadine K. Ruehr, Michael Schmitt, Andreas Gast, Georg Wohlfahrt, and Almut Arneth Biogeosciences Discuss., doi:10.5194/bg-2017-32,2017 Manuscript under review for BG (discussion: open, 0 comments) We studied the effects of two weeks long summer heat waves and drought on photosynthesis and isoprene emissions in black locust trees. While photosynthesis decreased, isoprene emission increased sharply in response to heat and heat-drought stress. Comparing isoprene emissions at the same temperature, however, demonstrated that stressed trees emitted less isoprene than expected. This reveals that in order to predict isoprene emissions during heat waves, model parameters need to be re-evaluated.

Description: An assessment of geographical distribution of different plant functional types over North America simulated using the CLASS-CTEM modelling framework Rudra K. Shrestha, Vivek K. Arora, Joe R. Melton, and Laxmi Sushama Biogeosciences Discuss., doi:10.5194/bg-2017-35,2017 Manuscript under review for BG (discussion: open, 0 comments) Computer models of vegetation provide a tool to assess how future changes in climate may affect geographical distribution of vegetation. However, such models must first be assessed for their ability to reproduce present day geographical distribution of vegetation. Here, we assess the ability of one such dynamic vegetation model. We find that while the model is broadly successful in reproducing the geographical distribution of trees and grasses in North America some limitations remain.

Description: The oxidative ratio (OR) is the ratio of O 2 :CO 2 associated with photosynthesis, respiration, and other ecosystem gas exchange processes, and can be reported on the scale of an individual leaf, an ecosystem, up to the entire terrestrial biosphere. The OR of the terrestrial biosphere is used to partition anthropogenic CO 2 between oceanic and terrestrial carbon sinks, and the ease of measurement of this property on smaller scales suggests its potential for other uses. However, controls on the natural variation of OR are not understood in either organic matter pools or fluxes, and this lack of basic information limits the use of the tracer. Here we assess the annual variability of the OR of photosynthesis over ~decade for two temperate forests, one coniferous and one deciduous, and show that the photosynthetic OR signature is strongly dominated by plant species. We determined the OR of this flux by measuring the OR of carbon pools that close on annual or shorter timescales (leaves and individual tree rings), via solid-state 13 C NMR spectroscopy and elemental analysis. Leaf litter OR is different between coniferous and deciduous forests, but tree bole OR is constant between species. There was no significant change in leaf litter OR with time, nor any correlations between leaf litter OR and temperature or precipitation. During this time growing season precipitation varied by 95% from the time period average, and growing season temperature by 22%, demonstrating that on the decadal scale photosynthetic OR is invariant over significant shifts in these climate parameters.

Description: Climate warming is expected to lengthen growing seasons of temperate forest ecosystems and increase gross primary productivity. Simultaneously, warming is expected to increase summer ecosystem respiration, which could offset gains accrued from longer growing seasons. These responses have been observed during anomalously warm years, but the role of future climate change on phenological tradeoffs and how they affect net primary productivity (NPP) at regional scales in temperate forests remains unexplored. We simulated scenarios of climate change on monthly forest NPP throughout 18 million hectares of temperate forests in New England, USA through year 2100. Using an ecophysiological model coupled to a forest landscape model, we simulated scenarios of climate change on monthly NPP. A high emission scenario (RCP 8.5), resulted in longer growing seasons that offset mid-summer ecosystem respiration costs and produced greater annual NPP throughout the study landscape compared to simulations using the current climate. In spring and autumn months, temperature was positively associated with greater NPP; in summer months, the relationship was negative. Spatially, the greatest increase in NPP occurred in the warmer southern region under a warm climate scenario with increased precipitation. Under a warm scenario with drier conditions, the greatest increase in NPP occurred in the cooler northern region. Phenological tradeoffs will affect NPP of future forests and their potential to serve as a negative feedback to climate change. Barring other limitations, longer growing seasons will offset greater respiratory demands and contribute to increases in NPP throughout the temperate forests of New England in the future.

Description: Storm events can drive highly variable behaviour in catchment nutrient and water fluxes, yet short-term event dynamics are frequently missed by low resolution sampling regimes. In addition, nutrient source zone contributions can vary significantly within and between storm events. Our inability to identify and characterise time-dynamic source zone contributions severely hampers the adequate design of land-use management practices in order to control nutrient exports from agricultural landscapes. Here, we utilise an 8-month high-frequency (hourly) time series of streamflow, nitrate (NO 3 -N), dissolved organic carbon (DOC), and hydroclimatic variables for a headwater agricultural catchment. We identified 29 distinct storm events across the monitoring period. These events represented 31% of the time series and contributed disproportionately to nutrient loads (42% of NO 3 -N and 43% of DOC) relative to their duration. Regression analysis identified a small subset of hydroclimatological variables (notably precipitation intensity and antecedent conditions) as key drivers of nutrient dynamics during storm events. Hysteresis analysis of nutrient concentration-discharge relationships highlighted the dynamic activation of discrete NO 3 -N and DOC source zones, which varied on an event-specific basis. Our results highlight the benefits of high-frequency in situ monitoring for characterising short-term nutrient fluxes and unravelling connections between hydroclimatological variability and river nutrient export and source zone activation under extreme flow conditions. These new process-based insights, which we summarise in a conceptual model, are fundamental to underpinning targeted management measures to reduce nutrient loading of surface waters.

Description: Elevated atmospheric CO 2 concentrations are expected to enhance photosynthesis and reduce stomatal conductance, thus increasing plant water use efficiency. A recent study based on eddy covariance flux observations from Northern Hemisphere forests showed a large increase in inherent water use efficiency (IWUE). Here, we used an updated version of the same dataset and robust uncertainty quantification to revisit these contemporary IWUE trends. We tested the hypothesis that the observed IWUE increase could be attributed to interannual trends in plant functional traits, potentially triggered by environmental change. We found that IWUE increased by ca. 1.3% yr -1 , which is less than previously reported but still larger than theoretical expectations. Numerical simulations with the T&C ecosystem model using temporally static plant functional traits cannot explain this increase. Simulations with plant functional trait plasticity, i.e., temporal changes in model parameters such as specific leaf area and maximum Rubisco capacity, match the observed trends in IWUE. Our results show that trends in plant functional traits, equal to 1.0% yr -1 , can explain the observed IWUE trends. Thus, at decadal or longer time scales, trait plasticity could potentially influence forest water, carbon and energy fluxes with profound implications for both the monitoring of temporal changes in plant functional traits and their representation in Earth system

Description: Soil decomposition models range from simple empirical functions to those that represent physical, chemical, and biological processes. Here we develop a parsimonious, modular C and N cycle model, the Dual Arrhenius Michaelis Menten – Microbial Carbon and Nitrogen Phyisology (DAMM-MCNiP), that generates testable hypotheses regarding the effect of temperature, moisture and substrate supply on C & N cycling. We compared this model to DAMM alone and an empirical model of heterotrophic respiration based on Harvard Forest data. We show that while different model structures explain similar amounts of variation in respiration, they differ in their ability to infer processes that affect C flux. We applied DAMM-MCNiP to explain an observed seasonal hysteresis in the relationship between respiration and temperature, and show using an exudation simulation that the strength of the priming effect depended on the stoichiometry of the inputs. Low C:N inputs stimulated priming of SOM decomposition but high C:N inputs were preferentially utilized by microbes as a C source with limited priming. The simplicity of DAMM-MCNiP's simultaneous representations of temperature, moisture, substrate supply, enzyme activity, and microbial growth processes is unique among microbial physiology models and is sufficiently parsimonious that it could be incorporated into larger scale models of C and N cycling.

Description: Measurements of late springtime nutrient concentrations in Arctic waters are relatively rare due to the extensive sea ice cover that makes sampling difficult. During the SUBICE cruise in May-June 2014, an extensive survey of hydrography and pre-bloom concentrations of inorganic macronutrients, oxygen, particulate organic carbon and nitrogen, and chlorophyll a was conducted in the northeastern Chukchi Sea. Cold (〈 -1.5°C) winter water was prevalent throughout the study area, and the water column was weakly stratified. Nitrate (NO 3 - ) concentration averaged 12.6±1.92 μM in surface waters and 14.0±1.91 μM near the bottom and was significantly correlated with salinity. The highest NO 3 - concentrations were associated with winter water within the Central Channel flow path. NO 3 - concentrations were much reduced near the northern shelfbreak within the upper halocline waters of the Canada Basin and along the eastern side of the shelf near the Alaskan coast. Net community production (NCP), estimated as the difference in depth-integrated NO 3 - content between spring (this study) and summer (historical), varied from 28-38 g C m -2 a -1 . This is much lower than previous NCP estimates that used NO 3 - concentrations from the southeastern Bering Sea as a baseline. These results demonstrate the importance of using profiles of NO 3 - measured as close to the beginning of the spring bloom as possible when estimating local NCP. They also show that once the snow melts in spring, increased light transmission through the sea ice to the waters below the ice could fuel large phytoplankton blooms over a much wider area than previously known.

Description: The lessngth of the fire cycle is a critical factor affecting the vegetation cover in boreal and temperate regions. However, its responses to climate change remain poorly understood. We re-analysed data from earlier studies of forest age structures at the landscape level, in order to map the evolution of regional fire cycles across Eastern North American boreal and temperate forests, following the termination of the Little Ice Age (LIA). We demonstrated a well-defined spatial pattern of post-LIA changes in the length of fire cycles towards lower fire activity during the 1800s and 1900s. The western section of Eastern North America (west of 77 °W) experienced a decline in fire activity as early as the first half of the 1800s. By contrast, the eastern section showed these declines as late as the early 1900s. During a regionally fire prone period of the 1910s-1920s, forests in the western section of Eastern boreal North America burned more than forests in the eastern section. The climate appeared to dominate over vegetation composition and human impacts in shaping the geographical pattern of the post-LIA change in fire activity. Changes in the atmospheric circulation patterns following the termination of the LIA, specifically changes in Arctic Oscillation and the strengthening of the Continental Polar Trough, were likely drivers of the regional fire dynamics.

Description: No abstract is available for this article.

Description: Beach aquifers host a dynamic and reactive mixing zone between fresh and saline groundwater of contrasting origin and composition. Seawater, driven up the beachface by waves and tides, infiltrates into the aquifer and meets the seaward-discharging fresh groundwater, creating and maintaining a reactive intertidal circulation cell. Within the cell, land-derived nutrients delivered by fresh groundwater are transformed or attenuated. We investigated this process by collecting porewater samples from multi-level wells along a shore-perpendicular transect on a beach near Cape Henlopen, Delaware and analyzing solute and particulate concentrations. Porewater incubation experiments were conducted to determine rates of oxygen consumption and nitrogen gas production. A numerical model was employed to support field and laboratory interpretations. Results showed that chemically-sensitive parameters such as pH and ORP diverged from salinity distribution patterns, indicating biogeochemical reactivity within the circulation cell. The highest respiration rates were found in the landward freshwater-saltwater mixing zone, supported by high dissolved inorganic carbon. Chlorophyll- a , a proxy for phytoplankton, and particulate carbon did not co-occur with the highest respiration rates, but were heterogeneously distributed in deeper and hypoxic areas of the cell. The highest rates of N 2 production were also found in the mixing zone coinciding with elevated O 2 consumption rates, but closer to the lower discharge point. Model results were consistent with these observations, showing heightened denitrification in the mixing zone. The results of this work emphasize the relationship between the physical flow processes of the circulation cell and its biogeochemical reactivity and highlight the environmental significance of sandy beaches.

Description: Mineral protection is known as an important mechanism stabilizing soil organic carbon (SOC). However, the composition, sources and variations of mineral-protected SOC remain poorly constrained. To fill this knowledge gap, we used hydrofluoric acid to demineralize soil matrix and compared the sources and distribution of mineral-protected lipids (ML) versus hydrolysable lipids (HL) of four typical Chinese shrubland soils. ML was found to represent a sizable fraction (9–32%) of total aliphatic lipids (including n -alkanols, n -alkanoic acids, α,ω-alkanedioic acids, hydroxyalkanoic acids and mid-chain-substituted acids) in all soils. Based on carbon chain length and branch positions, microbial- and plant-derived lipids were distinguished. No significant difference was found in the ratio of microbial- to plant-derived lipids in ML versus HL, implying that plant and microbial inputs are equally important for the mineral-associated soil lipids. However, ML contained a higher proportion of non-specific lipids, especially at depths. Furthermore, to evaluate key environmental variable(s) controlling the distribution of different lipid components, a multiple stepwise regression analysis was conducted. Notably, ML was mainly affected by SOC-to-nitrogen ratio instead of mineralogical properties, implying that the accrual of mineral-associated soil lipids relies strongly on organic matter properties. Collectively, our findings provide novel insights on sources and accumulation mechanisms of mineral-protected soil lipids. SOC decomposition and subsequent accretion of degradation products appear to be vital for the sequestration of mineral-associated soil lipids and warrant better recognition in the investigations of stable soil carbon accumulation mechanisms.

Description: Methane (CH 4 ) is a powerful greenhouse gas controlled by both biotic and abiotic processes. Few studies have investigated CH 4 fluxes in subarctic heath ecosystems, and climate change-induced shifts in CH 4 flux and the overall carbon budget are therefore largely unknown. Hence, there is an urgent need for long-term in situ experiments allowing for the study of ecosystem processes over time scales relevant to environmental change. Here, we present in situ CH 4 and CO 2 flux measurements from a wet heath ecosystem in northern Sweden subjected to 16 years of manipulations, including summer warming with open-top chambers, birch leaf litter addition, and the combination thereof. Throughout the snow-free season, the ecosystem was a net sink of CH 4 and CO 2 (CH 4 -0.27 mg C m - 2 d -1 ; net ecosystem exchange -1827 mg C m - 2 d -1 ), with highest CH 4 uptake rates (-0.70 mg C m - 2 d -1 ) during fall. Warming enhanced net CO 2 flux, while net CH 4 flux was governed by soil moisture. Litter addition and the combination with warming significantly increased CH 4 uptake rates, explained by a pronounced soil drying effect of up to 32 % relative to ambient conditions. Both warming and litter addition also increased the seasonal average concentration of dissolved organic carbon (DOC) in the soil. The site was a carbon sink with a net uptake of 60 g C m - 2 over the snow-free season. However, warming reduced net carbon uptake by 77 %, suggesting that this ecosystem type might shift from snow-free season sink to source with increasing summer temperatures.

Description: The importance of peatlands as sources of greenhouse gas emissions has been demonstrated in many studies during the last two decades. While most studies have shown the heterogeneous distribution of biogenic gas in peat soils at the field scale (sampling volumes in the order of meters), little information exists for sub-meter scales, particularly relevant to properly capture the dynamics of hot spots for gas accumulation and release when designing sampling routines with methods that use smaller (i.e. sub-meter) sampling volumes like flux chambers. In this study, ground penetrating radar (GPR) is used at the laboratory scale to evaluate biogenic gas dynamics at high spatial resolution (i.e. cm) in a peat monolith from the Everglades. The results indicate sharp changes (both spatially and temporally) in the dynamics of gas accumulation and release, representing hot spots for production and release of biogenic gases with surface areas ranging between 5 to 10 cm diameter and are associated with increases in porosity. Furthermore, changes in gas composition and inferred methane (CH 4 ) and carbon dioxide (CO 2 ) fluxes also displayed a high spatio-temporal variability associated with hot spots, resulting in CH 4 and CO 2 flux estimates showing differences up to one order of magnitude during the same day for different parts of the sample. This work follows on recent studies in the Everglades and questions the appropriateness of spatial and temporal scales of measurement when defining gas dynamics by showing how flux values may change both spatially and temporarily even when considering sub-meter spatial scales.

Description: Chronically elevated deposition of reactive nitrogen (N), as ammonium (NH 4 + ) and nitrate (NO 3 - ), in subtropical forests with monsoonal climate has caused widespread N leaching in southern China. So far, little is known about the effect of further increases in N input and changes in the relative proportion of NH 4 + and NO 3 - on turnover rate and fate of atmogenic N. Here, we report a 15 N tracer experiment in Tieshanping (TSP) forest, SW China, conducted as part of a long-term N fertilization experiment, using NH 4 NO 3 and NaNO 3 , where effects of a doubling of monthly N inputs were compared. In June 2012, the regular N fertilizers were replaced by their 15 N labeled forms, viz. 15 NH 4 NO 3 and Na 15 NO 3 , as a single dose addition. Mass balances of N for the initial 1.5 years following label addition showed that for both treatments, 70% to 80% of the annual N input was leached as NO 3 - , both at ambient and at double N input rates. This confirms the earlier reported extreme case of N saturation at TSP. The 15 N, added as Na 15 NO 3, showed recoveries of about 74% in soil leachates, indicating that NO 3 - input at TSP is subject to a rapid and nearly quantitative loss through direct leaching as a mobile anion. By contrast, recoveries of 15 N in soil leachates of only 33% were found if added as 15 NH 4 NO 3 . Much of the 15 N was immobilized in the soil, and to a lesser extent in the vegetation. Thus, immobilization of fresh N input is significantly greater if added as NH 4 + , than as NO 3 - .

Description: Despite being the only link between reduced and oxidized nitrogen, the impact of environmental factors on nitrification, temperature and particles in particular, remains unclear for coastal zones. By using the 15 NH 4 + -labeling technique, we determined nitrification rates in bulk (NTR B ) and free-living (NTR F , after removing particles 〉3μm) for water samples with varying particle concentrations (as sampled at different tidal stages) during autumn, winter and summer in a eutrophic coastal bay in southern China. The highest NTR B occurred in autumn, when particle concentrations were highest. In general, particle-associated nitrification rates (NTR P , 〉3 μm) were higher than NTR F and increased with particle abundance. Regardless of seasonally distinctive temperature and particle concentrations, nitrification exhibited consistent temperature dependence in all cases (including bulk, particle-associated, and free-living) with a Q 10 value of ~2.2. Meanwhile, the optimum temperature for NTR P was ~29 °C, 5 °C higher than that for NTR F although the causes for such a difference remained unclear. Strong temperature dependence and particle association suggests that nitrification is sensitive to temperature change (seasonality and global warming) and to ocean dynamics (wave and tide). Our results can potentially be applied to biogeochemical models of the nitrogen cycle for future predictions.

Description: Oceanic nontronite deposits have been identified to be closely related to low- temperature hydrothermal activities. However, their formation mechanisms associated with microbes in diffuse hydrothermal vents still remain largely unknown. The friable deposits, collected from the low-temperature diffuse flow at the Southern Atlantic Ridge, display a layered structure. Scanning electron microscope (SEM) and transmission electron microscope (TEM) analyses reveal that abundant filamentous, spherical and rod-shaped mineralized forms are preserved in the yellowish-green layer of the deposits. These mineralized forms primarily consist of Si and Fe. Selected area electron diffraction (SAED) patterns of the mineralized forms indicate that they are composed of nontronite. High intensities of 12 C and 12 C 14 N signals derived from cellular structures determined by nano secondary ion mass spectrometry (nanoSIMS) suggest the intimate relationship between nontronite and microbes. The results of 454 pyrosequencing analyses provide insights into the microbial communities involved in the biologically-induced mineralization in the yellowish-green layer. We propose an evolutionary model for establishing paragenetic sequences among nontronite, Mn-oxide and Fe-oxyhydroxide in the deposits. This paragenetic sequence could be widespread in modern and ancient low-temperature hydrothermal fields.

Description: Ammonium (NH 4 + ) pollution and associated processes causing environmental problems in aquatic ecosystems have attracted much attention. However, the microbial pathways of NH 4 + production and removal and associated influencing factors in the sediments of urban rivers remain unclear. In this study, microbial pathways of NH 4 + production and removal were quantified to examine the NH 4 + balance in the sediments of Shanghai river networks. The results indicated that potential rates of gross nitrogen mineralization, dissimilatory nitrate reduction to ammonium and nitrogen fixation ranged from 0.25 to 25.83 μg N g –1 d –1 , from undetectable to 3.47 μg N g –1 d –1 and from 0.07 to 3.05 μg N g –1 d –1 , respectively. The potential rates of gross NH 4 + immobilization, anammox and nitrification, as the NH 4 + removal processes, ranged from 0.24 to 26.27 μg N g -1 d –1 , from 0.01 to 7.97 μg N g –1 d –1 and from 0 to 9.62 μg N g –1 d –1 , respectively. Temperature, DO, NO 3 – , NH 4 + and TOC had great influences on these NH 4 + production and removal processes. In addition, the total amounts of NH 4 + production and removal through microbial pathways in sediments of Shanghai river networks were estimated at approximately 2.6 × 10 5 t N yr –1 and 3.9 × 10 5 t N yr –1 , respectively. Thus, the net sink of NH 4 + was 1.3 ×10 5 t N yr -1 , which accounts for 22% of total inputs of nitrogen in the river networks. These results indicate that microbial processes of removal in sediments can eliminate significant parts of NH 4 + generated from microbial pathways and/or inputs from anthropogenic activities in urban rivers. Overall, these results improve understanding of NH 4 + production and removal and associated influencing environmental factors, and highlight the environmental importance of these processes in regulating the NH 4 + budget in highly urbanized riverine ecosystems

Description: Sediment derived dissolved organic matter (DOM) can comprise a substantial portion of the organic carbon budget in coastal bottom waters, yet it is often neglected in coastal carbon cycle models. In most modern sediment-water column flux models, biologically mediated reactions that remineralize particulate organic matter (POM) into inorganic compounds are simplified. In reality, organic matter remineralization is a complex suite of reactions that include DOM intermediate compounds. To better represent the sequential breakdown of POM and remineralization of DOM, a DOM state variable was built into a widely used sediment flux model. In the model, DOM is created in the sediment by hydrolysis of POM, and all organic matter passes through the DOM pool before remineralization. The model was run for 11 years and tuned to reproduce observed sediment flux data collected in Chesapeake Bay and then used to assess the role of DOM in sediment organic matter dynamics. Sediment-water column fluxes of DOM are highly variable both on seasonal and inter-annual scales, with substantial variability among stations in both magnitude and flux direction. Across all stations, semi-labile and inert DOM is lost and labile DOM is taken up into the reactive first layer of the modeled sediment, with the net flux a balance of the two processes. The improved sediment flux model can be utilized to better understand the role of sediment biogeochemistry in the estuarine and coastal carbon cycle, and shed light on difficult to measure processes involving DOM intermediate compounds.

Description: The Pantanal is the largest tropical wetland on the planet and yet little information is available on the biome's carbon cycle. We used an automatic station to measure soil CO 2 concentrations and oxidation-reduction potential over the 2014 and 2015 flood cycles of a tree island in the Pantanal that is immune to inundation during the wetland's annual flooding. The soil CO 2 concentration profile was then used to estimate soil CO 2 efflux over the two periods. In 2014, subsurface soil saturation at 0.30 m depth created conditions in that layer that led to CO 2 buildup close to 200,000 ppm and soil oxidation-reduction potential below –300 mV, conditions that were not repeated in 2015 due to annual variability in soil saturation at the site. Mean CO 2 efflux over the 2015 flood cycle was 0.023 ± 0.103 mg CO 2 -C m -2 s -1 representing a total annual efflux of 593 ± 2690 mg CO 2 -C m -2 y -1 . Unlike a nearby tree island site that experiences full inundation during the wet season, here the soil dried quickly following repeated rain events throughout the year which led to the release of soil CO 2 pulses from the soil. This study not only highlights the complexity and heterogeneity in the Pantanal's carbon balance based on differences in topography, flood cycles and vegetation, but also the challenges of applying the gradient-method in the Pantanal due to deviations from steady-state conditions.

Description: Nitrogen sources and dynamics, one of the key issues in marine biogeochemical cycles, remain poorly constrained in marginal seas. Here, we examine the nitrogen cycle in the northern South China Sea (SCS) by combining data from previous reports with a new dataset of N isotopic compositions (δ 15 N) of nitrate, zooplankton and sinking particles. Average δ 15 N in subsurface nitrate is 4.8±0.3‰, similar to that of sinking particles (δ 15 N sink of 4.4‰) through the euphotic zone (EZ) collected by floating traps and to documented mean (4.6‰) for long-term mooring traps at 200 m. This along with oft-observed shallow nitracline (〈 100 m) suggests that subsurface nitrate is the primary source of new N to support export production. Moreover, δ 15 N sink at the bottom of the EZ resembles those of suspended particles (4.2±1.0‰) and zooplankton (5.4±1.0‰) inside the EZ. High similarity in δ 15 N among various types of particles including zooplankton in different size fractions in the EZ implies rapid N turnover in the ecosystem. In deep waters at 2000–3000 m, however, additional particulate N fluxes are found due to lateral transport, which contain 15 N-depleted particles, resulting in a downward decreasing trend of δ 15 N sink . Incorporation of lighter N by bacteria and selective export of picoplankton are proposed as alternative mechanisms contributing to low δ 15 N sink in the deep waters. The significant δ 15 N sink change in the deep water column makes the SCS different from most other marginal seas; thus, caution should be made when using sedimentary δ 15 N to reconstruct paleo- nitrogen processes.

Description: Most forests worldwide are located in upland landscapes. Previous studies have mainly focused on ground methane (CH 4 ) flux in upland forests, and living tree stem-based CH 4 processes and fluxes are thus relatively poorly understood. This study investigated the relationship between CH 4 concentration and water content in the heartwood of living trees in mid-temperate, warm temperate, and sub-tropical upland forests, and also measured seasonal changes of in situ stem CH 4 flux and the CH 4 concentration and water content in the heartwood of Populus davidiana Dode in a warm temperate upland forest. We found that approximately 4–13% of tree stems or approximately 8–31% of tree species had a substantial CH 4 concentration of ≥10000 μL L –1 in their heartwood across the three types of upland forests. The heartwood CH 4 concentration was related to water content by a power function. A threshold of water content occurred beyond which CH 4 was produced at high levels in the heartwood. The CH 4 emissions from the breast height stems of P. davidiana ranged from 202.1 to 331.6 μg m –2 h –1 on a stem surface area basis during Jul–Oct 2016 and were significantly linearly correlated with the CH 4 concentration or water content in the heartwood throughout the experimental period, but the linear correlation was not significant at daily and monthly scales. Temperature was not a limiting factor for CH 4 production during Jul–Oct 2016, and thus most of the CH 4 production may be explained by water content in the heartwood of living trees in upland forests.

Description: Nitrogen biogeochemistry during the beginning of a spring phytoplankton bloom in the Yellow Sea was investigated based on nutrient concentrations, benthic fluxes of nutrients, the nitrogen and oxygen isotope composition of NO 3 – , and the hydrological conditions. The δ 15 N and δ 18 O values for NO 3 – in the Yellow Sea were more variable in surface waters than in near-bottom waters and were generally 6.3–8.2‰ for δ 15 N and 6.2–9.7‰ for δ 18 O, except in the Yellow Sea Coastal Current Water (YSCCW), where the maximum δ 15 N and δ 18 O values found were 13.2‰ and 18.8‰, respectively. Both the δ 15 N and δ 18 O values varied among the different water masses with higher values in the YSCCW than in the Yellow Sea Mixed Water and the Yellow Sea Warm Current Water. Higher δ 18 O relative to δ 15 N in NO 3 – in the Yellow Sea reflects contributions from multiple NO 3 – sources, including nitrification and atmospheric deposition with the later accounting for 71% of total external NO 3 – input. A simple nitrogen budget indicated the importance of nutrient regeneration from sediments as a source for water column NO 3 – representing 74% of total NO 3 – input in the YS.

Description: Fjords are recognized as globally important sites for the burial and long-term storage of carbon (C) within sediments. The proximity of fjords to the terrestrial environment in combination with their geomorphology and hydrography results in the fjordic sediments being subsidized with organic carbon (OC) from the terrestrial environment. It has been well documented that terrestrial OC (OC terr ) is an important component of coastal sediments yet our understanding of the quantity of OC terr stored in these sediments remains poorly constrained. Utilizing Bayesian isotopic sediment fingerprinting techniques to the surface sediments of Loch Sunart we estimate that 42.0 ± 10.1 % of the OC is terrestrial in origin. Through combining these outputs with sedimentary OC stock estimates we have calculated the surface sediments (0-15 cm) hold 0.1 Mt OC terr and estimate that the postglacial sediment held within the fjord contains 3.96 Mt OC terr . When these totals are compared to the quantity of OC stored in the adjacent terrestrial environment it is clear that the fjord's catchment stores a greater amount of OC terr in the form of vegetation and soil. Though when normalized for area the results suggest that the marine sediments are a more effective long-term store of OC terr than the adjacent terrestrial environment. This striking result highlights the importance of the terrestrial environment as a source of OC to the coastal ocean and that the OC terr subsidy to the marine sediments is a significant mechanism for the long-term storage of OC in coastal marine sediments.

Description: Implications of sea-ice biogeochemistry for oceanic production and emissions of dimethyl sulfide in the Arctic Hakase Hayashida, Nadja Steiner, Adam Monahan, Virginie Galindo, Martine Lizotte, and Maurice Levasseur Biogeosciences, 14, 3129-3155, https://doi.org/10.5194/bg-14-3129-2017, 2017 In remote regions, cloud conditions may be strongly influenced by oceanic source of dimethylsulfide (DMS) produced by plankton and bacteria. In the Arctic, sea ice provides an additional source of these aerosols. The results of this study highlight the importance of taking into account both the sea-ice sulfur cycle and ecosystem in the flux estimates of oceanic DMS near the ice margins and identify key uncertainties in processes and rates that would be better constrained by new observations.

Description: The mutual dependence between sedimentary total organic carbon (TOC) and infaunal macrobenthos is here quantified by a mechanistic model. The model describes (i) the vertical distribution of infaunal macrobenthic biomass resulting from a trade-off between nutritional benefit (quantity and quality of TOC) and the costs of burial (respiration) and mortality, and (ii) the variable vertical distribution of TOC being in turn shaped by bioturbation of local macrobenthos. In contrast to conventional approaches, our model emphasizes variations of bioturbation both spatially and temporally depending on local food resources and macrobenthic biomass. Our implementation of the dynamic interaction between TOC and infaunal macrobenthos is able to capture a temporal benthic response to both depositional and erosional environments and provides improved estimates of the material exchange flux at the sediment-water interface. Applications to literature data for the North Sea demonstrate the robustness and accuracy of the model and its potential as an analysis tool for the status of TOC and macrobenthos in marine sediments. Results indicate that the vertical distribution of infaunal biomass is shaped by both the quantity and the quality of OC, while the community structure is determined only by the quality of OC. Bioturbation intensity may differ by one order of magnitude over different seasons owing to variations in the OC input, resulting in a significant modulation on the distribution of OC. Our relatively simple implementation may further improve models of early diagenesis and marine food-web dynamics by mechanistically connecting the vertical distribution of both TOC and macrobenthic biomass.

Description: The generation of elevated concentrations of sulfide in sediment porewaters that are toxic to rooted macrophytes is problematic in both marine and fresh waters. In marine waters, biogeochemical conditions that lead to toxic levels of sulfide generally relate to factors that affect oxygen dynamics or the sediment iron concentration. In freshwaters, increases in surface water sulfate have been implicated in decline of Zizania palustris (wild rice), which is important in wetlands across the Great Lakes region of North America. We developed a structural equation (SE) model to elucidate key variables that govern the evolution of sulfide in porewaters in shallow aquatic habitats that are potentially capable of supporting wild rice. The conceptual basis for the model is the hypothesis that dissimilatory sulfate reduction is limited by the availability of both sulfate and total organic carbon (TOC) in the sediment. The conceptual model also assumes that porewater sulfide concentrations are constrained by the availability of porewater iron, and that sediment iron supports the supply of dissolved iron to the porewater. A key result from the SE model is that variations in three external variables (sulfate, sediment TOC and sediment iron) contribute nearly equally to the observed variations in porewater sulfide. As a result, management efforts to mitigate against the toxic effects of porewater sulfide on macrophytes such as wild rice should approach defining a protective sulfate threshold as an exercise tailored to the geochemistry of each site that quantitatively considers the effects of ambient concentrations of sediment Fe and TOC.

Description: Field observations suggest that surface-water sulfate concentrations control the distribution of wild rice, an aquatic grass ( Zizania palustris ). However, hydroponic studies show that sulfate is not toxic to wild rice at even unrealistically high concentrations. To determine how sulfate might directly or indirectly affect wild rice, potential wild rice habitat was characterized for 64 chemical and physical variables in over 100 sites spanning a relatively steep climatic and geological gradient in Minnesota. Habitat suitability was assessed by comparing the occurrence of wild rice with the field variables, through binary logistic regression. This analysis demonstrated that sulfide in sediment porewater, generated by the microbial reduction of sulfate that diffuses or advects into the sediment, is the primary control of wild rice occurrence. Water temperature and water transparency independently control the suitability of habitat for wild rice. In addition to generating phytotoxic sulfide, sulfate reduction also supports anaerobic decomposition of organic matter, releasing nutrients that can compound the harm of direct sulfide toxicity. These results are important because they show that increases in sulfate loading to surface water can have multiple negative consequences for ecosystems, even though sulfate itself is relatively benign.

Description: Pyrogenic carbon (PyC) is hypothesized to play an important role in the carbon (C) cycle due to its resistance to decomposition; however, much uncertainty still exists regarding the stocks of PyC that persist on-site after the initial erosion in post-fire forests. Therefore, understanding how fire characteristics influence PyC stocks is vital, particularly in the context of California forests for which an increase of high-severity fires is predicted over the next decades. We measured forest C and persistent PyC stocks in areas burned by low-to-moderate and high-severity fire, as well as in adjacent unburned areas in a California mixed-conifer forest, two to three years after wildfire. We measured C and PyC stocks in the following compartments: standing trees, downed wood, forest floor and mineral soil (0–5 cm), and we identified PyC using the weak nitric acid digestion method. We found that the total stock of PyC did not differ among fire severity classes (overall mean 248 ± 30 g C m −2 ); however, fire severity influenced the distribution of PyC in the individual compartments. Areas burned by high-severity fire had 2.5 times more PyC stocked in the coarse woody debris (p 〈 0.05), 3.3 times more PyC stocked in standing trees (p 〈 0.05) and a lower PyC stock in the forest floor (−22%, p 〈 0.05) compared to low-to-moderate fire severity areas. These results have important implications for the permanence time of PyC, which is putatively higher in standing trees and coarse woody debris compared to the forest floor, where it is susceptible to rapid losses through erosion.

Description: No abstract is available for this article.

Description: Recent studies show sulfur redox processes in terrestrial settings are more important than previously considered, but much remains uncertain about how these processes respond to dynamic hydrologic conditions in natural field settings. We used field observations from a sulfate-impacted wetland and stream in the mining region of Minnesota (USA) to calibrate a reactive transport model and evaluate sulfur and coupled geochemical processes under contrasting hydrogeochemical scenarios. Simulations of different hydrological conditions showed that flux and chemistry differences between surface water and deeper groundwater strongly control hyporheic zone geochemical profiles. However, model results for the stream channel versus wetlands indicate sediment organic carbon content to be the more important driver of sulfate reduction rates. A complex nonlinear relationship between sulfate reduction rates and geochemical conditions is apparent from the model's higher sensitivity to sulfate concentrations in settings with higher organic content. Across all scenarios, simulated e − balance results unexpectedly showed that sulfate reduction dominates iron reduction, which is contrary to the traditional thermodynamic ladder but corroborates recent experimental findings by Hansel et al. [2015] that “cryptic” sulfur cycling could drive sulfate reduction in preference over iron reduction. Following the thermodynamic ladder, our models shows that high surface water sulfate slows methanogenesis in shallow sediments, but field observations suggest that sulfate reduction may not entirely suppress methane. Overall, our results show sulfate reduction may serve as a major component making up and influencing terrestrial redox processes, with dynamic hyporheic fluxes controlling sulfate concentrations and reaction rates, especially in high organic content settings.

Description: Central Asia covers a large land area of 5×10 6 km 2 and has unique temperate dryland ecosystems, with over 80% of the world's temperate deserts, which has been experiencing dramatic warming and drought in the recent decades. How the temperate dryland responds to complex climate change, however, is still far from clear. This study quantitatively investigates terrestrial net primary productivity (NPP) in responses to temperature, precipitation, and atmospheric CO 2 during 1980-2014, by using the Arid Ecosystem Model, which can realistically predicte ecosystems' responses to changes in climate and atmospheric CO 2 according to model evaluation against 28 field experiments/obervations. The simulation results show that unlike other mid/high latitude regions, NPP in Central Asia declined 10% (0.12×10 15 g C) since the 1980s in response to a warmer and drier climate The dryland's response to warming was weak while its cropland was sensitive to the CO 2 fertilization effect (CFE). However, the CFE was inhibited by the long-term drought from 1998-2008 and the positive effect of warming on photosynthesis was largely offset by the enhanced water deficit. The complex interactive effects among climate drivers, unique responses from diverse ecosystem types, and intensive and heterogeneous climate changes led to highly complex NPP changing patterns in Central Asia, of which 69% was dominated by precipitation variation and 20% and 9% was dominated by CO 2 and temperature, respectively. The Turgay Plateau in northern Kazakhstan and southern Xinjiang in China were hotspots of NPP degradation in response to climate changes during the past three decades and in the future.

Description: The purpose of this study was to determine how drainage impacts carbon densities and recent rates (past 50 years) of vertical accretion and carbon accumulation in southeastern forested peatlands. We compared these parameters in drained maple-gum (MAPL), Atlantic white cedar (CDR), and pocosin (POC) communities in the Great Dismal Swamp National Wildlife Refuge (GDS) of Virginia/North Carolina and in an intact (undrained) CDR swamp in the Alligator River National Wildlife Refuge (AR) of North Carolina. Cores were analyzed for bulk density, % organic carbon, and 137 Cs and 210 Pb. An uncertainty analysis of both 137 Cs and 210 Pb approaches was used to constrain error at least partially related to mobility of both radioisotopes. GDS peats had lower porosities (89.6 % (sd=1.71) vs. 95.3 % (0.18)) and higher carbon densities (0.082 (0.021) vs. 0.037 (0.009) g C cm -3 ) than AR. Vertical accretion rates (0.10 – 0.56 cm yr -1 ) were used to estimate a time period of ~84-362 years for re-establishment of peat lost during the 2011 Lateral West fire at the GDS. Carbon accumulation rates ranged from 51 - 389 g C m -2 yr -1 for all sites. In the drained (GDS) vs. intact (AR) CDR sites, carbon accumulation rates were similar with 137 Cs (87 GDS vs. 92 AR g C m -2 yr -1 ) and somewhat less at the GDS than AR as determined with 210 Pb (111 GDS vs. 159 AR g C m -2 yr -1 ). Heightened productivity and high polyphenol content of peat may be responsible for similar rates of carbon accumulation in both drained and intact CDR peatlands.

Description: Leveraging 35 years of forest research in the southeastern U.S. to constrain carbon cycle predictions: regional data assimilation using ecosystem experiment R. Quinn Thomas, Evan Brooks, Annika Jersild, Eric Ward, Randolph Wynne, Timothy J. Albaugh, Heather Dinon Aldridge, Harold E. Burkhart, Jean-Christophe Domec, Thomas R. Fox, Carlos A. Gonzalez-Benecke, Timothy A. Martin, Asko Noormets, David A. Sampson, and Robert O. Teskey Biogeosciences Discuss., doi:10.5194/bg-2017-46,2017 Manuscript under review for BG (discussion: open, 0 comments) Quantitative predictions of forest productivity in a changing world were improved when ecosystem manipulation experiments, including experiments that altered water and nutrient availability, were used to calibrate a mathematical forest model. When ecosystem experiments were not included in calibration, the predictions were overly sensitive to nutrient fertilization but not sensitive enough to drought in the Southeastern U.S., a region that supplies a large fraction of wood products to the world.

Description: Modelling past, present and future peatland carbon accumulation across the pan-Arctic Nitin Chaudhary, Paul A. Miller, and Benjamin Smith Biogeosciences Discuss., doi:10.5194/bg-2017-34,2017 Manuscript under review for BG (discussion: open, 0 comments) We employed an individual- and patch-based dynamic global ecosystem model to quantify long-term C accumulation rates and to assess the effects of historical and projected climate change on peatland C balances across the pan-Arctic. We found peatlands in Scandinavia, Europe, Russia and C. & E. Canada will become C source while Siberia, Far East Russia, Alaska and W. & N. Canada increase their sink capacity by the end of 21st century.

Description: Boreal forest ecosystems are experiencing changes in plant productivity that are likely to continue with ongoing climate change. Transpiration ( T ) and canopy stomatal conductance ( g c ) are a key influence on plant productivity, and a better understanding of drivers and limitations of T and g c is necessary for constraining estimates of boreal ecosystem change. We describe patterns in T and g c of a deciduous conifer, Larix cajanderi , in an arctic boreal forest in northeastern Russia across three growing seasons from 2013-2015. We examine the influence of environmental drivers on g c using a phenomenological model. T was highly variable across days, and varied between 0.03-0.75 L m -2 day -1 . T and g c largely co-varied with daily fluctuations in air temperature and vapor pressure deficit. g c was highly suppressed on days when the vapor pressure deficits exceeded 0.75 kPa with an average daily g c of 37.55 mmol m -2 s -1 , and the average daily g c was almost double (71.25 mmol m -2 s -1 ) when vapor pressure deficits stayed below 0.75 kPa. Daily variation in g c was significantly related air temperature, permafrost thaw depth, and past precipitation. The influence of past precipitation and permafrost thaw depth on g c indicates that belowground conditions relating to soil moisture status are a key limitation for T . Such limitations on g c and T suggest that soil water and plant water stress play an important role in plant productivity and water relations in far northeastern Siberia.

Description: Response of export production and dissolved oxygen concentrations in oxygen minimum zones to p CO 2 and temperature stabilization scenarios in the biogeochemical model HAMOCC 2.0 Teresa Beaty, Christoph Heinze, Taylor Hughlett, and Arne M. E. Winguth Biogeosciences, 14, 781-797, doi:10.5194/bg-14-781-2017, 2017 In this study HAMOCC2.0 is used to address how mechanisms of oxygen minimum zone (OMZ) expansion respond to changes in CO 2 radiative forcing within the model. Atmospheric p CO 2 is increased at a rate of 1 % annually until stabilized. Our study suggests that expansion in the Pacific Ocean within the model is controlled largely by changes in particulate organic carbon export (POC). The vertical expansion of the OMZs in the Atlantic and Indian oceans is linked to reduced oxygen solubility.

Description: Bayesian calibration of terrestrial ecosystem models: A study of advanced Markov chain Monte Carlo methods Dan Lu, Daniel Ricciuto, Anthony Walker, Cosmin Safta, and William Munger Biogeosciences Discuss., doi:10.5194/bg-2017-41,2017 Manuscript under review for BG (discussion: open, 0 comments) Calibration of terrestrial ecosystem models is important but challenging. Bayesian inference implemented by Markov chain Monte Carlo (MCMC) sampling provides a comprehensive framework to estimate model parameters and associated uncertainties using their posterior distributions. The effectiveness and efficiency of the method strongly depend on the MCMC algorithm used. In this study, a Differential Evolution Adaptive Metropolis (DREAM) algorithm was used to estimate posterior distributions of 21 parameters for the data assimilation linked ecosystem carbon (DALEC) model using 14 years of daily net ecosystem exchange data collected at the Harvard Forest Environmental Measurement Site eddy-flux tower. The DREAM is a multi-chain method and uses differential evolution technique for chain movement, allowing it to be efficiently applied to high-dimensional problems, and can reliably estimate heavy-tailed and multimodal distributions that are difficult for single-chain schemes using a Gaussian proposal distribution. The results were evaluated against the popular Adaptive Metropolis (AM) scheme. DREAM indicated that two parameters controlling autumn phenology have multiple modes in their posterior distributions while AM only identified one mode. The calibration of DREAM resulted in a better model fit and predictive performance compared to the AM. DREAM provides means for a good exploration of the posterior distributions of model parameters. It reduces the risk of false convergence to a local optimum and potentially improves the predictive performance of the calibrated model.

Description: Multi-frequency electrical impedance tomography as a non-invasive tool to characterize and monitor crop root systems Maximilian Weigand and Andreas Kemna Biogeosciences, 14, 921-939, doi:10.5194/bg-14-921-2017, 2017 Root systems are essential in nutrient uptake and translocation, but are difficult to characterize non-invasively with existing methods. We propose electrical impedance tomography (EIT) as a new tool for the imaging and monitoring of crop root systems. In a laboratory experiment we demonstrate the capability of the method to capture physiological responses of root systems with high spatial and temporal resolution. We conclude that EIT is a promising functional imaging technique for crop roots.

Description: The Oligotrophy to the UlTra-oligotrophy PACific Experiment (OUTPACE cruise, Feb. 18 to Apr. 3, 2015) Thierry Moutin, Andrea Doglioli, Alain De Verneil, and Sophie Bonnet Biogeosciences Discuss., doi:10.5194/bg-2017-50,2017 Manuscript under review for BG (discussion: open, 0 comments) The overall goal of OUTPACE was to obtain a successful representation of the interactions between planktonic organisms and the cycle of biogenic elements in the western tropical South Pacific Ocean across trophic and N 2 fixation gradients. The international OUTPACE cruise took place between 18 February and 3 April 2015 aboard the RV L'Atalante and involved 60 scientists. The transect covered ~ 4 000 km from the western part of the Melanesian Archipelago to the western boundary of the gyre.

Description: Daily variation in net primary production and net calcification in coral reef communities exposed to elevated pCO 2 Steeve Comeau, Peter J. Edmunds, Coulson A. Lantz, and Robert C. Carpenter Biogeosciences Discuss., doi:10.5194/bg-2017-58,2017 Manuscript under review for BG (discussion: open, 0 comments) Here we investigate how CO 2 enrichment affects in coral reef communities the relationships light–production and light–calcification. For the three communities tested, CO 2 did not affect the light–production, while calcification was lower at elevated CO 2 for all light levels. Our result indicates that CO 2 can modify the balance between net calcification and net photosynthesis of reef communities by depressing community calcification, but without affecting community photosynthesis.

Description: The applicability of the temperature proxy based on a tetraether index of 86 carbon atoms (TEX 86 ) from isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs) has increasingly been examined in the South China Sea (SCS). Views on the proxy differ on whether it is biased to a particular season or to depth in this sea. Here, we studied isoGDGTs in a large number of suspended particulate matters (SPM) in surface water during summer and winter as well as several surface sediments of the northern SCS. The integration of these novel data with already published data demonstrated that isoGDGTs were similar in distribution between surface water SPM and shallow shelf sediments. They can be characterized by low ratio (〈4) of [GDGT-2]/[GDGT-3] and low percentage abundance (〈4%) of the crenarchaeol regioisomer; however, those in deep basin sediments showed higher values for both parameters, suggesting a substantial contribution of “deep water” Thaumarchaeota . The relationship between TEX 86 in SPM and in-situ measured SST differed from a global calibration and showed distinct trends between summer and winter, reflecting archaeal seasonality in surface waters. By assuming the TEX 86 temperature signal in shallow sediments as the weighted mean of summer and winter signals, we revealed spatial changes in the relative contribution of summer and winter archaeal isoGDGTs to sediments, with a likely higher winter contribution further offshore. This occurrence may complicate the interpretation of the shallow sedimentary TEX 86 record, and further investigation is required to fully understand seasonal archaeal lipid production and sedimentation.

Description: Modeling the effects of tree species and temperature on soil's extracellular enzyme activity in 78-year-old tree plantations Xiaoqi Zhou, Shen S. J. Wang, and Chengrong Chen Biogeosciences Discuss., doi:10.5194/bg-2017-144,2017 Manuscript under review for BG (discussion: open, 0 comments) Here, we measured the responses of soil's extracellular enzyme activity to a gradient of temperatures using incubation methods in 78-year-old tree plantations. We established a new soil–enzyme–C/N model without considering plant C inputs. The modeling results help explain why exotic slash pine can grow faster, as it has longer residual soil N residence time than native hoop pine and kauri pine. This is helpful for understanding the mechanisms of soil C and N cycling by different tree species.

Description: Effects of long-term mowing on the fractions and chemical composition of soil organic matter in a semiarid grassland Jiangye Li, Qichun Zhang, Yong Li, Yimeng Liu, Jianming Xu, and Hongjie Di Biogeosciences, 14, 2685-2696, doi:10.5194/bg-14-2685-2017, 2017 (1) Solid state 13 C-NMR is a powerful tool to study the nature of soil organic carbon. (2) Moderate mowing improved the stability of the grassland soil organic carbon pool. (3) Heavy mowing decreased the stability of the grassland soil organic carbon pool. (4) Mowing once a year is the most recommended practice in the semiarid grassland. (5) Mowing twice a year should be avoided for the long term in the semiarid grassland.

Description: Carbon stocks and fluxes in the high latitudes: Using site-level data to evaluate Earth system models Sarah Chadburn, Gerhard Krinner, Philipp Porada, Annett Bartsch, Christian Beer, Luca Belelli Marchesini, Julia Boike, Bo Elberling, Thomas Friborg, Gustaf Hugelius, Margareta Johansson, Peter Kuhry, Lars Kutzbach, Moritz Langer, Magnus Lund, Frans-Jan Parmentier, Shushi Peng, Ko Van Huissteden, Tao Wang, Sebastian Westermann, Dan Zhu, and Eleanor Burke Biogeosciences Discuss., doi:10.5194/bg-2017-197,2017 Manuscript under review for BG (discussion: open, 0 comments) Earth System Models (ESMs) are our main tools to understand future climate. The Arctic is important for the future carbon cycle, particularly due to the large carbon stocks in permafrost. We evaluated the performance of the land component of three major ESMs at Arctic tundra sites, focussing on the fluxes and stocks of carbon. We show that the next steps for model improvement are to better represent the quantity of vegetation, to include mosses and to improve below-ground carbon processes.

Description: Functional diversity of microbial communities in pristine aquifers inferred by PLFA- and sequencing-based approaches Valérie F. Schwab, Martina Herrmann, Vanessa-Nina Roth, Gerd Gleixner, Robert Lehmann, Georg Pohnert, Susan Trumbore, Kirsten Küsel, and Kai U. Totsche Biogeosciences, 14, 2697-2714, https://doi.org/10.5194/bg-14-2697-2017, 2017 We used phospholipid fatty acids (PLFAs) to link specific microbial markers to the spatio-temporal changes of groundwater physico-chemistry. PLFA-based functional groups were directly supported by DNA/RNA results. O 2 resulted in increased eukaryotic biomass and abundance of nitrite-oxidizing bacteria but impeded anammox, sulphate-reducing and iron-reducing bacteria. Our study demonstrates the power of PLFA-based approaches to study the nature and activity of microorganisms in pristine aquifers.

Description: Estimating total alkalinity for coastal ocean acidification monitoring at regional to continental scales in Australian coastal waters Kimberlee Baldry, Nick Hardman-Mountford, and Jim Greenwood Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-221,2017 Manuscript under review for BG (discussion: open, 0 comments) Total alkalinity (TA) is a measure of the oceans ability to resist changes in pH. In the face of ocean acidification, having the ability to predict the distribution of TA in coastal waters may provide insight into which regions might be prone to larger pH changes. Here we test a number of commonly used models for reconstructing TA. We find that within Australian coastal waters, using salinity alone is not the best choice in this region and that models are regionally dependent.

Description: Effects of long-term mowing on the fractions and chemical composition of soil organic matter in a semiarid grassland Jiangye Li, Qichun Zhang, Yong Li, Yimeng Liu, Jianming Xu, and Hongjie Di Biogeosciences, 14, 2685-2696, https://doi.org/10.5194/bg-14-2685-2017, 2017 (1) Solid state 13 C-NMR is a powerful tool to study the nature of soil organic carbon. (2) Moderate mowing improved the stability of the grassland soil organic carbon pool. (3) Heavy mowing decreased the stability of the grassland soil organic carbon pool. (4) Mowing once a year is the most recommended practice in the semiarid grassland. (5) Mowing twice a year should be avoided for the long term in the semiarid grassland.

Description: Plant n-alkane production from litterfall altered the diversity and community structure of alkane degrading bacteria in litter layer in lowland subtropical rainforest in Taiwan Tung-Yi Huang, Bing-Mu Hsu, Wei-Chun Chao, and Cheng-Wei Fan Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-161,2017 Manuscript under review for BG (discussion: open, 0 comments) The n-alkane in litterfall and the microbial community in litter layer in different habitats of lowland subtropical rainforest were studied. We revealed that the plant vegetation of forest not only dominated the n-alkane input of habitats but also governed the diversity of microbial community of litter layer. In this study, we found that the habitat which had high n-alkane input induced a shift of relative abundance toward phylum of Actinobacteria and the growth of alkB gene contained bacteria.

Description: Modeling the effects of tree species and temperature on soil's extracellular enzyme activity in 78-year-old tree plantations Xiaoqi Zhou, Shen S. J. Wang, and Chengrong Chen Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-144,2017 Manuscript under review for BG (discussion: open, 0 comments) Here, we measured the responses of soil's extracellular enzyme activity to a gradient of temperatures using incubation methods in 78-year-old tree plantations. We established a new soil–enzyme–C/N model without considering plant C inputs. The modeling results help explain why exotic slash pine can grow faster, as it has longer residual soil N residence time than native hoop pine and kauri pine. This is helpful for understanding the mechanisms of soil C and N cycling by different tree species.

Description: Carbon stocks and fluxes in the high latitudes: Using site-level data to evaluate Earth system models Sarah Chadburn, Gerhard Krinner, Philipp Porada, Annett Bartsch, Christian Beer, Luca Belelli Marchesini, Julia Boike, Bo Elberling, Thomas Friborg, Gustaf Hugelius, Margareta Johansson, Peter Kuhry, Lars Kutzbach, Moritz Langer, Magnus Lund, Frans-Jan Parmentier, Shushi Peng, Ko Van Huissteden, Tao Wang, Sebastian Westermann, Dan Zhu, and Eleanor Burke Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-197,2017 Manuscript under review for BG (discussion: open, 0 comments) Earth System Models (ESMs) are our main tools to understand future climate. The Arctic is important for the future carbon cycle, particularly due to the large carbon stocks in permafrost. We evaluated the performance of the land component of three major ESMs at Arctic tundra sites, focussing on the fluxes and stocks of carbon. We show that the next steps for model improvement are to better represent the quantity of vegetation, to include mosses and to improve below-ground carbon processes.

Description: Technical Note: A minimally invasive experimental system for p CO 2 manipulation in plankton cultures using passive gas exchange (atmospheric carbon control simulator) Brooke A. Love, M. Brady Olson, and Tristen Wuori Biogeosciences, 14, 2675-2684, https://doi.org/10.5194/bg-14-2675-2017, 2017 This experimental system simulates future CO 2 conditions in the ocean. It mimics natural processes by allowing the CO 2 to move gently and across the surface of the water though gas exchange, making it well suited for delicate plankton. Researchers can use many small vessels, which allows tracking of the eggs from individual females, for instance. Multiple types of organisms can be grown at once, which facilitates feeding studies and other similar studies of species interactions.

Description: Assisting the Evolution of the Observing System for the Carbon Cycle through Quantitative Network Design Thomas Kaminski and Peter Julian Rayner Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-168,2017 Manuscript under review for BG (discussion: open, 0 comments) Quantitative Network Design (QND) assesses the impact of a given set of existing or hypothetical observations in a modelling framework. QND has been used to optimise in situ networks and assess the benefit from planned space missions. This paper describes recent progress and points at aspects that are not yet sufficiently addressed. It demonstrates the advantage of an integrated QND system that can simultaneously evaluate a multitude of observational data streams.

Description: The effects of carbon turnover time on terrestrial ecosystem carbon storage Yaner Yan, Xuhui Zhou, Lifeng Jiang, and Yiqi Luo Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-183,2017 Manuscript under review for BG (discussion: open, 0 comments) The effects of C turnover time on C storage have not been well quantified for previous researches, so we decomposed ecosystem C storage capacity into C turnover time and C fluxes for quantifying it. Our result showed that the resultant terrestrial C release driven by C turnover time decrease only accounted for about 13.5 % of than driven by NPP increase and the larger uncertainties in spatial variation of C turnover time than temporal changes would lead in a greater impact on ecosystem C storage.

Description: Expansion of oil palm and other cash crops causes an increase of land surface temperature in Indonesia Clifton R. Sabajo, Guerric le Maire, Tania June, Ana Meijide, Olivier Roupsard, and Alexander Knohl Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-203,2017 Manuscript under review for BG (discussion: open, 1 comment) From the analysis of MODIS and Landsat satellite data of the Jambi province in Indonesia, this first study on the effects of oil palm expansion on the surface temperature in Indonesia shows shows a local and regional warming effect caused by the expansion of oil palm plantations and other cash or tree crops between 2000 and 2015. The observed warming effects may affect ecosystem services, reduce water availabilty in the dry period and increase the vulnerability to fires in the province.

Description: Explaining CO 2 fluctuations observed in snowpacks Laura Graham and David Risk Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-172,2017 Manuscript under review for BG (discussion: open, 0 comments) Winter carbon dioxide (CO 2 ) respiration from soils is a significant and understudied component of the global carbon (C) cycle. In this study, we were able to show with a field campaign and a model how windy (advective) conditions can affect the usually slow (diffusive) transport of CO 2 from soils and out of snowpacks. This research is important to help with understanding winter CO 2 dynamics, especially for continued accurate accounting of the annual global C cycle.

Description: Optical properties of size and chemical fractions of suspended particulate matter in littoral waters of Quebec Gholamreza Mohammadpour, Jean-Pierre Gagné, Pierre Larouche, and Martin Montes Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-159,2017 Manuscript under review for BG (discussion: open, 0 comments) High mass-specific absorption coefficients of suspended particulate matter (SPM) were measured in waters having iron-enriched particles Particle chemical composition has a major influence on variability of mass-specific absorption coefficients of SPM. Conversely, size distribution mainly rules spatial changes of mass-specific scattering coefficients of SPM. Multiple optical proxies can be combined in optically complex waters for estimating changes on particle size fractions.

Description: A biophysical approach using drought stress factor for daily estimations of evapotranspiration and CO 2 uptake in high-energy water-limited environments David Helman, Itamar M. Lensky, Yagil Osem, Shani Rohatyn, Eyal Rotenberg, and Dan Yakir Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-204,2017 Manuscript under review for BG (discussion: open, 0 comments) A biophysical approach for estimating daily evapotranspiration and CO 2 uptake was tested in high-energy water-limited forest and non-forest sites in Israel after accounting for drought stress. A newly developed mobile lab system and an active Fluxnet station were used for validation. Models successfully tracked observed seasonal changes showing promise for reliable ecosystem-level flux estimations. Models were used to assess changes in water-use-efficiency due to afforestation in this region.

Description: Spatial variations in snowpack chemistry and isotopic composition of NO 3 − along a nitrogen deposition gradient in West Greenland Chris J. Curtis, Jan Kaiser, Alina Marca, N. John Anderson, Gavin Simpson, Vivienne Jones, and Erika Whiteford Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-140,2017 Manuscript under review for BG (discussion: open, 0 comments) Few studies have investigated the atmospheric deposition of nitrate in the Arctic or its impacts on Arctic ecosystems. We collected late season snowpack from 3 regions of W Greenland from the coast to the edge of the ice sheet. We found major differences in nitrate concentrations (lower at the coast) and deposition load (higher at the coast). Nitrate in snowpack undergoes losses and isotopic enrichment which are greatest in inland areas, hence deposition impacts may be greatest at the coast.

Description: The influence of episodic flooding on a pelagic ecosystem in the East China Sea Chung-Chi Chen, Gwo-Ching Gong, Wen-Chen Chou, Chih-Ching Chung, Chih-Hao Hsieh, Fuh-Kwo Shiah, and Kuo-Ping Chiang Biogeosciences, 14, 2597-2609, https://doi.org/10.5194/bg-14-2597-2017, 2017 To understand the flooding effects on a pelagic ecosystem in the East China Sea (ECS), a variety of variables were measured in 2009 (non-flood) and 2010 (flood). In 2010, the organic carbon consumption was higher than in 2009; this could be attributed to the vigorous plankton activities observed in low-salinity areas. A huge amount of f  CO 2 was also drawn down in the flood. This flood effect might become more pronounced as extreme rainfall events increase dramatically throughout the world.

Description: Limited protection of macro-aggregate-occluded organic carbon in Siberian steppe soils Norbert Bischoff, Robert Mikutta, Olga Shibistova, Alexander Puzanov, Marina Silanteva, Anna Grebennikova, Roland Fuß, and Georg Guggenberger Biogeosciences, 14, 2627-2640, https://doi.org/10.5194/bg-14-2627-2017, 2017 This study set out to determine the quantity of organic carbon (OC) which is protected from microbial decomposition in macro-aggregates of Siberian steppe soils under different land use. Our results imply that the quantity of macro-aggregate protected OC is smaller than in similar steppe ecosystems like the North American prairies. We conclude that the tillage-induced breakdown of macro-aggregates after grassland to arable land conversion has not reduced the OC contents in the studied soils.

Description: Anatomical structure overrides temperature controls on magnesium uptake -- calcification in the Arctic/subarctic coralline algae Leptophytum laeve and Kvaleya epilaeve (Rhodophyta; Corallinales) Merinda C. Nash and Walter Adey Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-180,2017 Manuscript under review for BG (discussion: open, 0 comments) Past seawater temperatures can be reconstructed using magnesium / calcium ratios of biogenic carbonates. As temperature increases, so does magnesium. Here we show that for these arctic/subarctic coralline algae, anatomy is the first control on Mg / Ca, not temperature. When using coralline algae for temperature reconstruction, it is first necessary to check for anatomical influences on Mg / Ca.

Description: Flooding-related increases in CO 2 and N 2 O emissions from a temperate coastal grassland ecosystem Amanuel W. Gebremichael, Bruce Osborne, and Patrick Orr Biogeosciences, 14, 2611-2626, https://doi.org/10.5194/bg-14-2611-2017, 2017 Given their increasing trend in Europe, an understanding of the role that flooding events play in carbon (C) and nitrogen (N) cycling and greenhouse gas (GHG) emissions will be important for improved assessments of local and regional GHG budgets. This study presents the results of an analysis of the CO 2 and N 2 O fluxes from a coastal grassland ecosystem affected by episodic flooding that was of either a relatively short (SFS) or long (LFS) duration. Compared to the SFS, the annual CO 2 and N 2 O emissions were 1.4 and 1.3 times higher at the LFS, respectively. Mean CO 2 emissions during the period of standing water were 144 ± 18.18 and 111 ± 9.51 mg CO 2 –C m −2  h −1 , respectively, for the LFS and SFS sites. During the growing season, when there was no standing water, the CO 2 emissions were significantly larger from the LFS (244 ± 24.88 mg CO 2 –C m −2  h −1 ) than the SFS (183 ± 14.90 mg CO 2 –C m −2  h −1 ). Fluxes of N 2 O ranged from −0.37 to 0.65 mg N 2 O–N m −2  h −1 at the LFS and from −0.50 to 0.55 mg N 2 O–N m −2  h −1 at the SFS, with the larger emissions associated with the presence of standing water at the LFS but during the growing season at the SFS. Overall, soil temperature and moisture were identified as the main drivers of the seasonal changes in CO 2 fluxes, but neither adequately explained the variations in N 2 O fluxes. Analysis of total C, N, microbial biomass and Q 10 values indicated that the higher CO 2 emissions from the LFS were linked to the flooding-associated influx of nutrients and alterations in soil microbial populations. These results demonstrate that annual CO 2 and N 2 O emissions can be higher in longer-term flooded sites that receive significant amounts of nutrients, although this may depend on the restriction of diffusional limitations due to the presence of standing water to periods of the year when the potential for gaseous emissions are low.

Description: 20th century changes in carbon isotopes and water-use efficiency: tree-ring-based evaluation of the CLM4.5 and LPX-Bern models Kathrin M. Keller, Sebastian Lienert, Anil Bozbiyik, Thomas F. Stocker, Olga V. Churakova (Sidorova), David C. Frank, Stefan Klesse, Charles D. Koven, Markus Leuenberger, William J. Riley, Matthias Saurer, Rolf Siegwolf, Rosemarie B. Weigt, and Fortunat Joos Biogeosciences, 14, 2641-2673, https://doi.org/10.5194/bg-14-2641-2017, 2017 Measurements of the stable carbon isotope ratio ( δ 13 C) on annual tree rings offer new opportunities to evaluate mechanisms of variations in photosynthesis and stomatal conductance under changing CO 2 and climate conditions, especially in conjunction with process-based biogeochemical model simulations. The isotopic discrimination is indicative of the ratio between the CO 2 partial pressure in the intercellular cavities and the atmosphere ( c i ∕ c a ) and of the ratio of assimilation to stomatal conductance, termed intrinsic water-use efficiency (iWUE). We performed isotope-enabled simulations over the industrial period with the land biosphere module (CLM4.5) of the Community Earth System Model and the Land Surface Processes and Exchanges (LPX-Bern) dynamic global vegetation model. Results for C3 tree species show good agreement with a global compilation of δ 13 C measurements on leaves, though modeled 13 C discrimination by C3 trees is smaller in arid regions than measured. A compilation of 76 tree-ring records, mainly from Europe, boreal Asia, and western North America, suggests on average small 20th century changes in isotopic discrimination and in c i ∕ c a and an increase in iWUE of about 27 % since 1900. LPX-Bern results match these century-scale reconstructions, supporting the idea that the physiology of stomata has evolved to optimize trade-offs between carbon gain by assimilation and water loss by transpiration. In contrast, CLM4.5 simulates an increase in discrimination and in turn a change in iWUE that is almost twice as large as that revealed by the tree-ring data. Factorial simulations show that these changes are mainly in response to rising atmospheric CO 2 . The results suggest that the downregulation of c i ∕ c a and of photosynthesis by nitrogen limitation is possibly too strong in the standard setup of CLM4.5 or that there may be problems associated with the implementation of conductance, assimilation, and related adjustment processes on long-term environmental changes.

Description: Land-use and land-cover change carbon emissions between 1901 and 2012 constrained by biomass observations Wei Li, Philippe Ciais, Shushi Peng, Chao Yue, Yilong Wang, Martin Thurner, Sassan S. Saatchi, Almut Arneth, Valerio Avitabile, Nuno Carvalhais, Anna B. Harper, Etsushi Kato, Charles Koven, Yi Y. Liu, Julia E. M. S. Nabel, Yude Pan, Julia Pongratz, Benjamin Poulter, Thomas A. M. Pugh, Maurizio Santoro, Stephen Sitch, Benjamin D. Stocker, Nicolas Viovy, Andy Wiltshire, Rasoul Yousefpour, and Sönke Zaehle Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-186,2017 Manuscript under review for BG (discussion: open, 0 comments) We used several observation-based biomass datasets to constrain the historical land use change carbon emissions simulated by models. Compared to the large range of the original modelled emissions (from 94 to 273 Pg C), the observationally constrained global cumulative emission estimate is of 155 ± 50 (1-σ Gaussian error) Pg C from 1901 to 2012. Our approach reduces the uncertainty and can be also applied to evaluate the LULCC impact of land-based climate mitigation policies.

Description: A zero power warming chamber for investigating plant responses to rising temperature Keith F. Lewin, Andrew McMahon, Kim S. Ely, Shawn P. Serbin, and Alistair Rogers Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-208,2017 Manuscript under review for BG (discussion: open, 0 comments) Experiments that manipulate the temperature of plants and ecosystems are used to improve understanding of how they will respond to climate change. In logistically challenging locations passive warming using solar energy is the the only viable option for warming experiments. Unfortunately current passive warming approaches can only raise air temperature by about 1.5 °C. We have developed a novel approach that doubles the warming possible using solar energy and requires no power.

Description: Temporal variability of chlorophyll distribution in the Gulf of Mexico: bio-optical data from profiling floats Orens Pasqueron de Fommervault, Paula Perez-Brunius, Pierre Damien, and Julio Sheinbaum Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-201,2017 Manuscript under review for BG (discussion: open, 0 comments) Chlorophyll concentration is a key oceanic biogeochemical variable. In the Gulf of Mexico (GOM), its distribution, which is mainly obtained from satellite surface observations and scarce in situ experiments, is still poorly understood. In 2011–2012, eight profiling floats equipped with biogeochemical sensors were deployed for the first time in the GOM and generated an unprecedented dataset that significantly increased the number of chlorophyll vertical distribution measurements in the region. The analysis of these data, once calibrated, permits us to reconsider the spatial and temporal variability of the chlorophyll concentration in the water column. At a seasonal scale, results confirm the surface signal seen by satellites, presenting maximum concentrations in winter and low values in summer. It is shown that the deepening of the mixed layer depth is the primary factor triggering the chlorophyll surface increase in winter. In the GOM, current belief is that this surface increase corresponds to a biomass increase. However, the present dataset reveals a vertically integrated content of chlorophyll which remains constant throughout the year, suggesting that the surface increase results from a vertical redistribution of subsurface chlorophyll or photoacclimation processes, rather than a net increase of primary productivity. One plausible explanation for this is the decoupling between the mixed layer depth and the deep nutrient reservoir since mixed layer depth only reaches the nitracline in sporadic events in the observations. Float measurements also provide evidence that the depth and the magnitude of the deep chlorophyll maximum is strongly controlled by the mesoscale variability, with higher chlorophyll biomass generally observed in cyclones rather than anticyclones.

Description: Modelling biogeochemical processes in sediments from the north western Adriatic Sea: response to enhanced POC fluxes Daniele Brigolin, Christophe Rabouille, Bruno Bombled, Silvia Colla, Salvatrice Vizzini, Roberto Pastres, and Fabio Pranovi Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-206,2017 Manuscript under review for BG (discussion: open, 0 comments) This work presents the result of a study carried out in the north-western Adriatic Sea, by combining two different types of biogeochemical models with field sampling efforts. A longline mussel farm was taken as a local source of perturbation to the natural POC downward flux. This flux was first quantified by means of a pelagic model of POC deposition coupled to sediment traps data, and its effects on sediment bioirrigation capacity and OM degradation pathways were investigated by constraining an early diagenesis model, linked to new data in sediment porewaters. The measurements were performed at stations located inside and outside the area affected by mussel farm deposition. Model-predicted POC fluxes showed marked spatial and temporal variability, which were mostly associated with the dynamics of the farming cycle. Sediment traps data at the two sampled stations (in and out of the mussel farm) showed average POC background flux of 20.0–24.2 mmol C m −2  d −1 . The difference of OC fluxes between the two stations was in agreement with model results, ranging between 3.3 and 14.2 mmol C m −2  d −1 , and primarily associated with mussel physiological conditions. Although restricted, these changes in POC fluxes induced visible effects on sediment biogeochemistry. Observed oxygen microprofiles presented a 50 % decrease in oxygen penetration depth (from 2.3 to 1.4 mm), accompanied by an increase in the O 2 influx at the station below the mussel farm (19–31 versus 10–12 mmol O 2  m −2  d −1 ) characterized by higher POC flux. DIC and NH 4 + concentrations had similar behavior, with a more evident effect of bioirrigation underneath the farm. This was confirmed through constraining the early diagenesis model, which calibration leads to an estimation of enhanced and shallower bioirrigation underneath the farm: bioirrigation rates of 40 y −1 and irrigation depth of 15 cm were estimated inside the shellfish deposition footprint versus 20 y −1 and 20 cm outside. These findings were confirmed by independent data on macrofauna composition collected at the study site. Early diagenesis model results indicated a larger organic matter mineralization below the mussel farm (11.1 versus 18.7 mmol m −2  d −1 ), characterized by similar proportions between oxic and anoxic degradation pathways at the two stations, with an increase in the absolute values of oxygen consumed by OM degradation and reduced substances re-oxidation underneath the mussel farm.

Description: Ocean acidification changes the structure of an Antarctic coastal protistan community Alyce M. Hancock, Andrew T. Davidson, John McKinlay, Andrew McMinn, Kai Schulz, and Rick L. van den Enden Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-224,2017 Manuscript under review for BG (discussion: open, 0 comments) Absorption of carbon dioxide (CO 2 ) realised by humans is decreasing the ocean pH (ocean acidification). Single celled organisms (microbes) support the Antarctic ecosystem, yet little is known about their sensitivity to ocean acidification. This study shows a shift in a natural Antarctic microbial community, with CO 2 levels exceeding 634 μatm changing the community composition and favouring small cells. This would have significant flow-effects for Antarctic food webs and elemental cycles.

Description: Soil nitrogen oxide fluxes from lowland forests converted to smallholder rubber and oil palm plantations in Sumatra, Indonesia Evelyn Hassler, Marife D. Corre, Syahrul Kurniawan, and Edzo Veldkamp Biogeosciences, 14, 2781-2798, https://doi.org/10.5194/bg-14-2781-2017, 2017 We measured the soil N-oxide gases, N 2 O and NO in four land uses of Jambi, Sumatra, Indonesia. We aimed to assess the impact of forest conversion to rubber and oil palm plantations on these N-oxide gases. We found that there were no differences in soil N-oxide fluxes among land uses. However, soil N-oxide fluxes increased following N-fertilizer application in oil palm plantations. We estimated an annual soil N-oxide emission of 361 t N yr −1 from N fertilization for the Jambi province.

Description: Continuous fluorescence-based monitoring of seawater pH in a temperate estuary John W. Runcie, Christian Krause, Sergio A. Torres Gabarda, and Maria Byrne Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-222,2017 Manuscript under review for BG (discussion: open, 0 comments) pH in coastal waters can be highly variable. A means to measure this variation is needed. A fully autonomous submersible fluorescence-based pH monitoring device incorporating additional sensors for temperature and salinity was deployed in or adjacent to a shallow estuary for week-long intervals. Results and calculated aragonite/calcite saturation values are presented. The device is well suited to continuous flow-through or standalone measurements with a precision of at least 0.022 pH units.

Description: Small peat-forming ecosystems in arctic landscapes may play a significant role in the regional biogeochemistry of high-latitude systems, yet they are understudied compared to arctic uplands and other major peat-forming regions of the North. We present a new dataset of 25 radiocarbon-dated permafrost peat cores sampled around eight low-arctic lake sites in northern Manitoba (Canada) to examine the timing of peat initiation and controls on peat accumulation throughout the Holocene. We used macrofossils and charcoal to characterize changes in the plant community and fire, and we explored potential impacts of these local factors, as well as regional climatic change, on rates of C accumulation and C stocks. Peat initiation was variable across and within sites, suggesting the influence of local topography, but 56% of the cores initiated after 3,000 B.P. Most cores initiated and remained as drier bog hummock communities, with few vegetation transitions in this landscape. C accumulation was relatively slow and did not appear to be correlated with Holocene-scale climatic variability, but C stocks in this landscape were substantial (mean = 45.4 kg-C m -2 ), potentially accounting for 13.2 Pg C in the Taiga Shield ecozone. To the extent that small peat-forming systems are underrepresented in peatland mapping, SOC stocks may be underestimated in arctic regions. Mean fire severity appeared to be negatively correlated with C accumulation rates. Initiation and accumulation of soil C may respond to both regional and local factors, and substantial lowland soil C stocks have the potential for biogeochemical impacts on adjacent aquatic ecosystems.

Description: Soil microbial biomass, activity and community composition along altitudinal gradients in the High Arctic (Billefjorden, Svalbard) Petr Kotas, Hana Šantrůčková, Josef Elster, and Eva Kaštovská Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-184,2017 Manuscript under review for BG (discussion: open, 0 comments) The soil microbial properties were investigated along altitudinal gradients in the Arctic. The significant shifts in composition, size and activity of microbial communities were mainly controlled by vegetation distribution (through its effect on edaphic conditions) and by bedrock chemistry. The upward migration of vegetation due to global warming will diminish the variability of microbial properties along the altitudinal gradients and could have negative implications for microbial diversity.

Description: Influence of infrastructure on water quality and greenhouse gas dynamics in urban streams Rose M. Smith, Sujay S. Kaushal, Jake J. Beaulieu, Michael J. Pennino, and Claire Welty Biogeosciences, 14, 2831-2849, https://doi.org/10.5194/bg-14-2831-2017, 2017 Urban streams receive excess nitrogen from numerous sources. We hypothesized that variations in carbon availability and subsurface infrastructure influence emissions of N 2 O and other greenhouse gases (CH 4 and CO 2 ) as excess N is utilized by microbes. We sampled eight streams draining four categories of stormwater and sanitary infrastructure. Dissolved nitrogen concentration was the strongest predictor of CO 2 and N 2 O concentrations, while C : N ratio was the strongest predictor of CH 4 in streams.

Description: Cyanobacterial carbon concentrating mechanisms facilitate sustained CO 2 depletion in eutrophic lakes Ana M. Morales-Williams, Alan D. Wanamaker Jr., and John A. Downing Biogeosciences, 14, 2865-2875, https://doi.org/10.5194/bg-14-2865-2017, 2017 Our study investigated the mechanisms sustaining cyanobacteria blooms when CO 2 is depleted in lake surface waters. We found that when lake CO 2 concentrations drop below those of the atmosphere, cyanobacteria switch on carbon concentrating mechanisms (CCMs), allowing them to actively take up bicarbonate. This may provide bloom-forming cyanobacteria with a competitive advantage over other algae. These results provide insight into the timing and duration of blooms in high-nutrient lakes.

Description: In this study, high quality soil moisture data derived from the Soil Moisture Active Passive (SMAP) satellite measurements are evaluated from a perspective of improving the estimation of the global Gross Primary Production (GPP) using a process-based ecosystem model, namely the Boreal Ecosystem Productivity Simulator (BEPS). The SMAP soil moisture data are assimilated into BEPS using an Ensemble Kalman Filter. The correlation coefficient (r) between simulated GPP from the sunlit leaves and Sun-Induced chlorophyll Fluorescence (SIF) measured by Global Ozone Monitoring Experiment–2 (GOME-2) is used as an indicator to evaluate the performance of the GPP simulation. Areas with SMAP data in low quality (i. e. forests), or with SIF in low magnitude (e. g. deserts) or both are excluded from the analysis. With the assimilated SMAP data, the r value is enhanced for Africa, Asia, and North America by 0.016, 0.013, and 0.013, respectively (p〈0.05). Significant improvement in r appears in single-cropping agricultural land where the irrigation is not considered in the model but well captured by SMAP (e. g. 0.09 in North America, p〈0.05). With the assimilation of SMAP, areas with weak model performances are identified in double or triple cropping cropland (e. g. part of North China Plain) and/or mountainous area (e. g. Spain and Turkey). The correlation coefficient is enhanced by 0.01 in global average for shrub, grass, and cropland. This enhancement is small and insignificant because non-water-stressed areas are included.

Description: Biogeochemical versus ecological consequences of modeled ocean physics Sophie Clayton, Stephanie Dutkiewicz, Oliver Jahn, Christopher Hill, Patrick Heimbach, and Michael J. Follows Biogeosciences, 14, 2877-2889, https://doi.org/10.5194/bg-14-2877-2017, 2017 We present a systematic study of the differences generated by coupling the same ecological–biogeochemical model to a 1°, coarse-resolution, and 1∕6°, eddy-permitting, global ocean circulation model to (a) biogeochemistry (e.g., primary production) and (b) phytoplankton community structure. Surprisingly, we find that the modeled phytoplankton community is largely unchanged, with the same phenotypes dominating in both cases. Conversely, there are large regional and seasonal variations in primary production, phytoplankton and zooplankton biomass. In the subtropics, mixed layer depths (MLDs) are, on average, deeper in the eddy-permitting model, resulting in higher nutrient supply driving increases in primary production and phytoplankton biomass. In the higher latitudes, differences in winter mixed layer depths, the timing of the onset of the spring bloom and vertical nutrient supply result in lower primary production in the eddy-permitting model. Counterintuitively, this does not drive a decrease in phytoplankton biomass but results in lower zooplankton biomass. We explain these similarities and differences in the model using the framework of resource competition theory, and find that they are the consequence of changes in the regional and seasonal nutrient supply and light environment, mediated by differences in the modeled mixed layer depths. Although previous work has suggested that complex models may respond chaotically and unpredictably to changes in forcing, we find that our model responds in a predictable way to different ocean circulation forcing, despite its complexity. The use of frameworks, such as resource competition theory, provides a tractable way to explore the differences and similarities that occur. As this model has many similarities to other widely used biogeochemical models that also resolve multiple phytoplankton phenotypes, this study provides important insights into how the results of running these models under different physical conditions might be more easily understood.

Description: Spatial variations in snowpack chemistry and isotopic composition of NO 3 − along a nitrogen deposition gradient in West Greenland Chris J. Curtis, Jan Kaiser, Alina Marca, N. John Anderson, Gavin Simpson, Vivienne Jones, and Erika Whiteford Biogeosciences Discuss., doi:10.5194/bg-2017-140,2017 Manuscript under review for BG (discussion: open, 0 comments) Few studies have investigated the atmospheric deposition of nitrate in the Arctic or its impacts on Arctic ecosystems. We collected late season snowpack from 3 regions of W Greenland from the coast to the edge of the ice sheet. We found major differences in nitrate concentrations (lower at the coast) and deposition load (higher at the coast). Nitrate in snowpack undergoes losses and isotopic enrichment which are greatest in inland areas, hence deposition impacts may be greatest at the coast.

Description: Plant functional types, nutrients and hydrology drive carbon cycling along a transect in an anthropogenically altered Canadian peatland complex Sina Berger, Leandra Praetzel, Marie Goebel, Christian Blodau, and Klaus-Holger Knorr Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-191,2017 Manuscript under review for BG (discussion: open, 0 comments) Peatlands play an important role in global carbon cycling, however, the response of peatland carbon fluxes to anthropogenically changed hydrologic conditions and long-term infiltration of nutrients is still understudied. Along a transect of 4 study sites, spanning from largely pristine to strongly altered conditions within the Wylde Lake peatland complex in Ontario (Canada), we monitored carbon dioxide (CO 2 ) and methane (CH 4 ) fluxes at the soil/atmosphere interface and DIC and CH 4 concentrations in the peat profiles from April 2014 through September 2015. Moreover, we applied δ 13 C-CH 4 and δ 13 C-CO 2 stable isotope abundance analyses to examine CH 4 and CO 2 production and consumption as well as the dominant CH 4 emission pathways during the growing season of 2015. We found that a graminoid-moss dominated site, which was exposed to wet conditions and long-term infiltration of nutrients, was a great sink of CO 2 (2260 ± 480 g CO 2  m −2 ) but a great source of CH 4 (61.4 ± 32 g CH 4  m −2 ). Comparably low δ 13 C-CH 4 signatures (−62.30 ± 5.54 ‰) indicated only low mitigation of CH 4 emission by methanotrophic activity here. On the contrary, a shrub dominated site, which has been subjected to similarly high moisture conditions and loads of nutrients, was a much weaker sink of CO 2 (1093 ± 794 g CO 2  m −2 ) as compared with all other sites. The shrub dominated site featured notably low DIC concentrations in the peat as well as comparably 13 C enriched CH 4 (δ 13 C-CH 4 : −57.81 ± 7.03 ‰) and depleted CO 2 (δ 13 C-CO 2 : −15.85 ± 3.61 ‰) in a more decomposed and surficial aerated peat, suggesting a higher share of CH 4 oxidation. Plant mediated transport was the prevailing methane emission pathway throughout the summer of 2015 among all sites, even where graminoids covered only 10 % of the area. Our study provides insight into the accelerated carbon cycling of a strongly altered peatland and our results supported earlier findings, that strongly altered, shrub dominated peatlands may turn into weak carbon sinks or even sources, while a graminoid-moss dominance may maintain the peatland's carbon storage function.

Description: Coastal sources, sinks and strong organic complexation of dissolved cobalt within the US North Atlantic GEOTRACES transect GA03 Abigail E. Noble, Daniel C. Ohnemus, Nicholas J. Hawco, Phoebe J. Lam, and Mak A. Saito Biogeosciences, 14, 2715-2739, https://doi.org/10.5194/bg-14-2715-2017, 2017 This study examines sources and sinks of dissolved and labile cobalt in the North Atlantic Ocean. The North and South Atlantic are influenced differently by dust, coastal margin sources, biota, and suspended particles. Dissolved cobalt in both basins is driven by a coastal margin source, leading to large plumes emanating from the north and south African coasts. These plumes are comparable in size despite the high dust flux observed in the North Atlantic that is absent in the South Atlantic.

Description: Planktonic foraminifera-derived environmental DNA extracted from abyssal sediments preserves patterns of plankton macroecology Raphaël Morard, Franck Lejzerowicz, Kate F. Darling, Béatrice Lecroq-Bennet, Mikkel Winther Pedersen, Ludovic Orlando, Jan Pawlowski, Stefan Mulitza, Colomban de Vargas, and Michal Kucera Biogeosciences, 14, 2741-2754, https://doi.org/10.5194/bg-14-2741-2017, 2017 The exploitation of deep-sea sedimentary archive relies on the recovery of mineralized skeletons of pelagic organisms. Planktonic groups leaving preserved remains represent only a fraction of the total marine diversity. Environmental DNA left by non-fossil organisms is a promising source of information for paleo-reconstructions. Here we show how planktonic-derived environmental DNA preserves ecological structure of planktonic communities. We use planktonic foraminifera as a case study.

Description: Changing patterns of fire occurrence in proximity to forest edges, roads and rivers between NW Amazonian countries Dolors Armenteras, Joan Sebastian Barreto, Karyn Tabor, Roberto Molowny-Horas, and Javier Retana Biogeosciences, 14, 2755-2765, https://doi.org/10.5194/bg-14-2755-2017, 2017 Tropical forests are highly threatened by the expansion of the agricultural frontier, use of fire and subsequent deforestation. NW Amazonia is the wettest part of the basin and the role of fire is still largely unknown in this subregion. In this study, we compared fire regimes in five countries sharing this tropical biome (Venezuela, Colombia, Ecuador, Peru and Brazil). We studied fire activity in relation to proximity to roads and rivers and how fire occurs in relation to forest fragmentation.

Description: Carbon uptake and biogeochemical change in the Southern Ocean, south of Tasmania Paula C. Pardo, Bronte Tilbrook, Clothilde Langlais, Tom W. Trull, and Steve R. Rintoul Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-213,2017 Manuscript under review for BG (discussion: open, 0 comments) The carbon content of the water masses of the Southern Ocean, South of Tasmania has increased over the period 1995–2011, leading to a general decrease of pH. The long-term change in the carbon system is mainly affected by the ocean circulation and atmospheric features. The winds play an important role in the biogeochemical changes of upper to intermediate layers of the ocean while the ocean circulation conditions the long-term changes in deep waters.

Description: Increased temperature causes different carbon and nitrogen processing patterns in two common intertidal foraminifera ( Ammonia tepida and Haynesina germanica ) Julia Wukovits, Annekatrin Julie Enge, Wolfgang Wanek, Margarete Watzka, and Petra Heinz Biogeosciences, 14, 2815-2829, https://doi.org/10.5194/bg-14-2815-2017, 2017 This study reports the response of two intertidal foraminifera to increased temperatures on the level of carbon and nitrogen uptake. Interspecific variations in the ability to cope with shifting environmental variables within the two commonly associated species show that temperature and food source might be critical factors that control their abundances. This should support the interpretation of sediment samples and increase knowledge about nutrient fluxes through foraminiferal communities.

Description: An Orphan Problem Looking for Adoption: Responding to Ocean Acidification Utilising Existing International Institutions Ellycia R. Harrould-Kolieb Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-230,2017 Manuscript under review for BG (discussion: open, 0 comments) This research finds that there is a dearth of policy making pertaining to ocean acidification at the international level. Indeed, only three institutions are found to have initiated rue-making or implementation activities with the goal of either preventing worsening acidification or responding to its impacts. In light of this, this paper proposes that there are a variety of institutions that could be utilized to enhance the international response to ocean acidification.

Description: Seasonal variability of the inorganic carbon system in a large coastal plain estuary Andrew Joesoef, David L. Kirchman, Christopher K. Sommerfield, and Wei-Jun Cai Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-233,2017 Manuscript under review for BG (discussion: open, 0 comments) In this paper, we focus on key, poorly understood properties of carbonate geochemistry in one of the largest estuaries in North America. We explore how varying environmental factors impact total input and export inorganic carbon fluxes to the estuary and overall internal net ecosystem production within the system. In turn, recent and ongoing conclusions gained over this study provide broad implications to the inorganic carbon behavior and budget of present and future coastal ecosystems.

Description: The influence of the ocean circulation state on ocean carbon storage and CO 2 drawdown potential in an Earth system model Malin Ödalen, Jonas Nycander, Kevin I. C. Oliver, Laurent Brodeau, and Andy Ridgwell Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-166,2017 Manuscript under review for BG (discussion: open, 0 comments) During the four most recent glacial cycles, atmospheric CO 2 during glacial maxima has been lowered by about 90–100 ppm with respect to interglacials. There is widespread consensus that most of this carbon was partitioned in the ocean. It is however still debated which processes were dominant in achieving this increased carbon storage. In this paper, we use an Earth system model of intermediate complexity to constrain the range in ocean carbon storage for an ensemble of ocean circulation equilibrium states. We do a set of simulations where we run the model to pre-industrial equilibrium, but where we achieve different ocean circulation by changing forcing parameters such as wind stress, ocean diffusivity and atmospheric heat diffusivity. As a consequence, the ensemble members also have different ocean carbon reservoirs, global ocean average temperatures, biological pump efficiencies and conditions for air-sea CO 2 disequilibrium. We analyse changes in total ocean carbon storage and separate it into contributions by the solubility pump, the biological pump and the CO 2 disequilibrium component. We also relate these contributions to differences in strength of ocean overturning circulation. In cases with weaker circulation, we see that the ocean's capacity for carbon storage is larger. Depending on which ocean forcing parameter that is tuned, the origin of the change in carbon storage is different. When wind stress or ocean vertical diffusivity is changed, the response of the biological pump gives the most important effect on ocean carbon storage, whereas when atmospheric heat diffusivity or ocean horizontal diffusivity is changed, the solubility pump and the disequilibrium component are also important and sometimes dominant. Finally, we do a drawdown experiment, where we investigate the capacity for increased carbon storage by maximising the efficiency of the biological pump in our ensemble members. We conclude that different initial states for an ocean model result in different capacities for ocean carbon storage, due to differences in the ocean circulation state. This could explain why it is difficult to achieve comparable responses of the ocean carbon pumps in model intercomparison studies, where the initial states vary between models. The drawdown experiment highlights the importance of the strength of the biological pump in the control state for model studies of increased biological efficiency.

Description: Evidence for microbial mediated nitrate cycling within floodplain sediments during groundwater fluctuations Nicholas J. Bouskill, Mark E. Conrad, Markus Bill, Eoin L. Brodie, Yiwei Cheng, Chad Hobson, Matthew Forbes, Karen L. Casciotti, and Kenneth H. Williams Biogeosciences Discuss., https//doi.org/10.5194/bg-2017-212,2017 Manuscript under review for BG (discussion: open, 0 comments) This work couples isotope geochemical techniques with mechanistic microbial modeling in an attempt to further unravel the major factors responsible for an observed reduction in nitrate concomitant with a rising water table within floodplain sediments. We focus on 3 depths below ground surface with different periods of saturation and varying degrees of microbial nitrate loss. Using a microbial model we identify the controlling factors on denitrification responsible for these differences.

Description: Biochar reduces yield-scaled emissions of reactive nitrogen gases from vegetable soils across China Changhua Fan, Hao Chen, Bo Li, and Zhengqin Xiong Biogeosciences, 14, 2851-2863, https://doi.org/10.5194/bg-14-2851-2017, 2017 Intensive vegetable fields suffered very low N use efficiency and very high N 2 O emissions as compared to other ecosystems. We have demonstrated that two contrasting biochars affected gaseous reactive nitrogen intensity (N 2 O, NO, NH 3 , yield) across four major vegetable soils in China. Biochar affects gaseous Nr or yield largely depending on soil types. Both wheat straw biochar ( B w ) and swine manure biochar ( B m ) decreased GNI with Bw mitigated gaseous Nr, whereas Bm improved yield.