ALBERT

Description: The oxidative ratio (OR) of the biosphere is the stoichiometric ratio (O 2 /CO 2 ) of gas exchange by photosynthesis and respiration—a key parameter in budgeting calculations of the land and ocean carbon sinks. Carbon cycle-climate feedbacks could alter the OR of the biosphere by affecting the quantity and quality of organic matter in plant biomass and soil carbon pools. This study considers the effect of elevated atmospheric carbon dioxide concentrations ([CO 2 ]) on the OR of a hardwood forest after 9 growing seasons of free-air CO 2 -enrichment (FACE). We measured changes in the carbon oxidation state (C ox ) of biomass and soil carbon pools as a proxy for the ecosystem OR. The OR of net primary production, 1.039, was not affected by elevated [CO 2 ]. However, the C ox of the soil carbon pool was 40% higher at elevated [CO 2 ], and the estimated OR values for soil respiration increased from 1.006 at ambient [CO 2 ] to 1.054 at elevated [CO 2 ]. A biochemical inventory of the soil organic matter ascribed the increases in C ox and OR to faster turnover of reduced substrates, lignin and lipids, at elevated [CO 2 ]. This implicates the heterotrophic soil community response to elevated [CO 2 ] as a driver of disequilibrium in the ecosystem OR. The oxidation of soil carbon pool constitutes an unexpected terrestrial O 2 sink. Carbon budgets constructed under the assumption of OR equilibrium would equate such a terrestrial O 2 sink to CO 2 uptake by the ocean. The potential for climate-driven disequilibrium in the cycling of O 2 and CO 2 in the terrestrial biosphere warrants further investigation.

Description: Impact of climate change and land use on watershed runoff involves multiattribute ecohydrologic interactions. This information is critical to development of comprehensive stormwater management policies. Watersheds in the continental United States have diverse temperatures and precipitation regimes and varying hydrogeomorphic features that influence runoff. This study investigates watershed-scale runoff using statistical modeling for stormwater policy optimization. Multivariate statistical modeling show that vegetative activity, annual evaporation, precipitation, temperature and soil moisture significantly influenced watershed runoff. Soil moisture has a strong influence on runoff with each percent increase causing five percent increase in runoff. Nonlinear modeling with quadratic and interaction effects shows a significant interaction between soil moisture and other climatic variables in influencing annual runoff patterns. Changes in climate affect ecohydrologic characters by altering available soil moisture, evaporation, precipitation patterns and runoff. Optimization of green infrastructure design can be a successful management tool for runoff with an understanding that changes to multiple attributes in ecohydrologic variables affects runoff. Multi-attribute based green infrastructure and incentive policies can result in comprehensive stormwater policies that incorporate climatic and ecohydrologic conditions of watershed systems.

Description: Previous studies investigating organic-rich tundra have reported that increasing biodegradation of Arctic tundra soil organic carbon (SOC) under warming climate regimes will cause increasing CO 2 and CH 4 emissions. Organic-poor, mineral cryosols, which comprise 87% of Arctic tundra, are not as well characterized. This study examined biogeochemical processes of one-meter-long intact mineral cryosol cores (1-6% SOC) collected in the Canadian high Arctic. Vertical profiles of gaseous and aqueous chemistry and microbial composition were related to surface CO 2 and CH 4 fluxes during a simulated spring/summer thaw under light versus dark and in situ versus water saturated treatments. CO 2 fluxes attained 0.8±0.4 mmol CO 2 m -2 hr -1 for in situ treatments, of which 85±11% was produced by aerobic SOC oxidation, consistent with field observations and metagenomic analyses indicating aerobic heterotrophs were the dominant phylotypes. The Q 10 values of CO 2 emissions ranged from 2-4 over the course of thawing. CH 4 degassing occurred during initial thaw, however all cores were CH 4 sinks at atmospheric concentration CH 4 . Atmospheric CH 4 uptake rates ranged from -126±77 to -207±7 nmol CH 4 m -2 hr -1 with CH 4 consumed between 0-35 cm depth. Metagenomic and gas chemistry analyses revealed that high-affinity Type II methanotrophic sequence abundance and activity were highest between 0- 35 cm depth. Microbial sulfate reduction dominated the anaerobic processes, outcompeting methanogenesis for H 2 and acetate. Fluxes, microbial community composition, and biogeochemical rates indicate that mineral cryosols of Axel Heiberg Island act as net CO 2 sources and atmospheric CH 4 sinks during summertime thaw under both in situ and water saturated states.

Description: Boreal lake sediments are important sites of organic carbon (OC) storage, which have accumulated substantial amounts of OC over the Holocene epoch; the temporal evolution and the strength of this Holocene carbon (C) sink is, however, not well constrained. In this study we investigated the temporal record of carbon mass accumulation rates (CMARs), and assessed qualitative changes of terrestrially derived OC in the sediment profiles of seven Swedish boreal lakes, in order to evaluate the variability of boreal lake sediments as a C sink over time. CMARs were resolved on a short-term (centennial) and long-term (i.e. over millennia of the Holocene) time scale, using radioactive lead ( 210 Pb) and carbon ( 14 C) isotope dating. Sources and degradation state of terrestrially derived OC were identified and characterized by molecular analyses of lignin phenols. We found that CMARs varied substantially on both short-term and long-term scales, and that the variability was mostly attributed to sedimentation rates and uncoupled from the OC content in the sediment profiles. The lignin phenol analyses revealed that woody material from gymnosperms was a dominant and constant OC source to the sediments over the Holocene. Furthermore, lignin-based degradation indices, such as acid-to-aldehyde ratios, indicated that post-depositional degradation in the sediments was very limited on longer time scales, implying that terrestrial OC is stabilized in the sediments on a permanent basis.

Description: We investigate the benefits of assimilating in situ and satellite data of the fraction of photosynthetically active radiation (FAPAR) relative to eddy-covariance flux measurements for the optimization of parameters of the ORCHIDEE biosphere model. We focus on model parameters related to carbon fixation, respiration and phenology. The study relies on two sites – Fontainebleau (deciduous broadleaf forest) and Puechabon (Mediterranean broadleaf evergreen forest) – where measurements of net carbon exchange (NEE) and latent heat (LE) fluxes are available at the same time as FAPAR products derived from ground measurements or derived from spaceborne observations at high (SPOT) and medium (MERIS) spatial resolutions. We compare the different FAPAR products, analyze their consistency with the in situ fluxes, and then evaluate the potential benefits of jointly assimilating flux and FAPAR data. The assimilation of FAPAR data leads to a degradation of the model-data agreement with respect to NEE at the two sites. It is caused by the change in leaf area required to fit the magnitude of the various FAPAR products. Assimilating daily NEE and LE fluxes however has a marginal impact on the simulated FAPAR. The results suggest that the main advantage of including FAPAR data is the ability to constrain the timing of leaf onset and senescence for deciduous ecosystems, which is best achieved by normalizing FAPAR time series. The joint assimilation of flux and FAPAR data lead to a similar model-data improvement across all variables than when each data-stream is used independently, corresponding however to different and likely improved parameter values.

Description: In order to investigate the importance of biogenic silica associated biopolymers on the scavenging of radionuclides, the diatom Phaeodactylum tricornutum was incubated together with the radionuclides 234 Th, 233 Pa, 210 Pb, and 7 Be during their growth phase. Normalized affinity coefficients were determined for the radionuclides bound with different organic compound classes (i.e., proteins, total carbohydrates, uronic acids) in extracellular (non-attached and attached exopolymeric substances, EPS), intracellular (ethylene diamine tetraacetic acid, EDTA and sodium dodecyl sulfate, SDS extractable), and frustule embedded biopolymeric fractions (BF). Results indicated that radionuclides were mostly concentrated in frustule BF. Among three measured organic components, URA showed the strongest affinities to all tested radionuclides. Confirmed by spectrophotometry and 2-dimensional heteronuclear single quantum coherence-nuclear magnetic resonance (2D HSQC-NMR) analyses, the frustule BF were mainly composed of carboxyl-rich, aliphatic-phosphoproteins, which were likely responsible for the strong binding of many of the radionuclides. Results from this study provide evidence for selective absorption of radionuclides with different kinds of diatom-associated biopolymers acting in concert rather than as a single compound. This clearly indicates the importance of these diatom related biopolymers, especially frustule biopolymers, in the scavenging and fractionation of radionuclides used as particle tracers in the ocean.

Description: Plant phenology is one of the preferred indicators of climate change, and its variation potentially impacts community dynamics and ecosystem functions. To better understand the responses of plants’ flowering phenology to rising temperatures, we investigated the temperature sensitivity (expressed as the date of changes in phenology per change in temperature in degree Celsius, days °C −1 ) of flowering phenology for more than 220 plant species at 59 sites in China during the period 1963–1988. Our results indicated that most flowerings in China were significantly sensitive to the temperature in the two months (60 days) prior to the flowering dates. Plants in warmer regions showed larger sensitivities to increased temperatures. Species flowering in the late spring and early summer were generally less sensitive to changing temperature than species flowering at other times of the year. For plants flowering in the spring, species that flower earlier showed higher temperature sensitivity; however, for plants flowering in the summer and autumn, species that flower earlier showed lower temperature sensitivity. The responses of the first and last flowering times to changing temperature were mostly consistent, so flowering durations were rarely (6.1%) sensitive to changing temperature. We hypothesize that plants in cold regions may have adapted to the more variable temperatures and thus showed lower temperature sensitivities than plants in warm regions. Overall, the responses of flowering phenology to temperature varied significantly among temperature zones and plant species, so it should be considered carefully when estimating the impacts of climate warming on the terrestrial biosphere.

Description: Northern peatlands have accumulated a large amount of organic carbon (C) in their thick peat profile. Climate change and associated variations in soil environments are expected to have significant impacts on the C balance of these ecosystems, but the magnitude is still highly uncertain. Verifying and understanding the influences of changes in environmental factors on C gas fluxes in biogeochemical models are essential for forecasting feedbacks between C gas fluxes and climate change. In this study, we applied a biogeochemical model, DeNitrification-DeComposition (DNDC), to assess impacts of air temperature (T A ) and water table (WT) on C gas fluxes in an Alaskan peatland. DNDC was validated against field measurements of net ecosystem exchange of CO 2 (NEE) and CH 4 fluxes under manipulated surface soil temperature and WT conditions in a moderate rich fen. The validation demonstrates that DNDC was able to capture the observed impacts of the manipulations in soil environments on C gas fluxes. To investigate responses of C gas fluxes to changes in T A and soil water condition, we conducted a series of simulations with varying T A and WT. The results demonstrate that: 1) uptake rates of CO 2 at the site were reduced by either too colder or warmer temperatures, and generally increased with increasing soil moisture; 2) CH 4 emissions showed an increasing trend as T A increased or WT rose toward the peat surface; and 3) the site could shift from a net GHG sink into a net GHG source under some warm and/or dry conditions. A sensitivity analysis evaluated the relative importance of T A and WT to C gas fluxes. The results indicate that both T A and WT played important roles in regulating NEE and CH 4 emissions, and that within the investigated ranges of the variations in T A and WT, changes in WT showed a greater impact than changes in T A on NEE, CH 4 fluxes, and net C gas fluxes at the study fen.

Description: To clarify the effect of differences in hydrophyte life forms on methane (CH 4 ) production and its carbon stable isotopic signature (δ 13 C-CH 4 ), we analyzed CH 4 and carbon dioxide (CO 2 ) concentrations, their stable carbon isotope values, and chemical constituents dissolved in porewater in a small floating peat bog in Japan. Because eutrophication has modified the surrounding water quality the bog vegetation on the mat has been, in part, replaced by fen-type vegetation. We hypothesized that differences in hydrophyte habitats affect redox conditions, including dissolved oxygen (DO) in water and therefore the amounts and carbon isotopic values of CH 4 and CO 2 dissolved in porewater. Between the habitats of two Sphagnum species, DO was considerably higher and CH 4 concentrations were significantly lower in Sphagnum cuspidatum Ehrh. habitats in hollow (DO; 0.62 ± 0.20 mg/L (SE) and CH 4 ; 0.18 ± 0.02 mmol/L) than in Sphagnum palustre L. habitats in hummock (DO; 0.29 ±0.08 and CH 4 ; 0.82 ±0.06) in porewater (10 cm depth). Both DO and CH 4 concentrations in three vascular plant habitats ( Rhynchospora fauriei Franch., Phragmites australis [reed], and Menyanthes trifoliata L.) in porewater (10 cm depth) were intermediate relative to the two Sphagnum species. However, CH 4 flux in M. trifoliata site was significantly higher than that at both Sphagnum sites, suggesting that the type of gas transport (diffusive or convective via root and stem) affected the depth profile of CH 4 concentrations and its flux. δ 13 C-CH 4 values in porewater also varied among the vegetation types, even within Sphagnum species (e.g., at 10-cm depth, δ 13 C-CH 4 : R. fauriei , −55.3 ± 1.8‰ [SE]; P. australis , −57.5 ± 1.6‰; M. trifoliata , −56.7 ± 1.5‰; S. cuspidatum , −71.2 ± 1.4‰; and S. palustre , −60.4 ± 0.6‰). Our results suggests that significant differences arise in CH 4 concentration and δ 13 C-CH 4 values among the hydrophyte habitats even within a small peat bog and that change in vegetation relative to trophic conditions can affect CH 4 emissions and associated δ 13 C-CH 4 values.

Description: Dynamics of carbon dioxide and energy exchange over a small boreal lake were investigated. Flux measurements have been carried out by the eddy covariance technique during two open water periods (June-October) at Lake Kuivajärvi in Finland. Sensible heat ( H ) flux peaked in the early morning and upward sensible heat flux at night results in unstable stratification over the lake. Minimum H was measured in the late afternoon, often resulting in adiabatic conditions or slightly stable stratification over the lake. The latent heat flux ( LE ) showed a different pattern, peaking in the afternoon, and having a minimum at night. High correlation (r 2 =0.75) between H and water-air temperature difference multiplied by wind speed ( U ) was found, while LE strongly correlated with the water vapour pressure deficit multiplied by U (r 2 =0.78). Monthly average values of energy balance closure ranged between 70 and 99%. The lake acted as net source of carbon dioxide, and the measured flux ( F CO2 ) averaged over the two open-water periods (0.7 µmol m −2 s −1 ) was up to three times higher than those reported in other studies. Furthermore, it was found that, during period of high wind speed (〉3 m s −1 ) shear induced water turbulence controls the water-air gas transfer efficiency. However, under calm night-time conditions, F CO2 was poorly correlated with the difference between the water and the equilibrium CO 2 concentrations multiplied by U . Night-time cooling of surface water enhances the gas transfer efficiency through buoyancy-driven turbulent mixing, and simple wind speed based transfer velocity models strongly underestimate F CO2 .

Description: While the importance of ecosystem functioning is undisputed in the context of climate change and earth system modeling, the role of short scale temporal variability of hydro-meteorological forcing (~1 hour) on the related ecosystem processes remains to be fully understood. Various impacts of meteorological forcing variability on water and carbon fluxes across a range of scales are explored here using numerical simulations. Synthetic meteorological drivers that highlight dynamic features of the short temporal scale in series of precipitation, temperature, and radiation are constructed. These drivers force a mechanistic ecohydrological model that propagates information content into the dynamics of water and carbon fluxes for an ensemble of representative ecosystems. The focus of the analysis is on a cross-scale effect of the short scale forcing variability on the modeled evapotranspiration and ecosystem carbon assimilation. Interannual variability of water and carbon fluxes is emphasized in the analysis. The main study inferences are summarized as follows: (a) short scale variability of meteorological input does affect water and carbon fluxes across a wide range of time scales, spanning from the hourly to the annual and longer scales; (b) different ecosystems respond to the various characteristics of the short scale variability of the climate forcing in various ways, depending on dominant factors limiting system productivity; (c) whenever short scale variability of meteorological forcing influences primarily fast processes such as photosynthesis, its impact on the slow scale variability of water and carbon fluxes is small; (d) whenever short scale variability of the meteorological forcing impacts slow processes such as movement and storage of water in the soil, the effects of the variability can propagate to annual and longer time scales.

Description: No abstract is available for this article.

Description: To explore the effects of sea level rise (SLR), sediment reduction (SR) and saltwater intrusion (SWI) on the vegetation patterns and primary production of one exotic ( Spartina alterniflora ) and two native dominant ( Scirpus mariqueter and Phragmites australis ) species in the coastal wetlands of East China, range expansion monitoring and stress experiments were conducted, followed by model prediction. After a rapid invasion period, the expansion rate of S. alterniflora slowed down due to the decreasing availability of suitable habitat under prolonged inundation. SLR was shown to decrease the colonization of S. alterniflora and the native P. australis up to 2100. In contrast, the native S. mariqueter that has a high tolerance of inundation increased in area following SLR, due to a reduction in competition from S. alterniflora in low-lying habitats and even recolonized areas previously invaded by the exotic species . The combination of SLR and SR resulted in further degradation of S. alterniflora and P. australis , while the area of S. mariqueter was not reduced significantly. The decrease in the area of vegetation would reduce the gross primary production under SLR and SR. SWI exacerbates the impacts, especially for P. australis , because S. alterniflora and S. mariqueter have a higher tolerance of salinity. Thus the coastal vegetation pattern was predicted to be modified due to species-specific adaption to changed geophysical features. This study indicated that the native species better adapted to prolonged inundation and increased salinity might once again become key contributors to primary production on the muddy coasts of East China.

Description: Empirical modelling approaches are frequently used to upscale local eddy-covariance observations of carbon, water and energy fluxes to regional and global scales. The predictive capacity of such models largely depends on the data used for parameterization and identification of input–output relationships, while prediction for conditions outside the training domain is generally uncertain. In this work, artificial neural networks (ANNs) were used for the prediction of gross primary production (GPP) and latent heat flux (LE) on local and European scales with the aim to assess the portion of uncertainties in extrapolation due to sample selection. ANNs were found to be a useful tool for GPP and LE prediction, in particular for extrapolation in time (mean absolute error MAE for GPP between 0.53 and 1.56 gC m −2 day −1 ). Extrapolation in space in similar climatic and vegetation conditions also gave good results (GPP MAE 0.7-1.41 gC m −2 day −1 ), while extrapolation in areas with different seasonal cycles and controlling factors (e.g. the tropical regions) showed noticeably higher errors (GPP MAE 0.8-2.09 gC m −2 day −1 ). The distribution and the number of sites used for ANN training had a remarkable effect on prediction uncertainty in both, regional GPP and LE budgets and their interannual variability. Results obtained show that for ANN upscaling for continents with relatively small networks of sites, the error due to the sampling can be large and needs to be considered and quantified. The analysis of the spatial variability of the uncertainty helped to identify the meteorological drivers driving the uncertainty.

Description: Permafrost-affected ecosystems are important components in the global carbon (C) cycle that, despite being vulnerable to disturbances under climate change, remain poorly understood. This study investigates ecosystem carbon storage in two contrasting continuous permafrost areas of NE and E Siberia. Detailed partitioning of soil organic carbon (SOC) and phytomass carbon (PC) is analyzed for one tundra (Kytalyk) and one taiga (Spasskaya Pad/Neleger) study area. In total 57 individual field sites (24 and 33 in the respective areas) have been sampled for PC and SOC, including the upper permafrost. Landscape partitioning of ecosystem C storage was derived from thematic up-scaling of field observations using a land cover classification (LCC) from very high resolution (2x2 m) satellite imagery. Non-Metric Multidimensional Scaling (NMDS) was used to explore patterns in C distribution. In both environments the ecosystem C is mostly stored in the soil (≥86%). At the landscape scale C stocks are primarily controlled by the presence of thermokarst depressions (alases). In the tundra landscape, site scale variability of C is controlled by periglacial geomorphological features, while in the taiga, local differences in catenary position, soil texture and forest successions are more important. Very high-resolution remote sensing is highly beneficial to the quantification of C storage. Detailed knowledge of ecosystem C storage and ground-ice distribution is needed to predict permafrost landscape vulnerability to projected climatic changes. We argue that vegetation dynamics are unlikely to offset mineralization of thawed permafrost C, and that landscape scale reworking of SOC represent the largest potential changes to C-cycling.

Description: Light-use efficiency (LUE) is a key biophysical parameter characterizing the ability of plants to convert absorbed light to carbohydrate. However, the environmental regulations on LUE, especially moisture stress, are poorly understood, leading to large uncertainties in primary productivity estimated by LUE models. The objective of this study is to investigate the effects of moisture stress on LUE for a wide range of ecosystems on daily, 8-day and monthly scales. Using the FLUXNET and MODIS data, we evaluated moisture stress along the soil-plant-atmosphere continuum, including soil water content (SWC) and soil water saturation (SWS), land surface wetness index (LSWI) and plant evaporative fraction (EF), and precipitation (Pre) and daytime atmospheric vapor pressure deficit (VPD). We found that LUE was most responsive to plant moisture indicators (EF & LSWI), least responsive to soil moisture (SWC & SWS) variations with the atmospheric indicator (VPD) falling in between. LUE showed higher sensitivity to SWC than VPD only for grassland ecosystems. For evergreen forest, LUE had better association with VPD than LSWI. All moisture indicators (except soil indicators) were generally less effective in affecting LUE on the daily and 8-day scales than on the monthly scale. Our study highlights the complexity of moisture stress on LUE, and suggests that a single moisture indicator or function in LUE models is not sufficient to capture the diverse responses of vegetation to moisture stress. LUE models should consider the variability identified in this study to more realistically reflect the environmental controls on ecosystem functions.

Description: A simple model of the global marine iron cycle is used to constrain the sources, sinks, and biological cycling of iron. The iron model is embedded in a data-assimilated steady-state circulation, with biological cycling driven by a prescribed, data-constrained phosphate cycle. Biogeochemical parameters are determined by minimizing a suitably weighted quadratic mismatch with available dissolved iron (dFe) observations, including GEOTRACES transects. Because the effective iron sources and sinks overlap, current dFe observations cannot constrain sources and sinks independently. We therefore determine a family of optimal solutions for a range of the aeolian source strength σ A from 0.3 to 6.1 Gmol/yr. We find that the dFe observations constrain the maximum Fe:P uptake ratio, R 0 , to be proportional to σ A , with a range that spans most available measurements. Thus, with either R 0 or σ A specified, a unique solution is determined. Global inventories of total and free iron are well constrained at (7.4 ± 0.2)×10 11 and (1.39 ± 0.04)×10 10 mol Fe, respectively. The dFe distributions are very similar across the family of solutions, with iron limitation in the known high-nutrient low-chlorophyll regions. Hydrothermal source strength ranges from 0.55 to 0.71 Gmol/yr and does not vary systematically with σ A suggesting that the hydrothermal and aeolian parts of the iron cycle are largely decoupled. The hydrothermal dFe anomaly in the euphotic zone is ∼10% and concentrated in subpolar regions of iron limitation. Enhanced ligand concentrations in old waters and in hydrothermal plumes are necessary to capture key features of the dFe observations.

Description: Tropical and subtropical forests are believed to be the largest source of methyl chloride (CH 3 Cl), which is a natural stratospheric ozone destroyer. However, very little is known about what controls the rate of emission from these forests or why biogenic CH 3 Cl emission is concentrated in the tropics and subtropics. In this study, we investigated the seasonal and spatial variation of the rate of CH 3 Cl emission from the fern Osmunda japonica , which has a broad distribution covering the subtropical, temperate, and sub-boreal climate zones. The average rates of CH 3 Cl emission from the fern were similar (~1–4 µg g(dw) −1 h −1 ) among three areas, and there was no significant seasonal change in the temperate zone, although the rate was highly variable among individual plants. These findings suggest that meteorological climate such as temperature and solar radiation is not a major environmental factor controlling biogenic CH 3 Cl emission of individual plants, but species with high CH 3 Cl emission activity are more abundant in tropical and subtropical zones. We also found that developmental stage might be an important factor controlling biogenic CH 3 Cl emission rates. These results have implications for predicting future global CH 3 Cl emission budgets and for understanding of the plant-atmosphere interaction.

Description: Although freshwaters are considered to be substantial natural sources of atmospheric methane (CH 4 ), in situ processes of CH 4 production and consumption in freshwater ecosystems are poorly understood, especially in subtropical areas, leading to uncertainties in the estimation of global CH 4 emissions. To improve our understanding of physical and biogeochemical factors affecting CH 4 dynamics in subtropical lakes, we examined vertical and seasonal profiles of dissolved CH 4 and its carbon isotope ratio (δ 13 C) and conducted incubation experiments to assess CH 4 production and oxidation in the deep subtropical Fei-Tsui Reservoir (FTR; Taiwan). The mixing pattern of the FTR is essentially monomixis, but the intensity of winter vertical mixing changes with climatic conditions. In years with incomplete vertical mixing (does not reach the bottom) and subsequent strong thermal stratification resulting in profundal hypoxia, we observed increases in sedimentary CH 4 production and thus profundal CH 4 storage with the development of reducing conditions. In contrast, in years with strong winter vertical mixing to the bottom of the reservoir, CH 4 production was suppressed under NO 3 − -rich conditions, during which denitrifiers have the competitive advantage over methanogens. Diffusive emission from profundal CH 4 storage appeared to be negligible due to the efficiency of CH 4 oxidation during ascent through methane-oxidizing bacteria (MOB) activity. Most of the profundal CH 4 was rapidly oxidized by MOB in both oxic and anoxic layers, as characterized by its carbon isotope signature. In contrast, aerobic CH 4 production in the subsurface layer, which may be enhanced under high temperatures in summer, may account for a large portion of atmospheric CH 4 emissions from this reservoir. Our CH 4 profiling results provide valuable information for future studies predicting CH 4 emissions from subtropical lakes with the progress of global warming.

Description: The role of terrestrial river systems in the global carbon cycle on a long timescale has been a subject of interest, especially in the context of past climate changes such as the global cooling in the Cenozoic. The discharges of water and carbon into the ocean from the Himalayan watersheds are among the highest in the world. However, there are few reliable geochemical data from the Ayeyarwady River. This study focused on reevaluating chemical weathering in the Himalayan watersheds based on samples taken from the Ayeyarwady, Mekong, and Chao Phraya rivers, and on chemical analysis of the composition of dissolved substances in these rivers. Comparisons of water quality showed that, unlike in previous studies, the total alkalinity budgets of the Ayeyarwady are dominated by carbonate rather than silicate weathering. Long-term CO 2 consumption by silicate weathering in the Ayeyarwady is estimated to be only 63–145 × 10 9 mol yr −1 , which is only 10% of the previous estimate. Our results also suggest that the total Himalayan watersheds account for only about 10% of the total global CO 2 consumption by silicate weathering. Although we need further studies, chemical weathering and associated CO 2 uptake in the Himalayas likely played a lesser role in long-term global cooling in the past than previously appreciated.

Description: No abstract is available for this article.

Description: Ebullition is an important pathway for methane emission from inland waters. However, the mechanisms controlling methane bubble formation and release in aquatic sediments remain unclear. A laboratory incubation experiment was conducted to investigate the dynamics of methane bubble formation, storage and release in response to hydrostatic head drops in three different types of natural sediment. Homogenized clayey, silty and sandy sediments (initially quasi-uniform through the depth of the columns) were incubated in chambers for three weeks. We observed three distinct stages of methane bubble formation and release: Stage I – formation of micro bubbles, displacing mobile water from sediment pores with negligible ebullition; Stage II – formation of large bubbles, displacing the surrounding sediment with concurrent increasing in ebullition; Stage III – formation of conduits, with relatively steady ebullition. The maximum depth-averaged volumetric gas content at steady state varied from 18.8% in clayey to 12.0% in silty and 13.2% in sandy sediment. Gas storage in the sediment columns showed a strong vertical stratification: most of the free gas was stored in an upper layer, whose thickness varied with sediment grain size. The magnitude of individual ebullition episodes was linearly correlated to hydrostatic head drop and decreased from clayey to sandy to silty sediment, and was in excess of that estimated from expansion alone indicating the release of porewater methane. These findings combined with a hydrodynamic model capable of determining dominant sediment type and depositional zones could help resolve spatial heterogeneities in methane ebullition at medium to larger scales in inland waters.

Description: Research on the subterranean CO 2 dynamics has focused individually on either surface soils or bedrock cavities, neglecting the interaction of both systems as a whole. In this regard, the vadose-zone contains CO 2 -enriched air ( ca. 5% by volume) in the first meters, and its exchange with the atmosphere can represent from 10 to 90% of total ecosystem CO 2 emissions. Despite its importance, to date still lacking are reliable and robust databases of vadose-zone CO 2 contents that would improve knowledge of seasonal-annual above-belowground CO 2 balances. Here we study two and a half years of vadose-zone CO 2 dynamics in a semi-arid ecosystem. The experimental design includes an integrative approach to continuously measure CO 2 in: vertical and horizontal soil profiles, following gradients from surface to deep horizons and from areas of net biological CO 2 production (under plants) to areas of lowest CO 2 production (bare soil), as well as a bedrock borehole representing karst cavities and ecosystem-scale exchanges. We found that CO 2 followed similar seasonal patterns for the different layers, with the maximum seasonal values of CO 2 delayed with depth (deeper more delayed). However, the behavior of CO 2 transport differed markedly among layers. Advective transport driven by wind induced CO 2 emission both in surface soil and bedrock, but with negligible effect on subsurface soil, which appear to act as a buffer impeding rapid CO 2 exchanges. Our study provides the first evidence of enrichment of CO 2 under plant, hypothesizing that CO 2 -rich air could come from root zone or by transport from deepest layers through cracks and fissures.

Description: Forest ecosystems play an important role in the global cycling of mercury (Hg). In this study, we characterized the Hg cycling at a remote evergreen broadleaf (EB) forest site in southwest China (Mt. Ailao). The annual Hg input via litterfall is estimated to be 75.0 ± 24.2 µg m -2 yr -1 at Mt. Ailao. Such a quantity is up to one order of magnitude greater than those observed at remote temperate/boreal (T/B) forest sites. Production of litter biomass is found to be the most influential factor causing the high Hg input to the EB forest. Given their large areal coverage, Hg deposition through litterfall in EB forests is appropriately 9 ± 5 Mg yr -1 in China and 1086 ± 775 Mg yr -1 globally. The observed wet Hg deposition at Mt. Ailao is 4.9 ± 4.5 µg m -2 yr -1 , falling in the lower range of those observed at 49 T/B forest sites in North America and Europe. Given the data, the Hg deposition flux through litterfall is approximately 15 times higher than the wet Hg deposition at Mt. Ailao. Steady Hg accumulation in decomposing litter biomass and Hg uptake from the environment were observed during a 25-month litter decomposition. The size of the Hg pool in the organic horizon of EB forest floors is estimated to be up to 2-10 times the typical pool size in T/B forests. This study highlights the importance of EB forest ecosystems in global Hg cycling, which requires further assessment when more data become available in tropical forests.

Description: Salt marshes provide numerous valuable ecological services. In particular, nitrogen (N) removal in salt marsh sediments alleviates N loading to the coastal ocean. N removal reduces the threat of eutrophication caused by increased N inputs from anthropogenic sources. It is unclear, however, whether chronic nutrient over-enrichment alters the capacity of salt marshes to remove anthropogenic N. To assess the effect of nutrient enrichment on N cycling in salt marsh sediments, we examined important N cycle pathways in experimental fertilization plots in a New England salt marsh. We determined rates of nitrification, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) using sediment slurry incubations with 15 N labeled ammonium or nitrate tracers under oxic headspace (20% oxygen / 80% helium). Nitrification and denitrification rates were more than ten-fold higher in fertilized plots compared to control plots. By contrast, DNRA, which retains N in the system, was high in control plots but not detected in fertilized plots. The relative contribution of DNRA to total nitrate reduction largely depends on the carbon/nitrate ratio in the sediment. These results suggest that long-term fertilization shifts N cycling in salt marsh sediments from predominantly retention to removal.

Description: Understanding how tropical rainforests respond to elevated atmospheric CO 2 concentration (eCO 2 ) is essential for predicting Earth's carbon, water and energy budgets under future climate change. Here we use long-term (1982-2010) precipitation ( P ) and runoff ( Q ) measurements to infer runoff coefficient ( Q / P ) and evapotranspiration ( E ) trends across 18 unimpaired tropical rainforest catchments. We complement that analysis by using satellite observations coupled with ecosystem process modelling (using both ‘top-down’ and ‘bottom-up’ perspectives) to examine trends in carbon uptake and relate that to the observed changes in Q / P and E . Our results show there have been only minor changes in the satellite-observed canopy leaf area over 1982-2010, suggesting that eCO 2 has not increased vegetation leaf area in tropical rainforests and therefore any plant response to eCO 2 occurs at the leaf-level. Meanwhile, observed Q / P and E also remained relatively constant in the 18 catchments, implying an unchanged hydrological partitioning and thus approximately conserved transpiration under eCO 2 . For the same period, using a ‘top-down’ model based on gas-exchange theory, we predict increases in plant assimilation ( A ) and light-use efficiency ( ε ) at the leaf-level under eCO 2 , the magnitude of which is essentially that of eCO 2 ( i.e ., ~12% over 1982-2010). Simulations from ten state-of-the-art ‘bottom-up’ ecosystem models over the same catchments also show the direct effect of eCO 2 is to mostly increase A and ε with little impact on E . Our findings add to the current limited pool of knowledge regarding the long-term eCO 2 impacts in tropical rainforests.

Description: The savanna vegetation of Brazil (Cerrado) accounts for 20-25% of the land cover of Brazil and is the second largest ecosystem following Amazonian forest; however, Cerrado mass and energy exchange is still highly uncertain. We used eddy covariance to measure the net ecosystem CO 2 exchange (NEE) of grass-dominated Cerrado ( campo sujo ) over three years. We hypothesized that soil water availability would be a key control over the seasonal and interannual variations in NEE. Multiple regression indicated that gross primary production (GPP) was positively correlated (Pearson's r = 0.69; p 〈 0.001) with soil water content, radiation, and the MODIS-derived Enhanced Vegetation Index (EVI), but negatively correlated with the vapor pressure deficit (VPD), indicating that drier conditions increased water limitations on GPP. Similarly, ecosystem respiration (Reco) was positively correlated (Pearson's r = 0.78; p 〈 0.001) with the EVI, radiation, soil water content, and temperature but slightly negatively correlated with rainfall and the VPD. While the NEE responded rapidly to temporal variations in soil water availability, the grass-dominated Cerrado stand was a net source of CO 2 to the atmosphere during the study period, which was drier compared to the long-term average rainfall. Cumulative NEE was approximately 842 gC m -2 , varying from 357 gC m -2 in 2011 to 242 gC m -2 in 2012. Our results indicate that grass-dominated Cerrado may be an important regional CO 2 source in response to the warming and drying that is expected to occur in the southern Amazon Basin under climate change.

Description: Large fires account for a disproportionally high percentage of area burned with potentially severe environmental and socioeconomic impacts. This study characterizes extremely large fires (ELF, 2500–24,843 ha) in Portugal (1998–2013) and the concomitant fuel and weather conditions, analyzing the response of ELF size to their variation. ELF burned less shrubland-grassland (33% of the total ELF area) than forest (59% of total), the latter primarily composed by pine and pine-eucalypt. High fuel hazard was the norm, as indicated by median values of 0.98 for fuel load as a fraction of potential (maximum) load, and time since fire 〉14 years over 91% of the burned area. ELF occurred under anticyclonic circulation patterns, especially ridging, and 78% of them coincided with extreme fire danger days (corresponding to infrequent conditions) in conjunction with unstable atmosphere. Containment time, fire growth rate and energy release metrics varied by one more order of magnitude than ELF size, hence indicating that size alone is insufficient to describe extreme fires. Distinct combinations between ambient weather conditions, atmospheric instability and drought defined three categories of ELF as defined by size. Quantile regression indicated that increasingly larger fires showed gradually stronger responses to fire weather severity, highlighting the difficulty in restraining fire spread in flammable landscapes in the absence of extensive fuel treatments. Data limitations inherent to the methods used are discussed, and improvements to further advance the understanding of extreme fires are suggested.

Description: Lakes are highly relevant players in the global carbon cycle as they can store large amounts of organic carbon (OC) in sediments, thereby removing OC from the actively cycling pool. However, sediment OC can be released to pore water under anoxic conditions and diffuse into the water column. In carbon budgets of lake ecosystems, this potential OC loss pathway from sediments is generally disregarded. Combining field observations and incubation experiments, we quantitatively investigated dissolved OC (DOC) diffusion from sediments into anoxic water of a boreal lake. We observed substantial increases of bottom-water DOC (26% in situ, 16% incubation), translating into a DOC flux from the sediment that was comparable to anoxic sediment respiration (3.3 vs. 5.1 mmol m –2 d –1 ). Optical characterization indicated that colored and aromatic DOC was preferentially released. Reactivity assays showed that DOC released from anoxic sediment enhanced water column respiration and flocculation in re-oxygenated water. Upon water oxygenation, flocculation was the most important loss pathway removing ~77% of released DOC, but the remaining ~23% was mineralized, constituting a pathway of permanent loss of sediment OC. DOC diffusion from lake sediment during anoxia and subsequent mineralization in oxic water during mixing increases overall OC loss from anoxic sediments by ~15%. This study enlarges our understanding of lake ecosystems by showing that under anoxic conditions, significant amounts of DOC can be released from OC stored in sediments and enter the active aquatic carbon cycle again.

Description: Boreal lakes can be ice-covered for a substantial portion of the year at which time methane (CH 4 ) can accumulate below ice. The amount of CH 4 emitted at ice-melt is partially determined by the interplay between CH 4 production and CH 4 oxidation, performed by methane-oxidizing bacteria (MOB). Yet, the balance between oxidation and emission and the potential for CH 4 oxidation in various lakes during winter is largely unknown. To address this we performed incubations at 2 °C to screen for wintertime CH 4 oxidation potential in seven lakes. Results showed that CH 4 oxidation was restricted to three lakes, where the phosphate concentrations were highest. Molecular analyses revealed that MOB were initially detected in all lakes, although an increase in type I MOB only occurred in the three lake water incubations where oxidation could be observed. Accordingly, the increase in CO 2 was on average five times higher in these three lake water incubations. For one lake where no oxidation was measured, we tested if temperature and CH 4 availability could trigger CH 4 oxidation. However, regardless of incubation temperatures and CH 4 concentrations, ranging from 2-20 °C and 1-500 μM respectively, no oxidation was observed. Our study indicates that some lakes with active wintertime CH 4 oxidation may have low emissions during ice-melt, while other and particularly nutrient poor lakes may accumulate large amounts of CH 4 below ice that, in the absence of CH 4 oxidation, will be emitted following ice-melt. This variability in CH 4 oxidation rates between lakes needs to be accounted for in large-scale CH 4 emission estimates.

Description: Rivers play a major role in the transport and processing of dissolved organic matter (DOM). Disturbances that impact DOM dynamics, such as river impoundments and flow regulation have consequences for biogeochemical cycling and aquatic ecosystems. In this study we examined how river impoundments and hydrologic regulation impact DOM quantity and quality by tracking spatial and seasonal patterns of DOM in a large, regulated river (Klamath River, USA). Dissolved organic carbon (DOC) concentrations decreased downstream and longitudinal patterns in DOC load varied by season. Export of DOM (as DOC) was largely driven by river flow, while DOM composition was strongly influenced by impoundments. Seasonal algal blooms in upstream lentic reaches provided a steady source of algal DOM that was processed in downstream reaches. DOM at upstream sites had an average spectral slope ratio (S R ) 〉 1, indicating algal-derived material, but decreased downstream to an average S R 〈1, more indicative of terrestrial-derived material. The increasingly terrestrial nature of DOM exported from reservoirs likely reflects degraded algal material that becomes increasingly more recalcitrant with distance from upstream source and additional processing. As a result, DOM delivered to free-flowing river reaches below impoundments was less variable in composition. Downstream of impoundments, tributary influences resulted in increasing contributions of terrestrial DOM from the surrounding watershed. Removal of the four lower dams on the Klamath River is scheduled to proceed in the next decade. These results suggest that management should consider the role of impoundments on altering DOM dynamics, particularly in the context of dam removal.

Description: Steep vegetation-free talus slopes in high mountain environments are prone to superficial slope failures and surface erosion. Eco-engineering measures can reduce slope instabilities and thus contribute to risk mitigation. In a field experiment, we established mycorrhizal and non-mycorrhizal research plots and determined their biophysical contribution to small scale soil fixation. Mycorrhizal inoculation impact on plant survival, aggregate stability and fine root development was analyzed. Here, we present plant survival (n total  = 1248) and soil core (n total  = 108) analyses of three consecutive years in the Swiss Alps. Soil cores were assayed for their aggregate stability coefficient (ASC), root-length density (RLD) and mean root diameter (MRD). Inoculation improved plant survival significantly, but it delayed aggregate stabilization relative to the non-inoculated site. Higher aggregate stability occurred only after three growing seasons. Then also RLD tended to be higher and MRD increased significantly at the mycorrhizal treated site. There was a positive correlation between RLD, ASC and roots 〈 0.5 mm, which had the strongest impact on soil aggregation. Our results revealed a temporal offset between inoculation effects tested in laboratory and field experiments. Consequently, we recommend to establish an intermediate to long-term field-experimental monitoring before transferring laboratory results to the field.

Description: This manuscript details investigations of a productive, mountain freshwater lake and examines the dynamic relationship between the chemical, stable isotope and microbial composition of lake-bed sediments with the geochemistry of the lake water column. A multi-disciplinary approach was used in order to better understand the lake water-sediment interactions including quantification and sequencing of microbial 16S rRNA genes in a sediment core as well as stable isotope analysis of C, S, N. One visit included the use of a pore water sampler to gain insight into the composition of dissolved solutes within the sediment matrix. Sediment cores showed a general decrease in total C with depth which included a decrease in the fraction of organic C combined with an increase in the fraction of inorganic C. One sediment core showed a maximum concentration of dissolved organic C, dissolved inorganic C and dissolved methane in pore water at the 4 cm depth which corresponded with a sharp increase in the abundance of 16S rRNA templates as a proxy for the microbial population size as well as the peak abundance of a sequence affiliated with a putative methanotroph. The isotopic separation between dissolved inorganic and dissolved organic carbon is consistent with largely aerobic microbial processes dominating the upper water column while anaerobic microbial activity dominates the sediment bed. Using sediment core carbon concentrations, predictions were made regarding the breakdown and return of stored carbon per year from this temperate climate lake with as much as 1.3 Gg C yr -1 being released in the form of CO 2 and CH 4 .

Description: To determine the availability of atmospheric NO 3 − deposition on forested ecosystems and to understand the interaction between the nitrogen cycle in a forest ecosystem and atmospheric nitrogen input/output, we quantitatively evaluated the atmospheric NO 3 − passing through forested watersheds by measuring δ 18 O NO3 leaching during rainfall events in two forest ecosystems (Su-A and Ab-S). Atmospheric NO 3 − leaching in rainfall events was clearly higher in Ab-S than in Su-A, even for a similar amount of rainfall, which demonstrated that atmospheric NO 3 − leaching differs among forested watersheds. Our observations suggest that a large part of the atmospheric NO 3 − leached from the watersheds was derived from surface soil, which was deposited before rainfall events occurred; however, direct atmospheric NO 3 − leaching via throughfall discharge also contributed, especially at the beginning of rainfall events. In Ab-S, 2.9–37.8% (average = 15.5%) of atmospheric NO 3 − deposition passed through the watershed, accounting for 3.1–49.8% (average, 26.4%) of the total NO 3 − leached during rainfall events. The NO 3 − input was not large, and the NO 3 − pool and net nitrification rate were small; therefore, nitrogen was not saturated in the soil at Ab-S. Nevertheless, some of the atmospheric NO 3 − deposition was not assimilated and was leached immediately. Moreover, our observations suggest that the hydrological characteristics of the watersheds, which control the ease of rainwater discharge, strongly influenced the rate of atmospheric NO 3 − leaching. This suggests that the hydrological characteristics of watersheds influence the availability of atmospheric NO 3 − deposition in forested ecosystems and the progression of nitrogen saturation.

Description: Key Points Regional differences in d13C and d18O from earlywood and latewood were observed, which reflect a gradient in seasonal monsoon development. Tree WUE inferred from latewood d13C exhibited greater sensitivity to moisture variation near the North limit of the monsoon system. Summer air humidity has a significant latitudinal influence on the relative d13C and d18O values in cellulose of earlywood and latewood.

Description: Aerobic respiration is an important component of in-stream metabolism. The larger part occurs in the streambed, where it is difficult to directly determine actual respiration rates. Existing methods for determining respiration are based on indirect estimates from whole-stream metabolism or provide time invariant results estimated from oxygen consumption measurements in enclosed chambers that do not account for the influence of hydrological changes. In this study we demonstrate a simple method for determining time-variable hyporheic respiration. We use a windowed cross-correlation approach for deriving time-variable travel times from the naturally changing electrical conductivity signal that is transferred into the sediment. By combining the results with continuous in situ dissolved oxygen measurements, variable oxygen consumption rate coefficients in the streambed are obtained. An empirical temperature relationship is derived and used for standardizing the respiration rate coefficients to isothermal conditions. For demonstrating the method, we compare two independent measurement spots in the streambed, which were located upstream and downstream of an in-stream gravel bar and thus exposed strongly diverse travel times. The derived respiration rate results are in accordance with findings of other stream studies. By comparing the travel time and respiration rate coefficient (i.e. Damköhler number) we estimate the contribution of each to the oxygen consumption in the streambed.

Description: This study uses an integrated modeling framework that couples the dynamics of hydrology, soil thermal regime, and ecosystem carbon and nitrogen to quantify the long-term peat carbon accumulation in Alaska during the Holocene. Modeled hydrology, soil thermal regime, carbon pools and fluxes and methane emissions are evaluated using observation data at several peatland sites in Minnesota, Alaska, and Canada. The model is then applied for a 10,000-year (15 ka to 5 ka; 1 ka = 1000 cal yr before present) simulation at four peatland sites. We find that model simulations match the observed carbon accumulation rates at fen sites during the Holocene ( R 2  = 0.88, 0.87, 0.38 and -0.05 using comparisons in 500-year bins). The simulated (2.04 m) and observed peat depths (on average 1.98 m) also compared well ( R 2  = 0.91). The early Holocene carbon accumulation rates, especially during the Holocene thermal maximum (HTM) (35.9 gCM –2 yr –1 ), are estimated up to 6-times higher than the rest of the Holocene (6.5 gCM –2 yr –1 ). Our analysis suggests that high summer temperature and the lengthened growing season resulted from the elevated insolation seasonality, along with wetter-than-before conditions might be major factors causing the rapid carbon accumulation in Alaska during the HTM. Our sensitivity tests indicate that, apart from climate, initial water-table depth and vegetation canopy are major drivers to the estimated peat carbon accumulation. When the modeling framework is evaluated for various peatland types in the Arctic, it can quantify peatland carbon accumulation at regional scales.

Description: Key Points Inflowing warm Atlantic water increases the net sea-air exchange in Siberian shelf seas. The increased volume transport has a larger impact on the CO2 flux than the warming of the water. The sea-air flux of CH4 is mainly affected by the increase in water temperature.

Description: Gross primary productivity (GPP) has been reported to increase with the fraction of diffuse solar radiation, for a given total irradiance. The correlation between GPP and diffuse radiation suggests effects of diffuse radiation on canopy light-use efficiency, but potentially confounding effects of vegetation phenology have not been fully explored. We applied several approaches to control for phenology, using 8 years of eddy-covariance measurements of winter wheat in the U.S. Southern Great Plains. The apparent enhancement of daily GPP due to diffuse radiation was reduced from 260% to 75%, after subsampling over the peak growing season or by subtracting a 15-day moving average of GPP, suggesting a role of phenology. The diffuse radiation effect was further reduced to 22% after normalizing GPP by a spectral reflectance index to account for phenological variations in LAI and canopy photosynthetic capacity. Canopy photosynthetic capacity covaries with diffuse fraction at a given solar irradiance at this site because both factors are dependent on day of year, or solar zenith angle. Using a two-leaf sun-shaded canopy radiative transfer model, we confirmed that the effects of phenological variations in photosynthetic capacity can appear qualitatively similar to the effects of diffuse radiation on GPP, and therefore can be difficult to distinguish using observations. The importance of controlling for phenology when inferring diffuse radiation effects on GPP raises new challenges and opportunities for using radiation measurements to improve carbon cycle models.

Description: The Finite-difference Ecosystem-scale Tree-Crown Hydrodynamics model version 2 (FETCH2) is a tree-scale hydrodynamic model of transpiration. The FETCH2 model employs a finite difference numerical methodology and a simplified single-beam conduit system to explicitly resolve xylem water potentials throughout the vertical extent of a tree. Empirical equations relate water potential within the stem to stomatal conductance of the leaves at each height throughout the crown. While highly simplified, this approach brings additional realism to the simulation of transpiration by linking stomatal responses to stem water potential rather than directly to soil moisture, as is currently the case in the majority of land-surface models. FETCH2 accounts for plant hydraulic traits, such as the degree of anisohydric/isohydric response of stomata, maximal xylem conductivity, vertical distribution of leaf area, and maximal and minimal xylem water content. We used FETCH2 along with sap flow and eddy covariance data sets collected from a mixed plot of two genera (oak/pine) in Silas Little Experimental Forest, NJ, USA, to conduct an analysis of the intergeneric variation of hydraulic strategies and their effects on diurnal and seasonal transpiration dynamics. We define these strategies through the parameters that describe the genus-level transpiration and xylem conductivity responses to changes in stem water potential. Our evaluation revealed that FETCH2 considerably improved the simulation of ecosystem transpiration and latent heat flux than more conventional models. A virtual experiment showed that the model was able to capture the effect of hydraulic strategies such as isohydric/anisohydric behavior on stomatal conductance under different soil-water availability conditions.

Description: Although many studies have considered the carbon or greenhouse gas budgets of peat ecosystems, only a few have considered the nutrient budget of peat soils and this, in turn, has limited the ability of studies to consider the impact of changes in climate and atmospheric deposition on the phosphorus budget of a peat soil. This study considered the total phosphorus (P) budget of an upland peat-covered catchment over the period 1993 to 2012. The study has shown: Total atmospheric deposition of phosphorus varied from 62 to 175 kg P/km 2 /yr; The carbon:phosphorus ratio of the peat profile declines significantly from values in the litter layer (C:P = 1326) to approximately constant at 30 cm depth (C:P = 4240); The total fluvial flux of phosphorus varied from 49 to 111 kg P/km 2 /yr, of which between 45 and 77% was dissolved P. The total phosphorus sink varied from -5.6 to +71.7 kg P/km 2 /yr with a median of +29.4 kg P/km 2 /yr, which is within the range of the estimated long-term accumulation rate of phosphorus in the peat profile of between 3 and 32 kg P/km 2 /yr. The phosphorus budget of the peat ecosystem relies on rapid recycling near the soil surface and this means that any vegetation management may critically deprive the ecosystem of this nutrient.

Description: Permafrost collapse, known as thermokarst, can alter soil properties and carbon emissions. However, little is known regarding the effects of permafrost collapse in upland landscapes on the biogeochemical processes that affect carbon balance. In this study, we measured soil carbon and physiochemical properties at a large thermokarst feature on a hillslope in the northeastern Tibetan Plateau. We categorized surfaces into three different micro-relief patches based on type and extent of collapse (control, drape and exposed areas). Permafrost collapse resulted in substantial decreases of surface soil carbon and nitrogen stocks, with losses of 29.6 ± 5.9% and 26.7 ± 8.8% for carbon and nitrogen, respectively, in the 0-10 cm soil layer. Laboratory incubation experiments indicated that control soil had significantly higher CO 2 production rates than that of drapes. The results from Fourier transform infrared (FTIR) spectroscopy analysis showed that exposed soils accumulated some organic matter due to their low position within the feature, which was accompanied by substantial changes in the chemical structure and characteristics of the soil carbon. Exposed soils had higher hydrocarbon and lignin/phenol backbone content than in control and drape soils in the 0-10 cm layer. This study demonstrates that permafrost collapse can cause abundant carbon and nitrogen loss, potentially from mineralization, leaching, photo-degradation and lateral displacement. These results demonstrate that permafrost collapse redistributes the soil organic matter, changes its chemical characteristics, and leads to losses of organic carbon due to the greenhouse gas emission.

Description: The discovery of anaerobic ammonium oxidation (anammox) highlighted the importance of alternative metabolic pathways to inorganic nitrogen removal in natural environments, particularly in those subjected to increased nitrate inputs, such as estuaries. Laboratory enrichment experiments were used to test the effect of increasing loads of nitrate (NO 3 - ), nitrite (NO 2 - ), and ammonium (NH 4 + ) on the anammox process. Three Atlantic temperate estuaries (NW Portugal) were investigated along a salinity gradient, and anammox activity was measured under different NO 3 - , NO 2 - and NH 4 + treatments, using the isotope pairing technique. Obtained results showed that NO 3 - stimulated denitrification but not anammox, whereas NO 2 - additions had a positive effect on anammox activity, confirming its role as a key environmental control. On the other hand, increasing NH 4 + concentrations seemed to inhibit anammox for low salinity sites. Our findings suggested an important role of the natural availability of nitrogen compounds in regulating anammox and the magnitude of anammox versus denitrification in estuarine environments.

Description: Understanding stream carbon export dynamics is needed to accurately predict how the carbon balance of peatland catchments will respond to climatic and environmental change. We used a twelve year record (2003-2014) of continuous streamflow and manual spot measurements of total organic carbon (TOC), dissolved inorganic carbon (DIC), methane (CH 4 ) and organic carbon quality (SUVA 254 ) to assess interannual and seasonal variability in stream carbon export for a peatland catchment (70% mire and 30% forest cover) in northern Sweden. Mean annual total carbon export for the twelve year period was 12.2 g C m −2  yr −1 , but individual years ranged between 6 and 18 g C m −2  yr −1 . TOC, which was primarily composed of dissolved organic carbon (〉99%), was the dominant form of carbon being exported, comprising 63% to 79% of total annual exports, and DIC contributed between 19% and 33%. CH 4 made up less than 5% of total export. When compared to previously published annual net ecosystem exchange (NEE) for the studied peatland system, stream carbon export typically accounted for 12 to 50% of NEE for most years. However, in 2006 stream carbon export accounted for 63 to 90% (estimated uncertainty range) of NEE due to a dry summer which suppressed NEE, followed by a wet autumn that resulted in considerable stream export. Runoff exerted a primary control on stream carbon export from this catchment; however, our findings suggest that seasonal variations in biologic and hydrologic processes responsible for production and transport of carbon within the peatland were secondary influences on stream carbon export. Consideration of these seasonal dynamics is needed when predicting stream carbon export response to environmental change.

Description: Streambed substrates harbor a rich biome responsible for biogeochemical processing in riverine waters. Beyond their biological role, the presence of benthic and hyporheic biofilms can play an important role in influencing large scale transport of solutes, even for conservative tracers. As biofilms grow and accumulate biomass, they actively interact with and influence surface and sub-surface flow patterns. To explore this effect we conducted experiments at the Notre Dame Linked Ecosystems Experimental Facility (ND-LEEF) in four outdoor streams, each with different gravel beds. Over the course of 20 weeks we conducted transport experiments in each of these streams and observed different patterns in breakthrough curves as biofilms grew on the substrate. Biofilms played a major role in shaping the observed conservative transport patterns. Overall, while the presence of biofilms led to a decreased exchange rate between the fast (mobile) and slow (immobile) parts of the flow domain, water that was exchanged tended to be stored in the slow regions for longer times once biofilms had established. More specifically, we observed enhanced longitudinal dispersion in breakthrough curves as well as broader residence time distributions when biofilms were present. Biofilm colonization over time homogenized transport patterns across the four streams that were originally very distinct. These results indicate that stream biofilms exert a strong control on conservative solute transport in streams, a role that to date has not received enough attention.

Description: Significant climate fluctuations in the Arctic over the recent past, and additional predicted future temperature changes, highlight the need for high-resolution Arctic paleoclimate records. Arctic coastal environments supplied with terrigenous sediment from Arctic rivers have the potential to provide annual to sub-decadal resolution records of climate variability over the last few millennia. A potential tool for paleotemperature reconstructions in these marine sediments is the MBT’/CBT proxy based on branched glycerol dialkyl glycerol tetraethers (brGDGTs). In this study, we examine the source of brGDGTs in the Colville River, Alaska and the adjacent Simpson Lagoon, and reconstruct temperatures from Simpson Lagoon sediments to evaluate the applicability of this proxy in Arctic estuarine environments. The Colville catchment soils, fluvial sediments, and estuarine sediments contain statistically similar brGDGT distributions, indicating that the brGDGTs throughout the system are soil-derived with little alteration from in situ brGDGT production in the river or coastal waters. Temperatures reconstructed from the MBT’/CBT indices for surface samples show good agreement with regional summer (June through September; JJAS) temperatures, suggesting a seasonal bias in Arctic temperature reconstructions from the Colville system. In addition, we reconstruct paleotemperatures from an estuarine sediment core that spans the last 75 y, revealing an overall warming trend in the 20 th century that is consistent with trends observed in regional instrumental records. These results support the application of this brGDGT-based paleotemperature proxy for sub-decadal scale summer temperature reconstructions in Arctic estuaries containing organic material derived from sediment-laden, episodic rivers.

Description: Global warming is expected to raise temperatures in freshwater lakes, which have been acknowledged to contribute up to 10% of the atmospheric methane concentrations. Increasing temperature enhances methane production and oxidation rates, but few studies have considered the balance between both processes at experimentally higher temperatures within lake sediments. The temperature dependence of methane concentrations, methane production rates and methanogenic ( mcrA ) and methanotrophic ( pmoA ) community size was investigated in intact sediment cores incubated with aerobic hypolimnion water at 4, 8 and 12 °C over three weeks. Sediment cores of 25 cm length were collected at two temperate lakes – Lake Stechlin (Germany) (meso-oligotrophic, maximum depth 69.5 m), and Lake Geneva (France/Switzerland) (mesotrophic, maximum depth 310 m). While methane production rates in Lake Stechlin sediments did not change with increasing temperatures, methane concentrations decreased significantly. In contrast, methane production rates increased in 20-25 cm in Lake Geneva sediments with increasing temperatures, but methane concentrations did not differ. Real-time PCR demonstrated the methanogenic and methanotrophic community size remained stable independently of the incubation temperature. Methane concentrations as well as community sizes were one to two magnitudes higher in Lake Stechlin than in Lake Geneva, while potential methane production rates after 24 h were similar in both lakes, with on average 2.5 and 1.9 nmol g -1 DW h -1 , respectively. Our results suggest that at higher temperatures methane oxidation could balance, and even exceed, methane production. This suggests anaerobic methane oxidation could be involved in the methane balance at a more important rate than previously anticipated.

Description: It is important to clarify the quantity and composition of hydrologic N export from terrestrial ecosystem and its primary controlling factors, because it affected N availability, productivity and C storage in natural ecosystems. The most previous investigations were focused on the effects of N deposition and human disturbance on the composition of hydrologic N export. However, few studies were aware of whether there were significant differences in the concentrations and composition of hydrologic N export from natural ecosystems in different climate zones, and what is the primary controlling factor. In the present study, three natural forest ecosystems and one natural grassland ecosystem that were located in different climate zones and with different soil pH range were selected. The concentrations of total dissolved N, DON, NH 4 + , NO 3 − in soil solution and stream water, soil properties, and soil gross N transformation rates were measured to answer above questions. Our results showed that NO 3 - concentrations and the composition pattern of hydrologic N export from natural ecosystems varied greatly in the different climate zones. The NO 3 - concentrations in stream water varied largely, ranging from 0.1 mg N L -1 to 1.6 mg N L -1 . While, DON concentration in stream water, ranging from 0.1 to 0.9 mg N L -1 , did not differ significantly and the concentrations of NH 4 + were uniformly low (average 0.1 mg N L -1 ) in all studied sites. There was a trade-off relationship between the proportions of NO 3 - and DON to total dissolved N in stream water. In subtropical strongly acidic forests soil site, DON was the dominance in total dissolved N in stream water. While, NO 3 - -N became dominance in temperate acidic forests soil site, subtropical alkaline forests soil region, and the alpine meadow sites on the Tibetan Plateau. The proportions of NO 3 - to total dissolved N in both soil solution and stream water significantly increased with the increasing of the gross autotrophic nitrification rates (p 〈 0.01). Our results indicated that the characteristics of soil N transformations were the most primary factor regulating the composition of hydrologic N losses from ecosystems. The nitrification was the central soil N transformation processes regulating N composition in soil solution and hydrologic N losses. These results provided important information on understanding easily the composition of hydrologic N export from terrestrial ecosystem.

Description: Dissolved organic matter (DOM) composition may be an important determinant of its fate in freshwaters, but little is known about temporal variability in DOM composition and the biodegradability of DOM in northern temperate watersheds. We measured biodegradable dissolved organic carbon (BDOC) via incubation assays and DOM composition using optical indices on eleven dates in three Lake Superior tributaries. Percent BDOC (%BDOC) and BDOC concentrations were seasonally synchronous across these watersheds, despite that they vary in size by orders of magnitude (1.7 to 3400 km 2 ). Relative to %BDOC, BDOC concentrations were more tightly constrained among sites on any given date. BDOC also varied within seasons; for example, %BDOC on two different dates in winter were among the highest (29% and 54%) and lowest (0%) values observed for each site (overall %BDOC range: 0 to 72%). DOM composition varied the most among tributaries during a summer storm event when BDOC (both as % and concentration) was elevated, but was remarkably similar among tributaries during fall, spring and winter. Multivariate models identified humic-like and tryptophan-like fluorophores as predictors of %BDOC, but DOM composition only described 21% of the overall variation in %BDOC. Collectively, these three rivers exported ~18 Gg C yr -1 as DOC and ~2Gg C yr -1 as BDOC, which corresponded to 9 to 17% of annual DOC exported in biodegradable form. Our results suggest much of the C exported from these northern temperate watersheds may be biodegradable within 28 days, and that large pulses of labile DOM can be exported during storm events and spring snowmelt.

Description: The predicted increase in the frequency and intensity of climate extremes is expected to impact terrestrial carbon fluxes to the atmosphere, potentially changing ecosystems from carbon sinks to sources, with positive feedbacks to climate change. As the second largest terrestrial carbon flux, soil CO 2 efflux or soil respiration (R s ), is strongly influenced by soil temperature and moisture. Thus, climate extremes such as heat waves and extreme drought should have substantial impacts on R s . We investigated the effects of such climate extremes on growing season R s in a mesic grassland by experimentally imposing two years of extreme drought combined with midsummer heat waves. After this two-year period, we continued to measure R s during a recovery year. Two consecutive drought years reduced R s by ~25% each growing season; however when normal rainfall returned during the recovery year, formerly droughted plots had higher rates of R s than control plots (up to +17%). The heat wave treatments had no effect on R s , alone or when combined with drought, and during the growing season, soil moisture was the primary driver of R s with little evidence for R s temperature sensitivity. When compared to aboveground net primary production, growing season R s was much less sensitive to drought, but was more responsive post-drought. These results are consistent with the hypothesis that ecosystems become sources of CO 2 during drought because carbon inputs (production) are decreased relatively more than outputs (respiration). Moreover, stimulation of R s post-drought may lengthen the time required for net carbon exchange to return to pre-drought levels.

Description: Global changes are altering many important drivers of ecosystem functioning, with precipitation amount and disturbance frequency being especially important. Carbon (C) and nitrogen (N) pools are key contemporary attributes of ecosystems that can also influence future C uptake via plant growth. Thus, understanding the impacts of altered precipitation amounts (through controls of primary production inputs) and disturbance regimes (through losses of C and N in biomass) is important to project how ecosystem services will respond to future global changes. A major difficulty inherent within this task is that drivers of ecosystem function and processes often interact, resulting in novel ecosystem responses. To examine how changes in precipitation affect grassland ecosystem responses under a frequent disturbance regime (annual fire), we assessed the biogeochemical and ecological consequences of more than two decades of irrigation in an annually burned mesic grassland in the central United States. In this experiment, precipitation amount was increased by 31% relative to ambient and 1 in 3 years were statistically extreme relative to the long-term historical record. Despite evidence that irrigation decreased root:shoot ratios and increased rates of N cycling – each expected to reduce soil C and N with annual burning – we detected no changes in these biogeochemical pools. This surprising biogeochemical resistance highlights the need to explore additional mechanisms within long-term experiments concerning the consequences of global change impacts on ecosystems.

Description: Foliage/atmosphere exchange is an important pathway of deposition and loss in the biogeochemical mercury (Hg) cycle of terrestrial ecosystems. The foliage/atmosphere fluxes of Hg 0 were observed over four seasons in a Masson pine (Pinus massoniana) forest in south China. Hg 0 exchange showed a bi-directional process, but without clear compensation point. Hg 0 emissions peaked mid-day in all four seasons, probably associated with Hg photoreduction on needle surface. Peaks in Hg 0 adsorption/deposition often occurred in the morning, especially in spring and autumn. Although current-year needles accumulated Hg at a rate of 19.4 µg · m -2  · yr -1 , they were a net Hg 0 source of 1.7 µg · m -2  · yr -1 to the atmosphere as their release of Hg exceeded inputs. In addition, previous-year needles emitted Hg 0 at an average rate of 9.2 µg · m -2  · yr -1 . Based on the mass balance of Hg in the forest canopy, the dry deposition of Hg was estimated 52.5 µg · m -2  · yr -1 , much higher than the wet deposition (to 14.4 µg · m -2  · yr -1 ). Although Hg in the atmosphere is considered the main source of Hg in folia, soil water may contribute to Hg 0 emission by plant transpiration. These processes should be further studied in the future.

Description: [1]  A small hot spring that is informally called “Fe-waterfall spring” and is located in the Rehai geothermal area discharges hot (42 to 73 °C), near-neutral (pH =7.65) Fe-rich water. Submerged reddish precipitates are composed largely of ferrihydrite, goethite, lepidocrocite, opal-A, quartz, and anorthite, as revealed by X-ray diffraction (XRD) and Mössbauer spectroscopy. Molecular phylogenetic analysis demonstrates that the bacterial community in these precipitates is mainly composed of Cyanobacteria, Planctomycetes, β-proteobacteria, Deinococci-Thermus and Chlorobi. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) examinations show that abundant sheath-like Fe oxyhydroxides, which exhibit different morphologies and sizes, are present in Fe-rich precipitates. These sheath-like structures are composed of ferrihydrite rather than more crystalline lepidocrocite or goethite. Energy dispersive X-ray spectrometer (EDS), scanning transmission electron microscopy (STEM) and nano secondary ion mass spectrometry (Nano-SIMS) reveal that they are mainly composed of Fe, Si and O, together with some trace elements. Most of the sheath-like structures are not morphologically comparable to biogenic Fe oxyhydroxides produced by known chemolithotrophic Fe oxidizers, which is consistent with the fact that no chemolithotrophic Fe oxidizers were identified by molecular analysis in the precipitates. We suggest that the sheath-like Fe oxyhydroxides are formed through passive Fe sorption and nucleation onto the cell walls of various thermophiles rather than by the direct metabolic activities of chemolithotrophic Fe oxidizers. Biogenic sheath-like Fe oxyhydroxides in Fe-waterfall spring have important implications for geochemical cycles driven by microorganisms, the origin of microfossils and the formation of banded iron formations (BIFs) in the Archean ocean.

Description: [1]  We present the concept of the Carbon Cycle Data Assimilation System and describe its evolution over the last two decades from an assimilation system around a simple diagnostic model of the terrestrial biosphere to a system for the calibration and initialization of the land component of a comprehensive earth system model. We critically review the capability of this modeling framework to integrate multiple data streams, to assess their mutual consistency and with the model, to reduce uncertainties in the simulation of the terrestrial carbon cycle, to provide, in a traceable manner, reanalysis products with documented uncertainty, and to assist the design of the observational network. We highlight some of the challenges we met and experience we gained, give recommendations for operating the system and suggest directions for future development.

Description: [1]  Hypolithic microbial communities are productive niches in deserts worldwide, but many facets of their basic ecology remain unknown. The Namib Desert is an important site for hypolith study because it has abundant quartz rocks suitable for colonization and extends west to east across a transition from fog to rain dominated moisture sources. We show fog sustains and impacts hypolithic ecology in several ways: (1) fog effectively replaces rainfall in the western zone of the central Namib to enable high (≥95%) hypolithic abundance at landscape (1- 10 km) and larger scales; and (2) high water availability, through fog (western zone) and/or rainfall (eastern zone), results in smaller size-class rocks being colonized (mean 6.3 ± 1.2 cm) at higher proportions (e.g. 98% vs. ca . 3%) than in previously studied hyperarid deserts. We measured 0.1% of incident sunlight as the lower limit for hypolithic growth on quartz rocks in the Namib, and found uncolonized ventral rock surfaces were limited by light rather than moisture. In-situ monitoring showed that, although rainfall supplied more liquid water (36 hrs) per event than fog (mean, 4 hrs), on an equivalent annual basis, fog provided nearly twice as much liquid water as rainfall to the hypolithic zone. Hypolithic abundance reaches 100% at a mean annual precipitation (MAP) ca . 40-60 mm, but at a much lower MAP ( ca . 25 mm) when moisture from fog is available.

Description: [1]  Mountain pine beetle ( Dendroctonus ponderosae ) outbreaks in North America are widespread and have potentially-large scale impacts on albedo and associated radiative forcing. Mountain pine beetle outbreaks in Colorado and southern Wyoming have resulted in persistent and significant increases in both winter albedo (change peaked 10 years post-outbreak at 0.06 ± 0.01 and 0.05 ± 0.01, in lodgepole pine ( Pinus contorta ) and ponderosa pine ( Pinus ponderosa ) stands, respectively) and spring albedo (change peaked 10 years post-outbreak at 0.06 ± 0.01 and 0.04 ± 0.01, in lodgepole pine and ponderosa pine stands, respectively). Instantaneous top-of-atmosphere radiative forcing peaked for both lodgepole pine and ponderosa pine stands in winter at 10 years post-outbreak at −1.7 ± 0.2 W m -2 and −1.4 ± 0.2 W m -2 , respectively. The persistent increase in albedo with time since mountain pine beetle disturbance combined with the continued progression of the attack across the landscape from 1994–2011, resulted in an exponential increase in winter and annual radiative cooling (MW) over time. In 2011 the rate of radiative forcing within the study area reached −982.7 ± 139.0 MW, -269.8 ± 38.2 MW, -31.1 ± 4.4 MW and −147.8 ± 20.9 MW in winter, spring, summer and fall, respectively. An increase in radiative cooling has the potential to decrease sensible and/or latent heat flux by reducing available energy. Such changes could affect current mountain pine beetle outbreaks which are influenced by climatic conditions.

Description: [1]  Characteristics of the natural fire regime are poorly resolved in the Arctic, even though fire may play an important role cycling carbon stored in tundra vegetation and soils to the atmosphere. In the course of studying vegetation and permafrost-terrain characteristics along a chronosequence of tundra burn sites from AD 1977, 1993, and 2007 on the North Slope of Alaska, we discovered two large, previously unrecognized tundra fires. The Meade River fire burned an estimated 500 km 2 and the Ketik River fire burned an estimated 1,200 km 2 . Based on radiocarbon dating of charred twigs, analysis of historic aerial photography, and regional climate proxy data, these fires likely occurred between AD 1880 and 1920. Together, these events double the estimated burn area on the North Slope of Alaska over the last ~100 to 150 years. Assessment of vegetation succession along the century-scale chronosequence of tundra fire disturbances demonstrates for the first time on the North Slope of Alaska that tundra fires can facilitate the invasion of tundra by shrubs. Degradation of ice-rich permafrost was also evident at the fire sites and likely aided in the presumed changes of the tundra vegetation post-fire. Other previously unrecognized tundra fire events likely exist in Alaska and other Arctic regions and identification of these sites is important for better understanding disturbance regimes and carbon cycling in Arctic tundra.

Description: [1]  Carbon sequestration occurs only when terrestrial ecosystems are at nonsteady states. Despite of their ubiquity in the real world, the nonsteady states of ecosystems have not been well quantified, especially at regional and global scales. In this study, we developed a two-step data assimilation scheme to estimate carbon sink strength in China's forest ecosystems. Specifically, the two-step scheme consists of a steady-state step and a nonsteady-state step. In the steady-state step, we constrained a process-based model (TECO-R model) against biometric data (NPP, biomass, litter, and soil organic carbon) in mature forests. With a subset of the parameter values estimated from the steady-state data assimilation being fixed, the nonsteady-state data assimilation was performed to estimate carbon sequestration in China's forest ecosystems. Our results indicated that 17 out of the 22 total parameters in the TECO-R model were well constrained by the biometric data with the steady-state data assimilation. When observations from both mature and developing forests were used, all the 10 parameters related to carbon sequestration in vegetation and soil carbon pools were well constrained at the nonsteady-state step. The estimated mean vegetation carbon sink in China's forests is 89.7 ± 16.8 gC m -2  yr -1 , comparable with the values estimated from the forest inventory and other process-based regional models. The estimated mean soil and litter carbon sinks in China's forests are 14.1 ± 20.7 and 4.7 ± 6.5 gCm -2  yr -1 . This study demonstrated that a two-step data assimilation scheme can be a potent tool to estimate regional carbon sequestration in nonsteady-state ecosystems.

Description: ABSTRACT [1]  It has often been hypothesized that the dissolved organic carbon (DOC) pool of algal origin in lakes is more bioavailable than its terrestrial counterpart, but this hypothesis has seldom been directly tested. Here we test this hypothesis by tracking the production and isotopic signature of bacterial respiratory CO 2 in 2-week lake water incubations, and use the resulting data to reconstruct and model the bacterial consumption dynamics of algal and terrestrial DOC. The proportion of algal DOC respired decreased systematically over time in all experiments, suggesting a rapid consumption and depletion of this substrate. Our results further show that the algal DOC pool was used in proportions and at rates twice and ten-time as high as the terrestrial DOC pool, respectively. On the other hand, the absolute amount of labile terrestrial DOC was on average four-time higher than labile algal DOC, accounting for almost the entire long-term residual C metabolism, but also contributing to short-term bacterial C consumption. The absolute amount of labile algal DOC increased with chlorophyll a concentrations, whereas total phosphorus appeared to enhance the amount of terrestrial DOC that bacteria could consume, suggesting that the degradation of these pools is not solely governed by their respective chemical properties, but also by interactions with nutrients. Our study shows that there is a highly reactive pool of terrestrial DOC that is processed in parallel to algal DOC, and because of interactions with nutrients, terrestrial DOC likely supports high levels of bacterial metabolism and CO 2 production even in more productive lakes.

Description: [1]  N-fertilization significantly increases N 2 O and NO soil fluxes to the atmosphere. In spite of the expansion of agricultural activities in tropical managed soils from the developing-world, there is little information about the loss of applied nitrogen (LAN) as NO and N 2 O from these areas. In this work, we determined LAN-N 2 O and LAN-NO from different crops during the growing season at a sandy soil experimental field and two active farms with loamy and clayed soils, respectively. Tillage (T) and no-tillage (NT) farming were separately evaluated. All of the evaluated areas were located in the Venezuela savanna region. [2]  A large range of LAN-N 2 O (0.30-6.1%) and LAN-NO (0.26-2.1%) were recorded, with overall average values of 1.9% and 0.9%, respectively. LAN values were mainly affected by soil texture and rainfall pattern, which affected soil moisture and WFPS. Also, soil management (T and NT) and the chemical composition of the N-fertilizer played important roles. The overall average of LAN-N 2 O is about two times higher than the IPCC default value of 1%, therefore, our results suggest that a higher factor should be considered for cropping systems in tropical savanna regions.

Description: [1]  Model-data fusion is a process in which field observations are used to constrain model parameters. How observations are used to constrain parameters has a direct impact on the carbon cycle dynamics simulated by ecosystem models. In this study, we present an evaluation of several options for the use of observations in modeling regional carbon dynamics and explore the implications of those options. We calibrated the Terrestrial Ecosystem Model (TEM) on a hierarchy of three vegetation classification levels for the Alaskan boreal forest: species-level, plant-functional-type-level (PFT-level) and biome-level, and we examined the differences in simulated carbon dynamics. Species specific field-based estimates were directly used to parameterize the model for species-level simulations, while weighted-averages based on species percent cover were used to generate estimates for PFT- and biome-level model parameterization. We found that calibrated key ecosystem process parameters differed substantially among species and overlapped for species that are categorized into different PFTs. Our analysis of parameter sets suggests that the PFT-level parameterizations primarily reflected the dominant species and that functional information of some species were lost from the PFT-level parameterizations. The biome-level parameterization was primarily representative of the needleleaf PFT and lost information on broadleaf species or PFT function. Our results indicate that PFT-level simulations may be potentially representative of the performance of species-level simulations while biome-level simulations may result in biased estimates. Improved theoretical and empirical justifications for grouping species into PFTs or biomes are needed to adequately represent the dynamics of ecosystem functioning and structure.

Description: [1]  Spatial and temporal heterogeneity of methane flux from boreal wetlands make prediction and up-scaling challenging, both within and among wetland systems. Drivers of methane production and emissions are also highly variable, making empirical model development difficult and leading to uncertainty in methane emissions estimates from wetlands. Previous studies have examined this problem using point-scale (static chamber method) and ecosystem-scale (flux tower methods) measurements, but few studies have investigated whether different processes are observed at these scales. We analyzed methane emissions from a boreal fen, measured by both techniques, using data from the Boreal Ecosystem-Atmosphere Study (BOREAS). We sought to identify driving processes associated with methane emissions at two scales and explain diurnal patterns in emissions measured by the tower. The mean methane emission rates from flux chambers were greater than the daytime, daily mean rates measured by the tower, but the nighttime, daily mean emissions from the tower were often an order of magnitude greater than emissions recorded during the daytime. Thus, daytime measurements from either the tower or chambers would lead to a biased estimate of total methane emissions from the wetland. We found the timing of nighttime emission events was coincident with the cooling and convective mixing within hollows, which occurred regularly during the growing season. We propose that diurnal thermal stratification in shallow pools traps methane by limiting turbulent transport. This methane stored during daytime heating is later released during evening cooling due to convective turbulent mixing.

Description: [1]  Disturbances affect forest-atmosphere exchanges of carbon, water, and energy, thereby influencing weather and climate. Bark beetle outbreaks are one such disturbance type that alters biogeochemical and biogeophysical processes in forests. Few studies have documented bark beetle impacts to leaf area index (LAI), gross primary productivity (GPP), evapotranspiration (ET), land surface temperature (LST), and surface albedo with satellite observations. Our objective was to use Landsat-derived estimates of bark beetle-caused tree mortality and Moderate Resolution Imaging Spectroradiometer (MODIS) land surface products to estimate beetle-caused changes in LAI, GPP, ET, LST, and surface albedo in northern Colorado. Following bark beetle-caused tree mortality, decreases occurred in LAI (0.02-0.80 m 2 m -2 , 1-40%), annual GPP (50-248 gC m -2  yr -1 , (5-26%), and daily summer ET (0.20-0.70 mm day -1 , 13-44%), whereas increases occurred in August LST (1-3.9 K) and February albedo (0.03-0.09, 19-52%). We found greater responses of these variables in areas of greater mortality severity. The extent and severity of tree mortality in northern Colorado caused substantial changes in land surface variables (9-23%) when averaged across all forested areas of our study area. Our results demonstrate that land surface variables are sensitive to bark beetle-caused tree mortality and that bark beetle outbreaks can significantly impact biogeochemical and biogeophysical processes.

Description: [1]  We studied the bio-optical properties and sediment dynamics of typical phytoplankton-dominated (PD) and macrophyte-dominated (MD) regions in shallow Lake Taihu in China, from long-term site-specific studies and short-term high-frequency observations. The long-term studies showed that 5 parameters were significantly lower in the MD region than in the PD region: PAR diffuse attenuation coefficient ( K d (PAR)), concentrations of total suspended matter ( C TSM ), tripton ( C Tripton ), and phytoplankton pigment ( C Chl a +P a ), and CDOM absorption coefficient ( a CDOM (350)). In winter in the MD region, with only scarce submerged aquatic vegetation (SAV) coverage, the K d (PAR), C TSM and C Tripton were significantly higher than in the PD region with no SAV; in contrast, C Chl a +P a and a CDOM (350) were significantly lower in the MD region than in the PD region. In the other three seasons with high SAV coverage, K d (PAR), C TSM , C Tripton , C Chl a +P a and a CDOM (350) were all significantly lower in the MD region than in the PD region. The appearance and growth of SAV decreased C TSM , C Tripton and K d (PAR). The short-term high-frequency study showed that phytoplankton and tripton absorption coefficients were significantly lower in the MD region than in the PD region. In the PD region, there were highly significant exponential relationships between wind speed, wave height, and wave shear stress, and C Tripton and K d (PAR), showing that wind-driven sediment resuspension could significantly affect both the tripton concentration and PAR attenuation. However, in the MD region, there were only weakly significant correlations, or no significant correlations, between wind speed, wave height, and wave shear stress, and C Tripton , and K d (PAR). The combination of the long-term site-specific and short-term high-frequency observations is an excellent tool for study of the bio-optical properties in lake environments.

Description: [1]  We examined two low-temperature hydrothermal deposits rich in Fe-Si-Mn collected from the recently discovered hydrothermal fields at the Southwest Indian Ridge using mineralogical, geochemical, and molecular biological techniques. The mineralogical and geochemical analyses indicated that the low-temperature hydrothermal fields would provide a warm and chemical species-rich habitat for chemosynthetic-based hydrothermal ecosystems. Analyses of 16S rRNA sequences showed that ζ -Proteobacteria , Pseudoalteromonas , Leptothrix , and Pseudomonas were potential Fe- and Mn-oxidizers in the low-temperature hydrothermal environments, but they were not present in equal abundance among the sub-niches. Some potential Fe- and Mn-reducers were also recovered; they were more commonly found in the exterior black Fe-Mn oxides. The difference between the exterior black Fe-Mn oxides and the interior Opal-A could be related to differences in in situ physicochemical conditions. We also identified microbial players that may participate in sulfur (S) geochemical cycling in these low-temperature hydrothermal environments via analyses of 16S rRNA sequences and the aprA functional gene. The results indicated that members of γ - and α -Proteobacteria were involved in the S-oxidation process, while members of δ -Proteobacteria , Nitrospirae , Firmicutes , and Archaea might participate in the S-reduction process. Fe-, Mn-, and S-oxidizers and reducers might actively participate in hydrothermal biogeochemical processes, which could influence the transfer of chemical species and the formation of biogenic minerals.

Description: [1]  Quantifying carbon fluxes at large spatial scales has attracted considerable scientific attentions. In this study, a novel approach was proposed to estimate the terrestrial ecosystem gross primary production (GPP) using imagery from the satellite-borne Moderate Resolution Imaging Spectroradiometer (MODIS) sensor. The new model (named as TGR) uses a combination of MODIS Enhanced Vegetation Index and Land Surface Temperature products, as well as in situ measurement of photosynthetically active radiation to estimate GPP at a 16-day interval. Three major advantages are included in the model: 1) the model follows strictly the logic of the light use efficiency model and each parameter has physical meaning; 2) the model reduces the dependency on ground-based meteorological measurements; 3) the overlap of information in correlated explanatory variables is avoided. The model was calibrated with data from 17 sites within the Ameriflux network and validated at another 13 sites, covering a wide range of climates and eight major vegetation types. Results show that the TGR model explains reasonably well the tower-based measurements of GPP for all vegetation types except for the evergreen broadleaf forest, with the coefficient of determination in a range from 0.67 to 0.91, and the root mean square error from 9.0 to 31.9 g C/m 2 /16-day. Comparisons with other two models (the TG and GR model) show that the TGR model generally gives better GPP estimates in nearly all vegetation types, especially under dry climate conditions. These results indicate that the TGR model can be potentially used to estimate GPP at regional scale.

Description: [1]  Boreal ecosystems represent a large carbon (C) reservoir and a substantial source of greenhouse gases. Hydrologic conditions dictate whether C leached from boreal soils is processed in catchments, or flushed to less productive environments via the stream. This study quantified hydrologic and biogeochemical C loss from a boreal catchment underlain by frozen silt, where flowpaths may deepen as the active layer thaws over the summer. We hypothesized a decrease in the magnitude of C mineralization over the summer associated with changing flow paths and decreasing hydrologic connectivity, organic matter lability, and nitrogen (N) availability. Conservative tracers were used to partition C and N loss between catchment export and biogeochemical processing. Coupling tracers with tributary and porewater chemistry indicated C and N cycling in soil flowpaths, with an exponential decrease over the summer. Nitrate was primarily reduced in hill slope flowpaths and the lack of N reaching the stream appeared to limit C mineralization. Stream export accounted for the greatest loss of C, removing 247 and 113 mol hr -1 in the early and late summer, respectively. Reactivity was related to hydrologic connectivity between the soils and stream, which was greatest early in the summer and following a large flood. While a warming climate may increase storage potential in thawed soils, the early season flush of labile material and late season runoff through mineral flowpaths may maintain high C export rates. Therefore, we highlight physical export as a dominant cause of aqueous C loss from silty catchments as the Arctic continues to thaw.

Description: [1]  African climate is changing at rates unprecedented in the Late Holocene with profound implications for tropical ecosystems and the global hydrologic cycle. Understanding the specific climate drivers behind tropical ecosystem change is critical for both future and paleo-modeling efforts. However, linkages between climate and vegetation in the tropics have been extremely controversial. The Normalized Difference Vegetation Index (NDVI) is a satellite derived index of vegetation productivity with a high spatial and temporal resolution. Here, we use regression analysis to show that NDVI variability in Africa is primarily correlated with the inter-annual extent of the Intertropical Convergence Zone (ITCZ). Our results indicate that inter-annual variability of the ITCZ, rather than sea surface temperatures or teleconnections to mid/high latitudes, drives patterns in African vegetation resulting from the effects of insolation anomalies and ENSO events on atmospheric circulation. Global controls on tropical atmospheric circulation allow for spatially coherent reconstruction of inter-annual vegetation variability throughout Africa on many time-scales through regulation of dry-season length and moisture convergence, rather than precipitation amount.

Description: [1]  In this paper, we investigate the relationship between seasonal climatic and environmental variables, and the skeletal δ 13 C of modern and mid-Holocene Porites lutea corals from the southern coast of Hainan Island in the northern South China Sea. No significant correlations were observed between δ 13 C in the modern coral and solar insolation or sea surface temperature (SST). However, seasonal variability of δ 13 C in the modern coral co-varies with rainfall on Hainan Island. Furthermore, the seasonal variations of δ 13 C in both the modern and mid-Holocene coral are synchronous with those of the coral Δδ 18 O, which is a proxy for seawater δ 18 O and, in turn, largely controlled by local rainfall. These observations suggest that coral δ 13 C variations are closely associated with rainfall in this region. Given that river runoff contains dissolved inorganic carbon (DIC) with a negative δ 13 C, we suggest that periods of high rainfall on Hainan Island deliver increased amounts of 13 C-depleted DIC to coastal seawater, resulting in an enhanced negative δ 13 C in the corals. Our findings, together with previous studies, appear to demonstrate that in coastal environments, coral skeletal δ 13 C levels are controlled mainly by terrestrial carbon input and are significantly influenced by terrestrial river runoff. Consequently, the geochemical interpretation of coral δ 13 C records may differ between coastal areas, and offshore areas or the open ocean.

Description: General circulation models (GCMs) forecast higher global vapor pressure deficit (VPD) but unchanged global relative humidity (RH) in future climates. A literature survey revealed that 50% of Earth system models (ESMs) and land surface models (LSMs) embedded within GCMs employ RH as an atmospheric aridity index when describing stomatal conductance ( gs ), whereas the remaining 50% employ VPD. The consequences of using RH or VPD in gs models for water cycling and vegetation productivity in future climates on large spatial and temporal scales remain to be explored. Process-based global dynamic vegetation model runs, changes in the hydrological cycle, and concomitant vegetation productivity for the 21st century projected climate were conducted by altering only gs responses to VPD or RH and not changing any other formulations. In the simulations of the African continent under a 21st century warming trend, both stomatal functions of VPD and RH resulted in similar gross geographic patterns in primary production (GPP). However, continental total GPP was larger for the VPD response than that for the RH response. Transpiration rates were lower, resulting in a 13% increase in water-use efficiency for the VPD response compared with its RH counterpart.

Description: The nitrate (NO 3 − ) dual isotope approach was applied to snowmelt, tundra active layer pore waters, and underlying permafrost in Barrow, Alaska, USA, to distinguish between NO 3 − derived from atmospheric deposition versus that derived from microbial nitrification. Snowmelt had an atmospheric NO 3 − signal with δ 15 N averaging −4.8 ± 1.0 (standard error of the mean) ‰ and δ 18 O averaging 70.2 ± 1.7 ‰. In active layer pore waters, NO 3 − primarily occurred at concentrations suitable for isotopic analysis in the relatively dry and oxic centers of high-centered polygons. The average δ 15 N and δ 18 O of NO 3 − from high-centered polygons were 0.5 ± 1.1 ‰ and −4.1 ± 0.6 ‰, respectively. When compared to the δ 15 N of reduced nitrogen (N) sources, and the δ 18 O of soil pore waters, it was evident that NO 3 − in high-centered polygons was primarily from microbial nitrification. Permafrost NO 3 − had δ 15 N ranging from approximately −6 to 10 ‰, similar to atmospheric and microbial NO 3 − , and highly variable δ 18 O ranging from approximately −2 to 38 ‰. Permafrost ice wedges contained a significant atmospheric component of NO 3 − , while permafrost textural ice contained a greater proportion of microbially-derived NO 3 − . Large-scale permafrost thaw in this environment would release NO 3 − with a δ 18 O signature intermediate to that of atmospheric and microbial NO 3 . Consequently, while atmospheric and microbial sources can be readily distinguished by the NO 3 − dual isotope technique in tundra environments, attribution of NO 3 − from thawing permafrost will not be straightforward. The NO 3 − isotopic signature, however, appears useful in identifying NO 3 − sources in extant permafrost ice.

Description: We report methane (CH 4 ) concentration and methane oxidation (MO x ) rate measurements from the eastern tropical north Pacific (ETNP) water column. This region comprises low-CH 4 waters and a depth interval (~200-760 m) of CH 4 supersaturation that is located within a regional oxygen minimum zone (OMZ). MO x rate measurements were made in parallel using tracer-based methods with low-level 14 C-CH 4 (LL 14 C) and 3 H-CH 4 ( 3 H). The two tracers showed similar trends in MO x rate with water depth, but consistent with previous work, the LL 14 C rates (range: 0.034-15 x 10 -3 nmol CH 4 L -1 d -1 ) were systematically slower than the parallel 3 H rates (range: 0.098-4000 x 10 -3 nmol CH 4 L -1 d -1 ). Priming and background effects associated with the 3 H-CH 4 tracer and LL 14 C filtering effects are implicated as the cause of the systematic difference. The MO x rates reported here include some of the slowest rates measured in the ocean to date, are the first rates for the ETNP region and show zones of slow CH 4 turnover within the OMZ that may permit CH 4 derived from coastal sediments to travel great lateral distances. The MO x rate constants correlate with both CH 4 and oxygen concentrations, suggesting their combined availability regulates MO x rates in the region. Depth-integrated MO x rates provide an upper limit on the magnitude of regional CH 4 sources and demonstrate the importance of water column MO x , even at slow rates, as a sink for CH 4 that limits the ocean-atmosphere CH 4 flux in the ETNP region.

Description: Evapotranspiration (ET), especially in the mainland of the Indochina peninsula, can impact and is impacted by the Asian monsoonal (AM) system, thereby prompting interest in its long-term variability. To separate the physical and biological factors controlling ET variability in a tropical deciduous forest under the AM influence, 7-year eddy-covariance and ancillary measurements were collected and analyzed. The 7-year mean rainfall ( P r ) and ET along with their standard deviations were 1335 ± 256 and 977 ±108 mm (about 85% of P r ), respectively, suggesting close coupling between these two hydrologic fluxes. However, other physical and biological drivers decouple seasonal and annual variations of ET from P r . To explore them, a big-leaf model complemented by perturbation analysis was employed. The big-leaf model agreed well with the measured ET at daily to multi-year timescales, lending confidence in its ability to separate biological and physical controls on ET. Using this formulation, both first-order and second-order Taylor series expansions of the total ET derivatives were applied to the big-leaf model and compared with measured changes in ET (dET). Higher-order and joint-terms in the second-order expansion were necessary for matching measured and analyzed dET. Vapor pressure deficit ( D ) was the primary external physical controlling driver of ET. Leaf area index (LAI) and bulk stomatal conductance ( g s ) were shown to be the main significant biological drivers of the transpiration component of ET. It can be surmised that rainfall variability controls long-term ET through physical (mainly D ) and biological (mainly LAI and g s ) factors in this ecosystem.

Description: Organic layers of living and dead vegetation cover the ground surface in many permafrost landscapes and play important roles in ecosystem processes. These soil-surface organic layers (SSOLs) store large amounts of carbon and buffer the underlying permafrost and its contained carbon from changes in aboveground climate. Understanding the dynamics of SSOLs is a prerequisite for predicting how permafrost and carbon stocks will respond to warming climate. Here we ask three questions about SSOLs in a representative area of the Arctic Foothills region of northern Alaska: 1) What environmental factors control the thickness of SSOLs and the carbon they store? 2) How long do SSOLs take to develop on newly stabilized point bars? 3) How do SSOLs affect temperature in the underlying ground? Results show SSOL thickness and distribution correlate with elevation, drainage area, vegetation productivity, and incoming solar radiation. A multiple regression model based on these correlations can simulate spatial distribution of SSOLs and estimate the organic carbon stored there. SSOLs develop within a few decades after a new, sandy, geomorphic surface stabilizes, but require 500–700 years to reach steady-state thickness. Mature SSOLs lower the growing-season temperature and mean annual temperature of the underlying mineral soil by 8 and 3 °C, respectively. We suggest that the proximate effects of warming climate on permafrost landscapes now covered by SSOLs will occur indirectly via climate's effects on the frequency, extent, and severity of disturbances like fires and landslides that disrupt the SSOLs and interfere with their protection of the underlying permafrost.

Description: Submarine permafrost degradation has been invoked as a cause for recent observations of methane emissions from the seabed to the water column and atmosphere of the East Siberian shelf. Sediment drilled 52 m down from the sea ice in Buor Khaya Bay, central Laptev Sea revealed unfrozen sediment overlying ice-bonded permafrost. Methane concentrations in the overlying unfrozen sediment were low (mean 20 μM), but higher in the underlying ice-bonded submarine permafrost (mean 380 μM). In contrast, sulfate concentrations were substantially higher in the unfrozen sediment (mean 2.5 mM) than in the underlying submarine permafrost (mean 0.1 mM). Using deduced permafrost degradation rates, we calculate potential mean methane efflux from degrading permafrost of 120 mg m -2 per year at this site. However, a drop of methane concentrations from 190 μM to 19 μM and a concomitant increase of methane δ 13 C from -63‰ to -35‰ directly above the ice-bonded permafrost suggest that methane is effectively oxidized within the overlying unfrozen sediment before it reaches the water column. High rates of methane ebullition into the water column observed elsewhere are thus unlikely to have ice-bonded permafrost as their source.

Description: Inland waters are an important component of the global carbon cycle through transport, storage, and direct emissions of CO 2 and CH 4 to the atmosphere. Despite predictions of high physical gas exchange rates due to turbulent flows and ubiquitous supersaturation of CO 2 —and perhaps also CH 4 —patterns of gas emissions are essentially undocumented for high mountain ecosystems. Much like other headwater networks around the globe, we found that high-elevation streams in Rocky Mountain National Park, U.S.A. were supersaturated with CO 2 during the growing season and were net sources to the atmosphere. CO 2 concentrations in lakes, on the other hand, tended to be less than atmospheric equilibrium during the open water season. CO 2 and CH 4 emissions from the aquatic conduit were relatively small compared to many parts of the globe. Irrespective of the physical template for high gas exchange (high k ), we found evidence of CO 2 source limitation to mountain streams during the growing season, which limits overall CO 2 emissions. Our results suggest a reduced importance of aquatic ecosystems for carbon cycling in high-elevation landscapes having limited soil development and high CO 2 consumption via mineral weathering.

Description: Dissolved organic carbon (DOC) in rivers contains a wide range of molecules that can be assimilated by microbes. However, there is today no integrated understanding of how the source and composition of this DOC regulate the extent to which the DOC can support microbial growth and respiration. We analyzed patterns in microbial metabolism of DOC from different streams and rivers in Québec, by combining short-term bacterial production and respiration measurements with long-term DOC loss and analyses of bacterial use of different single substrates. We show that distinct metabolic patterns indeed exist across catchments, reflecting the varying nature and sources of the DOC. For example, DOC from forest headwaters systematically supported the highest bacterial growth efficiency (BGE) that was recorded, while in contrast DOC in peat bog drainage was used with significantly lower BGE. The carbon consumption in clear mountain rivers, possibly represented by autochthonous algal DOC, supported the highest bacterial respiration rates and the highest long-term DOC losses. By using principle component analysis, we demonstrate how the major axes of variation in all of the measured metabolic responses are tightly connected to spectrofluorometrical DOC composition indicators and to isotopic indicators of DOC source. If causality is assumed, our results imply that changes in DOC supply from different sources, for example caused by land-use or climate change, should result in dramatic changes in the patterns of aquatic microbial metabolism, and thus in altered aquatic ecosystem functioning, with likely consequences for food-web structures and greenhouse gas balances.

Description: The Atacama Desert is the driest and one of the most life-limiting places on Earth. Despite the extreme conditions, microbial endolithic communities have been found inside halite rocks. The presence of these microbial communities is possible due to the hygroscopic properties of evaporitic rocks composed of sodium chloride. It is important to elucidate every possible water source in such a hyper-arid environment. Therefore, in the present study, an artificial neural network (ANN) based model has been designed to predict the presence of liquid water on the surface of halite pinnacles. The model predicts the moisture formation using two basic meteorological variables, air temperature and air relative humidity. ANNs have been successfully employed for the first time as a tool for predicting the appearance of liquid water, a key factor for the endolithic microbial communities living in the driest part of the Atacama Desert. The model developed is able to correctly predict the formation of water on the surface of the halite pinnacles 83% of the cases. We anticipate the future application of this model as an important tool for the prediction of the water availability, and, therefore, potential habitability of lithic substrates in extreme environments on Earth and perhaps elsewhere.

Description: In this study we investigate how the effect of rising atmospheric CO 2 levels on forest productivity is influenced by changes in nutrient availability caused by nitrogen (N) deposition. We used a dual-isotope approach (δ 15 N and δ 13 C), combined with dendrochronological and nutritional analyses, to evaluate the response of two dominant species in natural forest ecosystems near Mexico City ( Pinus hartwegii – pine; Abies religiosa – fir). Our analysis focused on changes that occurred over the past 50 years in two sites, one under high and one under low N deposition rates. Analyses of carbon isotopes indicate increasing water-use efficiency in response to rising CO 2 levels for both species and sites, but that effect did not lead to improved tree growth. The magnitude and direction of shifts in 13 C discrimination indicated a process of acclimation that varied with the rate of N deposition and species traits. Since the 1960s, strong negative responses to N deposition were observed in fir trees, which showed altered foliar nutrition and growth decline, while the negative impacts of N deposition on pine growth remained undetectable until the 1990s. In recent years, both species showed significant growth decline under high N deposition despite increasing atmospheric CO 2 . A multivariate analysis of leaf nutrients indicated that tree growth decline was caused by depleted macro (P, K, Ca) and micro (Cu, Fe, Zn, Mn) nutrient availability in high N deposition sites. At both sites, fir trees were a better indicator of N saturation due to differences in canopy rainfall interception.

Description: Anthropogenic nitrogen fixation and subsequent use of this nitrogen as fertilizer has greatly disturbed the global nitrogen cycle. Rivers are recognized hotspots of nitrogen removal in the landscape as interaction between surface water and sediments creates heterogeneous redox environments conducive for nitrogen transformations. Our understanding of riverbed nitrogen dynamics to date comes mainly from shallow sediments or hyporheic exchange flow pathways with comparatively little attention paid to groundwater-fed, gaining reaches. We have used 15 N techniques to quantify in situ rates of nitrate removal to 1m depth within a groundwater-fed riverbed where subsurface hydrology ranged from strong upwelling to predominantly horizontal water fluxes. We combine these rates with detailed hydrologic measurements to investigate the interplay between biogeochemical activity and water transport in controlling nitrogen attenuation along upwelling flow pathways. Nitrate attenuation occurred via denitrification rather than dissimilatory nitrate reduction to ammonium or anammox (range = 12 to 〉17000 nmol 15 N L −1 h −1 ). Overall, nitrate removal within the upwelling groundwater was controlled by water flux rather than reaction rate (i.e. Damköhler numbers 〈 1) with the exception of two hotspots of biogeochemical activity. Deep sediments were as important a nitrate sink as shallow sediments with fast rates of denitrification and short water residence time close to the riverbed surface balanced by slower rates of denitrification and water flux at depth. Within this permeable riverbed 〉80% of nitrate removal occurs within sediments not exposed to hyporheic exchange flows under baseflow conditions, illustrating the importance of deep sediments as nitrate sinks in upwelling systems.

Description: The dissolved oxygen budget in the upper Delaware Estuary between 1970 and 2014 was inferred from oxygen concentration measurements using a box-model approach. The region was found to be a net biogeochemical sink of oxygen, with net oxygen consumption greater in the tidal-fresh portion than in the oligohaline portion. Net oxygen consumption decreased from the 1970s to the 1990s by roughly a factor of two before increasing slightly in the 2000s. The dramatic decline in oxygen consumption was presumably due to improvements in wastewater treatment, though a comparison with biological oxygen demand measurements in wastewater was equivocal. Non-algal oxygen consumption (i.e., oxygen consumption due to heterotrophic respiration and nitrification) was computed as the sum of the estimated net oxygen consumption and historical measurements of primary production. Non-algal oxygen consumption was found to be highly seasonal and positively correlated with temperature, with Q 10 values ranging between 1.4 and 2.3. Annual non-algal oxygen consumption was found to be several times annual primary production. Exchange with the atmosphere is the main process that balances the net oxygen consumption throughout the study region, with advection also an important process in the tidal-fresh portion. Decadal-scale variability in oxygen concentration, including the recent decline in the 2000s, appears to be mainly driven by biological, not physical, processes.

Description: Key Points Net COS production in wheat field soil and roots scaled linearly by temperature Sterilized wheat field soil demonstrated abiotic COS production mechanism Lab observations of COS fluxes from soil confirmed previous field measurements

Description: Understanding how the sources of surface water change along river networks is an important challenge, with implications for soil-stream interactions, and our ability to predict hydrological and biogeochemical responses to environmental change. Network-scale patterns of stream water reflect distinct hydrological processes among headwater units, as well as variable contributions from deeper groundwater stores, which may vary non-linearly with drainage basin size. Here we explore the spatial variability of groundwater inputs to streams, and the corresponding implications for surface water chemistry, during winter baseflow in a boreal river network. The relative contribution of recent and older groundwater was determined using stable isotopes of water ( 18 O) at 78 locations ranging from small headwaters (0.12 km 2 ) to fourth-order streams (68 km 2 ) in combination with 79 precipitation and 10 deep groundwater samples. Results from a two end-member mixing model indicate that deeper groundwater inputs increased non-linearly with drainage area, ranging from ~20 % in smaller headwater sub-catchments to 70–80 % for catchments with a 10.6 km 2 area or larger. Increases in the groundwater contribution were positively correlated to network-scale patterns in surface stream pH and base cation concentrations, and negatively correlated to dissolved organic carbon (DOC). These trends in chemical variables are consistent with the production of weathering products and the mineralization of organic matter along groundwater flowpaths. Together, the use of stable isotopes and biogeochemical markers illustrate how variation in hydrologic routing and groundwater contributions shape network-scale patterns in stream chemistry as well as patchiness in the relative sensitivity of streams to environmental change and perturbation.

Description: No abstract is available for this article.

Description: Although lateral carbon (C) export from terrestrial to aquatic systems is known to be an important component in landscape C balances, most existing global studies are lacking empirical data on the soil C export. In this study, the concentration, character and export of dissolved organic carbon (DOC) were studied during two years in two hemiboreal headwater streams draining catchments with different soil characteristics (mineral vs. peat soils). The streams exposed surprisingly similar strong air temperature controls on the temporal variability in DOC concentration in spite of draining such different catchments. The temporal variability in DOC character (determined by absorbance metrics, SUVA 254 as a proxy for aromaticity and a254/a365 ratio as a proxy for mean molecular weight) was more complex but related to stream discharge. While the two streams showed similar ranges and patterns in SUVA 254 , we found a significant difference in median a254/a354 suggesting differences in the DOC character. Both streams responded similarly to hydrological changes with higher a254/a365 at higher discharge, although with rather small differences in a254/a365 between base- and high flows (〈0.3). The DOC exports (9.6-25.2 g C m −2 yr −1 ) were among the highest reported so far for Scandinavia and displayed large inter- and intra-annual variability mainly driven by irregular precipitation/discharge patterns. Our results show that air temperature and discharge affect the temporal variability in DOC quantity and character in different ways. This will have implications for the design of representative sampling programs, which in turn will affect the reliability of future estimates of landscape C budgets.

Description: Microbial reduction of Fe(III) can be one of the major factors controlling methane production from anaerobic sedimentary environments, such as paddy soils and wetlands. Although secondary iron mineralization following Fe(III) reduction is a process that occurs naturally over time, it has not yet been considered in methanogenic systems. This study performed a long-term anaerobic incubation of a paddy soil and ferrihydrite-supplemented soil cultures to investigate methanogenesis during ferrihydrite biomineralization. The results revealed that the long-term effect of ferrihydrite on methanogenesis may be enhancement rather than suppression documented in previous studies. During initial microbial ferrihydrite reduction, methanogenesis was suppressed, however, the secondary minerals of magnetite formation was simultaneous with facilitated methanogenesis in terms of average methane production rate and acetate utilization rate. In the phase of magnetite formation, microbial community analysis revealed a strong stimulation of the bacterial Geobacter , Bacillus and Sedimentibacter , and the archaeal Methanosarcina in the ferrihydrite-supplemented cultures. Direct electric syntrophy between Geobacter and Methanosarcina via conductive magnetite is the plausible mechanism for methanogenesis acceleration along with magnetite formation. Our data suggested that a change in iron mineralogy might affect the conversion of anaerobic organic matter to methane, and might provide a fresh perspective on the mitigation of methane emissions from paddy soils by ferric iron fertilization.

Description: Impacts of environmental changes on groundwater carbon cycling are poorly understood despite their potentially high relevance to terrestrial carbon budgets. This study focuses on streambed groundwater chemistry during a period of drought-induced river drying and consequent disconnection between surface water and groundwater. Shallow groundwater underlying vegetated and bare portions of a braided streambed in the Platte River (Nebraska, USA) was monitored during drought conditions in summer 2012. Water temperature and dissolved inorganic carbon (dominated by HCO 3 − ) in streambed groundwater were correlated over a three month period coinciding with a decline in river discharge from 35 to 0 m 3 s −1 . Physical, chemical, and isotopic parameters were monitored to investigate mechanisms affecting the HCO 3 − trend. Equilibrium thermodynamic modeling suggests that an increase of p CO 2 near the water table, coupled with carbonate mineral weathering, can explain the trend. Stronger temporal trends in Ca 2+ and Mg 2+ compared to Cl − are consistent with carbonate mineral re-equilibria rather than evaporative concentration as the primary mechanism of the increased HCO 3 − . Stable isotope trends are not apparent, providing further evidence of thermodynamic controls rather than evaporation from the water table. A combination of increased temperature and O 2 in the dewatered portion of the streambed is the most likely driver of increased p CO 2 near the water table. Results of this study highlight potential linkages between surface environmental changes and groundwater chemistry, and underscore the need for high-resolution chemical monitoring of alluvial groundwater in order to identify environmental change impacts.

Description: Hyporheic flow in aquatic sediment controls solute and heat transport thereby mediating the fate of nutrients and contaminants, dissolved oxygen, and temperature in the hyporheic zone (HZ). We conducted a series of numerical simulations of hyporheic processes within a dune with different uniform temperatures, coupling turbulent open-channel fluid flow, porous fluid flow, and reactive solute transport, to study the temperature dependence of nitrogen source/sink functionality and its efficiency. Two cases were considered: a polluted and a pristine stream. Sensitivity analysis was performed to investigate the influence of stream water [NO 3 − ] /[NH 4 + ]. The simulations showed that in both cases warmer temperatures resulted in shallower denitrification zones and oxic-anoxic zone boundaries, but the trend of net denitrification rate and nitrate removal or production efficiency of the HZ for these two cases differed. For both cases, at high [NO 3 − ] /[NH 4 + ], the HZ functioned as a NO 3 − sink with the nitrate removal efficiency increasing with temperature. But at low [NO 3 − ] /[NH 4 + ] for the polluted stream, the HZ is a NO 3 − sink at low temperature, but then switches to a NO 3 − source at warmer temperatures. For the pristine stream case, the HZ was always a NO 3 − source, with the NO 3 − production efficiency increasing monotonically with temperature. In addition, although the interfacial fluid flux expectedly increased with increasing temperature due to decreasing fluid viscosity, the total nitrate flux into the HZ did not follow this trend. This is because when HZ nitrification is high, uniformly elevated [NO 3 − ] lowers dispersive fluxes into the HZ. We found that there are numerous confounding and interacting factors that combined to lead to the final temperature-dependence of N transformation reaction rates. Although the temperature effect on the rate constant can be considered as the dominant factor, simply using the Arrhenius equation to predict the reaction rate would lead to incomplete insight by ignoring the changes in interfacial fluid and solute fluxes and reaction zone areas. Our study shows that HZ temperature and stream [NO 3 − ] /[NH 4 + ] are key controls for HZ sink/source functions.

Description: We present six and a half years of eddy covariance measurements of fluxes of methane (F CH4 ) and carbon dioxide (F CO2 ) from a flooded rice paddy in Northern California, USA. A pronounced warming trend throughout the study associated with drought and record high temperatures strongly influenced carbon (C) budgets and provided insights into biophysical controls of F CO2 and F CH4 . Wavelet analysis indicated that photosynthesis (GEP) induced the diel pattern in F CH4 , but soil temperature ( T s ) modulated its amplitude. Forward stepwise linear models and neural networking modeling were used to assess the variables regulating seasonal F CH4. As expected due to their competence in modeling non-linear relationships, neural network models explained considerably more of the variance in daily average F CH4 than linear models. During the growing season, GEP and water levels typically explained most of the variance in daily average F CH4 . However, T s explained much of the interannual variability in annual and growing season CH 4 sums. Higher T s also increased the annual and growing season ratio of F CH4 to GEP. The observation that the F CH4 to GEP ratio scales predictably with T s may help improve global estimates of F CH4 from rice agriculture. Additionally, T s strongly influenced ecosystem respiration, resulting in large interannual variability in the net C budget at the paddy, emphasizing the need for long-term measurements particularly under changing climatic conditions.

Description: The upwelling area off North-West Africa is characterized by high export production, high nitrate and low oxygen concentration in bottom waters. The underlying sediment consists of sands that cover most of the continental shelf. Due to their permeability sands allow for fast advective porewater transport and can exhibit high rates of nitrogen (N) loss via denitrification as reported for anthropogenically eutrophied regions. However, N-loss from sands underlying naturally eutrophied waters is not well studied and in particular N-loss from the North-West African shelf is poorly constrained. During two research cruises in April/May 2010/11, sediment was sampled along the North-West African shelf and volumetric denitrification rates were measured in sediment layers down to 8 cm depth using slurry incubations with 15 N-labelled nitrate. Areal N-loss was calculated by integrating volumetric rates down to the nitrate penetration depth derived from porewater profiles. Areal N-loss was neither correlated with water depth nor with bottom-water concentrations of nitrate and oxygen, but was strongly dependent on sediment grain size and permeability. The derived empirical relation between benthic N-loss and grains size suggests that porewater advection is an important regulating parameter for benthic denitrification in sands and further allowed extrapolating rates to an area of 53,000 km 2 using detailed sediment maps. Denitrification from this region amounts to 995 kt per year (average 3.6 mmol m -2 d -1 ) which is 4 times higher than previous estimates based on diffusive porewater transport. Sandy sediments cover 50-60% of the continental shelf and thus may contribute significantly to the global benthic N-loss.

Description: Present-day serpentinization generates groundwaters with conditions (pH 〉 11, Eh 〈 -550 mV) favorable for the microbial and abiotic production of organic compounds from inorganic precursors. Elevated concentrations of methane, C 2 -C 6 alkanes, acetate, and formate were detected at these sites, but the microbial or abiotic origin of these compounds remains unclear. While geochemical data indicate that methane at most sites of present-day serpentinization is abiotic, the stable carbon, hydrogen, and clumped isotope data as well as the hydrocarbon gas composition from The Cedars, CA, USA are consistent with a microbial origin for methane. However, there is no direct evidence of methanogenesis at this site of serpentinization. We report on laboratory experiments in which the microbial communities in fluids and sediments from The Cedars were incubated with 13 C labeled substrates. Increasing methane concentrations and the incorporation of 13 C into methane in live experiments, but not in killed controls, demonstrated that methanogens converted methanol, formate, acetate (methyl group), and bicarbonate to methane. The apparent fractionation between methane and potential substrates (α 13 C CH4-CO2(g)  = 1.059 to 1.105, α 13 C CH4-acetate  = 1.042 to 1.119) indicated that methanogenesis was dominated by the carbonate reduction pathway. Increasing concentrations of volatile organic acid anions indicated microbial acetogenesis. α 13 C CO2(g)-acetate values (0.999 to 1.000), however, were inconsistent with autotrophic acetogenesis, thus suggesting that acetate was produced through fermentation. This is the first study to show direct evidence of microbial methanogenesis and acetogenesis by the native microbial community at a site of present-day serpentinization.

Description: The underlying mechanisms driving the coupled interactions between inorganic nitrogen uptake and dissolved organic matter are not well understood, particularly in surface waters. To determine the relationship between dissolved organic carbon (DOC) quantity and nitrate (NO 3 - ) uptake kinetics in streams, we performed a series of NO 3 - Tracer Additions for Spiraling Curve Characterization (TASCC) experiments in four streams within the Lamprey River Watershed, New Hampshire, across a range in background DOC concentrations (1 – 8 mg C/L). Experiments were performed throughout the 2013 and 2014 growing seasons. Across streams and experimental dates, ambient uptake velocity (V f ) correlated positively with increasing DOC concentrations and DOC:NO 3 - ratios but was only weakly negatively associated with NO 3 - concentrations. Ambient NO 3 - V f was unrelated to pH, light, temperature, dissolved oxygen and SUVA 254 . Although there were general tendencies across the entire Lamprey River Watershed, individual sites behaved differently in their uptake kinetics. NO 3 - uptake dynamics in the Lamprey River Watershed are most strongly influenced by DOC concentrations rather than NO 3 - concentrations or physico-chemical parameters, which have been identified as regional- to continental-scale drivers in previous research. Understanding the fundamental relationships between dissolved organic matter and inorganic nutrients will be important as global and climatic changes influence the delivery and production of DOC and NO 3 - in aquatic ecosystems.

Description: Uncertainty in continental shelf air-sea CO 2 fluxes motivated us to investigate the impact of interannual and seasonal variability in atmospheric forcing on the capacity of three shelf regions along the U.S eastern continental shelf to act as a sink or source of atmospheric CO 2 . Our study uses a coupled biogeochemical-circulation model to simulate scenarios of “present-day” and “future-perturbed” mesoscale forcing variability. Overall, the U.S. eastern continental shelf acts as a sink for atmospheric CO 2 . There is a clear gradient in air-sea CO 2 flux along the shelf region, with estimates ranging from –0.6 Mt C y –1 in the South Atlantic Bight (SAB), to –1.0 Mt C y –1 in the Mid-Atlantic Bight (MAB) and –2.5 Mt C y –1 in the Gulf of Maine (GoM). These fluxes are associated with considerable interannual variability, with the largest interannual signal exhibited in the Gulf of Maine. Seasonal variability in the fluxes is also evident, with autumn and winter being the strongest CO 2 sink periods, and summer months exhibiting some outgassing. In our “future-perturbed” scenario spatial differences tend to cancel each other out when the fluxes are integrated over the MAB and GoM, resulting in only minor differences between “future-perturbed” and “present-day” air-sea CO 2 fluxes. This is not the case in the SAB where the position of the along-shelf gradient shifts northward and the SAB becomes a source of CO 2 to the atmosphere (0.7 Mt C y –1 ) primarily in response to surface warming. Our results highlight the importance of temperature in regulating air-sea CO 2 flux variability.

Description: Quantification and prediction of N 2 O emissions from croplands under different agricultural management practices are vital for sustainable agriculture and climate change mitigation. We simulated N 2 O emissions under tillage and no-tillage, and different nitrogen (N) fertilizer types and application methods (i.e. nitrification inhibitor, chicken manure and split applications) in a cornfield using the DeNitrification-DeComposition (DNDC) model. The model was parameterized with field experimental data collected in Nashville, Tennessee under various agricultural management treatments and run for a short term (3 yrs) and a long term (100 yrs). Results showed that the DNDC model could adequately simulate N 2 O emissions as well as soil properties under different agricultural management practices. The modeled emissions of N 2 O significantly increased by 35% with tillage, and decreased by 24% with the use of nitrification inhibitor, compared with no-tillage and normal N fertilization. Chicken manure amendment and split applications of N fertilizer had minor impact on N 2 O emission in a short term, but over a long term (100 yrs) the treatments significantly altered N 2 O emission (+35%, −10%, respectively). Sensitivity analysis showed that N 2 O emission was sensitive to mean annual precipitation, mean annual temperature, soil organic carbon, and the amount of total N fertilizer application. Our model results provide valuable information for determining agricultural best management practice to maintain highly productive corn yield while reducing greenhouse gas emissions.

Description: A key uncertainty concerning the effect of wildfire on carbon dynamics is the rate at which fire-killed biomass (e.g., dead trees) decays and emits carbon to the atmosphere. We used a ground-based approach to compute decomposition of forest biomass killed, but not combusted, in the Biscuit Fire of 2002, an exceptionally large wildfire that burned over 200,000 ha of mixed conifer forest in southwestern Oregon, USA. A combination of federal inventory data and supplementary ground measurements afforded the estimation of fire-caused mortality and subsequent 10-year decomposition for several functionally distinct carbon pools at 180 independent locations in the burn area. Decomposition was highest for fire-killed leaves and fine roots and lowest for large diameter wood. Decomposition rates varied somewhat among tree species and was only 35% lower for trees still standing than for trees fallen at the time of the fire. We estimate a total of 4.7 Tg C was killed but not combusted in the Biscuit Fire, 85% of which remains 10 years after. Biogenic carbon emissions from fire-killed necromass were estimated to be 1.0, 0.6, and 0.4 Mg C ha -1 yr -1 at 1, 10, and 50 years after the fire, respectively; compared to the one-time pyrogenic emission of nearly 17 Mg C ha -1 .

Description: Northern peatlands store ~500 Pg of carbon (C); however, controls on the spatial distribution of the stored C may differ regionally, owing to the complex interaction among climate, ecosystem processes, and geophysical controls. As a globally significant C sink, elucidation of controls on the distribution of C in the Hudson Bay Lowlands, Canada (HBL) is of particular importance. Although peat age is related to timing of land emergence and peat depth in the HBL, considerable variation in the total C mass (kg m -2 ) among sites of similar peat age suggests that other factors may explain spatial patterns in C storage (Pg) and sequestration. Here, we quantify the role of two key factors in explaining the spatial distribution the C mass in the HBL (n = 364 sites): (i) climate variability, and (ii) peat lithology, for two major peatland classes in the HBL (bogs and fens). We find that temperature, precipitation, and evapotranspiration each explained nearly half of the C mass variability. Regions characterized by warmer and wetter conditions stored the most C as peat. Our results show that bogs and fens store similar amounts of C within a given climate domain, although via distinct storage mechanisms. Namely, fen peats tend to be shallower and more C dense (kg m -3 ) compared to bogs. Following geophysical controls on the timing of peat initiation, our results reveal that both the widespread bog-fen patterning and variability in regional climate contribute to explaining the spatial distribution of the peat C mass in the HBL.

Description: The East China Sea (ECS) and the Southern Yellow Sea (SYS) ecosystem is undergoing dramatic changes, but the spatiotemporal patterns and forcing mechanisms of phytoplankton variations remain understudied. Phytoplankton lipid biomarkers are useful proxies for productivity and community structure changes, and they were measured in suspended particles of more than 81 sites from spring and summer of 2011 in the ECS and SYS. In spring, the concentrations of brassicasterol (4.7-127 ng L -1 ) and dinosterol (0.7-37 ng L -1 ) were markedly higher in the northern and central SYS, while C 37 alkenones (0-15 ng L -1 ) was detected at only seven sites in the ECS. In summer, brassicasterol (25.3-1178 ng L -1 ) and dinosterol (0-125 ng L -1 ) showed high values off the Changjiang River Estuary (CRE), while C 37 alkenones (0-410 ng L -1 ) had high values in the northwest and central SYS. The mean concentrations of the three lipid biomarkers in summer were three to 61 times higher than those in spring. Spatiotemporal patterns of biomarkers reveal higher ratios of diatom/dinoflagellate and diatom/haptophyte in higher productivity areas, off the CRE in summer and the northern and central SYS in spring. This study validates the applicability of brassicasterol, dinosterol and alkenones as proxies of productivity and community structure of the three phytoplankton taxa: diatoms, dinoflagellates and haptophytes. The results indicate that nutrients (in summer) and turbidity induced photosynthetic available radiation (in spring) play important roles in regulating spatiotemporal variations of phytoplankton in the ECS and SYS.

Description: Permafrost dynamics play an important role in high-latitude peatland carbon balance and are key to understanding the future response of soil carbon stocks. Permafrost aggradation can control the magnitude of the carbon feedback in peatlands through effects on peat properties. We compiled peatland plant macrofossil records for the northern permafrost zone (515 cores from 280 sites) and classified samples by vegetation type and environmental class (fen, bog, tundra and boreal permafrost, thawed permafrost). We examined differences in peat properties (bulk density, carbon (C), nitrogen (N) and organic matter content, C/N ratio) and C accumulation rates among vegetation types and environmental classes. Consequences of permafrost aggradation differed between boreal and tundra biomes, including differences in vegetation composition, C/N ratios, and N content. The vegetation composition of tundra permafrost peatlands was similar to permafrost-free fens, while boreal permafrost peatlands more closely resembled permafrost-free bogs. Nitrogen content in boreal permafrost and thawed permafrost peatlands was significantly lower than in permafrost-free bogs despite similar vegetation types (0.9% versus 1.5% N). Median long-term C accumulation rates were higher in fens (23 g C m -2 y -1 ) than in permafrost-free bogs (18 g C m -2 y -1 ), and were lowest in boreal permafrost peatlands (14 g C m -2 y -1 ). The plant macrofossil record demonstrated transitions from fens to bogs to permafrost peatlands, bogs to fens, permafrost aggradation within fens, and permafrost thaw and re-aggradation. Using data synthesis, we've identified predominant peatland successional pathways, changes in vegetation type, peat properties, and C accumulation rates associated with permafrost aggradation.

Description: Terrestrial carbon and water cycles are coupled at multiple spatiotemporal scales and are crucial to carbon sequestration. Water related climate extremes, such as drought and intense precipitation, can substantially affect the carbon cycle. Meanwhile, nitrogen is a limiting resource to plant, and has therefore the potential to alter the coupling of water and carbon cycles on land. Here, we assess the effect of nitrogen limitation on the response of the terrestrial carbon cycle to moisture anomalies using Geophysical Fluid Dynamics Laboratory's land surface model LM3V-N. We analyzed the response of three central carbon fluxes: net primary productivity (NPP), heterotrophic respiration (R h ) and net ecosystem productivity (NEP, the difference between NPP and R h ) and how these fluxes were altered under anomalies of the standardized precipitation and moisture index (SPEI). We found that globally, the correlations between each of the carbon flux and SPEI depended on the timescale and a strong legacy effect of SPEI anomalies on R h . Consideration of nitrogen constraints reduced anomalies in carbon fluxes in response to extreme dry/wet events. This nitrogen-induced buffer constrained the growth of plants under wet extremes and allowed for enhanced growth during droughts. Extra gain of soil moisture from the downregulation of canopy transpiration by nitrogen limitation, and shifts in the relative importance of water and nitrogen limitation during dry/wet extreme events are possible mechanisms contributing to the buffering of modeled NPP and NEP. Responses of R h to moisture anomalies were much weaker compared to NPP, and N buffering effects were less evident.

Description: In many northern temperate regions, the water color of lakes has increased over the past decades (“lake browning”), probably caused by an increased export of dissolved organic matter from soils. We investigated if the increase in water color in two lakes in Norway has resulted in increased burial of organic carbon (OC) and mercury (Hg) in the sediments, and if the Hg was prone to methylation. Lake Solbergvann experienced a 3-fold water color increase, and OC burial increased ~2-fold concomitant to the water color increase. This lake had prolonged periods of anoxic bottom water, and anoxic OC mineralization rates were only about half of the oxic OC mineralization rates (7.7 and 17.5 g C m -2 yr -1 , respectively), contributing to an efficient OC burial. In Lake Elvåga, where water color increase was only ~2-fold and bottom water was oxygenated, no recent increase in OC burial could be observed. Hg burial increased strongly in both lakes (3-fold and 1.6-fold in Lake Solbergvann and Lake Elvåga, respectively), again concomitant to the recent water color increase. The proportion of methylated Hg (MeHg) in surficial sediment was one order of magnitude higher in Lake Elvåga (up to 6% MeHg) than in Lake Solbergvann (0.2-0.6% MeHg), probably related to the different oxygenation regimes. We conclude that lake browning can result in increased OC and Hg burial in lake sediments, but the extent of browning and the dominating mode of sediment respiration (aerobic or anaerobic) strongly affect burial and fate of OC and Hg in sediments.