ALBERT

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: 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: 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: Permafrost soils currently store approximately 1672 Pg of carbon (C), but as high latitudes warm, this temperature-protected C reservoir will become vulnerable to higher rates of decomposition. In recent decades, air temperatures in the high latitudes have warmed more than any other region globally, particularly during the winter. Over the coming century, the arctic winter is also expected to experience the most warming of any region or season, yet it is notably understudied. Here we present non-summer season (NSS) CO 2 flux data from the Carbon in Permafrost Experimental Heating Research (CiPEHR) project, an ecosystem warming experiment of moist acidic tussock tundra in interior Alaska. Our goals were to quantify the relationship between environmental variables and winter CO 2 production, account for subnivean photosynthesis and late fall plant C uptake in our estimate of NSS CO 2 exchange, constrain NSS CO 2 loss estimates using multiple methods of measuring winter CO 2 flux, and quantify the effect of winter soil warming on total NSS CO 2 balance. We measured CO 2 flux using four methods: two chamber techniques (the snow pit method and one where a chamber is left under the snow for the entire season), eddy covariance, and soda lime adsorption, and found that NSS CO 2 loss varied up to 4 fold, depending on the method used. CO 2 production was dependent on soil temperature and day of season but atmospheric pressure and air temperature were also important in explaining CO 2 diffusion out of the soil. Warming stimulated both ecosystem respiration and productivity during the NSS and increased overall CO 2 loss during this period by 14% (this effect varied by year, ranging from 7 to 24%). When combined with the summertime CO 2 fluxes from the same site, our results suggest that this sub-arctic tundra ecosystem is shifting away from its historical function as a C sink to a C source.

Description: Nitrogen (N) is an important nutrient as it often limits productivity, but in excess can impair water quality. Most studies on watershed N cycling have occurred in upland forested catchments where snowmelt dominates N export; fewer studies have focused on low-relief watersheds that lack snow. We examined watershed N cycling in three adjacent, low-relief watersheds in the Upper Coastal Plain of the southeastern United States to better understand the role of hydrological flowpaths and biological transformations of N at the watershed scale. Groundwater was the dominant source of nitrified N to stream water in 2 of the 3 watersheds, while atmospheric deposition comprised 28% of stream water nitrate in one watershed. The greater atmospheric contribution may have been due to the larger stream channel area relative to total watershed area or the dominance of shallow subsurface flowpaths contributing to stream flow in this watershed. There was a positive relationship between temperature and stream water ammonium concentrations and a negative relationship between temperature and stream water nitrate concentrations in each watershed suggesting that N cycling processes (i.e., nitrification, denitrification) varied seasonally. However, there were no clear patterns in the importance of denitrification in different water pools possibly because a variety of factors (i.e., assimilatory uptake, dissimilatory uptake, mixing) affected nitrate concentrations. Together, these results highlight the hydrological and biological controls on N cycling in low-gradient watersheds, and variability in N delivery flowpaths among adjacent watersheds with similar physical characteristics.

Description: Projected changes in the seasonality of hydroclimatic regimes are likely to have important implications for water resources and terrestrial ecosystems in the U.S. Pacific Northwest. The tree-ring record, which has frequently been used to position recent changes in a longer-term context, typically relies on signals embedded in the total ring width of tree rings. Additional climatic inferences at a sub-annual temporal scale can be made using alternative tree-ring metrics such as earlywood and latewood widths and the density of tree-ring latewood. Here, we examine seasonal precipitation and temperature signals embedded in total ring width, earlywood width, adjusted latewood width, and blue intensity chronologies from a network of six Pinus ponderosa sites in and surrounding the upper Columbia River basin of the U.S. Pacific Northwest. We also evaluate the potential for combining multiple tree-ring metrics together in reconstructions of past cool- and warm-season precipitation. The common signal among all metrics and sites is related to warm-season precipitation. Earlywood and latewood widths differ primarily in their sensitivity to conditions in the year prior to growth. Total and earlywood widths from the lowest elevation sites also reflect cool-season moisture. Effective correlation analyses and composite-plus-scale tests suggest that combining multiple tree-ring metrics together may improve reconstructions of warm-season precipitation. For cool-season precipitation, total ring width alone explains more variance than any other individual metric or combination of metrics. The composite-plus-scale tests show that variance-scaled precipitation reconstructions in the upper Columbia River basin may be asymmetric in their ability to capture extreme events.

Description: The forest litter layer lies at the boundary between soil and atmosphere and is a major factor in biogeochemical cycles. While there are several studies on how the litter layer controls soil trace gas emissions, litter emissions itself are less well understood, and it is still unclear how important gases respond to changing temperature and moisture. In order to assess leaf litter gas exchange we conducted laboratory incubation experiments in which the full set of climate relevant gases, i.e. carbon dioxide (CO 2 ), nitrous oxide (N 2 O), methane (CH 4 ), and nitric oxide (NO) coming from deciduous and coniferous leaf litter were measured at 5 temperatures and 7 moisture contents. In addition, we compared litter and soil from different origin in terms of temperature / moisture responses of gas fluxes and investigated possible interactions between the two climate factors. Deciduous litter emitted more CO 2 (up to 335 mg CO 2 -C kg -1 h -1 ) than coniferous litter, whereas coniferous litter released maximum amounts of NO (207 µg NO-N kg -1 h -1 ). N 2 O was only emitted from litter under very moist and warm conditions (〉70 %wet weight, 〉10 °C). CH 4 emissions were close to zero. Temperature sensitivities of litter emissions were generally lower than for soil emissions. Nevertheless, wet and warm conditions always enhanced litter emissions, suggesting a strong feedback effect of the litter layer to predicted future climate change.

Description: Testing complex land surface models has often proceeded by asking the question: does the model prediction agree with the observation? Such an approach has yet led to high-performance terrestrial models that meet the challenges of climate and ecological studies. Here we test the Community Land Model (CLM) by asking the question: does the model behave like an ecosystem? We pursue its answer by testing CLM in the ecosystem functional space (EFS) at the Missouri Ozark AmeriFlux (MOFLUX) forest site in the central USA, focusing on carbon and water flux responses to precipitation regimes and associated stresses. In the observed EFS, precipitation regimes and associated water and heat stresses controlled seasonal and interannual variations of net ecosystem exchange (NEE) of CO 2 and evapotranspiration in this deciduous forest ecosystem. Such controls were exerted more strongly by precipitation variability than by the total precipitation amount per se. A few simply constructed climate variability indices captured these controls, suggesting a high degree of potential predictability. While the interannual fluctuation in NEE was large, a net carbon sink was maintained even during an extreme drought year. Although CLM predicted seasonal and interanual variations in evapotranspiration reasonably well, its predictions of net carbon uptake were too small across the observed range of climate variability. Also, the model systematically underestimated the sensitivities of NEE and evapotranspiration to climate variability and overestimated the coupling strength between carbon and water fluxes. We suspect that the modeled and observed trajectories of ecosystem fluxes did not overlap in the EFS and the model did not behave like the ecosystem it attempted to simulate. A definitive conclusion will require comprehensive parameter and structural sensitivity tests in a rigorous mathematical framework. We suggest that future model improvements should focus on better representation and parameterization of process responses to environmental stresses and on more complete and robust representations of carbon-specific processes so that adequate responses to climate variability and a proper degree of coupling between carbon and water exchanges are captured.

Description: Exchange between wetland surface water and the atmosphere is driven by a variety of motions, ranging from rainfall impact to thermal convection and animal locomotion. Here, we examine the effect of wind-driven vegetation movement. Wind causes the stems of emergent vegetation to wave back and forth, stirring the water column and facilitating air-water exchange. To understand the magnitude of this effect, a gas transfer velocity ( k 600 -value) was measured via laboratory experiments. Vegetation-waving was studied in isolation by mechanically forcing a model canopy to oscillate at a range of frequencies and amplitudes matching those found in the field. The results show that stirring due to vegetation-waving produces k 600 -values from 0.55 cm/hr to 1.60 cm/hr. The dependence of k 600 on waving amplitude and frequency are evident from the laboratory data. These results indicate that vegetation-waving has a non-negligible effect on gas transport; thus it can contribute to a mechanistic understanding of the fluxes underpinning biogeochemical processes.

Description: We partitioned the soil carbon dioxide flux (R s ) into its respective autotrophic and heterotrophic components in a mature temperate-boreal forest (Howland Forest in Maine, USA). We combined automated chamber measurements of R s with two different partitioning methods: (1) a classic root trenching experiment and (2) a radiocarbon ( 14 C) mass balance approach. With a model-data fusion approach, we used these data to constrain a parsimonious ecosystem model (FöBAAR), and we investigated differences in modeled C fluxes and pools under both current and future climate scenarios. The trenching experiment indicated that heterotrophic respiration accounted for 53 ± 11% of total R s . In comparison, using the 14 C method, the heterotrophic contribution was 42 ± 9%. For both current and future model runs, incorporating the partitioning data as constraints substantially reduced the uncertainties of autotrophic and heterotrophic respiration fluxes. Moreover, with best-fit model parameters, the two partitioning methods yielded fundamentally different estimates of the relative contributions of autotrophic and heterotrophic respiration to total R s , especially at the annual time scale. Surprisingly, however, modeled soil C and biomass C pool size trajectories did not differ significantly between model runs based on the different methods. Instead, model differences in partitioning were compensated for by changes in C allocation, resulting in similar, but still highly uncertain, soil C pool trajectories. Our findings show that incorporating constraints on the partitioning of R s can reduce model uncertainties of fluxes but not pools, and the results are sensitive to the partitioning method used.

Description: Uncertainty surrounding future climate makes it difficult to have confidence that current nutrient management strategies will remain effective. This study used monitoring and modelling to assess current effectiveness (% phosphorus reduction), and resilience (defined as continued effectiveness under a changing climate) of best management practices (BMPs) within 5 catchments of the Lake Simcoe watershed, Ontario. The model INCA-P was used, and monitoring data used to calibrate and validate a series of management scenarios. To assess current BMP effectiveness, models were run over a baseline period 1985-2014 with and without management scenarios. Climate simulations were run (2070-2099), and BMP resilience calculated as the % change in effectiveness between the baseline and future period. Results demonstrated that livestock removal from water courses was the most effective BMP, while manure storage adjustments were the least. Effectiveness varied between catchments, influenced by the dominant hydrological and nutrient transport pathways. Resilience of individual BMPs was associated with catchment sensitivity to climate change. BMPs were most resilient in catchments with high soil water storage capacity, and small projected changes in frozen-water availability and in soil moisture deficits. Conversely, BMPs were less resilient in catchments with larger changes in spring melt magnitude and in overland flow proportions. Results indicated that BMPs implemented are not always those most suited to catchment flow pathways, and a more site-specific approach would enhance prospects for maintaining P reduction targets. Furthermore, BMP resilience to climate change can be predicted from catchment physical properties and present day hydrochemical sensitivity to climate forcing.

Description: Biogeochemical processes driving the spatial variability of soil CO 2 production and flux are well studied, but little is known about the variability in the spatial distribution of the stable carbon isotopes that make up soil CO 2 , particularly in complex terrain. Spatial differences in stable isotopes of soil CO 2 could indicate fundamental differences in isotopic fractionation at the landscape level, and may be useful to inform modeling of carbon cycling over large areas. We measured the spatial and seasonal variability of the δ 13 C of soil CO 2 (δ S ) and the δ 13 C of soil CO 2 flux (δ P ) in a subalpine forest ecosystem located in the Rocky Mountains of Montana. We found consistently more isotopically depleted values of δ S and δ P in low and wet areas of the landscape relative to steep and dry areas. Our results suggest that the spatial patterns of δ S and δ P are strongly mediated by soil water and soil respiration rate. More interestingly, our analysis revealed different temporal trends in δ P across the landscape; in high landscape positions δ P became more positive whereas in low landscape positions δ P became more negative with time. These trends might be the result of differential dynamics in the seasonality of soil moisture and its effects on soil CO 2 production and flux. Our results suggest concomitant yet independent effects of water on physical (soil gas diffusivity) and biological (photosynthetic discrimination) processes that mediate δ S and δ P , and are important when evaluating the δ 13 C of CO 2 exchanged between soils and the atmosphere in complex terrain.

Description: The extent to which atmospheric nitrogen (N) deposition reflects land use differences and biogenic vs. fossil fuel reactive N sources remains unclear, yet represents a critical uncertainty in ecosystem N budgets. We compared N concentrations and isotopes in precipitation-event bulk (wet + dry) deposition across nearby valleys in northern Utah with contrasting land use (highly urban vs. intensive agriculture/low-density urban). We predicted greater nitrate (NO 3 - ) vs. ammonium (NH 4 + ) and higher δ 15 N of NO 3 - and NH 4 + in urban valley sites. Contrary to expectations, annual N deposition (3.5–5.1 kg N ha -1 y -1 ) and inorganic N concentrations were similar within and between valleys. Significant summertime decreases in δ 15 N of NO 3 - possibly reflected increasing biogenic emissions in the agricultural valley. Organic N was a relatively minor component of deposition (~13%). Nearby paired wildland sites had similar bulk deposition N concentrations as the urban and agricultural sites. Weighted bulk deposition δ 15 N was similar to natural ecosystems (-0.6 ± 0.7‰). Fine atmospheric particulate matter (PM 2.5 ) had consistently high values of bulk δ 15 N (15.6 ± 1.4‰), δ 15 N in NH 4 + (22.5 ± 1.6‰), and NO 3 - (8.8 ± 0.7‰), consistent with equilibrium fractionation with gaseous species. δ 15 N in bulk deposition NH 4 + varied by more than 40‰, and spatial variation in δ 15 N within storms exceeded 10‰. Sporadically high values of δ 15 N were thus consistent with increased particulate N contributions as well as potential N source variation. Despite large differences in reactive N sources, urban and agricultural landscapes are not always strongly reflected in the composition and fluxes of local N deposition—an important consideration for regional-scale ecosystem models.

Description: The magnitude of net soil nitrous oxide (N 2 O) production from a snow covered catchment in a northern temperate forest was investigated. There was considerable net soil N 2 O-N production and consumption through the snowpack, ranging from -6.6 to 26.2 g-N ha-1 d-1. There was no difference in net N 2 O production among topographic positions despite significant variation in soil moisture, reduction-oxidation conditions and pore water dissolved organic carbon and nitrate. Soil temperatures did not vary among topographic positions, suggesting that temperatures at or above the freezing point allow N 2 O production to proceed under the snowpack. Redox conditions were lower at wetland positions compared to lowlands and uplands, suggesting that the biogeochemical pathway of N 2 O production varies with topography. Over the entire non-growing season, 1.5 kg of N 2 O-N was exported to the atmosphere from the 6.33 ha catchment, representing 31% of the growing season N 2 O-N production. These results suggest winter is an active time for gaseous N production in these forests, and that N 2 O production under the snowpack represents an often unmonitored flux of N from catchments.

Description: We analyzed twenty years (1993-2013) of observations of dissolved inorganic macronutrients (nitrate, N; phosphate, P; silicate, Si) and chlorophyll a (Chl) at Palmer Station, Antarctica (64.8°S, 64.1°W) to elucidate how large-scale climate and local physical forcing affect the interannual variability in the seasonal phytoplankton bloom and associated drawdown of nutrients. The leading modes of nutrients (N, P, and Si Empirical Orthogonal Functions 1, EOF1) represent overall negative anomalies throughout growing seasons, showing a mixed signal of variability in the initial levels and drawdown thereafter (low-frequency dynamics). The second most common seasonal patterns of nitrate and phosphate (N, P EOF2) capture prolonged drawdown events during December-March, which are correlated to Chl EOF1. Si EOF2 captures a drawdown event during November-December, which is correlated to Chl EOF2. These different drawdown patterns are shaped by different sets of physical and climate forcing mechanisms. N and P drawdown events during December-March are influenced by the winter and spring Southern Annular Mode (SAM) phase, where nutrient utilization is enhanced in a stabilized upper water column as a consequence of SAM-driven winter sea ice and spring wind dynamics. Si drawdown during November-December is influenced by early sea ice retreat, where ice breakup may induce abrupt water column stratification and a subsequent diatom bloom or release of diatom cells from within the sea ice. Our findings underscore that seasonal nutrient dynamics in the coastal WAP are coupled to large-scale climate forcing and related physics, understanding of which may enable improved projections of biogeochemical responses to climate change.

Description: Ecosystem models often perform poorly in reproducing interannual variability in carbon and water fluxes, resulting in considerable uncertainty when estimating the land-carbon sink. While many aggregated variables (growing season length, seasonal precipitation or temperature) have been suggested as predictors for interannual variability in carbon fluxes, their explanatory power is limited and uncertainties remain as to their relative contributions. Recent results show that the annual count of hours where evapotranspiration (ET) is larger than its 95th percentile is strongly correlated with the annual variability of ET and gross primary production (GPP) in an ecosystem model. This suggests that the occurrence of favorable conditions has a strong influence on the annual carbon budget. Here, we analyzed data from 8 forest sites of the AmeriFlux network with at least 7 years of continuous measurements. We show that for ET and the carbon fluxes GPP, ecosystem respiration (RE), and net ecosystem productivity, counting the “most active hours/days” (i.e., hours/days when the flux exceeds a high percentile) correlates well with the respective annual sums, with correlation coefficients generally larger than 0.8. Phenological transitions have much weaker explanatory power. By exploiting the relationship between most active hours and interannual variability, we classify hours as most active or less active and largely explain interannual variability in ET and carbon fluxes, particularly for GPP and RE. Our results suggest that a better understanding and modeling of the occurrence of large values in high-frequency ecosystem fluxes will result in a better understanding of interannual variability of these fluxes.

Description: Radiocarbon in CO 2 ( 14 CO 2 ) measurements can aid in discriminating between fast (〈1 year) and slower (〉5-10 years) cycling of C between the atmosphere and the terrestrial biosphere due to the 14 C disequilibrium between atmospheric and terrestrial C. However, 14 CO 2 in the atmosphere is typically much more strongly impacted by fossil fuel emissions of CO 2 , and, thus, observations often provide little additional constraints on respiratory flux estimates at regional scales. Here, we describe a dataset of 14 CO 2 observations from a tall tower in northern Wisconsin (USA) where fossil fuel influence is far enough removed that, during the summer months, the biospheric component of the 14 CO 2 budget dominates. We find that the terrestrial biosphere is responsible for a significant contribution to 14 CO 2 that is 2-3 times higher than predicted by the CASA terrestrial ecosystem model for observations made in 2010. This likely includes a substantial contribution from the North American Boreal ecoregion, but transported biospheric emissions from outside the model domain cannot be ruled out. The 14 CO 2 enhancement also appears somewhat decreased in observations made over subsequent years, suggesting that 2010 may be anomalous. With these caveats acknowledged, we discuss the implications of the observation/model comparison in terms of possible systematic biases in the model vs short-term anomalies in the observations. Going forward, this isotopic signal could be exploited as an important indicator to better constrain both the long-term carbon balance of terrestrial ecosystems and the short-term impact of disturbance-based loss of carbon to the atmosphere.

Description: The prolonged Millennium drought in southeast Australia (2001–2009) provides a unique opportunity to analyze the responses of a semi-arid ecosystem to severe droughts. In this paper, we analyzed vegetation dynamics in the Millennium drought using visible/infrared observations, passive microwave observations, and a simple ecohydrological model. The satellite observations indicated that the ecosystem maintained its greenness in the Millennium drought, although the total aboveground biomass was significantly decreased by water scarcity. The results of our numerical experiments suggested that the resilience of vegetation greenness to the drought could be explained by a carbon allocation strategy that was sensitive to light and water availability and by temporal changes in vegetation traits. Our numerical experiments successfully simulated the decrease of total aboveground biomass with unchanged vegetation greenness during the Millennium drought.

Description: To test the hypothesis that particle composition has a stronger influence on the community structure of particle-attached than free-living bacteria, elemental (C/N, δ 13 C, and δ 15 N) and chemical composition of particles and the size-fractionated bacterial community composition were examined along the particle density gradient from the Pearl River to the open basin in the South China Sea. Microbial communities were collected at the three size fractions of 0.2–0.8, 0.8–3, and 〉3 µm and the community composition was analyzed using high-throughput sequencing of the 16S rRNA gene (V3–V4 regions). Multivariate analysis evaluating the similarities of bacterial community composition and chemical composition of particles revealed their general consistent spatial variations along the particle density gradient from freshwater to the sea basin. However, compositions of particulate organic matter were more strongly related to the free-living than to the particle-attached bacterial community composition, which was composed of relatively abundant anaerobic bacteria and those taxa preferring low-oxygen conditions. This latter result might be caused by low-oxygen micro-zones in particles. Community network models further revealed tighter interactions within the particle-attached than in free-living bacterial communities, suggesting that a relatively confined space in particles is more favorable for interactions within the community. These analyses indicated that particle micro-niche properties might be important in shaping particle-attached community structure. In contrast, particulate organic matter compositions significantly influenced the free-living bacterial community probably through the release of labile or semi-labile organic matter from particles contributing to the bioavailability of dissolved organic carbon.

Description: No abstract is available for this article.

Description: Knowledge of what controls the activity of subsurface microbial communities is critical for assessing and managing biogenic methane resources. In this study, 19 formation water and 5 gas samples were collected at depths of 800 to 1900 m from Quaternary biogenic gas fields of the Qaidam Basin, China. The formation waters were brines with chloride (Cl) concentrations from 1200 to 2700 mM. Bacterial 16S rRNA gene copies ranged from 3.75 × 10 4 to 2.23 × 10 6 copies mL -1 of water, and those of archaea ranged from 2.44 × 10 3 to 4.66 × 10 7 copies mL -1 of water. Both bacterial and archaea 16s rRNA gene copies were negatively correlated with Cl concentration. The microbial community structure differed significantly depending on Cl concentrations. At high Cl waters (〉1800 mM), the microbial community showed a halophilic signature made up of several abundant taxonomic groups within Firmicules , γ-Proteobacteria and methylotrophic Methanosarcinales. At low Cl, Firmicules and hydrogenotrophic methanogens were dominant members. The proportion of inferred hydrogenotrophic methanogens decreased from 89% to 14% of total archaeal reads with increasing Cl concentration; in contrast, methylotrophic species increased from 11% to 85%. Given that the proportion of hydrogenotrophic species was positively correlated with the archaeal gene abundances, we suggest that Cl concentrations primarily constrain the activity of archaea catalyzing H 2 reduction of CO 2 . Our results show that dilution of formation waters is critical in the process of biogenic gas formation, suggesting that an engineered decrease in Cl concentrations may induce methanogenesis as a potential method to increase gas reserves in such areas in the future.

Description: Land surface phenology (LSP) in the Sahara Desert is poorly understood due to the difficulty in detecting subtle variations in vegetation greenness. This study examined the spatial and temporal patterns of LSP and its responses to rainfall seasonality in the Sahara Desert. We first generated daily two-band Enhanced Vegetation Index (EVI2) from half-hourly observations acquired by the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on board the METEOSAT Second Generation series of geostationary satellites from 2006 to 2012. The EVI2 time series was used to retrieve LSP based on the Hybrid Piecewise Logistic Model. We further investigated the associations of spatial and temporal patterns in LSP with those in rainfall seasonality derived from the daily rainfall time series of the Tropical Rainfall Measurement Mission. Results show that the spatial shifts in the start of the vegetation growing season generally follows the rainy season onset that is controlled by the summer rainfall regime in the southern Sahara Desert. In contrast, the end of the growing season significantly lags the end of the rainy season without any significant dependence. Vegetation growing season can unfold during the dry seasons after onset is triggered during rainy seasons. Vegetation growing season can be as long as 300 days or more in some areas and years. However, the EVI2 amplitude and accumulation across the Sahara region was very low indicating sparse vegetation as expected in desert regions. EVI2 amplitude and accumulated EVI2 strongly depended on rainfall received during the growing season and the preceding dormancy period.

Description: CH 4 is the second largest contributor to human-induced global warming. However, large uncertainties still exist regarding the magnitude and temporal variation of CH 4 exchanges in China's natural ecosystems, especially under climate changes. In this study, we assessed its uncertainty and temporal variation during 1979–2012, by integrating a biogeochemical model, extensive in situ measurements, and various sources of wetland maps. Uncertainty analyses suggested that previous studies might have underestimated CH 4 emissions, primarily due to bias in wetland extents in NE China. After that, 1 km resolution wetland maps were used to drive the model, together with a 0.1° resolution climate dataset. The model showed that China's natural wetlands emitted 4.56 ± 1.24 Tg CH 4 yr −1 during the 1980s, which decreased to 3.86 ± 1.09 Tg CH 4 yr −1 in the 2000s, mainly due to wetland drainage in NE China. However, recent glacier-melt-induced wetland expansion has enhanced CH 4 emissions by 28% on the Tibetan Plateau since the 1980s. The magnitude of CH 4 uptake by the natural ecosystems has remained relatively stable, e.g., −2.57 ± 0.18 and −2.70 ± 0.19 Tg CH 4 yr −1 in the 1980s and 2000s, respectively. In summary, the net CH 4 balance of China's natural ecosystems has shown a decreasing pattern, i.e., 1.99 ± 1.42 and 1.16 ± 1.28 Tg CH 4 yr −1 in the 1980s and 2000s, respectively, despite distinct regional differences between NE China and the Tibetan Plateau. Furthermore, this study emphasizes the correct representation of wetland extent and its dynamics, i.e., wetland drainage in populated regions and wetland expansion in glacier-fed regions, in driving the decadal CH 4 exchange magnitude.

Description: Knowledge of nitrogen (N) and phosphorus (P) stoichiometry is essential for understanding biogeochemical cycle and ecosystem functioning. However, large-scale patterns in soil stoichiometry are not yet fully understood along environmental gradients nor over the temporal scale. Using a comprehensive dataset and artificial neural network approach (ANN), we evaluated spatial and temporal patterns in topsoil N and P concentrations and N:P ratio across China's forests. Our results revealed that soil weathering stage, climatic factors ( i.e., temperature and precipitation) and forest types jointly explained approximately 34.1% and 30.4% of spatial variation in soil N and P, respectively. By contrast, only precipitation could explain the variation in N:P ratio, with soil N:P ratio exhibiting a trend of increase along theprecipitation gradient. The observed spatial patterns in soil N:P ratio were consistent with previous findings derived from plants and microbes, suggesting that variation in precipitation may induce the imbalance of N:P stoichiometry in forest ecosystems. Our results also indicated that topsoil N:P ratios exhibited a significant increase from the 1980s to 2000s. However, the associations of N:P dynamics with a single element largely depended on forest type. In evergreen forests, soil N:P dynamics were caused by increasing N and decreasing P. Conversely, N:P changes in deciduous broadleaf forests were triggered only by soil N accumulation. Overall, these results demonstrated a stoichiometric shift in soil N:P both spatially and temporally, implying that nutrient imbalance between soil N and P may be accelerated under global change scenarios.

Description: No abstract is available for this article.

Description: Clouds scatter direct solar radiation, generating diffuse radiation and altering the ratio of direct to diffuse light. If diffuse light increases plant canopy CO 2 uptake, clouds may indirectly influence climate by altering the terrestrial carbon cycle. However, past research primarily uses proxies or qualitative categories of clouds to connect the effect of diffuse light on CO 2 uptake to sky conditions. We mechanistically link and quantify effects of cloud optical thickness ( τ c ) to surface light and plant canopy CO 2 uptake by comparing satellite retrievals of τ c to ground-based measurements of diffuse and total photosynthetically active radiation (PAR; 400–700 nm) and gross primary production (GPP) in forests and croplands. Overall, total PAR decreased with τ c , while diffuse PAR increased until an average τ c of 6.8 and decreased with larger τ c . When diffuse PAR increased with τ c , 7-24% of variation in diffuse PAR was explained by τ c . Light use efficiency (LUE) in this range increased 0.001-0.002 per unit increase in τ c . Although τ c explained 10-20% of the variation in LUE, there was no significant relationship between τ c and GPP ( p  〉 0.05) when diffuse PAR increased. We conclude that diffuse PAR increases under a narrow range of optically thin clouds and the dominant effect of clouds is to reduce total plant-available PAR. This decrease in total PAR offsets the increase in LUE under increasing diffuse PAR, providing evidence that changes within this range of low cloud optical thickness are unlikely to alter the magnitude of terrestrial CO 2 fluxes.

Description: The effect of surface water movement on methane emissions is not explicitly considered in most of the current methane models. In this study, a surface water routing was coupled into our previously developed large-scale methane model. The revised methane model was then used to simulate global methane emissions during 2006-2010. From our simulations, the global mean annual maximum inundation extent is 10.6 ± 1.9 km 2 and the methane emission is 297 ± 11 Tg C/yr in the study period. In comparison to the currently used TOPMODEL-based approach, we found that the incorporation of surface water routing leads to 24.7% increase in the annual maximum inundation extent and 30.8% increase in the methane emissions at the global scale for the study period, respectively. The effect of surface water transport on methane emissions varies in different regions: (1) the largest difference occurs in flat and moist regions, such as Eastern China; (2) high-latitude regions, hotspots in methane emissions, show a small increase in both inundation extent and methane emissions with the consideration of surface water movement; and (3) in arid regions, the new model yields significantly larger maximum flooded areas and a relatively small increase in the methane emissions. Although surface water is a small component in the terrestrial water balance, it plays an important role in determining inundation extent and methane emissions, especially in flat regions. This study indicates that future quantification of methane emissions shall consider the effects of surface water transport.

Description: Anaerobic ammonium oxidation (anammox) is a major microbial pathway for nitrogen (N) removal in estuarine and coastal environments. However, understanding of anammox bacterial dynamics and associations with anammox activity remains scarce along estuarine salinity gradient. In this study, the diversity, abundance, and activity of anammox bacteria, and their potential contributions to total N 2 production in the sediments along the salinity gradient (0.1-33.8) of the Yangtze estuarine and coastal zone were studied using 16S rRNA gene clone library, quantitative PCR assay, and isotope-tracing technique. Phylogenetic analysis showed a significant change in anammox bacterial community structure along the salinity gradient ( P  〈 0.01), with the dominant genus shifting from Brocadia in the freshwater region to Scalindua in the open ocean. Anammox bacterial abundance ranged from 3.67 × 10 5 to 8.22 × 10 7 copies 16S rRNA gene g -1 and related significantly with salinity ( P  〈 0.05). The anammox activity varied between 0.08 and 6.46 nmol N g -1 h -1 and related closely with anammox bacterial abundance ( P  〈 0.01). Contributions of anammox activity to total N loss were highly variable along the salinity gradient, ranging from 5-77%, and were significantly negatively correlated with salinity ( P  〈 0.01). Sediment organic matter was also recognized as an important factor in controlling the relative role of anammox to total N 2 production in the Yangtze estuarine and coastal zone. Overall, our data demonstrated a biogeographical distribution of anammox bacterial diversity, abundance, and activity along the estuarine salinity gradient, and suggested that salinity is a major environmental control on anammox process in the estuarine and coastal ecosystems.

Description: The controls on methane (CH 4 ) bubbling (ebullition) from peatlands are uncertain, but evidence suggests that physical factors related to gas transport and storage within the peat matrix are important. Variability in peat pore size and the permeability of layers within peat can produce ebullition that ranges from steady to erratic in time, and can affect the degree to which CH 4 bubbles bypass consumption by methanotrophic bacteria and enter the atmosphere. Here we investigate the role of peat structure on ebullition in structurally different peats using a physical model that replicates bubble production using air injection into peat. We find that the frequency distributions of number of ebullition events per time and the magnitude of bubble loss from the physical model were similar in shape to ebullition from peatlands and incubated peats. This indicates that the physical model could be a valid proxy for naturally occurring ebullition from peat. For the first time, data on bubble sizes from peat were collected to conceptualize ebullition, and we find that peat structure affects bubble sizes. Using a new method to measure peat macro structure, we collected evidence that supports the hypothesis that structural differences in peat determine if bubble release is steady or erratic and extreme. Collected pore size data suggests that erratic ebullition occurs when large amounts of gas stored at depth easily move through shallower layers of open peat. In contrast, steady ebullition occurs when dense shallower layers of peat regulate the flow of gas emitted from peat.

Description: Forest soils are considered a methane (CH 4 ) sink because dry soils can oxidize CH 4 ; however, previous studies on CH 4 fluxes in humid temperate forests indicated a high spatial and temporal variability in CH 4 fluxes, especially in CH 4 emissions from wet soils close to riparian zones, which can turn the soil of a whole forest from a CH 4 sink to a CH 4 source. In this study, the spatial and temporal variability of soil CH 4 fluxes was investigated in a Japanese coniferous forest, including a riparian wetland and a hillslope water-unsaturated forest floor, based on multipoint flux measurements using laser-based CH 4 analyzers over a period of two years. We identified CH 4 emission hotspots (60.2 ± 169.1 nmol m -2 s -1 from 117 sampling points) in the wetland in late summer, while the CH 4 absorption rate in the forest floor was comparatively lower (−1.2 ± 1.4 nmol m -2 s -1 from 119 sampling points). The temporal variability of watershed-scale CH 4 flux was amplified by a clear seasonal cycle of soil temperature and rainfall pattern under the Asian monsoon climate. The watershed-scale CH 4 budget showed that the forest turned into a CH 4 source during the summer owing to the high and variable CH 4 emissions from the riparian wetland and the lower part of the hillslope. Overall, our results indicated that CH 4 emissions from small riparian areas are important in controlling forest CH 4 dynamics at a watershed scale.

Description: Atmospheric wet nitrogen (N) and phosphorus (P) depositions are important sources of bioavailable N and P, and the input of N and P and their ratios significantly influence nutrient availability and balance in terrestrial as well as aquatic ecosystems. Here we monitored atmospheric P depositions by measuring monthly dissolved P concentration in rainfall at 41 field stations in China. Average deposition fluxes of N and P were 13.69 ± 8.69 kg N ha –1 a –1 (our previous study) and 0.21 ± 0.17 kg P ha –1 a –1 , respectively. Central and southern China had higher N and P deposition rates than northwest China, northeast China, Inner Mongolia, or Qinghai-Tibet. Atmospheric N and P depositions showed strong seasonal patterns and were dependent upon seasonal precipitation. Fertilizer and energy consumption were significantly correlated with N deposition but less correlated with P deposition. The N:P ratios of atmospheric wet deposition (with the average of 77 ± 40, by mass), were negatively correlated with current soil N:P ratios in different ecological regions, suggesting that the imbalanced atmospheric N and P deposition will alter nutrient availability and strengthen P limitation, which may further influence the structure and function of terrestrial ecosystems. The findings provide the assessments of both wet N and P deposition and their N:P ratio across China and indicate potential for strong impacts of atmospheric deposition on broad range of terrestrial ecosystems.

Description: To address the link between the composition and decomposition of freshwater dissolved organic matter (DOM), we manipulated the DOM from three boreal lakes using pre-incubations with UV light to cleave large aromatic molecules, and polyvinylpyrrolidone (PVP) to remove colored phenolic compounds. Subsequently, we monitored the dissolved organic carbon (DOC) loss over 4 months of microbial degradation in the dark to assess how compositional changes in DOM affected different aspects of the reactivity continuum, including the distribution of the apparent decay coefficients. We observed profound effects on decomposition kinetics, with pronounced shifts in the relative share of rapidly and more slowly decomposing fractions of the DOM. In the UV-exposed treatment initial apparent decay coefficient k 0 was almost three-fold higher than in the control. Significantly higher relative DOC loss in the UV-exposed treatment was sustained for two months of incubation, after which decay coefficients converged with those in the control. The PVP removed compounds with absorbance and fluorescence characteristics representative of aromatic compounds, which lead to slower decomposition, compared to that in the control. Our results demonstrate the reactivity continuum underlying the decomposition of DOM in freshwaters and highlight the importance of intrinsic properties of DOM in determining its decomposition kinetics.

Description: Living (rose Bengal stained) benthic foraminifera were investigated on surface sediments from 23 stations from the river-dominated north-western Portuguese margin. Samples were collected in March 2011, following the period of the maximum rainfall over the Iberian Peninsula, between 20 and 2000 m water depth along five cross-margin transects. Four of them are located off the Douro, Mondego, Tagus and Sado rivers and one off the Estremadura coast. The major objectives of this study are 1) to assess the impact of organic matter of various origin and quality on the benthic foraminifera and 2) to investigate the spatial differences of faunal distribution from coastal waters to the deep sea under river influences. To do this, sedimentological and biogeochemical characteristics of the sediments were identified by measuring grain size, oxygen penetration depth (OPD), total organic carbon (TOC) content, stable carbon isotopic composition of TOC (δ 13 C TOC ) and concentration of pigments and amino acids. Based on the principal component (PCA) and cluster analyses of the environmental data, three major geographical groups are identified: (1) deepstations, (2) coastal and mid-slopestations, and (3) shelf stations under river influence.At the deepest stations, species are associated with high organic matter (OM) quantity but low OM quality, where Uvigerina mediterranea , Hoeglundina elegans and agglutinated species such as Reophax scorpiurus or Bigenerina nodosaria are dominant. All stations off the Sado River, which is the most affected area by the anthropogenic influence, are also characterized by high quantity but low quality of OM with the minimum faunal density and diversity within the study area. Mid-slope stations are associated with low OM content and coarse sediments (Q 50 ) with the predominance of N. scaphum .Shallow shelf stations close to the Douro and Tagus river mouths show a dominance of taxa (e.g. Ammonia beccarii , Bulimina aculeata , Eggerelloides scaber , Nonion scaphum , Cancris auriculus and Quinqueloculina seminula ) adapted to environments characterized by high OM quality (high fresh chlorophyll (Chl-a/Phaeo) and available amino acids (EHAA/THAA)).The Biotic and Environmental linking (BIOENV) analysis suggests that the benthic foraminiferal distribution is mostly controlled by three environmental parameters, i.e. TOC (quantity), EHAA/THAA (quality), and δ 13 C TOC (source). Hence, this study clearly highlights that the quantitative and qualitative inputs of OM and its source are the most important factors controlling the living benthic foraminiferal distribution with clear influences between the different rivers. This study also suggests a good tolerance of several species for river discharges where the OM quality is high.

Description: Woodland encroachment into grasslands is a globally pervasive phenomenon attributed to land use change, fire suppression, and climate change. This vegetation shift impacts ecosystem services such as ground water allocation, carbon (C) and nutrient status of soils, above- and belowground biodiversity, and soil structure. We hypothesized that woodland encroachment would alter microbial community structure and function and would be related to patterns in soil C accumulation. To address this hypothesis, we measured the composition and δ 13 C values of soil microbial phospholipids (PLFAs) along successional chronosequences from C 4 dominated grasslands to C 3 dominated woodlands (small discrete clusters and larger groves) spanning up to 134 years. Woodland development increased microbial biomass, soil C and nitrogen (N) concentrations, and altered microbial community composition. The relative abundance of gram-negative bacteria (cy19:0) increased linearly with stand age, consistent with decreases in soil pH and/or greater rhizosphere development and corresponding increases in C inputs. δ 13 C values of all PLFAs decreased with time following woody encroachment, indicating assimilation of woodland C sources. Among the microbial groups, fungi and actinobacteria in woodland soils selectively assimilated grassland C to a greater extent than its contribution to bulk soil. Between the two woodland types, microbes in the groves incorporated relatively more of the relict C 4 -C than those in the clusters, potentially due to differences in below ground plant C allocation and organo-mineral association. Changes in plant productivity and C accessibility (rather than C chemistry) dictated microbial C utilization in this system in response to shrub encroachment.

Description: The impacts of large-scale conversion of cattle pastures to cropland on soil carbon (C) and nitrogen (N) stocks are poorly understood in the Amazon region. The objective of this research was to determine whether soybean cultivation on a previously deforested and pastured soil has changed C and N stocks and dynamics. We sampled a chronosequence of soybean fields in 2009 and again in 2013. We hypothesized that detecting statistically significant changes in total soil C and N stocks would be difficult, but that fluxes of C and N through the soil would be sufficiently large to significantly decrease the stable isotope ratios of soil organic matter. We observed statistically significant decreases in the 13 C and 15 N enrichments and C:N ratio. When combined with estimates of crop biomass production, harvest yield, and biological nitrogen fixation, these measurements provided sufficient constraints for C and N budgets to infer modest rates of net change in soil N (+15 to + 27 kg N ha -1 yr -1 ) and soil C (-0.15 to -0.30 Mg C ha -1 yr -1 ) in the top 10 cm of soil. These results indicate that this intensive soybean cropping system is having minimal impacts on N loss to the environment, but likely is a small net source of C to the atmosphere.

Description: Linking variation in ecosystem functioning to physiological and landscape drivers has become an important research need for understanding ecosystem responses to global changes. We investigate how these contrasting scale dependent ecosystem drivers influence soil respiration (R s ), a key ecosystem process, using in-situ landscape surveys and experimental subsidies of water and labile carbon. Surveys and experiments were conducted in summer and winter seasons and were distributed along a coastal to desert climate gradient and among the dominant land-use classes in southern California, USA. We found R s decreased from lawn to agricultural and wildland land-uses for both seasons and along the climate gradient in the summer while increasing along the climate gradient in the winter. R s variation was positively correlated with soil temperature, and negatively to soil moisture and substrate. Water additions increased R s in wildland land-uses, while urban land-uses responded little or negatively. However, most land-uses exhibited carbon limitation, with wildlands experiencing largest responses to labile carbon additions. These findings show intensively managed land-uses have increased rates, decreased spatial variation, and decreased sensitivity to environmental conditions in R s compared to wild lands while increasing aridity has the opposite effect. In linking scales, physiological drivers were correlated with R s but landscape position influenced R s by altering both the physiological drivers and the sensitivity to the drivers. Systematic evaluation of physiological and landscape variation provides a framework for understanding the effects of interactive global change drivers to ecosystem metabolism across multiple scales.

Description: Carbonyl sulfide (COS) is a promising tracer for partitioning terrestrial photosynthesis and respiration from net carbon fluxes, based on its daytime co-uptake alongside CO 2 through leaf stomata. Because ecosystem COS fluxes are the sum of plant and soil fluxes, using COS as a photosynthesis tracer requires accurate knowledge of soil COS fluxes. At an oak woodland in southern California, we monitored below-canopy surface (soil + litter) COS and CO 2 fluxes for 40 days using chambers and laser spectroscopy. We also measured litter fluxes separately, and used a depth-resolved diffusion-reaction model to quantify the role of litter uptake in surface COS fluxes. Soil and litter were primarily COS sinks, and mean surface COS uptake was small (∼1 pmol m −2 s −1 ). After rainfall, uptake rates were higher (6–8 pmol m −2 s −1 ), and litter contributed a significant fraction (up to 90%) to surface fluxes. We observed rapid concurrent increases in COS uptake and CO 2 efflux following the onset of rain. The patterns were similar to the Birch effect widely documented for soils, however, both COS and CO 2 flux increases originated mainly in the litter. The synchronous COS-CO 2 litter Birch effect indicates that it results from a rapid increase in litter microbial activity after rainfall. We expect that the drying-rewetting cycles typical for mediterranean and other semi-arid ecosystems create a pronounced seasonality in surface COS fluxes. Our results highlight that litter uptake is an important component of surface COS exchange that needs to be taken into account in ecosystem COS budgets and model simulations.

Description: We evaluated variation in DOM export as a function of hydrology and land use from a large arid river basin in northwestern China. Two soil-derived, humic-like (C1, C2) and three protein-like fluorescence components (C3, C4, C5) were identified. During high discharges, river water DOM had higher values of DOC concentration, percent humic fluorescence, and humification index, but lower values of fluorescence index and percent protein fluorescence than found at base flow, suggesting that flow paths shifted to shallower depths flushing out topsoil OM. Loading of DOC and soil-derived humic fluorescence were driven largely by discharge, with values over 10 times higher during high discharges than at base flow. Furthermore, both δ 13 C-DOC and C1 at high flows positively correlated with %agricultural lands within 1 km river buffers, demonstrating that near-river agricultural activities enhanced storm export of soil DOM. At base flow, C4 positively correlated with %agricultural lands, showing stimulation of aquatic bacterial carbon production as a result of elevated nutrient inputs from agricultural lands. Percent contributions of humic fluorescence in groundwater varied with well depths in shallow wells but this pattern was not observed for deeper groundwater, suggesting that humic DOM could serve as a water source tracer indicating deeper aquifers were isolated from river water and shallow groundwater. Together, our data demonstrate that hydrology and land use controlled the sources and amount of riverine DOM in this large agricultural basin, and that regulating storm runoff and near-river agricultural activities should be incorporated in ecosystem-based management of water resources.

Description: Litterfall is important for returning nutrients and carbon to the forest floor, and microbes decompose the litterfall to release CO 2 into the atmosphere. Litterfall is a pivotal component in the forest biogeochemical cycle, which is sensitive to climate variability and plant physiology. In this study, we combined field litterfall estimates and time-series (2001–2011) climate (the Moderate Resolution Imaging Spectroradiometer [MODIS] land surface temperature [LST] and Tropical Rainfall Measuring Mission [TRMM] precipitation) and green vegetation (MODIS photosynthetically active vegetation cover [PV]) variables to estimate regional annual litterfall in tropical/subtropical forests in Taiwan. We found that time-series MODIS LST- and PV-derived metrics, the annual accumulated MODIS LST and coefficient of variation of PV, respectively, but not the TRMM precipitation variables were salient factors for the estimation ( r 2 = 0.548, p 〈 0.001). The mean (± standard deviation) annual litterfall was 5.1 ± 1.2 Mg ha -1 yr -1 during the observation period. The temporal dynamics of the litterfall revealed that typhoons and consecutive drought events might affect the litterfall temporal variation. Overall, the annual litterfall decreased along the elevation gradient, which may reflect a change in the vegetation type. The northeast and northwest facing slopes yielded the highest amount of annual litterfall (≥5.9 Mg ha -1 yr -1 ), which was in contrast with the southern aspect (5.1 Mg ha -1 yr -1 ). This variation may be associated with the dryness of the microclimate influenced by solar radiation. This study demonstrates the feasibility of utilizing time-series MODIS LST and PV data to predict large-scale field litterfall, which may facilitate large-scale monitoring of biogeochemical cycles in forest ecosystems.

Description: Terrestrial hydrology is central to the Arctic system and its freshwater circulation. Water transport and water constituents vary, however, across a very diverse geography. In this paper, which is a component of the Arctic Freshwater Synthesis, we review the central freshwater processes in the terrestrial Arctic drainage and how they function and change across seven hydro-physiographical regions (Arctic tundra, boreal plains, shield, mountains, grasslands, glaciers/ice caps, and wetlands). We also highlight links between terrestrial hydrology and other components of the Arctic freshwater system. In terms of key processes, snow cover extent and duration is generally decreasing on a pan-Arctic scale, but snow depth is likely to increase in the Arctic tundra. Evapotranspiration will likely increase overall, but as it is coupled to shifts in landscape characteristics, regional changes are uncertain and may vary over time. Streamflow will generally increase with increasing precipitation, but high and low flows may decrease in some regions. Continued permafrost thaw will trigger hydrological change in multiple ways, particularly through increasing connectivity between groundwater and surface water and changing water storage in lakes and soils, which will influence exchange of moisture with the atmosphere. Other effects of hydrological change include increased risks to infrastructure and water resource planning, ecosystem shifts, and growing flows of water, nutrients, sediment and carbon to the ocean. Coordinated efforts in monitoring, modeling, and process studies at various scales are required to improve the understanding of change, in particular at the interfaces between hydrology, atmosphere, ecology, resources, and oceans.

Description: This study seeks an improved understanding of how matrix-association affects the redistribution and degradation of terrigenous organic carbon (TerrOC) during cross-shelf transport in the Siberian margin. Sediments were collected at increasing distance from two river outlets (Lena and Kolyma rivers) and one coastal region affected by erosion. Samples were fractionated according to density, size and settling velocity. The chemical composition in each fraction was characterized using elemental analyses and terrigenous biomarkers. In addition, a dual-carbon-isotope mixing model (δ 13 C and Δ 14 C) was used to quantify the relative TerrOC contributions from active layer (Topsoil) and Pleistocene Ice Complex Deposits (ICD). Results indicate that physical properties of particles exert first-order control on the redistribution of different TerrOC pools. Because of its coarse nature, plant debris is hydraulically retained in the coastal region. With increasing distance from the coast, the OC is mainly associated with fine/ultrafine mineral particles. Furthermore, biomarkers indicate that the selective transport of fine-grained sediment results in mobilizing HMW lipid-rich, diagenetically-altered TerrOC while lignin-rich, less degraded TerrOC is retained near the coast. The loading (µg/m 2 ) of lignin and HMW wax lipids on the fine/ultrafine fraction drastically decreases with increasing distance from the coast (98% and 90%, respectively), which indicates extensive degradation during cross-shelf transport. Topsoil-C degrades more readily (90±3.5%) compared to the ICD-C (60±11%) during transport.. Altogether our results indicate that TerrOC is highly reactive and its accelerated remobilization from thawing permafrost followed by cross-shelf transport will likely represent a positive feedback to climate warming.

Description: Decomposing litter accumulated on the soil surface in forests plays a major role in several ecosystem processes; its detailed characterization is therefore essential for thorough understanding of ecosystem functioning. In addition, litter is known to affect remote sensing radar data over forested areas and their proper processing requires accurate quantification of litter scattering properties. In the present study, ultra wideband (0.8-2.2 GHz) ground-penetrating radar (GPR) data were collected in situ for a wide range of litter types to investigate the potential of the technique to reconstruct litter horizons in undisturbed natural conditions. Radar data were processed resorting to full-wave inversion. Good agreement was generally found between estimated and measured litter layer thicknesses, with RMSE values around 1 cm for recently fallen litter (OL layer) and around 2 cm for fragmented litter in partial decomposition (OF layer) and total litter (OL+OF). Nevertheless, significant correlations between estimated and measured thicknesses were found for total litter only. Inaccuracies in the reconstruction of the individual litter horizons were mainly attributed to weak dielectric contrasts amongst litter layers, with absolute differences in relative dielectric permittivity values often lower than 2 between humus horizons, and to uncertainties in the ground-truth values. Radar signal inversions also provided reliable estimates of litter electromagnetic properties, with average relative dielectric permittivity values around 2.9 and 6.3 for OL and OF litters, respectively. These results are encouraging for the use of GPR for non-invasive characterization and mapping of forest litter. Perspectives for the application of the technique in biogeosciences are discussed.

Description: The study uses satellite MODIS Albedo products (MCD43A3) to assess changes in albedo at two sites in the treeless tundra region of Alaska, both within the foothills region of the Brooks Range, the 2007 Anaktuvuk River Fire (ARF) and 2012 Kucher Creek Fire (KCF). Results are compared to each other and other studies to assess the magnitude of albedo change and the longevity of impact of fire on land surface albedo. In both sites there was a marked decrease of albedo in the year following the fire. In the ARF, albedo slowly increased until four years after the fire, when it returned to albedo values prior to the fire. For the year immediately after the fire, a three-fold difference in the shortwave albedo decrease was found between the two sites. ARF showed a 45.3% decrease while the KCF showed a 14.1% decrease in shortwave albedo, and albedo is more variable in the KCF site than ARF site one year after the fire. These differences are possibly the result of differences in burn severity of the two fires, wherein the ARF burned more completely with more contiguous patches of complete burn than KCF. The impact of fire on average growing season (April - September) surface shortwave forcing in the year following fire is estimated to be 13.24 ± 6.52 Wm -2 at the ARF site, a forcing comparable to studies in other treeless ecosystems. Comparison to boreal studies and the implications to energy flux are discussed in the context of future increases in fire occurrence and severity in a warming climate.

Description: Recent studies suggest that increase in extent and duration of winter soil frost increase dissolved organic carbon (DOC) concentrations in boreal riparian soils and connected aquatic systems during the subsequent spring and summer. However, little is known about the impact of frost on DOC character and its degradability. We applied three experimental treatments to riparian soils in northern Sweden – shallow soil frost (insulated), deep soil frost (snow removed) and control plots – to test the effect of different soil frost regimes on the chemical characteristics and degradability of soil DOC. Soil pore water samples were analyzed using excitation-emission fluorescence (PARAFAC analysis) combined with biological and photochemical degradation experiments. We found that the absolute bacterial metabolic rates were significantly lower in samples from the shallow soil frost treatments, compared with the other treatments. Explorative multivariate analyses indicate that increasing soil frost is contributing to increased protein-like fluorescence and to increased biological degradability of the DOC. Our study shows that decreases in riparian soil frost due to climate warming may not only contribute to decreased riparian DOC concentrations, but also lead to shifts in the DOC composition, resulting in decreased bio-degradability (yet similar photo-degradability) of the DOC that is exported from riparian soils to streams.

Description: Boreal and northern temperate lakes (hereinafter referred to as northern lakes) are sites of intense processing of dissolved organic carbon (DOC), which is reflected in part in the persistent CO 2 supersaturation of their surface waters. These ecosystems are subjected to strong seasonal fluctuations in both irradiance and DOC amount and quality, which in turn should result in temporal shifts in the magnitude of DOC photo-degradation. Here we explore the temporal patterns in the magnitude of water column DOC photo-mineralization, and its potential contribution to pelagic CO 2 production in three northern lakes of different DOC content. We performed laboratory DOC photo-degradation incubations, and combined the resulting rates with field measurements and modeling to reconstruct the annual cycle in depth-integrated DOC photo-mineralization. We found that areal rates of DOC photo-mineralization were driven by both irradiance and intrinsic DOC photo-reactivity, both of which showed seasonality. Over an annual cycle, depth-integrated DOC photo-mineralization rates were remarkably similar across lakes, averaging 4.4 (SD = 0.7) g C m -2 yr -1 , and where daily rates followed an apparent seasonal pattern. The contribution of DOC photo-mineralization to total pelagic CO 2 production (as the sum of respiration and DOC photo-mineralization) peaked after ice melt (up to 49%), averaging 14% for the entire open water season. Our study identifies potential hot periods of photochemical activity that result from the interplay between DOC properties and environmental conditions, which should be incorporated into models of lake functioning.

Description: Spatial and temporal patterns of forest background (understory) reflectance are crucial for retrieving biophysical parameters of forest canopies (overstory) and subsequently for ecosystem modeling. In this communication, we retrieved seasonal courses of understory Normalized Difference Vegetation Index (NDVI) from multi-angular MODIS BRDF/Albedo data. We compared satellite-based seasonal courses of understory NDVI to understory NDVI values measured in different types of forests distributed along a wide latitudinal gradient (65.12° N - 31.35° N). Our results indicated the retrieval method performs well particularly over open forests of different types. We also demonstrated the limitations of the method for closed canopies, where the understory signal retrieval is much attenuated.

Description: Centennial-scale climate-ecosystem feedbacks are a major source of predictive uncertainty for land-atmosphere fluxes of energy, carbon, and water. Accurate representations of plant functional type (PFT) distributions through time and space are required for modeling centennial-scale feedbacks within earth system models (ESMs). We tested the ability of ESMs from the Coupled Model Intercomparison Project, Phase 5 (CMIP5) to capture historical PFT distributions at the time of Euro-American settlement in the Northeastern United States against a new subcontinental-scale dataset of historical tree abundances derived from forest composition surveys. To identify and diagnose errors in ESM-simulated PFT distributions and quantify impacts on modeled albedo, net primary productivity, and transpiration, we analyzed actual and modeled PFT distributions with respect to historical mean annual climate and modeled elasticity among PFTs, climate, and vegetation-atmosphere fluxes. Historical PFT distributions were poorly matched between ESMs and the settlement-era data, often due to inaccurate PFT-climate relationships within ESMs, particularly for evergreen trees. Some models exhibited large local, but regionally compensating, errors in simulated albedo, net primary productivity, and transpiration due to inaccurate PFT distributions, while others had systematic regional biases in vegetation-atmosphere fluxes. Internal variable elasticity varied among ESMs, and these differences closely corresponded to model skill in predicting PFT distributions. New historical benchmarks like the settlement-era vegetation data provide opportunities to confront ESMs, parse sources of error, and improve simulations of historical and future vegetation-atmosphere feedbacks.

Description: The Canadian Land Surface Scheme (CLASS) coupled to the Canadian Terrestrial Ecosystem Model (CTEM) is used to simulate competition between the model's seven non-crop plant functional types (PFTs) for available space. Our objective is to assess if the model is successfully able to reproduce the observed mix of PFTs and their fractional coverages, and to what extent the simulated competition is affected by the manner in which the sub-grid scale variability of vegetation is represented. The model can be run either in a composite (single tile) configuration, where structural vegetation attributes of PFTs are aggregated for use in grid-averaged energy and water balance calculations, or a mosaic (multiple tiles) configuration, where separate energy and water balance calculations are performed for each PFT. The model realistically simulates the fractional coverages of trees, grasses and bare ground, as well as that of individual tree and grass PFTs and their succession patterns. Our results show that the model is not overly sensitive to the manner in which sub-grid scale variability of vegetation is represented. Of the seven sites chosen across the globe to illustrate the difference between the two configurations, the simulated fractional coverage of PFTs are generally very similar (root mean square difference, RMSD, 〈 5 %) between the composite and mosaic configurations at locations characterized by low heterogeneity (e.g. Amazonia, Vancouver Island, and the Tibetan Plateau), whereas at locations characterized by high heterogeneity (e.g. India, South Sudan and California), the two configurations yield somewhat different results (RMSD 〉 5 %).

Description: Microphysical processes of fog and their spatial and temporal pattern are a challenge to study under natural conditions. This work focuses on the development of bi-directional fluxes of fog droplets above a forest canopy in north-eastern Taiwan. Bi-directional fluxes occurred regularly, start from the smallest droplet class (〈2.66 µm diameter), and subsequently extend to larger droplets up to 7.41 µm diameter. The development of the bi-directional fluxes with positive (upward) fluxes of smaller droplets and downward fluxes of larger fluxes is associated with a temperature gradient and with the activation of fog droplets according to the Köhler-Theory. Small fog droplets develop close to the canopy as result of evapotranspiration and subsequent condensation. The rapid growth of small fog droplets and the accelerated growth of activated droplets, a process which is more likely to occur at higher levels of the fog layer, lead to a sink of small droplets and a source of larger droplets within the fog. This is in accordance with the observation that positive droplet number fluxes of small fog droplets outnumber the negative fluxes from the larger fog droplets. For liquid water, the net flux is negative.

Description: The kinetics of bacterial Fe(II)-oxidation was investigated 297 m underground at the Äspö Hard Rock Laboratory (near Oskarshamn, Sweden) under steady state groundwater flow conditions in a flow-through cell containing well developed flocculent mats of bacteriogenic iron oxides (BIOS). Pseudo-first order rate constants of 0.004 min -1 and 0.009 min -1 were obtained for chemical and bacterial Fe(II)-oxidation, respectively, based on the 104 minute retention time of groundwater in the flow cell, inlet Fe(II) concentration of 21.0 ± 0.5 μM, outlet Fe(II) concentration of 8.5 ± 0.7 μM, as well constant pH of 7.42 ± 0.01, dissolved O 2 concentration of 0.11 ± 0.01 mg/L, and groundwater temperature of 12.4 ± 0.1 °C. Redox potential was lower at the BIOS-free inlet (-135.4 ± 1.16 mV) compared to inside BIOS within the flow cell (-112.6 ± 1.91 mV), consistent with the Nernst relationship and oxidation of Fe(II) to Fe(III). Further evaluation of the redox potential time series data using detrended fluctuation analysis (DFA) revealed power-law scaling in the amplitude of fluctuations over increasing intervals of time with significantly different ( p  〈 0.01) DFA α scaling exponents of 1.89 ± 0.03 for BIOS and 1.67 ± 0.06 at the inlet. These α values not only signal the presence of long-range correlation in the redox potential time series measurements, but also distinguish between the slower rate of chemical Fe(II)-oxidation at the inlet and faster rate accelerated by FeOB in BIOS.

Description: Mediterranean-type shublands are subject to periodic fire and high levels of nitrogen (N) deposition, but little is known how chronic N deposition affects carbon (C) and N storage during succession. We conducted a long-term experiment in Californian chaparral to test the hypothesis that chronic N-enrichment would increase post-fire C and N accumulation. The experimental layout consisted of a randomized design where four-10 x 10 m plots received 5 gN m −2 annually since 2003 and four-10 x 10 m plots served as controls. Aboveground and belowground C and N pools and fluxes were measured seasonally (every 3 months) for a period of 10 years. Added N rapidly increased soil extractable N pools and decreased soil pH; however, total soil C and N storage were not affected. Added N plots initially had significantly lower C and N storage than control plots, presumably because of nutrient losses from leaching and/or higher belowground C allocation. However, rates of aboveground N and C storage became significantly higher in added N plots after 4–5 years of exposure, thus increasing fuel buildup, which has implications for future fire intensity. This recovering chaparral stand is not yet “N-saturated” after 10 years of chronic N input. However, N leaching continues to be higher in added N plots, indicating that post-fire chaparral stands in high-N deposition areas can be important sources of N to groundwater/aquatic systems even if productivity is stimulated by N input.

Description: In the Great Plains, grassland carbon dynamics differ across broad gradients of precipitation and temperature, yet finer-scale variation in these variables may also affect grassland processes. Despite the importance of grasslands, there is little information on how fine-scale relationships compare between them regionally. We compared grassland C exchanges, energy partitioning and precipitation variability in eight sites in the eastern and western Great Plains using eddy covariance and meteorological data. During our study, both eastern and western grasslands varied between an average net carbon sink and a net source. Eastern grasslands had a moderate vapor pressure deficit (VPD = 0.95 kPa) and high growing season gross primary productivity (GPP = 1010 ± 218 g C m −2 m −2 y −1 ). Western grasslands had a growing season with higher VPD (1.43 kPa) and lower GPP (360 ± 127 g C m −2 m −2 y −1 ). Western grasslands were sensitive to precipitation at daily timescales, whereas eastern grasslands were sensitive at monthly and seasonal timescales. Our results support the expectation that C exchanges in these grasslands differ as a result of varying precipitation regimes. Because eastern grasslands are less influenced by short-term variability in rainfall than western grasslands, the effects of precipitation change are likely to be more predictable in eastern grasslands because the timescales of variability that must be resolved are relatively longer. We postulate increasing regional heterogeneity in grassland C exchanges in the Great Plains in coming decades.

Description: No abstract is available for this article.

Description: Recent studies report trends of strongly increasing iron (Fe) concentrations in freshwaters. Since Fe is a key element with a decisive role in the biogeochemical cycling of major elements, it is important to understand the mechanisms behind these trends. We hypothesized that variations in Fe concentration are driven mainly by redox dynamics in hydraulically connected soils. Notably, Fe(III), which is the favored oxidation state except in environments where microbial activity provide strong reducing intensity, has several orders of magnitude lower water solubility than Fe(II). To test our hypothesis, seasonal variation in water chemistry, discharge, and air temperature was studied in three Swedish rivers. Methylmercury and sulfate were used as indicators of seasonal redox changes. Seasonal variability in water chemistry, discharge, and air temperature in the Emån and Lyckeby rivers implied that the variation in Fe was primarily driven by the prevalence of reducing conditions in the catchment. In general high Fe concentrations were observed when methylmercury was high and sulfate was low, indicative of reducing conditions. The Fe concentrations showed no or weak relationships with variations in dissolved organic matter concentration and aromaticity. The seasonal variation in Fe concentration Ume river was primarily dependent on timing of the snowmelt in high versus low altitude areas of the catchment. There were long-term trends of increasing temperature in all catchments and also trends of increasing discharge in the southern rivers, which should increase the probability for anaerobic conditions in space and time and thereby increase Fe transport to the aquatic systems.

Description: Using a comparative spatial analysis of sediment cores from 8 lakes in tundra uplands adjacent to the Mackenzie Delta, NT, we examined how the presence of retrogressive thaw slumps on lake shores affected persistent organic pollutant (POPs, including polychlorinated biphenyls (PCBs) and organochlorine (OC) pesticides) accumulation in lake sediments. Sediments of slump-affected lakes contained higher total organic carbon (TOC)-normalized POP concentrations than nearby reference lakes that were unaffected by thaw slumps. Mean focus-corrected inorganic sedimentation rates were positively related to TOC-normalized contaminant concentrations, explaining 58 – 94% of the variation in POP concentrations in sediment, suggesting that reduced organic carbon in slump-affected lake water results in higher concentrations of POPs on sedimentary organic matter. This explanation was corroborated by an inverse relationship between sedimentary POP concentrations and TOC content of the lake water. Inferred chlorophyll a , S2 and S3 carbon fluxes to sediment were not significantly correlated to POP fluxes. Higher POP concentrations observed in sediment of slump-affected lakes are best explained by simple solvent switching processes of hydrophobic organic contaminants onto a smaller pool of available organic carbon when compared to neighboring lakes unaffected by thaw slump development.

Description: Water-limited ecosystems occupy nearly thirty percent of the Earth, but arguably the controls on their ecosystem processes remain largely uncertain. We analyzed six site-years of eddy covariance measurements of evapotranspiration (ET) from 2008–2010 at two water-limited shrublands: one dominated by winter precipitation (WP-site) and another dominated by summer precipitation (SP-site), but with similar solar radiation patterns in the northern hemisphere. We determined how physical forcing factors (i.e., net radiation (Rn), soil water content (SWC), air temperature (Ta), vapor pressure deficit (VPD)) influence annual and seasonal variability of ET. Mean annual ET at SP-site was 455 ± 91 mm yr −1 , was mainly influenced by SWC during the dry season, by Rn during the wet season, and was highly sensitive to changes in annual precipitation (P). Mean annual ET at WP-site was 363 ± 52 mm yr −1 , had less inter-annual variability, but multiple variables (i.e., SWC, Ta, VPD, Rn) were needed to explain ET among years and seasons. Wavelet coherence analysis showed that ET at SP-site has a consistent temporal coherency with Ta and P, but this was not the case for ET at WP-site. Our results support the paradigm that SWC is the main control of ET in water-limited ecosystems when radiation and temperature are not limiting factors. In contrast, when P and SWC are decoupled from available energy (i.e., radiation and temperature), then ET is controlled by an interaction of multiple variables. Our results bring attention to the need for better understanding how climate and soil dynamics influence ET across these globally distributed ecosystems.

Description: Soil organic matter is one of the most important carbon (C) pools in terrestrial ecosystems, and future warming from climate change will likely alter soil C storage via temperature effects on microbial respiration. In this study, we collected forest soils from eight locations along a 3,700 km North–South transect in eastern China (NSTEC). For eight weeks these soils were incubated under a periodically changing temperature range of 6–30°C while frequently measuring soil microbial respiration rate ( Rs ; each sample about every 20 minutes). This experimental design allowed us to investigate Rs and the temperature sensitivity of Rs ( Q 10 ) along the NSTEC. Both Rs at 20°C ( R 20 ) and Q 10 significantly increased (logarithmically) with increasing latitude along the NSTEC suggesting that the sensitivity of soil microbial respiration to changing temperatures is higher in forest soils from locations with lower temperature. Our findings from an incubation experiment provide support for the hypothesis that temperature sensitivity of soil microbial respiration increases with biochemical recalcitrance (C quality-temperature hypothesis) across forest soils on a large spatial scale. Furthermore, microbial properties primarily controlled the observed patterns of R 20 , whereas both substrate and microbial properties collectively controlled the observed patterns of Q 10 . These findings advance our understanding of the driving factors (microbial vs . substrate properties) of R 20 and Q 10 as well as the general relationships between temperature sensitivity of soil microbial respiration and environmental factors.

Description: Tundra soils store large amounts of carbon (C) that could be released through enhanced ecosystem respiration (ER) as the Arctic warms. Over time, this may change the quantity and quality of available soil C pools which in-turn may feedback and regulate ER responses to climate warming. Therefore, short-term increases in ER rates due to experimental warming may not be sustained over longer periods, as observed in other studies. One important aspect, which is often overlooked, is how climatic changes affecting ER in one season may carry-over and determine ER in following seasons. Using snow fences, we increased snow depth and thereby winter soil temperatures in a high arctic site in Svalbard (78°N) and a low arctic site in the Northwest Territories, Canada (64°N), for 5 and 9 years, respectively. Deepened snow enhanced winter ER while having negligible effect on growing season soil temperatures and soil moisture. Growing season ER at the high arctic site was not affected by the snow treatment after two years. However, surprisingly, the deepened snow treatments significantly reduced growing season ER rates after 5 years at the high arctic site and after 8-9 years at the low arctic site. We speculate that the reduction in ER rates, that became apparent only after several years of experimental manipulation, may, at least in part, be due to prolonged depletion of labile C substrate as a result of warmer soils over multiple cold seasons. Long-term changes in winter climate may therefore significantly influence annual net C balance not just because of increased wintertime C loss but also because of ‘legacy’ effects on ER rates during the following growing seasons.

Description: A combined data set, combining data from field campaigns and oceanographic cruises was used to ascertain the influence of both river discharges and upwelling processes, covering spatial and temporal variation in DIC and aragonite saturation state. This work was conducted in one of the most productive river-influenced upwelling area in the South Pacific coasts (36°S). Additionally, further work was also conducted to ascertain the contribution of different DIC sources, influencing the dynamics of DIC along the land-ocean range.. Six sampling campaigns were conducted across seven stations at the Biobío River basin, covering approximately 200 km. Three research cruises were undertaken simultaneously, covering the adjacent continental shelf, including 12 sampling stations for hydrographic measurements. Additionally, 6 stations were also sampled for chemical analyses, covering summer, winter and spring conditions over 2010 and 2011. Our results evidenced that seaward extent of the river plume was more evident during the winter field campaign, when highest riverine DIC fluxes were observed. The carbonate system along the river-ocean continuum was very heterogeneous varying over spatial and temporal scales. High DIC and p CO 2 were observed in river areas with larger anthropogenic effects. CO 2 supersaturation at the river plume was observed during all campaigns due to the influence of low pH river waters in winter/spring and high- p CO 2 upwelling waters in summer. δ 13 C DIC evidenced that main DIC sources along the river and river plume corresponded to the respiration of terrestrial organic matter. We have linked this natural process to the carbonate saturation on the adjacent river-influenced coastal area, suggesting that Ω aragonite undersaturation in surface/sub-surface waters is largely modulated by the influence of both river discharge and coastal upwelling events in this productive coastal area. Conditions of low Ω aragonite might impact negatively physiological traits for marine organisms, such as, bivalves, gastropods, and crustaceans. Therefore, local populations from river-influenced sites could be inherently more tolerant to ocean acidification than organisms living in regions with lower Ω aragonite variability.

Description: The study of phenological patterns and their dynamics provides insights into the impacts of climate change on terrestrial ecosystems. Here we present a novel analytical workflow, based on co-clustering, that enables the concurrent study of spatio-temporal patterns in spring phenology. The workflow is illustrated with a long-term time series of first leaf dates ( FLD ) over Europe, northern Africa and Turkey calculated using the extended spring index models and the European E-OBS daily maximum and minimum temperature datasets (1950 to 2011 with a spatial resolution of 0.25 degrees). This FLD dataset was co-clustered using the Bregman block average co-clustering with I-divergence (BBAC_I) and the results were refined using k -means. These refined co-clusters were mapped to provide a first spatially-continuous delineation of phenoregions in Europe. Our results show that the study area exhibits four main spatial phenological patterns of spring onset. The temporal dynamics of these phenological patterns indicate that the first years of the study period tend to have late spring onsets and the recent years have early spring onsets. Our results also show that the study period exhibits twelve main temporal phenological patterns of spring onset. The spatial distributions of these temporal phenological patterns show that western Turkey tends to have the most variable spring onsets. Changes in the boundaries of other phenoregions can also be observed. These results indicate that this co-clustering based analytical workflow effectively enables the simultaneous study of both spatial patterns and their temporal dynamics and of temporal patterns and their spatial dynamics in spring phenology.

Description: Here we present direct measurements of the biological breakdown of 13 C-labeled substrates to CO 2 at seven locations along the lower Amazon River, from Óbidos to the mouth. Dark incubation experiments were performed at high and low water periods using vanillin, a lignin phenol derived from vascular plants, and at the high water period using four different 13 C-labeled plant litter leachates. Leachates derived from oak wood were degraded most slowly with vanillin monomers, macrophyte leaves, macrophyte stems, and whole grass leachates being converted to CO 2 1.2, 1.3, 1.7, and 2.3 times faster, respectively, at the upstream boundary, Óbidos. Relative to Óbidos, the sum degradation rate of all four leachates was 3.3 and 2.6 times faster in the algae-rich Tapajós and Xingu rivers, respectively. Likewise, the leachates were broken down 3.2 times more quickly at Óbidos when algal biomass from the Tapajós River was simultaneously added. Leachate reactivity similarly increased from Óbidos to the mouth with leachates breaking down 1.7 times more quickly at Almeirim (midway to the mouth) and 2.8 times more quickly across the river mouth. There was no discernible correlation between in situ nutrient levels and remineralization rates, suggesting that priming effects were an important factor controlling reactivity along the continuum. Further, continuous measurements of CO 2 , O 2 , and conductivity along the confluence of the Tapajós and Amazon rivers and the Xingu and Jarauçu rivers revealed in situ evidence for enhanced O 2 drawdown and CO 2 production along the mixing zone of these confluences.

Description: We measured soil respiration across a soil moisture gradient ranging from dry to wet snow-scoured alpine tundra soils throughout three winters and two summers. In the absence of snow accumulation, soil moisture variability was principally determined by the combination of meso-topographical hydrological focusing and shallow subsurface permeability, which resulted in a patchwork of co-mingled ecosystem types along a single alpine ridge. To constrain the subsequent carbon cycling variability, we compared three measures of effective diffusivity and three methods to calculate gradient method soil respiration from four typical vegetation communities. Over-winter soil respiration was primarily restricted to wet meadow locations, and a conservative estimate of the rate of over-winter soil respiration from snow-scoured wet meadow tundra was 69 – 90% of the maximum carbon dioxide (CO 2 ) respired by seasonally snow-covered soils within this same catchment. This was attributed to higher over-winter soil temperatures at wet meadow locations relative to fellfield, dry meadow, and moist meadow communities, which supported liquid water and heterotrophic respiration throughout the winter. These results were corroborated by eddy covariance-based measurements that demonstrated an average of 272 g C m -2 over-winter carbon loss during the study period. As a result, we updated a conceptual model of soil respiration versus snow cover to express the potential for soil respiration variability from snow-scoured alpine tundra.

Description: Multiple isotopes ( 13 C-DIC, 34 S and 18 O-SO 4 2− , 15 N and 18 O-NO 3 − ) and water chemistry were used to evaluate weathering rates and associated CO 2 consumption by carbonic acid and strong acids (H 2 SO 4 and HNO 3 ) in a typical karst watershed (Wujiang River, Southwest China). The dual sulfate isotopes indicate that sulfate is mainly derived from sulfide oxidation in coal stratum and sulfide-containing minerals, and dual nitrate isotopes indicate that nitrate is mainly derived from soil N and nitrification. The correlation between isotopic compositions and water chemistry suggests that sulfuric and nitric acids, in addition to carbonic acid, are involved in carbonate weathering. The silicate and carbonate weathering rates are 7.2 t km −2 yr −1 and 76 t km −2 yr −1 , respectively. In comparison with carbonate weathering rates (43 t km −2 yr −1 ) by carbonic acid alone, the subsequent increase in rates indicates significant enhancement of weathering when combined with sulfuric and nitric acid. Therefore, the role of sulfuric and nitric acid in the rock weathering should be considered in the global carbon cycle.

Description: Riverine biogeochemical processes are understudied relative to headwaters, and reach-scale processes in rivers reflect both the water column and sediment. Denitrification in streams is difficult to measure, and is often assumed to occur only in sediment, but the water column is potentially important in rivers. Dissolved nitrogen (N) gas flux (as dinitrogen (N 2 )) and open-channel N 2 exchange methods avoid many of the artificial conditions and expenses of common denitrification methods like acetylene block and 15 N-tracer techniques. We used membrane-inlet mass spectrometry and microcosm incubations to quantify net N 2 and oxygen flux from the sediment and water column of five Midwestern rivers spanning a land use gradient. Sediment and water column denitrification ranged from below detection to 1.8 mg N m -2 h -1 , and from below detection to 4.9 mg N m -2 h -1 , respectively. Water column activity was variable across rivers, accounting for 0-85% of combined microcosm denitrification, and 39-85% of combined microcosm respiration. Finally, we estimated reach-scale denitrification at one Midwestern river using a diel, open-channel N 2 exchange approach based on reach-scale metabolism methods, providing an integrative estimate of riverine denitrification. Reach-scale denitrification was 8.8 mg N m -2 h -1 (95% credible interval: 7.8-9.7 mg N m -2 h -1 ), higher than combined sediment and water column microcosm estimates from the same river (4.3 mg N m -2 h -1 ), and other estimates of reach-scale denitrification from streams. Our denitrification estimates, which span habitats and spatial scales, suggest that rivers can remove N via denitrification at equivalent or higher rates than headwater streams.