ALBERT

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

Description: Inland waters are hotspots for carbon (C) cycling and therefore important for landscape C budgets. Small streams and lakes are particularly important, however quantifying C fluxes is difficult and has rarely been done for the entire aquatic continuum, comprised of connected streams and lakes within the same catchment. We investigated carbon dioxide (CO 2 ) evasion and fluvial fluxes of dissolved inorganic and organic carbon (DIC and DOC) in stream and lake systems within the 2.3 km 2 catchment of a small boreal lake. Our results show pronounced spatial and temporal variability in C fluxes even at a small spatial scale. C loss from the catchment through CO 2 evasion from headwaters for the total open water-sampling period was 9.7 g C m -2 catchment, dominating the total catchment C loss (including CO 2 evasion, DIC and DOC export from the lake, which were 2.7, 0.2 and 5.2 g C m -2 catchment, respectively). Aquatic CO 2 evasion was dominated by headwater streams that occupy ~0.1% of the catchment, but contributed 65% to the total aquatic CO 2 evasion from the catchment. The importance of streams was mainly an effect of the higher gas transfer velocities than compared to lakes (median, 67 and 2.2 cm h -1 , respectively). Accurately estimating the contribution of C fluxes from headwater streams, particularly the temporal and spatial dynamics in their gas transfer velocity, is key to landscape-scale C budgets. This study demonstrates that CO 2 evasion from headwaters can be the major pathway of C loss from boreal catchments, even at a small spatial scale.

Description: Worldwide dam building in large river basins has substantially altered the carbon cycle by trapping much of the riverine transported particulate organic carbon (POC) in terrestrial reservoirs. Here we take the Changjiang (Yangtze) River basin, in which ~50,000 dams were built over the past 50 years, as an example to evaluate the effect of dam building on POC sequestration. We report the characteristics (elemental composition, radiocarbon and stable carbon isotopic compositions, and Raman spectra) of bulk POC in the lower Changjiang from October 2007 to September 2008, and we estimate the POC sequestration induced by dam building since the 1950s for the Changjiang Basin. Using radiocarbon measurements, we quantify the fraction of biospheric POC (POC bio ) and petrogenic POC (POC petro ) in Changjiang POC. Over the study period, around 25% of the Changjiang POC is radiocarbon-dead POC petro ; the remaining is POC bio with a mean radiocarbon age of ~3.5 kyr. Studies on the East China Sea (ECS) shelf, along with an oxidation experiment, suggest that, prior to dam building, the Changjiang POC bio was significantly oxidized in the ECS margin. In contrast, high preservation of POC is observed in Changjiang reservoirs. Combining our POC data with hydrometric datasets, our study indicates that, over the past five decades, dam building may have largely shifted the Changjiang POC burial site from the ECS margin to terrestrial reservoirs. This shift in burial site preserved labile POC bio that would have been oxidized, suggesting a new temporary carbon sink. We estimate that dam building in the Changjiang has sequestered ~4.9 ± 1.9 Mt POC bio every year since 2003, approximately 10% of the global riverine POC burial flux to the oceans.

Description: [1]  Shrublands in semi-arid regions are heterogeneous landscapes consisting of infertile bare areas separated by nutrient rich vegetated areas known as resource islands. Spatial patterns in these landscapes are structured by feedbacks driven by the transport of water and nutrient resources from the intershrub space to areas below shrubs, and the retention of these resources to locally drive productivity and tight biogeochemical cycles. Most understanding of plant-soil feedbacks is based predominantly on studies of low topographic gradient landscapes, and it is unclear whether the patterns of association between soils and vegetation, and the autogenic processes that create them, also occur on more steeply sloping terrain. Here we analyze the spatial patterns of soils, vegetation and micro-topography on hillslopes of contrasting lithology (one granite at 16 ∘ , one schist at 27 ∘ ) in the Sonoran desert foothills of the Catalina Mountains. We also describe a method of extracting vegetation density from terrestrial laser scanning point-cloud data at 5 cm × 5 cm scales, and find that it correlates well with soil organic carbon measurements. Vegetation was associated with microtopographic mounds (relative to the mean slope) extending 0.3 m downslope and 1.8 m (schist) and 0.9 m (granite) upslope on the study hillslopes. Soils below the shrub canopies exhibited 2-3 times more soil organic matter and 2-4 times higher hydraulic conductivity than the interspaces. Soils enriched with organic matter were found to extend at least two canopy radii downslope of woody shrubs, but not upslope. These plumes were clearest in the lower-gradient granite site where vegetation mounds created distinct patterns of microtopographic convergence and divergence. At the steeper schist site microtopography appeared to have a weaker control on topographic flow accumulation. Collectively our findings suggest that the spatial structure of association between soils and microtopography and vegetation on these slopes exhibit many of the features observed in lower-gradient areas. However microtopography and soils are more asymmetric along the downslope axis of the hillslopes than lower-gradient areas, and vary with lithology. Alluvial and colluvial processes are likely more important in shaping vegetation and soil dynamics on hillslopes, and these factors need further consideration in scaling results to the landscape level.

Description: [1]  Plant phenology, a sensitive indicator of climate change, influences vegetation-atmosphere interactions by changing the carbon and water cycles from local to global scales. Camera-based phenological observations of the color changes of the vegetation canopy with throughout the growing season have become popular in recent years. However, the linkages between camera phenological metrics and leaf biochemical, biophysical and spectral properties are elusive. We measured key leaf properties including chlorophyll and carotenoids concentrations and leaf reflectance on a weekly basis from June to November, 2011 in a white oak forest on the island of Martha's Vineyard, Massachusetts, USA. Concurrently, we used a digital camera to automatically acquire daily pictures of the tree canopies. We found that there was a mismatch between the camera-based phenological metric for the canopy greenness (green chromatic coordinate, g cc ) and the total chlorophyll and carotenoids concentration and leaf mass per area during late spring/early summer. The seasonal peak of g cc is approximately 20 days earlier than the peak of the total chlorophyll concentration. During the summer we observed a gradual decline of g cc and leaf-level vegetation index in a period of insignificant change in total chlorophyll concentration. During the fall, both canopy and leaf redness (red chromatic coordinate, r cc ) were significantly correlated with the vegetation index for anthocyanin concentration, opening a new window to quantify vegetation senescence remotely. Satellite and camera-based vegetation indices agreed well, suggesting that camera-based observations can be used as the ground validation for satellites. Using the high temporal resolution dataset of leaf biochemical, biophysical and spectral properties, our results show the strengths and potential uncertainties to use canopy color as the proxy of ecosystem functioning. Our results suggest an urgent need for additional research that bridges the gap between leaf physiological properties and camera-based phenological metrics.

Description: [1]  Long-term land carbon-cycle feedback to climate change is largely determined by dynamics of soil organic carbon (SOC). However, most evaluation studies conducted so far indicate that global land models predict SOC poorly. This study was to evaluate predictions of SOC by the Community Land Model with Carnegie-Ames-Stanford Approach biogeochemistry sub-model (CLM-CASA’), investigate underlying causes of mismatches between model predictions and observations, and calibrate model parameters to improve the prediction of SOC. We compared modeled SOC to the SOC pools provided by IGBP-DIS globally gridded data product and found that CLM-CASA’ on average underestimated SOC pools by 65% ( r 2  = 0.28). We extracted the C cycle component from CLM-CASA’ and applied data assimilation to it to estimate SOC residence times, C partitioning coefficients among the pools, as well as temperature sensitivity of C decomposition. The model with calibrated parameters explained 41% of the global variability in the observed SOC, which was substantial improvement from the initial 27%. The SOC and litter C feedbacks to changing climate differed between models with original and calibrated parameters: after 95 years of climate change (2006-2100) the amount of C released from soils was 48 Pg C lower in the calibrated than in the non-calibrated model, and the amount of C released from litter was 6.5 Pg C lower in the calibrated than the non-calibrated model. Thus, assimilating estimated soil carbon data into the model improved model parameterization and reduced the amount of C released under changing climate. To further reduce the uncertainty in the soil carbon prediction, we need to explore alternative model structures, improve representation of ecosystems, and develop additional global datasets for constraining model parameters.

Description: [1]  Carbon C storage in lakes is now recognized as a significant sink of C at a global scale, but the pathways that lead to this storage remain poorly understood. In this study, we attempt to reconstruct and connect the processes that lead to long-term C accumulation in boreal lakes. These include the rate of POC sedimentation in the water column and sediment metabolism operating at a temporal scale of weeks to months, organic C accumulation in the top sediment layers integrated over scales of tens of years, and long-term organic C burial in lake sediment integrated over hundreds to thousands of years. The sinking POC flux was tenfold higher than the short-term sediment C accumulation rates in all systems, and we found no direct relationship between this downward C flux and either the short-term or long-term C accumulation rates. However, the resulting C burial efficiency (which ranged from 5 to 62%) was strongly related to lake shape, which ultimately constrains the time freshly deposited material is exposed to oxygen and thereby regulates the fraction of the carbon sinking flux that is mineralized back to the atmosphere or permanently buried in the sediments. Small and deep lakes act as more efficient C sinks than large and flat lakes. We also show that long-term burial rates are nearly identical to current centennial scale accumulation rates, and that therefore there is little degradation occurs after a few decades. Sediment C storage tends to be small (〈5%) relative to lake C emissions, but that this balance is also strongly related to lake morphometry.

Description: [1]  Boreal peatlands are a major long-term reservoir of atmospheric carbon (C) and play an important role in the global C cycle. It is unclear how C accumulation in peatlands responds to changing temperatures and nutrients (specifically, nitrogen and sulfur). In this study, we assessed how the C input rate and C accumulation rate in decadal old peat layers respond to increased air temperatures (+3.6 ºC) during the growing season and the annual additions of nitrogen (N) and sulfur (S) (30 and 20 kg ha -1  yr -1 , respectively) over 12 years of field treatments in a boreal mire. An empirical mass-balance model was applied to 210 Pb dated peat cores to evaluate changes in C inputs, C mass-loss and net C accumulation rates in response to the treatments. We found that: i) none of the treatments generated a significant effect on peat mass-loss decay rates, ii) C input rates were positively affected by N additions and negatively affected by S additions, iii) the C accumulation rate in the uppermost (10 to 12 cm) peat was increased by N additions and decreased by S additions, and iv) only air temperature significantly affected the main effects induced by N and S additions. Based on our findings, we argue that C accumulation rates in surface peat layers of nutrient-poor boreal mires can increase despite the predicted rise in air temperatures as long as N loads increase and acid atmospheric S remains low.

Description: [1]  Previous studies have reported that atmospheric CO 2 enrichment would increase the ion concentrations in the soil water. However, none of these studies could exactly quantify the amount of ions changes in the soil water induced by elevated CO 2 and all of these experiments were carried out only in the temperate areas. Using an Open-Top Chamber design, we studied the effects of CO 2 enrichment alone and together with nitrogen (N) addition on soil water chemistry in subtropics. Three years of exposure to an atmospheric CO 2 concentration of 700 ppm resulted in accelerated base cation loss via leaching water below the 70 cm soil profile. The total of base cation (K + +Na + +Ca 2+ +Mg 2+ ) loss in the elevated CO 2 treatment were higher than that of the control by 220%, 115%, and 106% in 2006, 2007, and 2008, respectively. The N treatment decreased the effect of high CO 2 treatment on the base cation loss in the leachates. Compared to the control, N addition induced greater metal cation (Al 3+ and Mn 2+ ) leaching loss in 2008, and net Al 3+ and Mn 2+ loss in the high N treatment increased by 100% and 67%, respectively. However, the CO 2 treatment decreased the effect of high N treatment on the metal cation loss. Changes of ion export following by the exposure to the elevated CO 2 and N treatments were related to both ion concentrations and leached water amount. We hypothesize that forests in subtropical China might suffer from nutrient limitation and some poisonous metal activation in plant biomass under future global change.

Description: Partitioning of CO 2 exchange into canopy ( F A ) and soil ( F R ) flux components is essential to improve our understanding ecosystem processes. The stable isotope C 18 OO can be used for flux partitioning, but this approach depends on the magnitude and consistency of the isotope disequilibrium ( D eq ), i.e. difference between the isotope compositions of F R ( δ A ) and F A ( δ R ). In this study high temporal resolution isotopic data were used: 1) to test the suitability of existing steady state and non-steady models to estimate H 2 18 O enrichment in a mixed forest canopy, 2) to investigate the temporal dynamics of δ A using a big-leaf parameterization, and 3) to quantify the magnitude of the C 18 OO disequilibrium ( D eq ) in a temperate deciduous forest throughout the growing season and to determine the sensitivity of this variable to the CO 2 hydration efficiency ( θ eq ). A departure from steady state conditions was observed even at mid-day in this study, so the non-steady state formulation provided better estimates of leaf water isotope composition. The dynamics of δ R was mainly driven by changes in soil water isotope composition, caused by precipitation events. Large D eq values (up to 11‰) were predicted; however the magnitude of the disequilibrium was variable throughout the season. The magnitude of D eq was also very sensitive to the hydration efficiencies in the canopy. For this temperate forest during most of the growing season, the magnitude of D eq was inversely proportional to θ eq , due to the very negative δ R signal, which is contrary to observations for other ecosystems investigated in previous studies.

Description: This paper presents a continental-scale phenological analysis of African savannas and woodlands. We apply an array of synergistic vegetation and hydrological data records from satellite remote sensing and model simulations to explore the influence of rainy season timing and duration on regional land surface phenology and ecosystem structure. We find that: (i) the rainy season onset precedes and is an effective predictor of the growing season onset in African grasslands. (ii) African woodlands generally have early green-up before rainy season onset, and have a variable delayed senescence period after the rainy season, with this delay correlated non-linearly with tree fraction. These woodland responses suggest their complex water use mechanisms (either from potential groundwater use by relatively deep roots or stem-water reserve) to to maintain dry season activity. (iii) We empirically find that the rainy season length has strong non-linear impacts on tree fractional cover in the annual rainfall range from 600-1800 mm/year, which may lend some support to the previous modeling study that given the same amount of total rainfall, the tree fraction may first increases with the lengthening of rainy season unitl reaching an “optimal rainy season length”, after which tree fraction decreases with the further lengthening of rainy season. This non-linear response is resulted from compound mechanisms of hydrological cycle, fire and other factors. We conclude that African savannas and deciduous woodlands have distinctive responses in their phenology and ecosystem functioning to rainy season. Further research is needed to address interaction between groundwater and tropical woodland as well as to explicitly consider the ecological significance of rainy season length under climate change.

Description: This study combined a process-based ecosystem model with a fire regime model to understand the effect of changes in fire regime and climate pattern on woody plants of miombo woodland in African savanna. Miombo woodland covers wide areas in Africa and is subject to frequent anthropogenic fires. The model was developed based on observations of tree topkill rates in individual tree size classes for fire intensity, and re-sprouting. Using current and near-future climate patterns, the model simulated the dynamics of miombo woodland for various fire return intervals and grass cover fractions, allowing fire intensity to be estimated. There was a significant relationship between aboveground woody biomass and long-term fire regimes. An abrupt increase in fire intensity and/or fire frequency applied as a model forcing led to reduced long-term average aboveground woody biomass and mean tree size. Fire intensity increased with increasing living-grass biomass (which provides increased flammable fuel), thereby affecting the relationship between fire regime and tree size, creating a demographic bottleneck on the route to tree maturity. For the current fire regime in miombo woodland, with a fire return interval of about 1.6–3 years, the model predicted fire intensity lower than 930–1700 kW m −1 is necessary to maintain today's aboveground woody biomass under current climate conditions. Future climate change was predicted to have a significant positive effect on woody plants in miombo woodland associated with elevated CO 2 concentration and warming, allowing woody plants to survive more effectively against periodic fires.

Description: Understanding and predicting the extent, location, and function of biogeochemical hotspots at the watershed scale is a frontier in environmental science. We applied a hydropedologic approach to identify 1) biogeochemical differences among morphologically distinct hydropedologic settings and 2) hotspots of microbial carbon (C) and nitrogen (N) cycling activity in a northern hardwood forest in Hubbard Brook Experimental Forest, New Hampshire, USA. We assessed variables related to C and N cycling in spodic hydropedologic settings (typical podzols, bimodal podzols, and Bh podzols) and groundwater seeps during August 2010. We found that soil horizons (Oi/Oe, Oa/A, and B) differed significantly for most variables. B horizons (〉 10 cm) accounted for 71% (± 11%) of C pools and 62% (± 10%) of microbial biomass C in the sampled soil profile, whereas the surface horizons (Oi/Oe and Oa/A; 0-10 cm) were dominant zones for N-cycle-related variables. Watershed-wide estimates of C and N cycling were higher by 34 to 43% (± 17-19%) when rates, horizon thickness, and areal extent of each hydropedologic setting were incorporated, versus conventionally calculated estimates for typical podzols that included only the top 10 cm of mineral soil. Despite variation in profile development in typical, bimodal, and Bh podzols, we did not detect significant differences in C and N cycling among them. Across all soil horizons and hydropedologic settings, we found strong links between biogeochemical cycling and soil C, suggesting that the accumulation of C in soils may be a robust indicator of microbial C and N cycling capacity in the landscape.

Description: Regional scale drought-induced forest mortality events are projected to become more frequent under future climates due to changes in rainfall patterns. The occurrence of these mortality events is driven by exogenous factors such as frequency and severity of drought and endogenous factors such as tree water and carbon use strategies. To explore the link between these exogenous and endogenous factors underlying forest mortality, a stochastic ecohydrological framework that accounts for random arrival and length of droughts as well as responses of tree water and carbon balance to soil water deficit is proposed. The main dynamics of this system are characterized with respect to the spectrum of anisohydric-isohydric stomatal control strategies. Using results from a controlled drought experiment, a maximum tolerable drought length at the point where carbon starvation and hydraulic failure occur simultaneously is predicted, supporting the notion of coordinated hydraulic function and metabolism. We find qualitative agreement between the model predictions and observed regional-scale canopy die-back across a precipitation gradient during the 2002–2003 southwestern United States drought. Both the model and data suggest a rapid increase of mortality frequency below a precipitation threshold. The model also provides estimates of mortality frequency for given plant drought strategies and climate regimes. The proposed ecohydrological approach can be expanded to estimate the effect of anticipated climate change on drought-induced forest mortality and associated consequences for the water and carbon balances.

Description: The amount and timing of snow-cover control the cycling of carbon (C), water and energy in arctic ecosystems. The implications of changing snow-cover for regional C budgets, biogeochemistry, hydrology and albedo due to climate change are rudimentary, especially for the High Arctic. In a polar semi-desert of NW Greenland, we used a ~10-year-old snow manipulation experiment to quantify how deeper snow affects magnitude, seasonality and 14 C content of summer C emissions. We monitored ecosystem respiration (R eco ), soil CO 2 and their 14 C contents over three summers in vegetated and bare areas. Additional snowpack, elevated soil water content (SWC) and temperature throughout the growing-season in vegetated, but not in bare areas. Daily R eco was positively correlated to temperature, but negatively correlated to SWC; consequently we found no effect of increased snow on daily flux. Cumulative summertime R eco was not related to annual snowfall, but to water-year precipitation (winter snow plus summer rain). Experimentally increased snowpack shortened the growing-season length and reduced summertime R eco up to 40%. Soil CO 2 was older under increased snow. However, we found no effect of snow-depth on the R eco age because older C emissions were masked by younger CO 2 produced from the litter layer or plant respiration. In the High Arctic, anticipated changes in precipitation regime associated with warming are a key uncertainty for understanding future C cycling. In polar semi-deserts, water-year precipitation is an important driver of summertime R eco . Permafrost C is vulnerable to changes in snowpack, with a deeper snowpack promoting decomposition of older soil C.

Description: The net plankton community metabolism of oceanic surface waters is particularly important as it more directly affects the partial pressure of CO 2 in surface waters and, thus, the air-sea fluxes of CO 2 . Plankton communities in surface waters are exposed to high irradiance that includes significant ultraviolet B radiation (UVB, 280–315 nm). UVB radiation affects both photosynthetic and respiration rates, increase plankton mortality rates, and other metabolic and chemical processes. Here we test the sensitivity of net community production (NCP) to UVB of planktonic communities in surface waters across contrasting regions of the ocean. We observed here that UVB radiation affects net plankton community production at the ocean surface, imposing a shift in NCP by, on average, 50 % relative to the values measured when excluding partly UVB. Our results show that under full solar radiation, the metabolic balance shows the prevalence of net heterotrophic community production. The demonstration of an important effect of UVB radiation on NCP in surface waters presented here is of particular relevance in relation to the increased UVB radiation derived from the erosion of the stratospheric ozone layer. Our results encourage design future research to further our understanding of UVB effects on the metabolic balance of plankton communities.

Description: Rivers receive and process large quantities of terrestrial dissolved organic carbon (DOC). Biologically available (unstable) DOC leached from primary producers may stimulate (i.e., prime) the consumption of more stable terrestrially-derived DOC by heterotrophic microbes. We measured microbial DOC consumption (i.e., decay rates) from contrasting C sources in ten rivers in the Western and Midwestern United States using short-term bioassays of river water, soil and algal leachates, glucose, and commercial humate. We added inorganic nutrients (ammonium and phosphorus) to a subset of bioassays. We also amended a subset of river, soil, and commercial humate bioassays with glucose or algal leachates to test the hypothesis that unstable DOC primes consumption of more stable DOC. We used prior measurements of source-specific DOC bioavailability, linked with a Bayesian process model, to estimate means and posterior probability distributions for source-specific DOC decay rates in multi-source bioassays. Modeled priming effects ranged from a −130 to +370% change in more stable DOC decay when incubated with unstable DOC. Glucose increased modeled river DOC decay by an average of 87% among all rivers. Glucose and algal leachates increased soil leachate and commercial humate decay by an average of 25% above background rates. Inorganic nutrient additions did not have consistent effects on DOC decay, likely because most of the study rivers had high ambient background nutrients. Our results demonstrate that the priming effect can augment DOC decay in rivers. In addition, Bayesian models can be used to estimate mechanisms driving aquatic ecosystem processes that are difficult to measure directly.

Description: At the interface between stream water, groundwater and the hyporheic zone (HZ), important biogeochemical processes occur, that play a crucial role in fluvial ecology. Solutes that infiltrate into the HZ can react with each other and possibly also with upwelling solutes from the groundwater. In this study, we systematically evaluate how variations of gaining and losing conditions, stream discharge and pool-riffle morphology affect aerobic respiration (AR) and denitrification (DN) in the HZ. For this purpose, a computational fluid dynamics (CFD) model of stream water flow is coupled to a reactive transport model. Scenarios of variations of the solute concentration in the upwelling groundwater were conducted. Our results show that solute influx, residence time and the size of reactive zones strongly depend on presence, magnitude and direction of ambient groundwater flow. High magnitudes of ambient groundwater flow lower AR efficiency by up to four times and DN by up to three orders of magnitude, compared to neutral conditions. The influence of stream discharge and morphology on the efficiency of AR and DN are minor, in comparison to that of ambient groundwater flow. Different scenarios of O 2 and NO 3 concentrations in the upwelling groundwater reveal that DN efficiency of the HZ is highest under low upwelling magnitudes accompanied with low concentrations of O 2 and NO 3 . Our results demonstrate how ambient groundwater flow influences solute transport, AR and DN in the HZ. Neglecting groundwater flow in stream-groundwater interactions would lead to a significant overestimation of the efficiency of biogeochemical reactions in fluvial systems.

Description: Free Air CO 2 Enrichment (FACE) experiments provide a remarkable wealth of data which can be used to evaluate and improve terrestrial ecosystem models (TEMs). In the FACE Model-Data Synthesis project (FACE-MDS), 11 TEMs were applied to two decade-long FACE experiments in temperate forests of the south eastern US—the evergreen Duke Forest and the deciduous Oak Ridge forest. In this baseline paper, we demonstrate our approach to model-data synthesis by evaluating the models' ability to reproduce observed Net Primary Productivity (NPP), transpiration and Leaf Area Index (LAI) in ambient CO 2 treatments. Model outputs were compared against observations using a range of goodness-of-fit statistics. Many models simulated annual NPP and transpiration within observed uncertainty. We demonstrate however, that high goodness-of-fit values do not necessarily indicate a successful model, because simulation accuracy may be achieved through compensating biases in component variables. For example, transpiration accuracy was sometimes achieved with compensating biases in leaf area index and transpiration per unit leaf area. Our approach to model-data synthesis therefore goes beyond goodness-of-fit to investigate the success of alternative representations of component processes. Here, we demonstrate this approach by comparing competing model hypotheses determining peak LAI. Of three alternative hypotheses—(1) optimisation to maximise carbon export, (2) increasing specific leaf area (SLA) with canopy depth and (3) the pipe model—the pipe model produced peak LAI closest to the observations. This example illustrates how datasets from intensive field experiments such as FACE can be used to reduce model uncertainty despite compensating biases, by evaluating individual model assumptions.

Description: Disturbances are increasing globally due to anthropogenic changes in land use and climate. This study determines whether a disturbance that affects the physiology of individual trees can be used to predict the response of the ecosystem by weighing two competing hypothesis at annual time scales: (a) changes in ecosystem fluxes are proportional to observable patterns of mortality or (b) to explain ecosystem fluxes the physiology of dying trees must also be incorporated. We evaluate these hypotheses by analyzing six years of eddy-covariance flux data collected throughout the progression of a spruce beetle ( Dendroctonus rufipennis ) epidemic in a Wyoming Engelmann spruce ( Picea engelmannii )-subalpine fir ( Abies lasiocarpa ) forest and testing for changes in canopy conductance ( g c ), evapotranspiration (ET), and net ecosystem exchange (NEE) of CO 2 . We predict from these hypotheses that (a) g c , ET, and NEE all diminish (decrease in absolute magnitude) as trees die or (b) that (1) g c and ET decline as trees are attacked (hydraulic failure from beetle associated blue-stain fungi) and (2) NEE diminishes both as trees are attacked (restricted gas exchange) and when they die. Ecosystem fluxes declined as the outbreak progressed and the epidemic was best described as two phases: (I) hydraulic failure caused restricted g c , ET (28 ± 4% decline, Bayesian posterior mean ± standard deviation), and gas exchange (NEE diminished 13 ± 6%) and (II) trees died (NEE diminished 51 ± 3% with minimal further change in ET to 36 ± 4%). These results support hypothesis (b) and suggest that model predictions of ecosystem fluxes following massive disturbances must be modified to account for changes in tree physiological controls and not simply observed mortality.

Description: To improve understanding and prediction of dissolved organic carbon (DOC) sources and fluxes in northern peat-dominated catchments we present the development and application of a parsimonious tracer-aided rainfall-runoff model coupled with a biogeochemistry sub-routine able to concurrently simulate streamflow and DOC dynamics. The modelling approach which included quantitative assessment of associated uncertainties was conditioned by geochemical tracers which discriminate dominant water sources. Integration of DOC was predicated on statistical time series models which identified air temperature and stream flow as the key proxies that capture DOC supply and transport processes in two upland catchments in Scotland, UK. Conceptualizing the non-linear partitioning of quick near-surface and slower groundwater runoff sources in combination with a DOC mass balance resulted in a coupled, low-parameter mechanistic model. Model tests showed mostly sensitive parameters and reasonable simulation results with seasonally-controlled DOC supply and event-based DOC transport. Transport is facilitated even for smaller events by overland flow from saturated histosols connected to the stream network. However, during prolonged dry periods near-surface runoff “switches off” and stream DOC is dominated by low concentration groundwaters. Furthermore, the model was able to explain subtle differences in DOC dynamics between the two catchments mainly reflecting the distribution of saturated soils and available storage. We conclude that tracers and statistical time series models can successfully guide the development of parsimonious, yet structurally consistent water quality models. Parsimonious models provide tools for estimating DOC dynamics and loads with reduced uncertainty and potentially greater transferability.

Description: We measured CH 4 flux at high temporal resolution with triplicate autochambers from three different plant communities at the ombrotrophic Mer Bleue bog in Canada to investigate the spatial and temporal variations, and factors that related to the CH 4 flux. Our results show that seasonal mean CH 4 fluxes from the Eriophorum -dominated community were 1.4-2.2 and 3.7-5.5 times higher than those from Maianthemum/Ledum and Chamaedaphne communities, respectively. Significant interannual variations in CH 4 flux were observed in Maianthemum/Ledum and Chamaedaphne communities, attributable to a 55-60% reduction of mean summer (July-September) CH 4 flux in 2010 as a consequence of a 5.5-9.0 cm lower mean summer water table compared to 2009. The Eriophorum community showed a much larger rate of increase in CH 4 flux with peat temperature in the early growing season than in mid-summer that might be caused by a concomitant increase in root exudation of labile carbon for methanogenesis. Temporal variability of log-transformed CH 4 flux was correlated ( r  ≥ 0.4) with peat temperature only when water table was less than 20, 30 and 40 cm below the peat surface for Maianthemum/Ledum , Chamaedaphne , and Eriophorum communities, respectively. This difference in water table threshold among communities might partly be related to differences in rooting depth and hence the ability of plants to sustain CH 4 flux in dry conditions. These results suggest that modelling of CH 4 flux from ombrotrophic peatlands over time should take into account the role of different vegetation types, since the relationships between CH 4 emissions and environmental factors vary among vascular plant communities.

Description: Drought has been a concern in global and regional water, carbon and energy cycles. From 1999-2011, Northern China experienced a multiyear precipitation reduction that significantly decreased water availability as indicated by the Palmer Drought Severity Index and soil moisture measurements. In this study, a light use efficiency model (EC-LUE) and an ecosystem physiological model (IBIS) were used to characterize the impacts of long-term drought on terrestrial carbon fluxes in Northern China. EC-LUE and IBIS models showed that the reduction of averaged GPP of 0.09 and 0.05 Pg C yr -1 during 1999-2011 compared with 1982-1998. Based on the IBIS model, simulated ecosystem respiration underwent an insignificant decrease from 1999-2011. The multiyear precipitation reduction changed the regional carbon uptake of 0.011 Pg C yr -1 from 1982-1998 to a net source of 0.018 Pg C yr -1 from 1999-2011. Moreover, a pronounced decrease in maize yield in almost all provinces in the study region was found from 1999-2011 versus the average of yield from1978-2011. The largest maize yield reduction occurred in Beijing (2499 kg ha -1  yr -1 ), Jilin (2180 kg ha -1  yr -1 ), Tianjing (1923 kg ha -1  yr -1 ) and Heilongjiang (1791 kg ha -1  yr -1 ), and the maize yield anomaly was significantly correlated with the annual precipitation over the entire study area. Our results revealed that recent climate change, especially drought-induced water stress, is the dominant cause of the reduction in the terrestrial carbon sink over Northern China.

Description: Estimates of peat depth are required to inform understanding of peatland development, functioning, and ecosystem services such as carbon storage. However, there is a considerable lack of peat depth data at local, national and global scales. Recent studies have attempted to address this knowledge deficit by using manual probing and Ground Penetrating Radar (GPR) to estimate depth. Despite increasing application, little consideration has been given to the accuracy of either of these techniques. This study examines the accuracy of probing and GPR for measuring peat depth. Corresponding GPR and probing surveys were carried out at a catchment scale in a blanket peatland. GPR depth estimations, calibrated using common mid-point (CMP) surveys, were found to be on average 35% greater than probe measurements. The source of disagreement was found to be predominantly caused by depth probes becoming obstructed by artifacts within the peat body, although occasionally probing rods also penetrated sediments underlying the peat. Using the Complex Refractive Index Model (CRIM), it was found that applying a single velocity of 0.036 m ns -1 across a single site may also result in -8 to +17% error in estimation of peat depth due to spatial variability in water content and porosity. It is suggested that GPR calibrated at each site using CMP surveys may provide a more accurate method for measuring peat depth.

Description: [1]  A nested sampling network on the Colorado (CR) and Missouri Rivers (MR) provided data to assess impacts of large-scale reservoir systems and climate on carbon export. The LOADEST model was used to estimate both DIC and DOC fluxes for a total of 22 sites along the main stem of the CR and MR. Both the upper CR and MR DIC and DOC fluxes increased longitudinally, but the lower CR fluxes decreased while the lower MR's continued to increase. We examined multiple factors through space and time that help explain these flux patterns. Seasonal variability in precipitation and temperature, along with site-level concentration versus discharge relationships proved to be significant factors explaining much of the difference between sites located below reservoirs as compared to sites located in more free-flowing segments of the river. The characterization of variability in C exports over space and time provides a basis for understanding C cycling and transport within river basins affected by large reservoir systems, particular in arid-to semi-arid ecosystems.

Description: No abstract is available for this article.

Description: No abstract is available for this article.

Description: Measurements of the stable isotope composition of soil flux have many uses, from separating autotrophic and heterotrophic components of respiration to teasing apart information about gas transport physics. While soil flux chambers are typically used for these measurements, subsurface approaches are becoming more accessible with the introduction of field-deployable isotope analyzers. These subsurface measurements have the unique benefit of offering depth-resolved isotopologue flux data, which can help to disentangle the many soil respiration processes that occur throughout the soil profile. These methods are likely to grow in popularity in the coming years and a solid methodological basis needs to be formed in order for data collected in these subsurface studies to be interpreted properly. Here we explore the range of possible techniques that could be used for subsurface isotopologue gas interpretation and rigorously test the assumptions and application of each approach using a combination of numerical modeling, laboratory experiments and field studies. Our results suggest that methodological uncertainties arise due to poor assumptions and mathematical instabilities but certain methods, particularly those based on diffusion physics, are able to cope with these uncertainties well and produce excellent depth-resolved isotopologue flux data.

Description: ABSTRACT Although snags and coarse woody debris are a small component of ecosystem respiration, disturbances can significantly increase the mass and respiration from these carbon (C) pools. The objectives of this study were to 1) measure respiration rates of snags and coarse woody debris throughout the year in a forest previously defoliated by gypsy moths, 2) develop models for dead stem respiration rates 3) model stand-level respiration rates of dead stems using forest inventory and analysis datasets and environmental variables pre- and post-disturbance and 4) compare total dead stem respiration rates with total ecosystem respiration and net ecosystem exchange. Respiration rates were measured on selected Pinus and Quercus snags and coarse woody debris each month for one year in a northeastern US temperate forest. Multiple linear regression using environmental and biometric variables including wood temperature, diameter, density, species and decay class was used to model respiration rates of dead stems. The mass of snags and coarse woody debris increased more than five-fold after disturbance and respiration rates increased more than three-fold. The contribution of dead stems to total ecosystem respiration more than tripled from 0.85% to almost 3% and respiration from dead stems alone was approximately equal to the net ecosystem exchange of the forest in 2011 (fourth year post-disturbance). This study highlights the importance of dead stem C pools and fluxes particularly during disturbance and recovery cycles. With climate change increasing the ranges of many forest pests and pathogens, these data become particularly important for accurately modeling future C cycling.

Description: We characterized peat decomposition at the Marcell Experimental Forest (MEF), Minnesota, USA, to a depth of 2 meters to ascertain the underlying chemical changes using Fourier Transform Infrared (FT IR) and 13 C Nuclear Magnetic Resonance ( 13 C NMR) spectroscopy) and related these changes to decomposition proxies C:N ratio, δ 13 C and δ 15 N, bulk density and water content. FT IR determined peat humification increased rapidly between 30 and 75 cm, indicating a highly reactive intermediate-depth zone consistent with changes in C: N ratio, δ 13 C and δ 15 N, bulk density and water content. Peat decomposition at the MEF, especially in the intermediate depth zone, is mainly characterized by preferential utilization of O-alkyl-C, carboxyl-C, and other oxygenated functionalities with a concomitant increase in the abundance of alkyl- and nitrogen-containing compounds. Below 75-cm, less change was observed but aromatic functionalities and lignin accumulated with depth. Significant correlations with humification indices, identified by FT IR spectroscopy, were found for C: N ratios. Incubation studies at 22 °C revealed the highest methane production rates, greatest CH 4 :CO 2 production ratios and significant O-alkyl-C utilization within this 30 and 75 cm zone. Oxygen-containing functionalities, especially O-alkyl-C, appear to serve as excellent proxies for soil decomposition rate, and should be a sensitive indicator of the response of the solid phase peat to increased temperatures caused by climate change and the field study manipulations that are planned to occur at this site. Radiocarbon signatures of microbial respiration products in deeper porewaters at the MEF resembled the signatures of more modern dissolved organic carbon (DOC) rather than solid phase peat, indicating that recently photosynthesized organic matter fueled the bulk of subsurface microbial respiration. These results indicate that carbon cycling at depth at the MEF is not isolated from surface processes.

Description: Dissolved organic carbon (DOC) and inorganic carbon (DIC and p CO 2 ), lignin biomarkers and the optical properties of dissolved organic matter (DOM) were measured in a gradient of streams and rivers within the Congo Basin (Republic of Congo), with the aim of examining how vegetation cover and hydrology influences the composition and concentration of exported fluvial carbon (C). Three sampling campaigns (February 2010, November 2010 and August 2011) spanning 56 sites are compared by sub-basin watershed land cover type (savannah, tropical forest, and swamp) and hydrologic regime (high, intermediate, and low). Land cover properties predominately controlled the amount and quality of DOC, chromophoric DOM (CDOM) and lignin phenol concentrations (∑ 8 ) exported in streams and rivers throughout the Congo Basin. Higher DIC concentrations and changing DOM composition (lower molecular weight, less aromatic C) during periods of low hydrologic flow indicated a shift from rapid overland supply pathways in wet conditions to deeper groundwater inputs during drier periods. Lower DOC concentrations in forest and swamp sub-basins were apparent with increasing catchment area, indicating enhanced DOC loss with extended water residence time. Surface water p CO 2 in savannah and tropical forest catchments ranged between 2600 and 11922 µatm, and swamp regions contained extremely high p CO 2 (10598-15802 µatm), highlighting their potential as significant pathways for water-air efflux. Our data suggest that the quantity and quality of DOM exported to streams and rivers is largely driven by terrestrial ecosystem structure and that anthropogenic land-use or climate change may impact the composition and reactivity of fluvial C, with ramifications for regional C budgets and future climate scenarios.

Description: Non-methane biogenic volatile organic compounds (BVOCs) play key roles in the atmosphere, where they can influence a wide range of chemical processes, and in soils, where they can alter the rates of biogeochemical cycles and impact the growth of plants and soil organisms. However, the diversity and quantities of BVOCs released from or taken up by soils remain poorly characterized as do the biotic and abiotic controls on these fluxes. Here, we used proton transfer reaction mass spectrometry (PTR-MS) to quantify BVOC flux rates from soils with and without active root systems in a subalpine coniferous forest. The total measured BVOC flux averaged 102 nmol m -2  h -1 (an estimated 2.0 µg-C m -2  h -1 ). The individual BVOCs with the highest net emissions from soil included monoterpenes and methanol (averaging 646 and 641 ng-C m -2  h -1 , respectively) while soil represented a net sink of isoprene (-98 ng-C m -2  h -1 ) and formaldehyde (-37 ng-C m -2  h -1 ). Tree roots, directly or indirectly, contributed an average of 53% of the total carbon emitted from the soil as BVOCs, with methanol and acetaldehyde among those BVOCs most strongly associated with active root presence. The fluxes of most of the dominant BVOCs emitted from soil, including methanol, increased linearly with increasing temperature. Together the fluxes of certain BVOCs into or out of the forest floor (particularly methanol, isoprene, and monoterpenes) are likely relevant to ecosystem-level processes and belowground ecology, but these fluxes are highly variable and are strongly controlled by both root presence and soil abiotic conditions.

Description: Dissolved oxygen (DO) and heat exchanges across the water sediment interface (WSI) of a shallow lagoon are controlled by processes occurring on both sides of the WSI, particularly volumetric source and sink on the sediment side and turbulent transport on the water side. This article presents and analyzes measurements of DO ( J s ) and heat ( H g ) fluxes across the WSI in the extremely shallow lagoon of Salar del Huasco (20.274 o S, 68.883 o W, 3800 m above sea level), where volumetric source of DO and heat exists in the sediment layer, related to benthic primary production and absorption of solar radiation, respectively. Micro-profiles of temperature and DO were measured, and they were used for measuring J s and H g , and volumetric source/sink terms in the sediments. This information was used to propose and validate the simple theoretical framework to predict both the magnitude and direction of J s and H g . On the one hand, J s can be predicted with a simple algebraic expression, where the diffusional mass transfer coefficient defines the magnitude of J s while the direction is controlled by the balance between DO production and consumption in the sediments. On the other hand, solar radiation is absorbed in the upper sediments, and this heat diffuses toward the water column and the sediments. The heat flux toward the water column also induces unstable convection that promotes vertical transport across the WSI. The theoretical framework proposed here will help to understand DO and heat budgets of shallow aquatic systems in which solar radiation reaches the WSI.

Description: Dynamic vegetation models have been used to assess the resilience of tropical forests to climate change, but the global application of these modeling experiments often misrepresent carbon dynamics at a regional level, limiting the validity of future projections. Here, a dynamic vegetation model (LPJ-GUESS) was adapted to simulate present day potential vegetation as a baseline for climate change impact assessments in the evergreen and deciduous forests of Bolivia. Results were compared to biomass measurements (819 plots), and remote sensing data. Using regional parameter values for allometric relations, specific leaf area, wood density and disturbance interval, a realistic transition from the evergreen Amazon to the deciduous dry forest was simulated. This transition coincided with threshold values for precipitation (1400 mm yr −1 ) and water deficit (i.e. potential evapotranspiration minus precipitation) (−830 mm yr −1 ), beyond which leaf abscission became a competitive advantage. Significant correlations were found between modeled and observed values of seasonal leaf abscission (R2 = 0.6, p 〈0.001) and vegetation carbon (R 2  = 0.31, p 〈0.01). Modeled Gross Primary Productivity (GPP) and remotely sensed Normalized Difference Vegetation Index (NDVI) showed that dry forests were more sensitive to rainfall anomalies than wet forests. GPP was positively correlated to the El Niño Southern Oscillation index in the Amazon, and negatively correlated to consecutive dry days. Decreasing rainfall trends were simulated to reduce GPP in the Amazon. The current model set-up provides a baseline for assessing the potential impacts of climate change in the transition zone from wet to dry tropical forests in Bolivia.

Description: Dissolved organic carbon (DOC) production, consumption, and quality displayed differences after long-term (~55 years) hydrological alterations in a poor fen peatland in northern Michigan. The construction of an earthen levee resulted in areas of a raised and lowered water table (WT) relative to an unaltered intermediate WT site. The lowered WT site had greater peat aeration and larger seasonal vertical WT fluctuations that likely elevated peat decomposition and subsidence with subsequent increases in bulk density, vertical hydraulic gradient, decreased hydraulic conductivity (K sat ), and a greater pore water residence time relative to the unaltered site. The raised WT site displayed a decreased K sat combined with seasonal upwelling events that contributed to a longer residence time in comparison to the unaltered site. These differences are potentially contributing to elevated DOC concentrations at the lowered and raised WT site relative to the unaltered site. Additionally, spectrophotometric indices and chemical constituent assays indicated the lowered site DOC was more aromatic and contained elevated concentrations of phenolics compared to the intermediate site. The raised site DOC was less aromatic, more humified, and also had a greater phenolic content than the intermediate site. Furthermore, a microbial mineralization incubation showed DOC in the raised site contained the greatest labile carbon source. Based on our results, long-term WT alterations will likely impose significant effects on DOC dynamics in these peatlands; however, WT position alone was not a good predictor of DOC concentrations, though impoundment appears to produce a more labile DOC whereas drainage increases DOC aromaticity.

Description: Soils are large sources of atmospheric greenhouse gases, and both the magnitude and composition of soil gas emissions are strongly controlled by redox conditions. Though the effect of redox dynamics on greenhouse gas emissions has been well studied in flooded soils, less research has focused on redox dynamics without total soil inundation. For the latter, all that is required are soil conditions where the rate of oxygen (O 2 ) consumption exceeds the rate of atmospheric replenishment. We investigated the effects of soil anaerobiosis, generated with and without flooding, on greenhouse gas emissions and redox-sensitive biogeochemistry. We collected a Histosol from a regularly flooded peatland pasture and an Ultisol from a humid tropical forest where soil experiences frequent low redox events. We used a factorial design of flooding and anaerobic dinitrogen (N 2 ) headspace treatments applied to replicate soil microcosms. An N 2 headspace suppressed carbon dioxide (CO 2 ) emissions by 50 % in both soils. Flooding, however, led to greater anaerobic CO 2 emissions from the Ultisol. Methane emissions under N 2 were also significantly greater with flooding in the Ultisol. Flooding led to very low N 2 O emissions after an initial pulse in the Histosol, while higher emission rates were maintained in control and N 2 treatments. We conclude that soil greenhouse gas emissions are sensitive to the redox effects of O 2 depletion as a driver of anaerobiosis, and that flooding can have additional effects independent of O 2 depletion. We emphasize that changes to the soil diffusive environment under flooding impacts transport of all gases, not only O 2 , and changes in dissolved solute availability under flooding may lead to increased mineralization of C.

Description: Thawing subsea permafrost controls methane release from the Russian Arctic shelf having a considerable impact on the climate-sensitive Arctic environment. Expulsions of methane from shallow Russian Arctic shelf areas may continue to rise in response to intense degradation of relict subsea permafrost. Here, we show modeling of the permafrost evolution from the Late Pleistocene to present time at the West Yamal shelf. Modeling results suggest a highly-dynamic permafrost system that directly responds to even minor variations of lower and upper boundary conditions, e.g. geothermal heat flux from below and/or bottom water temperature changes from above permafrost. Scenarios of permafrost evolution show a potentially nearest landward modern extent of the permafrost at the West Yamal shelf limited by ~17 m isobaths, whereas its farthest seaward extent coincides with ~100 m isobaths. The model also predicts seaward tapering of relict permafrost with a maximal thickness of 275–390 m near the shore line. Previous field observations detected extensive emissions of free gas into the water column at the transition zone between today's shallow-water permafrost (〈20 m) and deeper water non-permafrost areas (〉20 m). The model adapts well to corresponding heat flux and ocean temperature data, providing crucial information about the modern permafrost conditions. It shows current locations of upper and lower permafrost boundaries and evidences for possible release of methane from the seabed to the hydrosphere in a warming Arctic.

Description: Measuring methane (CH 4 ) flux at upland forests is challenging due to high levels of heterogeneity in upscaling chamber measurements and the detection limits of currently available micrometeorological methods. We estimated CH 4 fluxes in an upland forest from vertical concentration profiles using three different inverse multi-layer models: the Lagrangian localized near field theory, Eulerian, and hybrid Lagrangian–Eulerian models. The approach could estimate spatially representative fluxes, and use of higher gradients within canopies than above them could minimize uncertainties associated with sensor noises. Comparing fluxes by the models and measurements by the micrometeorological hyperbolic relaxed eddy accumulation and chamber methods, daytime fluxes were reasonably reproduced, but nighttime fluxes were overestimated most likely due to an underestimation of stable conditions and storage effects. The models and measurements show that the forest acted as a CH 4 sink during the study period, and the soil acted as the dominant sink. The estimated sink increased with increasing soil temperatures and decreasing soil water content. The CH 4 sink estimated during the study period were 1.5 ± 0.2 nmol m −2  s −1 by the micrometeorological method, 2.4 ± 0.5 nmol m −2  s −1 by chambers, 2.8 ± 1.1 nmol m −2  s −1 by the Lagrangian model, 2.7 ± 1.0 nmol m −2  s −1 by the Eulerian model, and 3.7 ± 2.8 nmol m −2  s −1 by the hybrid model. The performance of the Lagrangian and hybrid models increased when the CH 4 sink/source was assumed to only exist in the soil.

Description: The objective of this research was to measure temporal variability in accretion and mass sedimentation rates (including organic carbon (OC), total nitrogen (TN), and total phosphorous (TP)) from the past century in a mangrove forest on the Shark River in Everglades National Park, USA. The 210 Pb Constant Rate of Supply model was applied to six soil cores to calculate annual rates over the most recent 10-, 50-, and 100-year time spans. Our results show that rates integrated over longer timeframes are lower than those for shorter, recent periods of observation. Additionally, the substantial spatial variability between cores over the 10-year period is diminished over the 100-year record, raising two important implications. First, a multiple-decade assessment of soil accretion and OC burial provides a more conservative estimate, and is likely to be most relevant for forecasting these rates relative to long-term processes of sea level rise and climate change mitigation. Secondly, a small number of sampling locations are better able to account for spatial variability over the longer periods than for the shorter periods. The site average 100-year OC burial rate, 123 ± 19 (SD) g m -2  yr -1 , is low compared with global mangrove values. High TN and TP burial rates in recent decades may lead to increased soil carbon remineralization, contributing to the low carbon burial rates. Finally, the strong correlation between OC burial and accretion across this site signals the substantial contribution of OC to soil building in addition to the ecosystem service of CO 2 sequestration.

Description: The distribution and sources of particulate organic carbon (POC) and nitrogen (PN) in 27 Indian estuaries were examined during the monsoon using the content and isotopic composition of carbon and nitrogen. Higher phytoplankton biomass was noticed in estuaries with deeper photic zone than other estuaries receiving higher suspended matter. The δ 13 C POC and δ 15 N PN data suggests that relatively higher δ 13 C POC (−27.9 to −22.6‰) and lower δ 15 N PN (0.7 to 5.8‰) were noticed in the estuaries located in the northern India, north of 16°N, and lower δ 13 C POC (−31.4 to −28.2‰) and higher δ 15 N PN (5 to 10.3‰) in the estuaries located in the southern India. This is associated with higher Chl-a in the northern than southern estuaries suggesting that in situ production contributed significantly to the POC pool in the former whereas terrestrial sources are important in the latter estuaries. The spatial distribution pattern of δ 15 N PN is consistent with fertilizer consumption in the Indian subcontinent, which is twice as much in the northern India as in the south whereas δ 13 C POC suggests that in situ production is a dominant source in the southern and terrestrial sources are important in the northern estuaries. Based on the SIAR (Stable Isotope Analysis in R) model, 40-90% (70-90%) of organic matter is contributed by C 3 plants (freshwater algae) in the estuaries located in the northern (southern) India.

Description: No abstract is available for this article.

Description: Hydraulic redistribution is the process of soil water transport through the low-resistance pathway provided by plant roots. It has been observed in field studies and proposed to be one of the processes that enable the Amazon rainforest to resist periodical dry spells without experiencing water limitations. How and to what extent hydraulic redistribution may increase vegetation resistance to longer or more severe droughts than seasonal dryness has not been investigated yet, which is the focus of this study. The artificially prolonged drought produced by the rainfall exclusion experiment is used as an example of long drought and the 2005 drought is used as a severe drought. The parameterization of hydraulic redistribution proposed by Ryel et al . [2002] was incorporated into the Community Land Model version 4 (CLM4). Three paired numerical experiments were conducted, one set using the default model (CTL) and the other using the model with considerations of hydraulic redistribution (HR). Results show that the vegetation response (including evapotranspiration, biomass, and LAI) to dryness of all the three types is better captured with hydraulic redistribution incorporated. Plants are more resistant to dryness when hydraulic redistribution increases plant water availability and thus facilitates their growth. When a drought is long lasting, the vegetation response is delayed by hydraulic redistribution. Therefore, if a drought ends earlier than permanent damage is made, the magnitude of vegetation response will be lowered by this mechanism, i.e. the vegetation will be more resistant to dryness.

Description: The thick permanent ice cover on the lakes of the McMurdo Dry Valleys, Antarctica inhibits spatial lake sampling due to logistical constraints of penetrating the ice-cover. To date most sampling of these lakes has been made at only a few sites with the assumption that there is a spatial homogeneity of the physical and biogeochemical properties of the ice cover and the water column at any given depth. To test this underlying assumption, an autonomous underwater vehicle (AUV) was deployed in Lake Bonney, Taylor Valley. Measurements were obtained over the course of two years in a 100 x 100 meter horizontal sampling grid (at a 0.2 m vertical resolution). Additionally, the AUV measured the ice thickness (in water equivalent) and collected images looking up through the ice, which were used to quantify sediment distribution on the surface and within the ice. Satellite imagery was used to map sediment distribution on the surface of the ice. We present results of the spatial investigation of the sediment distribution on the ice cover and its effects on biological processes, with particular emphasis on photosynthetically active radiation (PAR). The surface sediment is a secondary controller of the ice cover thickness, which in turn controls the depth-integrated PAR in the water column. Our data revealed that depth-integrated PAR was negatively correlated with depth-integrated chlorophyll-a (r = 0.88, p 〈 0.001, n = 83), which appears to be related to short-term photoadaptation of phytoplanktonic communities to spatial and temporal variation in PAR within the water column.

Description: Coastal freshwater wetland chemistry is rapidly changing due to increased frequency of salt water incursion, a consequence of global change. Seasonal salt water incursion introduces sulfate, which microbially reduces to sulfide. Sulfide binds with reduced iron, producing iron sulfide (FeS), recognizable in wetland soils by its characteristic black color. The objective of this study is to document iron and sulfate reduction rates, as well as product formation (acid- volatile and chromium reducible sulfide, AVS and CRS) in a coastal freshwater wetland undergoing seasonal salt water incursion. Understanding iron and sulfur cycling, as well as their reduction products, allows us to calculate the Degree of Sulfidization (DOS), from which we can estimate how long soil iron will buffer against chemical effects of sea-level rise. We show that soil chloride, a direct indicator of the degree of incursion, best predicted iron and sulfate reduction rates. Correlations between soil chloride and iron or sulfur reduction rates were strongest in the surface layer (0-3 cm), indicative of surface water incursion, rather than groundwater intrusion at our site. The interaction between soil moisture and extractable chloride was significantly related to increased AVS, whereas increased soil chloride was a stronger predictor of CRS. The current DOS in this coastal plains wetland is very low, resulting from high soil iron content and relatively small degree of salt water incursion. However, with time and continuous salt water exposure, iron will bind with incoming sulfur, creating FeS complexes, and DOS will increase.

Description: This study focused on the time series analysis of passive microwave and optical satellite data collected from six Southern Hemisphere ecosystems in Australia and Argentina. The selected ecosystems represent a wide range of land cover types, including deciduous open forest, temperate forest, tropical and semi-arid savannas, and grasslands. We used two microwave indices, the Frequency Index (FI) and Polarization Index (PI), to assess the relative contributions of soil and vegetation properties (moisture and structure) to the observations. Optical-based satellite vegetation products from the Moderate Resolution Imaging Spectroradiometer (MODIS) were also included to aid in the analysis. We studied the X and Ka bands of the Advanced Microwave Scanning Radiometer-EOS (AMSR-E) and Wind Satellite (WINDSAT), resulting in up to four observations per day (1:30, 6:00, 13:30 and 18:00 h). Both the seasonal and hourly variations of each of the indices were examined. Environmental drivers (precipitation and temperature) and eddy covariance measurements (gross ecosystem productivity and latent energy) were also analyzed. It was found that in moderately dense forests FI was dependent on canopy properties (LAI and vegetation moisture). In tropical woody savannas, a significant regression (R 2 ) was found between FI and PI with precipitation (R 2  〉 0.5) and soil moisture (R 2  〉 0.6). In the areas of semi-arid savanna and grassland ecosystems, FI variations found to be significant related to soil moisture (R 2  〉 0.7) and evapotranspiration (R 2  〉 0.5), while PI varied with vegetation phenology. Significant differences (p 〈0.01) were found among FI values calculated at the four local times.

Description: Recent warming at high latitudes has accelerated permafrost thaw, which can modify soil carbon dynamics and watershed hydrology. The flux and composition of dissolved organic matter (DOM) from soils to rivers is sensitive to permafrost configuration and its impact on subsurface hydrology and groundwater discharge. Here, we evaluate the utility of DOM composition and age as a tool for detecting permafrost thaw in three rivers (Beaver, Birch and Hess Creeks) within the discontinuous permafrost zone of interior Alaska. We observed strong temporal controls on Δ 14 C content of hydrophobic acid isolates (Δ 14 C-HPOA) across all rivers, with the most enriched values occurring during spring snowmelt (75 ± 8 ‰) and most depleted during winter flow (−21 ± 8 ‰). Radiocarbon ages of winter flow samples ranged from 35 to 445 y BP, closely tracking estimated median baseflow travel times for this region (335 y). During spring snowmelt, young DOM was composed of highly aromatic, high molecular-weight compounds, whereas older DOM of winter flow had lower aromaticity and molecular weight. We observed a significant correlation between Δ 14 C-HPOA and UV absorbance coefficient at 254 nm ( α 254 ) across all study rivers. Using α 254 as an optical indicator for Δ 14 C-HPOA, we also observed a long-term decline in α 254 during maximum annual thaw depth over the last decade at the Hess Creek study site. These findings suggest a shift in watershed hydrology associated with increasing active layer thickness. Further development of DOM optical indicators may serve as a novel and inexpensive tool for detecting permafrost degradation in northern watersheds.

Description: Model data integration (MDI) studies are key to parameterize ecosystem models that synthesize our knowledge about ecosystem function. The use of diverse datasets, however, results in strongly imbalanced contributions of data streams with model fits favoring the largest data stream. This imbalance poses new challenges in the identification of model deficiencies. A standard approach for balancing is to attribute weights to different data streams in the cost function. However, this may result in overestimation of posterior uncertainty. In this study, we propose an alternative: the parameter-block approach. The proposed method enables joint optimization of different blocks, i.e., subsets of the parameters, against particular data streams. This method is applicable when specific parameter blocks are related to processes that are more strongly associated with specific observations, i.e. data streams. A comparison of different approaches using simple artificial examples and the DALEC ecosystem model is presented. The unweighted inversion of a DALEC model variant, where artificial structural errors in photosynthesis calculation had been introduced, failed to reveal the resulting biases in fast processes (e.g., turnover). The posterior bias emerged only in parameters related to slower processes (e.g., carbon allocation) constrained by fewer datasets. On the other hand, when weighted or blocked approaches were used, the introduced biases were revealed, as expected, in parameters of fast processes. Ultimately, with the parameter-block approach, the transfer of model error was diminished and at the same time the overestimation of posterior uncertainty associated with weighting was prevented.

Description: Tundra ecosystem fire regimes are intensifying with important implications for regional and global carbon (C) and energy dynamics. Although a substantial portion of the tundra biome is located in Russia the vast majority of studies accessible describe North American tundra fires. Here we use field observations and high-resolution satellite remote sensing observations to describe the effects of wildfire on ecosystem C pools and vegetation communities four decades after fire for a tundra ecosystem in northeastern Siberia. Our analyses reveal no differences between soil physical properties and C pools in burned and unburned tundra, which we attribute to low combustion of organic soil associated with low-severity fire. Field and remote sensing data show no differences in aboveground C pools and vegetation communities indicating recovery to pre-fire conditions. These results are comparable to observations of ecosystem recovery in North American tundra. An assessment of literature data indicate that the average annual area burned in Russian tundra is an order of magnitude larger than that of Alaskan tundra, highlighting a crucial need to assess Russian tundra fire regimes in order to understand the current and future role of the biome wide fire regime in regional and global C and energy dynamics.

Description: Methane (CH 4 ) emissions were measured at the Wilma H. Schiermeier Olentangy River Wetland Research Park (ORWRP) over three summers and two winters using an eddy covariance system. We used an empirical model to determine the main environmental drivers of methane emissions. Methane emissions co-vary strongly with water vapor fluxes, CO 2 fluxes, and soil temperature. We adjust our models to account for the heterogeneous environment of the wetland by including the flux footprint distribution among different microsites as a predictive variable in the methane model. We used a forward linear stepwise model in combination with an Akaike Information Criteria-based model selection process and neural network modeling to determine which environmental variables are most effective in modeling methane emissions in our site. Different models and environmental variables best represented methane fluxes in the winter and summer and also during the day or night within each season. We parameterized an optimal empirical model for methane emissions from the ORWRP that is used for gap-filling of site-level methane fluxes over 2 years. Some of the most effective variables for modeling methane were carbon, water vapor and heat fluxes, all of which typically have the same data gaps as the time series of methane flux. In order to determine if these variables were useful for modeling methane despite the additional gap-filling error, we determined through an error propagation experiment that eddy covariance gap-filling models for methane may be best developed by including other gap-filled fluxes as predictors, despite the high level of shared gaps and subsequent gap-fill error propagation.

Description: In this paper, a global carbon assimilation system (GCAS) is developed for optimizing the global land surface carbon flux at 1° resolution using multiple ecosystem models. In GCAS, three ecosystem models, BEPS, CASA, and CABLE, produce the prior fluxes, and an atmospheric transport model MOZART is used to calculate atmospheric CO 2 concentrations resulting from these prior fluxes. A local ensemble Kalman filter is developed to assimilate atmospheric CO 2 data observed at 92 stations to optimize the carbon flux for six land regions, and the Bayesian model averaging (BMA) method is implemented in GCAS to calculate the weighted average of the optimized fluxes based on individual ecosystem models. The weights for the models are found according to the closeness of their forecasted CO 2 concentration to observation. Results of this study show that the model weights vary in time and space, allowing for an optimum utilization of different strengths of different ecosystem models. It is also demonstrated that spatial localization is an effective technique to avoid spurious optimization results for regions that are not well constrained by the atmospheric data. Based on the multi-model optimized flux from GCAS, we found that the average global terrestrial carbon sink over the 2002-2008 period is 2.97 ± 1.1 PgC year -1 , and the sinks are 0.88 ± 0.52, 0.27 ± 0.33, 0.67 ± 0.39, 0.90 ± 0.68, 0.21 ± 0.31, 0.04 ± 0.08 PgC year -1 for the North America, South America, Africa, Eurasia, Tropical Asia and Australia, respectively. This multi-model GCAS can be used to improve global carbon cycle estimation.

Description: Due to its marked vegetation phenology and precipitation gradients, the North American Monsoon Region (NAMR) is a useful domain for studying ecosystem responses to climate variability and change. To this end, we analyze long-term dynamics (1982–2004) in monsoon precipitation (Pr), time-integrated Normalized Difference Vegetation Index (TINDVI) used as proxy of net primary productivity, and rain-use efficiency (RUE). The analysis focuses on six ecoregions, spanning from desert environments to tropical dry forests, to investigate: (1) how net primary productivity and rain-use efficiency vary along a precipitation gradient; (2) if interannual variability in net primary productivity is linked to the interannual variability in precipitation; and (3) if there is evidence of a long-term signal imposed on the interannual variability in rain-use efficiency. Variations in TINDVI and RUE with Pr along the NAMR precipitation gradient differ among ecoregions exhibiting intensive or extensive water use strategies. We explain the nonlinear behaviors along the precipitation gradient as resulting from different physiological responses to climatological means and the impact of topographic effects. Statistical analysis indicates that the interannual variability in vegetation response is significantly related to the interannual variability in Pr, but their correlation declines with time. A long-term positive signal in RUE imposed on its interannual variability is identified and results from a constant TINDVI under negative long-term trends of Pr. This important finding suggests the combined long-term effects of ecosystem acclimation to reduced water availability and increasing CO 2 concentration across the varied ecosystems of the North American monsoon region.

Description: As high latitudes warm, a portion of the large organic carbon pool stored in permafrost will become available for transport to aquatic ecosystems as dissolved organic carbon (DOC). If permafrost DOC is biodegradable, much will be mineralized to the atmosphere in freshwater systems before reaching the ocean, accelerating carbon transfer from permafrost to the atmosphere, whereas if recalcitrant, it will reach marine ecosystems where it may persist over long time periods. We measured biodegradable DOC (BDOC) in water flowing from collapsing permafrost (thermokarst) on the North Slope of Alaska and tested the role of DOC chemical composition and nutrient concentration in determining biodegradability. DOC from collapsing permafrost was some of the most biodegradable reported in natural systems. However, elevated BDOC only persisted during active permafrost degradation, with a return to pre-disturbance levels once thermokarst features stabilized. Biodegradability was correlated with background nutrient concentration, but nutrient addition did not increase overall BDOC, suggesting that chemical composition may be a more important control on DOC processing. Despite its high biodegradability, permafrost DOC showed evidence of substantial previous microbial processing and we present four hypotheses explaining this incongruity. Because thermokarst features form preferentially on river banks and lake shores and can remain active for decades, thermokarst may be the dominant short-term mechanism delivering sediment, nutrients, and biodegradable organic matter to aquatic systems as the Arctic warms.

Description: Net ecosystem exchange (NEE) of tidal brackish wetlands in urban areas is largely unknown, albeit it is an important ecosystem service. High carbon dioxide (CO 2 ) uptake of estuaries can potentially be achieved by creating conditions that foster CO 2 uptake and sequestration. Thus, this study sought to assess NEE in a mesohaline tidal urban wetland that has been restored and determine the biophysical drivers of NEE in order to investigate uptake strength and drivers thereof. Beginning in 2009, NEE was measured using the eddy covariance technique in a restored urban estuarine wetland. Maximum NEE rates observed were −30 µmol m −2  s −1 under high light conditions in the summer. Monthly mean NEE showed this ecosystem to be a CO 2 source in the winter, but a CO 2 sink in summer. Conditional Granger causality showed influence of net radiation on half daily to biweekly timescales on NEE and the influence of water level at half daily time scales. The overall productivity of this wetland is within the expected range of tidal brackish marshes and it was a sink for atmospheric CO 2 in two out of the three years of this study and had a continued increase over the study period.

Description: The flocculation of dissolved organic matter (DOM) was studied along transects through three boreal estuaries. Besides the bulk concentration parameters, a suite of DOM quality parameters were investigated, including colored DOM (CDOM), fluorescent DOM and the molecular weight of DOM as well as associated dissolved iron concentrations. We observed significant deviations from conservative mixing at low salinities (〈2) in the estuarine samples of dissolved organic carbon (DOC), UV absorption ( a (CDOM254) ) and humic-like fluorescence. The maximum deviation from conservative mixing for DOC concentration was -16 %, at salinities between 1 and 2. An associated laboratory experiment was conducted where an artificial salinity gradient between 0 and 6 was created. The experiment confirmed the findings from the estuarine transects, since part of the DOC and dissolved iron pools were transformed to particulate fraction (〉0.2 µm), and thereby removing them from the dissolved phase. We also measured flocculation of CDOM, especially in the UV region of the absorption spectrum. Protein-like fluorescence of DOM decreased, while humic-like fluorescence increased because of salt-induced flocculation. Additionally, there was a decrease in molecular weight of DOM. Consequently, the quantity and quality of the remaining DOM pool was significantly changed after influenced to flocculation. Based on these results, we constructed a mechanistic, two-component flocculation model. Our findings underline the importance of the coastal filter, where riverine organic matter is flocculated and exported to the sediments.

Description: According to recent studies, dissolved organic carbon (DOC) concentrations in rivers throughout the boreal zone are increasing. However, the mechanistic explanation of this phenomenon is not yet well known. We studied how the short and long-term changes in precipitation, soil temperature, soil water content and net ecosystem exchange (NEE) are reflected to DOC concentrations and runoff DOC fluxes in two small forested upland catchments in Southern Finland. We used continuous eddy covariance measurements above the forest and runoff flow measurements from the catchment areas conducted over a 15-year-long time period to study the correlation between NEE, GPP, TER, litter production and runoff DOC. In addition, we looked for the most important environmental variables in explaining the inter-annual changes in runoff DOC by using multiple linear regression. Finally, we studied the temporal connection between runoff DOC concentrations, precipitation, soil water content and NEE by using wavelet coherence analysis technique. Our results indicate that the DOC concentrations have increased over the last 15 years. The DOC flux was to a large extent determined by the amount of precipitation, but the previous year's NEE and litter production had also a small but significant effect on runoff DOC fluxes.

Description: Studies of carbon allocation in forests provide essential information for understanding spatial and temporal differences in carbon cycling that can inform models and predict possible responses to changes in climate. Amazon forests play a particularly significant role in the global carbon balance, but there are still large uncertainties regarding abiotic controls on the rates of net primary production (NPP) and the allocation of photosynthetic products to different ecosystem components. We evaluated three different aspects of stand-level carbon allocation (biomass, NPP, and its partitioning) in two amazon forests on different soils (nutrient-rich clay soils versus nutrient-poor sandy soils), but otherwise growing under similar conditions. We found differences in carbon allocation patterns between these two forests, showing that the forest on clay soil had a higher aboveground and total biomass as well as a higher aboveground NPP than the sandy forest. However, differences between the two forest types in terms of total NPP were smaller, as a consequence of different patterns in the carbon allocation of above- and belowground components. The proportional allocation of NPP to new foliage was relatively similar between them. Our results of aboveground biomass increments and fine-root production suggest a possible trade-off between carbon allocation to fine-roots versus aboveground compartments, as opposed to the most commonly assumed trade-off between total above- and belowground production. Despite these differences among forests in terms of carbon allocation, the leaf area index showed only small differences, suggesting that this index is more indicative of total NPP than its above or belowground components.

Description: Recent studies have recognized sea ice as a source of reactive iodine to the Antarctic boundary layer. Volatile iodinated compounds (iodocarbons) are released from sea ice, and they have been suggested to contribute to the formation of iodine oxide (IO), which takes part in tropospheric ozone destruction in the polar spring. We measured iodocarbons (CH 3 I, CH 2 ClI, CH 2 BrI and CH 2 I 2 ) in sea ice, snow, brine and air during two expeditions to Antarctica, OSO 10/11 to the Amundsen Sea during austral summer, and ANT XXIX/6 to the Weddell Sea in austral winter. These are the first reported measurements of iodocarbons from the Antarctic winter. Iodocarbons were enriched in sea ice in relation to seawater in both summer and winter. During summer the positive relationship to Chl a biomass indicated a biological origin. We suggest that CH 3 I is formed biotically in sea ice during both summer and winter. For CH 2 ClI, CH 2 BrI and CH 2 I 2 an additional abiotic source at the snow-ice interface in winter is suggested . Elevated air concentrations of CH 3 I and CH 2 ClI during winter indicate that they are enriched in lower troposphere and may take part in formation of IO at polar sunrise.

Description: Intermediate disturbances shape forest structure and composition, which may in turn alter carbon, nitrogen, and water cycling. We used a large-scale experiment in a forest in northern lower Michigan where we prescribed an intermediate disturbance by stem-girdling all canopy-dominant early successional trees to simulate an accelerated age-related senescence associated with natural succession. Using three years of eddy covariance and sap flux measurements in the disturbed area and an adjacent control plot, we analyzed disturbance induced changes to plot level and species-specific transpiration and stomatal conductance. We found transpiration to be ~15% lower in disturbed plots than in unmanipulated control plots. However, species-specific responses to changes in microclimate varied. While red oak and white pine showed increases in stomatal conductance during post-disturbance (62.5 and 132.2%, respectively), red maple reduced stomatal conductance by 36.8%. We used the hysteresis between sap flux and vapor pressure deficit to quantify diurnal hydraulic stress incurred by each species in both plots. Red oak, a ring porous anisohydric species, demonstrated the largest mean relative hysteresis, while red maple, bigtooth aspen, and paper birch, all diffuse porous isohydric species, had the lowest relative hysteresis. We employed the Penman-Monteith model for LE to demonstrate that these species-specific responses to disturbance are not well captured using current modeling strategies, and that accounting for changes to leaf area index ( LAI ) and plot microclimate are insufficient to fully describe the effects of disturbance on transpiration.

Description: The three-north region in China (northeastern, northwestern, and northern China) is one of the most environmentally vulnerable regions in the country. To improve the local natural environment, the Chinese government launched the Three-North Shelter Forest Program (TNSFP), one of the largest afforestation/reforestation programs in the world. This program has led to significant changes in vegetation. Although many studies have evaluated the impacts of vegetation changes on local climate in this region, their results are highly inconsistent. In this study, evidence for local monthly climate impacts of vegetation change was investigated using remotely sensed data and ground meteorological measurements during the growing season (May to September) from 1982 to 2011 using the bivariate Granger causality test. The results showed that the local near-surface climate is sensitive mostly to vegetation changes characterized by the normalized difference vegetation index (NDVI) in arid and semi-arid regions, and that vegetation plays a more important role in influencing hydro-climate in the arid/semi-arid zones than in other zones, which has great implications for water resources in this dry region. Moreover, NDVI changes in northeastern China have a significantly negative influence on air temperature but no other climatic variables, whereas the test results in northern China is not as objective as other zones due to the rapid urbanization. All these results suggest that the local climate is very sensitive to the variations in vegetation in arid and semi-arid regions, so extra caution should be taken when planting trees in this area.

Description: Warming in northern high latitudes has changed the energy balance between terrestrial ecosystems and the atmosphere. This study evaluated changes in regional surface energy exchange in Alaska from 2000 to 2011 when substantial declines in spring snow cover due to spring warming and large scale fire events were observed. Energy fluxes from a network of 20 eddy covariance sites were upscaled using a support vector regression (SVR) model, by combining satellite remote sensing data, and global climate data. Based on site scale analysis, SVR reproduced observed net radiation, sensible heat flux, latent heat flux, and ground heat flux; eight-day root mean square errors for these variables were 15, 10, 9, and 3 W m -2 , respectively. Based on upscaled fluxes, decreases in spring snow cover induced an increase in surface net radiation, a net heating effect, of 0.56 W m -2 decade -1 . This heating effect was comparable to the net cooling effect due to increased fire extent during the study period (up to 0.59 W m -2 decade -1 ). These land cover effects were larger than the change in the energy forcing associated with CO 2 balance for the Alaska region. Spring warming and post-fire land cover change increased the regional latent heat flux. The regional sensible heat flux decreased with the post-fire land cover change. Our results highlight the importance of positive spring snow-albedo feedback to climate and a post-fire negative feedback under the expected warming climate in the Arctic.

Description: This study proposes a multi-component, multi-process scheme to explain the turnover of organic matter (particulate and dissolved organic matter) in streams. The scheme allows for production and degradation of organic matter by both photic and aphotic processes with transformation of DOC to increasingly refractory forms. The proposed scheme was compared to 10 months of experimental observations of the turnover and fate of particulate and dissolved organic matter in streamwater from a peat-covered catchment. The scheme was able to explain average decline in DOC concentration of 65% over 70 hours with a 13% mean average percentage error (MAPE) based on turnover in three types of organic matter (particulate, labile dissolved, refractory dissolved) although the order and rate of reactions did change between sets of experimental observations. The modelling suggests that activation energies are low for all except the most refractory forms of DOC in turn suggesting that processes are not sensitive to temperature change. Application of the modelling scheme to organic matter turnover in the River Tees, northern England, showed that annual removal of total organic carbon (TOC) was equivalent to between 13 and 33 tonnes C/km 2 /yr from an at source export of between 22 and 56 tonnes C/km 2 /yr giving a total in-stream loss rate of between 53 and 62% over a median in-stream residence time of 35 hours.

Description: Spatially separated electron donors and acceptors in sediment can be exploited by so-called ‘cable bacteria’. Electric potential microelectrodes (EPMs) were constructed to measure the electric fields that should appear when cable bacteria conduct electrons over centimeter distances. The EPMs were needle-shaped, shielded Ag/AgCl half-cells rendered insensitive to redox-active species in the environment. Tip diameters of 40 to 100 microns and signal resolution of approximately 10 μV were achieved. A test in marine sediments with active cable bacteria showed an electric potential increase by approximately 2 mV from the sediment-water interface to a depth of approximately 20 millimeter, in accordance with the location and direction of the electric currents estimated from oxygen, pH and H 2 S microprofiles. The EPM also captured emergence and decay of electric diffusion potentials in the upper millimeters of artificial sediment in response to changes in ion concentrations in the overlying water. The results suggest that the EPM can be used to track electric current sources and sinks with sub-millimeter resolution in microbial, biogeochemical, and geophysical studies.

Description: Conventional Q10 soil organic matter decomposition models and more complex microbial models are available for making projections of future soil carbon dynamics. However, it is unclear (1) how well the conceptually different approaches can simulate observed decomposition, and (2) to what extent the trajectories of long-term simulations differ when using the different approaches. In this study, we compared three structurally different soil carbon (C) decomposition models (one Q10 and two microbial models of different complexity), each with a one- and two-horizon version. The models were calibrated and validated using four years of measurements of heterotrophic soil CO 2 efflux from trenched plots in a Dahurian larch ( Larix gmelinii Rupr.) plantation. All models reproduced the observed heterotrophic component of soil CO 2 efflux, but the trajectories of soil carbon dynamics differed substantially in 100-year simulations with and without warming and increased litterfall input, with microbial models produced better agreement with observed changes in soil organic C in long-term warming experiments. Our results also suggest that both constant and varying carbon use efficiency are plausible when modeling future decomposition dynamics, and that the use of a short-term (e.g. a few years) period of measurement is insufficient to adequately constrain model parameters that represent long-term responses of microbial thermal adaption. These results highlight the need to reframe the representation of decomposition models and to constrain parameters with long-term observations and multiple data streams. We urge caution in interpreting future soil carbon responses derived from existing decomposition models because both conceptual and parameter uncertainty is substantial.

Description: The short-term (hourly, daily) variation in chromophoric dissolved organic matter (CDOM) in lakes is largely unknown. We assessed the spectral characteristics of light absorption by CDOM in a eutrophic, humic shallow mixed lake of temperate Sweden at a high-frequency (30 minutes) interval, and during a full growing season (May to October). Physical time series, such as solar radiation, temperature, wind, and partial pressures of carbon dioxide in water and air were measured synchronously. We identified a strong radiation-induced summer CDOM loss (25 to 50%) that developed over four months, which was accompanied by strong changes in CDOM absorption spectral shape. The magnitude of the CDOM loss exceeded sub-hourly to daily variability by an order of magnitude. Applying Fourier analysis we demonstrate that variation in CDOM remained largely unaffected by rapid shifts in weather, and no apparent response to in-lake dissolved organic carbon (DOC) production was found. In autumn, CDOM occasionally showed variation at hourly to daily time scales, reaching a maximum daily coefficient of variation of 15%. We suggest that lake-internal effects on CDOM are quenched in humic lake waters by dominating effects associated with imported CDOM and solar exposure. Since humic lake waters belong to one of the most abundant lake type on Earth, our results have important implications for the understanding of global CDOM cycling.

Description: No abstract is available for this article.

Description: Inverse empirical models can inform and improve more complex process-based models by quantifying the principal factors that control water quality variation. Here, we developed a multiple regression model that explains 81% of the variation in filtered methylmercury (FMeHg) concentrations in Fishing Brook, a 4 th -order stream in the Adirondack Mountains, New York, a known “hot spot” of Hg bioaccumulation. This model builds on previous observations that wetland-dominated riparian areas are the principal source of MeHg to this stream, and was based on 43 samples collected during a 33-mo period in 2007–09. Explanatory variables include those that represent the effects of water temperature, streamflow, and modeled riparian water table depth on seasonal and annual patterns of FMeHg concentrations. An additional variable represents the effects of an upstream pond on decreasing FMeHg concentrations. Model results suggest that temperature-driven effects on net Hg methylation rates is the principal control on annual FMeHg concentration patterns. Additionally, streamflow dilutes FMeHg concentrations during the cold dormant season. The model further indicates that depth and persistence of the riparian water table as simulated by TOPMODEL are dominant controls on FMeHg concentration patterns during the warm growing season, especially evident when concentrations during the dry summer of 2007 were less than half of those in the wetter summers of 2008 and 2009. This modeling approach may help identify the principal factors that control variation in surface water FMeHg concentrations in other settings, which can guide the appropriate application of process-based models.

Description: Information on the contribution of nitrogen (N) cycling processes in bed sediments to river nutrient fluxes in large northern latitude river systems is limited. This study examined the relationship between N-cycling processes in bed sediments and N speciation and loading in the Yukon River near its mouth at the Bering Sea. We conducted laboratory bioassays to measure N-cycling processes in sediment samples collected over distinct water cycle seasons. In conjunction, the microbial community composition in the bed sediments using genes involved in N-cycling ( narG , napA , nosZ and amoA ) and 16S rRNA gene pyrosequences was examined. Temporal variation was observed in net N mineralization, nitrate uptake and denitrification rate potentials and correlated strongly with sediment carbon (C) and extractable N content and microbial community composition rather than with river water nutrient concentrations. The C content of the bed sediment was notably impacted by the spring flood, ranging from 1.1% in the midst of an ice-jam to 0.1% immediately after ice-out, suggesting a build-up of organic material (OM) prior to scouring of the bed sediments during ice break-up. The dominant members of the microbial community that explained differences in N processing rates belonged to the genera Crenothrix , Flavobacterium and the family of Comamonadaceae. Our results suggest that biogeochemical processing rates in the bed sediments appear to be more coupled to hydrology, nutrient availability in the sediments and microbial community composition rather than river nutrient concentrations at Pilot Station.

Description: Two years of eddy covariance measurements of lake carbon dioxide (CO 2 ) fluxes reveal a diel cycle with higher fluxes during night. Measurements of partial pressure in the air ( pCO 2a ) and in the water (pCO 2w ), during four months, show that the high nighttime fluxes are not explained by changes in the difference between pCO 2a and pCO 2w . Analyzing the transfer velocity ( k 600,meas ), which is a measure of the efficiency of the gas transfer, with respect to wind speed, shows that variations in wind speed do not explain the diel cycle. During nighttime when the fluxes are high the wind is normally low. Thus, a solely wind based parameterization of the transfer velocity (k u,CC ) results in large errors compared to k 600,meas , especially for wind speeds lower than 6 m s −1 . The mean absolute percentage error between k u,CC and k 600,meas is 79%. By subtracting k u,CC from k 600,meas , we investigate how waterside convection, influence k 600,meas . Our results show that the difference ( k 600,meas – k u,CC ) increases with increasing waterside convection. Separating the transfer velocity parameterization in two parts, one depending on the wind speed and one depending on waterside convection, the mean absolute percentage error compared to the measurements reduces to 22%. The results in this paper show that the high nighttime CO 2 fluxes can to a large extent be explained by waterside convection and that a transfer velocity parameterization based on both wind speed and waterside convection better fits the measurements compared to a parameterization based solely on wind speed.

Description: Enhanced winter streamflow is characteristic of a nival / pluvial regime that has emerged in parts of the subarctic Canadian Shield because of increasingly common late summer rains. This phenomenon is part of a widespread trend towards higher winter streamflow in watersheds across the circumpolar north. There may be implications for biogeochemical systems as streamflow regimes undergo these types of changes associated with climate warming. . Streamflow and geochemical fluxes were observed over two years with different winter flow conditions in a subarctic Canadian Shield catchment. Results show that higher wintertime loads of carbon and solutes associated with enhanced winter streamflow were in association with an expansion of contributing areas to runoff over what would have existed during typical winter recession. Furthermore, the wet fall conditions that lead to enhanced winter streamflow require water tables close to the topographic surface in highly conductive organic soil layers, which is a similar to the condition during the spring melt. Fall rainfall-runoff leaves an ample volume of water in the lakes that are ubiquitous in this landscape. This water maintains winter streamflow during a time when it traditionally would have ceased. A slowing of biological activity under lake ice increases net mineralization and nitrification rates. This convergence of nitrogen cycling and winter streamflow produced a disproportionate flux of inorganic nitrogen from the study catchment. A conceptual model of how enhanced winter streamflow changes water chemistry in a lake dominated Shield landscape is proposed and may be used as a benchmark to guide hypotheses of process interactions, change in other landscapes or across scales.

Description: In contrast to upland croplands, carbon dioxide (CO 2 ) emission from soils has rarely been investigated previously in fields with paddy rice cultivation. In this study, we hypothesized that CO 2 emission from paddy soils is suppressed to be a low level due the soil submergence for months for paddy rice cultivation, and conducted a continuous measurement of net CO 2 flux from the soil/water surface of a paddy field throughout the year, including both the submerged and drained periods. The net CO 2 flux was generally near zero during the submerged period with paddy rice cultivation and showed a slight CO 2 influx in the daytime and efflux at nighttime, indicating dominance of photosynthetic CO 2 uptake and respiratory CO 2 release by aquatic weeds and algae in paddy water. The diurnal variations in net CO 2 flux and dissolved CO 2 concentration had negative correlations with the pH of paddy water. A remarkably high CO 2 efflux was observed during the period with intermittent drainage in summer. Unexpectedly, the cumulative CO 2 emissions throughout the year were not considerably lower than those reported in upland croplands, ranging from 1309 to 2160 g CO 2 m −2 y −1 , of which 41 - 48% was emitted from the first drainage in summer to the rice harvest in autumn. In summary, in this study, we revealed that CO 2 emission from soil in paddy fields is strictly suppressed during the submerged period, but considerably enhanced by the succeeding drainage, which may negate the suppressed CO 2 emission during the submerged period.

Description: No abstract is available for this article.

Description: Oil palm plantation expansion into tropical forests may alter physical and biogeochemical inputs to streams, thereby changing hydrological function. In West Kalimantan, Indonesia, we assessed streams draining watersheds characterized by five land uses: intact forest, logged forest, mixed agroforest, and young (〈3 yrs) and mature (〉10 yrs) oil palm plantation. We quantified suspended sediments, stream temperature, and metabolism using high-frequency submersible sonde measurements during month-long intervals between 2009 and 2012. Streams draining oil palm plantations had markedly higher sediment concentrations and yields, and stream temperatures, compared to other streams. Mean sediment concentrations were 4-fold to 550-fold greater in young oil palm than in all other streams, and remained elevated even under baseflow conditions. After controlling for precipitation, the mature oil palm stream exhibited significantly greater sediment yield than other streams. Young and mature oil palm streams were 3.9 °C and 3.0 °C warmer than the forested stream (25 °C). Across all streams, baseflow periods were significantly warmer than times of stormflow, and these differences were especially large in oil palm catchments. Ecosystem respiration rates were also influenced by low precipitation. During an El Niño Southern Oscillation-associated drought, the mature oil palm stream consumed a maximum 21 g O 2 m −2  day −1 in ecosystem respiration, in contrast with 2.8 ± 3.1 g O 2 m −2  day −1 during non-drought sampling. Given that 23% of Kalimantan's land area is occupied by watersheds similar to those studied here, our findings inform potential hydrologic outcomes of regional periodic drought coupled with continued oil palm plantation expansion.

Description: Wildfires exhibit a strong seasonality that is driven by climatic factors and human activities. Although fire seasonality is commonly determined using burned area and fire frequency, it could also be quantified using biomass consumption estimates that directly represent biomass loss (a combination of the area burned and the fuel loading). Therefore, in this study a dataset of long-term biomass consumed was derived from geostationary satellite data to explore the interannual variation in fire seasonality and the possible impacts of climate change and land management practices across the Contiguous United States (CONUS). Specifically, daily biomass consumed data were derived using the fire radiative power (FRP) retrieved from Geostationary Operational Environmental Satellites (GOES) series with a pixel size of 4-10 km from 1995 to 2011. Annual fire seasonality metrics including the fire season duration, the timing of the start, peak, and end of the fire season, and interannual variation and trends, were derived from the 17 year biomass consumed record. These metrics were associated with climatic factors to examine drivers and mediators of fire seasonality. The results indicate that biomass consumed significantly increased at a rate of 2.87Tg/yr, however, the derived fire season duration exhibited a shortening trend in various states over the western CONUS and no significant trend in most other regions. This suggests that the frequency of extreme fire events has increased, which is perhaps associated with an observed increase of extreme weather conditions. Further, both the start and the end of fire season exhibited an early shift (1.5-5d/yr) in various eastern states although a late shift occurred in Arizona and Oregon. The interannual variation and trend in the fire seasonality was more strongly related to temperature in the western CONUS and to precipitation in the south east. The Palmer Drought Severity Index was found to effectively reflect interannual variations in total biomass consumed although it was poorly correlated to the fire seasonality metrics. The results indicate that across the CONUS the spatial patterns of the start, peak, and end of the fire season shift regularly in various regions in response to latitudinal gradients of temperature variation.

Description: Few studies have evaluated land-surface models for African ecosystems. Here, we evaluate the ORCHIDEE process-based model for the inter-annual variability (IAV) of the fraction of Absorbed Active Radiation (fAPAR), the gross primary productivity (GPP), soil moisture and evapotranspiration (ET). Two ORCHIDEE versions are tested, which differ by their soil hydrology parameterization, one with a 2-layer simple bucket and the other a more complex 11-layer soil-water diffusion. In addition, we evaluate the sensitivity of climate forcing data, atmospheric CO 2 , and soil depth. Beside a very generic vegetation parameterization, ORCHIDEE simulates rather well the IAV of GPP and ET (0.5 〈 r 〈 0.9 inter-annual correlation) over Africa except in forestlands. The ORCHIDEE 11-layer version outperforms the 2-layer version for simulating IAV of soil moisture, whereas both versions have similar performance of GPP and ET. Effects of CO 2 trends, and of variable soil depth on the IAV of GPP, ET and soil moisture are small, although these drivers influence the trends of these variables. The meteorological forcing data appears to be quite important for faithfully reproducing the IAV of simulated variables, suggesting that in regions with sparse weather station data, the model uncertainty is strongly related to uncertain meteorological forcing. Simulated variables are positively and strongly correlated with precipitation but negatively and weakly correlated with temperature and solar radiation. Model-derived and observation-based sensitivities are in agreement for the driving role of precipitation. However the modelled GPP is too sensitive to precipitation, suggesting that processes such as increased water use efficiency during drought need to be incorporated in ORCHIDEE.

Description: The bio-reactivity or susceptibility of dissolved organic matter (DOM) to microbial degradation in streams and rivers is of critical importance to global change studies, but a comprehensive understanding of DOM bio-reactivity has been elusive due, in part, to the stunningly diverse assemblages of organic molecules within DOM. We approach this problem by employing a range of techniques to characterize DOM as it flows through biofilm reactors: dissolved organic carbon (DOC) concentrations, excitation emission matrix spectroscopy (EEMs), and ultrahigh resolution mass spectrometry. The EEMs and mass spectral data were analyzed using a combination of multivariate statistical approaches. We found that 45% of stream water DOC was biodegraded by microorganisms, including 31-45% of the humic DOC. This bio-reactive DOM separated into 2 different groups: 1) H/C centered at 1.5 with O/C 0.1-0.5, or 2) low H/C of 0.5-1.0 spanning O/C 0.2-0.7, that were positively correlated (Spearman ranking) with chromophoric and fluorescent DOM (CDOM and FDOM, respectively). DOM that was more recalcitrant and resistant to microbial degradation aligned tightly in the center of the van Krevelen space (H/C 1.0-1.5, O/C 0.25-0.6) and negatively correlated (Spearman ranking) with CDOM and FDOM. These findings were supported further by principal component analysis and 2D correlation analysis of the relative magnitudes of the mass spectral peaks assigned to molecular formulas. This study demonstrates that our approach of processing stream water through bioreactors followed by EEMs and FTICR-MS analyses, in combination with multivariate statistical analysis, allows for precise, robust characterization of compound bio-reactivity and associated molecular level composition.

Description: ABSTRACT Continued warming of the Arctic may cause permafrost to thaw and speed the decomposition of large stores of soil organic carbon (OC), thereby accentuating global warming. However, it is unclear if recent warming has raised the current rates of permafrost OC release to anomalous levels or to what extent soil carbon release is sensitive to climate forcing. Here we use a time series of radiocarbon age-offsets ( 14 C) between the bulk lake sediment and plant macrofossils deposited in an arctic lake as an archive for soil and permafrost OC release over the last 14,500-years. The lake traps and archives OC imported from the watershed and allows us to test whether prior warming events stimulated old carbon release and heightened age-offsets. Today, the age-offset (2 ka; thousand of calendar years before AD 1950) and the depositional rate of ancient OC from the watershed into the lake are relatively low and similar to those during the Younger Dryas cold interval (occurring 12.9-11.7 ka). In contrast, age-offsets were higher (3.0-5.0 ka) when summer air temperatures were warmer-than-present during the Holocene Thermal Maximum (11.7-9.0 ka) and Bølling-Allerød periods (14.5-12.9 ka). During these warm times, permafrost thaw contributed to ancient OC depositional rates that were ~10 times greater than today. Although permafrost OC was vulnerable to climate warming in the past, we suggest surface soil organic horizons and peat are presently limiting summer thaw and carbon release. As a result, the temperature threshold to trigger widespread permafrost OC release is higher than during previous warming events.

Description: The spatial and temporal variability in accumulation and release of greenhouse gases (mainly methane and carbon dioxide) to the atmosphere from peat soils remains very uncertain. The use of near surface geophysical methods such as Ground Penetrating Radar (GPR) has proven useful during the last decade to expand scales of measurement as related to in-situ gas distribution and dynamics beyond traditional methods (i.e. gas chambers). However, this approach has focused exclusively on boreal peatlands, while no studies in subtropical systems like the Everglades using these techniques exist. In this paper GPR is combined with gas traps, time lapse cameras, gas chromatography, and surface deformation measurements to explore biogenic gas dynamics (mainly gas build-up and release) in two locations in the Everglades. Similar to previous studies in northern peatlands, our data in the Everglades show a statistically significant correlation between: 1) GPR estimated gas content and gas fluxes; 2) GPR estimated gas content and surface deformation; and 3) atmospheric pressure and both GPR estimated gas content and gas flux. From these results several gas releasing events ranging between 33.8 and 718.8 mg CH 4 m −2 d −1 were detected as identified by: 1) decreases in GPR estimated gas content within the peat matrix; 2) increases in gas fluxes captured by gas traps and time-lapse cameras; 3) decreases in surface deformation. Furthermore, gas releasing events corresponded to periods of high atmospheric pressure. Changes in gas accumulation and release were attributed to differences in seasonality and peat soil type between sites. These results suggest that biogenic gas releases in the Everglades are spatially and temporarily variable. For example flux events measured at hourly scales were up to threefold larger when compared to daily fluxes, therefore suggesting that flux measurements decline when averaged over longer time-spans. This research therefore questions what the appropriate spatial and temporal scale of measurement is necessary to properly capture dynamics of biogenic gas release in subtropical peat soils.

Description: We estimated annual fluxes of suspended matter and different carbon (C) pools at three sites along the lower Tana River (Kenya), based on monthly sampling between January 2009 and December 2011. Concentrations of total suspended matter (TSM), particulate organic carbon (POC), dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) were monitored, as was the stable isotope composition of the carbon pools. Both TSM and POC concentrations showed strong seasonality, varying over several orders of magnitude, while DOC and DIC concentrations showed no seasonal variations. A strong shift in the origin of POC was observed, which was dominated by C3-derived C during dry conditions (low δ 13 C POC between -28 ‰ and -25 ‰ ), but had significant C4 contributions during high flow events ( δ 13 C POC up to -19.5 ‰ ). Between Garissa and the most downstream sampling point, a clear decrease in suspended matter and organic C fluxes was observed, being most pronounced during high discharge conditions: on an annual basis, fluxes of TSM, POC and DIC decreased by 34 % to 65 % for the 3-year study period. Our results suggest that floodplains along the lower Tana River could play an important role in regulating the transport of suspended matter and organic C. A comparison of current flux estimates with data collected prior to the construction of several hydropower dams reveals that the sediment loading is reduced, predominantly during low discharge conditions.

Description: No abstract is available for this article.

Description: ABSTRACT [1]  Temporal scales of variability for the partial pressure of CO 2 ( p CO 2 ) in the surface waters of two stratified Mediterranean reservoirs were examined through the temporal decomposition of 5-month time series with hourly sampling frequency. p CO 2 time series included similar patterns of variability at daily, biweekly and seasonal scales regardless of the difference in amplitude of the p CO 2 variation in the two reservoirs studied. Daily variability was strongly related to the day-night cycles of metabolic activity, accounting for about one-third of the total amplitude in p CO 2 variation. At a biweekly scale, wind forcing led to higher rates of air-water CO 2 exchange and subsequently temporary partial mixing events associated to relevant increase of CO 2 concentration in surface waters. Seasonal variability accounted for one-third of the amplitude of the p CO 2 variability, and was coupled to the seasonal dynamics of water temperature and thermal stratification of the water column. Our results provide evidence that CO 2 emission from stratified water bodies show significant variability at daily, biweekly and seasonal scales; all of which should be taken into consideration in the analyses of the carbon fluxes. The wind-induced mixing events, operating at temporal scales between daily and seasonal cycles, may become a major factor controlling the p CO 2 dynamics. Hence, some of the most common models for computing CO 2 fluxes from p CO 2 were not able to reproduce the biweekly response patterns of CO 2 emissions to wind forcing.

Description: [1]  Terrestrial biosphere models are designed to synthesize our current understanding of how ecosystems function, test competing hypotheses of ecosystem function against observations, and predict responses to novel conditions such as those expected under climate change. Reducing uncertainties in such models can improve both basic scientific understanding and our predictive capacity, but rarely are ecosystem models employed in the design of field campaigns. We provide a synthesis of carbon cycle uncertainty analyses conducted using the Predictive Ecosystem Analyzer (PEcAn) ecoinformatics workflow with the Ecosystem Demography model v2 (ED). This work is a synthesis of multiple projects, using Bayesian data assimilation techniques to incorporate field data and trait databases across temperate forests, grasslands, agriculture, short rotation forestry, boreal forests, and tundra. We report on a number of data needs that span a wide array of diverse biomes, such as the need for better constraint on growth respiration, mortality, stomatal conductance, and water uptake. We also identify data needs that are biome specific, such as photosynthetic quantum efficiency at high latitudes. We recommend that future data collection efforts balance the bias of past measurements toward aboveground processes in temperate biomes with the sensitivities of different processes as represented by ecosystem models.

Description: [1]  The functioning of Arctic ecosystems is not only critically affected by climate change, but it also has the potential for major positive feedbacks on climate. There is however relatively little information on the role, patterns, and vulnerabilities of CO 2 fluxes during the non-summer seasons in Arctic ecosystems. Presented here is a year-around study of CO 2 fluxes in an Alaskan Arctic tussock tundra ecosystem, and key environmental controls on these fluxes. Important controls on fluxes vary by season. This paper also presents a new empirical quantification of seasons in the Arctic based on net radiation. The fluxes were computed using standard FluxNet methodology and corrected using standard Web-Pearman-Leuning (WPL) density terms adjusted for influences of open-path instrument surface heating. The results showed that the non-summer season comprises a significant source of carbon to the atmosphere. The summer period was a net sink of 24.3 g C m -2 , while the non-summer seasons released 37.9 g C m -2 . This release is 1.6 times the summer uptake, resulting in a net annual source of +13.6 g C m -2 to the atmosphere. These findings support early observations of a change in this particular region of the Arctic from a long-term annual sink of CO 2 to an annual source from the terrestrial ecosystem and soils to the atmosphere. The results presented here demonstrate that nearly continuous observations may be required in order to accurately calculate the annual NEE of Arctic ecosystems, and to build predictive understanding that can be used to estimate, with confidence, Arctic fluxes under future conditions.

Description: [1]  Air-lake methane flux ( FCH 4 ) and partial pressure of methane in the atmosphere ( pCH 4a ) were measured using the eddy covariance method over a Swedish lake for an extended period. The measurements show a diurnal cycle in both FCH 4 and pCH 4a with high values during nighttime ( FCH 4  ≈ 300 nmol m -2  s -1 , pCH 4a  ≈ 2.5 µatm) and low values during day ( FCH 4  ≈ 0 nmol m -2  s -1 , pCH 4a  ≈ 2.0 µatm) for a large part of the data set. This diurnal cycle persist in all open water season however the magnitude of the diurnal cycle is largest in the spring months. Estimations of buoyancy in the water show that high nighttime fluxes coincide with convective periods. Our interpretation of these results is that the convective mixing enhances the diffusive flux, in analogy to previous studies. We also suggest that the convection may bring methane-rich water from the bottom to the surface and trigger bubble release from the sediment. A diurnal cycle is not observed for all convective occasions, indicating that the presence of convection is not sufficient for enhanced night time flux, other factors are also necessary. The observed diurnal cycle of pCH 4a is explained with the variation of FCH 4 and a changing internal boundary layer above the lake. The presence of a diurnal cycle of FCH 4 stress the importance of making long term continuous flux measurements. A lack of FCH 4 measurements during night may significantly bias estimations of total CH 4 emissions from lakes to the atmosphere.

Description: [1]  Extending phenological records into the past is essential for the understanding of past ecological change and evaluating the effects of climate change on ecosystems. A growing body of historical phenological information is now available for Europe, North America and Asia. In East Asia, long-term phenological series are still relatively scarce. This study extracted plant phenological observations from old diaries in the 1834-1962 period. A spring phenology index (SPI) for the modern period (1963–2009) was defined as the mean flowering time of three shrubs (first flowering of Amygdalus davidiana and Cercis chinensis , 50 % of full flowering of Paeonia suffruticosa ) according to the data availability. Applying calibrated transfer functions from the modern period to the historical data, we reconstructed a continuous SPI time series across eastern China from 1834 to 2009. In the recent 30 years, the SPI is 2.1-6.3 days earlier than during any other consecutive 30-year period before 1970. A moving linear trend analysis shows that the advancing trend of SPI over the past three decades reaches upwards of 4.1 days/decade, which exceeds all previously observed trends in the past 30-year period. In addition, the SPI series correlates significantly with spring (February-April) temperatures in the study area, with an increase in spring temperature of 1 °C inducing an earlier SPI by 3.1 days. These shifts of SPI provide important information regarding regional vegetation-climate relationships, and they are helpful to assess longterm of climate change impacts on biophysical systems and biodiversity.

Description: [1]  Stable isotopic measurements of water provide a promising tool for partitioning of ecosystem evapotranspiration (ET). This approach, however, is still facing some challenges due to the uncertainties in estimating the isotopic compositions of ET and its components. In this study, a tunable diode laser analyzer was deployed for in situ measurements of the oxygen isotopic compositions of water vapor. Using these measurements together with samples of water in plant and soil pools, we attempted to partition ET via estimating the oxygen isotopic compositions of ET ( δ ET ) and that of its two components, i.e., plant transpiration ( δ T ) and soil water evaporation ( δ E ). A new δ T model was developed in this study, which illustrated consistent estimations with the traditional model. Most of the variables and parameters in the new model can be measured directly with high accuracy, making its potential to be used at other sites high. Our results indicate that the ratio of plant transpiration to evapotranspiration ( T /ET) illustrates a ‘U’ shape diurnal pattern. Mean T /ET at 0630-1830 during the sampling days was 83%. Soil depth of 15 cm is a reasonable depth for soil water sampling for estimating δ E at this site. Sampling water at a too shallow depth may bring in biased δ E estimation when soil moisture is very low. We also investigated the uncertainties in estimating these three terms and their effects on partitioning. Overall, in terms of partitioning, the uncertainties are relatively small from δ T and δ E , but quite large from δ ET . Quantifying and improving the precision of δ ET should be a priority in future endeavors of ET partitioning via the stable isotopic approach.

Description: [1]  Permafrost thaw in peat plateaus leads to the flooding of surface soils and the formation of collapse scar bogs, which have the potential to be large emitters of methane (CH 4 ) from surface peat as well as deeper, previously frozen, permafrost carbon (C). We used a network of bubble traps, permanently installed 20 cm and 60 cm beneath the moss surface, to examine controls on ebullition from three collapse bogs in interior Alaska. Overall, ebullition was dominated by episodic events that were associated with changes in atmospheric pressure and ebullition was mainly a surface process regulated by both seasonal ice dynamics and plant phenology. The majority (〉90%) of ebullition occurred in surface peat layers, with little bubble production in deeper peat. During periods of peak plant biomass, bubbles contained acetate-derived CH 4 dominated (〉90%) by modern C fixed from the atmosphere following permafrost thaw. Post senescence, the contribution of CH 4 derived from thawing permafrost C was more variable and accounted for up to 22%, although on average only 7%, in the most recently thawed site. Thus, the formation of thermokarst features resulting from permafrost thaw in peatlands stimulates ebullition and CH 4 release both by creating flooded surface conditions conducive to CH 4 production and bubbling as well as by exposing thawing permafrost C to mineralization.

Description: An emergent linear relationship between the long-term sensitivity of tropical land carbon storage to climate warming (γ LT ) and the short-term sensitivity of atmospheric carbon dioxide (CO 2 ) to interannual temperature variability (γ IAV ) has previously been identified by Cox et al. (2013) across an ensemble of Earth System models (ESMs) participating in the Coupled Climate-Carbon Cycle Model Intercomparison Project (C 4 MIP). Here, we examine whether such a constraint also holds for a new set of eight ESMs participating in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). A wide spread in tropical land carbon storage is found for the quadrupling of atmospheric CO 2 , which is of the order of 252 ± 112 GtC when carbon climate feedbacks are enabled. Correspondingly, the spread in γ LT is wide (-49 ± 40 GtC/K) and thus remains one of the key uncertainties in climate projections. A tight correlation is found between the long-term sensitivity of tropical land carbon and the short-term sensitivity of atmospheric CO 2 (γ LT vs. γ IAV ), which enables the projections to be constrained with observations. The observed short-term sensitivity of CO 2 (-4.9 ± 0.9 GtC/yr/K) sharpens the range of γ LT -44 ± 14 GtC/K. Which overlaps with the probability density function (PDF) derived from the C 4 MIP models (-53 ± 17 GtC/K) by Cox et al. (2013), even though the lines relating γ LT and γ IAV differ in the two cases. Emergent constraints of this type provide a means to focus ESM evaluation against observations on the metrics most relevant to projections of future climate change.

Description: The extent of peat decomposition was investigated in four cores collected along a latitudinal gradient from 56˚N to 66˚N in the West Siberian Lowland. The acid:aldehyde ratios of lignin phenols were significantly higher in the two northern cores compared with the two southern cores, indicating peats at the northern sites were more highly decomposed. Yields of hydroxyproline, an amino acid found in plant structural glycoproteins, were also significantly higher in northern cores compared with southern cores. Hydroxyproline-rich glycoproteins are not synthesized by microbes and are generally less reactive than bulk plant carbon, so elevated yields indicated northern cores were more extensively decomposed than the southern cores. The southern cores experienced warmer temperatures, but were less decomposed, indicating temperature was not the primary control of peat decomposition. The plant community oscillated between Sphagnum and vascular plant dominance in the southern cores, but vegetation type did not appear to affect the extent of decomposition. Oxygen exposure time appeared to be the strongest control of the extent of peat decomposition. The northern cores had lower accumulation rates and drier conditions, so these peats were exposed to oxic conditions for a longer time before burial in the catotelm, where anoxic conditions prevail and rates of decomposition are generally lower by an order of magnitude.

Description: ABSTRACT Ecosystem phenology plays an important role in carbon exchange processes and can be derived from continuous records of carbon dioxide (CO 2 ) exchange data. In this study we examine the potential use of phenological indices for characterizing cumulative annual CO 2 exchange in four contrasting northern peatland ecosystems. We used the approach of Jonsson and Eklundh [2004] to derive a set of phenological indices based on the daily times series of gross primary production (GPP), ecosystem respiration (R e ) and net ecosystem production (NEP) measured in the four peatland sites. The main objectives of this study were: a) to examine the variation in phenological indices across sites; and b) to determine the relationships among phenological indices, environmental conditions, and cumulative annual CO 2 exchange. The phenological index used to define the “start of the growing season” showed good potential for differentiation among sites based on their average annual site GPP. Sites with earlier growing seasons had the highest average annual site GPP. The “peak CO 2 exchange rate” phenological index performed best in reflecting variations among sites and for estimating annual values of GPP, R e and NEP (Pearson correlation coefficients ranged between 0.77 and 0.99, p 〈 0.05 for all). The phenological indices and annual GPP, R e and NEP were sensitive to winter (January - March) and summer (July - September) temperature and precipitation; but correlations, though significant, were weak.

Description: The surface energy balance of a high-elevation groundwater-fed wetland (High Creek Fen) in central Colorado was measured from 9-June 2000 through 18-January 2005. In agreement with observations and predictions for decreased winter snowcover in the region, low snowcover in 2001-2002 allowed for an examination of the impact of winter drought on the wetland. During years with an average snowpack, summer evaporation far exceeded precipitation. Despite near-normal summer precipitation following a winter drought, the summer-time surface energy balance was affected with decreased latent heat fluxes and increased sensible heat fluxes. Leaf area index and the fraction of photosynthetic radiation absorbed were reduced following the winter drought. A shift in the primary controls on evaporation occurred, as the surface's response to vapor pressure deficits and soil moisture increased following the winter drought. The earlier snowmelt coupled with earlier increase in soil temperature and moisture following winter drought did not increase evaporation since vegetation was not yet developed and evaporation from soil water was low during the early spring period.

Description: Land surface models use different formulations of stomatal conductance and plant hydraulics, and it is unclear which type of model best matches observed surface-atmosphere water flux. We use the NACP (North American Carbon Program) dataset of latent heat flux (LE) measurements from 25 sites and predictions from 9 models to evaluate models’ ability to resolve sub-daily dynamics of transpiration. Despite overall good forecast at the seasonal scale, models have difficulty resolving the dynamics of intra-daily hysteresis. The majority of models tend to underestimate LE in the pre-noon hours and overestimate in the evening. We hypothesize that this is a result of un-resolved afternoon stomatal closure due to hydrodynamic stresses. Although no model or stomata parameterization was consistently best or worst in terms of ability to predict LE, errors in model-simulated LE were consistently largest and most variable when soil moisture was moderate and VPD was moderate to limiting. Nearly all models demonstrate a tendency to underestimate the degree of maximum hysteresis which, across all sites studied, is most pronounced during moisture limited conditions. These diurnal error patterns are consistent with models’ diminished ability to accurately simulate the natural hysteresis of transpiration. We propose that lack of representation of plant hydrodynamics is, in part, responsible for these error patterns.

Description: [1]  Fires in croplands, plantations, grasslands, and rangelands contribute significantly to fire emissions in the United States, yet these management fires are often overshadowed by wildland fires in terms of efforts to estimate climate impacts or mitigation potential. Here we quantified decadal trends, interannual variability, and seasonality of Terra Moderate Resolution Imaging Spectroradiometer (MODIS) observations of active fires (thermal anomalies) as a function of management type in the contiguous U.S. during 2001-2010. We used the Monitoring Trends in Burn Severity (MTBS) database to identify the perimeters of large wildland fires and MODIS land cover products to identify fires in croplands. A third class of fires defined as prescribed/other included all residual satellite active fire detections not attributed to large wildland or cropland fire types. Large wildland fires were the most variable of all three fire types and had no significant annual trend for the contiguous U.S. during 2001-2010. Active fires in croplands, in contrast, increased at a rate of 3.4% per year during the decade. Cropland and prescribed/other fire types combined were responsible for 77% of total active fire detections within the U.S. Most of these were located in the south and southeastern United States, where contributions from large wildland fires were relatively small. In the west, cropland active fires had a decreasing trend (6% per year) likely in response to intensive air quality policies. Prescribed/other fires were in-phase with cropland fires across the U.S. on both seasonal and interannual timescales, suggesting these fires had similar management and climate controls. Potential evaporation (PE) was a dominant regulator of the interannual variability of large wildland fires, but had a weaker influence on cropland and prescribed/other fires. Our analyses suggest that potential exists to modify or reduce landscape fire emissions in the U.S. by changing the way fires are used in heavily managed ecosystems.

Description: [1]  Gross primary production (GPP) is an important parameter for carbon cycle and climate change research. Previous estimations of GPP on the Tibetan Plateau were usually reported without quantitative uncertainty analyses. This study sought to quantify the uncertainty and its partitioning in GPP estimation across Tibetan alpine grasslands during 2003-2008 with the modified Vegetation Photosynthesis Model (VPM). Monte Carlo analysis was used to provide a quantitative assessment of the uncertainty in model simulations, and Sobol’ variance decomposition method was applied to determine the relative contribution of each source of uncertainty to the total uncertainty. The results showed that the modified VPM successfully reproduced the seasonal dynamics and magnitude of GPP of 10 flux tower sites on the plateau (R 2  = 0.77-0.95, p 〈 0.001). The six-year mean GPP in Tibetan alpine grasslands was estimated at 223.3 Tg C yr -1 (312.3 g C m -2  yr -1 ). The mean annual GPP increased from western to eastern plateau, with the increase of annual temperature and precipitation and the decrease of elevation; whilst the decrease of GPP from southern to northern plateau was primarily driven by air temperature. Furthermore, the mean relative uncertainty of the annual GPP was 18.30%, with larger uncertainty occurring in regions with lower GPP. Photosynthetic active radiation (PAR), enhanced vegetation index (EVI) and the maximum light use efficiency (LUE) are the primary sources of uncertainty in GPP estimation, contributing 36.84%, 26.86% and 21.99% respectively. This emphasizes the importance of uncertainty in driving variables as well as that of maximum LUE in LUE model simulation.

Description: Event-driven and diel dynamics of soil respiration (R s ) strongly influence terrestrial carbon (C) emissions and are difficult to predict. Wetting events may cause a large pulse or strong inhibition of R s . Complex diel dynamics include hysteresis in the relationship between R s and soil temperature. The mechanistic basis for these dynamics is not well understood, resulting in large discrepancies between predicted and observed R s . We present a unifying approach for interpreting these phenomena in a hot arid agricultural environment. We performed a whole ecosystem wetting experiment with continuous measurement of R s to study pulse responses to wetting in a heterotrophic system. We also investigated R s during cultivation of Sorghum bicolor to evaluate the role of photosynthetic C in the regulation of diel variation in R s . Finally, we adapted a R s model with sensitivity to soil O 2 and water content by incorporating two soil C pools differing in lability. We observed a large wetting-induced pulse of R s from the fallow field and were able to accurately simulate the pulse via release of labile soil C. During the exponential phase of plant growth, R s was inhibited in response to wetting, which was accurately simulated through depletion of soil O 2 . Without plants, hysteresis was not observed, however, with growing plants, an increasingly significant counterclockwise hysteresis developed. Hysteresis was simulated via a dynamic photosynthetic C pool and was not likely controlled by physical processes. These results help characterize the complex regulation of R s and improve understanding of these phenomena under warmer and more variable conditions.

Description: The Changjiang River supplies huge amounts of freshwater and dissolved and particulate substances to the East China Sea, thereby exerting a great influence on the coastal ecosystem. Meanwhile, the construction of the Three Gorges Reservoir (TGR) has re-allocated the annual discharge and likely affecting the transportation of carbon in its various forms. The transport and transformation of carbon in Changjiang River and the effect of the TGR were discussed based on three field campaigns, a one-year time-series investigation, and historical data. Our results indicated that: (1) Dissolved inorganic carbon (DIC) was derived from the upper stream and was significantly diluted downstream by the low-DIC waters from two large lakes. Dissolved organic carbon (DOC) was a product of anthropogenic input and showed no clear relationship with discharge. POC within total suspended matter (POC%) was below the global average. (2) The TGR has not measurably affected the transport of DOC downstream of the reservoir dam. However, downstream grain size has decreased and autochthonous processes have increased, resulting in a sharp increase in POC% since reservoir construction. (3) For the period 1997–2010, estimated annual DIC flux was 16.9 Tg yr -1 . The regulation of river flow by the TGR has decreased the river DIC flux to the East China Sea in the autumn and increased it in the spring. Furthermore, the South-North Water Diversion will reduce the high DIC water from the upper reach, thus affecting the biogeochemistry of the Changjiang estuary and the ecosystem of the nearby coastal ocean.

Description: Marine phytoplankton and associated organic materials absorb a substantial quantity of solar shortwave energy penetrating the upper ocean. Most of this absorbed energy is lost as heat and thereby contributes to the warming of near surface waters. Here we examine this biothermal feedback effect on upper ocean physics and air-sea energy exchange using a fully integrated ocean-atmosphere-biological modeling system. Our model simulations show that a local phytoplankton bloom may impact upper ocean physics in such a way as to promote the spatiotemporal persistence of the bloom itself within a semi-enclosed coastal embayment. This is accomplished primarily via enhanced thermal stratification that promotes vertical stability and more efficient utilization of macronutrients. Modulations of wind stress patterns due to perturbations in the local surface pressure gradients also arise as a result of the simulated biothermal warming of surface waters. The model evidence suggests that the observed persistence of phytoplankton blooms in the northern Monterey Bay, California may be enhanced by similar synergistic interactions between ocean biology and physics.

Description: Peatland vegetation is controlled primarily by the depth of the water table, making peat paleovegetation a useful climate archive. We applied a biochemical approach to quantitatively estimate the plant sources of peat carbon based on (1) neutral sugar compositions of Sphagnum , vascular plants, and lichens; and (2) lignin phenol compositions of vascular plants. We used these biochemical indices to characterize vegetation change over the last 2000 years in four peat cores from the West Siberian Lowland (Russia), to investigate climate change during the Medieval Climate Anomaly and Little Ice Age. The vegetation was dominated by Sphagnum in all four cores, but was punctuated by several rapid but transient transitions to vascular plant dominance in the two cores from the southern West Siberian Lowland (〈60°N latitude). Lichen contributions were evident at the end of the Medieval Climate Anomaly and during the Little Ice Age in the two cores from northern West Siberian Lowland (〉60°N), possibly indicating permafrost development. However, there was no evidence for sustained vegetation change in response to either climatic event in cores from southern West Siberian Lowland. This suggests these climatic events were relatively mild in the southern West Siberian Lowland, although the sensitivity of bog plant communities to climate change remains poorly understood.

Description: The 2011 flood in the Lower Mississippi resulted in the second highest recorded river flow diverted into the Atchafalaya River Basin (ARB). The higher water levels during the flood peak resulted in high hydrologic connectivity between the Atchafalaya River and floodplain, with up to 50% of the Atchafalaya River water moving off channel. Water quality samples were collected throughout the ARB over the course of the flood event. Significant nitrate (NO 3 - ) reduction (75%) occurred within the floodplain, resulting in a total NO 3 - reduction of 16.6% over the flood. The floodplain was a small but measurable source of dissolved reactive phosphorus (SRP) and ammonium (NH 4 + ). Collectively, these results from this large flood event suggest that enhancing river-floodplain connectivity through freshwater diversions will reduce NO 3 - loads to the Gulf of Mexico during large annual floods.

Description: Net ecosystem carbon dioxide (F CO2 ) and methane (F CH4 ) exchanges were measured by using the eddy covariance method to quantify the atmospheric carbon budget at a Typha - and Nymphaea -dominated freshwater marsh (March 2011 to March 2013) and a soybean cropland (May 2011 to May 2012) in northwestern Ohio, USA. Two year average annual F CH4 (49.7 gC-CH 4 m -2  yr -1 ) from the marsh was high and compatible with its net annual CO 2 uptake (F CO2 : -21.0 gC-CO 2 m -2  yr -1 ). In contrast, F CH4 was small (2.3 gC-CH 4 m -2  yr -1 ) and accounted for a minor portion of the atmospheric carbon budget (F CO2 : -151.8 gC-CO 2 m -2  yr -1 ) at the cropland. At the seasonal scale, soil temperature associated with methane (CH 4 ) production provided the dominant regulator of F CH4 at the marsh (R 2  = 0.86). At the diurnal scale, plant-modulated gas flow was the major pathway for CH 4 outgassing in the growing season at the marsh. Diffusion and ebullition became the major pathways in the non-growing season and were regulated by friction velocity. Our findings highlight the importance of freshwater marshes for their efficiency in turning over and releasing newly fixed carbon as CH 4 . Despite marshes accounting for only ~4% of area in the agriculture-dominated landscape, their high F CH4 should be carefully addressed in the regional carbon budget.

Description: No abstract is available for this article.