Search

Peatlands cover only 3–4% of the Earth's surface, but they store nearly 30% of global soil carbon stock. This significant carbon store is under threat as peatlands continue to be degraded at alarming rates around the world. It has prompted countries worldwide to establish regulations to conserve and reduce emissions from this carbon rich ecosystem. For example, the EU has implemented new rules that mandate sustainable management of peatlands, critical to reaching the goal of carbon neutrality by 2050. However, a lack of information on the extent and condition of peatlands has hindered the development of national policies and restoration efforts. This paper reviews the current state of knowledge on mapping and monitoring peatlands from field sites to the globe and identifies areas where further research is needed. It presents an overview of the different methodologies used to map peatlands in nine countries, which vary in definition of peat soil and peatland, mapping coverage, and mapping detail. Whereas mapping peatlands across the world with only one approach is hardly possible, the paper highlights the need for more consistent approaches within regions having comparable peatland types and climates to inform their protection and urgent restoration. The review further summarises various approaches used for monitoring peatland conditions and functions. These include monitoring at the plot scale for degree of humification and stoichiometric ratio, and proximal sensing such as gamma radiometrics and electromagnetic induction at the field to landscape scale for mapping peat thickness and identifying hotspots for greenhouse gas (GHG) emissions. Remote sensing techniques with passive and active sensors at regional to national scale can help in monitoring subsidence rate, water table, peat moisture, landslides, and GHG emissions. Although the use of water table depth as a proxy for interannual GHG emissions from peatlands has been well established, there is no single remote sensing method or data product yet that has been verified beyond local or regional scales. Broader land-use change and fire monitoring at a global scale may further assist national GHG inventory reporting. Monitoring of peatland conditions to evaluate the success of individual restoration schemes still requires field work to assess local proxies combined with remote sensing and modeling. Long-term monitoring is necessary to draw valid conclusions on revegetation outcomes and associated GHG emissions in rewetted peatlands, as their dynamics are not fully understood at the site level. Monitoring vegetation development and hydrology of restored peatlands is needed as a proxy to assess the return of water and changes in nutrient cycling and biodiversity. [ABSTRACT FROM AUTHOR]

Despite the documented increase in greenhouse gas (GHG) emissions from Southeast Asian peat swamp forest degradation and conversion to oil palm over recent decades, reliable estimates of emissions of nitrous oxide (N2O) and methane (CH4) are lacking. We measured soil fluxes of N2O and CH4 and their environmental controls along a peatland transition from primary forest (PF) to degraded drained forest (DF) to oil palm plantation (OP) over 18 months in Jambi, Sumatra, Indonesia. Sampling was conducted monthly at all sites and more intensively following two fertilization events in the OP. Mean annual emissions of N2O (kg N ha−1 yr−1) were 1.7 ± 0.2 for the PF, 2.3 ± 0.2 for the DF and for the OP 8.1 ± 0.8 without drainage canals (DC) and 7.7 ± 0.7 including DC. High N2O emissions in the OP were driven by peat decomposition, not by N fertilizer addition. Mean CH4 annual fluxes (kg C ha−1 yr−1) were 8.2 ± 1.9 for the PF, 1.9 ± 0.4 for the DF, and 1.6 ± 0.3 for the OP with DC and 1.1 ± 0.2 without. Considering their 20-year global warming potentials (GWP), the combined non-CO2 GHG emission (Mg CO2-equivalent ha−1 yr−1) was 3.3 ± 0.6 for the PF and 1.6 ± 0.2 for the DF. The emission in the OP (3.8 ± 0.3 with or without DC) was similar to the PF because reductions in CH4 emissions offset N2O increases. However, considering 100-year GWP, the combined non-CO2 GHG emission was larger in the OP (3.4 ± 0.3 with DC and 3.5 ± 0.3 without) compared to both the PF and the DF (1.5 ± 0.2 and 1.2 ± 0.1, respectively). The increase in peat N2O emissions associated with the land-use change transition from primary forest to oil palm plantation at our sites provides further evidence of the urgent need to protect tropical peat swamp forests from drainage and conversion. [ABSTRACT FROM AUTHOR]

There is increasing interest in utilizing the high carbon sequestration capacity of wetlands as a rationale for their restoration, but this requires careful assessment of their greenhouse gas (GHG) fluxes. We used a spatially extensive sampling approach across salinity gradients and management regimes (disturbed, restored, and reference) in 22 wetland sites across two Oregon, USA estuaries, Tillamook and Coos Bays, to measure fluxes of methane (CH4), ecosystem respiration as carbon dioxide (CO2), and nitrous oxide (N2O) over 1 year and related them to environmental forcing variables. Boosted regression tree (BRT) models explained drivers of GHG fluxes reasonably well despite highly nonlinear and interactive relationships and many flux measurements below detection. We used the BRT models to predict annual GHG fluxes in a subset of restored and reference sites where continuous environmental data were recorded and compared them to previously published soil carbon sequestration rates. Most sites had net removal of CO2-equivalents from the atmosphere over both 20 and 100 year timeframes. Our results show that a spatially extensive GHG flux sampling scheme and machine-learning statistical techniques can be used to estimate GHG fluxes in other current and former wetlands within the region if environmental data are collected at a spatial resolution that reflects site variability and at sufficient duration to reflect seasonality (i.e., at least one full year). Such an approach can save time and money in determining the feasibility of wetland restoration as a climate mitigation strategy. We use our results to suggest wetland restoration strategies that optimize climate benefits. [ABSTRACT FROM AUTHOR]

The major greenhouse gases in streams and rivers, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), can contribute significantly to regional greenhouse gas (GHG) budgets, and each appears to be responding to multiple drivers. Recent work suggests that tropical water bodies may be hot spots of GHG emissions due to high primary productivity in their watersheds, but tropical streams and rivers have historically been underrepresented in studies of GHG concentration and emissions. We use a five-year record of weekly water chemistry and dissolved gas data from eight tropical watersheds of varying lithology and redox conditions in the Luquillo Mountains of Puerto Rico to examine controls on GHG variability and estimate gas flux. Streams were frequently supersaturated in all three gases indicating that streams in this tropical landscape are sources of GHGs to the atmosphere. Concentrations of CO2 and N2O were associated with lateral inputs from the surrounding landscape, whereas CH4 concentrations correlated with in-stream oxygen availability and lithology. Our results underscore the importance of including tropical sites in global syntheses and budgets and the role of both in-stream biological and physical processes as well as landscape attributes that contribute to the export of gases to the fluvial network and atmosphere. [ABSTRACT FROM AUTHOR]

Lake-rich Arctic deltas differ biogeochemically from tundra lakes, and their role as sources and sinks of greenhouse gases remains poorly understood. Under-ice and open-water changes in methane (CH4) storage (43 lakes, 2014), floating chamber measurements of total and diffusive CH4 evasion to the atmosphere (6 lakes, 2014–2015), and water-column CH4 oxidation (MOX) (6 lakes, 2014–2015) permitted evaluation of how CH4 emissions vary among lakes with differing river-to-lake connection times within the Mackenzie Delta. CH4 emissions during ice-out were considerable, followed by substantial declines as open-water progressed. Water-column MOX rates were highest after ice-out, and declined throughout open-water. After accounting for a strong effect of CH4 substrate levels, MOX rates were inversely related to pH, which can increase to high levels during open-water because of high macrophyte production. Comparisons of water-column CH4 storage versus open-water fluxes (6 lakes) showed that diffusive evasion plus MOX removed most CH4 in the water columns every 1–2 days with only modest changes in storage, suggesting that counter-balancing water-column replenishment is substantial. Lakes with short river-connection times (i.e. most strongly autotrophic and strongly CO2-absorbing in this delta) and thermokarst lakes contribute disproportionately to CH4 flux, relative to lakes with long river-connection times. Thus, this great Arctic delta represents an important system of greenhouse-gas emitting lakes despite prior work showing their net absorption of CO2 during open-water, and having a low landscape area of CO2-saturated thermokarst lakes. Autotrophically absorbed CO2 becomes labile carbon substrate, and is microbially shunted back to the atmosphere as the more potent greenhouse gas CH4. [ABSTRACT FROM AUTHOR]

Eutrophication has been identified as the primary cause of water quality deterioration in inland waters worldwide, often associated with algal blooms or fish kills. Eutrophication can be controlled through watershed management and in-lake measures. An extreme heatwave event, through its impact on mineralization rates and internal nutrient loading (phosphorus—P, and nitrogen—N), could counteract eutrophication control measures. We investigated how the effectiveness of a nutrient abatement technique is impacted by an extreme heatwave, and to what extent biogeochemical processes are modulated by exposure to heatwaves. To this end, we carried out a sediment-incubation experiment, testing the effectiveness of lanthanum-modified bentonite (LMB) in reducing nutrients and greenhouse gas emissions from eutrophic sediments, with and without exposure to an extreme heatwave. Our results indicate that the effectiveness of LMB may be compromised upon exposure to an extreme heatwave event. This was evidenced by an increase in concentration of 0.08 ± 0.03 mg P/L with an overlying water volume of 863 ± 21 mL, equalling an 11% increase, with effects lasting to the end of the experiment. LMB application generally showed no effect on nitrogen species, while the heatwave stimulated nitrification, resulting in ammonium loss and accumulation of dissolved oxidized nitrogen species as well as increased dissolved nitrous oxide concentrations. In addition, carbon dioxide (CO2)-equivalent was more than doubled during the heatwave relative to the reference temperature, and LMB application had no effect on mitigating them. Our sediment incubation experiment indicates that the rates of biogeochemical processes can be significantly accelerated upon heatwave exposure, resulting in a change in fluxes of nutrient and greenhouse gas between sediment and water. The current efforts in eutrophication control will face more challenges under future climate scenarios with more frequent and intense extreme events as predicted by the IPCC. [ABSTRACT FROM AUTHOR]

In a preliminary experiment we found that methane evolved from a sandy subsoil during aerobic incubation of shaken soil slurries. In the study presented here the methane was found to be released from the sand particles by mechanical weathering, caused by the grinding effect of the shaking. Large amounts of gas (about 0.5 ml gas g-1soil) were extracted by intense grinding of the soil in gas tight serum vials. Methane was the main hydrocarbon in the emitted gas, but also a considerable amount of ethane was present, as well as minor amounts of heavier hydrocarbons (up to C6). The delta13C-values of the emitted methane and ethane were -3.3% and -2.9%,respectively. Together these results demonstrate a thermogenic origin of the gas. This paper also reports the results of an incubation experiment where possible methane oxidation was looked for. If a possible release of methane is not accounted for, methane oxidation may be overlooked, as illustrated in this paper. Methane consumption was detected only in soil from 40 cm, in contrast to soil sampled at 100 cm and deeper where a slight production was measured. When methane oxidation was inhibited by dimethyl-ether, a significant release of methane was seen. The release was probably caused by chemical weathering. When this methane release was taken into account, methane oxidation was found to be present at all measured depths (40 to 200cm). Fertilization with urea inhibited the methane oxidation at 40cm but not at deeper layers. It is hypothesized that ammonia oxidizing bacteria were the main methane oxidizers in this mineral subsoil (deeper than 1 m), and that oxidation of methane might be a survival mechanism for ammonia oxidizers in ammonia limited environments. [ABSTRACT FROM AUTHOR]

Organic matter (OM) loading drives benthic metabolism and controls nitrogen (N), phosphorus (P), and carbon (C) cycling in coastal sediments. To better understand the influence of OM on benthic metabolism, we conducted sediment core incubations in Long Island Sound, NY (USA), an estuary heavily influenced by anthropogenic nutrient loading. Cores were collected during the summer and winter at five stations along a west to east transect representing a gradient of high to low nutrient inputs and subsequent OM deposition. Here we provide a comprehensive analysis of environmental drivers of benthic fluxes as well as the first directly measured rates of sediment denitrification and N-fixation, thus providing a missing piece of the N budget for Long Island Sound. Our findings show greatest sediment oxygen demand (SOD) in the western basin during the summer. Sediments were a source of dissolved inorganic N and P. Compared to historic SOD and ammonium fluxes, our measurements were about 50% lower, potentially reflecting larger scale perturbations (e.g., warming temperatures, decrease wind speed) or reduced N loading through improved management of wastewater treatment facilities. Sediments were a source of nitrous oxide (mean (± standard error): 8.63 ± 3.18 nmol m−2 h−1) and methane (23.80 ± 18.05 nmol m−2 h−1). Net sediment denitrification rates were low (10.34 ± 7.61 µmol N2–N m−2 h−1) and did not vary significantly across stations or seasons. These sediments have a N removal efficiency of ~ 30% with the potential to remove approximately 9% of the total land-based N load entering Long Island Sound. [ABSTRACT FROM AUTHOR]

Nutrient pollution and greenhouse gas emissions related to crop agriculture and confined animal feeding operations (CAFOs) in the US have changed substantially in recent years, in amounts and forms. This review is intended to provide a broad view of how nutrient inputs—from fertilizer and CAFOs—as well as atmospheric NH3 and greenhouse gas emissions, are changing regionally within the US and how these changes compare with nutrient inputs from human wastewater. Use of commercial nitrogen (N) fertilizer in the US, which now exceeds 12,000,000 metric tonnes (MT) continues to increase, at a rate of 60,000 MT per year, while that of phosphorus (P) has remained nearly constant over the past decade at around 1,800,000 MT. The number of CAFOs in the US has increased nearly 10% since 2012, driven largely by a near 13% increase in hog production. The annualized inventory of cattle, dairy cows, hogs, broiler chickens and turkeys is approximately 8.7 billion, but CAFOs are highly regionally concentrated by animal sector. Country-wide, N applied by fertilizer is about threefold greater than manure N inputs, but for P these inputs are more comparable. Total manure inputs now exceed 4,000,000 MT as N and 1,400,000 MT as P. For both N and P, inputs and proportions vary widely by US region. The waste from hog and dairy operations is mainly held in open lagoons that contribute to NH3 and greenhouse gas (as CH4 and N2O) emissions. Emissions of NH3 from animal waste in 2019 were estimated at > 4,500,000 MT. Emissions of CH4 from manure management increased 66% from 1990 to 2017 (that from dairy increased 134%, cattle 9.6%, hogs 29% and poultry 3%), while those of N2O increased 34% over the same time period (dairy 15%, cattle 46%, hogs 58%, and poultry 14%). Waste from CAFOs contribute substantially to nutrient pollution when spread on fields, often at higher N and P application rates than those of commercial fertilizer. Managing the runoff associated with fertilizer use has improved with best management practices, but reducing the growing waste from CAFO operations is essential if eutrophication and its effects on fresh and marine waters–namely hypoxia and harmful algal blooms (HABs)—are to be reduced. [ABSTRACT FROM AUTHOR]

Ecosystem fires are stochastic and anthropogenic phenomena that affect critical soil processes. Nevertheless, environmental managers, policy-makers, and even scientists have often overlooked the induced transformations that fire does to soil organic matter (SOM), which sustains an ecosystem's overall health. Here, we investigated the effects of simulated fire conditions on bulk SOM, water-extractable organic matter (WEOM), and vulnerability to microbial degradation. Sequential thermal decomposition experiments were carried out to investigate the effect of increasing temperature on C, N, δ13C, and δ15N of bulk SOM and WEOM. A microbial decomposition experiment was also done to determine the effects of burning on SOM degradability. Intermediate-intensity burns caused significant decreases in C and N concentrations in soils, as well as alterations in bulk SOM and WEOM properties. Conversely, the effects of low-intensity burns were less apparent in terms of bulk SOM content and stable C and N isotope composition. However, the results of the microbial decomposition experiment revealed that low-intensity burning resulted in elevated CO2 emission that were significantly correlated with C concentration, δ13C values, and the C/N ratio of WEOM. These results provide evidence that low-intensity burning can have important consequences for soils, altering its organic components and releasing significant amounts of greenhouse gases. Thus, particular focus must be given to managing ecosystem fires and evaluating their impacts, especially because fire is still widely used in agriculture and forest management with frequency projected to increase in the coming years. [ABSTRACT FROM AUTHOR]

Tropical peatland ecosystems are a significant component of the global carbon cycle and feature a range of distinct vegetation types, but the extent of links between contrasting plant species, peat biogeochemistry and greenhouse gas fluxes remains unclear. Here we assessed how vegetation affects small scale variation of tropical peatland carbon dynamics by quantifying in situ greenhouse gas emissions over 1 month using the closed chamber technique, and peat organic matter properties using Rock-Eval 6 pyrolysis within the rooting zones of canopy palms and broadleaved evergreen trees. Mean methane fluxes ranged from 0.56 to 1.2 mg m−2 h−1 and were significantly greater closer to plant stems. In addition, pH, ranging from 3.95 to 4.16, was significantly greater closer to stems. A three pool model of organic matter thermal stability (labile, intermediate and passive pools) indicated a large labile pool in surface peat (35-42%), with equivalent carbon stocks of 2236-3065 g m−2. Methane fluxes were driven by overall substrate availability rather than any specific carbon pool. No peat properties correlated with carbon dioxide fluxes, suggesting a significant role for root respiration, aerobic decomposition and/or methane oxidation. These results demonstrate how vegetation type and inputs, and peat organic matter properties are important determinants of small scale spatial variation of methane fluxes in tropical peatlands that are affected by climate and land use change. [ABSTRACT FROM AUTHOR]

Coastal forested wetlands provide important ecosystem services such as carbon sequestration, nutrient retention, and flood protection, but they are also important sources of greenhouse gas emissions. Human appropriation of surface water and extensive ditching and draining of coastal plain landscapes are interacting with rising sea levels to increase the frequency and magnitude of saltwater incursion into formerly freshwater coastal wetlands. Both hydrologic change and saltwater incursion are expected to alter carbon and nutrient cycling in coastal forested wetlands. We performed a full factorial experiment in which we exposed intact soil cores from a coastal forested wetland to experimental marine salt treatments and two hydrologic treatments. We measured the resulting treatment effects on the emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) over 112 days. Salinity effects were compared across four treatments to isolate the effects of increases in ionic strength from the impact of adding a terminal electron acceptor (SO42−). We compared control treatments (DI addition), to artificial saltwater (ASW, target salinity of 5 parts per thousand) and to two treatments that added sulfate alone (SO42−, at the concentration found in 5 ppt saltwater) and saltwater with the sulfate removed (ASW-SO42−, with the 5 ppt target salinity maintained by adding additional NaCl). We found that all salt treatments suppressed CO2 production, in both drought and flooded treatments. Contrary to our expectations, CH4 fluxes from our flooded cores increased between 300 and 1200% relative to controls in the ASW and ASW-SO42− treatments respectively. In the drought treatments, we saw virtually no CH4 release from any core, while artificial seawater with sulfate increased N2O fluxes by 160% above DI control. In contrast, salt and sulfate decreased N2O fluxes by 72% in our flooded treatments. Our results indicate that salinization of forested wetlands of the coastal plain may have important climate feedbacks resulting from enhanced greenhouse gas emissions and that the magnitude and direction of these emissions are contingent upon wetland hydrology. [ABSTRACT FROM AUTHOR]

Tidal freshwater ecosystems experience acute seawater intrusion associated with periodic droughts, but are expected to become chronically salinized as sea level rises. Here we report the results from an experimental manipulation in a tidal freshwater Zizaniopsis miliacea marsh on the Altamaha River, GA where diluted seawater was added to replicate marsh plots on either a press (constant) or pulse (2 months per year) basis. We measured changes in porewater chemistry (SO42−, Cl−, organic C, inorganic nitrogen and phosphorus), ecosystem CO2 and CH4 exchange, and microbial extracellular enzyme activity. We found that press (chronic) seawater additions increased porewater chloride and sulfate almost immediately, and ammonium and phosphate after 2-4 months. Chronic increases in salinity also decreased net ecosystem exchange, resulting in reduced CO2 and CH4 emissions from press plots. Our pulse treatment, designed to mimic natural salinity incursion in the Altamaha River (September and October), temporarily increased porewater ammonium concentrations but had few lasting effects on porewater chemistry or ecosystem carbon balance. Our findings suggest that long-term, chronic saltwater intrusion will lead to reduced C fixation and the potential for increased nutrient (N, P) export while acute pulses of saltwater will have temporary effects. [ABSTRACT FROM AUTHOR]

Many lacustrine systems, despite management efforts to control eutrophication, are hypoxic during stratified periods. Hypoxia is a major concern, not only for its impact on aquatic life but also for its potential to stimulate production of the greenhouse gases, methane (CH) and nitrous oxide (NO). We investigated the drivers of hypoxia in Muskegon Lake, a temperate dimictic freshwater estuary that experiences frequent hypolimnetic mixing due to atmospheric forces, riverine inputs, and intrusion of oxic water from coastal upwelling in Lake Michigan. Primary production and respiration (R) rates obtained from a δO mass balance model were similar to other mesotrophic environments (0.56-26.31 and 0.57-13.15 mmol O m day, respectively), although high P/R (≥2 in mid-summer) indicated there is sufficient autochthonous production to support hypoxia development and persistence. The isotopic enrichment factor for respiration (ε) varied markedly and was least negative in August of both sampling years, consistent with high R rates. Hypoxic conditions were associated with accumulation of NO but not CH, and emissions of NO are among the highest reported from lakes. The average NO site preference value of 25.4‰ indicates that the majority of NO was produced by nitrification via hydroxylamine oxidation, despite the presence of resilient hypoxia. While it has been hypothesized that denitrification acts as a sink for NO in hypoxic lakes, it is clear that Muskegon Lake functions as a strong source of NO via nitrification. Further considerations of lakes as global sources of NO thus warrant a closer evaluation of nitrification-fueled NO production. [ABSTRACT FROM AUTHOR]

Rivers transport and transform significant quantities of carbon to coastal zones globally. Urbanization and climate change impact the transport and transformation of carbon by altering hydrology, water temperatures, and in-stream metabolism rates. Changes in exports, sources, and metabolism of carbon influence ecosystem processes, food webs, and greenhouse gases. We characterized exports, sources, and metabolism of carbon in four urban watersheds using a combination of discrete stream chemistry measurements and continuous water-quality sensors. Over three years, watershed DOC exports in the Baltimore-Washington D.C. metropolitan area ranged from 9 to 23 kg ha year. DIC exports ranged from 19 to 59 kg ha year. Daily contributions from in-stream metabolism varied between −65 and 90 % of DIC export depending on stream size and streamflow conditions. Negative contributions from metabolism occurred on days when streams were autotrophic. All streams were heterotrophic during 60 to 87 % of each year, but showed significant peaks in autotrophy during spring and summer. Differences in the timing and magnitude of peaks in springtime net ecosystem productivity were likely driven by varying light availability across streams of different sizes and riparian shading. CO was consistently over-saturated with respect to the atmosphere on all sampling dates and was 0.25-2.9 mg C L. Exports, sources, and metabolism of DOC and DIC showed strong predictable patterns across streamflow. Thus, we present a new conceptual model for predicting carbon transport and transformation across changing streamflow and light availability (with impacts on sources and fluxes of DOC, DIC, and CO). Overall, our results and conceptual model suggest that urbanization accelerates the transition of streams from transporters to transformers of carbon across streamflow, with implications for timing and magnitude of CO fluxes, river alkalinization, and oxygen demand in downstream waters. [ABSTRACT FROM AUTHOR]

Concurrent changes in climate, atmospheric nitrogen (N) deposition, and increasing levels of atmospheric carbon dioxide (CO) affect ecosystems in complex ways. The DayCent-Chem model was used to investigate the combined effects of these human-caused drivers of change over the period 1980-2075 at seven forested montane and two alpine watersheds in the United States. Net ecosystem production (NEP) increased linearly with increasing N deposition for six out of seven forested watersheds; warming directly increased NEP at only two of these sites. Warming reduced soil organic carbon storage at all sites by increasing heterotrophic respiration. At most sites, warming together with high N deposition increased nitrous oxide (NO) emissions enough to negate the greenhouse benefit of soil carbon sequestration alone, though there was a net greenhouse gas sink across nearly all sites mainly due to the effect of CO fertilization and associated sequestration by plants. Over the simulation period, an increase in atmospheric CO from 350 to 600 ppm was the main driver of change in net ecosystem greenhouse gas sequestration at all forested sites and one of two alpine sites, but an additional increase in CO from 600 to 760 ppm produced smaller effects. Warming either increased or decreased net greenhouse gas sequestration, depending on the site. The N contribution to net ecosystem greenhouse gas sequestration averaged across forest sites was only 5-7 % and was negligible for the alpine. Stream nitrate (NO) fluxes increased sharply with N-loading, primarily at three watersheds where initial N deposition values were high relative to terrestrial N uptake capacity. The simulated results displayed fewer synergistic responses to warming, N-loading, and CO fertilization than expected. Overall, simulations with DayCent-Chem suggest individual site characteristics and historical patterns of N deposition are important determinants of forest or alpine ecosystem responses to global change. [ABSTRACT FROM AUTHOR]

To gain a more mechanistic understanding of how soil organic matter (OM) characteristics can affect carbon mineralization in tidal freshwater wetlands, we conducted a long-term in situ field manipulation of OM type and monitored associated changes in carbon dioxide (CO) and methane (CH) production. In addition, we characterized microbial community structure and quantified the activity of several extracellular enzymes (EEA) involved in the acquisition of carbon, nitrogen, and phosphorus. Treatments included a plant litter addition, prepared using naturally-senescing vegetation from the site, and a compost amendment, designed to increase the concentration of aged, partially humified, OM. Both types of OM-amended soils had CO production rates 40-50 % higher than unamended control soils, suggesting that the added OM had inherently higher quality and/or availability than the native soil OM. Rates of CO production were not correlated with microbial community structure or EEA except a modest relationship with cellulose breakdown via the K of β-1,4-glucosidase. We interpret this lack of correlation to be a consequence of high functional redundancy of microorganisms that are capable of producing CO. Rates of CH production were also influenced by OM quality, increasing by an order of magnitude with plant litter additions relative to compost-amended and control soils. Unlike CO, rates of CH production were significantly correlated with the microbial community structure and with enzyme kinetic parameters (V and K) for both carbon (β-1,4-glucosidase, 1,4-β-cellobiosidase, and β- d-xylosidase) and nitrogen acquisition (leucyl aminopeptidase). The monophyletic nature of methanogenic archaea, combined with their reliance on a small select group of organic substrates produced via enzyme-mediated hydrolysis and subsequent bacterial fermentation, provides a basis for the strong links between microbial community structure, EEA, and CH production. Our results suggest that incorporating microbial community structure and EEA into conceptual models of wetland OM decomposition may enhance our mechanistic understanding of, and predictive capacity for, biogeochemical process rates. [ABSTRACT FROM AUTHOR]

Agriculture in the United States (US) cycles large quantities of nitrogen (N) to produce food, fuel, and fiber and is a major source of excess reactive nitrogen (Nr) in the environment. Nitrogen lost from cropping systems and animal operations moves to waterways, groundwater, and the atmosphere. Changes in climate and climate variability may further affect the ability of agricultural systems to conserve N. The N that escapes affects climate directly through the emissions of nitrous oxide (NO), and indirectly through the loss of nitrate (NO), nitrogen oxides (NO) and ammonia to downstream and downwind ecosystems that then emit some of the N received as NO and NO. Emissions of NO lead to the formation of tropospheric ozone, a greenhouse gas that can also harm crops directly. There are many opportunities to mitigate the impact of agricultural N on climate and the impact of climate on agricultural N. Some are available today; many need further research; and all await effective incentives to become adopted. Research needs can be grouped into four major categories: (1) an improved understanding of agricultural N cycle responses to changing climate; (2) a systems-level understanding of important crop and animal systems sufficient to identify key interactions and feedbacks; (3) the further development and testing of quantitative models capable of predicting N-climate interactions with confidence across a wide variety of crop-soil-climate combinations; and (4) socioecological research to better understand the incentives necessary to achieve meaningful deployment of realistic solutions. [ABSTRACT FROM AUTHOR]

Microbial oxidation in aerobic soils is the primary biotic sink for atmospheric methane (CH), a powerful greenhouse gas. Although tropical forest soils are estimated to globally account for about 28% of annual soil CH consumption (6.2 Tg CH year), limited data are available on CH exchange from tropical montane forests. We present the results of an extensive study on CH exchange from tropical montane forest soils along an elevation gradient (1,000, 2,000, 3,000 m) at different topographic positions (lower slope, mid-slope, ridge position) in southern Ecuador. All soils were net atmospheric CH sinks, with decreasing annual uptake rates from 5.9 kg CH-C ha year at 1,000 m to 0.6 kg CH-C ha year at 3,000 m. Topography had no effect on soil atmospheric CH uptake. We detected some unexpected factors controlling net methane fluxes: positive correlations between CH uptake rates, mineral nitrogen content of the mineral soil and with CO emissions indicated that the largest CH uptake corresponded with favorable conditions for microbial activity. Furthermore, we found indications that CH uptake was N limited instead of inhibited by NH. Finally, we showed that in contrast to temperate regions, substantial high affinity methane oxidation occurred in the thick organic layers which can influence the CH budget of these tropical montane forest soils. Inclusion of elevation as a co-variable will improve regional estimates of methane exchange in these tropical montane forests. [ABSTRACT FROM AUTHOR]

Most terrestrial ecosystems support a similar suite of biogeochemical processes largely dependent on the availability of water and labile carbon (C). Here, we explored the biogeochemical potential of soils from Earth's driest ecosystem, the Atacama Desert, characterized by extremely low moisture and organic C. We sampled surface soil horizons from sites ranging from the Atacama's hyper-arid core to less-arid locations at higher elevation that supported sparse vegetation. We performed laboratory incubations and measured fluxes of the greenhouse gases carbon dioxide (CO), nitrous oxide (NO), and methane (CH) as indices of potential biogeochemical activity across this gradient. We were able to stimulate trace gas production at all sites, and treatment responses often suggested the influence of microbial processes. Sites with extant vegetation had higher C concentrations (0.13-0.68%) and produced more CO under oxic than sub-oxic conditions, suggesting the presence of aerobic microbial decomposers. In contrast, abiotic CO production appeared to predominate in the most arid and C-poor (<0.08% C) sites without plants, with one notable exception. Soils were either a weak source or sink of CH under oxic conditions, whereas anoxia stimulated CH production across all sites. Several sites were rich in nitrate, and we stimulated NO fluxes in all soils by headspace manipulation or dissolved organic matter addition. Peak NO fluxes in the most C-poor soil (0.02% C) were very high, exceeding 3 ng nitrogen g h under anoxic conditions. These results provide evidence of resilience of at least some soil biogeochemical capacity to long-term water and C deprivation in the world's driest ecosystem. Atacama soils appear capable of responding biogeochemically to moisture inputs, and could conceivably constitute a regionally-important source of NO under altered rainfall regimes, analogous to other temperate deserts. [ABSTRACT FROM AUTHOR]

Methane emissions and below ground methane pore water concentrations were determined in an alpine fen at 1,915 m a.s.l. in central Switzerland. The fen represented an acidic (pH 4.5-4.9), nutrient-poor to mesotrophic habitat dominated by Carex limosa, Carex rostrata, Trichophorum caespitosum and Sphagnum species. From late fall to late spring the fen was snow-covered. Throughout winter the temperatures never dropped below 0°C at 5 cm below the vegetation surface. Methane emissions in June, July, August and September were in the range of 125 (±26)-313 (±71) mg CH m day with a tendency to decrease along the summer season. Mean methane pore water concentrations at a depth of 20-40 cm below the vegetation surface were 526 (±32) μM in June and in the range of 144 (±10)-233 (±7) μM in July, August and September. At a depth of 0-20 cm the mean methane pore water concentrations dropped back to <20 μM with an almost linear decrease between 0 and 15 cm. Oxygen pore water concentrations were close to air saturation in the first few centimeters and dropped back below detection limit at a depth of 20 cm. In July and August the pore water concentrations of dissolved organic carbon (DOC) were in the range of 7.2-10.1 mg C l at all depths. The pore water concentrations of acetate, formate and oxalate were in the range of 2.0-8.2 μM at all depths. Methanotrophic and methanogenic communities were quantified using pmoA and mcrA, respectively, as marker genes. The abundances of both communities showed a distinct peak at a depth of 10-15 cm below the vegetation surface. [ABSTRACT FROM AUTHOR]

Organic matter decomposition regulates rates of carbon loss (CO and CH) in wetlands and has implications for carbon sequestration in the context of changing global temperature. Here we determined the influence of temperature and vegetation type on both aerobic and anaerobic decomposition of organic matter in subtropical wetland soils. As in many other studies, increased temperature resulted in higher rates of respiration and methanogenesis under both aerobic and anaerobic conditions, and the positive effect of temperature depended on vegetation (source of carbon substrate to soil). Under anaerobic incubations, the proportion of gaseous C (CO and CH) lost as CH increased with temperature indicating a greater sensitivity of methanogenesis to temperature. This was further supported by a wider range of Q values (1.4-3.6) for methane production as compared with anaerobic CO (1.3-2.5) or aerobic CO (1.4-2.1) production. The increasing strength of positive linear correlation between CO:CH ratio and the soil organic matter ligno-cellulose index at higher temperature indicated that the temperature sensitivity of methanogenesis was likely the result of increased C availability at higher temperature. This information adds to our basic understanding of decomposition in warmer subtropical and tropical wetland systems and has implications for C models in wetlands with different vegetation types. [ABSTRACT FROM AUTHOR]

Small streams that drain agricultural landscapes have come under close scrutiny as potentially significant indirect sources of greenhouse gases (GHGs) to the atmosphere. By exploring the stream-ground water connection in three dimensional space (horizontally and vertically beneath the stream channel, and longitudinally along the stream corridor) our results show (1) ground water can be a significant source of greenhouse gases to streams draining agricultural watersheds with concentrations in excess of atmospheric equilibrium by 221 μmol C L carbon dioxide, 0.64 μmol C L methane, and 0.65 μmol N L nitrous oxide (NO); (2) changes in the stream-ground water connection can create seemingly erratic patterns in GHG concentrations over short longitudinal distances (order of meters); (3) soil-stream interfaces are hotspots for denitrification and methanogenesis; however, no significant NO production was observed at such an interface under a riparian forest; and (4) nitrate (NO) and NO can be preserved as electron acceptors in oxic ground waters draining agriculture landscapes; hence, soil nitrification was the major source of NO to stream water, with a legacy in ground water dating back to the 1960s; NO tracked the seepage of NO into surface waters. In this study, we demonstrate the utility of detailed measurements of multiple trace gases towards revealing spatial and temporal patterns of surface-ground water interactions and biogeochemistry across several small baseflow-dominated stream ecosystems in central Wisconsin, USA. [ABSTRACT FROM AUTHOR]

Denitrification is known as an important pathway for nitrate loss in agroecosystems. It is important to estimate denitrification fluxes to close field and watershed N mass balances, determine greenhouse gas emissions (N2O), and help constrain estimates of other major N fluxes (e.g., nitrate leaching, mineralization, nitrification). We compared predicted denitrification estimates for a typical corn and soybean agroecosystem on a tile drained Mollisol from five models (DAYCENT, SWAT, EPIC, DRAINMOD-N II and two versions of DNDC, 82a and 82h), after first calibrating each model to crop yields, water flux, and nitrate leaching. Known annual crop yields and daily flux values (water, nitrate-N) for 1993–2006 were provided, along with daily environmental variables (air temperature, precipitation) and soil characteristics. Measured denitrification fluxes were not available. Model output for 1997–2006 was then compared for a range of annual, monthly and daily fluxes. Each model was able to estimate corn and soybean yields accurately, and most did well in estimating riverine water and nitrate-N fluxes (1997–2006 mean measured nitrate-N loss 28 kg N ha−1 year−1, model range 21–28 kg N ha−1 year−1). Monthly patterns in observed riverine nitrate-N flux were generally reflected in model output ( r2 values ranged from 0.51 to 0.76). Nitrogen fluxes that did not have corresponding measurements were quite variable across the models, including 10-year average denitrification estimates, ranging from 3.8 to 21 kg N ha−1 year−1 and substantial variability in simulated soybean N2 fixation, N harvest, and the change in soil organic N pools. DNDC82a and DAYCENT gave comparatively low estimates of total denitrification flux (3.8 and 5.6 kg N ha−1 year−1, respectively) with similar patterns controlled primarily by moisture. DNDC82h predicted similar fluxes until 2003, when estimates were abruptly much greater. SWAT and DRAINMOD predicted larger denitrification fluxes (about 17–18 kg N ha−1 year−1) with monthly values that were similar. EPIC denitrification was intermediate between all models (11 kg N ha−1 year−1). Predicted daily fluxes during a high precipitation year (2002) varied considerably among models regardless of whether the models had comparable annual fluxes for the years. Some models predicted large denitrification fluxes for a few days, whereas others predicted large fluxes persisting for several weeks to months. Modeled denitrification fluxes were controlled mainly by soil moisture status and nitrate available to be denitrified, and the way denitrification in each model responded to moisture status greatly determined the flux. Because denitrification is dependent on the amount of nitrate available at any given time, modeled differences in other components of the N cycle (e.g., N2 fixation, N harvest, change in soil N storage) no doubt led to differences in predicted denitrification. Model comparisons suggest our ability to accurately predict denitrification fluxes (without known values) from the dominant agroecosystem in the midwestern Illinois is quite uncertain at this time. [ABSTRACT FROM AUTHOR]

To clarify the reason for the higher CH4 uptake rate in Japanese forest soils, twenty-seven sites were established for CH4 flux measurement. The first order rate constant for CH4 uptake was also determined using soil core incubation at 14 sites. The CH4 uptake rate had a seasonal fluctuation, high in summer and low in winter, and the rate correlated with soil temperature at 17 sites. The annual CH4 uptake rates ranged from 2.7 to 24.8 kg CH4 ha−1 y−1 (the average of these rates was 9.7 or 10.9 kg CH4 ha−1 y−1, depending on method of calculation), which is somewhat higher than the uptake rates reported in previous literature. The averaged CH4 uptake rate correlated closely with the CH4 oxidation rate of the topsoil (0–5 cm) in the study sites. The CH4 oxidation constant of the topsoil was explained by a multiple regression model using total pore volume of the soil, nitrate content, and C/N ratio ( p < 0.05, R2 = 0.684). This result and comparison with literature data suggest that the high CH4 uptake rate in Japanese forest soils depends on the high porosity probably due to volcanic ash parent materials. According to our review of the literature, the CH4 uptake rate in temperate forests in Europe is significantly different from that in Asia and North America. A new global CH4 uptake rate in temperate forests was estimated to be 5.4 Tg y−1 (1 SE is 1.1 Tg y−1) on a continental basis. [ABSTRACT FROM AUTHOR]

This study investigates soil N2O dynamics in forest soils representing early (3-years) and late (>50 years) post-harvest succession in Atlantic Canada over a 9-month snow-free period in order to develop a better understanding of the role of managed forests as sources and sinks of N2O. We couple measurement of surface flux with detailed measurements of subsurface N2O concentrations at four mineral soil depths (0, 5, 20 and 35 cm) at 40 plots located within four sites. Median surface fluxes were similar at all sites regardless of the management stage (−5 to +19 ugN2O–N/m2/day), with all sites behaving as net sinks and sources of N2O over the measurement period. Subsurface mineral soil N2O concentrations at early (3-year) post-harvest succession sites, which ranged from median values of 362 ppbv at 0 cm to 1783 ppbv at 35 cm depth, were significantly higher than late post-harvest succession sites where median concentrations ranged from 329 ppbv at 0 cm to 460 ppbv at 35 cm depth. Examination of relationships between subsurface gas storage and surface flux magnitudes, suggested although recently harvested forest soils may be producing N2O at a greater rate than mature forest soils, observed patterns are consistent with a strong sink for this gas that prevents its conservative transport through the soil profile, and ultimate emission to the atmosphere through the majority of the measurement period. [ABSTRACT FROM AUTHOR]

The objective of this study was to evaluate the effect of N fertilization and the presence of N2 fixing leguminous trees on soil fluxes of greenhouse gases. For a one year period, we measured soil fluxes of nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4), related soil parameters (temperature, water-filled pore space, mineral nitrogen content, N mineralization potential) and litterfall in two highly fertilized (250 kg N ha−1 year−1) coffee cultivation: a monoculture (CM) and a culture shaded by the N2 fixing legume species Inga densiflora (CIn). Nitrogen fertilizer addition significantly influenced N2O emissions with 84% of the annual N2O emitted during the post fertilization periods, and temporarily increased soil respiration and decreased CH4 uptakes. The higher annual N2O emissions from the shaded plantation (5.8 ± 0.3 kg N ha−1 year−1) when compared to that from the monoculture (4.3 ± 0.1 kg N ha−1 year−1) was related to the higher N input through litterfall (246 ± 16 kg N ha−1 year−1) and higher potential soil N mineralization rate (3.7 ± 0.2 mg N kg−1 d.w. d−1) in the shaded cultivation when compared to the monoculture (153 ± 6.8 kg N ha−1 year−1 and 2.2 ± 0.2 mg N kg−1 d.w. d−1). This confirms that the presence of N2 fixing shade trees can increase N2O emissions. Annual CO2 and CH4 fluxes of both systems were similar (8.4 ± 2.6 and 7.5 ± 2.3 t C-CO2 ha−1 year−1, −1.1 ± 1.5 and 3.3 ± 1.1 kg C-CH4 ha−1 year−1, respectively in the CIn and CM plantations) but, unexpectedly increased during the dry season. [ABSTRACT FROM AUTHOR]

Methane (CH4) is the second most important greenhouse gas after carbon dioxide (CO2). To understand CH4 cycling, quantitative information about microbial CH4 oxidation in soils is essential. Field methods such as the gas push-pull test (GPPT) to quantify CH4 oxidation are often used in combination with specific inhibitors, such as acetylene (C2H2). Acetylene irreversibly binds to the enzyme methane monooxygenase, but little is known about recovery of CH4 oxidation activity after C2H2 inhibition in situ, which is important when performing several experiments at the same location. To assess recovery of CH4 oxidation activity following C2H2 inhibition, we performed a series of GPPTs over 8 weeks at two different locations in the vadose zone above a petroleum hydrocarbon-contaminated aquifer in Studen, Switzerland. After 4 weeks a maximum recovery of 30% and 50% of the respective initial activity was reached, with a subsequent slight drop in activity at both locations. Likely, CH4 oxidation activity and CH4 concentrations were too low to allow for rapid recovery following C2H2 inhibition at the studied locations. Therefore, alternative competitive inhibitors have to be evaluated for application in conjunction with GPPTs, especially for sites with low activity. [ABSTRACT FROM AUTHOR]

During the last decades, decision makers and policy have increasingly demanded for regional and national inventories of greenhouse gas emission, such as nitrous oxide (N2O), to develop appropriate strategies and mitigation options. A potential way to derive large-scale estimates of N2O emission is the use of process-based models, such as PnET-N-DNDC or Wetland-DNDC. While PnET-N-DNDC has been effectively applied for various upland forest ecosystems, the Wetland-DNDC model has not yet been validated with regard to N2O emission. We calibrated and validated the Wetland-DNDC model on the basis of a 4-year field data set of two water-logged soils ( Humic Gleysol and Histic Gleysol) of a spruce forest ecosystem. Model calibration by means of the Levenberg–Marquardt algorithm considerably improved the model performance for the period of calibration (2001–2002). The error variance was reduced by up to a factor of two and the modelling efficiency was increased from −1.24 to −0.15 ( Humic Gleysol) and from −0.42 to 0.1 ( Histic Gleysol). However, the model performance for the period of validation (2003–2004) and particularly for the extreme dry period in summer 2003 was not fully satisfying, notably with regard to the temporal pattern of the N2O emission. [ABSTRACT FROM AUTHOR]

Restored forested wetlands reduce N loads in surface discharge through plant uptake and denitrification. While removal of reactive N reduces impact on receiving waters, it is unclear whether enhanced denitrification also enhances emissions of the greenhouse gas N2O, thus compromising the water-quality benefits of restoration. This study compares denitrification rates and N2O:N2 emission ratios from Sharkey clay soil in a mature bottomland forest to those from an adjacent cultivated site in the Lower Mississippi Alluvial Valley. Potential denitrification of forested soil was 2.4 times of cultivated soil. Using intact soil cores, denitrification rates of forested soil were 5.2, 6.6 and 2.0 times those of cultivated soil at 70, 85 and 100% water-filled pore space (WFPS), respectively. When NO3 was added, N2O emissions from forested soil were 2.2 times those of cultivated soil at 70% WFPS. At 85 and 100% WFPS, N2O emissions were not significantly different despite much greater denitrification rates in the forested soil because N2O:N2 emission ratios declined more rapidly in forested soil as WFPS increased. These findings suggest that restoration of forested wetlands to reduce NO3 in surface discharge will not contribute significantly to the atmospheric burden of N2O. [ABSTRACT FROM AUTHOR]

This study reports the linkage between MIKE SHE and Wetland-DNDC for carbon dynamics and greenhouse gases (GHGs) emissions simulation in forested wetland.Wetland-DNDC was modified by parameterizing management measures, refining anaerobic biogeochemical processes, and was linked to the hydrological model – MIKE SHE. As a preliminary application, we simulated the effect of water table position and forest management practices on GHGs emissions and carbon dynamics to test the capabilities of the models for simulating seasonal and long-term carbon budget. Simulation results show that water table changes had a remarkable effect on GHGs fluxes. Anaerobic conditions in forested wetland soils reduce organic matter decomposition and stimulate CH4 production. Decrease in the water table from the wetland surface decreases methane flux, while CO2 emission was lower with a rise in the water table. When there is a drop in water availability, wetlands can become a net source of atmospheric CO2 as photosynthesis is decreased and respiration loss enhanced. Forest management activities i.e. harvest, fertilization and reforestation practices were parameterized in the model. We predicted carbon fluxes and stores on a pine forest under different forest management scenarios during 160 years. Results show that average long-term carbon storage in ecosystem pools increased with increasing rotation length; Soil carbon showed only minor, long-term responses to harvesting events. In contrast, carbon sequestered in tree biomass and litter fluctuated widely, in concert with the harvest cycle. Application of nitrogen fertilizer increased average carbon storage in all ecosystem pools and wood products. We presented the linkage of MIKE SHE and Wetland-DNDC as a way to use of simulation modeling tools for assessing GHGs mitigation strategies, carbon budgeting and forest management. [ABSTRACT FROM AUTHOR]

We monitored soil emissions of NO, NO2, N2O, and CO2 throughout the summer dry season at a remote North American sagebrush-steppe ecosystem following application of several resources, including water, NH4+, NO3- and sucrose. Despite low levels of soil NH4+ (5.60+/-0.95 mg NH4+-N per kg soil,mean +/- S.E.), and NO3--N (1.34+/-0.20 mg NO3--N per kg soil), NO emissions ranged from about 0.2 to 2.8 ng NO-N m-2 s-1, comparable to rates measured from many agricultural, tropical, and other undisturbed ecosystems. Soil wetting increased NO emissions as much as 400-fold when initial gravimetric soil moisture contents were less than about 50mg kgsoil-1 and soil temperature was greater than or equal to 20 deg. C. Wetting treatments with 20 mg NH4+-N kgsoil-1 raised NO emission rates to a level that was nearly an order of magnitude higher than thatobserved after water addition alone. Wetting treatments with 20 mg NO3--N kgsoil-1, 240 mg sucrose-C kgsoil-1, or NO3-plus sucrose had no statistically significant effect upon NO emissions. Soil denitrifying enzyme activity was low at this site, and N2O emissions in the field were below detection limits. Soil nitrifying enzyme activity was extremely high at this site, indicating that the NH4+ released by ammonification would be consumed at least once every 1.7 days. These observations indicatethat NO emissions from this undisturbed ecosystem were likely a consequence of high nitrification activity, and that sagebrush-steppe ecosystems may be a more important NO source than has been previously assumed. [ABSTRACT FROM AUTHOR]

Potential rates of both methane production and methane consumption vary over three orders of magnitude and their distribution is skew. These rates are weakly correlated with ecosystem type, incubation temperature, in situ aeration, latitude, depth and distance to oxic/anoxicinterface. Anaerobic carbon mineralisation is a major control of methane production. The large range in anaerobic CH4:CO2 production rates indicate that a large part of the anaerobically mineralised carbon is used for reduction of electron acceptors, and, hence, is not available for methanogenesis. Consequently, cycling of electron acceptors needs to be studied to understand methane production. Methane and oxygen half saturation constants for methane oxidation vary about one order of magnitude. Potential methane oxidation seems to be correlated with methanotrophic biomass. Therefore, variation in potential methane oxidation could be related to site characteristics with a model of methanotrophic biomass. [ABSTRACT FROM AUTHOR]

The vertical distribution of dissolved greenhouse gases (CH{sub}4{end}, sigmaCO{sub}2{end} and N{sub}2{end}O), NO{sub}2{end}-, and delta {sup}13{end}C Of sigma CO{sub}2{end} in Lake Biwa during a stagnant period was precisely determined. SigmaCO{sub}2{end} as well as NO{sub}3{end}- was accumulated in the hypolimnion, whereas NO{sub}2{end}- and CH{sub}4{end} concentrations were generally higher in the epilimnion than in the hypolimnion. In August, NO{sub}2{end}- and CH{sub}4{end} were ephemerally accumulated at the thermocline. The concentration of CH{sub}4{end} always exceeded equilibrium with respect to air/water exchange. N{sub}2{end}O was rather uniformly distributed in both time and space, and remained near equilibrium with respect to air/waterexchange. All of these observations are similar to other stratified,oligotrophic lakes, in which the hypolimnia were well oxygenated. The delta {sup}13{end}C of sigmaCO{sub}2{end} became more negative withincreasing depth, and showed a strong negative correlation with apparent oxygen utilization. From the data, the delta {sup}13{end}C value of organic matter decomposed into sigmaCO{sub}2{end} in the hypolimnion was calculated by isotope mass-balance, and found to be in a similar range to delta {sup}13{end}C of phytoplankton and benthic algae and distinctively higher than delta {sup}13{end}C of both terrestrialand sedimentary organic matters. This suggests that autochthonous organic matter was the major source of sigmaCO{sub}2{end}. [ABSTRACT FROM AUTHOR]