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1.
Wetlands are the single largest natural source of atmospheric methane (CH 4), a greenhouse gas, and occur extensively in the northern hemisphere. Large discrepancies remain between “bottom‐up” and “top‐down” estimates of northern CH 4 emissions. To explore whether these discrepancies are due to poor representation of nongrowing season CH 4 emissions, we synthesized nongrowing season and annual CH 4 flux measurements from temperate, boreal, and tundra wetlands and uplands. Median nongrowing season wetland emissions ranged from 0.9 g/m 2 in bogs to 5.2 g/m 2 in marshes and were dependent on moisture, vegetation, and permafrost. Annual wetland emissions ranged from 0.9 g m ?2 year ?1 in tundra bogs to 78 g m ?2 year ?1 in temperate marshes. Uplands varied from CH 4 sinks to CH 4 sources with a median annual flux of 0.0 ± 0.2 g m ?2 year ?1. The measured fraction of annual CH 4 emissions during the nongrowing season (observed: 13% to 47%) was significantly larger than that was predicted by two process‐based model ensembles, especially between 40° and 60°N (modeled: 4% to 17%). Constraining the model ensembles with the measured nongrowing fraction increased total nongrowing season and annual CH 4 emissions. Using this constraint, the modeled nongrowing season wetland CH 4 flux from >40° north was 6.1 ± 1.5 Tg/year, three times greater than the nongrowing season emissions of the unconstrained model ensemble. The annual wetland CH 4 flux was 37 ± 7 Tg/year from the data‐constrained model ensemble, 25% larger than the unconstrained ensemble. Considering nongrowing season processes is critical for accurately estimating CH 4 emissions from high‐latitude ecosystems, and necessary for constraining the role of wetland emissions in a warming climate. 相似文献
2.
Shallow fresh water bodies in peat areas are important contributors to greenhouse gas fluxes to the atmosphere. In this study we determined the magnitude of CH 4 and CO 2 fluxes from 12 water bodies in Dutch wetlands during the summer season and studied the factors that might regulate emissions of CH 4 and CO 2 from these lakes and ditches. The lakes and ditches acted as CO 2 and CH 4 sources of emissions to the atmosphere; the fluxes from the ditches were significantly larger than the fluxes from the lakes. The mean greenhouse gas flux from ditches and lakes amounted to 129.1 ± 8.2 (mean ± SE) and 61.5 ± 7.1 mg m ?2 h ?1 for CO 2 and 33.7 ± 9.3 and 3.9 ± 1.6 mg m ?2 h ?1 for CH 4, respectively. In most water bodies CH 4 was the dominant greenhouse gas in terms of warming potential. Trophic status of the water and the sediment was an important factor regulating emissions. By using multiple linear regression 87% of the variation in CH 4 could be explained by PO 4 3? concentration in the sediment and Fe 2+ concentration in the water, and 89% of the CO 2 flux could be explained by depth, EC and pH of the water. Decreasing the nutrient loads and input of organic substrates to ditches and lakes by for example reducing application of fertilizers and manure within the catchments and decreasing upward seepage of nutrient rich water from the surrounding area will likely reduce summer emissions of CO 2 and CH 4 from these water bodies. 相似文献
3.
Vernal pools are small, seasonal wetlands that are a common landscape feature contributing to biodiversity in northeastern North American forests. Basic information about their biogeochemical functions, such as carbon cycling, is limited. Concentrations of dissolved methane (CH 4) and carbon dioxide (CO 2) and other water chemistry parameters were monitored weekly at the bottom and surface of four vernal pools in central and eastern Maine, USA, from April to August 2016. The vernal pools were supersaturated with respect to CH 4 and CO 2 at all sampling dates and locations. Concentrations of dissolved CH 4 and CO 2 ranged from 0.4 to 210 μmol L ?1 and 72–2300 μmol L ?1, respectively. Diffusive fluxes of CH 4 and CO 2 into the atmosphere ranged from 0.2 to 73 mmol m ?2 d ?1, and 30–590 mmol m ?2 d ?1, respectively. During the study period, the four vernal pools emitted 0.1–5.8 kg C m ?2 and 9.6–120 kg C m ?2 as CH 4 and CO 2, respectively. The production fluxes (production rates normalized to surface area) of CH 4 and CO 2 ranged from ? 0.02 to 0.66 and 0.40–4.6 g C m ?2 d ?1, respectively, and increased significantly over the season. Methane concentrations were best predicted by alkalinity, ortho-phosphate and depth, while CO 2 concentrations were best predicted with only alkalinity. Alkalinity as a predictor variable highlights the importance of anaerobic respiration in production of both gases. Our study pools had large concentrations and effluxes of CH 4 and CO 2 compared to permanently inundated wetlands, indicating vernal pools are metabolically active sites and may be important contributors to the global carbon budget. 相似文献
4.
Methane efflux was studied in stands of three emergent macrophyte species ( Equisetum fluviatile, Schoenoplectus lacustris and Phragmites australis) commonly found in the littoral zone of boreal lakes. In vegetation stands with relatively low methane (CH 4) emissions (<0.3 mol m ?2 (ice‐free period) ?1), the seasonal variation of CH 4 efflux was better correlated with the dynamics of plant growth than variation in sediment temperature. In dense and productive vegetation stands that released high amounts of CH 4 (2.3–7.7 mol m ?2 (ice‐free period) ?1), the seasonal variation in CH 4 efflux was correlated with sediment temperature, indicating that methanogens were more limited by temperature than substrate supply. The bottom type at the growth site of the emergent plants significantly influenced the ratio of CH 4 efflux to aboveground biomass of plants (Eff : B). The lowest Eff : B ratio was found in E. fluviatile stands growing on sand bottom under experimental conditions and the highest in P. australis‐dominated littoral areas accumulating detritus from external sources. The future changes expected in the hydrology of boreal lakes and rivers because of climatic warming may impact the growth conditions of aquatic macrophytes as well as decomposition and accumulation of detritus and, thus, CH 4 effluxes from boreal lakes. 相似文献
5.
We measured CO 2 and CH 4 concentrations throughout the water columns of two boreal lakes with contrasting trophic status and water color during a wet summer. Previous work suggested that rainfall was important for carbon gas evasion. During the stratified period, precipitation generated unexpected variabilities in CO 2, CH 4, and DOC concentrations below the euphotic zone, especially in the metalimnion. The DOC concentrations after the rains rose to 22 and 10 mg L ?1 from the initial 13 and 8 mg L ?1, in the humic and clear-water lakes respectively, simultaneously with an increase in carbon gas concentrations. In both lakes, the water column was stable, suggesting that the high gas concentrations were not due to transport from hypolimnia rich in carbon gases. The high concentrations of CH 4, which can only be produced in anoxic conditions, in the oxic metalimnion and epilimnion in comparison to the hypolimnetic concentrations indicated that a considerable proportion of the pelagic CH 4 originated from the catchment and/or the littoral zone. Thus, as a consequence of high levels of precipitation, carbon gas concentrations during summer stratification can increase, which can have overall importance in annual carbon budgets. 相似文献
6.
The literature concerning methane (CH 4) emissions from temperate and boreal lakes is extensive, but emissions from tropical and subtropical lakes have been less documented. In particular, methane emissions from Mexican lakes, which are often polluted by anthropogenic carbon and nutrient inputs, have not been reported previously. In this work, methane emissions from six Mexican lakes were measured, covering a broad range of organic inputs, trophic states, and climatic conditions. Methane emissions ranged from 5 to 5,000 mg CH 4 m ?2 day ?1. Water samples from several depths in each lake were analyzed for correlation between water quality indicators and methane emissions. Trophic state and water quality indexes were most strongly correlated with methane fluxes. The global methane flux from Mexican freshwater lakes was estimated to be approximately 1.3 Tg CH 4 year ?1, which is about 20% of methane and 4.4% of total national greenhouse gas emissions. Data for untreated wastewater releases to the environment gave an emission factor of 0.19 kg CH 4 kg ?1 of Biochemical Oxygen Demand, which is superior to that previously estimated by the IPCC for lake discharges. Thus, the large volume of untreated wastewater in Mexico implies higher methane emission than previously estimated. 相似文献
7.
Wetlands are important sources of methane (CH 4) and sinks of carbon dioxide (CO 2). However, little is known about CH 4 and CO 2 fluxes and dynamics of seasonally flooded tropical forests of South America in relation to local carbon (C) balances and atmospheric exchange. We measured net ecosystem fluxes of CH 4 and CO 2 in the Pantanal over 2014–2017 using tower‐based eddy covariance along with C measurements in soil, biomass and water. Our data indicate that seasonally flooded tropical forests are potentially large sinks for CO 2 but strong sources of CH 4, particularly during inundation when reducing conditions in soils increase CH 4 production and limit CO 2 release. During inundation when soils were anaerobic, the flooded forest emitted 0.11 ± 0.002 g CH 4‐C m ?2 d ?1 and absorbed 1.6 ± 0.2 g CO 2‐C m ?2 d ?1 (mean ± 95% confidence interval for the entire study period). Following the recession of floodwaters, soils rapidly became aerobic and CH 4 emissions decreased significantly (0.002 ± 0.001 g CH 4‐C m ?2 d ?1) but remained a net source, while the net CO 2 flux flipped from being a net sink during anaerobic periods to acting as a source during aerobic periods. CH 4 fluxes were 50 times higher in the wet season; DOC was a minor component in the net ecosystem carbon balance. Daily fluxes of CO 2 and CH 4 were similar in all years for each season, but annual net fluxes varied primarily in relation to flood duration. While the ecosystem was a net C sink on an annual basis (absorbing 218 g C m ?2 (as CH 4‐C + CO 2‐C) in anaerobic phases and emitting 76 g C m ?2in aerobic phases), high CH 4 effluxes during the anaerobic flooded phase and modest CH 4 effluxes during the aerobic phase indicate that seasonally flooded tropical forests can be a net source of radiative forcings on an annual basis, thus acting as an amplifying feedback on global warming. 相似文献
8.
Sources of methane (CH 4) become highly variable for countries undergoing a heightened period of development due to both human activity and climate change. An urgent need therefore exists to budget key sources of CH 4, such as wetlands (rice paddies and natural wetlands) and lakes (including reservoirs and ponds), which are sensitive to these changes. For this study, references in relation to CH 4 emissions from rice paddies, natural wetlands, and lakes in China were first reviewed and then reestimated based on the review itself. Total emissions from the three CH 4 sources were 11.25 Tg CH 4 yr ?1 (ranging from 7.98 to 15.16 Tg CH 4 yr ?1). Among the emissions, 8.11 Tg CH 4 yr ?1 (ranging from 5.20 to 11.36 Tg CH 4 yr ?1) derived from rice paddies, 2.69 Tg CH 4 yr ?1 (ranging from 2.46 to 3.20 Tg CH 4 yr ?1) from natural wetlands, and 0.46 Tg CH 4 yr ?1 (ranging from 0.33 to 0.59 Tg CH 4 yr ?1) from lakes (including reservoirs and ponds). Plentiful water and warm conditions, as well as its large rice paddy area make rice paddies in southeastern China the greatest overall source of CH 4, accounting for approximately 55% of total paddy emissions. Natural wetland estimates were slightly higher than the other estimates owing to the higher CH 4 emissions recorded within Qinghai‐Tibetan Plateau peatlands. Total CH 4 emissions from lakes were estimated for the first time by this study, with three quarters from the littoral zone and one quarter from lake surfaces. Rice paddies, natural wetlands, and lakes are not constant sources of CH 4, but decreasing ones influenced by anthropogenic activity and climate change. A new progress‐based model used in conjunction with more observations through model‐data fusion approach could help obtain better estimates and insights with regard to CH 4 emissions deriving from wetlands and lakes in China. 相似文献
9.
Microbial oxidation in aerobic soils is the primary biotic sink for atmospheric methane (CH 4), a powerful greenhouse gas. Although tropical forest soils are estimated to globally account for about 28% of annual soil CH 4 consumption (6.2 Tg CH 4 year ?1), limited data are available on CH 4 exchange from tropical montane forests. We present the results of an extensive study on CH 4 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 4 sinks, with decreasing annual uptake rates from 5.9 kg CH 4–C ha ?1 year ?1 at 1,000 m to 0.6 kg CH 4–C ha ?1 year ?1 at 3,000 m. Topography had no effect on soil atmospheric CH 4 uptake. We detected some unexpected factors controlling net methane fluxes: positive correlations between CH 4 uptake rates, mineral nitrogen content of the mineral soil and with CO 2 emissions indicated that the largest CH 4 uptake corresponded with favorable conditions for microbial activity. Furthermore, we found indications that CH 4 uptake was N limited instead of inhibited by NH 4 +. 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 4 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. 相似文献
10.
Aims and methods To evaluate the seasonal and spatial variations of methane (CH 4) emissions and understand the controlling factors, we measured CH 4 fluxes and their environmental variables for the first time by a static chamber technique in high Suaeda salsa marsh (HSM), middle S. salsa marsh (MSM), low S. salsa marsh (LSM) and bare flat (BF) in the northern Yellow River estuary throughout a year. Results CH 4 emissions from coastal marsh varied throughout different times of the day and significant differences were observed in some sampling periods ( p?<?0.05). Over all sampling periods, CH 4 fluxes averaged between ?0.392 mgCH 4 m ?2?h ?1 and 0.495 mgCH 4 m ?2?h ?1, and emissions occurred during spring (0.008 mgCH 4 m ?2?h ?1) and autumn (0.068 mgCH 4 m ?2?h ?1) while sinks were observed during summer (?0.110 mgCH 4 m ?2?h ?1) and winter (?0.009 mgCH 4 m ?2?h ?1). CH 4 fluxes from the four marshes were not significantly different ( p?>?0.05), and emissions occurred in LSM (0.026 mgCH 4 m ?2?h ?1) and BF (0.055 mgCH 4 m ?2?h ?1) while sinks were observed in HSM (?0.035 mgCH 4 m ?2?h ?1) and MSM (?0.022 mgCH 4 m ?2?h ?1). The annual average CH 4 flux from the intertidal zone was 0.002 mgCH 4 m ?2?h ?1, indicating that coastal marsh acted as a weak CH 4 source. Temporal variations of CH 4 emission were related to the interactions of abiotic factors (temperatures, soil moisture and salinity) and the variations of limited C and mineral N in sediments, while spatial variations were mainly affected by the vegetation composition at spatial scale. Conclusions This study observed a large spatial variation of CH 4 fluxes across the coastal marsh of the Yellow River estuary (CV?=?7856.25 %), suggesting that the need to increase the spatial replicates at fine scales before the regional CH 4 budget was evaluated precisely. With increasing exogenous nitrogen loading to the Yellow River estuary, the magnitude of CH 4 emission might be enhanced, which should also be paid more attentions as the annual CH 4 inventory was assessed accurately. 相似文献
11.
Global warming is associated with the continued increase in the atmospheric concentrations of greenhouse gases; carbon dioxide, methane (CH 4) and nitrous oxide. Wetlands constitute the largest single natural source of atmospheric CH 4 in the world contributing between 100 and 231 Tg year ?1 to the total budget of 503–610 Tg year ?1, approximately 60 % of which is emitted from tropical wetlands. We conducted diffusive CH 4 emission measurements using static chambers in river channels, floodplains and lagoons in permanent and seasonal swamps in the Okavango Delta, Botswana. Diffusive CH 4 emission rates varied between 0.24 and 293 mg CH 4 m ?2 h ?1, with a mean (±SE) emission of 23.2 ± 2.2 mg CH 4 m ?2 h ?1 or 558 ± 53 mg CH 4 m ?2 day ?1. These emission rates lie within the range reported for other tropical wetlands. The emission rates were significantly higher ( P < 0.007) in permanent than in seasonal swamps. River channels exhibited the highest average fluxes at 31.3 ± 5.4 mg CH 4 m ?2 h ?1 than in floodplains (20.4 ± 2.5 mg CH 4 m ?2 h ?1) and lagoons (16.9 ± 2.6 mg CH 4 m ?2 h ?1). Diffusive CH 4 emissions in the Delta were probably regulated by temperature since emissions were highest (20–300 mg CH 4 m ?2 h ?1) and lowest (0.2–3.0 mg m ?2 h ?1) during the warmer-rainy and cooler winter seasons, respectively. Surface water temperatures between December 2010 and January 2012 varied from 15.3 °C in winter to 33 °C in summer. Assuming mean inundation of 9,000 km 2, the Delta’s annual diffusive emission was estimated at 1.8 ± 0.2 Tg, accounting for 2.8 ± 0.3 % of the total CH 4 emission from global tropical wetlands. 相似文献
12.
Natural wetlands release about 20% of global emissions of CH 4, an effective greenhouse gas contributing to the total radiative forcing. Thus, changes in the carbon cycle in wetlands could have significant impacts on climate. The effect of raised supply of CO 2 or NH 4NO 3 on the annual CH 4 efflux from the lawn of a boreal oligotrophic mire was investigated over two years. Ten study plots were enclosed with mini‐FACE rings, five vented with CO 2‐enriched air and the other five with ambient air. In addition, five plots were sprayed with NH 4NO 3 so that the cumulative addition of N was 3 g m ?2 y ?1; and five plots were controls. The CO 2 enrichment (target concentration 560 ppm v) increased CH 4 efflux about 30–40%, but half of this increase seemed to be caused by the air‐blowing system. The increasing atmospheric concentration of CO 2 would promote CH 4 release in boreal mires, but the increase in CH 4 efflux would be clearly smaller than that reported in studies made in temperate or subtropical temperature conditions. Addition of N enhanced the annual release of CH 4 only slightly. At least over the short‐term, the increase in N deposition would have little effect on CH 4 effluxes. The increase in CH 4 release would probably increase radiative forcing and thus accelerate climate change. However, CH 4 effluxes are only a small part in the whole matter balance in mires and thus further studies are needed to define the net effects of raised supply of CO 2 or N for carbon accumulation, trace gas fluxes and radiative forcing. 相似文献
13.
Dissolved CH 4 concentrations in the Belgian coastal zone (North Sea) ranged between 670 nmol l ?1 nearshore and 4 nmol l ?1 offshore. Spatial variations of CH 4 were related to sediment organic matter (OM) content and gassy sediments. In nearshore stations with fine sand or muddy sediments, the CH 4 seasonal cycle followed water temperature, suggesting methanogenesis control by temperature in these OM-rich sediments. In offshore stations with permeable sediments, the CH 4 seasonal cycle showed a yearly peak following the chlorophyll- a spring peak, suggesting that in these OM-poor sediments, methanogenesis depended on freshly produced OM delivery. This does not exclude the possibility that some CH 4 might originate from dimethylsulfide (DMS) or dimethylsulfoniopropionate (DMSP) or methylphosphonate transformations in the most offshore stations. Yet, the average seasonal CH 4 cycle was unrelated to those of DMS(P), very abundant during the Phaeocystis bloom. The annual average CH 4 emission was 126 mmol m ?2 y ?1 in the most nearshore stations (~4 km from the coast) and 28 mmol m ?2 y ?1 in the most offshore stations (~23 km from the coast), 1260–280 times higher than the open ocean average value (0.1 mmol m ?2 y ?1). The strong control of CH 4 by sediment OM content and by temperature suggests that marine coastal CH 4 emissions, in particular in shallow areas, should respond to future eutrophication and warming of climate. This is supported by the comparison of CH 4 concentrations at five stations obtained in March 1990 and 2016, showing a decreasing trend consistent with alleviation of eutrophication in the area. 相似文献
14.
Climate changes are likely to be significantly affected by reservoirs/lakes due to emission of greenhouse gas (GHG), change in the magnitude and seasonality of river runoffs and severe extreme events. In this study, a coupled GHG Risk Assessment Tool (GRAT) and Soil Water Assessment Tool (SWAT) are used to predict the GHG risk of Koteshwar reservoirs located in Uttarakhand, India. Before running the GRAT model, SWAT model was used to simulate the runoffs (one of the major input of GRAT). The model was calibrated (2004–09) and validated (2010–13) at Uttarkashi station using monthly discharge data. The model performance was checked by R 2, NSE, RSR, and p-value as 0.785, 0.60, 0.63, and 0.04, respectively, during calibration and 0.790, 0.66, 0.57, and 0.06 during validation and shows satisfactory model performance on monthly time step. Further, GRAT model is also applied and the results show that Koteshwar reservoir is found to be under high risk of CO 2 (CO 2 > 645 mg m ?2 d ?1) and medium risk of CH 4 (CH 4 < 45 mg m ?2 d ?1) till 2023. Subsequently, the GHG risk is minimized after passage of time over 100 years. These models may be used by the policy-makers to know the potential of GHG and its vulnerability to the reservoirs after the impoundment. 相似文献
15.
Peatlands are large terrestrial stores of carbon, and sustained CO 2 sinks, but over the last century large areas have been drained for agriculture and forestry, potentially converting them into net carbon sources. More recently, some peatlands have been re-wetted by blocking drainage ditches, with the aims of enhancing biodiversity, mitigating flooding, and promoting carbon storage. One potential detrimental consequence of peatland re-wetting is an increase in methane (CH 4) emissions, offsetting the benefits of increased CO 2 sequestration. We examined differences in CH 4 emissions between an area of ditch-drained blanket bog, and an adjacent area where drainage ditches were recently infilled. Results showed that Eriophorum vaginatum colonization led to a “hotspot” of CH 4 emissions from the infilled ditches themselves, with smaller increases in CH 4 from other re-wetted areas. Extrapolated to the area of blanket bog surrounding the study site, we estimated that CH 4 emissions were around 60 kg CH 4 ha ?1 y ?1 prior to drainage, reducing to 44 kg CH 4 ha ?1 y ?1 after drainage. We calculated that fully re-wetting this area would initially increase emissions to a peak of around 120 kg CH 4 ha ?1 y ?1, with around two-thirds of the increase (and 90% of the increase over pre-drainage conditions) attributable to CH 4 emissions from E. vaginatum-colonized infilled ditches, despite these areas only occupying 7% of the landscape. We predicted that emissions should eventually decline toward pre-drainage values as the ecosystem recovers, but only if Sphagnum mosses displace E. vaginatum from the infilled ditches. These results have implications for peatland management for climate change mitigation, suggesting that restoration methods should aim, if possible, to avoid the colonization of infilled ditches by aerenchymatous species such as E. vaginatum, and to encourage Sphagnum establishment. 相似文献
16.
We report a data-set of dissolved methane (CH 4) in three rivers (Comoé, Bia and Tanoé) and five lagoons (Grand-Lahou, Ebrié, Potou, Aby and Tendo) of Ivory Coast (West Africa), during the four main climatic seasons (high dry season, high rainy season, low dry season and low rainy season). The surface waters of the three rivers were over-saturated in CH 4 with respect to atmospheric equilibrium (2221–38719%), and the seasonal variability of CH 4 seemed to be largely controlled by dilution during the flooding period. The strong correlation of CH 4 concentrations with the partial pressure of CO 2 (pCO 2) and dissolved silicate (DSi) confirm the dominance of a continental sources (from soils) for both CO 2 and CH 4 in these rivers. Diffusive air–water CH 4 fluxes ranged between 25 and 1187 μmol m ?2 day ?1, and annual integrated values were 288 ± 107, 155 ± 38, and 241 ± 91 μmol m ?2 day ?1 in the Comoé, Bia and Tanoé rivers, respectively. In the five lagoons, surface waters were also over-saturated in CH 4 (ranging from 1496 to 51843%). Diffusive air–water CH 4 fluxes ranged between 20 and 2403 μmol m ?2 day ?1, and annual integrated values were 78 ± 34, 338 ± 217, 227 ± 79, 330 ± 153 and 326 ± 181 μmol m ?2 day ?1 in the Grand-Lahou, Ebrié, Potou, Aby and Tendo lagoons, respectively. The largest CH 4 over-saturations were observed in the Tendo and Aby lagoons that are permanently stratified systems (unlike the other three lagoons), leading to anoxic bottom waters favorable for a large CH 4 production. In addition, these two stratified lagoons showed low pCO 2 values due to high primary production, which suggests an efficient transfer of organic matter across the pycnocline. As a result, the stratified Tendo and Aby lagoons were respectively, a low source of CO 2 to the atmosphere and a sink of atmospheric CO 2 while the other three well-mixed lagoons were strong sources of CO 2 to the atmosphere but less over-saturated in CH 4. 相似文献
17.
Northern lakes are a source of greenhouse gases to the atmosphere and contribute substantially to the global carbon budget. However, the sources of methane (CH4) to northern lakes are poorly constrained limiting our ability to the assess impacts of future Arctic change. Here we present measurements of the natural groundwater tracer, radon, and CH4 in a shallow lake on the Yukon-Kuskokwim Delta, AK and quantify groundwater discharge rates and fluxes of groundwater-derived CH4. We found that groundwater was significantly enriched (2000%) in radon and CH4 relative to lake water. Using a mass balance approach, we calculated average groundwater fluxes of 1.2 ± 0.6 and 4.3 ± 2.0 cm day−1, respectively as conservative and upper limit estimates. Groundwater CH4 fluxes were 7—24 mmol m−2 day−1 and significantly exceeded diffusive air–water CH4 fluxes (1.3–2.3 mmol m−2 day−1) from the lake to the atmosphere, suggesting that groundwater is an important source of CH4 to Arctic lakes and may drive observed CH4 emissions. Isotopic signatures of CH4 were depleted in groundwaters, consistent with microbial production. Higher methane concentrations in groundwater compared to other high latitude lakes were likely the source of the comparatively higher CH4 diffusive fluxes, as compared to those reported previously in high latitude lakes. These findings indicate that deltaic lakes across warmer permafrost regions may act as important hotspots for CH4 release across Arctic landscapes. 相似文献
18.
Small lakes in northern latitudes represent a significant source of CH 4 to the atmosphere that is predicted to increase with warming in the Arctic. Yet, whole-lake CH 4 budgets are lacking as are measurements of δ 13C-CH 4 and δ 2H-CH 4. In this study, we quantify spatial variability of diffusive and ebullitive fluxes of CH 4 and corresponding δ 13C-CH 4 and δ 2H-CH 4 in a small, Arctic lake system with fringing wetland in southwestern Greenland during summer. Net CH 4 flux was highly variable, ranging from an average flux of 7 mg CH 4 m ?2 d ?1 in the deep-water zone to 154 mg CH 4 m ?2 d ?1 along the lake margin. Diffusive flux accounted for ~8.5 % of mean net CH 4 flux, with plant-mediated and ebullitive flux accounting for the balance of the total net flux. Methane content of emitted ebullition was low (mean ± SD 10 ± 17 %) compared to previous studies from boreal lakes and wetlands. Isotopic composition of net CH 4 emissions varied widely throughout the system, with δ 13C-CH 4 ranging from ?66.2 to ?55.5 ‰, and δ 2H-CH 4 ranging from ?345 to ?258 ‰. Carbon isotope composition of CH 4 in ebullitive flux showed wider variation compared to net flux, ranging from ?69.2 to ?49.2 ‰. Dissolved CH 4 concentrations were highest in the sediment and decreased up the water column. Higher concentrations of CH 4 in the hypoxic deep water coincided with decreasing dissolved O 2 concentrations, while methanotrophic oxidation dominated in the epilimnion based upon decreasing concentrations and increasing values of δ 13C-CH 4 and δ 2H-CH 4. The most depleted 13C- and 2H-isotopic values were observed in profundal bottom waters and in subsurface profundal sediments. Based upon paired δ 13C and δ 2H observations of CH 4, acetate fermentation was likely the dominant production pathway throughout the system. However, isotopic ratios of CH 4 in deeper sediments were consistent with mixing/transition between CH 4 production pathways, indicating a higher contribution of the CO 2 reduction pathway. The large spatial variability in fluxes of CH 4 and in isotopic composition of CH 4 throughout a single lake system indicates that the underlying mechanisms controlling CH 4 cycling (production, consumption and transport) are spatially heterogeneous. Net flux along the lake margin dominated whole-lake flux, suggesting the nearshore littoral area dominates CH 4 emissions in these systems. Future studies of whole-lake CH 4 budgets should consider this significant spatial heterogeneity. 相似文献
19.
Sheepfolds represent significant hot spot sources of greenhouse gases (GHG) in semi-arid grassland regions, such as Inner Mongolia in China. However, the annual contribution of sheepfolds to regional GHG emissions is still unknown. In order to quantify its annual contribution, we conducted measurements of carbon dioxide (CO 2), methane (CH 4) and nitrous oxide (N 2O) fluxes at two sheepfold sites in the Baiyinxile administrative region of Inner Mongolia for 1 year, using static opaque chamber and gas chromatography methods. Our data show that, at an annual scale, both sheepfolds functioned as net sources of CO 2, CH 4 and N 2O. Temperatures primarily determined the seasonal pattern of CO 2 emission; 60–84% of the CO 2 flux variation could be explained by temperature changes. High rates of net CH 4 emissions from sheepfold soils were only observed when animals (sheep and goats) were present. While nitrous oxide emissions were also stimulated by the presence of animals, pulses of N 2O emissions were also be related to rainfall and spring-thaw events. The total annual cumulative GHG emissions in CO 2 equivalents (CO 2: 1; CH 4: 25; and N 2O: 298) were quantified as 87.4?±?18.4 t ha ?1 for the sheepfold that was used during the non-grazing period (i.e., winter sheepfold) and 136.7?±?15.9 t ha ?1 used during the grazing period (i.e., summer sheepfold). Of the annual total GHG emissions, CH 4 release accounted for approximately 1% of emissions, while CO 2 and N 2O emissions contributed to approximately 59% and 40%, respectively. The total GHG emission factor (CO 2?+?CH 4?+?N 2O) per animal for the sheepfolds investigated in this study was 30.3 kg CO 2 eq yr ?1 head ?1, which translates to 0.3, 18.8 and 11.2 kg CO 2 eq yr ?1 head ?1 for CH 4, CO 2 and N 2O, respectively. Sheepfolds accounted for approximately 34% of overall N 2O emissions in the Baiyinxile administrative region, a typical steppe region within Inner Mongolia. The contribution of sheepfolds to the regional CO 2 or CH 4 exchange is marginal. 相似文献
20.
In northeastern Canada, at the ecotonal limit of the forest tundra and lichen woodland, a rise of the regional water table in the peatland systems was registered since Little Ice Age resulting in increasing pool compartment at the expense of terrestrial surfaces. We hypothesized that, with a mean water table closer to peat surface and higher pool density, these ecosystems would be great CH 4 emitters. In summers 2009 and 2010, methane fluxes were measured in a patterned fen located in the northeastern portion of the La Grande river watershed to determine the contribution of the different microforms (lawns, hollows, hummocks, string, pools) to the annual CH 4 budget. Mean seasonal CH 4 fluxes from terrestrial microforms ranged between 12.9 and 49.4 mg m ?2 day ?1 in 2009 and 15.4 and 47.3 mg m ?2 day ?1 in 2010. Pool fluxes (which do not include ebullition fluxes) ranged between 102.6 and 197.6 mg CH 4 m ?2 day ?1 in 2009 and 76.5 and 188.1 mg CH 4 m ?2 day ?1 in 2010. Highest fluxes were measured in microforms with water table closer to peat surface but no significant relationship was observed between water table depth and CH 4 fluxes. Spatially weighted CH 4 budget demonstrates that, during the growing season, the studied peatland emitted 66 ± 31 in 2009 and 55 ± 26 mg CH 4 m ?2 day ?1 in 2010, 79 % of which is accounted by pool fluxes. In a context where climate projections predict greater precipitations in northeastern Canada, these results indicate that this type of peatlands could contribute to modify the methane balance in the atmosphere. 相似文献
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