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1.
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
2.
Northern peatlands constitute a significant source of atmospheric methane (CH 4). However, management of undisturbed peatlands, as well as the restoration of disturbed peatlands, will alter the exchange of CH 4 with the atmosphere. The aim of this systematic review and meta‐analysis was to collate and analyze published studies to improve our understanding of the factors that control CH 4 emissions and the impacts of management on the gas flux from northern (latitude 40° to 70°N) peatlands. The analysis includes a total of 87 studies reporting measurements of CH 4 emissions taken at 186 sites covering different countries, peatland types, and management systems. Results show that CH 4 emissions from natural northern peatlands are highly variable with a 95% CI of 7.6–15.7 g C m ?2 year ?1 for the mean and 3.3–6.3 g C m ?2 year ?1 for the median. The overall annual average (mean ± SD) is 12 ± 21 g C m ?2 year ?1 with the highest emissions from fen ecosystems. Methane emissions from natural peatlands are mainly controlled by water table (WT) depth, plant community composition, and soil pH. Although mean annual air temperature is not a good predictor of CH 4 emissions by itself, the interaction between temperature, plant community cover, WT depth, and soil pH is important. According to short‐term forecasts of climate change, these complex interactions will be the main determinant of CH 4 emissions from northern peatlands. Drainage significantly ( p < .05) reduces CH 4 emissions to the atmosphere, on average by 84%. Restoration of drained peatlands by rewetting or vegetation/rewetting increases CH 4 emissions on average by 46% compared to the original premanagement CH 4 fluxes. However, to fully evaluate the net effect of management practice on the greenhouse gas balance from high latitude peatlands, both net ecosystem exchange (NEE) and carbon exports need to be considered. 相似文献
3.
Inland waters were recently recognized to be important sources of methane (CH 4) and carbon dioxide (CO 2) to the atmosphere, and including inland water emissions in large scale greenhouse gas (GHG) budgets may potentially offset the estimated carbon sink in many areas. However, the lack of GHG flux measurements and well‐defined inland water areas for extrapolation, make the magnitude of the potential offset unclear. This study presents coordinated flux measurements of CH 4 and CO 2 in multiple lakes, ponds, rivers, open wells, reservoirs, springs, and canals in India. All these inland water types, representative of common aquatic ecosystems in India, emitted substantial amounts of CH 4 and a major fraction also emitted CO 2. The total CH 4 flux (including ebullition and diffusion) from all the 45 systems ranged from 0.01 to 52.1 mmol m ?2 d ?1, with a mean of 7.8 ± 12.7 (mean ± 1 SD) mmol m ?2 d ?1. The mean surface water CH 4 concentration was 3.8 ± 14.5 μm (range 0.03–92.1 μm ). The CO 2 fluxes ranged from ?28.2 to 262.4 mmol m ?2 d ?1 and the mean flux was 51.9 ± 71.1 mmol m ?2 d ?1. The mean partial pressure of CO 2 was 2927 ± 3269 μatm (range: 400–11 467 μatm). Conservative extrapolation to whole India, considering the specific area of the different water types studied, yielded average emissions of 2.1 Tg CH 4 yr ?1 and 22.0 Tg CO 2 yr ?1 from India's inland waters. When expressed as CO 2 equivalents, this amounts to 75 Tg CO 2 equivalents yr ?1 (53–98 Tg CO 2 equivalents yr ?1; ± 1 SD) , with CH 4 contributing 71%. Hence, average inland water GHG emissions, which were not previously considered, correspond to 42% (30–55%) of the estimated land carbon sink of India. Thereby this study illustrates the importance of considering inland water GHG exchange in large scale assessments. 相似文献
4.
Nearly 5000 chamber measurements of CH 4 flux were collated from 21 sites across the United Kingdom, covering a range of soil and vegetation types, to derive a parsimonious model that explains as much of the variability as possible, with the least input requirements. Mean fluxes ranged from ?0.3 to 27.4 nmol CH 4 m ?2 s ?1, with small emissions or low rates of net uptake in mineral soils (site means of ?0.3 to 0.7 nmol m ?2 s ?1) and much larger emissions from organic soils (site means of ?0.3 to 27.4 nmol m ?2 s ?1). Less than half of the observed variability in instantaneous fluxes could be explained by independent variables measured. The reasons for this include measurement error, stochastic processes and, probably most importantly, poor correspondence between the independent variables measured and the actual variables influencing the processes underlying methane production, transport and oxidation. When temporal variation was accounted for, and the fluxes averaged at larger spatial scales, simple models explained up to ca. 75% of the variance in CH 4 fluxes. Soil carbon, peat depth, soil moisture and pH together provided the best sub‐set of explanatory variables. However, where plant species composition data were available, this provided the highest explanatory power. Linear and nonlinear models generally fitted the data equally well, with the exception that soil moisture required a power transformation. To estimate the impact of changes in peatland water table on CH 4 emissions in the United Kingdom, an emission factor of +0.4 g CH 4 m ?2 yr ?1 per cm increase in water table height was derived from the data. 相似文献
5.
Sources and sinks of methane (CH 4) are critical for understanding global biogeochemical cycles and their role in climate change. A growing number of studies have reported that CH 4 concentrations in cave ecosystems are depleted, leading to the notion that these subterranean environments may act as sinks for atmospheric CH 4. Recently, it was hypothesized that this CH 4 depletion may be caused by radiolysis, an abiotic process whereby CH 4 is oxidized via interactions with ionizing radiation derived from radioactive decay. An alternate explanation is that the depletion of CH 4 concentrations in caves could be due to biological processes, specifically oxidation by methanotrophic bacteria. We theoretically explored the radiolysis hypothesis and conclude that it is a kinetically constrained process that is unlikely to lead to the rapid loss of CH 4 in subterranean environments. We present results from a controlled laboratory experiment to support this claim. We then tested the microbial oxidation hypothesis with a set of mesocosm experiments that were conducted in two Vietnamese caves. Our results reveal that methanotrophic bacteria associated with cave rocks consume CH 4 at a rate of 1.3–2.7 mg CH 4 · m ?2 · d ?1. These CH 4 oxidation rates equal or exceed what has been reported in other habitats, including agricultural systems, grasslands, deciduous forests, and Arctic tundra. Together, our results suggest that depleted concentrations of CH 4 in caves are most likely due to microbial activity, not radiolysis as has been recently claimed. Microbial methanotrophy has the potential to oxidize CH 4 not only in caves, but also in smaller‐size open subterranean spaces, such as cracks, fissures, and other pores that are connected to and rapidly exchange with the atmosphere. Future studies are needed to understand how subterranean CH 4 oxidation scales up to affect local, regional, and global CH 4 cycling. 相似文献
6.
At the southern margin of permafrost in North America, climate change causes widespread permafrost thaw. In boreal lowlands, thawing forested permafrost peat plateaus (‘forest’) lead to expansion of permafrost‐free wetlands (‘wetland’). Expanding wetland area with saturated and warmer organic soils is expected to increase landscape methane (CH 4) emissions. Here, we quantify the thaw‐induced increase in CH 4 emissions for a boreal forest‐wetland landscape in the southern Taiga Plains, Canada, and evaluate its impact on net radiative forcing relative to potential long‐term net carbon dioxide (CO 2) exchange. Using nested wetland and landscape eddy covariance net CH 4 flux measurements in combination with flux footprint modeling, we find that landscape CH 4 emissions increase with increasing wetland‐to‐forest ratio. Landscape CH 4 emissions are most sensitive to this ratio during peak emission periods, when wetland soils are up to 10 °C warmer than forest soils. The cumulative growing season (May–October) wetland CH 4 emission of ~13 g CH 4 m ?2 is the dominating contribution to the landscape CH 4 emission of ~7 g CH 4 m ?2. In contrast, forest contributions to landscape CH 4 emissions appear to be negligible. The rapid wetland expansion of 0.26 ± 0.05% yr ?1 in this region causes an estimated growing season increase of 0.034 ± 0.007 g CH 4 m ?2 yr ?1 in landscape CH 4 emissions. A long‐term net CO 2 uptake of >200 g CO 2 m ?2 yr ?1 is required to offset the positive radiative forcing of increasing CH 4 emissions until the end of the 21st century as indicated by an atmospheric CH 4 and CO 2 concentration model. However, long‐term apparent carbon accumulation rates in similar boreal forest‐wetland landscapes and eddy covariance landscape net CO 2 flux measurements suggest a long‐term net CO 2 uptake between 49 and 157 g CO 2 m ?2 yr ?1. Thus, thaw‐induced CH 4 emission increases likely exert a positive net radiative greenhouse gas forcing through the 21st century. 相似文献
7.
Arctic winter precipitation is projected to increase with global warming, but some areas will experience decreases in snow accumulation. Although Arctic CH 4 emissions may represent a significant climate forcing feedback, long‐term impacts of changes in snow accumulation on CH 4 fluxes remain uncertain. We measured ecosystem CH 4 fluxes and soil CH 4 and CO 2 concentrations and 13C composition to investigate the metabolic pathways and transport mechanisms driving moist acidic tundra CH 4 flux over the growing season (Jun–Aug) after 18 years of experimental snow depth increases and decreases. Deeper snow increased soil wetness and warming, reducing soil %O 2 levels and increasing thaw depth. Soil moisture, through changes in soil %O 2 saturation, determined predominance of methanotrophy or methanogenesis, with soil temperature regulating the ecosystem CH 4 sink or source strength. Reduced snow (RS) increased the fraction of oxidized CH 4 (Fox) by 75–120% compared to Ambient, switching the system from a small source to a net CH 4 sink (21 ± 2 and ?31 ± 1 mg CH 4 m ?2 season ?1 at Ambient and RS). Deeper snow reduced Fox by 35–40% and 90–100% in medium‐ (MS) and high‐ (HS) snow additions relative to Ambient, contributing to increasing the CH 4 source strength of moist acidic tundra (464 ± 15 and 3561 ± 97 mg CH 4 m ?2 season ?1 at MS and HS). Decreases in Fox with deeper snow were partly due to increases in plant‐mediated CH 4 transport associated with the expansion of tall graminoids. Deeper snow enhanced CH 4 production within newly thawed soils, responding mainly to soil warming rather than to increases in acetate fermentation expected from thaw‐induced increases in SOC availability. Our results suggest that increased winter precipitation will increase the CH 4 source strength of Arctic tundra, but the resulting positive feedback on climate change will depend on the balance between areas with more or less snow accumulation than they are currently facing. 相似文献
8.
This paper reports the range and statistical distribution of oxidation rates of atmospheric CH 4 in soils found in Northern Europe in an international study, and compares them with published data for various other ecosystems. It reassesses the size, and the uncertainty in, the global terrestrial CH 4 sink, and examines the effect of land‐use change and other factors on the oxidation rate. Only soils with a very high water table were sources of CH 4; all others were sinks. Oxidation rates varied from 1 to nearly 200 μg CH 4 m ?2 h ?1; annual rates for sites measured for ≥1 y were 0.1–9.1 kg CH 4 ha ?1 y ?1, with a log‐normal distribution (log‐mean ≈ 1.6 kg CH 4 ha ?1 y ?1). Conversion of natural soils to agriculture reduced oxidation rates by two‐thirds –‐ closely similar to results reported for other regions. N inputs also decreased oxidation rates. Full recovery of rates after these disturbances takes > 100 y. Soil bulk density, water content and gas diffusivity had major impacts on oxidation rates. Trends were similar to those derived from other published work. Increasing acidity reduced oxidation, partially but not wholly explained by poor diffusion through litter layers which did not themselves contribute to the oxidation. The effect of temperature was small, attributed to substrate limitation and low atmospheric concentration. Analysis of all available data for CH 4 oxidation rates in situ showed similar log‐normal distributions to those obtained for our results, with generally little difference between different natural ecosystems, or between short‐and longer‐term studies. The overall global terrestrial sink was estimated at 29 Tg CH 4 y ?1, close to the current IPCC assessment, but with a much wider uncertainty range (7 to > 100 Tg CH 4 y ?1). Little or no information is available for many major ecosystems; these should receive high priority in future research. 相似文献
9.
In the root zone of rice plants aerobic methanotrophic bacteria catalyze the oxidation of CH 4 to CO 2, thereby reducing CH 4 emissions from paddy soils to the atmosphere. However, methods for in situ quantification of microbial processes in paddy soils are scarce. Here we adapted the push–pull tracer-test (PPT) method to quantify CH 4 oxidation in the root zone of potted rice plants. During a PPT, a test solution containing CH 4 ± O 2 as reactant(s), Cl ? and Ar as nonreactive tracers, and BES as an inhibitor of CH 4 production was injected into the root zone at different times throughout the circadian cycle (daytime, early nighttime, late nighttime). After a 2-h incubation phase, the test solution/pore-water mixture was extracted from the same location and rates of CH 4 oxidation were calculated from the ratio of measured reactant and nonreactive tracer concentrations. In separate rice pots, O 2 concentrations in the vicinity of rice roots were measured throughout the circadian cycle using a fiber-optic sensor. Results indicated highly variable CH 4 oxidation rates following a circadian pattern. Mean rates at daytime and early nighttime varied from 62 up to 451 μmol l ?1 h ?1, whereas at late nighttime CH 4 oxidation rates were low, ranging from 13 to 37 μmol l ?1 h ?1. Similarly, daytime O 2 concentration in the vicinity of rice roots increased to up to 250% air saturation, while nighttime O 2 concentration dropped to below detection (<0.15% air saturation). Our results suggest a functional link between root-zone CH 4 oxidation and photosynthetic O 2 supply. 相似文献
10.
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
11.
During two intensive field campaigns in summer and autumn 2004 nitrogen (N 2O, NO/NO 2) and carbon (CO 2, CH 4) trace gas exchange between soil and the atmosphere was measured in a sessile oak ( Quercus petraea (Matt.) Liebl.) forest in Hungary. The climate can be described as continental temperate. Fluxes were measured with a fully automatic measuring system allowing for high temporal resolution. Mean N 2O emission rates were 1.5 μg N m −2 h −1 in summer and 3.4 μg N m −2 h −1 in autumn, respectively. Also mean NO emission rates were higher in autumn (8.4 μg N m −2 h −1) as compared to summer (6.0 μg N m −2 h −1). However, as NO 2 deposition rates continuously exceeded NO emission rates (−9.7 μg N m −2 h −1 in summer and −18.3 μg N m −2 h −1 in autumn), the forest soil always acted as a net NO
x
sink. The mean value of CO 2 fluxes showed only little seasonal differences between summer (81.1 mg C m −2 h −1) and autumn (74.2 mg C m −2 h −1) measurements, likewise CH 4uptake (summer: −52.6 μg C m −2 h −1; autumn: −56.5 μg C m −2 h −1). In addition, the microbial soil processes net/gross N mineralization, net/gross nitrification and heterotrophic soil respiration as well as inorganic soil nitrogen concentrations and N 2O/CH 4 soil air concentrations in different soil depths were determined. The respiratory quotient (ΔCO 2 resp ΔO 2 resp−1) for the uppermost mineral soil, which is needed for the calculation of gross nitrification via the Barometric Process Separation (BaPS) technique, was 0.8978 ± 0.008. The mean value of gross nitrification rates showed only little seasonal differences between summer (0.99 μg N kg −1 SDW d −1) and autumn measurements (0.89 μg N kg −1 SDW d −1). Gross rates of N mineralization were highest in the organic layer (20.1–137.9 μg N kg −1 SDW d −1) and significantly lower in the uppermost mineral layer (1.3–2.9 μg N kg −1 SDW d −1). Only for the organic layer seasonality in gross N mineralization rates could be demonstrated, with highest mean values in autumn, most likely caused by fresh litter decomposition. Gross mineralization rates of the organic layer were positively correlated with N 2O emissions and negatively correlated with CH 4 uptake, whereas soil CO 2 emissions were positively correlated with heterotrophic respiration in the uppermost mineral soil layer. The most important abiotic factor influencing C and N trace gas fluxes was soil moisture, while the influence of soil temperature on trace gas exchange rates was high only in autumn. 相似文献
12.
Methane oxidation rates were measured in boreal forest soils using seven techniques that provide a range of information on soil CH 4 oxidation. These include: (a) short-term static chamber experiments with a free-air (1.7 ppm CH 4) headspace, (b) estimating CH 4 oxidation rates from soil CH 4 distributions and (c) 222Rn-calibrated flux measurements, (d) day-long static chamber experiments with free-air and amended (+20 to 2000 PPM CH 4) headspaces, (e) jar experiments on soil core sections using free-air and (f) amended (+500 ppm CH 4) headspaces, and (g) jar experiments on core sections involving tracer additions of 14CH 4. Short-term unamended chamber measurements, 222Rn-calibrated flux measurements, and soil CH 4 distributions show independently that the soils are capable of oxidizing atmospheric CH 4 at rates ranging to < 2 mg m –2 d –1. Jar experiments with free-air headspaces and soil CH 4 profiles show that CH 4 oxidation occurs to a soil depth of 60 cm and is maximum in the 10 to 20 cm zone. Jar experiments and chamber measurements with free-air headspaces show that CH 4 oxidation occurs at low (< 0.9 ppm) thresholds. The 14CH 4-amended jar experiments show the distribution of end products of CH 4 oxidation; 60% is transformed to CO 2 and the remainder is incorporated in biomass. Chamber and jar experiments under amended atmospheres show that these soils have a high capacity for CH 4 oxidation and indicate potential CH 4 oxidation rates as high as 867 mg m –2 d –1. Methane oxidation in moist soils modulates CH 4 emission and can serve as a negative feedback on atmospheric CH 4 increases. 相似文献
13.
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
14.
Arctic permafrost ecosystems store ~50% of global belowground carbon (C) that is vulnerable to increased microbial degradation with warmer active layer temperatures and thawing of the near surface permafrost. We used anoxic laboratory incubations to estimate anaerobic CO 2 production and methanogenesis in active layer (organic and mineral soil horizons) and permafrost samples from center, ridge and trough positions of water‐saturated low‐centered polygon in Barrow Environmental Observatory, Barrow AK, USA. Methane (CH 4) and CO 2 production rates and concentrations were determined at ?2, +4, or +8 °C for 60 day incubation period. Temporal dynamics of CO 2 production and methanogenesis at ?2 °C showed evidence of fundamentally different mechanisms of substrate limitation and inhibited microbial growth at soil water freezing points compared to warmer temperatures. Nonlinear regression better modeled the initial rates and estimates of Q10 values for CO 2 that showed higher sensitivity in the organic‐rich soils of polygon center and trough than the relatively drier ridge soils. Methanogenesis generally exhibited a lag phase in the mineral soils that was significantly longer at ?2 °C in all horizons. Such discontinuity in CH 4 production between ?2 °C and the elevated temperatures (+4 and +8 °C) indicated the insufficient representation of methanogenesis on the basis of Q10 values estimated from both linear and nonlinear models. Production rates for both CH 4 and CO 2 were substantially higher in organic horizons (20% to 40% wt. C) at all temperatures relative to mineral horizons (<20% wt. C). Permafrost horizon (~12% wt. C) produced ~5‐fold less CO 2 than the active layer and negligible CH 4. High concentrations of initial exchangeable Fe(II) and increasing accumulation rates signified the role of iron as terminal electron acceptors for anaerobic C degradation in the mineral horizons. 相似文献
15.
Temperate grasslands are considered to be a significant sink for CH 3Br, although large uncertainties exist about the magnitude of this sink because of a paucity of field measurements. Here, we report the results of a combined field and laboratory study that investigated the effects of water, temperature, and plant community composition on CH 3Cl and CH 3Br fluxes in a semiarid temperate grassland. A novel stable isotope tracer technique was also employed to deconvolute simultaneous production and oxidation of CH 3Cl and CH 3Br. Net and gross fluxes were measured from different landforms (ridges, floodplains) and cover types (grass‐dominated, shrub‐dominated) to capture a representative range of hydrologic regimes, temperatures, and plant communities. In field experiments, net CH 3Cl and CH 3Br uptake was observed at all grass‐dominated sites (?400±77 nmol CH 3Cl m ?2 day ?1 and ?3.4±0.9 nmol CH 3Br m ?2 day ?1), while net CH 3Cl emission (439±58 nmol CH 3Cl m ?2 day ?1) was observed at sites dominated by the shrub Atriplex canescens, indicating that this plant is a strong CH 3Cl producer. Gross CH 3Cl and CH 3Br oxidation were comparable with estimates from other dryland ecosystems (507±115 nmol CH 3Cl m ?2 day ?1 and 9.1±2.2 nmol CH 3Br m ?2 day ?1), although CH 3Br oxidation rates were at least five times lower than those observed in more mesic temperate grasslands. We suggest that estimates of the temperate grassland CH 3Br sink should be reduced by ≥19% (≥1.8 Gg yr ?1) to account for the weaker sink strength of semiarid environments. Identification of A. canescens as a ‘new’ CH 3Cl source may have important ramifications for the global atmospheric budget of CH 3Cl, given the global distribution of this plant and its congeners and their widespread presence in many dryland ecosystems. Laboratory experiments revealed that soil water was the chief regulator of CH 3Cl and CH 3Br oxidation, while temperature had no observed effect between 14 and 26 °C. Oxidation rates rose most rapidly between 0.4% and 5% volumetric water content, suggesting that methyl halide‐oxidizing bacteria respond strongly to small inputs of water under the very driest conditions. Soil drying and rewetting experiments did not appear to affect the oxidation of CH 3Cl and CH 3Br by soil microorganisms, which are presumably adapted to frequent wet/dry cycles. 相似文献
16.
Lakes are a major component of boreal landscapes, and whereas lake CO 2 emissions are recognized as a major component of regional C budgets, there is still much uncertainty associated to lake CH 4 fluxes. Here, we present a large‐scale study of the magnitude and regulation of boreal lake summer diffusive CH 4 fluxes, and their contribution to total lake carbon (C) emissions, based on in situ measurements of concentration and fluxes of CH 4 and CO 2 in 224 lakes across a wide range of lake type and environmental gradients in Québec. The diffusive CH 4 flux was highly variable (mean 11.6 ± 26.4 SD mg m ?2 d ?1), and it was positively correlated with temperature and lake nutrient status, and negatively correlated with lake area and colored dissolved organic matter (CDOM). The relationship between CH 4 and CO 2 concentrations fluxes was weak, suggesting major differences in their respective sources and/or regulation. For example, increasing water temperature leads to higher CH 4 flux but does not significantly affect CO 2 flux, whereas increasing CDOM concentration leads to higher CO 2 flux but lower CH 4 flux. CH 4 contributed to 8 ± 23% to the total lake C emissions (CH 4 + CO 2), but 18 ± 25% to the total flux in terms of atmospheric warming potential, expressed as CO 2‐equivalents. The incorporation of ebullition and plant‐mediated CH 4 fluxes would further increase the importance of lake CH 4. The average Q 10 of CH 4 flux was 3.7, once other covarying factors were accounted for, but this apparent Q 10 varied with lake morphometry and was higher for shallow lakes. We conclude that global climate change and the resulting shifts in temperature will strongly influence lake CH 4 fluxes across the boreal biome, but these climate effects may be altered by regional patterns in lake morphometry, nutrient status, and browning. 相似文献
17.
Anthropogenic nutrient inputs fuel eutrophication and hypoxia ([O2]?<?2 mg L?1), threatening coastal and near shore environments across the globe. The world’s second largest anthropogenic coastal hypoxic zone occurs annually along the Louisiana (LA) shelf. Springtime loading of dissolved inorganic nitrogen (DIN) from the Mississippi River, combined with summertime stratification and increased water residence time on the shelf, promotes establishment of an extensive hypoxic zone that persists throughout the summer. We investigated the patterns of pelagic denitrification and methane (CH4) oxidation along the LA shelf. Microbial activity rates were determined along with concentrations of dissolved nutrients and greenhouse gases, nitrous oxide (N2O) and CH4, during summer in 2013, 2015, and 2016. We documented denitrification rates up to 1900 nmol N L?1 d?1 and CH4 oxidation rates as high as 192 nmol L?1 d?1 in hypoxic waters characterized by high concentrations of N2O (range: 1 to 102 nM) and CH4 (range: 3 to 641 nM). Ecosystem scaling estimates suggest that pelagic denitrification could remove between 0.1 and 47% of the DIN input from the Mississippi River, whereas CH4 oxidation does not function as an effective removal process with CH4 escaping to the atmosphere. Denitrification and CH4 oxidizing bacteria within the LA shelf hypoxic zone were largely unable to keep up with the DIN and CH4 inputs to the water column. Rates were variable and physiochemical dynamics appeared to regulate the microbial removal capacity for both nitrate/nitrite and CH4 in this ecosystem. 相似文献
18.
The biosphere–atmosphere exchange of methane (CH 4) was estimated for a temperate/boreal lowland and wetland forest ecosystem in northern Wisconsin for 1997–1999 using the modified Bowen ratio (MBR) method. Gradients of CH 4 and CO 2 and CO 2 flux were measured on the 447‐m WLEF‐TV tower as part of the Chequamegon Ecosystem–Atmosphere Study (ChEAS). No systematic diurnal variability was observed in regional CH 4 fluxes measured using the MBR method. In all 3 years, regional CH 4 emissions reached maximum values during June–August (24±14.4 mg m ?2 day ?1), coinciding with periods of maximum soil temperatures. In 1997 and 1998, the onset in CH 4 emission was coincident with increases in ground temperatures following the melting of the snow cover. The onset of emission in 1999 lagged 100 days behind the 1997 and 1998 onsets, and was likely related to postdrought recovery of the regional water table to typical levels. The net regional emissions were 3.0, 3.1, and 2.1 g CH 4 m ?2 for 1997, 1998, and 1999, respectively. Annual emissions for wetland regions within the source area (28% of the land area) were 13.2, 13.8, and 10.3 g CH 4 m ?2 assuming moderate rates of oxidation of CH 4 in upland regions in 1997, 1998, and 1999, respectively. Scaling these measurements to the Chequamegon Ecosystem (CNNF) and comparing with average wetland emissions between 40°N and 50°N suggests that wetlands in the CNNF emit approximately 40% less than average wetlands at this latitude. Differences in mean monthly air temperatures did not affect the magnitude of CH 4 emissions; however, reduced precipitation and water table levels suppressed CH 4 emission during 1999, suggesting that long‐term climatic changes that reduce the water table will likely transform this landscape to a reduced source or possibly a sink for atmospheric CH 4. 相似文献
19.
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
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