The positive net radiative greenhouse gas forcing of increasing methane emissions from a thawing boreal forest‐wetland landscape

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 (CH₄) emissions. Here, we quantify the thaw‐induced increase in CH₄ 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 (CO₂) exchange. Using nested wetland and landscape eddy covariance net CH₄ flux measurements in combination with flux footprint modeling, we find that landscape CH₄ emissions increase with increasing wetland‐to‐forest ratio. Landscape CH₄ 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 CH₄ emission of ~13 g CH₄ m^?2 is the dominating contribution to the landscape CH₄ emission of ~7 g CH₄ m^?2. In contrast, forest contributions to landscape CH₄ 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 CH₄ m^?2 yr^?1 in landscape CH₄ emissions. A long‐term net CO₂ uptake of >200 g CO₂ m^?2 yr^?1 is required to offset the positive radiative forcing of increasing CH₄ emissions until the end of the 21st century as indicated by an atmospheric CH₄ and CO₂ concentration model. However, long‐term apparent carbon accumulation rates in similar boreal forest‐wetland landscapes and eddy covariance landscape net CO₂ flux measurements suggest a long‐term net CO₂ uptake between 49 and 157 g CO₂ m^?2 yr^?1. Thus, thaw‐induced CH₄ emission increases likely exert a positive net radiative greenhouse gas forcing through the 21st century.     

Nongrowing season methane emissions–a significant component of annual emissions across northern ecosystems

Wetlands are the single largest natural source of atmospheric methane (CH₄), a greenhouse gas, and occur extensively in the northern hemisphere. Large discrepancies remain between “bottom‐up” and “top‐down” estimates of northern CH₄ emissions. To explore whether these discrepancies are due to poor representation of nongrowing season CH₄ emissions, we synthesized nongrowing season and annual CH₄ flux measurements from temperate, boreal, and tundra wetlands and uplands. Median nongrowing season wetland emissions ranged from 0.9 g/m² in bogs to 5.2 g/m² 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 CH₄ sinks to CH₄ sources with a median annual flux of 0.0 ± 0.2 g m^?2 year^?1. The measured fraction of annual CH₄ 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 CH₄ emissions. Using this constraint, the modeled nongrowing season wetland CH₄ 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 CH₄ 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 CH₄ emissions from high‐latitude ecosystems, and necessary for constraining the role of wetland emissions in a warming climate.     

基于过程模型的青藏高原湿地甲烷排放格局评估

甲烷(CH_4)是大气中最丰富的碳氢化合物,是仅次于二氧化碳(CO_2)的温室气体。湿地是甲烷的重要来源,在全球碳循环中发挥着重要作用,其排放的甲烷占所有天然甲烷排放源的70%,占全球甲烷排放总量的24.8%。青藏高原平均海拔4000 m以上,占有中国约三分之一的湿地。近几十年来,由于全球气候变暖和降水增加,该地区甲烷排放率和湿地面积都发生着巨大变化,因此,青藏高原湿地CH_4排放的长期变化在很大程度上仍存在较大的不确定性。利用TRIPLEX-GHG模型模拟了青藏高原湿地1978—2008年CH_4排放的动态特征,研究结果表明:(1)1978—2008年青藏高原湿地CH_4排放速率呈逐渐增加趋势。(2)青藏高原大多数湿地区域CH_4排放速率为0—6.13 g CH_4 m~(-2 )a~(-1);东北部分湿地区域CH_4排放速率为6.14—20.19 g CH_4 m~(-2 )a~(-1);较高的CH_4排放速率分布于青藏高原南部湿地区域,为56.14—74.97 g CH_4 m~(-2 )a~(-1)。(3)青藏高原湿地CH_4排放量在1978、1990、2000年和2008年分别为0.21、0.23、0.27和0.32 Tg CH_4 a~(-1)。在1978—1990年,尽管CH_4排放速率增加,但湿地面积减少,因此这一时期青藏高原湿地CH_4排放量并未发生明显变化。随后由于降水增加和冰川融化,使得湿地面积逐渐增加,青藏高原湿地CH_4排放量也呈现增加趋势。     

Ecosystem–atmosphere exchange of CH4 and N2O and ecosystem respiration in wetlands in the Sanjiang Plain, Northeastern China

Natural wetlands are critically important to global change because of their role in modulating atmospheric concentrations of CO₂, CH₄, and N₂O. One 4‐year continuous observation was conducted to examine the exchanges of CH₄ and N₂O between three wetland ecosystems and the atmosphere as well as the ecosystem respiration in the Sanjiang Plain in Northeastern China. From 2002 to 2005, the mean annual budgets of CH₄ and N₂O, and ecosystem respiration were 39.40 ± 6.99 g C m^?2 yr^?1, 0.124 ± 0.05 g N m^?2 yr^?1, and 513.55 ± 8.58 g C m^?2 yr^?1 for permanently inundated wetland; 4.36 ± 1.79 g C m^?2 yr^?1, 0.11 ± 0.12 g N m^?2 yr^?1, and 880.50 ± 71.72 g C m^?2 yr^?1 for seasonally inundated wetland; and 0.21 ± 0.1 g C m^?2 yr^?1, 0.28 ± 0.11 g N m^?2 yr^?1, and 1212.83 ± 191.98 g C m^?2 yr^?1 for shrub swamp. The substantial interannual variation of gas fluxes was due to the significant climatic variability which underscores the importance of long‐term continuous observations. The apparent seasonal pattern of gas emissions associated with a significant relationship of gas fluxes to air temperature implied the potential effect of global warming on greenhouse gas emissions from natural wetlands. The budgets of CH₄ and N₂O fluxes and ecosystem respiration were highly variable among three wetland types, which suggest the uncertainties in previous studies in which all kinds of natural wetlands were treated as one or two functional types. New classification of global natural wetlands in more detailed level is highly expected.     

Methane emissions from wetlands and their relationship with vascular plants: an Arctic example

This paper investigates the relationship between vascular plant production and CH₄ emissions from an arctic wet tundra ecosystem in north‐east Greenland. Light intensity was manipulated by shading during three consecutive growing seasons (1998–2000). The shading treatment resulted in lower carbon cycling in the ecosystem as mean seasonal net ecosystem exchange (NEE) decreased from ?336 to ?196 mg CO₂ m^?2 h^?1 and from ?476 to ?212 mg CO₂ m^?2 h^?1 in 1999 and 2000, respectively, and total ecosystem respiration decreased from 125 to 94 mg CO₂ m^?2 h^?1 in 1999 and from 409 to 306 mg CO₂ m^?2 h^?1 in 2000. Seasonal mean CH₄ emissions in controls and shaded plots were, respectively, 6.5 and 4.5 mg CH₄ m^?2 h^?1 in 1999 and 8.3 and 6.2 mg CH₄ m^?2 h^?1 in 2000. We found that CH₄ emission was sensitive to NEE and carbon turnover, and it is reasonable to assume that the correlation was due to a combined effect of vegetative CH₄ transport and substrate quality coupled to vascular plant production. Total above‐ground biomass was correlated to mean seasonal CH₄ emission, but separation into species showed that plant‐mediated CH₄ transport was highly species dependent. Potential CH₄ production peaked at the same depth as maximum root density (5–15 cm) and treatment differences further suggest that substrate quality was negatively affected by decreased NEE in the shaded plots. The concentration of dissolved CH₄ decreased in the control plots as the growing season progressed while it was relatively stable in the shaded plots. This suggests that a progressively better developed root system in the controls increased the capacity to transport CH₄ from the soil to the atmosphere. In conclusion, vascular plant photosynthetic rate and subsequent allocation of recently fixed carbon to below‐ground structures seemed to influence both vegetative CH₄ transport and substrate quality.     

Seasonal nitrous oxide and methane emissions across a subtropical estuarine salinity gradient

Currently, there is a lack of knowledge about GHG emissions, specifically N₂O and CH₄, in subtropical coastal freshwater wetland and mangroves in the southern hemisphere. In this study, we quantified the gas fluxes and substrate availability in a subtropical coastal wetland off the coast of southeast Queensland, Australia over a complete wet-dry seasonal cycle. Sites were selected along a salinity gradient ranging from marine (34 psu) in a mangrove forest to freshwater (0.05 psu) wetland, encompassing the range of tidal influence. Fluxes were quantified for CH₄ (range ?0.4–483 mg C–CH₄ h^?1 m^?2) and N₂O (?5.5–126.4 μg N–N₂O h^?1 m^?2), with the system acting as an overall source for CH₄ and N₂O (mean N₂O and CH₄ fluxes: 52.8 μg N–N₂O h^?1 m^?2 and 48.7 mg C–CH₄ h^?1 m^?2, respectively). Significantly higher N₂O fluxes were measured during the summer months (summer mean 64.2 ± 22.2 μg N–N₂O h^?1 m^?2; winter mean 33.1 ± 24.4 µg N–N₂O h^–1 m^?2) but not CH₄ fluxes (summer mean 30.2 ± 81.1 mg C–CH₄ h^?1 m^?2; winter mean 37.4 ± 79.6 mg C–CH₄ h^?1 m^?2). The changes with season are primarily driven by temperature and precipitation controls on the dissolved inorganic nitrogen (DIN) concentration. A significant spatial pattern was observed based on location within the study site, with highest fluxes observed in the freshwater tidal wetland and decreasing through the mangrove forest. The dissolved organic carbon (DOC) varied throughout the landscape and was correlated with higher CH₄ fluxes, but this was a nonlinear trend. DIN availability was dominated by N–NH₄ and correlated to changes in N₂O fluxes throughout the landscape. Overall, we did not observe linear relationships between CH₄ and N₂O fluxes and salinity, oxygen or substrate availability along the fresh-marine continuum, suggesting that this ecosystem is a mosaic of processes and responses to environmental changes.     

Effect of water table on greenhouse gas emissions from peatland mesocosms

Peatland landscapes typically exhibit large variations in greenhouse gas (GHG) emissions due to microtopographic and vegetation heterogeneity. As many peatland budgets are extrapolated from small-scale chamber measurements it is important to both quantify and understand the processes underlying this spatial variability. Here we carried out a mesocosm study which allowed a comparison to be made between different microtopographic features and vegetation communities, in response to conditions of both static and changing water table. Three mesocosm types (hummocks?+?Juncus effusus, hummocks?+?Eriophorum vaginatum, and hollows dominated by moss) were subjected to two water table treatments (0–5 cm and 30–35 cm depth). Measurements were made of soil-atmosphere GHG exchange, GHG concentration within the peat profile and soil water solute concentrations. After 14 weeks the high water table group was drained and the low water table group flooded. Measurement intensity was then increased to examine the immediate response to change in water table position. Mean CO₂, CH₄ and N₂O exchange across all chambers was 39.8 μg m^?2 s^?1, 54.7 μg m^?2 h^?1 and ?2.9 μg m^?2 h^?1, respectively. Hence the GHG budget was dominated in this case by CO₂ exchange. CO₂ and N₂O emissions were highest in the low water table treatment group; CH₄ emissions were highest in the saturated mesocosms. We observed a strong interaction between mesocosm type and water table for CH₄ emissions. In contrast to many previous studies, we found that the presence of aerenchyma-containing vegetation reduced CH₄ emissions. A significant pulse in both CH₄ and N₂O emissions occurred within 1–2 days of switching the water table treatments. This pulsing could potentially lead to significant underestimation of landscape annual GHG budgets when widely spaced chamber measurements are upscaled.     

Plant-mediated CH4 transport and C gas dynamics quantified in-situ in a Phalaris arundinacea-dominant wetland

Northern peatland methane (CH₄) budgets are important for global CH₄ emissions. This study aims to determine the ecosystem CH₄ budget and specifically to quantify the importance of Phalaris arundinacea by using different chamber techniques in a temperate wetland. Annually, roughly 70?±?35% of ecosystem CH₄ emissions were plant-mediated, but data show no evidence of significant diurnal variations related to convective gas flow regardless of season or plant growth stages. Therefore, despite a high percentage of arenchyma, P. arundinacea-mediated CH₄ transport is interpreted to be predominantly passive. Thus, diurnal variations are less important in contrast to wetland vascular plants facilitating convective gas flow. Despite of plant-dominant CH₄ transport, net CH₄ fluxes were low (–?0.005–0.016 μmol m^?2 s^?1) and annually less than 1% of the annual C-CO₂ assimilation. This is considered a result of an effective root zone oxygenation resulting in increased CH₄ oxidation in the rhizosphere at high water levels. This study shows that although CH₄, having a global warming potential 25 times greater than CO₂, is emitted from this P. arundinacea wetland, less than 9% of the C sequestered counterbalances the CH₄ emissions to the atmosphere. It is concluded that P. arundinacea-dominant wetlands are an attractive C-sequestration ecosystem.     

Emissions of methane from northern peatlands: a review of management impacts and implications for future management options

Northern peatlands constitute a significant source of atmospheric methane (CH₄). However, management of undisturbed peatlands, as well as the restoration of disturbed peatlands, will alter the exchange of CH₄ 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 CH₄ 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 CH₄ emissions taken at 186 sites covering different countries, peatland types, and management systems. Results show that CH₄ 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 CH₄ 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 CH₄ emissions from northern peatlands. Drainage significantly (p < .05) reduces CH₄ emissions to the atmosphere, on average by 84%. Restoration of drained peatlands by rewetting or vegetation/rewetting increases CH₄ emissions on average by 46% compared to the original premanagement CH₄ 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.     

Rewetting of Cutaway Peatlands: Are We Re‐Creating Hot Spots of Methane Emissions?

Hot spots of CH₄ emissions are a typical feature of pristine peatlands at the microsite and landscape scale. To determine whether rewetting and lake construction in a cutaway peatland would result in the re‐creation of hot spots, we first measured CH₄ fluxes over a 2‐year period with static chambers and estimated annual emissions. Second, to assess whether rewetting and lake creation would produce hot spots at the landscape level, we hypothesized a number of alternative land use scenarios for the peatland following the cessation of peat extraction. Using the results from this study and other studies from literature, we calculated the global warming potential (GWP) of each scenario and the respective contribution of CH₄. The results showed that hot spots of CH₄ fluxes were observed as a consequence of microsite‐specific differences in water table (WT) position and plant productivity. CH₄ fluxes were closely related to peat temperature at 10 cm depth and WT position. Annual emissions ranged from 4.3 to 38.8 g CH₄ m^?2 yr^?1 in 2002 and 3.2 to 28.8 g CH₄ m^?2 yr^?1 in 2003. The scenario results suggest that lake creation is likely to result in the re‐creation of a hot spot at the landscape level. However, the transition from cutaway to wetland ecosystem may lead to a reduction in the GWP of the peatland.     

Quantifying landscape‐level methane fluxes in subarctic Finland using a multiscale approach

Quantifying landscape‐scale methane (CH₄) fluxes from boreal and arctic regions, and determining how they are controlled, is critical for predicting the magnitude of any CH₄ emission feedback to climate change. Furthermore, there remains uncertainty regarding the relative importance of small areas of strong methanogenic activity, vs. larger areas with net CH₄ uptake, in controlling landscape‐level fluxes. We measured CH₄ fluxes from multiple microtopographical subunits (sedge‐dominated lawns, interhummocks and hummocks) within an aapa mire in subarctic Finland, as well as in drier ecosystems present in the wider landscape, lichen heath and mountain birch forest. An intercomparison was carried out between fluxes measured using static chambers, up‐scaled using a high‐resolution landcover map derived from aerial photography and eddy covariance. Strong agreement was observed between the two methodologies, with emission rates greatest in lawns. CH₄ fluxes from lawns were strongly related to seasonal fluctuations in temperature, but their floating nature meant that water‐table depth was not a key factor in controlling CH₄ release. In contrast, chamber measurements identified net CH₄ uptake in birch forest soils. An intercomparison between the aerial photography and satellite remote sensing demonstrated that quantifying the distribution of the key CH₄ emitting and consuming plant communities was possible from satellite, allowing fluxes to be scaled up to a 100 km² area. For the full growing season (May to October), ~ 1.1–1.4 g CH₄ m^?2 was released across the 100 km² area. This was based on up‐scaled lawn emissions of 1.2–1.5 g CH₄ m^?2, vs. an up‐scaled uptake of 0.07–0.15 g CH₄ m^?2 by the wider landscape. Given the strong temperature sensitivity of the dominant lawn fluxes, and the fact that lawns are unlikely to dry out, climate warming may substantially increase CH₄ emissions in northern Finland, and in aapa mire regions in general.     

Effects of seasonality,transport pathway,and spatial structure on greenhouse gas fluxes in a restored wetland

Wetlands can influence global climate via greenhouse gas (GHG) exchange of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Few studies have quantified the full GHG budget of wetlands due to the high spatial and temporal variability of fluxes. We report annual open‐water diffusion and ebullition fluxes of CO₂, CH₄, and N₂O from a restored emergent marsh ecosystem. We combined these data with concurrent eddy‐covariance measurements of whole‐ecosystem CO₂ and CH₄ exchange to estimate GHG fluxes and associated radiative forcing effects for the whole wetland, and separately for open‐water and vegetated cover types. Annual open‐water CO₂, CH₄, and N₂O emissions were 915 ± 95 g C‐CO₂ m^?2 yr^?1, 2.9 ± 0.5 g C‐CH₄ m^?2 yr^?1, and 62 ± 17 mg N‐N₂O m^?2 yr^?1, respectively. Diffusion dominated open‐water GHG transport, accounting for >99% of CO₂ and N₂O emissions, and ~71% of CH₄ emissions. Seasonality was minor for CO₂ emissions, whereas CH₄ and N₂O fluxes displayed strong and asynchronous seasonal dynamics. Notably, the overall radiative forcing of open‐water fluxes (3.5 ± 0.3 kg CO₂‐eq m^?2 yr^?1) exceeded that of vegetated zones (1.4 ± 0.4 kg CO₂‐eq m^?2 yr^?1) due to high ecosystem respiration. After scaling results to the entire wetland using object‐based cover classification of remote sensing imagery, net uptake of CO₂ (?1.4 ± 0.6 kt CO₂‐eq yr^?1) did not offset CH₄ emission (3.7 ± 0.03 kt CO₂‐eq yr^?1), producing an overall positive radiative forcing effect of 2.4 ± 0.3 kt CO₂‐eq yr^?1. These results demonstrate clear effects of seasonality, spatial structure, and transport pathway on the magnitude and composition of wetland GHG emissions, and the efficacy of multiscale flux measurement to overcome challenges of wetland heterogeneity.     

Methane Emissions from Paludified Boreal Soils in European Russia as Measured and Modelled

Spatial or temporal forest–peatland transition zones were proposed as potential hot spots of methane (CH₄) emissions. Consequently, paludified soils are an important component of boreal landscape biogeochemistry. However, their role in the regional carbon cycle remains unclear. This study presents CH₄ fluxes from two forest–peatland transition zones, two wet forest sites and two clear-cut sites which were compared to fluxes of open peatlands and dry forest. The median fluxes measured using the closed-chamber technique varied from ? 0.04 to 12.6 mg m^?2 h^?1 during three climatically different years. The annual mean CH₄ emissions of the forest–peatland transition zone were significantly lower than the fluxes of the open peatland sites, 7.9 ± 0.5 and 21.9 ± 1.6 g m^?2a^?1, respectively. The dry forest site was characterized by a small uptake of CH₄ (? 2.3 ± 0.2 g m^?2a^?1). Although clear-cut forest area drastically increased in European Russia during the last two decades, if water level depths in these forests remains below 10 cm they do not act as CH₄ sources. Fluxes of CH₄ from the transition zone sites showed a higher response to soil temperature than to water table level. Fluxes of CH₄ between the atmosphere and the two investigated peatlands were not significantly different, although a significant difference in water table level could be observed. The meteorological conditions of the investigated summers changed from being hot and dry in 2013 to cold and wet in 2014; the summer of 2015 was characterized as warmer and drier in the first half and colder and wetter in the second half. Significant differences in CH₄ fluxes were measured only between 2014 and 2013. Significant differences in CH₄ fluxes and in nonlinear regressions showed that the CH₄ fluxes of the different site types such as dry forests, transition zones and open peatlands need to be modelled separately on a landscape level. Obviously, underlying processes vary with the ecosystem and (along with regional aspects) have to be understood first before large-scale modelling is possible.     

Effects of an Experimental Water-level Drawdown on Methane Emissions from a Eutrophic Reservoir

Reservoirs are a globally significant source of methane (CH₄) to the atmosphere. However, emission rate estimates may be biased low due to inadequate monitoring during brief periods of elevated emission rates (that is, hot moments). Here we investigate CH₄ bubbling (that is, ebullition) during periods of falling water levels in a eutrophic reservoir in the Midwestern USA. We hypothesized that periods of water-level decline trigger the release of CH₄-rich bubbles from the sediments and that these emissions constitute a substantial fraction of the annual CH₄ flux. We explored this hypothesis by monitoring CH₄ ebullition in a eutrophic reservoir over a 7-month period, which included an experimental water-level drawdown. We found that the ebullitive CH₄ flux rate was among the highest ever reported for a reservoir (mean = 32.3 mg CH₄ m^?2 h^?1). The already high ebullitive flux rates increased by factors of 1.4–77 across the nine monitoring sites during the 24-h experimental water-level drawdown, but these emissions constituted only 3% of the CH₄ flux during the 7-month monitoring period due to the naturally high ebullitive CH₄ flux rates that persist throughout the warm weather season. Although drawdown emissions were found to be a minor component of annual CH₄ emissions in this reservoir, our findings demonstrate a link between water-level change and CH₄ ebullition, suggesting that CH₄ emissions may be mitigated through water-level management in some reservoirs.     

Methane emissions from soils: synthesis and analysis of a large UK data set

Nearly 5000 chamber measurements of CH₄ 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 CH₄ 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 CH₄ 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 CH₄ emissions in the United Kingdom, an emission factor of +0.4 g CH₄ m^?2 yr^?1 per cm increase in water table height was derived from the data.     

Seasonal variations of methane fluxes from an unvegetated tidal freshwater mudflat (Hammersmith Creek, GA)

Wetlands are estimated to contribute nearly 40 % of global annual methane (CH₄) emissions to the atmosphere. However, because CH₄ fluxes from these systems vary spatially, seasonally, and by wetland type, there is a large uncertainty associated with scaling up the CH₄ flux from these environments. We monitored seasonal patterns of CH₄ cycling from tidal mudflat wetland sediments adjacent to a vegetated freshwater wetland in coastal Georgia between 2008 and 2009. CH₄ emissions were significantly correlated with CH₄ production and sediment saturation state with respect to CH₄ but not with temperature. CH₄ cycling displayed distinct seasonal patterns. Winter months were characterized by low CH₄ production and emissions. During the spring, summer and fall, CH₄ fluxes exceeded CH₄ production in the top 40 cm. Comparison of CH₄ sources and sinks in conjunction with the interpretation of CH₄ concentration profiles using a 1D reactive transport model indicated that CH₄ delivered via lateral tidal pumping likely provided additional CH₄ to the upper sediment column. Seasonally high CH₄ ebullition rates reflected increased CH₄ production and decreased CH₄ solubility. The annual CH₄ flux was estimated to be on the order of 10 mol CH₄ m^?2 y^?1 which is 2–4 times the global average for wetland CH₄ emissions. Thus, even though tidal freshwater mudflats are of limited spatial extent, these environments may serve as globally significant sources of CH₄ to the atmosphere. This study highlights the importance of these dynamic environments to the global CH₄ cycle and their relevance to climate change.     

Interannual variation in methane emissions from tropical wetlands triggered by repeated El Niño Southern Oscillation

Methane (CH₄) emissions from tropical wetlands contribute 60%–80% of global natural wetland CH₄ emissions. Decreased wetland CH₄ emissions can act as a negative feedback mechanism for future climate warming and vice versa. The impact of the El Niño–Southern Oscillation (ENSO) on CH₄ emissions from wetlands remains poorly quantified at both regional and global scales, and El Niño events are expected to become more severe based on climate models’ projections. We use a process‐based model of global wetland CH₄ emissions to investigate the impacts of the ENSO on CH₄ emissions in tropical wetlands for the period from 1950 to 2012. The results show that CH₄ emissions from tropical wetlands respond strongly to repeated ENSO events, with negative anomalies occurring during El Niño periods and with positive anomalies occurring during La Niña periods. An approximately 8‐month time lag was detected between tropical wetland CH₄ emissions and ENSO events, which was caused by the combined time lag effects of ENSO events on precipitation and temperature over tropical wetlands. The ENSO can explain 49% of interannual variations for tropical wetland CH₄ emissions. Furthermore, relative to neutral years, changes in temperature have much stronger effects on tropical wetland CH₄ emissions than the changes in precipitation during ENSO periods. The occurrence of several El Niño events contributed to a lower decadal mean growth rate in atmospheric CH₄ concentrations throughout the 1980s and 1990s and to stable atmospheric CH₄ concentrations from 1999 to 2006, resulting in negative feedback to global warming.     

Microbial and Environmental Controls of Methane Fluxes Along a Soil Moisture Gradient in a Pacific Coastal Temperate Rainforest

Most studies of greenhouse gas fluxes from forest soils in the coastal rainforest have considered carbon dioxide (CO₂), whereas methane (CH₄) has not received the same attention. Soil hydrology is a key driver of CH₄ dynamics in ecosystems, but the impact on the function and distribution of the underlying microbial communities involved in CH₄ cycling and the resultant net CH₄ exchange is not well understood at this scale. We studied the growing season variations of in situ CH₄ fluxes, microbial gene abundances of methanotrophs (CH₄ oxidizers) and methanogens (CH₄ producers), soil hydrology, and nutrient availability in three typical forest types across a soil moisture gradient. CH₄ displayed a spatial variability changing from a net uptake in the upland soils (3.9–46 µmol CH₄ m^?2 h^?1) to a net emission in the wetter soils (0–90 μmol CH₄ m^?2 h^?1). Seasonal variations of CH₄ fluxes were related to soil hydrology in both upland and wet soils. Thus, in the upland soils, uptake rates increased with the decreasing soil moisture, whereas CH₄ emission was inversely related to the water table depth in the wet soils. Spatial variability of CH₄ exchange was related to the abundance of genes involved in CH₄ oxidation and production, but there was no indication of a temporal link between microbial groups and CH₄ exchange. Our data show that the abundances of genes involved in CH₄ oxidation and production are strongly influenced by soil moisture and each other and grouped by the upland–wetland classification but not forest type.     

Modelling carbon balances of coastal arctic tundra under changing climate

Rising air temperatures are believed to be hastening heterotrophic respiration (R_h) in arctic tundra ecosystems, which could lead to substantial losses of soil carbon (C). In order to improve confidence in predicting the likelihood of such loss, the comprehensive ecosystem model ecosys was first tested with carbon dioxide (CO₂) fluxes measured over a tundra soil in a growth chamber under various temperatures and soil‐water contents (θ). The model was then tested with CO₂ and energy fluxes measured over a coastal arctic tundra near Barrow, Alaska, under a range of weather conditions during 1998–1999. A rise in growth chamber temperature from 7 to 15 °C caused large, but commensurate, rises in respiration and CO₂ fixation, and so no significant effect on net CO₂ exchange was modelled or measured. An increase in growth chamber θ from field capacity to saturation caused substantial reductions in respiration but not in CO₂ fixation, and so an increase in net CO₂ exchange was modelled and measured. Long daylengths over the coastal tundra at Barrow caused an almost continuous C sink to be modelled and measured during most of July (2–4 g C m^?2 d^?1), but shortening daylengths and declining air temperatures caused a C source to be modelled and measured by early September (～1 g C m^?2 d^?1). At an annual time scale, the coastal tundra was modelled to be a small C sink (4 g C m^?2 y^?1) during 1998 when average air temperatures were 4 °C above normal, and a larger C sink (16 g C m^?2 y^?1) during 1999 when air temperatures were close to long‐term normals. During 100 years under rising atmospheric CO₂ concentration (C_a), air temperature and precipitation driven by the IS92a emissions scenario, modelled R_h rose commensurately with net primary productivity (NPP) under both current and elevated rates of atmospheric nitrogen (N) deposition, so that changes in soil C remained small. However, methane (CH₄) emissions were predicted to rise substantially in coastal tundra with IS92a‐driven climate change (from ～20 to ～40 g C m^?2 y^?1), causing a substantial increase in the emission of CO₂ equivalents. If the rate of temperature increase hypothesized in the IS92a emissions scenario had been raised by 50%, substantial losses of soil C (～1 kg C m^?2) would have been modelled after 100 years, including additional emissions of CH₄.     

Carbon fluxes from a tropical peat swamp forest floor

A tropical ombrotrophic peatland ecosystem is one of the largest terrestrial carbon stores. Flux rates of carbon dioxide (CO₂) and methane (CH₄) were studied at various peat water table depths in a mixed‐type peat swamp forest floor in Central Kalimantan, Indonesia. Temporary gas fluxes on microtopographically differing hummock and hollow peat surfaces were combined with peat water table data to produce annual cumulative flux estimates. Hummocks formed mainly from living and dead tree roots and decaying debris maintained a relatively steady CO₂ emission rate regardless of the water table position in peat. In nearly vegetation‐free hollows, CO₂ emission rates were progressively smaller as the water table rose towards the peat surface. Methane emissions from the peat surface remained small and were detected only in water‐saturated peat. By applying long‐term peat water table data, annual gas emissions from the peat swamp forest floor were estimated to be 3493±316 g CO₂ m^?2 and less than 1.36±0.57 g CH₄ m^?2. On the basis of the carbon emitted, CO₂ is clearly a more important greenhouse gas than CH₄. CO₂ emissions from peat are the highest during the dry season, when the oxic peat layer is at its thickest because of water table lowering.