Aircraft‐derived regional scale CO2 fluxes from vegetated drained thaw‐lake basins and interstitial tundra on the Arctic Coastal Plain of Alaska

The landscape surface of the Barrow Peninsula of Alaska is a mosaic of small ponds, thaw lakes, different aged vegetated drained thaw‐lake basins (VDTLBs), and interstitial tundra which have been dynamically formed by both short‐ and long‐term processes. We used a combination of tower‐ and aircraft‐based eddy covariance measurements to characterize the spatial and temporal patterns of CO₂, latent, and sensible heat fluxes along with MODIS NDVI, and were able to scale the aircraft‐based CO₂ fluxes to the 1802 km² Barrow Peninsula region. During typical 2006 summer conditions, the midday hourly CO₂ flux over the region was ?2.04 × 10⁵ kg CO₂ h^?1. The CO₂ fluxes among the interstitial tundra, Ancient, and Old VDTLBs, as well as between the Medium and Young VDTLBs were not significantly different. Combined, the interstitial tundra and Old and Ancient VDTLBs represent～67% of the Barrow Peninsula surface area, accounting for ～59% of the regional flux signal. Although the Medium and Young VDTLBs represent ～11% of the surface area, they account for a large portion, ～35%, of the total regional flux. The remaining ～22% of the surface area are lakes and contributed the remaining ～6% of the total regional flux. Previous studies treated vegetated areas of the region as a single surface type with measurements from a few study sites; doing so could underestimate the regional flux by ～22%. Here, we demonstrate that aircraft‐based systems have the ability to cover large spatial scales while measuring the turbulent fluxes across a number of surfaces and combined with ground‐ and satellite‐based measurements provide a valuable tool for both scaling and validation of regional‐scale fluxes.     

Spatial variation in landscape‐level CO2 and CH4 fluxes from arctic coastal tundra: influence from vegetation,wetness, and the thaw lake cycle

Regional quantification of arctic CO₂ and CH₄ fluxes remains difficult due to high landscape heterogeneity coupled with a sparse measurement network. Most of the arctic coastal tundra near Barrow, Alaska is part of the thaw lake cycle, which includes current thaw lakes and a 5500‐year chronosequence of vegetated thaw lake basins. However, spatial variability in carbon fluxes from these features remains grossly understudied. Here, we present an analysis of whole‐ecosystem CO₂ and CH₄ fluxes from 20 thaw lake cycle features during the 2011 growing season. We found that the thaw lake cycle was largely responsible for spatial variation in CO₂ flux, mostly due to its control on gross primary productivity (GPP). Current lakes were significant CO₂ sources that varied little. Vegetated basins showed declining GPP and CO₂ sink with age (R² = 67% and 57%, respectively). CH₄ fluxes measured from a subset of 12 vegetated basins showed no relationship with age or CO₂ flux components. Instead, higher CH₄ fluxes were related to greater landscape wetness (R² = 57%) and thaw depth (additional R² = 28%). Spatial variation in CO₂ and CH₄ fluxes had good satellite remote sensing indicators, and we estimated the region to be a small CO₂ sink of ?4.9 ± 2.4 (SE) g C m^?2 between 11 June and 25 August, which was countered by a CH₄ source of 2.1 ± 0.2 (SE) g C m^?2. Results from our scaling exercise showed that developing or validating regional estimates based on single tower sites can result in significant bias, on average by a factor 4 for CO₂ flux and 30% for CH₄ flux. Although our results are specific to the Arctic Coastal Plain of Alaska, the degree of landscape‐scale variability, large‐scale controls on carbon exchange, and implications for regional estimation seen here likely have wide relevance to other arctic landscapes.     

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.     

Isotopic insights into methane production,oxidation, and emissions in Arctic polygon tundra

Arctic wetlands are currently net sources of atmospheric CH₄. Due to their complex biogeochemical controls and high spatial and temporal variability, current net CH₄ emissions and gross CH₄ processes have been difficult to quantify, and their predicted responses to climate change remain uncertain. We investigated CH₄ production, oxidation, and surface emissions in Arctic polygon tundra, across a wet‐to‐dry permafrost degradation gradient from low‐centered (intact) to flat‐ and high‐centered (degraded) polygons. From 3 microtopographic positions (polygon centers, rims, and troughs) along the permafrost degradation gradient, we measured surface CH₄ and CO₂ fluxes, concentrations and stable isotope compositions of CH₄ and DIC at three depths in the soil, and soil moisture and temperature. More degraded sites had lower CH₄ emissions, a different primary methanogenic pathway, and greater CH₄ oxidation than did intact permafrost sites, to a greater degree than soil moisture or temperature could explain. Surface CH₄ flux decreased from 64 nmol m^?2 s^?1 in intact polygons to 7 nmol m^?2 s^?1 in degraded polygons, and stable isotope signatures of CH₄ and DIC showed that acetate cleavage dominated CH₄ production in low‐centered polygons, while CO₂ reduction was the primary pathway in degraded polygons. We see evidence that differences in water flow and vegetation between intact and degraded polygons contributed to these observations. In contrast to many previous studies, these findings document a mechanism whereby permafrost degradation can lead to local decreases in tundra CH₄ emissions.     

Rising plant‐mediated methane emissions from arctic wetlands

Plant‐mediated CH₄ flux is an important pathway for land–atmosphere CH₄ emissions, but the magnitude, timing, and environmental controls, spanning scales of space and time, remain poorly understood in arctic tundra wetlands, particularly under the long‐term effects of climate change. CH₄ fluxes were measured in situ during peak growing season for the dominant aquatic emergent plants in the Alaskan arctic coastal plain, Carex aquatilis and Arctophila fulva, to assess the magnitude and species‐specific controls on CH₄ flux. Plant biomass was a strong predictor of A. fulva CH₄ flux while water depth and thaw depth were copredictors for C. aquatilis CH₄ flux. We used plant and environmental data from 1971 to 1972 from the historic International Biological Program (IBP) research site near Barrow, Alaska, which we resampled in 2010–2013, to quantify changes in plant biomass and thaw depth, and used these to estimate species‐specific decadal‐scale changes in CH₄ fluxes. A ~60% increase in CH₄ flux was estimated from the observed plant biomass and thaw depth increases in tundra ponds over the past 40 years. Despite covering only ~5% of the landscape, we estimate that aquatic C. aquatilis and A. fulva account for two‐thirds of the total regional CH₄ flux of the Barrow Peninsula. The regionally observed increases in plant biomass and active layer thickening over the past 40 years not only have major implications for energy and water balance, but also have significantly altered land–atmosphere CH₄ emissions for this region, potentially acting as a positive feedback to climate warming.     

Winter precipitation and snow accumulation drive the methane sink or source strength of Arctic tussock tundra

Arctic winter precipitation is projected to increase with global warming, but some areas will experience decreases in snow accumulation. Although Arctic CH₄ emissions may represent a significant climate forcing feedback, long‐term impacts of changes in snow accumulation on CH₄ fluxes remain uncertain. We measured ecosystem CH₄ fluxes and soil CH₄ and CO₂ concentrations and ¹³C composition to investigate the metabolic pathways and transport mechanisms driving moist acidic tundra CH₄ 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 %O₂ levels and increasing thaw depth. Soil moisture, through changes in soil %O₂ saturation, determined predominance of methanotrophy or methanogenesis, with soil temperature regulating the ecosystem CH₄ sink or source strength. Reduced snow (RS) increased the fraction of oxidized CH₄ (Fox) by 75–120% compared to Ambient, switching the system from a small source to a net CH₄ sink (21 ± 2 and ?31 ± 1 mg CH₄ 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 CH₄ source strength of moist acidic tundra (464 ± 15 and 3561 ± 97 mg CH₄ m^?2 season^?1 at MS and HS). Decreases in Fox with deeper snow were partly due to increases in plant‐mediated CH₄ transport associated with the expansion of tall graminoids. Deeper snow enhanced CH₄ 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 CH₄ 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.     

Ecosystem carbon response of an Arctic peatland to simulated permafrost thaw

Permafrost peatlands are biogeochemical hot spots in the Arctic as they store vast amounts of carbon. Permafrost thaw could release part of these long‐term immobile carbon stocks as the greenhouse gases (GHGs) carbon dioxide (CO₂) and methane (CH₄) to the atmosphere, but how much, at which time‐span and as which gaseous carbon species is still highly uncertain. Here we assess the effect of permafrost thaw on GHG dynamics under different moisture and vegetation scenarios in a permafrost peatland. A novel experimental approach using intact plant–soil systems (mesocosms) allowed us to simulate permafrost thaw under near‐natural conditions. We monitored GHG flux dynamics via high‐resolution flow‐through gas measurements, combined with detailed monitoring of soil GHG concentration dynamics, yielding insights into GHG production and consumption potential of individual soil layers. Thawing the upper 10–15 cm of permafrost under dry conditions increased CO₂ emissions to the atmosphere (without vegetation: 0.74 ± 0.49 vs. 0.84 ± 0.60 g CO₂–C m^?2 day^?1; with vegetation: 1.20 ± 0.50 vs. 1.32 ± 0.60 g CO₂–C m^?2 day^?1, mean ± SD, pre‐ and post‐thaw, respectively). Radiocarbon dating (¹⁴C) of respired CO₂, supported by an independent curve‐fitting approach, showed a clear contribution (9%–27%) of old carbon to this enhanced post‐thaw CO₂ flux. Elevated concentrations of CO₂, CH₄, and dissolved organic carbon at depth indicated not just pulse emissions during the thawing process, but sustained decomposition and GHG production from thawed permafrost. Oxidation of CH₄ in the peat column, however, prevented CH₄ release to the atmosphere. Importantly, we show here that, under dry conditions, peatlands strengthen the permafrost–carbon feedback by adding to the atmospheric CO₂ burden post‐thaw. However, as long as the water table remains low, our results reveal a strong CH₄ sink capacity in these types of Arctic ecosystems pre‐ and post‐thaw, with the potential to compensate part of the permafrost CO₂ losses over longer timescales.     

Biogeochemistry of organic carbon, CO2, CH4, and trace elements in thermokarst water bodies in discontinuous permafrost zones of Western Siberia

Active processes of permafrost thaw in Western Siberia increase the number of soil subsidencies, thermokarst lakes and thaw ponds. In continuous permafrost zones, this process promotes soil carbon mobilisation to water reservoirs, as well as organic matter (OM) biodegradation, which produces a permanent flux of carbon dioxide (CO₂) to the atmosphere. At the same time, the biogeochemical evolution of aquatic ecosystems situated in the transition zone between continuous permafrost and permafrost-free terrain remains poorly known. In order to better understand the biogeochemical processes that occur in thaw ponds and lakes located in discontinuous permafrost zones, we studied ~30 small (1–100,000 m²) shallow (<1 m depth) lakes and ponds formed as a result of permafrost subsidence and thaw of the palsa bog located in the transition zone between the tundra and forest-tundra (central part of Western Siberia). There is a significant increase in dissolved CO₂ and methane (CH₄) concentration with decreasing water body surface area, with the largest supersaturation with respect to atmospheric CO₂ and CH₄ in small (<100 m²) permafrost depressions filled with thaw water. Dissolved organic carbon (DOC), conductivity, and metal concentrations also progressively increase from large lakes to thaw ponds and depressions. As such, small water bodies with surface areas of 1–100 m² that are not accounted for in the existing lake and pond databases may significantly contribute to CO₂ and CH₄ fluxes to the atmosphere, as well as to the stocks of dissolved trace elements and organic carbon. In situ lake water incubation experiments yielded negligible primary productivity but significant oxygen consumption linked to the mineralisation rate of dissolved OM by heterotrophic bacterioplankton, which produce a net CO₂ flux to the atmosphere of 5 ± 2.5 mol C m² year^?1. The most significant result of this study, which has long-term consequences on our prediction of aquatic ecosystem development in the course of permafrost degradation is CO₂, CH₄, and DOC concentrations increase with decreasing lake age and size. As a consequence, upon future permafrost thaw, the increase in the number of small water bodies, accompanied by the drainage of large thermokarst lakes to the hydrological network, will likely favour (i) the increase of DOC and colloidal metal stocks in surface aquatic systems, and (ii) the enhancement of CO₂ and CH₄ fluxes from the water surface to the atmosphere. According to a conservative estimation that considers that the total area occupied by water bodies in Western Siberia will not change, this increase in stocks and fluxes could be as high as a factor of ten.     

Methane oxidation in contrasting soil types: responses to experimental warming with implication for landscape‐integrated CH4 budget

Arctic ecosystems are characterized by a wide range of soil moisture conditions and thermal regimes and contribute differently to the net methane (CH₄) budget. Yet, it is unclear how climate change will affect the capacity of those systems to act as a net source or sink of CH₄. Here, we present results of in situ CH₄ flux measurements made during the growing season 2014 on Disko Island (west Greenland) and quantify the contribution of contrasting soil and landscape types to the net CH₄ budget and responses to summer warming. We compared gas flux measurements from a bare soil and a dry heath, at ambient conditions and increased air temperature, using open‐top chambers (OTCs). Throughout the growing season, bare soil consumed 0.22 ± 0.03 g CH₄‐C m^?2 (8.1 ± 1.2 g CO₂‐eq m^?2) at ambient conditions, while the dry heath consumed 0.10 ± 0.02 g CH₄‐C m^?2 (3.9 ± 0.6 g CO₂‐eq m^?2). These uptake rates were subsequently scaled to the entire study area of 0.15 km², a landscape also consisting of wetlands with a seasonally integrated methane release of 0.10 ± 0.01 g CH₄‐C m^?2 (3.7 ± 1.2 g CO₂‐eq m^?2). The result was a net landscape sink of 12.71 kg CH₄‐C (0.48 tonne CO₂‐eq) during the growing season. A nonsignificant trend was noticed in seasonal CH₄ uptake rates with experimental warming, corresponding to a 2% reduction at the bare soil, and 33% increase at the dry heath. This was due to the indirect effect of OTCs on soil moisture, which exerted the main control on CH₄ fluxes. Overall, the net landscape sink of CH₄ tended to increase by 20% with OTCs. Bare and dry tundra ecosystems should be considered in the net CH₄ budget of the Arctic due to their potential role in counterbalancing CH₄ emissions from wetlands – not the least when taking the future climatic scenarios of the Arctic into account.     

Stoichiometry and temperature sensitivity of methanogenesis and CO2 production from saturated polygonal tundra in Barrow,Alaska

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 CO₂ 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 (CH₄) and CO₂ production rates and concentrations were determined at ?2, +4, or +8 °C for 60 day incubation period. Temporal dynamics of CO₂ 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 CO₂ 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 CH₄ 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 CH₄ and CO₂ 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 CO₂ than the active layer and negligible CH₄. 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.     

High carbon emissions from thermokarst lakes and their determinants in the Tibet Plateau

Thermokarst lakes are potentially important sources of methane (CH₄) and carbon dioxide (CO₂). However, considerable uncertainty exists regarding carbon emissions from thermokarst lakes owing to a limited understanding of their patterns and motivators. In this study, we measured CH₄ and CO₂ diffusive fluxes in 163 thermokarst lakes in the Qinghai–Tibet Plateau (QTP) over 3 years from May to October. The median carbon emissions from the QTP thermokarst lakes were 1440 mg CO₂ m⁻² day⁻¹ and 60 mg CH₄ m⁻² day⁻¹, respectively. The diffusive rates of CO₂ and CH₄ are related to the catchment land cover type. Sediment microbial abundance and hydrochemistry explain 51.9% and 38.3% of the total variance in CH₄ diffusive emissions, respectively, while CO₂ emissions show no significant relationship with environmental factors. When upscaling carbon emissions from the QTP thermokarst lakes, the annual average CH₄ release per lake area is equal to that of the pan-Arctic region. Our findings highlight the importance of incorporating in situ observation data with different emission pathways for different land cover types in predicting carbon emissions from thermokarst lakes in the future.     

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.     

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.     

Methane emission from Siberian arctic polygonal tundra: eddy covariance measurements and modeling

Eddy covariance measurements of methane flux were carried out in an arctic tundra landscape in the central Lena River Delta at 72°N. The measurements covered the seasonal course of mid‐summer to early winter in 2003 and early spring to mid‐summer in 2004, including the periods of spring thaw and autumnal freeze back. The study site is characterized by very cold and deep permafrost and a continental climate with a mean annual air temperature of ?14.7 °C. The surface is characterized by wet polygonal tundra, with a micro‐relief consisting of raised moderately dry sites, depressed wet sites, polygonal ponds, and lakes. We found relatively low fluxes of typically 30 mg CH₄ m^?2 day^?1 during mid‐summer and identified soil temperature and near‐surface atmospheric turbulence as the factors controlling methane emission. The influence of atmospheric turbulence was attributed to the high coverage of open water surfaces in the tundra. The soil thaw depth and water table position were found to have no clear effect on methane fluxes. The excess emission during spring thaw was estimated to be about 3% of the total flux measured during June–October. Winter emissions were modeled based on the functional relationships found in the measured data. The annual methane emission was estimated to be 3.15 g m^?2. This is low compared with values reported for similar ecosystems. Reason for this were thought to be the very low permafrost temperature in the study region, the sandy soil texture and low bio‐availability of nutrients in the soils, and the high surface coverage of moist to dry micro‐sites. The methane emission accounted for about 14% of the annual ecosystem carbon balance. Considering the global warming potential of methane, the methane emission turned the tundra into an effective greenhouse gas source.     

Large‐scale patterns in summer diffusive CH4 fluxes across boreal lakes,and contribution to diffusive C emissions

Lakes are a major component of boreal landscapes, and whereas lake CO₂ emissions are recognized as a major component of regional C budgets, there is still much uncertainty associated to lake CH₄ fluxes. Here, we present a large‐scale study of the magnitude and regulation of boreal lake summer diffusive CH₄ fluxes, and their contribution to total lake carbon (C) emissions, based on in situ measurements of concentration and fluxes of CH₄ and CO₂ in 224 lakes across a wide range of lake type and environmental gradients in Québec. The diffusive CH₄ 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 CH₄ and CO₂ concentrations fluxes was weak, suggesting major differences in their respective sources and/or regulation. For example, increasing water temperature leads to higher CH₄ flux but does not significantly affect CO₂ flux, whereas increasing CDOM concentration leads to higher CO₂ flux but lower CH₄ flux. CH₄ contributed to 8 ± 23% to the total lake C emissions (CH₄ + CO₂), but 18 ± 25% to the total flux in terms of atmospheric warming potential, expressed as CO₂‐equivalents. The incorporation of ebullition and plant‐mediated CH₄ fluxes would further increase the importance of lake CH₄. The average Q₁₀ of CH₄ flux was 3.7, once other covarying factors were accounted for, but this apparent Q₁₀ 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 CH₄ fluxes across the boreal biome, but these climate effects may be altered by regional patterns in lake morphometry, nutrient status, and browning.     

Paired-tower measurements of carbon and energy fluxes following disturbance in the boreal forest

Disturbances by fire and harvesting are thought to regulate the carbon balance of the Canadian boreal forest over scales of several decades. However, there are few direct measurements of carbon fluxes following disturbances to provide data needed to refine mathematical models. The eddy covariance technique was used with paired towers to measure fluxes simultaneously at disturbed and undisturbed sites over periods of about one week during the growing season in 1998 and 1999. Comparisons were conducted at three sites: a 1‐y‐old burned jackpine stand subjected to an intense crown fire at the International Crown Fire Modelling Experiment site near Fort Providence, North‐west Territories; a 1‐y‐old clearcut aspen area at the EMEND project near Peace River, Alberta; and a 10‐y‐old burned, mixed forest near Prince Albert National Park, Saskatchewan. Nearby mature forest stands of the same types were also measured as controls. The harvested site had lower net radiation (R_n), sensible (H) and latent (LE) heat fluxes, and greater ground heat fluxes (G) than the mature forest. Daytime CO₂ fluxes were much reduced, but night‐time CO₂ fluxes were identical to that of the mature aspen forest. It is hypothesized that the aspen roots remained alive following harvesting, and dominated soil respiration. The overall effect was that the harvested site was a carbon source of about 1.6 gC m^?2 day^?1, while the mature site was a sink of about ?3.8 gC m^?2 day^?1. The one‐year‐old burn had lower R_n, H and LE than the mature jackpine forest, and had a continuous CO₂ efflux of about 0.8 gC m^–2 day^?1 compared to the mature forest sink of ? 0.5 g C m^?2 day^?1. The carbon source was likely caused by decomposition of fire‐killed vegetation. The 10‐y‐old burned site had similar H, LE, and G to the mature mixed forest site. Although the diurnal amplitude of the CO₂ fluxes were slightly lower at the 10‐y‐old site, there was no significant difference between the daily integrals (? 1.3 gC m^?2 day^?1 at both sites). It appears that most of the change in carbon flux occurs within the first 10 years following disturbance, but more data are needed on other forest and disturbance types for the first 20 years following the disturbance event.     

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 and carbon dioxide emissions from inland waters in India – implications for large scale greenhouse gas balances

Inland waters were recently recognized to be important sources of methane (CH₄) and carbon dioxide (CO₂) 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 CH₄ and CO₂ 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 CH₄ and a major fraction also emitted CO₂. The total CH₄ 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 CH₄ concentration was 3.8 ± 14.5 μm (range 0.03–92.1 μm ). The CO₂ 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 CO₂ 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 CH₄ yr^?1 and 22.0 Tg CO₂ yr^?1 from India's inland waters. When expressed as CO₂ equivalents, this amounts to 75 Tg CO₂ equivalents yr^?1 (53–98 Tg CO₂ equivalents yr^?1; ± 1 SD)_, with CH₄ 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.     

Environmental factors controlling temporal and spatial variability in the soil‐atmosphere exchange of CO2, CH4 and N2O from an Australian subtropical rainforest

The temporal variations in CO₂, CH₄ and N₂O fluxes were measured over two consecutive years from February 2007 to March 2009 from a subtropical rainforest in south‐eastern Queensland, Australia, using an automated sampling system. A concurrent study using an additional 30 manual chambers examined the spatial variability of emissions distributed across three nearby remnant rainforest sites with similar vegetation and climatic conditions. Interannual variation in fluxes of all gases over the 2 years was minimal, despite large discrepancies in rainfall, whereas a pronounced seasonal variation could only be observed for CO₂ fluxes. High infiltration, drainage and subsequent high soil aeration under the rainforest limited N₂O loss while promoting substantial CH₄ uptake. The average annual N₂O loss of 0.5 ± 0.1 kg N₂O‐N ha^?1 over the 2‐year measurement period was at the lower end of reported fluxes from rainforest soils. The rainforest soil functioned as a sink for atmospheric CH₄ throughout the entire 2‐year period, despite periods of substantial rainfall. A clear linear correlation between soil moisture and CH₄ uptake was found. Rates of uptake ranged from greater than 15 g CH₄‐C ha^?1 day^?1 during extended dry periods to less than 2–5 g CH₄‐C ha^?1 day^?1 when soil water content was high. The calculated annual CH₄ uptake at the site was 3.65 kg CH₄‐C ha^?1 yr^?1. This is amongst the highest reported for rainforest systems, reiterating the ability of aerated subtropical rainforests to act as substantial sinks of CH₄. The spatial study showed N₂O fluxes almost eight times higher, and CH₄ uptake reduced by over one‐third, as clay content of the rainforest soil increased from 12% to more than 23%. This demonstrates that for some rainforest ecosystems, soil texture and related water infiltration and drainage capacity constraints may play a more important role in controlling fluxes than either vegetation or seasonal variability.     

Spatio-temporal variability and environmental controls of methane fluxes at the forest–tundra ecotone in the Fennoscandian mountains

We report on temporal and spatial variability in net methane (CH₄) fluxes measured during the thaw period of 1999 and 2000 at three study sites along a c. 8° latitudinal gradient in the Fennoscandian mountain range and across the mountain birch‐tundra ecotone. All of the sites studied here were underlain by well‐drained mesic soils. In addition, we conducted warming experiments in the field to simulate future climate change. Our results show significant CH₄ uptake at mesic sites spanning the forest‐tundra ecotone: on average 0.031 and 0.0065 mg CH₄ m^?2 h^?1 during the 1999 and 2000 thaw periods, respectively, in Abisko (Sweden), and 0.019 and 0.032 mg CH₄ m^?2 h^?1 during 2000 in Dovrefjell and Joatka (Norway), respectively. These values were both temporally and spatially highly variable, and multiple regression analysis of data from Abisko showed no consistent relationship with soil‐moisture status and temperature. Also, there was no consistent difference in CH₄ fluxes between forest and tundra plots; our data, therefore, provide no support for the hypothesis that conversion of tundra to mountain birch forest, or vice versa, would result in a systematic change in the magnitude or direction of net CH₄ fluxes in this region. Experimental warming treatments were associated with a 2.4 °C increase in soil temperatures (5 cm depth) in 1999 in Abisko, but no consistent soil warming was noted at any of the three field locations during 2000. In spite of this, there were significant treatment effects, principally early during the thaw period, with increased CH₄ uptake compared with control (ambient) plots. These results suggest that direct effects of air warming on vegetation processes (e.g. transpiration, root exudation and nutrient assimilation) can influence CH₄ fluxes even in predominantly methanotrophic environments. We conclude that net CH₄ oxidation is significant in these cold, mesic soils and could be strengthened in an environmental change scenario involving a combination of (i) an increase in the length of the thaw period and (ii) increased mean temperatures during this period in combination with decreased soil‐moisture content.