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.     

Polygonal tundra geomorphological change in response to warming alters future CO2 and CH4 flux on the Barrow Peninsula

The landscape of the Barrow Peninsula in northern Alaska is thought to have formed over centuries to millennia, and is now dominated by ice‐wedge polygonal tundra that spans drained thaw‐lake basins and interstitial tundra. In nearby tundra regions, studies have identified a rapid increase in thermokarst formation (i.e., pits) over recent decades in response to climate warming, facilitating changes in polygonal tundra geomorphology. We assessed the future impact of 100 years of tundra geomorphic change on peak growing season carbon exchange in response to: (i) landscape succession associated with the thaw‐lake cycle; and (ii) low, moderate, and extreme scenarios of thermokarst pit formation (10%, 30%, and 50%) reported for Alaskan arctic tundra sites. We developed a 30 × 30 m resolution tundra geomorphology map (overall accuracy:75%; Kappa:0.69) for our ~1800 km² study area composed of ten classes; drained slope, high center polygon, flat‐center polygon, low center polygon, coalescent low center polygon, polygon trough, meadow, ponds, rivers, and lakes, to determine their spatial distribution across the Barrow Peninsula. Land‐atmosphere CO₂ and CH₄ flux data were collected for the summers of 2006–2010 at eighty‐two sites near Barrow, across the mapped classes. The developed geomorphic map was used for the regional assessment of carbon flux. Results indicate (i) at present during peak growing season on the Barrow Peninsula, CO₂ uptake occurs at ‐902.3 10⁶gC‐CO₂ day^?1 (uncertainty using 95% CI is between ?438.3 and ?1366 10⁶gC‐CO₂ day^?1) and CH₄ flux at 28.9 10⁶gC‐CH₄ day^?1(uncertainty using 95% CI is between 12.9 and 44.9 10⁶gC‐CH₄ day^?1), (ii) one century of future landscape change associated with the thaw‐lake cycle only slightly alter CO₂ and CH₄ exchange, while (iii) moderate increases in thermokarst pits would strengthen both CO₂ uptake (?166.9 10⁶gC‐CO₂ day^?1) and CH₄ flux (2.8 10⁶gC‐CH₄ day^?1) with geomorphic change from low to high center polygons, cumulatively resulting in an estimated negative feedback to warming during peak growing season.     

Characterization of the carbon fluxes of a vegetated drained lake basin chronosequence on the Alaskan Arctic Coastal Plain

Greenhouse gas fluxes from vegetated drained lake basins have been largely unstudied, although these land features constitute up to 47% of the land cover in the Arctic Coastal Plain in northern Alaska. To describe current and to better predict future sink/source activity of the Arctic tundra, it is important to assess these vegetated drained lake basins with respect to the patterns of and controls on gross primary production (GPP), net ecosystem exchange, and ecosystem respiration (ER). We measured CO₂ fluxes and key environmental variables during the 2007 growing season (June through August) in 12 vegetated drained lake basins representing three age classes (young, drained about 50 years ago; medium, drained between 50 and 300 years ago; and old, drained between 300 and 2000 years ago, as determined by Hinkel et al., 2003) in the Arctic Coastal Plain. Young vegetated drained lake basins had both the highest average GPP over the summer (11.4 gCO₂ m^?2 day^?1) and the highest average summer ER (7.3 gCO₂ m^?2 day^?1), while medium and old vegetated drained lake basins showed lower and similar GPP (7.9 and 7.2 gCO₂ m^?2 day^?1, respectively), and ER (5.2 and 4 gCO₂ m^?2 day^?1, respectively). Productivity decreases with age as nutrients are locked up in living plant material and dead organic matter. However, we showed that old vegetated drained lakes basins maintained relatively high productivity because of the increased development of ice‐wedge polygons, the formation of ponds, and the re‐establishment of very productive species. Comparison of the seasonal CO₂ fluxes and concomitant environmental factors over this chronosequence provides the basis for better understanding the patterns and controls on CO₂ flux across the coastal plain of the North Slope of Alaska and for more accurately estimating current and future contribution of the Arctic to the global carbon budget.     

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.     

Direct and indirect climate change effects on carbon dioxide fluxes in a thawing boreal forest–wetland landscape

In the sporadic permafrost zone of northwestern Canada, boreal forest carbon dioxide (CO₂) fluxes will be altered directly by climate change through changing meteorological forcing and indirectly through changes in landscape functioning associated with thaw‐induced collapse‐scar bog (‘wetland’) expansion. However, their combined effect on landscape‐scale net ecosystem CO₂ exchange (NEE_LAND), resulting from changing gross primary productivity (GPP) and ecosystem respiration (ER), remains unknown. Here, we quantify indirect land cover change impacts on NEE_LAND and direct climate change impacts on modeled temperature‐ and light‐limited NEE_LAND of a boreal forest–wetland landscape. Using nested eddy covariance flux towers, we find both GPP and ER to be larger at the landscape compared to the wetland level. However, annual NEE_LAND (?20 g C m^?2) and wetland NEE (?24 g C m^?2) were similar, suggesting negligible wetland expansion effects on NEE_LAND. In contrast, we find non‐negligible direct climate change impacts when modeling NEE_LAND using projected air temperature and incoming shortwave radiation. At the end of the 21st century, modeled GPP mainly increases in spring and fall due to reduced temperature limitation, but becomes more frequently light‐limited in fall. In a warmer climate, ER increases year‐round in the absence of moisture stress resulting in net CO₂ uptake increases in the shoulder seasons and decreases during the summer. Annually, landscape net CO₂ uptake is projected to decline by 25 ± 14 g C m^?2 for a moderate and 103 ± 38 g C m^?2 for a high warming scenario, potentially reversing recently observed positive net CO₂ uptake trends across the boreal biome. Thus, even without moisture stress, net CO₂ uptake of boreal forest–wetland landscapes may decline, and ultimately, these landscapes may turn into net CO₂ sources under continued anthropogenic CO₂ emissions. We conclude that NEE_LAND changes are more likely to be driven by direct climate change rather than by indirect land cover change impacts.     

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.     

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.     

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.     

Climatic variability,hydrologic anomaly,and methane emission can turn productive freshwater marshes into net carbon sources

Freshwater marshes are well‐known for their ecological functions in carbon sequestration, but complete carbon budgets that include both methane (CH₄) and lateral carbon fluxes for these ecosystems are rarely available. To the best of our knowledge, this is the first full carbon balance for a freshwater marsh where vertical gaseous [carbon dioxide (CO₂) and CH₄] and lateral hydrologic fluxes (dissolved and particulate organic carbon) have been simultaneously measured for multiple years (2011–2013). Carbon accumulation in the sediments suggested that the marsh was a long‐term carbon sink and accumulated ~96.9 ± 10.3 (±95% CI) g C m^?2 yr^?1 during the last ~50 years. However, abnormal climate conditions in the last 3 years turned the marsh to a source of carbon (42.7 ± 23.4 g C m^?2 yr^?1). Gross ecosystem production and ecosystem respiration were the two largest fluxes in the annual carbon budget. Yet, these two fluxes compensated each other to a large extent and led to the marsh being a CO₂ sink in 2011 (?78.8 ± 33.6 g C m^?2 yr^?1), near CO₂‐neutral in 2012 (29.7 ± 37.2 g C m^?2 yr^?1), and a CO₂ source in 2013 (92.9 ± 28.0 g C m^?2 yr^?1). The CH₄ emission was consistently high with a three‐year average of 50.8 ± 1.0 g C m^?2 yr^?1. Considerable hydrologic carbon flowed laterally both into and out of the marsh (108.3 ± 5.4 and 86.2 ± 10.5 g C m^?2 yr^?1, respectively). In total, hydrologic carbon fluxes contributed ~23 ± 13 g C m^?2 yr^?1 to the three‐year carbon budget. Our findings highlight the importance of lateral hydrologic inflows/outflows in wetland carbon budgets, especially in those characterized by a flow‐through hydrologic regime. In addition, different carbon fluxes responded unequally to climate variability/anomalies and, thus, the total carbon budgets may vary drastically among years.     

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.     

Predicting long‐term carbon mineralization and trace gas production from thawing permafrost of Northeast Siberia

The currently observed Arctic warming will increase permafrost degradation followed by mineralization of formerly frozen organic matter to carbon dioxide (CO₂) and methane (CH₄). Despite increasing awareness of permafrost carbon vulnerability, the potential long‐term formation of trace gases from thawing permafrost remains unclear. The objective of the current study is to quantify the potential long‐term release of trace gases from permafrost organic matter. Therefore, Holocene and Pleistocene permafrost deposits were sampled in the Lena River Delta, Northeast Siberia. The sampled permafrost contained between 0.6% and 12.4% organic carbon. CO₂ and CH₄ production was measured for 1200 days in aerobic and anaerobic incubations at 4 °C. The derived fluxes were used to estimate parameters of a two pool carbon degradation model. Total CO₂ production was similar in Holocene permafrost (1.3 ± 0.8 mg CO₂‐C gdw^?1 aerobically, 0.25 ± 0.13 mg CO₂‐C gdw^?1 anaerobically) as in 34 000–42 000‐year‐old Pleistocene permafrost (1.6 ± 1.2 mg CO₂‐C gdw^?1 aerobically, 0.26 ± 0.10 mg CO₂‐C gdw^?1 anaerobically). The main predictor for carbon mineralization was the content of organic matter. Anaerobic conditions strongly reduced carbon mineralization since only 25% of aerobically mineralized carbon was released as CO₂ and CH₄ in the absence of oxygen. CH₄ production was low or absent in most of the Pleistocene permafrost and always started after a significant delay. After 1200 days on average 3.1% of initial carbon was mineralized to CO₂ under aerobic conditions while without oxygen 0.55% were released as CO₂ and 0.28% as CH₄. The calibrated carbon degradation model predicted cumulative CO₂ production over a period of 100 years accounting for 15.1% (aerobic) and 1.8% (anaerobic) of initial organic carbon, which is significantly less than recent estimates. The multiyear time series from the incubation experiments helps to more reliably constrain projections of future trace gas fluxes from thawing permafrost landscapes.     

Soil carbon and belowground carbon balance of a short‐rotation coppice: assessments from three different approaches

Uncertainty in soil carbon (C) fluxes across different land‐use transitions is an issue that needs to be addressed for the further deployment of perennial bioenergy crops. A large‐scale short‐rotation coppice (SRC) site with poplar (Populus) and willow (Salix) was established to examine the land‐use transitions of arable and pasture to bioenergy. Soil C pools, output fluxes of soil CO₂, CH₄, dissolved organic carbon (DOC) and volatile organic compounds, as well as input fluxes from litter fall and from roots, were measured over a 4‐year period, along with environmental parameters. Three approaches were used to estimate changes in the soil C. The largest C pool in the soil was the soil organic carbon (SOC) pool and increased after four years of SRC from 10.9 to 13.9 kg C m^?2. The belowground woody biomass (coarse roots) represented the second largest C pool, followed by the fine roots (Fr). The annual leaf fall represented the largest C input to the soil, followed by weeds and Fr. After the first harvest, we observed a very large C input into the soil from high Fr mortality. The weed inputs decreased as trees grew older and bigger. Soil respiration averaged 568.9 g C m^?2 yr^?1. Leaching of DOC increased over the three years from 7.9 to 14.5 g C m^?2. The pool‐based approach indicated an increase of 3360 g C m^?2 in the SOC pool over the 4‐year period, which was high when compared with the ?27 g C m^?2 estimated by the flux‐based approach and the ?956 g C m^?2 of the combined eddy‐covariance + biometric approach. High uncertainties were associated to the pool‐based approach. Our results suggest using the C flux approach for the assessment of the short‐/medium‐term SOC balance at our site, while SOC pool changes can only be used for long‐term C balance assessments.     

Effect of catchment characteristics on aquatic carbon export from a boreal catchment and its importance in regional carbon cycling

Inland waters transport and emit into the atmosphere large amounts of carbon (C), which originates from terrestrial ecosystems. The effect of land cover and land‐use practises on C export from terrestrial ecosystems to inland waters is not fully understood, especially in heterogeneous landscapes under human influence. We sampled for dissolved C species in five tributaries with well‐determined subcatchments (total size 174.5 km²), as well as in various points of two of the subcatchments draining to a boreal lake in southern Finland over a full year. Our aim was to find out how land cover and land‐use affect C export from the catchments, as well as CH₄ and CO₂ concentrations of the streams, and if the origin of C in stream water can be determined from proxies for quality of dissolved organic matter (DOM). We further estimated the gas evasion from stream surfaces and the role of aquatic fluxes in regional C cycling. The export rate of C from the terrestrial system through an aquatic conduit was 19.3 g C m^?2(catchment) yr^?1, which corresponds to 19% of the estimated terrestrial net ecosystem exchange of the catchment. Most of the C load to the recipient lake consisted of dissolved organic carbon (DOC, 6.1 ± 1.0 g C m^?2 yr^?1); the share of dissolved inorganic carbon (DIC) was much smaller (1.0 ± 0.2 g C m^?2 yr^?1). CO₂ and CH₄ emissions from stream and ditch surfaces were 7.0 ± 2.4 g C m^?2 yr^?1 and 0.1 ± 0.04 g C m^?2 yr^?1, respectively, C emissions being thus equal with C load to the lake. The proportion of peatland in the catchment and the drainage density of peatland increased DOC in streams, whereas the proportion of agricultural land in the catchment decreased it. The opposite was true for DIC. Drained peatlands were an important CH₄ source for streams.     

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.     

Integration of CO2 flux and remotely-sensed data for primary production and ecosystem respiration analyses in the Northern Great Plains: potential for quantitative spatial extrapolation

Aim Extrapolation of tower CO₂ fluxes will be greatly facilitated if robust relationships between flux components and remotely sensed factors are established. Long‐term measurements at five Northern Great Plains locations were used to obtain relationships between CO₂ fluxes and photosynthetically active radiation (Q), other on‐site factors, and Normalized Difference Vegetation Index (NDVI) from the SPOT VEGETATION data set. Location CO₂ flux data from the following stations and years were analysed: Lethbridge, Alberta 1998–2001; Fort Peck, MT 2000, 2002; Miles City, MT 2000–01; Mandan, ND 1999–2001; and Cheyenne, WY 1997–98. Results Analyses based on light‐response functions allowed partitioning net CO₂ flux (F) into gross primary productivity (P_g) and ecosystem respiration (R_e). Weekly averages of daytime respiration, γ_day, estimated from light responses were closely correlated with weekly averages of measured night‐time respiration, γ_night (R² 0.64 to 0.95). Daytime respiration tended to be higher than night‐time respiration, and regressions of γ_day on γ_night for all sites were different from 1 : 1 relationships. Over 13 site‐years, gross primary production varied from 459 to 2491 g CO₂ m^?2 year^?1, ecosystem respiration from 996 to 1881 g CO₂ m^?2 year^?1, and net ecosystem exchange from ?537 (source) to +610 g CO₂ m^?2 year^?1 (sink). Maximum daily ecological light‐use efficiencies, ?_d,max = P_g/Q, were in the range 0.014 to 0.032 mol CO₂ (mol incident quanta)^?1. Main conclusions Ten‐day average P_g was significantly more highly correlated with NDVI than 10‐day average daytime flux, P_d (R² = 0.46 to 0.77 for P_g‐NDVI and 0.05 to 0.58 for P_d‐NDVI relationships). Ten‐day average R_e was also positively correlated with NDVI, with R² values from 0.57 to 0.77. Patterns of the relationships of P_g and R_e with NDVI and other factors indicate possibilities for establishing multivariate functions allowing scaling‐up local fluxes to larger areas using GIS data, temporal NDVI, and other factors.     

Evaluation of terrestrial carbon cycle models for their response to climate variability and to CO2 trends

The purpose of this study was to evaluate 10 process‐based terrestrial biosphere models that were used for the IPCC fifth Assessment Report. The simulated gross primary productivity (GPP) is compared with flux‐tower‐based estimates by Jung et al. [Journal of Geophysical Research 116 (2011) G00J07] (JU11). The net primary productivity (NPP) apparent sensitivity to climate variability and atmospheric CO₂ trends is diagnosed from each model output, using statistical functions. The temperature sensitivity is compared against ecosystem field warming experiments results. The CO₂ sensitivity of NPP is compared to the results from four Free‐Air CO₂ Enrichment (FACE) experiments. The simulated global net biome productivity (NBP) is compared with the residual land sink (RLS) of the global carbon budget from Friedlingstein et al. [Nature Geoscience 3 (2010) 811] (FR10). We found that models produce a higher GPP (133 ± 15 Pg C yr^?1) than JU11 (118 ± 6 Pg C yr^?1). In response to rising atmospheric CO₂ concentration, modeled NPP increases on average by 16% (5–20%) per 100 ppm, a slightly larger apparent sensitivity of NPP to CO₂ than that measured at the FACE experiment locations (13% per 100 ppm). Global NBP differs markedly among individual models, although the mean value of 2.0 ± 0.8 Pg C yr^?1 is remarkably close to the mean value of RLS (2.1 ± 1.2 Pg C yr^?1). The interannual variability in modeled NBP is significantly correlated with that of RLS for the period 1980–2009. Both model‐to‐model and interannual variation in model GPP is larger than that in model NBP due to the strong coupling causing a positive correlation between ecosystem respiration and GPP in the model. The average linear regression slope of global NBP vs. temperature across the 10 models is ?3.0 ± 1.5 Pg C yr^?1 °C^?1, within the uncertainty of what derived from RLS (?3.9 ± 1.1 Pg C yr^?1 °C^?1). However, 9 of 10 models overestimate the regression slope of NBP vs. precipitation, compared with the slope of the observed RLS vs. precipitation. With most models lacking processes that control GPP and NBP in addition to CO₂ and climate, the agreement between modeled and observation‐based GPP and NBP can be fortuitous. Carbon–nitrogen interactions (only separable in one model) significantly influence the simulated response of carbon cycle to temperature and atmospheric CO₂ concentration, suggesting that nutrients limitations should be included in the next generation of terrestrial biosphere models.     

Autumn warming reduces the CO2 sink of a black spruce forest in interior Alaska based on a nine‐year eddy covariance measurement

Nine years (2003–2011) of carbon dioxide (CO₂) flux were measured at a black spruce forest in interior Alaska using the eddy covariance method. Seasonal and interannual variations in the gross primary productivity (GPP) and ecosystem respiration (RE) were associated primarily with air temperature: warmer conditions enhanced GPP and RE. Meanwhile, interannual variation in annual CO₂ balance was controlled predominantly by RE, and not GPP. During these 9 years of measurement, the annual CO₂ balance shifted from a CO₂ sink to a CO₂ source, with a 9‐year average near zero. The increase in autumn RE was associated with autumn warming and was mostly attributed to a shift in the annual CO₂ balance. The increase in autumn air temperature (0.22 °C yr^?1) during the 9 years of study was 15 times greater than the long‐term warming trend between 1905 and 2011 (0.015 °C yr^?1) due to decadal climate oscillation. This result indicates that most of the shifts in observed CO₂ fluxes were associated with decadal climate variability. Because the natural climate varies in a cycle of 10–30 years, a long‐term study covering at least one full cycle of decadal climate oscillation is important to quantify the CO₂ balance and its interaction with the climate.     

Environmental controls of temporal and spatial variability in CO2 and CH4 fluxes in a neotropical peatland

Tropical peatlands play an important role in the global storage and cycling of carbon (C) but information on carbon dioxide (CO₂) and methane (CH₄) fluxes from these systems is sparse, particularly in the Neotropics. We quantified short and long‐term temporal and small scale spatial variation in CO₂ and CH₄ fluxes from three contrasting vegetation communities in a domed ombrotrophic peatland in Panama. There was significant variation in CO₂ fluxes among vegetation communities in the order Campnosperma panamensis > Raphia taedigera > Cyperus. There was no consistent variation among sites and no discernible seasonal pattern of CH₄ flux despite the considerable range of values recorded (e.g. ?1.0 to 12.6 mg m^?2 h^?1 in 2007). CO₂ fluxes varied seasonally in 2007, being greatest in drier periods (300–400 mg m^?2 h^?1) and lowest during the wet period (60–132 mg m^?2 h^?1) while very high emissions were found during the 2009 wet period, suggesting that peak CO₂ fluxes may occur following both low and high rainfall. In contrast, only weak relationships between CH₄ flux and rainfall (positive at the C. panamensis site) and solar radiation (negative at the C. panamensis and Cyperus sites) was found. CO₂ fluxes showed a diurnal pattern across sites and at the Cyperus sp. site CO₂ and CH₄ fluxes were positively correlated. The amount of dissolved carbon and nutrients were strong predictors of small scale within‐site variability in gas release but the effect was site‐specific. We conclude that (i) temporal variability in CO₂ was greater than variation among vegetation communities; (ii) rainfall may be a good predictor of CO₂ emissions from tropical peatlands but temporal variation in CH₄ does not follow seasonal rainfall patterns; and (iii) diurnal variation in CO₂ fluxes across different vegetation communities can be described by a Fourier model.     

Measurement and modelling of CO2 flux from a drained fen peatland cultivated with reed canary grass and spring barley

Cultivation of bioenergy crops has been suggested as a promising option for reduction of greenhouse gas (GHG) emissions from arable organic soils (Histosols). Here, we report the annual net ecosystem exchange (NEE) fluxes of CO₂ as measured with a dynamic closed chamber method at a drained fen peatland grown with reed canary grass (RCG) and spring barley (SB) in a plot experiment (n = 3 for each cropping system). The CO₂ flux was partitioned into gross photosynthesis (GP) and ecosystem respiration (R_E). For the data analysis, simple yet useful GP and R_E models were developed which introduce plot‐scale ratio vegetation index as an active vegetation proxy. The GP model captures the effect of temperature and vegetation status, and the R_E model estimates the proportion of foliar biomass dependent respiration (R_fb) in the total R_E. Annual R_E was 1887 ± 7 (mean ± standard error, n = 3) and 1288 ± 19 g CO₂‐C m^?2 in RCG and SB plots, respectively, with R_fb accounting for 32 and 22% respectively. Total estimated annual GP was ?1818 ± 42 and ?1329 ± 66 g CO₂‐C m^?2 in RCG and SB plots leading to a NEE of 69 ± 36 g CO₂‐C m^?2 yr^?1 in RCG plots (i.e., a weak net source) and ?41 ± 47 g CO₂‐C m^?2 yr^?1 in SB plots (i.e., a weak net sink). Standard errors related to spatial variation were small (as shown above), but more significant uncertainties were related to the modelling approach for establishment of annual budgets. In conclusion, the bioenergy cropping system was not more favourable than the food cropping system when looking at the atmospheric CO₂ emissions during cultivation. However, in a broader GHG life‐cycle perspective, the lower fertilizer N input and the higher biomass yield in bioenergy cropping systems could be beneficial.     

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.