Northern peatlands have accumulated large stocks of organic carbon (C) and nitrogen (N), but their spatial distribution and vulnerability to climate warming remain uncertain. Here, we used machine-learning techniques with extensive peat core data (n > 7,000) to create observation-based maps of northern peatland C and N stocks, and to assess their response to warming and permafrost thaw. We estimate that northern peatlands cover 3.7 ± 0.5 million km2 and store 415 ± 150 Pg C and 10 ± 7 Pg N. Nearly half of the peatland area and peat C stocks are permafrost affected. Using modeled global warming stabilization scenarios (from 1.5 to 6 °C warming), we project that the current sink of atmospheric C (0.10 ± 0.02 Pg C⋅y−1) in northern peatlands will shift to a C source as 0.8 to 1.9 million km2 of permafrost-affected peatlands thaw. The projected thaw would cause peatland greenhouse gas emissions equal to ∼1% of anthropogenic radiative forcing in this century. The main forcing is from methane emissions (0.7 to 3 Pg cumulative CH4-C) with smaller carbon dioxide forcing (1 to 2 Pg CO2-C) and minor nitrous oxide losses. We project that initial CO2-C losses reverse after ∼200 y, as warming strengthens peatland C-sinks. We project substantial, but highly uncertain, additional losses of peat into fluvial systems of 10 to 30 Pg C and 0.4 to 0.9 Pg N. The combined gaseous and fluvial peatland C loss estimated here adds 30 to 50% onto previous estimates of permafrost-thaw C losses, with southern permafrost regions being the most vulnerable.

Recent findings on large nitrous oxide (N 2 O) emissions from permafrost peatlands have shown that tundra soils can support high N 2 O release, which is on the contrary to what was thought previously. However, field data on this topic have been very limited, and the spatial and temporal extent of the phenomenon has not been known. To address this question, we studied N 2 O dynamics in two types of subarctic permafrost peatlands, a peat plateau in Russia and three palsa mires in Finland, including also adjacent upland soils. The peatlands studied have surfaces that are uplifted by frost (palsas and peat plateaus) and partly unvegetated as a result of wind erosion and frost action. Unvegetated peat surfaces with high N 2 O emissions were found from all the studied peatlands. Very high N 2 O emissions were measured from peat circles at the Russian site (1.40 AE 0.15 g N 2 O m À2 yr À1 ). Elevated, sparsely vegetated peat mounds at the same site had significantly lower N 2 O release. The N 2 O emissions from bare palsa surfaces in Northern Finland were highly variable but reached high rates, similar to those measured from the peat circles. All the vegetated soils studied had negligible N 2 O release. At the bare peat surfaces, the large N 2 O emissions were supported by the absence of plant N uptake, the low C : N ratio of the peat, the relatively high gross N mineralization rate and favourable moisture content, together increasing availability of mineral N for N 2 O production. We hypothesize that frost heave is crucial for high N 2 O emissions, since it lifts the peat above the water table, increasing oxygen availability and making it vulnerable to the the physical processes that may remove the vegetation cover. In the future, permafrost thawing may change the distribution of wet and dry surfaces in permafrost peatlands, which will affect N 2 O emissions.

Rapidly rising temperatures in the Arctic might cause a greater release of greenhouse gases (GHGs) to the atmosphere. To study the effect of warming on GHG dynamics, we deployed open-top chambers in a subarctic tundra site in Northeast European Russia. We determined carbon dioxide (CO ), methane (CH ), and nitrous oxide (N O) fluxes as well as the concentration of those gases, inorganic nitrogen (N) and dissolved organic carbon (DOC) along the soil profile. Studied tundra surfaces ranged from mineral to organic soils and from vegetated to unvegetated areas. As a result of air warming, the seasonal GHG budget of the vegetated tundra surfaces shifted from a GHG sink of -300 to -198 g CO -eq m to a source of 105 to 144 g CO -eq m . At bare peat surfaces, we observed increased release of all three GHGs. While the positive warming response was dominated by CO , we provide here the first in situ evidence of increasing N O emissions from tundra soils with warming. Warming promoted N O release not only from bare peat, previously identified as a strong N O source, but also from the abundant, vegetated peat surfaces that do not emit N O under present climate. At these surfaces, elevated temperatures had an adverse effect on plant growth, resulting in lower plant N uptake and, consequently, better N availability for soil microbes. Although the warming was limited to the soil surface and did not alter thaw depth, it increased concentrations of DOC, CO and CH in the soil down to the permafrost table. This can be attributed to downward DOC leaching, fueling microbial activity at depth. Taken together, our results emphasize the tight linkages between plant and soil processes, and different soil layers, which need to be taken into account when predicting the climate change feedback of the Arctic.

Wetlands are the single largest natural source of atmospheric methane (CH ), a greenhouse gas, and occur extensively in the northern hemisphere. Large discrepancies remain between "bottom-up" and "top-down" estimates of northern CH emissions. To explore whether these discrepancies are due to poor representation of nongrowing season CH emissions, we synthesized nongrowing season and annual CH flux measurements from temperate, boreal, and tundra wetlands and uplands. Median nongrowing season wetland emissions ranged from 0.9 g/m in bogs to 5.2 g/m in marshes and were dependent on moisture, vegetation, and permafrost. Annual wetland emissions ranged from 0.9 g m year in tundra bogs to 78 g m year in temperate marshes. Uplands varied from CH sinks to CH sources with a median annual flux of 0.0 ± 0.2 g m year . The measured fraction of annual CH emissions during the nongrowing season (observed: 13% to 47%) was significantly larger than that was predicted by two process-based model ensembles, especially between 40° and 60°N (modeled: 4% to 17%). Constraining the model ensembles with the measured nongrowing fraction increased total nongrowing season and annual CH emissions. Using this constraint, the modeled nongrowing season wetland CH flux from >40° north was 6.1 ± 1.5 Tg/year, three times greater than the nongrowing season emissions of the unconstrained model ensemble. The annual wetland CH flux was 37 ± 7 Tg/year from the data-constrained model ensemble, 25% larger than the unconstrained ensemble. Considering nongrowing season processes is critical for accurately estimating CH emissions from high-latitude ecosystems, and necessary for constraining the role of wetland emissions in a warming climate.

No abstract

Permafrost in the Arctic is thawing, exposing large carbon and nitrogen stocks for decomposition. Gaseous carbon release from Arctic soils due to permafrost thawing is known to be substantial, but growing evidence suggests that Arctic soils may also be relevant sources of nitrous oxide (N 2 O). Here we show that N 2 O emissions from subarctic peatlands increase as the permafrost thaws. In our study, the highest postthaw emissions occurred from bare peat surfaces, a typical landform in permafrost peatlands, where permafrost thaw caused a fivefold increase in emissions (0.56 ± 0.11 vs. 2.81 ± 0.6 mg N 2 O m −2 d −1 ). These emission rates match those from tropical forest soils, the world's largest natural terrestrial N 2 O source. The presence of vegetation, known to limit N 2 O emissions in tundra, did decrease (by ∼90%) but did not prevent thaw-induced N 2 O release, whereas waterlogged conditions suppressed the emissions. We show that regions with high probability for N 2 O emissions cover one-fourth of the Arctic. Our results imply that the Arctic N 2 O budget will depend strongly on moisture changes, and that a gradual deepening of the active layer will create a strong noncarbon climate change feedback.

The regional variability in tundra and boreal carbon dioxide (CO2) fluxes can be high, complicating efforts to quantify sink‐source patterns across the entire region. Statistical models are increasingly used to predict (i.e., upscale) CO2 fluxes across large spatial domains, but the reliability of different modeling techniques, each with different specifications and assumptions, has not been assessed in detail. Here, we compile eddy covariance and chamber measurements of annual and growing season CO2 fluxes of gross primary productivity (GPP), ecosystem respiration (ER), and net ecosystem exchange (NEE) during 1990–2015 from 148 terrestrial high‐latitude (i.e., tundra and boreal) sites to analyze the spatial patterns and drivers of CO2 fluxes and test the accuracy and uncertainty of different statistical models. CO2 fluxes were upscaled at relatively high spatial resolution (1 km2) across the high‐latitude region using five commonly used statistical models and their ensemble, that is, the median of all five models, using climatic, vegetation, and soil predictors. We found the performance of machine learning and ensemble predictions to outperform traditional regression methods. We also found the predictive performance of NEE‐focused models to be low, relative to models predicting GPP and ER. Our data compilation and ensemble predictions showed that CO2 sink strength was larger in the boreal biome (observed and predicted average annual NEE −46 and −29 g C m−2 yr−1, respectively) compared to tundra (average annual NEE +10 and −2 g C m−2 yr−1). This pattern was associated with large spatial variability, reflecting local heterogeneity in soil organic carbon stocks, climate, and vegetation productivity. The terrestrial ecosystem CO2 budget, estimated using the annual NEE ensemble prediction, suggests the high‐latitude region was on average an annual CO2 sink during 1990–2015, although uncertainty remains high.

We report here the carbon dioxide (CO₂) budget of a 98.6 km² subarctic tundra area in northeast European Russia based on measurements at two different scales and two independent upscaling approaches. Plot-scale measurements (chambers on terrestrial surfaces, gas gradient method and bubble collectors on lakes) were carried out from July 2007 to October 2008. The landscape-scale eddy covariance (EC) measurements covered the snow-free period of 2008. The annual net ecosystem exchange (NEE) of different land cover types ranged from −251 to 84 g C m⁻². Leaf area index (LAI) was an excellent predictor of the spatial variability in gross photosynthesis (GP), NEE and ecosystem respiration (ER). The plot-scale CO₂ fluxes were first scaled up to the EC source area and then to the whole study area using two data sets: a land cover classification and a LAI map, both based on field data and a 2.4 m pixel-sized QuickBird satellite image. The good agreement of the CO₂ balances for the EC footprint based on the different measuring techniques (−105 to −81 g C m⁻² vs. −79 g C m⁻²; growing season 2008) justified the integration of the plot-scale measurements over the larger area. The regional CO₂ balance based on area-integrated plot-scale measurements was −41 or −79 g C m⁻² yr⁻¹ according to the two upscaling methods, the land cover classification and the LAI map, respectively. Due to the heterogeneity of tundra, the effect of climate change on CO₂ uptake will vary strongly according to the land cover type and, moreover, likely changes in their relative coverage in the future will have great impact on the regional CO₂ balance