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Significant progress in permafrost carbon science made over the past decades include the identification of vast permafrost carbon stocks, the development of new pan‐Arctic permafrost maps, an increase in terrestrial measurement sites for CO2and methane fluxes, and important factors affecting carbon cycling, including vegetation changes, periods of soil freezing and thawing, wildfire, and other disturbance events. Process‐based modeling studies now include key elements of permafrost carbon cycling and advances in statistical modeling and inverse modeling enhance understanding of permafrost region C budgets. By combining existing data syntheses and model outputs, the permafrost region is likely a wetland methane source and small terrestrial ecosystem CO2sink with lower net CO2uptake toward higher latitudes, excluding wildfire emissions. For 2002–2014, the strongest CO2sink was located in western Canada (median: −52 g C m−2y−1) and smallest sinks in Alaska, Canadian tundra, and Siberian tundra (medians: −5 to −9 g C m−2y−1). Eurasian regions had the largest median wetland methane fluxes (16–18 g CH4m−2y−1). Quantifying the regional scale carbon balance remains challenging because of high spatial and temporal variability and relatively low density of observations. More accurate permafrost region carbon fluxes require: (a) the development of better maps characterizing wetlands and dynamics of vegetation and disturbances, including abrupt permafrost thaw; (b) the establishment of new year‐round CO2and methane flux sites in underrepresented areas; and (c) improved models that better represent important permafrost carbon cycle dynamics, including non‐growing season emissions and disturbance effects. Climate change and the consequent thawing of permafrost threatens to transform the permafrost region from a carbon sink into a carbon source, posing a challenge to global climate goals. Numerous studies over the past decades have identified important factors affecting carbon cycling, including vegetation changes, periods of soil freezing and thawing, wildfire, and other disturbance events. Overall, studies show high wetland methane emissions and a small net carbon dioxide sink strength over the terrestrial permafrost region but results differ among modeling and upscaling approaches. Continued and coordinated efforts among field, modeling, and remote sensing communities are needed to integrate new knowledge from observations to modeling and predictions and finally to policy. Rapid warming of northern permafrost region threatens ecosystems, soil carbon stocks, and global climate targetsLong‐term observations show importance of disturbance and cold season periods but are unable to detect spatiotemporal trends in C fluxCombined modeling and syntheses show the permafrost region is a small terrestrial CO2sink with large spatial variability and net CH4source Rapid warming of northern permafrost region threatens ecosystems, soil carbon stocks, and global climate targets Long‐term observations show importance of disturbance and cold season periods but are unable to detect spatiotemporal trends in C flux Combined modeling and syntheses show the permafrost region is a small terrestrial CO2sink with large spatial variability and net CH4source

Thermokarst can rapidly mobilize vast amounts of sediment, solutes, and organic carbon previously maintained in frozen soils to inland waters. Streams provide a critical pathway for transforming these materials into carbon dioxide (CO2) and methane (CH4), yet the direct effects of thermokarst on fluvial C gas efflux from streams to the atmosphere are largely unknown. Working on the Peel Plateau in the western Canadian Arctic, we show that CO2efflux in rill runoff thaw streams (runoff) within retrogressive thaw slumps (RTSs) was four times greater than in adjacent streams and contributed modestly but disproportionately to efflux at the landscape scale. In contrast, CH4efflux was generally greater in adjacent streams than in RTS runoff and, overall, was within the range of values reported for other northern streams. While RTS occurrence was a primary driver of CO2efflux, CH4efflux was more strongly associated with conditions reflective of biological activity. Transects downstream of two RTSs revealed that CH4consistently and rapidly degassed to the atmosphere, while elevated CO2was sustained downstream of one RTS feature. At the watershed scale, streams adjacent to RTSs rather than runoff streams within RTSs dominated fluvial CO2and CH4efflux. Intensifying thermokarst activity in the western Canadian Arctic will likely amplify contributions from runoff streams in RTSs to watershed‐scale fluvial C gas efflux. Rapid Arctic warming is thawing perennially frozen ground (permafrost) and driving terrain subsidence in ice‐rich areas (thermokarst). In glaciated permafrost landscapes, thermokarst features called retrogressive thaw slumps, which are caused by thawing of ice‐rich permafrost, release sediment, solutes, and organic materials that have been preserved in permafrost for millennia. Following thaw, permafrost carbon can rapidly transform into the greenhouse gas carbon dioxide and methane in streams. Yet the effects of thermokarst on fluvial gas flux are largely unknown. Working on the Peel Plateau in northwest Canada, we found that carbon dioxide efflux to the atmosphere from runoff draining from thaw slumps was four times greater than in streams adjacent to thaw slumps. In contrast, methane efflux in adjacent streams was twice that in runoff streams and both were similar to streams in other northern regions. While thaw slump occurrence was a primary driver of carbon dioxide efflux, methane efflux was more strongly associated with conditions reflective of biological activity. Generally, both carbon dioxide and methane were rapidly degassed downstream. While efflux of carbon dioxide and methane in thaw slumps were a small component of watershed‐scale fluvial C gas efflux, these fluxes will likely increase as regional thermokarst activity accelerates. CO2efflux in thaw slump runoff streams was four times greater than in adjacent streams, but a small part of watershed‐scale effluxCH4efflux was generally small in runoff streams, variable in adjacent streams, and associated with conditions reflecting biological activityElevated CO2partial pressures persisted downstream of a large thaw slump but dissipated downstream of a smaller thaw slump

Northern high‐latitude deltas are hotspots of biogeochemical processing, terrestrial‐aquatic connectivity, and, in Alaska's Yukon‐Kuskokwim Delta (YKD), tundra wildfire. Yet, wildfire effects on aquatic biogeochemistry remain understudied in northern delta regions, thus limiting a more comprehensive understanding of high latitude biogeochemical cycles. In this study, we assess wildfire impacts on summertime aquatic biogeochemistry in YKD tundra using a multi‐year (2015–2019) data set of water chemistry measurements (n= 406) from five aquatic environments: peat plateau ponds, fen ponds, fen channels, lakes, and streams. We aimed to (a) characterize variation in hydrochemistry among aquatic environments; (b) determine wildfire effects on hydrochemistry; and (c) assess post‐fire multi‐year patterns in hydrochemistry in lakes (lower terrestrial‐freshwater connectivity) and fen ponds (higher connectivity). Variation in hydrochemistry among environments was more strongly associated with watershed characteristics (e.g., terrestrial‐aquatic connectivity) than wildfire. However, certain hydrochemical constituents showed consistent wildfire effects. Decreases in dissolved organic carbon (DOC) and CO2, and increases in pH, specific conductance, NH4+, and NO3−indicate that, by combusting soil organic matter, wildfire reduces organics available for hydrologic transport and microbial respiration, and mobilizes nitrogen into freshwaters. Multi‐year post‐fire variation in specific conductance, DOC, and CO2in lakes and fen ponds suggest that watershed characteristics underlie ecosystem response and recovery to wildfire in the YKD. Together, these results indicate that increasing tundra wildfire occurrence at northern high latitudes could drive multi‐year shifts toward stronger aquatic inorganic nutrient cycling, and that variation in terrain characteristics is likely to underlie wildfire effects on aquatic ecosystems across broader scales. Warming at northern high latitudes is expected to increase wildfire activity, permafrost thaw, and hydrologic connectivity. Yet, contrasting evidence for wildfire effects on aquatic biogeochemistry hinders a more concrete understanding of potential future changes to northern ecosystems, especially in delta environments, where land‐freshwater linkages are strong. To address this, we made ∼400 measurements of hydrochemistry within the Yukon‐Kuskokwim Delta, Alaska, following high wildfire activity in 2015. Variation in hydrochemistry among five aquatic environments—peat plateau ponds, fen ponds, fen channels, lakes, and streams—was more strongly associated with watershed characteristics (e.g., land‐freshwater linkages) than wildfire. However, wildfire consistently affected some parameters. For instance, dissolved organic carbon and CO2were lower in burned watersheds, likely from combustion of organic matter, whereas, pH, specific conductance, and nitrogen were higher, reflecting solute release from combusted organics. Multi‐year measurements (2015–2019) allowed us to assess post‐fire variation in hydrochemistry among lakes and fen ponds, and indicate that ecosystem response and recovery to wildfire depend on watershed characteristics. Results indicate that increasing tundra wildfire at northern high latitudes could drive multi‐year shifts toward stronger aquatic inorganic nutrient cycling. Across broader spatiotemporal scales, variation in terrain characteristics is likely to underlie wildfire effects on aquatic ecosystems. Variation in hydrochemistry across tundra aquatic environments was more strongly coupled to physiographic factors than to wildfire effectsWildfire generally reduced dissolved organic carbon (DOC) and CO2, and enhanced ion mobility and nitrogen cycling, but effects variedWildfire effects on some hydrochemical constituents persisted beyond 3 years (e.g., DOC, CO2) and were brief for others (conductivity) Variation in hydrochemistry across tundra aquatic environments was more strongly coupled to physiographic factors than to wildfire effects Wildfire generally reduced dissolved organic carbon (DOC) and CO2, and enhanced ion mobility and nitrogen cycling, but effects varied Wildfire effects on some hydrochemical constituents persisted beyond 3 years (e.g., DOC, CO2) and were brief for others (conductivity)