Impacts of Changes in Winter Precipitation on C Stocks and Fluxes in Arctic Tussock Tundra

The alteration of winter precipitation patterns in Arctic regions represents a potentially important climate forcing agent. However, climate/carbon (C) cycle forcing feedbacks from Arctic regions remain largely unresolved due to uncertainties in the strength, form (CO2 and CH4), direction and timing of ecosystem C fluxes under future precipitation scenarios. I combined C flux measurements and soil organic carbon (SOC) inventories with stable isotope and radioisotope methods in a multi-year, multi-level snow manipulation experiment in Arctic tundra to investigate: i) the rate at which permafrost C will become available for decomposition and will be released relative to ecosystem C inputs under future precipitation scenarios, ii) the magnitude, form and direction of derived climate/C-cycle feedbacks, and iii) the mechanisms driving long-term impacts of changes in winter precipitation on Arctic tundra C budget and fluxes. Results indicated the potential of Arctic tundra to become a transient C source through accelerated soil organic carbon (SOC) mineralization rates under future precipitation scenarios, but also to act as an additional long-term C sink with persistent increases in winter precipitation, as recently thawed SOC may remain largely immobilized over decades under thaw-induced near-water saturated conditions. This additional C sink however, may come at the cost of a substantial positive feedback on climate derived from increases in the net CH4 source strength of Arctic tundra, as warmer and wetter active layer stimulate CH4 production above CH4 oxidation further subsidized by enhanced plant-mediated transport associated to transitions in supported vegetation over the course of progressive permafrost degradation. Results suggested that much of current divergence among model predicted Arctic climate/C-cycle feedbacks may stem from inaccurate representations of the sensitivity of both physical and biological processes to changes in winter precipitation over time. Findings presented here indicate that projected precipitation scenarios will drive the Arctic tundra C budget and shape the radiative forcing from Arctic regions, critically affecting future climate

Permafrost Carbon: Progress on Understanding Stocks and Fluxes Across Northern Terrestrial Ecosystems

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 CO2 and 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 CO2 sink with lower net CO2 uptake toward higher latitudes, excluding wildfire emissions. For 2002–2014, the strongest CO2 sink was located in western Canada (median: −52 g C m−2 y−1) and smallest sinks in Alaska, Canadian tundra, and Siberian tundra (medians: −5 to −9 g C m−2 y−1). Eurasian regions had the largest median wetland methane fluxes (16–18 g CH4 m−2 y−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 CO2 and 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.</jats:p