| Summary: | As the Arctic warms, the ~277 Pg of carbon stored in permafrost peatlands faces an uncertain fate. Arctic and Subarctic peatlands are likely to release more methane (CH4) as permafrost thaw releases formerly-frozen carbon, thaw-induced land subsidence and inundation lead to anaerobic conditions, and higher temperatures allow more rapid decomposition. In addition to these effects, CH4 and carbon dioxide (CO2) emissions may also change due to shifts in plant inputs and consequent changes in organic matter quality, but the exact relationships between organic matter and CH4 production are not well understood. In this study, we examined microbial CH4 and CO2 production and their relationship to organic matter chemistry in Stordalen Mire, a thawing Subarctic peatland in northern Sweden. We also used stable carbon isotopes (δ13C) of CH4 and CO2, and their apparent fractionation factor (αC), to examine the effect of thaw on the proportion of methanogenesis by hydrogenotrophic or acetoclastic pathways. At Stordalen, permafrost thaw causes dry, aerobic permafrost plateaus (palsas) to collapse and become inundated. These wet depressions are then colonized first by Sphagnum mosses and then by sedges as permafrost thaw and plant succession progress. In our study, we examined a chronosequence of sites with varying permafrost status and plant community composition. These sites included dry, intact palsas; recently-thawed collapsed palsa sinkholes; partially-thawed Sphagnum-dominated bogs; mostly-thawed poor fens with a combination of Sphagnum and tall sedges; and fully-thawed rich fens with mature stands of tall sedges and no Sphagnum. The changes in potential CH4 and CO2 production along the thaw progression were examined with anaerobic peat incubations, which were all performed with identical temperature and water saturation. These incubations showed increases in potential decomposition rates and CH4/CO2 production ratios along the thaw progression. Methanogenesis pathways also shifted from predominately hydrogenotrophic to ...
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