Abrupt Thaw in a Finnish Palsa: Potential CH ₄ Production Driven by Vegetation Adaptation in the Transition From Permafrost to Post‐Thaw Soils

Abstract Permafrost peatlands store substantial amounts of carbon, though persistence of this soil carbon is unknown in a rapidly warming Arctic. To investigate potential carbon production from soils at different stages of permafrost degradation, we incubated soils from a palsa mire in northern Fennoscandia. Three soil horizons from four thaw stages were included within the transect, beginning with intact permafrost and ending in an established post‐thaw wetland. Samples were incubated anaerobically for a year at different temperatures (4°C, 20°C) with the aim of investigating drivers of carbon degradation rates. Additional subsamples from the intact palsa were incubated under aerobic conditions, or inoculated with thermokarst pond water to further explore thaw processes on soil. Total CO 2 and CH 4 produced ranged from 9,910 ± 626 (from the surface peat of the established post‐thaw wetland, at 20°C) to 1,921 ± 126 μg C g −1 DW (from the intermediate thaw stage of the palsa permafrost, incubated at 20°C). The CH 4 temperature sensitivity was markedly higher in permafrost soils, with Q 10 s more than four times larger than that of the active layer (active layer average: 1.7 ± 1.6, permafrost average: 8.4 ± 5). Methanogenesis generally increased with thaw, but the largest increase of cumulative methane production was between the wetland thaw stages (from 633 to 2,880 μg CH 4 ‐C g −1 DW), where graminoids colonized the post‐thaw environment. This uptick in CH 4 production 30+ years after post‐thaw wetland establishment implies that increases in CH 4 production are largely due to vegetation inputs rather than thawed permafrost carbon contributions.