Long lasting greenhouse gas emissions beyond abrupt permafrost thaw event in permafrost peatlands

Abrupt permafrost thawing is expected to release large amounts of greenhouse gasses to the atmosphere, creating a positive feedback to climate warming. There is, however, still large uncertainty in the timing, duration, magnitude, and mechanisms controlling this process, which hampers accurate quantification of permafrost carbon climate feedback cycles. The current understanding supports that abrupt permafrost thaw will lead to surface inundation and create anaerobic landscapes, which dominantly produce methane during the decomposition process. Over time, natural succession and vegetation growth may decrease methane release and increase net carbon uptake. We investigated how rapid permafrost thawing and subsequent natural succession over time affect CO₂, CH₄, and N₂O release at a field site in northern Norway (69ᵒN), where recent abrupt degradation of permafrost created thaw ponds in palsa peat plateau-mire ecosystems. The site exhibits a natural gradient of permafrost thaw, which also corresponds to a strong hydrological gradient (i.e. dry peat plateau underlain by intact permafrost, seasonally inundated thaw slumps, thaw ponds, and natural succession ponds covered by sphagnum and sedges). Since 2017, we used a range of manual and automated techniques to measure changes in vegetation, soil and water microclimate, biogeochemistry, and soil CO₂, CH₄, and N₂O concentrations and fluxes across the permafrost thaw gradient. In the three-year observations, we show that abrupt permafrost thaw and land surface subsidence – both intermediate slumping and pond formation – increase net annual carbon loss. Permafrost thaw accelerated CO₂ release greatly in thaw slumps (177.5 gCO₂ m^-2) compared to intact permafrost peat plateau (59.0 gCO₂ m^-2). During the growing season, peat plateau was a small sink of atmospheric CH₄ (-2.5 gCH₄ m^-2), whereas permafrost thaw slumping and pond formation increased CH₄ release dramatically (ranging from 9.7 to 36.1 gCH₄ m^-2). Furthermore, CH₄ release continues to increase even in natural succession pond likely due to aerenchyma transport of CH₄ from deeper soil. The overall N₂O release was negligeable except in the bare soil peat plateau. The net radiative forcing of ecosystem carbon balance will depend on the carbon uptake from the natural succession of vegetation, but we show that greenhouse gas emissions continue to increase beyond abrupt permafrost thaw event towards natural succession.