Abrupt thaw processes linked to enhanced intermediate C cycle steps within palsa soil mesocosms: a metagenomic analysis

Permafrost underlies 25% of the land surface area of the northern hemisphere and stores approximately a third of the world's organic soil carbon (C). When permafrost thaws, organic C that was frozen in a suspended state of decomposition rejoins the active layer and can be respired by microbial organisms within the soil as CO₂ or CH₄. As climate warming advances, permafrost thaw is likely to occur more within abrupt (seasons to decades) timelines as opposed to the generally better understood gradual thaw timelines (decades to centuries). Abrupt timelines increase C emissions over a shorter time scale. This increase in C respiration can be further spurred by the reintroduction of nutrients that were frozen in the permafrost, alongside the soil C. In this study, 1m intact soil cores were collected from a palsa in Northern Finland, and incubated in ex-situ mesocosms for 12 weeks with continuous GHG production measurements. In tandem, subsamples of the soil cores were collected pre- and post- simulated abrupt and gradual thaw scenarios for metagenomic analysis. The coupling of these methods revealed a significant increase of GHG production in the abrupt thaw simulation, as measured by the mesocosm incubations. In the permafrost horizon, this was coupled with a shift to an increase of activity of the intermediate C cycle steps leading to respiration. Additionally, a large taxonomic shift was observed in the permafrost microbial community structure when comparing samples before and after the thaw simulations. Gene abundances associated with nitrogen cycling increased in the abrupt thaw simulation, while there was little discernible change in the Fe and S cycling dynamics pre- and post- thaw. This multidisciplinary approach lays groundwork for our evolving understanding of abrupt permafrost thaw and emphasizes the differences in C cycling strategies microbial communities utilize in abrupt and gradual thaw timescales.