The Northern high latitudes are warming twice as fast as the global average and permafrost has become vulnerable to thaw. Changes to the environment during thaw leads to shifts in microbial communities and their associated functions, such as carbon emissions. However, little is known regarding the processes structuring the identity and abundance (i.e. assembly) of pre- and post-thaw communities. Deterministic processes are driven by abiotic and biotic selection pressures and stochastic processes, which include inherent randomness, are unpredictable. We characterized microbial community assembly during permafrost thaw using in situ observation and soil incubations. We hypothesized (1) in situ surface active later and permafrost communities were dominated by deterministic assembly due to selection and (2) transition zone (i.e. the permafrost-active layer interface) communities were dominated by stochastic assembly due to recent shifts in the abiotic environment. Furthermore, we hypothesized an increase in stochastic processes at all depths after lab induced thaw compared to in situ assembly. We tested these hypotheses using 16S and ITS amplicon sequencing across replicate soil depth profiles from the Storflaket Mire in Abisko, Sweden where permafrost thaw has occurred over the past decade. Null modeling was used to determine the dominant assembly processes at eight depths, encompassing active layer, transition zone, and permafrost soils. We found in situ active layer soils were dominated by deterministic processes and a shift towards stochastic processes in transition zone and permafrost soils. This suggests environmental selection predominately structures active layer communities whereas processes that include inherent randomness structure intact and newly thawed permafrost. Identification of dominant microbial community assembly processes has the potential to improve our understanding of the ecological impact of permafrost thaw and the permafrost–climate feedback.
