The transition from stochastic to deterministic bacterial community assembly during permafrost thaw succession

The Northern high latitudes are warming twice as fast as the global average, andpermafrost has become vulnerable to thaw. Changes to the environment duringthaw leads to shifts in microbial communities and their associated functions, suchas greenhouse gas emissions. Understanding the ecological processes that structurethe identity and abundance (i.e., assembly) of pre- and post-thaw communities mayimprove predictions of the functional outcomes of permafrost thaw. We characterizedmicrobial community assembly during permafrost thaw using in situ observationsand a laboratory incubation of soils from the Storflaket Mire in Abisko, Sweden,where permafrost thaw has occurred over the past decade. In situ observationsindicated that bacterial community assembly was driven by randomness (i.e., stochasticprocesses) immediately after thaw with drift and dispersal limitation being thedominant processes. As post-thaw succession progressed, environmentally driven(i.e., deterministic) processes became increasingly important in structuring microbialcommunities where homogenizing selection was the only process structuring upperactive layer soils. Furthermore, laboratory-induced thaw reflected assembly dynamicsimmediately after thaw indicated by an increase in drift, but did not capture the longtermeffects of permafrost thaw on microbial community dynamics. Our results didnot reflect a link between assembly dynamics and carbon emissions, likely becauserespiration is the product of many processes in microbial communities. Identificationof dominant microbial community assembly processes has the potential to improveour understanding of the ecological impact of permafrost thaw and the permafrost–climate feedback.