Assembly of microbial communities in thawed permafrost and implications for carbon processing

The Northern high latitudes are warming twice as fast as the global average and permafrost has become vulnerable to thaw leading to physical, chemical, and biological changes across the landscape. Of particular interest is the change in soil microbial communities and the potential effect on ecosystem functions, such as increased carbon emissions. Changes to the environment during thaw can lead to shifts in microbial communities, the assembly of which is heavily influenced by biotic and abiotic factors. Little is known regarding permafrost microbial assembly both in situ or post-thaw including how assembly might influence ecosystem function post-thaw. We hypothesize that (i) microbial communities residing in well-established active layer and intact permafrost are the result of niche-based assembly and (ii) the assembly of microbial communities in newly formed active layer (i.e. the permafrost-active layer interface) is a result of neutral processes. We tested these hypotheses by determining the microbial community structure through 16S and ITS amplicon sequencing along replicate soil depth profiles from a fourteen-year field thaw experiment at the Storflaket Mire, Sweden. A null modeling approach was used to determine the dominant assembly processes at eight depths, encompassing both active layer, transition zone, and permafrost soils. We also compared the post-thaw carbon processing potential at each depth to determine the link between community assembly and function. Preliminary results show a four-fold decrease in CO₂ respiration along the depth profile with active layer soils exhibiting higher respiration rates compared to permafrost soils. This trend holds true at both 15°C and 4°C with a significant difference in respiration observed between the uppermost active layer and permafrost soil at 15°C (p=0.03). This suggests active layer microbes process carbon faster than permafrost microbes regardless of temperature. Identification of dominant microbial community assembly processes and insights of the functional implications of assembly (e.g., carbon processing rates) will improve our understanding of the ecological impact of permafrost thaw and the permafrost-climate feedback.