Arctic tundra soil depth, more than seasonality, determines active layer bacterial community variation down to the permafrost transition

Enhanced microbial decomposition of the Arctic's vast soil carbon pool due to climate warming could result in globally significant CO₂/CH₄ emissions. Our understanding of tundra soil microbial communities is generally based on studies of either the top surface or the deep frozen permafrost. In contrast, there has been relatively little exploration of bacterial communities and soil biogeochemistry down through entire soil depth profiles to the permafrost, and none that has specifically incorporated temporal dynamics with depth as the summer thaw proceeds. Here we report bacterial community composition in 10-cm increments from the surface to the permafrost from late winter through to autumn, at a mesic low Arctic tundra site. Bacterial community composition and phylogenetic diversity varied substantially with soil depth and much less among sampling times. Correlation analysis indicated that the surface organic, subsoil mineral, and permafrost transition soil layers each had distinct community interaction networks. Acidobacteriota-affiliated taxa dominated surface communities but declined significantly with soil depth and increasing pH, while Bacteroidetes taxa relative abundances were also associated with pH, and were largest in the carbon-restricted permafrost transition layer. Thus, patterns in community assembly were primarily associated with depth and correlated with edaphic factors, with relatively little impact of the seasonal changes between frozen and thawed assemblages within each sampling depth interval. Nevertheless, total microbial biomass within each depth interval generally declined during the seasonal transition from frozen to thawed state. Finally, our data indicate that soil pH, which is known to govern tundra microbial community composition horizontally across the surface organic layer at multiple spatial scales, is also an important determinant of vertical differences in bacterial community composition from the top surface down to the permafrost.