Soil depth is more important than season in determining mesic low Arctic tundra bacterial community composition

Climatic warming may stimulate microbial decomposition of the Arctic's vast pool of thermally protected soil carbon, resulting in globally significant greenhouse-gas emissions. Although significant temperature increases are anticipated for Arctic soil profiles, our current understanding of the controls on bacterial community composition in permafrost-affected soils is restricted to the uppermost surface layer. Here we investigate the relative influence of soil depth and seasonality on bacterial community composition in 166 soil profile samples collected from the surface down into the underlying permafrost at multiple times from late-winter to fall in a mesic Low Arctic tundra site. Bacterial community composition and phylogenetic diversity varied greatly with soil depth, and much less among sampling times, with patterns in community assembly best explained by pH. An OTU correlation analysis indicated that the surface organic and subsoil mineral soils contained almost completely separate interaction networks. Acidobacteria dominated surface communities but declined significantly with soil depth and increasing pH, whereas Bacteroidetes abundance increased with pH. We conclude that tundra soil bacterial community composition is more strongly influenced by soil depth rather than season and that soil pH influences microbial community composition, not just horizontally across the uppermost surface layers, but also down through Arctic soil profiles.