Bacterial community diversity and underlying assembly patterns along vertical soil profiles in wetland and meadow habitats on the Zoige Plateau, China

Microorganisms are found throughout the soil, however, our understanding of microbial diversity and the underlying assembling patterns is mostly limited to the surface horizons (0–20 cm). In this study, soil samples were collected from a total of 12 integral vertical soil profiles (from the surface layer to the parent material, up to 5 m depth) from two typical alpine habitats (wetland and meadow) in the Zoige Plateau, China. As determined from 16S rRNA sequencing, soil bacterial diversity and community composition were unique to each habitat. Similar decays in bacterial similarity over distance along soil vertical profiles were observed in both habitats, and taxonomic turnover rates were higher than phylogenetic ones. Bacterial distribution greatly depended on soil depth, with most oligotrophic taxa residing in deeper soil layers. Soil macro-nutrients were major factors regulating bacterial assembly, however, in deeper soil layers with lower nutrient effectiveness, minerals such as manganese, potassium, and sulfur were more influential. Along the whole profile, community assembly was driven mainly by deterministic processes (mostly heterogeneous selection) and to a lesser extent by stochastic processes, as shown by both a null model-based β-nearest taxon index and a normalized stochastic ratio. The habitats differed in the importance of these processes, however, with deterministic processes playing a larger role in bacterial assembly in meadows than in wetlands, and with community members in alpine meadows having narrower environmental thresholds and stronger phylogenetic signals. Additionally, bacterial communities in the subsoil exhibited a narrower niche breadth and were susceptible to deterministic processes. This trend was accompanied by the weaker phylogenetic conservation, suggesting their lower adaptation to environmental changes compared to the topsoil. Overall, our study investigated bacterial biodiversity patterns and the potential processes underlying bacterial assemblage along the vertical soil profile, and provided the first data regarding the vertical geography of soil bacterial communities in typical alpine habitats.