Bacterial community composition and assembly along a natural sodicity/salinity gradient in surface and subsurface soils

Abstract Soil microbes play a key role in terrestrial ecosystems. In the saline-sodic environment, the microbial communities and their community assembly processes, especially throughout soil profiles, however, are not well understood. Here, we characterized the soil microbial communities across three sodicity/salinity gradients to 1.0 m depth in the western Songnen Plain, Northeast China. Soil sodicity (exchangeable sodium percentage) and salinity (electrical conductivity) exerted a strong influence on bacterial community compositions compared with other soil properties. These filtering effects of sodicity/salinity were strong on the microbial communities in the layer of 0–60 cm across the three sites, with more contributions of deterministic processes over stochastic processes. As sodicity/salinity decreased, some potential biomarkers for a high community tolerance to salt or soda, such as KSA1, Halomonas, Euzebya and Bacillus, were depleted; meanwhile, some taxa associated with decomposition of organic matter and nutrient transformations, such as Arthrobacter, Mesorhizobium, Pseudomonas and Nitrospira, were enriched. Within each soil profile, the turnover in community composition was lower than between sites, and the stochastic processes (homogenizing dispersal) dominated the microbial assembly. Yet, the surface and deepest depth (80–100 cm) of saline-sodic soils still harbored some distinct microbial communities. The near-surface (20–40 cm) may serve as a gating role for limiting surface microbial penetrance into the deeper depth. Taken together, our results show that sodicity/salinity can strongly structure soil microbial communities, and emphasize the importance of soil depth as an environmental gradient on the assembly processes in saline-sodic soils.