Species pool and local ecological assembly processes shape the β-diversity of diazotrophs in grassland soils

Based on the contemporary coexistence theory developed from macroecology, the regional species pool, environmental processes, dispersal assembly and biotic interactions are considered to determine the assembly of a local community. However, we do not fully understand whether these kinds of mechanisms are all-important for the whole microbial community or for specific functional groups. Here we studied the underlying processes driving the β-diversity of soil-based diazotrophic communities across a large geographical distance in Chinese grasslands by considering the roles of species pools and ecological assembly processes. Results indicated that the species pool (γ-diversity) showed significant and positive correlations to the observed β-diversity of the diazotrophic community; that is, diazotrophic community turnover increased with the size of the species pool. Environmental filtering played a greater role than dispersal effects or biotic interactions on β-diversity. Biotic interactions drove the assemblage of diazotrophs by influencing the co-occurrence pattern of species. Overall, these results demonstrated the importance of the species pool and local community assembly processes in shaping diazotrophic β-diversity patterns in grassland soils.