Different stochastic processes regulate bacterial and fungal community assembly in estuarine wetland soils

The long-standing challenge in microbial ecology is to quantify the relative importance of deterministic and stochastic processes in controlling the community structures through space and time. Deterministic processes are important when the environmental selection and species interactions are strong, whereas stochastic processes may dominate when there is large number of neutral births, death, speciation of microorganism. The existing studies are still lacking in understanding how these two processes structure microbial communities in an ecosystem whose environmental gradients change sharply at a very short geographic scale, such as in the fresh-saltwater transition zone. Here, we investigated the bacterial and fungal assembly processes in topsoil (0–10 cm) in a wetland transect with fresh-saltwater transition zone in the Yellow River Estuary, China, by calculating beta-mean-nearest taxon distance (βMNTD), beta nearest taxon index (βNTI) and Raup–Crick index. We found that stochastic processes primarily shaped the two communities. The bacterial assembly was controlled by homogenizing dispersal and dispersal limitation within and across the wetland transect, respectively. The fungal assembly, on the other hand, was mostly regulated by undominated processes, which are processes induced by weak selection/dispersal, diversification, and drift. Dispersal limitation had more pronounced effects on the bacterial diversity and cooccurrence in comparison with the effects of undominated processes on the fungal community. Nevertheless, environmental factors, such as pH, salinity and C/N, explained ∼30% (p < 0.01) of compositional variations with more explanatory power for bacteria than fungi. Our study highlights the different types of stochastic processes in regulating bacterial and fungal communities along the short natural environmental gradients.