Assembly Processes Underlying Biotic Homogenization of Soil Microbial Communities in an Urban Ecosystem

Urbanization is known to cause biotic homogenization, but the processes controlling biotic homogenization are not well understood. Here, we analyzed microbial communities from 258 soil samples covering the large landscape heterogeneity of the entire Shanghai megacity. We measured the urbanization intensity by incorporating habitat fragmentation, connectivity, and distance to the city center. We determined the extent to which bacterial and fungal community composition varied with urbanization intensity and how different assembly processes contributed to the variations. We found significantly positive effects of urbanization on the compositional homogenization of bacteria and fungi, and the proportions of generalists and specialists were significantly related to homogenization. Dispersal and ecological drift explained at least 60% of bacterial and fungal compositional variations, with increased influences of dispersal and ecological drift reducing the specialists. Environmental variables explained < 28% of compositional variations, and higher urbanization intensity led to a simplified co‐occurrence network and an increased proportion of generalists in the network. These results indicate that dispersal and ecological drift homogenized soil microbial communities in the city by shifting the proportions of generalist and specialist microbes, with weak effects from environmental selection. Therefore, to conserve urban biodiversity and ecosystem functioning in the face of complex human impacts, management strategies should consider not only environmental conditions but also influences of dispersal and drift, as well as species habitat preferences, to increase the effectiveness of management actions.