Dispersal is increasingly recognized as an important assembly mechanism in microbial communities, altering biogeographical patterns and microbial community function. Dispersal can also affect community resilience; high rates of immigration can ‘rescue’ community function under abiotic stress. But knowledge of the ecological role of microbial dispersal in situ is limited. Rain water hosts a diverse community of microorganisms, making it a major source of microorganisms in soil and litter communities. To assess the ecological impact of dispersal through rain on soil processes, we established mesocosms of intact soil cores collected at Kellogg Biological Station. After natural rain events, cores received either filter-sterilized rain (no dispersal) or ambient rain (dispersal). We also overlaid a drought treatment on these cores to test whether zero dispersal constrained community and functional resilience under stress. Finally, we mimicked this design in a model environment to study changes in growth, death, and diversity under dispersal-limited conditions.
We found that excluding rain immigrants significantly altered microbial and ecosystem function. Specifically, dispersal limited treatments had lower water holding capacity and higher nitrate export. Although microbial biomass was similar among dispersal levels, we found that functional diversity was reduced under dispersal limitation, possibly due to reduced community evenness. Drought treatments also had reduced functional capacity and diversity, but were unaffected by dispersal treatment. Thus, we observed an interaction between drought and dispersal, but saw no evidence of dispersal rescuing function. Limiting dispersal in our theoretical model led to drift, suggesting this was the mechanism for observed shifts in community composition and function. Overall, our data demonstrate that rain-dispersed microbes are important for maintaining soil microbial function, but dispersal may play different roles in community assembly and function under stressful versus ambient conditions.