Responses of soil rare and abundant microorganisms to recurring biotic disturbances

Periodic inoculations of soil-beneficial microbes can increase their populations, but they also act as recurring biotic disturbances on the native microbial community. Soil rare and abundant microorganisms disproportionally shape the community diversity and stability. Uncovering their dynamic responses to recurring biotic disturbances and the underlying driving factors helps improve our understanding of the inoculation effects. Here, we imposed temporally recurring biotic disturbances by inoculating soils with phosphate-solubilizing bacteria, nitrogen-fixing bacteria, and the combination of both, with the overall aim of studying the successive responses of bacterial and fungal subcommunities along a rarity index. Our results showed that, in both bacterial and fungal communities, the relatively rare taxa exhibited higher diversity than the abundant taxa, and the relative abundance of rare taxa increased with recurring disturbances. However, the responses of rare and abundant taxa to inoculations were different between bacteria and fungi and were related to time and inoculation frequency. The rarer bacteria and the more abundant fungi explained most of the effects of inoculations on the resident microbial community. About 20 percent of the microbes changed their rarity categories over time, and most of the changes and interactions occurred within the rarer taxa during the first 45 days. Modeling analyses and co-occurrence networks indicated that microbial interactions, soil biochemical factors, and inoculation time drove the shifts of subcommunities. In summary, relatively rare bacteria and relatively abundant fungi play major roles in understanding the impacts of recurring biotic disturbances, while the conditionality of microbial rarity is dependent on both biotic and abiotic factors.