Bison grazing and prairie fire shape soil microbial community diversity and assembly

Soil microbes mediate ecosystem function in grasslands, but the impacts of megaherbivore grazing and fire on microbial community assembly are poorly understood, despite widespread changes in these drivers following bison extermination and fire suppression across North America in the 19th century. To better understand how grazing and fire influence microbial metacommunity assembly processes such as dispersal and environmental filtering, we analyzed five years of soil microbial community and soil chemistry data from a long-term, landscape-scale grazing and fire frequency field experiment. Bison grazing reduced soil microbial alpha diversity, while both drivers altered microbial composition. Grazing and fire interactively increased microbial community dissimilarity over distance, with the strongest distance-dissimilarity patterns in grazed, 20-year burn interval treatments, suggesting complex controls over microbial community assembly across large spatial scales. Increases in soil nitrogen and pH as a result of bison reintroduction, along with significant relationships between these factors and microbial composition, highlight environmental filtering as an important assembly process. Temporal patterns showed that grazing and fire effects on microbial composition did not covary with interannual differences in precipitation, but instead with soil environment differences, pointing again to the influence of the abiotic habitat on microbial community assembly. However, geographic distance and the soil environment independently influenced microbial composition, suggesting fire and grazing drive community assembly by influencing both local-scale environmental filtering and regional-scale dispersal processes. Notably, because bison grazing increases plant diversity at this experimental site while decreasing soil microbial diversity, our study reinforces how microbial communities can respond to the same drivers as plant communities, but with different patterns that reflect common ecological mechanisms operating at smaller spatial scales. Overall, this study shows that grazing and fire can shape soil microbiomes through multiple mechanisms, suggesting that soil microbial communities across the Great Plains shifted dramatically following bison extermination and fire suppression.