Grassland soil microbial community composition and distribution response to grazing by Bison bison

Abstract

In Great Plains prairie ecosystems, bison were historically a keystone species, and still exert significant influence on the composition of aboveground communities through their grazing activity. Given that soil microorganisms also support essential grassland ecosystem services, there is a need for greater understanding of how bison grazing activity affects soil microbial communities. Although the mechanisms controlling soil microbial community assembly at different spatial scales are known to be the same as for all larger organisms – environmental filtering, drift, dispersal, and mutation – the context in which each mechanism becomes important is not well understood. I predicted that bison would weaken the soil microbial community distance-dissimilarity relationship, one of the most common spatial patterns, making microbial communities more similar across space. More specifically, I predicted this pattern would be a result of bison physically distributing microbial cells or altering the environment to increase competitive dominance of certain taxa, and that bison dung would be a main contributor to these mechanisms. To address these predictions, I carried out an observational project evaluating regional soil microbial composition and distribution and an experimental project investigating dispersal mechanisms. For the observational project, surface soils were collected from bison grazed and ungrazed areas at nine grassland sites across the Great Plains for analysis of microbial community composition (as bacterial and archaeal 16S rRNA gene sequence libraries), along with information on soil chemistry and plant community cover. The experimental project involved manipulating the openness of the soil microbial community to passive dispersal or the addition of bison dung to simulate active dispersal, in combination with the presence or absence of bison and spring burning at a focal tallgrass prairie site (Konza Prairie Biological Station). To assess soil microbial dispersal rates under these contrasting conditions, change in soil microbial community composition was measured over time. Results indicate that bison grazing does weaken the soil microbial distance-dissimilarity relationship when evaluated at a regional level, but at a local site level the strength and direction of this relationship relative to ungrazed areas is mediated by plant community structure and soil factors. Still, variation in the strength of the grazing effect on distance-dissimilarity relationships could be driven by both relative ease of microbial dispersal and environmental filtering at the small-scale sample level. Experimental results show that passive dispersal occurred throughout the duration of the project, but dispersal limitation of microbial taxa does not vary with grazing or fire management. Furthermore, bison dung can directly disperse microbes and influence community assembly over time. Overall, both projects support the importance of bison grazing in structuring and mediating soil microbial community dynamics across Great Plains grasslands, and provide impetus for future research and conservation of soil microbial communities, especially in relation to belowground ecosystem services.