Prairie biota evolved with and were thus maintained by fires and large herbivore grazing. Extensive research has explored how grazers impact plant communities at multiple scales. In particular, Bison bison (bison) are thought to have been a keystone species because of their selective grazing, which promotes plant species richness and heterogeneity. However, it is unclear how bison influence soil microbial diversity both locally and regionally. Previous data suggest that a history of bison grazing significantly impacts soil bacterial and archaeal diversity, composition, and spatial distribution at Konza Prairie Biological Station in eastern Kansas, USA, but we do not know if this is true at larger scales. We wanted to understand whether bison affect soil microbial distribution similarly among grassland sites with variable soils, climates and land use histories. To address this question, we used a repeated sampling design at nine bison grazed grasslands across the Great Plains, consisting of thee replicate linear transects in both grazed and ungrazed areas at each site. During the summer of 2019, plant functional cover was recorded and six 10-cm soil cores were collected from each transect. Soil chemical variables were measured and corresponding microbial communities characterized using amplicon sequencing targeting the 16S rRNA gene.
Results/Conclusions
We used both traditional and spatial statistics to elucidate the role of bison grazing and site characteristics on soil microbial diversity and distribution. PERMANOVA of Bray-Curtis distances among communities of 16S rRNA gene actual sequence variants (ASVs; Qiime2 and R-vegan adonis) showed significant (P<0.0001) effects of both site and bison grazing status, and a relatively strong interaction between the two (R2=0.383, 0.012, 0.072, respectively). ASV richness was higher on average in grazed soils (F=15.5, P=0.00011), with significant site interactions (F=4.2, P=0.00010); soil water content was the best predictor of microbial richness (P<0.001, R=-0.53). Further analysis will evaluate how other site-specific factors predict the level of grazing impact, including bison density, fire history, and plant cover. Further, variance partitioning will be used to compare the relative contribution of grazing, soil, and spatial distribution to soil microbial heterogeneity, and distance-dissimilarity of community change in bison-grazed versus ungrazed soils will provide evidence on whether the presence of bison reduces microbial dispersal limitation. Soil microbes are important mediators of many ecosystem processes and differences in community diversity and distribution could have important consequences for ecosystem function. Therefore, understanding if and how bison alter the soil community can lead to better grassland conservation and management.