Plant host effects on soil bacterial community diversity, nutrient characteristics, and microbial antagonistic phenotypes depend on plant community diversity.

While both plant host and community diversity have been shown to influence soil microbial communities, there is little understanding of how microbial communities may vary in association with the same plant host growing within communities that vary in plant species richness.  Here, we evaluate relationships between plant species and soil bacterial community composition, structure, and antagonistic activities in association with 4 prairie plant hosts growing in communities of 1, 4, 8, or 16 plant species. We found that plant diversity, but not plant species, significantly influenced soil bacterial community composition, diversity, and antagonist populations. Bacterial diversity and antagonistic activity varied significantly with soil nutrient characteristics (N, P, K, C), but the patterns of variation often differed among plant species richness treatments. For example, soil K was positively correlated with bacterial diversity in monoculture plots, but negatively correlated with bacterial bacterial diversity in high diversity plots. This suggests that plant community richness, and specifically the presence of heterospecific competitors, alters relationships between plant hosts and their rhizosphere bacterial communities. Variation in plant-microbe interactions has significant implications for plant fitness and the role of pathogen antagonists across a plant diversity gradient.