Soil mycobiome dissimilarity, independent of fungal guild, is associated with increased probability of plant coexistence

Major theories regarding microbe-mediated plant community dynamics assume that plant species cultivate distinct microbial communities. However, few studies empirically assess the role of species-associated microbial community dissimilarity in plant competitive dynamics. In this study, we paired a competition experiment between eight annual forbs with characterization of species-associated fungal communities to assess whether mycobiome dissimilarity is associated with pairwise competitive dynamics. Using a quantitative approach informed by modern coexistence theory, we found that fungal dissimilarity was correlated with increased stabilizing niche differences and even steeper increased fitness inequalities. Though these results suggest fungal dissimilarity, per se, should be associated with exclusion between our species, we also found that the probability of coexistence increased with mycobiome dissimilarity. When subsetting the community into different fungal functional groups (pathotrophs, saprotrophs, symbiotrophs), overall relationships between dissimilarity and competitive dynamics were independent of these functional groups. These results suggest that fungal community divergence may play an important role in mediating plant competitive dynamics. Although fungal community dissimilarity is strongly associated with fitness differences, complex biotic and/or abiotic interactions belowground may result in an observed correlation between fungal community dissimilarity and plant coexistence. Ultimately, this study implements a novel approach to studying the role of microbiome dissimilarity in host community dynamics.