The effect of plant diversity on ecosystem function has long been debated, but it has largely focused on plant-mediated processes despite the many ecosystem processes mediated by soil microbial communities.  Here, we provide the rationale and experimental evidence that plant diversity influences soil processes by affecting the composition, diversity, and activity of heterotrophic microbial communities.  Our previous work at the Cedar Creek Biodiversity experiment has revealed two important patterns: 1) that fungal diversity is a unimodal function of resource availability, which increases with plant species richness and 2) that the relative abundance of soil fungi increases with plant species richness.  We reasoned that plant diversity increases the relative abundance of soil fungi because, as greater plant diversity increases the availability of limiting resources (organic substrates in detritus), soil fungi out compete bacteria in the long term.  Moreover, greater fungal abundance increases the occurrence of fungal-specific functional genes and soil enzymes mediating organic matter decomposition, thereby affecting soil C and N cycling.  Our results demonstrate that competition is an underlying mechanism for the unimodal relationship between fungal diversity and resource availability, providing a clear ecological mechanism which links plant diversity to microbial processes mediating the biogeochemical cycling of C and N in soil.