Findings of immense microbial diversity are at odds with observed functional redundancy, as competitive exclusion should hinder coexistence. Tradeoffs between dispersal and competitive ability could resolve this contradiction, but the extent to which they influence microbial community assembly is unclear. Because fungi influence the biogeochemical cycles upon which life on earth depends, understanding the mechanisms that maintain the richness of their communities is critically important. Here, we focus on ectomycorrhizal fungi, which are microbial plant mutualists that significantly affect global carbon dynamics and the ecology of host plants. Synthesizing theory with a decade of empirical research at our study site, we show that competition-colonization tradeoffs structure diversity in situ and that models calibrated only with empirically derived competition-colonization tradeoffs can accurately predict species-area relationships in this group of key eukaryotic microbes. These findings provide evidence that competition-colonization tradeoffs can sustain the landscape-scale diversity of microbes that compete for a single limiting resource.