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The widely observed positive relationship between plant diversity and ecosystem functioning is thought to be substantially driven by complementary resource use of plant species. Recent work suggests that biotic interactions among plants and between plants and soil organisms drive key aspects of resource use complementarity. Here, we provide a conceptual framework for integrating positive biotic interactions across guilds of organisms, more specifically between plants and mycorrhizal types, to explain resource use complementarity in plants and its consequences for plant competition. Our overarching hypothesis is that ecosystem functioning increases when more plant species associate with functionally dissimilar mycorrhizal fungi because differing mycorrhizal types will increase coverage of habitat space for and reduce competition among plants. We introduce a recently established field experiment (MyDiv) that uses different pools of tree species that associate with either arbuscular or ectomycorrhizal fungi to create orthogonal experimental gradients in tree species richness and mycorrhizal associations and present initial results. Finally, we discuss options for future mechanistic studies on resource use complementarity within MyDiv. We show how mycorrhizal types and biotic interactions in MyDiv can be used in the future to test novel questions regarding the mechanisms underlying biodiversity-ecosystem function relationships.