Soil microbes promote complementarity effects among co-existing trees through soil nitrogen partitioning

1. Plant resource partitioning is a mechanism promoting species coexistence and ecosystem functioning. Yet, we still have limited understanding of how soil microbes, especially plant symbiotic microbes, influence resource partitioning. We hypothesized that soil borne microbes, in particular mycorrhizal fungi, facilitate differential performance of tree species depending on different nitrogen sources and that this leads to a positive plant diversity–community productivity relationship. 2. We conducted two complementing glasshouse experiments. In a “monoculture experiment”, we supplied nitrogen as ammonium, nitrate or glycine and tested the growth response of three tree species associated with different root symbionts: one associated with ectomycorrhizal fungi, one associated with arbuscular mycorrhizal fungi and the third associated with both arbuscular mycorrhizal fungi and N-fixing bacteria. In an “intermixed experiment”, we grew the tree species at three richness levels (one, two or t...