Contrasting effects of intra- and interspecific identity and richness of ectomycorrhizal fungi on host plants, nutrient retention and multifunctionality.

A major gap in our understanding of biodiversity-ecosystem function relationships concerns the role of intra- and interspecific diversity of mycorrhizal fungi, which are critical for plant fitness, biogeochemical cycling and other processes. Here, we test the hypothesis that the identity and richness of ectomycorrhizal (ECM) fungi at the intra- and interspecific levels affect ecosystem multifunctionality by regulating plant and fungal productivity, soil CO2 efflux and nutrient retention. Microcosms containing Scots pine (Pinus sylvestris) seedlings colonized by different ECM fungal isolates, in monocultures and mixtures, enabled us to test for both intra- and interspecific identity and richness effects, and transgressive overyielding. Intra- and interspecific identity had modest but significant effects on plant and fungal productivity and nutrient retention, but no effect on CO2 efflux. Intraspecific richness increased plant root productivity and ECM root tips but decreased hyphal length, whereas interspecific richness had no effects. Interspecific mixtures outperformed the most productive monocultures in only 10% of the cases, compared with 42% for the intraspecific mixtures. Both intra- and interspecific identity and richness of ECM fungi regulate ecosystem multifunctionality, but their effects on the direction and magnitude of individual variables differ. Transgressive overyielding suggests that positive niche complementarity effects are driving some of the responses to intraspecific richness.