Species richness and nitrogen supply regulate the productivity and respiration of ectomycorrhizal fungi in pure culture

► Effects of EM species richness on respiration and biomass were measured in-vitro. ► Mycelium biomass and CO2 efflux increase significantly alongside species richness. ► Biomass increase in communities results from selection and complementarity effects. ► Community composition and substrate C:N ratio alter biodiversity effects. ► Facilitative interactions underpin net biodiversity effects under high N conditions.