Soil microorganisms are vital for forest ecosystem functioning, and tree species richness is expected to enhance soil microbial functionality. Yet, evidence remains inconclusive, possibly because local small-scale tree–tree interactions and their associated mycorrhizal relationships introduce additional complexity. Variation in belowground competition, root distributions, mycorrhizal hyphal networks, and microsite nutrient availability around individual trees may modify microbial activity and, consequently, influencing how multiple soil functions operate together.

We assessed how tree species richness, mycorrhizal associations, and local fine-scale differences affect soil microbial functioning using a temperate forest biodiversity experiment (MyDiv). Plots contained monocultures, 2-species, or 4-species mixtures composed of tree species associated with arbuscular mycorrhizal (AM), ectomycorrhizal (EM), or mixed (MIX) communities. We sampled soil near target trees to assess individual effects and in interaction zones to capture tree–tree interaction effects. We analysed microbial biomass and respiration, enzyme activities, aggregate stability, and calculated soil multifunctionality. Environmental variables (soil water and carbon content, pH, and tree basal area) were assessed to potentially explain tree community effects on multifunctionality.

We found that tree species richness increased soil multifunctionality by enhancing microbial biomass and enzyme activities related to nitrogen and phosphorus cycling. Mycorrhizal type strongly affected soil multifunctionality in EM plots, while mixing mycorrhizal types did not yield synergistic effects. However, individual soil functions showed distinct patterns: microbial biomass and nitrogen cycle-related enzyme activity peaked in EM plots, whereas carbon cycle-related enzyme activity was highest in AM plots. Tree species richness increased soil multifunctionality close to the target tree but not in the interaction zone, and the environmental variables measured could not explain these relationships.

Overall, tree species richness enhanced soil multifunctionality, particularly in EM-associated plots. Importantly, positive diversity effects were highly localized, suggesting that individual tree responses and mycorrhizal-mediated interactions may play a stronger role than broader tree–tree interactions.