Tree diversity effects on soil multifunctionality differ across biomes and spatial scales

Forest biodiversity is widely assumed to enhance soil ecosystem functioning, yet empirical evidence remains inconsistent, partly due to the strong spatial heterogeneity and multiple interaction pathways linking plants and soil microorganisms. Ecosystem multifunctionality offers a useful framework to integrate these complex responses, but biodiversity effects may depend on scale as well as abiotic and biotic context. Here, we compare two forest biodiversity experiments conducted in contrasting biomes to assess how tree diversity, vegetation structure, and local fine-scale interactions shape soil microbial multifunctionality.In a temperate forest biodiversity experiment (MyDiv), we investigated how tree species richness and mycorrhizal type (arbuscular vs. ectomycorrhizal) influence soil microbial functioning at the scale of individual trees and tree–tree interaction zones. Soil multifunctionality, derived from microbial biomass, respiration, enzyme activities, and aggregate stability, increased with tree species richness, particularly in ectomycorrhizal-associated plots. Importantly, positive biodiversity effects were spatially constrained to soils close to target trees and did not extend into interaction zones, highlighting the importance of localized root–microbe and mycorrhizal-mediated processes.In contrast, a subtropical forest biodiversity experiment (BEF-China) examined the combined influence of tree species richness (up to 24 species), understory shrub presence, and shrub–tree interactions on soil microbial multifunctionality. Preliminary analyses indicate that soil multifunctionality and individual microbial functions are comparatively stable across gradients of tree species richness, suggesting a weaker or more buffered biodiversity effect under higher structural complexity and environmental heterogeneity.Together, these experiments reveal that biodiversity–multifunctionality relationships are strongly context-dependent, varying across biomes, vegetation layers, and spatial scales. Our comparison suggests that localized plant–microbe interactions and mycorrhizal strategies may be key drivers of soil multifunctionality in simpler systems, whereas increasing community complexity may dampen detectable biodiversity effects. These findings underscore the need to integrate spatial scale and environmental context when assessing biodiversity–ecosystem functioning relationships.