Soil fauna niche complementarity explains long-term soil multifunctionality differences between planted and natural subalpine forests

• Planted forests exhibit 76 % lower soil multifunctionality than natural forests. • Soil fauna niche complementarity explains soil multifunctionality. • Plant resources mediate the niche complementarity of soil fauna. • Niche complementarity effects outweigh mass ratio or quantity effects. Understanding soil biodiversity − ecosystem functioning relationships (BEFs) remains a key challenge. Although the mass ratio, niche complementarity, and quantity hypotheses offer mechanistic explanations for BEFs, their relative roles have not been disentangled in forest soil fauna communities. Here, we compared soil multifunctionality across 30–80-year sequences of planted coniferous forests ( Picea asperata ) and adjacent naturally regenerated forests ( Betula albosinensis ). To mechanistically link soil multifunctionality with fauna, we pioneer the testing of the mass ratio (functional identities), niche complementarity (functional diversity), and quantity (biomass) hypotheses. Coniferous afforestation, which displaces native broadleaved forests, reduced soil multifunctionality by 40–76 %, with this degradation intensifying over time. Key predictors of these declines included reduced plant resource availability and diminished soil faunal niche complementarity in coniferous forests. Notably, the niche complementarity effect of soil fauna was found to be more critical than either the mass ratio or quantity effect, and this complementarity is regulated more by plant resources and less by microbial biomass. This study advances soil BEFs theory by providing the empirical validation that niche complementarity mechanisms dominate over mass ratio or quantity mechanisms. These results provide new insights into trait-based forest soil management.