Soil biotic complementarity is associated with plant diversity effects on ecosystem multifunctionality

The partitioning of biodiversity effects into selection and complementarity effects has substantially advanced our understanding of biodiversity–ecosystem functioning relationships. While this framework is well established for plant communities, its extension to soil biota remains underexplored, limiting our understanding of how plant diversity influences ecosystem multifunctionality via belowground pathways. Here, we integrate a three-year monoculture experiment with surveys of naturally assembled plant communities to evaluate how plant diversity shapes soil biotic attributes and ecosystem multifunctionality. Across bacteria, fungi, and nematodes, net diversity effects on soil biota were largely accounted for by complementarity effects, whereas selection effects were generally weak. Notably, selection effects of plant diversity on soil biotic diversity and biomass were associated with ecosystem multifunctionality in contrasting ways, while complementarity effects, particularly those observed for bacterial biomass, were positively related to multifunctionality. Our results indicate that plant diversity is consistently associated with higher ecosystem multifunctionality, a relationship that corresponds strongly with belowground complementarity effects expressed in soil biotic biomass, highlighting the importance of resource partitioning and facilitative interactions in soil communities. We also highlight that these inferences rely on integrating experimental monocultures with observational natural communities and thus represent strong statistical associations rather than direct experimental manipulations of soil biota.