Summary Resource complementarity can contribute to enhanced ecosystem functioning in diverse plant communities, but the role of facilitation in the enhanced complementarity is poorly understood. Here, we use leaf manganese concentration ([Mn]) as a proxy for rhizosheath carboxylate concentration to explore novel mechanisms of complementarity mediated by phosphorus (P) facilitation. In pot experiments, we showed that mixtures involving Carex korshinskyi, an efficient P‐mobilizing species, exhibited greater biomass and relative complementarity effect than combinations without C. korshinskyi on P‐deficient soils. Compared with monocultures, leaf [Mn] and [P] of species that are inefficient at P mobilization increased by 27% and 21% when grown with C. korshinskyi (i.e. interspecific P facilitation via carboxylates) rather than next to another inefficient P‐mobilizing species. This experimental result was supported by a meta‐analysis including a range of efficient P‐mobilizing species. Phosphorus facilitation enhanced the relative complementarity effect in low‐P environments, related to a greater change in several facilitated species of their root morphological traits relative to those in monoculture. Using leaf [Mn] as a proxy, we highlight a vital mechanism of interspecific P facilitation via belowground processes and provide evidence for the pivotal role of P facilitation mediated by the plasticity of root traits in biodiversity research.

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