Legume introduction enhances rhizosphere phosphorus availability through organic acid-induced dissolution and microbial transformation: Insights from an 11-year field experiment in grassland

• Legume increase available P via increasing organic acid and genes for P activation • P addition reduces microbial P transformation • N addition accelerates depletion of available P While chemical fertilization offers a direct solution to restore soil fertility in degraded ecosystem, revegetation through legume introduction represents a more sustainable management strategy. However, the mechanisms by which legume introduction influence soil phosphorus (P) transformation, and how these processes respond to nutrient fertilization, remain poorly understood. Using one-time sampling from an 11-year field experiment, we investigated how the introduction of a legume ( Medicago falcata L.) influenced rhizosphere soil P fractions, both alone and in combination with nitrogen (N; 5 g m −2 yr −1 ) and P (3.2 g m −2 yr −1 ) fertilization. Our findings reveal that legume introduction stimulated the mobilization of soil P by increasing organic acid concentrations and microbial P demand, as indicated by the microbial biomass N:P ratio. This resulted in significant changes in P pools, marked by a 97.4% increase in labile inorganic P, a 22.9% decrease in moderately labile inorganic P, a 9.6% decrease in moderately labile organic P, and a 3.7% decrease in non-labile P pool. In contrast, while N fertilization promoted the solubilization of moderately labile inorganic P and the dissolution of the non-labile P pool by lowering soil pH and enhancing the abundance of genes for inorganic P solubilization, it ultimately led to a 10.6% depletion of the labile P pool. Phosphorus fertilization increased labile inorganic P, moderately labile inorganic P, and non-labile P, yet it inhibited microbial P transformation processes. Importantly, legume introduction mitigated the negative impacts of N fertilization on bioavailable P and the negative effects of P fertilization on microbial P mineralization genes. These findings suggest that legume introduction is a sustainable practice to stimulate P cycling in natural grassland, highlighting the importance of activating microbial functions in grassland management and restoration.