Tree species richness is known to enhance soil phosphorus (P) availability; however, the mechanisms by which tree leaf functional trait diversity regulates P bioavailability through plant–microbe interactions remain unclear. Here, we conducted a tree diversity experiment with four species gradients (1, 2, 4, and 6 species) in southern subtropical China. We quantified four bioavailable P fractions (CaCl₂-P, Citrate-P, Enzyme-P, and HCl-P), and specifically targeted P bioavailability in the rhizosphere. We found that tree leaf functional trait diversity exerted a stronger and more direct effect on rhizosphere P bioavailability than did tree species richness. This enhancement was closely associated with increased forest productivity and soil organic carbon (SOC). Communities dominated by resource-acquisitive species showed higher P bioavailability. Further, soil microbial biomass and the relative abundance of Bacteroidetes were positively associated with P bioavailability. Notably, mixtures of nitrogen-fixing and non-fixing species exhibited higher rhizosphere P bioavailability than either type in monoculture. Our findings suggest that subtropical plantation management should prioritize functional diversity over simply maximizing species richness. In particular, including P-mobilizing species in mixtures may enhance rhizosphere P bioavailability and support long-term forest productivity.