Unlocking rhizosphere phosphorus: Root exudate-microbe synergy drives phosphorus activation in mixed Chinese fir species plantation

Cunninghamia lanceolata (Lamb.) Hook., also termed as Chinese fir, is a rapidly growing plantation species of significant economic value in China. However, long-term pure plantations have exacerbated soil phosphorus (P) limitations by reducing both P availability and turnover. Mixed-species plantations offer a promising strategy to enhance nutrient availability, yet the underlying mechanisms by which they alleviate soil P limitation remain largely unexplored. This study conducted a 15-year transformation experiment in subtropical China and found that interplanting Schima superba with Chinese fir significantly increased rhizosphere P availability. Sequential P fractionation revealed a notable shift from stable P forms to moderately labile and labile and P pools in mixed plantations, with NaHCO3-Po increasing by 108 % and NaOH-Po by 58.3 % compared to pure plantations. Metabolomic analysis identified four organic acids, oxoproline, alanine-2, aconitic acid, and N-carbamylglutamate-4 (NCG 4), secreted exclusively by Chinese fir roots in the mixed-species system. These root exudates were strongly correlated with NaHCO3-Po levels and activated a microbial P-solubilization gene network. This included upregulation of inorganic P-solubilizing genes (ppk, ppx, and pqqD), high-affinity phosphate transporters (pstA, pstB, and pstC), and P-starvation response regulators (phoB, phoP, and phoU). Partial least squares path explaining 99 %, 91 %, and 81 % of the variations in labile, moderately labile, and stable phosphorus fractions and modeling further indicated that changes in rhizosphere P fractions were driven by the interactions among root exudates, microbial functional genes, and environmental factors. Together, these findings support a synergistic “root exudate-microbial gene” mechanism that transforms stable P forms into plant-available P, offering a blueprint for designing rhizosphere-engineered mixed-species plantations to overcome P limitation and promote sustainable forestry.