Abstract: Underplanting represents a traditional silvicultural practice, and optimal underplanting can enhance plant acquisition of limiting nutrients, especially phosphorus (P). Chinese fir (Cunninghamia lanceolata) is widely planted in low-latitude P-deficient regions. However, the beneficial companion species and species-specific rhizospheric mechanisms governing soil P availability in these systems remain poorly resolved. To address this, the study evaluated rhizospheric P cycling mechanisms in a 46-year-old plantation underplanted for 10 years with four treatments: autospecific Chinese fir (CF), Schima superba (CS), Liriodendron chinense (CL), and Manglietia fordiana (CM). After ten years, CF exhibited significantly higher available P (AP), organic P, and total P compared to broadleaf systems (CM, CS, CL), along with elevated microbial biomass P (MBP) and alkaline phosphatase (ALP) activity. Functional gene analysis reveals higher abundances of P-solubilizing (ppx, pqqC, gcd) and mineralizing (phoD, phnK, phoX) genes in CF. Root exudation profiles demonstrate that CF secreted more oxalic acid than CS and CL, and exhibited significantly higher ALP activity, which was a primary driver of organic P mineralization. These results highlight that intraspecific underplanting enhances rhizospheric P availability primarily through microbial-driven organic P mineralization, supported by organic acid-mediated solubilization of mineral-bound P. CF systems maintained higher microbial biomass and phosphatase activity while minimizing interspecific competition, unlike broadleaf treatments, where nutrient competition reduced microbial efficiency. This study proposes CF intraspecific underplanting as a sustainable strategy to alleviate P limitation during the early stages of underplantation in mature subtropical plantations via microbial-driven organic P mineralization and organic acid mediated solubilization, offering a sustainable strategy to alleviate P limitation in subtropical plantations.

Key words: Phosphorus mobilization, rhizosphere microbial biomass, acid phosphatase activity, organic acid secretion, intraspecific underplanting