Rhizosphere microbial roles in phosphorus cycling during successive plantings of Chinese fir plantations

The rhizosphere, a critical interface involving soil, plant roots, and microorganisms, plays a vital role in the feedback processes between plants and soil, especially under phosphorus (P) limiting conditions typical of subtropical forests. This study used a chronosequence design to investigate first-fourth successive planting rotations of Chinese fir (Cunninghamia lanceolata) plantations in Fujian Province, China, with stands around 17 years old. We employed a modified P fractionation assay and metagenomic sequencing to explore P cycling in the rhizosphere across different plantation rotations. Rhizosphere labile P concentrations increased significantly in the fourth rotation, alongside a consistent upward trend in moderately labile P throughout successive plantings. Conversely, stable P and residual P concentrations declined during successive plantings, signaling a shift toward more accessible P forms. From metagenomic analyses, the proportion of P transport processes (transportation of phosphonate, phosphate, and inorganic phosphate) gradually increased. Notably, abundances were significantly higher in the rhizosphere soil of the fourth Chinese fir planting rotation of the K01126 gene (involved in phosphate ester mineralization), the phnD gene (associated with phosphonate transport), functional genes related to the solubilization of inorganic phosphate, such as pqqB, pqqC, pqqE, and ppa, and the phoB gene (linked to P-starvation response regulation). The results indicate that functional microbes of the rhizosphere, dominated by Proteobacteria and Acidobacteria, are instrumental in changing P cycling processes during successive plantings. The successive planting rotations of Chinese fir plantations significantly and positively impacted on the gene abundance related to the activation and uptake of P in the rhizosphere. Based on these insights, specific strategies, such as regular monitoring and application of phosphate fertilizer and adjusting rotation timing based on the soil rhizosphere P status, and incorporating native broad-leaved tree species are suggested to promote efficient P cycling, thus supporting sustainable forest management practices.