Microbial genomic strategies control the soil iron-phosphorus nexus in successive rotations of Chinese fir plantation

In subtropical China, successive rotations of Chinese fir (Cunninghamia lanceolata) plantations exacerbate soil phosphorus (P) limitation, yet the iron (Fe)-P coupling mechanisms mediated by microbial adaptation remain poorly understood. This study integrated soil P fractionation, Fe mineral speciation via Mössbauer spectroscopy, and metagenomic sequencing to unravel how Fe oxides and microbial functional traits collectively regulate P availability across four rotation stages of Chinese fir. The results demonstrated that successive rotations shifted the composition of soil Fe minerals toward forms with a stronger capacity to adsorb P, thereby increasing soil Fe-occluded phosphorus concentrations. A significant depletion of soil organic P pools was observed: from the first to the fourth rotation, labile organic phosphorus decreased from 60 to 23 mg kg−1, and moderately labile organic phosphorus decreased from 218 to 139 mg kg−1. In response, soil microorganisms selectively increased the abundance of genes related to organic P mineralization and P transport. This enhanced their capacity to decompose and utilize soil organic P sources to maintain P availability, a strategy that nonetheless accelerated the depletion of soil organic P. In contrast, adaptation of Fe-cycling microorganisms, marked by an increased abundance of the fbpABC gene (encoding an Fe transporter), played a particularly critical role by unlocking occluded P from Fe oxides. These findings indicate the potential for soil microbiome management to promote sustainable phosphorus utilization in intensively managed forestry plantations.