Organic Fertilization Promotes the Microbial Formation of Moderately Active Soil Phosphorus Pools to Sustain Phosphorus Availability: Insights from 180 years of Fertilization

Phosphorus (P) fertilization is essential for crop production, but most applied P is rapidly fixed into mineral-associated forms. Although fertilization regulates P distribution in soils, its effects on microbe-mediated processes that regulate P availability and stabilization remain unclear. Here, we investigated P transformations under organic fertilization (FYM), inorganic fertilization (NPK), and no fertilization (NIL) using the 180 year Broadbalk experiment. Through 33P isotopic tracing, metagenomics, and enzymatic profiling, we found that FYM and NPK stimulate distinct P transformation pathways. FYM, through sustained organic carbon inputs, enhanced microbial immobilization and phosphatase activity, causing a 41% reduction in stable P formation and 47% increase in moderately active P levels and shifting P dynamics toward more bioavailable forms. NPK fertilization reduced pH and limited microbial carbon availability; 33% of 33P was recovered in the stable P fraction, indicating abiotic immobilization into inorganic P pools. The microbial community under NPK adapted to P fixation by enriching P-cycling-related genes and acid-tolerant taxa, enhancing P turnover relative to NIL but preventing long-term immobilization less effectively than FYM. Thus, organic fertilization maintains P in more biologically available forms and mitigates abiotic P fixation; our research provides a mechanistic foundation for more efficient and resilient P management in agroecosystems.