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 33 P 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 33 P 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.