Iron-organic carbon coupling controls phosphorus transformation and release in decades manure-enriched paddy soil

Manure fertilizer, as high P source, is widely used to enhance soil P availability, achieving organic waste recycling and sustainable agriculture. However, the excessive application of manure fertilizer poses risks of P loss. Herein, we address this critical challenge through a 25-year in-situ field experiment combined with a 60-days anaerobic incubation experiments, exploring the vital roles of iron (Fe) and organic carbon (OC) in regulating P transformation. Our in-situ field results reveal that prolonged manure fertilizer inputs resulted in the decoupling of the Fe dissolution and inorganic P (P_i) release in paddy soil. Under the further anoxic incubations, the results showed both chemical fertilizer (CF) and manure fertilizer (pig manure, CPM) lifted the P_i concentration due to dissimilatory Fe reduction, but through different mechanisms. In CF treatment, the increased P mainly derived from the release of Fe-bound P_i (Fe-P_i). CPM enhanced the Fe transformation from crystalline to amorphous, resulting in 18.8 ​% loss of Fe-bound OC, this resulted in the coupled Fe-bound organic P (Fe-P_o) decreased from 455.4 to 7.9 ​mg ​kg⁻¹, ultimately leading to a sharp increase in labile P from 177.2 to 353.4 ​mg ​kg⁻¹. Notably, Fe-bound lipids and proteins were more prone to microbial degradation, accelerating the breakdown of organic matter and enhancing P mobilization. Our findings underscore the pivotal role of Fe-OC interactions in controlling P release and provide critical insights for developing more effective strategies to optimize P management in sustainable agricultural practices.