Molecular mechanism of flavonoid-induced iron oxide transformation and implication for phosphorus loss risk in paddy soils

Phosphorus (P), a non-renewable resource essential for sustaining life, faces increasing environmental losses from agricultural systems. However, the effects of flavonoid-induced iron oxide on soil P behavior remain poorly understood, particularly in paddy soils. Here, we conducted a 7-year field trial under four fertilization regimes combined with laboratory incubations to reveal how flavonoid-modified iron oxides regulate P at the molecular scale. Results showed that manure-applied (NPKM) soils increased the content of flavonoids by 48.7% and short-range-order minerals (SROs) by 8.7% compared with chemically fertilized (NPK) soils. Through interfacial reactions between Fe-P minerals and quercetin, the formation of 2-line ferrihydrite was promoted by quercetin with an interplanar distance of 0.26/0.30 nm at pH 7. It was found that 14.25% of the Fe-phosphate group was incorporated at a quercetin concentration of 1 mM, and Fe oxides acted as the "core" for retaining P in these complexes. Further, phosphate release was observed during the interfacial reaction with increasing quercetin concentrations, suggesting a potential trade-off between P fixation and release. Despite these benefits, NPKM soils exhibited the highest degree of phosphorus saturation (DPS) (18.61%) and the lowest soil phosphorus storage capacity (1.70 mg kg^-1), indicating an elevated risk of P loss. A significant positive correlation was identified among SROs, flavonoids, and DPS in paddy soils. Collectively, our findings demonstrate that flavonoids can modify the morphology of Fe oxides in paddy soils, thereby enhancing P fixation. This presents a promising approach for mitigating diffuse P pollution and promoting sustainable agriculture.