Reductive dissolution-induced phosphorus release mediated by iron speciation in flooded Jie River sediments

The release of sediment-bound phosphorus (P) into overlying water is a critical driver of eutrophication in freshwater ecosystems. In seasonal rivers, alternating dry and wet periods create dynamic redox conditions that can mobilize legacy P, but the specific mechanisms governing P release under continuous inundation, particularly the role of iron (Fe) redox cycling, remain inadequately understood. This study investigates P release mechanisms mediated by Fe speciation transformations in sediments of the seasonal Jie River under prolonged flooding. Inundation triggered reductive dissolution of crystalline iron oxides (Fed), which in turn mobilized sediment phosphorus. Phosphorus speciation shifted significantly, with Fe-bound phosphorus (Fe-P) and occluded phosphorus (Oc-P) decreasing, while more bioavailable fractions such as exchangeable phosphorus (Ex-P) and aluminum-bound phosphorus (Al-P) increased. Strong correlations between Fed and both Fe-P and Ex-P confirmed that iron redox cycling is the principal mechanism controlling phosphorus release and redistribution. Declining Fe-P and rising soluble Al-P indicate high eutrophication potential despite TP accumulation (1465.34 mg/kg; 67.52% inorganic). Iron redox cycling is the primary driver of phosphorus release and redistribution in inundated river sediments. Reductive dissolution of iron oxides mobilizes legacy phosphorus (notably Fe-P), increasing bioavailable fractions. This mechanism sustains eutrophication risks even after external load reduction.