Phosphorus Release and Speciation in Manganese(IV) Oxide and Zeolite-Amended Flooded Soils

Phosphorus (P) losses from flooded soils and subsequent transport to waterways contribute to eutrophication of surface waters. This study evaluated the effectiveness of MnO₂ and a zeolite Y amendment in reducing P release from flooded soils and explored the underlying mechanisms controlling P release. Unamended and amended (MnO₂ or zeolite, surface-amended at 5 Mg ha^-1) soil monoliths from four clayey-alkaline soils were flooded at 22 ± 2 °C for 56 days. Soil redox potential and dissolved reactive P (DRP), pH, and concentrations of major cations and anions in porewater and floodwater were analyzed periodically. Soil P speciation was simulated using Visual MINTEQ at 1, 28, and 56 days after flooding (DAF) and P K-edge X-ray absorption near-edge structure spectroscopy and sequential fractionation at 56 DAF. Porewater DRP increased with DAF and correlated negatively with pe+pH and positively with dissolved Fe. Reductive dissolution of Fe-associated P was the dominant mechanism of flooding-induced P release. The MnO₂ amendment reduced porewater DRP by 30%-50% by favoring calcium phosphates (Ca-P) precipitation and delaying the reductive dissolution reactions. In three soils, the zeolite amendment at some DAF increased porewater and/or floodwater DRP through dissolution of Ca-P and thus was not effective in reducing P release from flooded soils.