LAUSR

Anaerobic conditions developed during flooding can increase phosphorus (P) losses from soils to waterways. Soil amendment with gypsum (CaSO4 •2H2 O) can effectively reduce flooding-induced P release, but its effectiveness is soil-dependent, and the reasons are poorly understood. The objectives of the study were to reveal the possible inorganic P transformations during flooding of two soils [acidic-Neuenberg sandy loam (NBG-SL) and alkaline-Fyala clay (FYL-Cl)], with and without gypsum amendment prior to flooding. Porewater collected at 0, 35, and 70 days after flooding (DAF) from soils incubated in vessels were analyzed for dissolved reactive P (DRP), pH, concentrations of calcium (Ca), magnesium, iron (Fe), manganese, chloride, nitrate, sulfate and fluoride. Thermodynamic modeling using Visual MINTEQ software and chemical fractionation of soil P were used to infer P transformations. Soil redox potential (Eh) decreased with flooding and favored reductive dissolution of Fe associated P increasing porewater DRP concentrations. Greater solubility of Ca-P under acidic pH maintained a higher DRP concentration in NBG-SL during early stages of flooding. Subsequent increase in pH with flooding and higher Ca concentration with added gypsum, enhanced the stability of Ca-P (β-Tricalcium phosphate and octacalcium phosphate) reducing the DRP concentration in gypsum-amended NBG-SL. Stability of Ca-P was less affected with flooding and gypsum amendment in FYL-Cl soil as it had an alkaline pH and inherently higher Ca concentration. FYL-Cl with a more rapid decrease in Eh than NBG-SL, became severely reduced, releasing more P and Fe by 70 DAF. These conditions favored vivianite formation in FYL-Cl, but not in NBG-SL. This article is protected by copyright. All rights reserved.