Redox reactions and phosphorus release in re-flooded soils of an altered wetland

Phosphorus loss from land can be a major factor affecting surface water quality. We studied P‐release mechanisms in wetland soils that had been drained and cultivated for four decades and then re‐flooded. We measured redox, pH and solution composition in two sites in the field and in four peat and calcareous soils incubated in biogeochemical microcosms. The redox and pH measurements during the 120 days of incubation and the resulting soil solution composition indicated that the main process leading to P release is reductive dissolution of ferric hydroxides on which P was adsorbed and in which P was occluded. The molar Fe:P ratio increased with period of reduction from below 1 in the first week of re‐flooding to 15–60 after 120 days. This suggests an increased P‐retention capacity upon reoxidation of the soil solution, whether within the soil profile or in the drainage canals. Prolonged flooding of the calcite‐poor, gypsum‐rich peat soils increased the oversaturation of soil solutions with respect to hydroxyapatite and occasionally β‐Ca₃(PO₄)₂(c), indicating that in spite of the large Ca concentration, the rate of Ca‐P precipitation was insufficient to maintain the saturation status of the Ca‐P system. In the calcareous soils the Ca‐P system effectively controlled the P activity in soil solution throughout the incubation period. In both cases the precipitation of Ca‐P minerals could be an important P‐retention mechanism.