Sediment from Agricultural Constructed Wetland Immobilizes Soil Phosphorus

Phosphorus (P) losses from agricultural soils impair the quality of receiving surface waters by enhancing eutrophication. This study tested the potential of using sediment from agricultural constructed wetlands (CWs) to immobilize soil P using two soils differing in texture and soil test P (STP). A silty clay soil (SIC) with high STP (24 mg ammonium acetate-extractable P [PAc] L⁻¹) and a sandy loam soil (SL) with excessive STP (210 mg PAc L⁻¹) were incubated with increasing amounts of clayey CW sediment. The soil-sediment mixtures were studied with the quantity/intensity (Q/I) technique, using chemical extractions, and by exposing the mixtures to simulated rainfall. In both Q/I and simulated rainfall tests, P solubility steadily decreased with increasing sediment proportion in the mixtures. However, in chemical extractions this effect was observed only at high sediment addition rates (10 or 50% [v/v] sediment). At a practically feasible sediment addition rate of 5%, dissolved reactive P (DRP) in percolating water from simulated rainfall decreased by 55% in SIC and by 54% in SL (p < 0.001 in both cases). Particulate P (PP) also showed a decreasing trend with increasing sediment addition rate. Upon prolonged simulated rainfall, the decreasing effect of sediment on DRP and PP declined somewhat. The effects of sediment addition can be attributed partly to increased salt concentrations in the sediment, which have a short-term effect on P mobilization, but mostly to increased concentrations of Al and Fe (hydr)oxides, increasing long-term P sorption capacity. Adding CW sediment at a rate of up to 5% of surface soil volume to soils could provide an alternative to chemical treatment (e.g., with metal salts) for immobilizing P in small, high-risk P leaching areas, such as around drinking troughs in pastures.