Dissolution of phosphorus into pore-water flowing through an organic soil

Abstract

Understanding dissolution and transport of phosphorus (P) from organic soils in agricultural fields and restored wetlands is critical for devising management strategies to mitigate P losses. The objective of this study was to evaluate the dissolution of P in water flowing through the vadose zone-shallow ground water continuum of an organic soil. Three 90cm×50cm×8cm flow cells were uniformly packed with organic soil material from a Ponzer muck (Terric Haplosaprists) collected from an area that had been in agricultural production for at least 30years. The packed flow cells were instrumented with soil solution samplers and platinum-tipped redox electrodes at approximately 5cm below a simulated water table (WT), and in the capillary fringe (CF) at approximately 5 and 20cm above the WT to collect soil solution and monitor the reduction potential (Eh), respectively. Distilled water was continuously supplied at constant rates of 1.2, 2.4, and 3.6Ld−1 to one end, and drained at the other end of the flow cells while maintaining a WT at 12cm above the bottom of the midpoint of each flow cell. Phosphorus concentration in the outflow solution was consistently above the USEPA water quality criteria of 0.1mgL−1. Changes in pore-water velocity did not alter the amounts of P leached within the time frame of the experiment. Dissolved phosphorus concentrations at both 5cm below and 5cm above the WT were significantly higher than at 20cm above the WT. This observation indicated that P that leached out of the flow cells was not only from the saturated zone but also from the lower part of the CF. These results suggest that controlling the height of the WT to limit saturation of P enriched surficial soils, while considering the contribution of the CF, may be an effective management tool for limiting P export from restored wetland sited on former agricultural fields.