Long-term phosphorus fertility in wastewater-irrigated cropland

Land treatment of municipal wastewater effluent is a proven method for augmenting freshwater resources and avoiding direct nutrient discharges to surface waters. We assessed changes in soil test phosphorus (P) of the Ap horizon of cropped fields continuously irrigated for 26 yr with secondary effluent from the Penn State University wastewater treatment plant. For annual P additions averaging 97 kg P ha^-1, Mehlich-3 P (M3P) response in the 0- to 20-cm surface soil (initially <20 mg kg^-1) was represented by two lines. For the first 12 yr of irrigation, soil test P increased, with 14.5 kg P ha^-1 needed to increase M3P by 1 mg P kg^-1. After the initial buildup, M3P maintained a quasi-steady-state value of ∼110 mg kg^-1. Over time, the surface soil equilibrium P concentration at zero sorption increased markedly (from <1 to 5.5 mg P L^-1), and extractable aluminum (Al) decreased significantly (P < 0.001). Speciation modeling using Visual MINTEQ suggests complexation of Al by dissolved organic carbon at site pH conditions. Loss of Al from the surface layer lowered its P-sorbing capacity, causing added effluent-P to move into the subsoil. Results suggest that current management practices can continue for many years without exceeding the surface soil M3P environmental threshold (200 mg kg^-1) used in state P-based nutrient policies.