Long-Term Phosphorus Fertility in Wastewater Irrigated Cropland.

Land treatment of municipal wastewater effluents is a proven method for augmenting declining freshwater resources and avoiding direct nutrient discharge to surface waters. Cropland continuously irrigated for 26 yr with secondary effluent from Penn State University has experienced a buildup of soil phosphorus (P) in excess of crop requirements in response to average annual P addition of ~105 kg P ha^-1. The accumulation of available P in the 0 to 0.20-m surface soil layer increased linearly for the first 9 (±1) yr of irrigation with a net 9.1 kg P ha^-1 needed to raise Mehlich-3 extractable P (M3P) by 1 mg P kg^-1. Despite continuous P addition in excess of crop removal, the M3P appears to have approached a steady-state value of ~115 mg kg^-1 after the initial M3P buildup phase. The surface soil equilibrium P concentration at zero sorption (EPC₀) has increased markedly (from <1 to ~5 mg P L^-1) and extractable Al has decreased significantly over the site life. Geochemical speciation modeling using visual MINTEQ confirms complexation of Al by dissolved organic carbon (DOC) at soil pH conditions of the site. Loss of Al from surface soils has reduced P sorbing capacity, causing added effluent P to move deeper in the soil profile rather than accumulating in the solum. Results suggest that current management practices are sustainable long-term without soil P levels reaching the M3P environmental threshold (200 mg kg^-1) used in state P-based nutrient management policies.