Phosphorus management for sustainable agricultural irrigation of reclaimed water

Irrigation reuse of municipal effluents conserves freshwater resources and avoids direct nutrient discharges to surface waters. The sustainability of agricultural reuse programs, however, may be challenged by evolving phosphorus (P)-based nutrient management policies. This paper assesses the agricultural and environmental implications of soil P accumulation from effluent irrigation and discusses sustainability under guidelines to control P export in agricultural runoff and drainage. For typical regulatory maximum irrigation rates (5.1-cm-wk-1), secondary effluents containing 3-4-mg-P-L-1 provide ∼90-kg-P-ha-1-yr-1 and cause soil P enrichment beyond levels needed for optimum crop yields. Applied P potentially falls short of crop needs for advanced wastewater treatment processes with low (<0.5-mg-L-1) effluent P or where hydraulic rates are restricted by soil, climatic, or regulatory conditions. Except for some locations where well-drained low P-retention soils overlie shallow groundwater, effluent-irrigated P has minimal impact on water quality. Most site assessment tools (P indices) were developed to identify fields with high P loss vulnerability from manure and commercial fertilizer application and do not explicitly address effluent-applied P. Adoption of P-management regulations not accounting for the differential behavior of P in effluent-irrigated fields may unnecessarily restrict agricultural irrigation of reclaimed water.