Orthophosphorus Contributions to Total Phosphorus Concentrations and Loads in Iowa Agricultural Watersheds

Abstract

© American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. Phosphorus (P) is delivered to streams as episodic particulate P and more continuous soluble P (orthophosphorus [OP]), and it is important to determine the proportion of each P form in river water to more effectively design remedial measures. In this study, we evaluated the annual mean ratios of OP to total P (TP) concentrations and loads in 12 Iowa rivers and found systematic variation in the ratios. The OP/TP ratios were > 60% in two tiledrained watersheds of the Des Moines Lobe and in a shallow fractured bedrock watershed in northeast Iowa, whereas in southern and western Iowa, OP contributions to TP were < 30%. Higher OP/TP ratios were associated with greater row crop intensity in the watershed and a greater proportion of baseflow in the river. Orthophosphorus contributions from croplands would be greater in watersheds characterized by widespread tile drainage and well-drained soils, whereas cropland TP export would be dominated by particulate P in dissected till plains with poorly drained soils. Understanding the dominant form and transport pathway of P from agricultural areas in a watershed is seen as an important first step in determining appropriate conservation practices to reduce P loads.