Phosphorus management at the watershed scale: a modification of the phosphorus index

We considered hydrologic and chemical factors controlling P export from a 3 9.5-ha mixed land use watershed in east-central Pennsylvania, focusing our evaluation on watershed vulnerability to P loss. The spatial variations of P source factors, soil P, and P inputs from fertilizer and manure were eval uated. Distribution of Mehlich-3 soil P on a 30-m grid over the watershed s howed that soil P varied with land use. Soils in wooded areas had low Mehli ch-3 P (<30 mg kg(-1)); grazed pasture had Mehlich-3 P values between 100 a nd 200 mg kg(-1); and cropped fields receiving manure and fertilizer applic ations were mostly >200 mg kg(-1). Phosphorus sources and transport control s on P loss were evaluated by examining in-stream P concentrations during s torm hydrographs. Phosphorus concentrations decreased 50% downstream from h eadwaters to watershed outlet, and were more closely related to near-stream (within 60 m) distribution of high-P soils than to that of the whole water shed. This suggests that near-stream surface runoff and soil P are controll ing P export from the watershed. Based on these findings, we modified the P hosphorus Index (PI), a user-oriented tool developed by the NRCS-USDA to id entify critical source areas controlling P export from agricultural watersh eds. The modification separately evaluates P source and transport factors, and incorporates the hydrologic return period to describe contributing area s. The modified PI was applied to the watershed to illustrate interactions between P source and transport processes controlling P export, and approach es for managing P loss.