Sources of phosphorus exported from an agricultural watershed in Pennsylvania

Freshwater eutrophication is usually controlled by inputs of phosphorus (P). As point source controls have decreased P inputs, the relative contributions of nonpoint inputs from agriculture have increased. Thus, remedial strategies are now being directed towards minimizing P export from agriculture. To identify critical sources of P, we investigated chemical and hydrologic factors controlling P export from a mixed land use (30% wooded, 40% cultivated, 30% pasture) 39.5 ha watershed in east-central Pennsylvania. The watershed was divided into four hydrologically distinct segments and streamflow and P concentration from each segment measured since June 1996. Mehlich-3 extractable soil P, determined on a 30-m grid over the watershed, ranged from 7 to 788 mg kg-1. Generally, soils in wooded areas had low Mehlich-3 P (<30 mg kg-1), grazed pasture had Mehlich-3 P values between 100 and 200 mg kg-1, and cropped fields receiving manure and fertilizer applications were in most cases above 200 mg kg-1. Average flow-weighted P concentrations for ten storms during August-November 1996 decreased 60% downstream from segments 4-1 (watershed outlet). Streamflow P concentrations were more closely related to the near-stream (within 60 m) than whole watershed distribution of high-P soils in each watershed segment. This suggests that near-stream surface runoff and soil P, control P export form the watershed. Thus, managing P application in the primary surface runoff-producing areas near the stream has a greater potential to decrease P export in streamflow than for areas further from the channel. Clearly, the integration of areas of high soil P with areas of surface runoff production must be considered when guidelines are developed for P applications. Considering the distribution of high P soils alone may unnecessarily restrict farm management options without providing the desired reduction in P export from watersheds.