A conceptual approach for integrating phosphorus and nitrogen management at watershed scales

Since the late 1960s, point-sources of water pollution have been reduced du e to their ease of identification and treatment. As water quality problems remain and Further point-source measures become less cost-effective, attent ion is directed toward reducing agricultural nonpoint-sources of P and N. I n the past, separate strategies for P and N were developed and implemented at farm or watershed scales, Because of differing biology, chemistry, and f low pathways of P and N in sop, these narrowly targeted strategies may lead to mixed results, In some cases, N management of manures has increased sai l P and subsequent P enrichment of surface runoff, while no-till has reduce d P losses but increased nitrate leaching. Thus, an integrated approach to nutrient management is needed, with best management practices (BMPs) target ed to critical areas of a watershed that contribute most of the P and N exp orted, We have developed indices that identify critical sources and transpo rt pathways controlling P and N export. These indites are applied to a mixe d land use watershed in Pennsylvania, Areas most vulnerable to P loss are l imited to small, well-defined areas of the watershed (<20% of area) near th e stream channel. In contrast to P, larger areas contribute to nitrate leac hing and generally occur on the upper boundaries of the watershed (60%), wh ere freely draining seas and high manure and fertilizer N applications are made. Thus, differing levels of nutrient management may be appropriate for different areas of a watershed.