ARS | Publication request: INTEGRATING P AND N DECISION MANAGEMENT AT WATERSHED SCALES

Submitted to: Proceedings of the Symposium on Watershed Management
Publication Type: Proceedings
Publication Acceptance Date: May 20, 2001
Publication Date: August 20, 2001
Citation: Mcdowell, R.W., Sharpley, A.N., Kleinman, P.J. 2001. Integrating p and n decision management at watershed scales. Proceedings of the Symposium on Watershed Management. [CD-ROM] 18 p.

Technical Abstract: The persistence of water quality problems has directed attention towards the reduction of agricultural non-point sources of phosphorus (P) and nitrogen (N). We assessed the practical impact of three management scenarios to reduce P and N losses from a mixed land use watershed in central PA. Using an agronomic soil P threshold of 100 mg P/kg Mehlich-3, above which no crop response is expected, 80% of our watershed would receive no P as fertilizer or manure. An environmental soil P threshold of 200 mg P/kg Mehlich-3, above which the loss of P in surface runoff and subsurface drainage increases greatly, restricts future P inputs in only 40% of the watershed. Finally, site assessment P and N indices were developed and imposed. These indices contained factors, which account for likely source and transport risks and showed that 20% of the watershed was at high risk or greater of P loss, while 60% of the watershed was classified as of high risk of nitrate leaching. Areas at risk of P loss were near the stream channel, while areas at risk of nitrate leaching were near the boundaries of the watershed, where freely draining soils and high manure and fertilizer N applications coincide. It is suggested that remedial measures to minimize P export should focus on critical source areas, while remedial measures to reduce N losses should be source based, concentrating on the more efficient use of N by crops.