ASSESSING SITE VULNERABILITY TO PHOSPHORUS LOSS IN AN AGRICULTURAL WATERSHED

Authors: Sharpley, Andrew; Kleinman, Peter; MCDOWELL, RICHARD - PENN STATE UNIVERSITY

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2001
Publication Date: 11/20/2001
Citation: Sharpley, A.N., Kleinman, P.J., Mcdowell, R.W. 2001. Assessing site vulnerability to phosphorus loss in an agricultural watershed. Journal Of Environmental Quality. 30(6):2026-2036.

Interpretive Summary: Although phosphorus inputs are essential to crop and livestock production, phosphorus export in runoff can accelerate the eutrophication of receiving fresh waters. The concentration of crop and livestock production in separate areas of the country has led to local accumulations of phosphorus in excess of local crop needs. Increases in surface soil phosphorus concentrations exacerbate phosphorus losses in surface runoff and subsurface flow. In response to these trends as well as to frequent outbreaks of harmful algal blooms (e.g., Pfiesteria and cyanobacteria), the U.S. Department of Agriculture and U.S. Environmental Protection Agency have developed a joint strategy for sustainable nutrient management. Three management options for land application of phosphorus are proposed in the new strategy: (1) managing phosphorus based on crop needs; (2) managing phosphorus based upon environmental soil phosphorus thresholds, above which hrunoff phosphorus enrichment is unacceptable; (3) using a Phosphorus Index to limit phosphorus applications on fields at greatest risk for phosphorus loss. Although crop and environmental soil phosphorus threshold approaches provide incomplete assessments of the risk of phosphorus loss, there has been little site evaluation of the Phosphorus Index. The Phosphorus Index, which considers transport and source effects on phosphorus loss potential, accounted for 80% of the variability in surface runoff phosphorus from an agricultural watershed in south central Pennsylvania. Based on the relatively small export of phosphorus from the studied watershed, manure management projections using the Phosphorus Index were more defensible than the restrictive projections derived from a soil phosphorus threshold.

Technical Abstract: The P index was developed as a tool to rank agricultural fields on the basis of P loss vulnerability, helping to target remedial P management options within watersheds. We evaluated two approaches, a soil P threshold approach and the P index approach, by comparing site vulnerability estimates derived from these two approaches with measured runoff P losses in an agricultural watershed in Pennsylvania. Rainfall-surface runoff simulations (7 cm/hr for 30 min) were conducted on 57 sites representing the full range of soil P concentrations and management conditions found in the watershed. Each site was comprised of two, abutting 1 x 2 m runoff plots, serving as duplicate observations. For sites that had not received P additions for at least six months prior to the study, Mehlich-3 P concentration was strongly associated with dissolved P concentrations (r2 = 0.86) and losses (r2 = 0.83) in surface runoff, as well as with total P concentration (r2 = 0.80) and loss (r2 = 0.74). However, Mehlich-3 P alon was poorly correlated with runoff P from sites with recent histories of manure application. The P index effectively described 88 and 83% of the variability in dissolved P concentrations and losses from all sites in the watershed; and, P index ratings exhibited strong associations with total P concentrations (r2 = 0.81) and losses (r2 = 0.79). When site-specific observations were extrapolated to all fields in the watershed, management recommendations derived from the P-index approach were less restrictive than those derived from the soil P threshold approach, better reflecting the low P loads exported from the watershed.