Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/14/2012
Publication Date: 10/16/2012
Publication URL: http://handle.nal.usda.gov/10113/55937
Citation: Sharpley, A., Beegle, D., Bolster, C.H., Good, L., Joern, B., Ketterings, Q., Lory, J., Mikkelsen, R., Osmond, D., Vadas, P.A. 2012. Phosphorus indices: Why we need to take stock of how we are doing. Journal of Environmental Quality. 41(6):1711-1719. Interpretive Summary: The goal of a Phosphorus (P) Index is to estimate the risk of P loss from any given field in order to guide field management. Currently, 48 U.S. states have adopted the P Index as a site assessment tool to identify critical source areas and target remedial practices, required by the Natural Resources Conservation Service (NRCS) 590 Nutrient Management Conservation Standard and other state and federal programs. As different versions of the P Index have emerged, to account for regional differences in soil types, land management, climate, physiographic and hydrologic controls, manure management strategies, and policy conditions; so have differences in P and manure management recommendations under relatively similar site conditions. Such inconsistencies among P Indices of different states contributed to NRCS initiating a revision of the 590 Standard in late 2009. The need for revision was heightened by a slower than expected decrease in P-related water quality impairment. Further, in certain areas of the U.S., there is growing concern that P-based nutrient management is not bringing about as great a reduction in elevated soil P levels and P loss from agricultural lands as expected or desired. This paper discusses the major issues related to future P Index use by: 1) outlining the important issues that need to be addressed as P Index development and implementation proceeds, 2) demonstrating the complexity of these issues and why they need to be thoroughly vetted, and 3) documenting some of the existing and proposed science behind the issues.
Technical Abstract: Many states have invested a great deal of research into defining the necessary components of their P Index, particularly source factors, which reliably estimates the risk of phosphorus (P) loss and incentivizes conservation management. However, differences in management recommendations and outcomes among state P Indices that result in a range of allowable P application rates for similar field conditions, predicated the need for revision of the 590 Nutrient Management Standard by Natural Resources Conservation Service (NRCS). Also, little reduction in extent and degree of P-impaired surface waters or declines in elevated soil test P (STP), fuelled concern that Indices were not initiating sufficient management change. Thus, a review of current Indices, in terms of their scientific underpinnings, need to independently evaluate their ability to assign risk, and need for unified outcomes were undertaken. While some states have initiated STP or P sorption saturation thresholds, above which no application of P is allowed in conjunction with Indices, these alone cannot define a site’s risk of P loss. Even so, there is a point above which the risk of P loss from a field is too great to warrant the application of P in any form, either from an environmental or finite resource conservation perspective. Most critically, there needs to be a rigorous evaluation of any P Index to determine if they are directionally and magnitudinally correct. While use of observed P loss data under various management scenarios is ideal, such data are not widely available. Alternatively, use of a locally relevant and validated water quality model may be the most expedient option to conduct Index assessments in the short time required by the newly revised 590 Standard.