Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2006
Publication Date: 10/27/2006
Citation: Elliot, H.A., Brandt, R.C., Kleinman, P.J., Sharpley, A.N., Beegle, D.B. 2006. Estimating Source Coefficients for Phosphorus Site Indices. Journal of Environmental Quality. 35:2195-2201. Interpretive Summary: The advent of phosphorus-based management in U.S. agriculture has resulted in widespread implementation of the Phosphorus Index to guide land application of phosphorus in fertilizers, manures and biosolids. While there has been widespread recognition that different types of applied phosphorus sources have different potentials to release phosphorus to runoff, few states have found a means to recognize these differences in the Phosphorus Index. This study documents the development of one approach to using manure and biosolid testing data to improve the ability of the Phosphorus Index to represent the vulnerability of different phosphorus sources to phosphorus loss.
Technical Abstract: Phosphorus (P) release to runoff varies widely for different land-applied organic-P sources even when applied at equivalent total P rates. To address this variability, some P site indices include tabulated P source coefficients (PSCs) for differential weighting of applied P materials based on their runoff enrichment potential. Because runoff P can vary widely even within source categories depending on composition, storage, and treatment differences, this study explored a method for estimating PSCs based on the water extractable P (WEP) content of the applied amendment. Using seven published rainfall-runoff studies that followed National Phosphorus Research Project protocols, runoff dissolved P (RDP) was correlated (r-squared = 0.80) with WEP for multiple applied manures and biosolids. Assuming amendments with WEP greater than or equal to 10 g per kg behave as highly soluble P sources and have a maximum PSC of 1.0, an empirical equation was developed for computing source-specific PSCs from laboratory-determined WEP values [PSC = 0.102 ×WEP**0.99]. For two independent runoff experiments, correlations between RDP loss and P source loading rate were improved when loading rates were multiplied by the computed (r-squared = 0.73-0.86) versus generic (r-squared = 0.45-0.48) PSCs. Source-specific PSCs should enhance the ability of assessment tools to identify vulnerable sites and P loss management alternatives, although the exact inclusion process depends on index scaling and conceptual framework.