RELATING SOIL PHOSPHORUS TO DISSOLVED PHOSPHORUS IN RUNOFF: A SINGLE EXTRACTION COOEFFICIENT FOR WATER QUALITY MODELING

Authors: Vadas, Peter; Kleinman, Peter; Sharpley, Andrew

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2004
Publication Date: 3/9/2005
Citation: Vadas, P.A., Kleinman, P.J., Sharpley, A.N. 2005. Relating soil phosphorus to dissolved phosphorus in runoff: a single extraction cooefficient for water quality modeling. Journal of Environmental Quality. 34:572-580.

Interpretive Summary: Phosphorus loss from agricultural soils contributes to the eutrophication of fresh waters. Conducting field experiments to determine the extent of P loss from all soils in a watershed or region is too expensive and time consuming, so simulation models are often the best method to achieve this goal. Most models use extraction coefficients to predict dissolved P in runoff from the concentration of soil P. Because P in runoff need not be great to constitute an environmental risk, accurate values for extraction coefficients are critical for model reliability. Much research suggests that model extraction coefficients should vary based on soil type, land use and management, and runoff hydrology. However, using data from 14 independent studies, we found a consistent relationship between soil P and runoff dissolved P. These results show that a single extraction coefficient could be used in models to approximate the effect of soil P on runoff dissolved P for a wide variety of soil, hydrologic, and management conditions. Thus, the effort made by model users to determine accurate values for extraction coefficients can be greatly simplified. More general yet accurate models to predict P loss will aid the evaluation and development of management practices that reduce P loads in surface waters from watersheds.

Technical Abstract: Phosphorus (P) loss from agricultural soils has been implicated in the eutrophication of fresh waters. Computer simulation models represent a relatively rapid, inexpensive method to identify areas in an agricultural watershed where the potential for P loss is high. Such models typically use a constant extraction coefficient to predict dissolved P loss in runoff from soil P. Although researchers have suggested that such extraction coefficients vary as a function of soil type, land use and management, and runoff hydrologic conditions, no effort has been made to determine if specific effects of soil, management, or hydrology are consistent across a number of studies. We collected published data from 14 independent studies that related soil P, extracted by either Mehlich-3 or Bray-1, to dissolved P in runoff. We found that explanations for variability in extraction coefficients within a given study did not necessarily hold true across a number of studies. In combining the data from all studies in a single plot (450 observations), we found that there was a consistent, strong relationship between Mehlich-3 extractable soil P and runoff dissolved P that could be described by a single regression line (R2 = 0.73). These results show that a single extraction coefficient could be used in water quality models to approximate the effect of soil P on runoff dissolved P for a variety of soil, hydrologic, and management conditions.