Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2000
Publication Date: 11/1/2000
Citation: Kleinman, P.J.A, Bryant, R.B., Reid, W.S., Sharpley, A.N., Pimentel, D. 2000. Using soil phosphorus behavior to identify environmental thresholds. Soil Science. 165:943-950. Interpretive Summary: Concern over the transport of phosphorus from agricultural soils to surface waters has focused attention on environmental thresholds of soil phosphorus. Such thresholds demarcate levels of soil phosphorus above which potential for phosphorus loss to runoff or leach water increases dramatically. By relating the concentration of phosphorus extracted by various compounds, it is possible to construct phosphorus Quantity/ Intensity relationships, with Quantity representing solid phase phosphorus, and Intensity representing solution phase phosphorus. Soil Intensity is considered an indicator of phosphorus loss potential. This study compares an existing method of determining thresholds in Quantity/Intensity relationships with a new method devised specifically for phosphorus sorption isotherms. Both methods identify similar thresholds, suggesting that these thresholds represent a fundamental quality of soil phosphorus behavior, and, hence, the potential for soil phosphorus loss in runoff and leaching.
Technical Abstract: Concern over the transport of phosphorus from agricultural soils to surface waters has focused attention on environmental thresholds of soil P. This study explores the existence of natural thresholds in soil chemical behavior as expressed by P Quantity/Intensity relationships. Fifty-nine samples were collected from agricultural soils in New York's Delaware River rWatershed, and Morgan, Mehlich III and 0.01 M CaCl2 extractable P determined. Soil P sorption saturation was calculated as a function of oxalate extractable P, Fe and Al. In addition, P sorption isotherms were constructed for all soils. Thresholds in the relationships between CaCl2 P and Morgan P, Mehlich III P and P sorption saturation were identified by change point analysis. A change point in the relationship between CaCl2 P and Morgan P, Mehlich III P and P sorption saturation occurred at a CaCl2 P concentration of 0.9 mg kg-1, suggesting a common chemical behavior threshold for soil P. A P sorption threshold was identified by segmented, quadratic-linear regression of the sorption isotherms. Results appear to describe a fundamental property of soils: a non-linear behavior of P in soils that exhibits a threshold, above which the potential for P release increases. This threshold describes a critical point in the release of P from soil to water, and therefore may be of importance as an environmental threshold considering the potential for P loss in transport pathways such as runoff and leaching.