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Estimating Labile and Dissolved Inorganic Phosphate Concentrations in Surface Runoff¹

ABSTRACT

Mathematical models are needed to better estimate effects of alternative management methods on the amounts of bioavailable P lost in surface runoff over a variety of soil, topographic, and climatic conditions. In this study, methods for estimating concentrations of soil-derived labile (dissolved plus adsorbed) and dissolved P in runoff are proposed and tested. The methods are based on the assumption that most labile P contributed by soil to runoff is derived from soil particles that are detached and transported with runoff water. Effects of erosive selectivity are accounted for by dividing soil and sediment into five undispersed size fractions. Isotherms relating adsorbed labile and dissolved P levels were developed for each from soil analyses. Labile P concentrations in runoff were estimated by summing the product of labile P concentrations prior to entrainment in runoff and measured sediment concentrations for each of the size fractions. Dissolved P concentrations were estimated by distributing labile P between dissolved and adsorbed forms using a mass balance approach. Methods of estimation were tested with runoff from 14 natural rainstorms occurring on 10 fallow plots having soil P levels ranging from about 1.3 to 3.2 mmol kg^–1 as labile P. Linear regressions of observed on estimated values of both labile and dissolved P were significant. For most events, slopes and intercepts of regression lines were not significantly different from one and zero, respectively, indicating good absolute agreement. Results indicate that soil analyses in combination with models for predicting the amount and size distribution of eroded soil form a basis for estimating movement of bioavailable P forms in runoff. However, additional studies are needed to determine the range of soil types for which the assumptions inherent in the development of the approach are valid.

Key Words: bioavailable P • eutrophication • nonpoint-source pollution • aggregates • adsorption isotherms

¹ Contribution from the Watershed Research Unit, USDA-ARS, Columbia, MO 65203.

² Soil scientists, USDA-ARS, Columbia, MO.

Received for publication December 28, 1983.

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