The Release of Soil Phosphorus to Runoff in Relation to the Kinetics of Desorption'

The release of P from surface soil to runoff water under simulated rainfall Conditions was investigated for five soils at several rates of fertilizer P application. A simplified kinetic model proposed earlier, describing the desorption of soil P, adequately described both the concentration of soluble P in runoff during an event and mean values for successive events. During an event the logarithm of soluble P Concentration decreased linearly as the logarithm of time increased. Mean soluble P Concentrations of individual runoff events increased linearly with amount of desorbable P in the surface soil (top 0.5 cm), while the effect of increased rain intensity on concentrations could be explained by a power function related to water/soil ratio. The average depth of runoff-surface soil interaction calculated from the data using the kinetic model ranged from 1.5 to 3.0 mm for the five soils, and was significantly related to degree of soil aggregation. These depths agreed closely with those determined using "P as a tracer in earlier studies. The depth of interaction increased with increase in soil slope, kinetic energy of the raindrops, and, to a lesser extent, rainfall intensity. The use of the kinetic model would improve modeling of soluble P loading in runoff. Additional Index Words: phosphate release, water-extractable P, soil aggregation. Sharpley, A. N. , L. R. Ahuja, and R. G. Menzel. 1981. The release of soil phosphorus to runoff in relation to the kinetics of desorption. J. Environ. Qual. 10:386-391. The transport of phosphorus (P) in runoff water from agricultural watersheds 'to streams and reservoirs has 'Contribution of the Dep. of Agronomy, Oklahoma Agric. Exp. Stn. , Oklahoma State Univ. Stillwater; and Southern Plains Watershed and Water Quality Lab. , USDA-SEA, Durant, Okla. Oklahoma Agric. Exp. Stn. Journ. Ser. no. 3783. Received 15 Aug. 1980. 'Soil Scientists, USDA-SEA-AR. The senior author is under a cooperative agreement with USDA-SEA, Durant, and Oklahoma State Univ., Agreement no. 58-71330-8-22. Address: Southern Plains Watershed and Water Quality Lab. , USDA-SEA, Durant, OK 74701. 386 J. Environ. Qual. , Vol. 10, no. 3, 1981 been the subject of several recent studies (Taylor and Kunishi, 1971; Kunishi et al., 1972; Gburek and Heald, 1974; Duffy et al., 1978; Green et al., 1978). Although most of the P is moved with eroded sediment, the solution component can be appreciable under certain soil conditions and times and methods of fertilizer P application (Schuman et al., 1973; Romkens and Nelson, 1974). Soluble P in runoff water is released from a thin layer of surface soil including plant material, which interacts with rainfall and runoff. A linear relationship has been found between water-extractable p in surface soil and the average soluble P concentration of surface runoff (Romkens and Nelson, 1974; Sharpley et al., 1977, 1978). However, very little is known about the kinetics of P release during a rainstorm, as influenced by various soil and rain parameters, or of the extent and dynamics of the interacting layer of surface soil. Current nonpoint Pollution models assume a complete or partial mixing between extractable P in a thin soil surface zone and the rainfall-runoff water, in order to calculate the amount extracted by runoff water (Donigian et al., 1977; Davis and Donigian, 1979; Frere et al., 1980). These modelers have emphasized that information on the release of soil chemicals to runoff water is the weakest part of their models. This is especially true for P, whose release to runoff water is controlled by kinetics rather than by an instantaneous equilibrium between solution and solidphase P. In an earlier paper (Sharpley et al., 1981), we proposed an empirical model that described the kinetics of P desorption in water as a function of rate of P addition to soil and of water/soil ratio for five different soils. The present paper reports on the use of soil P desorption kinetics and the simplified model presented in the previous paper to describe the release of P to runoff water from soil under simulated rainfall. The effective mass or thickness of the surface soil layer interacting with rainfall and runoff was determined in the process. The Simplified Model in Relation to Release of P in Runoff In an earlier paper (Sharpley et al., 1981) we proposed and tested the following equation to describe P desorption for short to intermediate times and water/soil ratios: