Modeling Phosphorus Transfer between Labile and Nonlabile Soil Pools: Updating the EPIC Model

Phosphorus transfer from agricultural soils to surface waters is an important environmental issue. Commonly used computer models like EPIC have not always been appropriately updated to reflect our improved understanding of soil P transformations and transfer to runoff. Our objectives were to determine if replacing EPIC’s constant sorption and desorption rate factor (0.1) with more dynamic rate factors can more accurately predict changes in soil labile P on addition to and depletion of P from soils. From published data, methods were developed to easily determine dynamic sorption and desorption rate constants from soil properties. These methods were tested with data from new soil P incubation experiments where changes in soil labile P after P addition to and depletion from nine U.S. soils were measured. Replacing constant 0.1 P sorption rate factors with dynamic factors improved prediction of soil labile P with time after P additions but more so for high-clay than low-clay soils. EPIC’s constant 0.1 P desorption rate factor greatly underpredicted soil P desorption. Increasing the constant to 0.6 improved predictions, whereas dynamic P desorption rate factors most accurately predicted P desorption. Soil P simulations showed that replacing constant P sorption and desorption rate factors with dynamic ones may change dissolved P loads (kg ha) in runoff for common soil, cropping, and runoff scenarios by only 1 to 8% in the long term but by 8 to 30% in the short term. These improvements are recommended given the simplicity of making EPIC’s sorption and desorption rate factors dynamic. PHOSPHORUS (P) transfer from agricultural soils to P-limited surface waters can stimulate eutrophication, which limits water use for drinking, recreation, and industry (Bennett et al., 2001; Sharpley et al., 1999). Over the past decade, substantial effort has been put toward eliminating excess inputs of agricultural P to surface waters. Understanding of sources and pathways of P transfer has greatly improved (Gburek et al., 2000; Sims et al., 2000), but computer models used to simulate P transport from agricultural soils to the environment have not always been updated to reflect this improved understanding (Sharpley et al., 2002). Because they are relatively rapid and cost effective, models such as EPIC (Williams et al., 1983), GLEAMS (Leonard et al., 1987), ANSWERS (Bouraoui and Dillaha, 1996), and SWAT (Arnold et al., 1998) are used to identify agricultural areas in watersheds with a high potential for P transfer to runoff, to quantify the P transfer, and to assess the ability of management practices to minimize the transfer. Soil P routines in these models were developed from EPIC, often with modifications. Soil P routines in EPIC provide for pools of Active andLabile inorganic P (Jones et al., 1984).Any inorganic P added to soil becomes labile at application but may be quickly transferred to Active P to simulate soil P sorption. When Labile P decreases due to transfer to runoff or plant uptake, P is transferred from the Active to the Labile P pool to simulate soil P buffering. Flow between Labile and Active P is governed by an equilibrium equation with a constant rate factor of 0.1 d. Because experiments have shown that inorganic P sorption from a labile to a more stable form is initially rapid and slows with time (Indiati et al., 1999; Javid and Rowell, 2002), a constant rate factor may not best simulate soil P sorption dynamics. The 0.1 rate constant for soil P buffering exists in EPIC with no apparent justification. EPIC was designed to simulate soil erosion and its effects on crop productivity for a variety of soils, climates, crops, and conservation practices. A constant rate of P transfer between Labile and Active P was adequate for these purposes. As the effort to control P transfer from agricultural soils has intensified, EPIC and other models with its soil P routines are used to simulate dissolved and sediment P transfer from soil to runoff (Pierson et al., 2001, Sharpley et al., 2002). Because soil Labile P in such models is the sole source for dissolved P in runoff and contributes to sediment P in runoff, Labile P dynamics must be accurately simulated if a model is to reliably estimate runoff P. Because most annual P transfer from soils can often occur during a few, intense storms (Pionke et al., 1996, 2000), especially after recent additions of P to soils (Shreve et al., 1995), models must accurately simulate shortand long-term soil P dynamics. As the uses of EPIC and othermodels change, wemust examine model routines and determine if they are appropriate for new uses. Our objective was to develop dynamic rate factors to update the constant 0.1 factor controlling P flow between the inorganic Labile andActive P pools for EPIC and other models that use its P routines. MATERIALS AND METHODS Description of Phosphorus Subroutines in EPIC Soil P routines in EPIC provide for pools of Active and Labile inorganic P (Jones et al., 1984). Labile P represents easily desorbable P immediately available for plant uptake or transfer to runoff and is defined as P extracted by anion exchange resin (Sharpley et al., 1984). Model users must input an initial value for Labile P. Active P represents more stable P that is not easily desorbable but is in equilibrium with Labile P. Active P is initialized from Labile P and a P sorption coefficient (PSC) as: Active P5 (LabileP) (1 2 PSC)/PSC [1] P.A. Vadas and A.N. Sharpley, USDA-ARS, Pasture Systems and Watershed Management Research Unit, Building 3702, Curtin Rd., University Park, PA 16802-3702. T. Krogstad, Dep. of Plant Environ. Sci., Norwegian Univ. Life Sci., N-1432 Aas, Norway. Received 1 Mar. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:736–743 (2006).