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(-1)) 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.