Agricultural P transport in runoff is an environmental concern. An important source of P runoff is surface-applied, unincorporated manures, but computer models used to assess P transport do not adequately simulate P release and transport from surface manures. We developed a model to address this limitation. The model operates on a daily basis and simulates manure application to the soil surface, letting 60% of manure P infiltrate into soil if manure slurry with less than 15% solids is applied. The model divides manure P into four pools, water-extractable inorganic and organic P, and stable inorganic and organic P. The model simulates manure dry matter decomposition, and manure stable P transformation to water-extractable P. Manure dry matter and P are assimilated into soil to simulate bioturbation. Waterextractable P is leached from manure when it rains, and a portion of leached P can be transferred to surface runoff. Eighty percent of manure P leached into soil by rain remains in the top 2 cm, while 20% leaches deeper. This 2-cm soil layer contributes P to runoff via desorption. We used data from field studies in Texas, Pennsylvania, Georgia, and Arkansas to build and validate the model. Validation results show the model accurately predicted cumulative P loads in runoff, reflecting successful simulation of the dynamics of manure dry matter, manure and soil P pools, and storm-event runoff P concentrations. Predicted runoff P concentrations were significantly related to (r 2 5 0.57) but slightly less than measured concentrations. Our model thus represents an important modification for field or watershed scale models that assess P loss from manured soils.
NONPOINT-SOURCE pollution of fresh waters by agricultural P can accelerate eutrophication and limit water use for drinking, recreation, and industry (Sharpley and Rekolainen, 1997;Carpenter et al., 1998;Gibson et al., 2000). A major pathway of P transport from agricultural soils is surface runoff, to which mismanaged or excessive surface application of unincorporated animal manures can be important contributors (Kleinman and Sharpley, 2003;DeLaune et al., 2004aDeLaune et al., , 2004b. Recent research has concentrated on better understanding and minimizing P transport in runoff from surface-applied, unincorporated manures (Harmel et al., 2004;Moore et al., 2000). Computer simulation models that quantify field or watershed-scale P transport, such as EPIC (Williams et al., 1983), GLEAMS (Leonard et al., 1987), ANSWERS (Bouraoui and Dillaha, 1996), or SWAT (Arnold et al., 1998), are also used to improve management to minimize offsite P transport . The models generally use similar P routines, which adequately simulate manure applications for tilled systems where manure is well mixed into soil. However, the models do not adequately simulate surface application of manures or direct transfer of P from manures on the soil surface to runoff (Pierson et al., 2001b;Sharpley et al., 2002). If such models are used where surface-applied, unincorporated manures are comm...
