Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2006
Publication Date: 1/9/2007
Citation: Vadas, P.A., Gburek, W., Sharpley, A.N., Kleinman, P.J., Moore Jr, P.A., Cabrera, M.L., Harmel, R.D. 2007. A model for phosphorus transformation and runoff loss for surface-applied manures. Journal of Environmental Quality. 36:324-332. Interpretive Summary: Phosphorus loss in runoff from agricultural fields can contribute to non-point source pollution of water bodies. An important source of phosphorus in runoff is animal manures that have been applied to fields and left unincorporated. Computer models are used to help idenitfy farm practices that can reduce phospphours loss to the envrionment, but most models do not simulate direct transport of phosphorus from surface manures. We developed a new model that simulates surface application of manure, dividing manure phosphorus into water-extractable and stable pools. Manure solids and stable phosphorus decompose with time, and manure solids and phosphorus are assimilated into soil to simulate insect or freeze/thaw action. Manure water-extractable phosphorus is leached by rain, with leached phosphours transferred either to runoff or into soil. Soil also contributes phosphorus to runoff. Data from field studies in Texas, Pennsylvania, Georgia, and Arkansas show that the model accurately predicts daily changes in manure solids, manure and soil phosphorus pools, and storm-by-storm and cumulative phosphorus loss in runoff. The new model can improve existing models that help quantify and decrease agricultural phosphorus loss to the environment.
Technical Abstract: 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 (r2 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.