Modification and Validation of GLEAMS for Prediction of Phosphorus Leaching in Waste-Amended Soils
Excess phosphorus applied to soils with low P adsorption capacities can enter surface water via leaching and subsurface transport, thereby negatively impacting water quality. Computer simulation models can be used to describe the effects of management practices on P leaching losses, provided the models are appropriately validated. The objectives of this research were to modify and validate P subroutines in the GLEAMS (Groundwater Loading Effects of Agricultural Management Systems) model to more accurately reflect P sorption and desorption, and then use the modified model to determine crop and waste management effects on long-term P leaching losses below the root zone of a grazed pasture with >20-year history of swine lagoon liquid application and considerable P buildup in the soil profile. GLEAMS was modified with the Langmuir equation to partition labile P between adsorbed and solution phases. The modification improved predictions of percolate P concentrations and soil P accumulation in acid sandy soils receiving waste-based P additions. The modification also increased model sensitivity to changes in crop and P management. The modified model predicted that P-based swine lagoon liquid applications would decrease P leaching by >20 kg ha-1 year-1 compared to N-based applications. Eliminating all P applications decreased the predicted P leaching losses by less than 1 kg ha-1 year-1 compared to P-based swine lagoon liquid application. Results show that P can continue leaching from P-saturated soils even in the absence of P additions.