Modeling Effectiveness of Broiler Litter Application Method and Timing for Reducing Phosphorus and Nitrogen losses in Big Creek Watershed, Alabama

Abstract

Agriculture plays an important role in the economy of Alabama (AL) and the United States (US). In AL, broiler litter is commonly used to fertilize pastures. However, repeated application of broiler litter to same pasture fields year after year results in build-up of nutrients in soils. Excessive loss of nutrients to surface waters via agricultural runoff results in toxic algal blooms, reduces dissolved oxygen levels, and causes fish kills. The linkages among the best management practices implemented at the field-level and downstream water quality improvement at the watershed level are complex, because the processes that link management practices and watershed-level water quality span a range of scales. However, it is important to understand the effect of nutrient management strategies on watershed-level water quality because most of the water quality management occurs at the watershed scale. Specific objectives of this study were: (a) quantify the effect of broiler litter application method (surface vs. subsurface application) on P and N losses in surface runoff, and (b) determine the effect of timing of broiler litter application (with respect to the occurrence of a storm event) on P and N losses in surface runoff. The research was conducted in the Big Creek watershed (8024 ha) located in Mobile County, AL. The Soil and Water Assessment Tool (SWAT) model was used to evaluate the effectiveness of best management practices to reduce P and N losses on a long-term basis at the hydrologic response unit (HRU), subwatershed and watershed level. The results show that SWAT successfully simulated streamflow and N and P losses at the watershed outlet. Subsurface application of broiler litter helped to reduce N and P losses in surface runoff compared to surface application of broiler litter. Losses of P and N were greater in winter followed by spring, summer, and fall. Application of broiler litter with respect to the occurrence of a storm event did not affect P and N losses in surface runoff. Overall, results of this study suggest that subsurface application of broiler litter helps to reduce nutrient losses in surface runoff on a long-term basis at the watershed level.