Nitrogen and phosphorus runoff from cropland and pasture fields fertilized with poultry litter

Application of litter and other organic by-products to agricultural land off site of animal production facilities has created both environmental concerns and agro-economic opportunities, but limited long-term, field-scale data are available to guide management decisions. Thus, the objective of this study was to determine the water quality effects of repeated annual poultry litter application as a cropland and pasture fertilizer. Eight years of data collected on ten field-scale watersheds indicated several significant water quality differences based on litter rate (0.0 to 13.4 Mg ha⁻¹ [0 to 6 ton ac⁻¹]) and land use (cropland and pasture). On cropland fields, increasing litter rates (with corresponding decreases in supplemental inorganic nitrogen [N]) increased runoff orthophosphate phosphorus (PO₄-P) concentrations but reduced extreme high nitrate nitrogen (NO₃-N) concentrations. Whereas runoff PO₄-P concentrations were somewhat similar between land uses, NO₃-N concentrations were much lower in pasture runoff because of supplemental inorganic N application, reduced nutrient uptake potential, and faster litter mineralization on cropland. Although considerable variability was observed, intra-annual runoff NO₃-N and PO₄-P concentrations generally exhibited curvilinear decay based on time since fertilizer application. In spite of repeated annual litter application and buildup of soil phosphorus (P) at high litter rates, few long-term trends in N and P runoff were evident due to the dynamic interaction between transport and source factors. These results support several practical implications, specifically: (1) combining organic and inorganic nutrient sources can be environmentally friendly and economically sound if application rates are carefully managed; (2) high runoff N and P concentrations can occur from well-managed fields, which presents difficulty in regulating edge-of-field water quality; and (3) change in the animal industry mindset to view by-products as marketable resources could mitigate environmental problems, provide alternative fertilizer sources, and enhance animal industry revenue opportunities.