UTILIZING PHOSPHORUS BUDGETS AND ISOTOPIC TRACERS TO EVALUATE PHOSPHORUS FATE IN SOILS WITH LONG TERM POULTRY LITTER APPLICATION

Converting a nutrient management plan from commercial fertilizers to poultry litter helps effectively utilize waste from the nearly 10 billion broiler birds across the United States. Nine field scale watersheds from the USDA ARS Grassland, Soil and Water Research Laboratory near Riesel, TX were evaluated for P inputs and P outputs to determine phosphorus budgets for 15 years of annual application of poultry litter ranging from 75 – 219 kg P ha^-1 yr^-1 on cultivated and pasture/grazed fields. The cumulative net P continued to increase regardless of the application rate and had a positive relationship with soil level P (Mehlich-3 P) and flow weighted mean concentration (FWMC) for dissolved reactive P for both cultivated and pasture managed fields. We assessed hydrological connectivity within two nested watersheds by using the before-after-control-impact (BACI) design. Results showed hydrological connectivity during high rainfall years whereas low rainfall years had minimal connectivity compared to the controls. These results suggest the P contributions from upstream fields receiving poultry litter, even at high application rates, did not exhibit a treatment effect during the low rainfall years at downslope monitoring stations.

As nutrient source variability increases in nutrient management plans, improving our ability to differentiate P sources and their fate in soils is critical. We evaluated soils with unique P inputs: inorganic P, poultry litter, and cattle grazing for isotopic signatures by forming silver phosphate and determining the δ¹⁸O_P. Isotopic signatures of the oxygen molecules which are strongly bound to P, provided signatures of 17.09‰, 18.00‰, and 17.20‰ for fields receiving commercial fertilizer, poultry manure, and cattle grazed, respectively. Significant effort was made to determine critical steps in the method to successfully precipitate Ag₃PO₄ for analysis. Results show adding a cation removal step as well as monitoring and adjusting pH throughout the method increases probability of successful Ag₃PO₄ precipitation. Findings from this study provide a valuable framework for future analysis to confirm unique δ¹⁸O_P signatures which can be used to differentiate the fate of different phosphorus sources in agricultural systems.