QUANTIFYING LEGACY PHOSPHORUS USING A MASS BALANCE APPROACH AND UNCERTAINTY ANALYSIS

Classic agricultural-conservation practices may not address decades of phosphorus (P) accumulation, known as legacy P. Identifying and quantifying legacy P sources are necessary to identify the most costefficient conservation practices. A method was developed to identify and quantify legacy P at the watershed scale using a mass-balance approach and uncertainty analysis. The method was applied to two nutrient-rich watersheds in northeast Oklahoma and northwest Arkansas. Each P import and export to and from the two watersheds was identified and quantified using a probability distribution and uncertainty analysis. The P retained in the soils, reservoirs, and stream systems were estimated from 1925 to 2015. Over 8.5 and 6.1 kg/ha/year of P were added to the Illinois River and Eucha-Spavinaw watersheds with 53 and 55% from poultry production, respectively. Other major historical sources were attributed to human population and commercial fertilizer. Though currently the net addition of P in the watersheds is small due to the export of approximately 90% of the poultry litter, historically only 14-19% of all P imported to the Illinois River and Eucha-Spavinaw watersheds was removed via the reservoir spillways, poultry litter, and food exports. The majority of the retained P is located in the soil, 3.6-5.8 kg/ha/year, and stream systems, 0.01-3.0 per ha/year.