ASSESSING ENVIRONMENTAL SUSTAINABILITY OF AGRICULTURAL SYSTEMS BY SIMULATION OF NITROGEN AND PHOSPHORUS LOSS IN RUNOFF

Inputs of nitrogen (N) and phosphorus (P) in agricultural runoff can p romote coastal and fresh a water eutrophication. Thus, information on the effect of agricultural management on N and P loss in runoff is nee ded to develop sustainable management systems. While reliable field da ta require several years of study, simulation models can quickly estim ate the relative effects of management on nutrient loss. A kinetic equ ation describing the desorption of soil P was used to predict dissolve d P (DP) loss in runoff; enrichment ratio approaches were used to pred ict particulate N (PN), particulate P (PP), and bioavailable PP (BPP) losses. Accurate predictions of N and P loss in runoff were obtained f or watersheds under conservation and conventional till wheat; reduced and no till rotational small grain crops; winter wheat with and withou t a sorghum cover crop; and native and set-aside grasses. Overall, pre diction errors were 25 per cent of measured values. Although conservat ion practices reduced PN and PP losses in runoff up to 50 per cent, al gal available DP losses increased 56 per cent compared to conventional practices. Simulated losses of N and P from soils receiving animal ma nure (beef, poultry, and swine) for 10 to 35 years, indicate the need to minimize surface soil accumulations of N and P in excess of crop re quirements. Threshold soil P levels, above which DP concentration of r unoff is expected to exceed water quality standards associated with eu trophication, were predicted by the kinetic equation. The kinetic and enrichment ratio approaches used can provide valuable information on t he relative effects of watershed management on soil and water resource s and thereby aid development of sustainable agricultural systems.