Thoroughly tested agricultural systems models can be used to quantify the long-term effects of crop management practices under conditions where measurements are lacking. In a field near Story City, Iowa, ten years (1996-2005) of measured data were collected from plots receiving low, medium, and high (57-67, 114-135, and 172-202 kg N ha(-1)) nitrogen (N) fertilizer application rates during corn (Zea mays L.) years. Using these data, the Root Zone Water Quality Model linked with the CERES and CROPGRO plant growth models (RZWQM-DSSAT) was evaluated for simulating the various N application rates to corn. The evaluated model was then used with a sequence of historical weather data (1961-2005) to quantify the long-term effects of different N rates on corn yield and nitrogen dynamics for this agricultural system. Simulated and measured dry-weight corn yields, averaged over plots and years, were 7452 and 7343 kg ha(-1) for the low N rate, 8982 and 9224 kg ha(-1) for the medium N rate, and 9143 and 9484 kg ha(-1) for the high N rate, respectively. Simulated and measured flow-weighted average nitrate concentrations (FWANC) in drainage water were 10.6 and 10.3 mg L-1 for the low N rate, 13.4 and 13.2 mg L-1 for the medium N rate, and 18.0 and 19.1 mg L-1 for the high N rate, respectively. The simulated N rate for optimum corn yield over the long term was between 100 and 150 kg N ha(-1). Currently, the owner-operator of the farm applies 180 kg N ha(-1) to corn in nearby production fields. Reducing long-term N rates from 180 to 130 kg N ha(-1) corresponded to an 18% simulated long-term reduction in N mass lost to water resources. Median annual FWANC in subsurface drainage water decreased from 19.5 to 16.4 mg N L-1 with this change in management. Current goals for diminishing the hypoxic zone in the Gulf of Mexico call for N loss reductions of 30% and greater. Thus, long-term simulations suggest that at least half of this N loss reduction goal could be met by reducing N application rates to the production optimum. However, additional changes in management will be necessary to completely satisfy N loss reduction goals while maintaining acceptable crop production for the soil and meteorological conditions of this study. The results suggest that after calibration and thorough testing, RZWQM-DSSAT can be used to quantify the long-term effects of different N application rates on corn production and subsurface drainage FWANC in Iowa.
