Increased level of NO3-N in the drinking water supplies is a major health concern these days. The long-term effects of actual nitrogen (N) fertilizer management practices are not well understood. The use of computer models allows the simulation of different N management practices on a long-term basis and their related effects on water quality. The RZWQM (Root Zone Water Quality Model, Version 3.0) was used to simulate the long-term (1978–1992) impacts of N management practices (single N applications at 50, 100, 150, and 200 kg per ha; and single and split N applications at 150 and 200 kg per ha) on NO3-N losses with subsurface drain flows and crop yields under two tillage systems (moldboard plow (MB) and no till (NT)). Simulations conducted in this study were based on input parameters calibrated by Singh et al. (J. Environ. Qual., in press) for NO3-N transport to subsurface drains. However, calibration of some additional parameters was required in this study for long-term simulations. The long-term climatic data and soil properties data for these simulations were obtained from a water quality research site at Nashua, Iowa.

The results of this study showed that increasing rates of N applications (50, 100, 150, and 200 kg per ha) resulted in increased NO3-N losses with subsurface drain flows and increased crop yields. However, increasing rates of NO3-N losses and crop yields were not linearly proportional with increasing rates of N applications. These trends were similar for both MB and NT treatments. Also, NO3-N losses and crop yields were not significantly different under single and split N applications at both 150 and 200 kg per ha levels of application. The single N application of 150 kg per ha was considered the best N application practice as the simulated NO3-N losses under this practice were reduced considerably (40.3% less in MB and 52.4% less in NT) when compared with the single N application of 200 kg per ha. At the same time, the reduction in crop yields at 150 kg per ha single N application was very small (5.9% reduction under MB and about 6.1% under NT) when compared with the crop yields at 200 kg per ha single N application. This study also shows that RZWQM can be used successfully in evaluating similar N management schemes for other geographic regions of the world by utilizing site-specific data on soils, geological features, crops, and climatic parameters such as rainfall and evaporation.