SIMULATING NITROGEN MANAGEMENT EFFECTS ON SUBSURFACE DRAINAGE WATER-QUALITY

Increased level of NO3-N in the drinking water supplies is a major hea lth concern these days. The long-term effects of actual nitrogen (N) f ertilizer management practices are not well understood. The use of com puter 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 simulat e 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 spli t N applications at 150 and 200 kg per ha) on NO3-N lasses with subsur face drain pools and crop yields under two tillage systems (moldboard plow (MB) and no till (NT)). Simulations conducted in this study were based an input parameters calibrated by Singh et al. (J. Environ. Qual ., in press) for NO3-N transport to subsurface drains. However, calibr ation of some additional parameters was required in this study for lon g-term simulations. The long-term climatic data and soil properties da ta for these simulations were obtained from a water quality research s ite at Nashua, Iowa. The results of this study showed that increasing tares 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. Th ese trends were similar for both MB and NT treatments. Also, NO3-N los ses and crop yields were not significantly different under single and split N applications at both 150 and 200 kg per ha levels of applicati on. The single N application of 150 kg per ha was considered the best N application practice as the simulated NO3-N losses under this practi ce were reduced considerably (40.3% less in MB and 52.4% less in NT) w hen compared with the single N application of 200 kg per ha. At the sa me time, the reduction in crop yields at 150 kg per ha single N applic ation 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 applicat ion. This study also shows that RZWQM can be used successfully in eval uating similar N management schemes for other geographic regions of th e world by utilizing site-specific data on soils, geological features, crops, and climatic parameters such as rainfall and evaporation. (C) 1997 Elsevier Science Ltd.