Since the 1950s, agriculture and food production in the U.S. have become increasing dependent on the application of nitrogen fertilizers. However, the increase in fertilizer application in the Mississippi Basin is also largely responsible for a three-fold increase in nitrate export by the Mississippi River and the growth of the hypoxic zone in the Gulf of Mexico. In this study, we use the IBIS terrestrial ecosystem model and the HYDRA hydrological transport model to simulate crop yield and aquatic nitrate export in the Upper Mississippi Basin from 1974-1994 and to assess the impact of a 30% change in historical N-fertilizer use.

The modeling system accurately simulates the annual and spatial variability in historical maize and soybean yields and nitrogen loading to rivers. There is strong agreement (r²=0.77) between simulated and USGS estimated annual nitrate export by the Mississippi at Clinton, Iowa. The 30% increase in fertilizer causes only a 4% increase in mean maize yield, but a 59% increase in mean nitrate export; whereas a 30% decrease in fertilizer causes a 10% decrease in yield, but a 42% decrease in nitrate export. As fertilizer application increases, nitrogen leaching becomes increasingly sensitive to the hydrologic conditions, particularly if there is ample residual soil nitrogen. Controlling N-fertilizer application could reduce nitrate export without significantly affecting crop yields, but it will depend largely on climate variability and the soil nitrogen storage.