| The intensification of agricultural and industrial activity has more than doubled the rate of nitrogen (N) fixation in the biosphere. The increased loading of N to terrestrial and aquatic systems threatens the health of aquatic ecosystems worldwide. The problem is particularly acute in the Mississippi River Basin, where a massive increase in N-fertilizer use since the 1950s is believed to be the cause of the “dead zone” in the Gulf of Mexico.; This dissertation examines how climate and land use have influenced N cycling across the Mississippi River Basin and contributed to the increase in nitrate export by the Mississippi River. For this study, the IBIS terrestrial ecosystem model and HYDRA hydrological transport model were adapted to simulate terrestrial and aquatic N cycling. The modeling system enabled the first integrated analysis of biogeochemical cycling in terrestrial and aquatic systems across a large river basin.; This dissertation demonstrates that the increase in nitrate export by the Mississippi River was due to an increase in fertilizer use, primarily on corn, an expansion of soybean cultivation and an increase in precipitation. The increase in fertilizer use and soil-N storage made nitrate leaching across the Mississippi Basin increasingly sensitive to precipitation over time. A continuation of modern-day cropping practices in the future could lead to persistently high spring nitrate export by the Mississippi and a worsening in the extent of Gulf hypoxia.; The study also demonstrates the large variability in nitrate leaching across the Mississippi Basin. Highly cultivated lands in the central U.S., especially those underlain by sandy soils, are prone to extremely high leaching losses and low crop yields during wet years (e.g., 1993). The sensitivity of nitrate leaching to precipitation makes such highly fertilized lands—particularly those close to higher order streams—disproportionately responsible for the nitrate exported to the Gulf. The results suggest reducing nitrate export by the Mississippi without threatening crop production will require the coordination of activities at a variety of scales, from precision agriculture at individual farms to improved seasonal weather forecasting for major growing regions to informed federal agricultural policy. |
