HR: 12:05h
AN: H51G-07    [PDF]
TI: The impact of changing land use practices and climate variability on nitrate export by the Mississippi River
AU: * Donner, S D
EM: sddonner@princeton.edu
AF: Woodrow Wilson School, Princeton University, 402 Robertson Hall, Princeton, NJ 08544 United States
AU: Oppenheimer, M
EM: omichael@princeton.edu
AF: Woodrow Wilson School, Princeton University, 402 Robertson Hall, Princeton, NJ 08544 United States
AU: Kucharik, C J
EM: kucharik@wisc.edu
AF: Center for Sustainability and the Global Environment, University of Wisconsin-Madison 1710 University Ave., Madison, WI 53726 United States
AU: Foley, J A
EM: jfoley@wisc.edu
AF: Center for Sustainability and the Global Environment, University of Wisconsin-Madison 1710 University Ave., Madison, WI 53726 United States
AB: The increased use of nitrogen fertilizer in the Mississippi River Basin since the 1950s has been blamed for declining water quality, the degradation of aquatic ecosystems and the growth of a seasonal hypoxic zone in the Gulf of Mexico. In this study, we use the IBIS terrestrial and agro-ecosystem model and the HYDRA aquatic transport model to examine how agricultural practices and climate variability influenced terrestrial and aquatic nitrogen cycling across the Mississippi Basin and the export of nitrate to the Gulf. The modeling system accurately depicts the observed trends and interannual variability in nitrate export by the Mississippi River (r$^{2}$ $>$ 0.8), and several of the major tributaries, between 1960 and 1994. The challenge of simulating nitrate export from the western sub-basins highlights the key role of nitrogen retention processes like denitrification. The simulations demonstrate that three factors led to the doubling of nitrate export by the Mississippi River since 1960: (1) an increase in fertilizer application rates, particularly on corn; (2) the expansion of soybean cultivation; and (3) an increase in runoff across the basin. By the early 1990s, crops alone may have accounted for almost 90% of the nitrate leached to the river system, despite representing only 20% of the watershed area. The majority of the nitrate exported to the Gulf now appears to originate from several "hot spots", including a stretch of the `Corn Belt' across Iowa, Illinois and Indiana. The relative contribution of such heavily fertilized lands is even greater during wet periods, due to high leaching losses and relatively low in-stream nitrogen retention. These findings demonstrate that the effort to reduce the flux of agriculturally-derived nitrogen to the Gulf of Mexico could be further complicated by increasing variability in climate.
DE: 1615 Biogeochemical processes (4805)
DE: 1803 Anthropogenic effects
DE: 1871 Surface water quality
DE: 4842 Modeling
DE: 4845 Nutrients and nutrient cycling
SC: Hydrology [H]
MN: 2003 Fall Meeting