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In Depth...
Investigating the Impacts of Land Use Change, Land Management, and Climate Variability on Nitrate Transport and Crop Yield in the Upper Mississippi Basin: Coupling Large-Scale Integrated Models of Terrestrial and Freshwater Systems
a study by Chris Kucharik
Since the 1950s, agriculture and food production have become increasingly dependent on the application of nitrogen (N) fertilizer. A substantial proportion of applied N-fertilizer is not utilized by plants, and leaches to surface waters and groundwater supplies. The leached nitrogen represents a significant economic loss and a serious threat to human health and both the freshwater and marine environment. The problem is particularly acute in the Mississippi River Basin, the world’s third largest river basin; A tripling in nitrate (NO3-) export by the Mississippi River since the 1950s, largely due to increased application of N-fertilizers, has been blamed for an increase in the severity and extent of bottom water hypoxia in the Gulf of Mexico.
The challenge of reducing nitrate export while sustaining crop production is complicated by climate variability. Climate variability, as well as the increased dependency on N-fertilizers, may be partially responsible for the increased variability in crop production and nitrate flux since the 1980s. As the soil and groundwater system becomes increasingly saturated with N, the loading to surface water becomes increasingly sensitive to changes in hydrology. Thus, any changes in mean climate and climate variability pose a significant threat to crop yields and food security, nitrogen levels in groundwater, and the health of aquatic ecosystems.
The SAGE research team has recently developed integrated models of terrestrial and aquatic systems over the past several years in a first attempt to assess the impacts of climate variability and land management on nitrate export and crop yields across a large agricultural region. We used the IBIS model, which now includes mechanistic representations of crop phenology and management, and the HYDRA model to simulate crop yield, nitrogen cycling and aquatic nitrate transport in the Upper Mississippi Basin (UMB) during the 1974-1994 time period. We used sensitivity studies to quantify the effect of varied N-fertilizer use in maize on crop yield and nitrate export across the basin during the past several decades.
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Map of the Upper Mississippi River Basin and the five sub-basins examined in this study. This map of the simulated Mississippi Basin was generated from topographic data and manually corrected river directions. The simulated area of each basin is within 10% of observations (Goolsby et al., 1999).
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Our initial work in the UMB had two specific objectives; 1) validate model output against U.S. Department of Agriculture (USDA) estimates of historical crop yield, literature estimates of individual components of the nitrogen cycle and U.S. Geological Survey (USGS) estimates of river nitrate export throughout the UMB from 1974-1994; 2) use the IBIS/HYDRA modeling system to assess the impacts of a 30% change in historical N-fertilizer application on maize yields and nitrate export from the basin.
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1992 fractional coverage of (a) maize and (b) soybean at 5' x 5' spatial resolution for the Upper Mississippi Basin (upstream of Clinton, IA) from Donner (subm).
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Simulated annual mean maize and soybean yield were within 20% of USDA historical estimates for each of the Upper Mississippi sub-basins. There was also strong agreement between simulated and USGS estimated annual nitrate export for the Mississippi River at Clinton, Iowa (r2=0.81), the outlet of the basin, and the Minnesota River at Jordan, Minnesota (r2=0.78). The model also indicated a 30% increase in N-fertilizer application across the basin would have caused only a 4% increase in mean maize yield, but a 53% increase in mean dissolved inorganic nitrogen (DIN) leaching, while a 30% decrease in N-fertilizer application would have caused a 10% decrease in maize yield, but a 37% decrease in DIN leaching. At higher levels of N-fertilizer usage, DIN loading and river nitrate export becomes increasingly sensitive to the hydrologic conditions, particularly when there is ample residual N in the soil. Therefore, any effort to reduce nitrate export without significantly affecting crop yields must account for the impact of historical land use practices on N storage in the soil and groundwater system.
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Simulated and observed maize yield (Mg ha-1) for a) 1985-1994 average, b) 1993 and c) 1994. Yield is displayed for each 5' x 5' grid cell with at least 1% maize cover. Observed maize yield was determined from USDA county-level estimates. The percent difference in yield is (sim-obs)/obs * 100.
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Simulated (dashed line) and USGS estimated (solid line) annual nitrate export (ton yr-1) by the Mississippi River at Clinton, IA from 1974-1994. Nitrate export is simulated with historical fertillizer use on maize and soybeans.
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Simulated annual mean maize yield (Mg ha-1) and nitrogen leaching from maize (kg ha-1) for the Upper Mississippi Basin from 1974-1994 under the three fertilizer scenarios. They are plotted at relative scales to illustrate the relative effect of increasing fertilizer use on maize yield and nitrogen leaching.
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Relevant Literature
Donner, S.D and C.J. Kucharik, 2003. Evaluating the impacts of land management and climate variability on crop production and nitrate export across the Upper Mississippi Basin. Submitted to Global Biogeochemical Cycles (in press).
Donner, S.D., M.T. Coe, J.D. Lenters, T.E. Twine, and J.A. Foley. Modeling the impact of hydrological changes on nitrate transport in the Mississippi River Basin from 1955-1994 (2002). Global Biogeochemical Cycles 10.1029/2001GB001396
Donner, S.D., and C.J. Kucharik. The Impact of Changing Cropping Practices on Nitrate Export by the Mississippi River Since 1960. (in prep).
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