Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 4, 1998
Publication Date: N/A
Interpretive Summary: Residual agricultural nitrogen is transported through the Mississippi River and is responsible for hypoxia, or reduced oxygen content, in the Gulf of Mexico. We mapped the geographic distribution of sources and losses of nitrogen in agricultural systems in the Mississippi River basin by applying a geographic information system (GIS) to several national data bases. The largest differences between sources and losses (residual) were found in the Upper Mississippi and Ohio River basins, in the heart of the corn belt. However, these basins also were most efficient in using nitrogen inputs for the production of crops. The Tennessee, Arkansas, Red, and Lower Mississippi River basins were the least efficient in utilizing nitrogen inputs in crop production. Knowing the geographic distribution of sources and losses of nitrogen will help scientists and policy planners to isolate areas where research and policy changes may be implemented to reduce the residual nitrogen available to streams.
Nitrate is the principal nutrient transported through the Mississippi basin that is related to hypoxia in the Gulf of Mexico. Agriculture is a major contributor to the nitrogen load. This paper defines the geographic distribution, by hydrologic unit, of major agricultural sources and dominant losses of nitrogen in the basin. Sources include imported nitrogen such as inorganic fertilizer, manure, and atmospheric deposition, and sources generated in situ such as potentially mineralizable nitrogen from soil, nitrogen fixed by legumes, and redeposition of locally derived ammonia. The dominant nitrogen losses include crop harvests, losses to the atmosphere through volatilization of manure and inorganic fertilizer, plant senescence, and denitrification of soil nitrate. National data bases used in the analysis include the State Soils Geographic Database, 1992 Census of Agriculture, and the National Atmospheric Deposition Program/National Trends Network. The hydrologic units with the largest residual nitrogen contributions potentially available to streams are located in the Upper Mississippi River and the Ohio River basins. Mineralizable nitrogen in the organic rich soils, inorganic nitrogen fertilizer, legume fixation, and redeposition of locally derived ammonia constitute the major sources in this part of the Mississippi basin, although manure adds to the sources as well. However, these northern hydrologic regions utilize a greater fraction of the sources to produce crop nitrogen than do the southern hydrologic regions. Residual contributions to the Tennessee, Arkansas/Red, and Lower Mississippi hydrologic regions are greatest when analyzed as a percent of the total sources.