Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 7, 1999
Publication Date: N/A
Interpretive Summary: Excess agricultural nitrogen is transported through the Mississippi River and contributes to hypoxia, or reduced oxygen content, in the Gulf of Mexico. We mapped the geographic distribution of sources, losses, and immobilization of nitrogen in agricultural systems in the Mississippi River basin by applying a geographic information system (GIS) to several national ldata bases. The hydrologic units with largest excesses of nitrogen 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.
Technical Abstract: Nitrate is the principal nutrient transported through the Mississippi River basin that is related to hypoxia in the Gulf of Mexico. Agriculture is a major contributor to the nitrogen load. Knowledge of the geographic distribution of sources, losses, and immobilized nitrogen within the basin is critical to understanding the problem and identifying potential solutions. This paper defines the distribution by hydrologic unit of majo agricultural sources, dominant losses, and immobilized nitrogen in the basin. Sources include imported nitrogen such as inorganic fertilizer, manure, and atmospheric deposition, and in situ sources such as mineralized nitrogen from soil organic matter, 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. Estimates of inorganic fertilizer and manure that are immobilized in soil are also included to compute the distribution of excess nitrogen. 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 excess nitrogen available to streams are located in the Upper Mississippi River, Tennessee, and the Arkansas/Red River regions. However, the Upper Mississippi River region utilizes a greater fraction of the sources to produce crop nitrogen (83%) than do the other two regions (< 63%).