Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: January 20, 2000
Publication Date: N/A
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 temporal changes in the distribution by hydrologic unit of major agricultural sources, dominant losses, and immobilized nitrogen in the basin. This distribution is correlated to measurements of nitrogen in rivers 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, plan 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%).