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United States Department of Agriculture

Agricultural Research Service

Title: Agricultural Nitrogen Trends in the Mississippi Basin 1949-1997

Authors
item Burkart, Michael
item James, David

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: April 30, 2001
Publication Date: N/A

Interpretive Summary: Agriculture is the major contributor to nitrate loads in the Mississippi River that causes hypoxia in the Gulf of Mexico. The distribution of excess agricultural nitrogen was estimated using sources of nitrogen, including fertilizer, manure, nitrogen fixed by legumes, mineralization of organic nitrogen, and redeposition of ammonia. Losses include harvest, volatilization, plant senescence, and denitrification. The Upper Mississippi and Ohio hydrologic regions had the largest reductions in excess nitrogen since 1949. Only the Tennessee River showed little change in excess nitrogen. The efficiency of nitrogen use for crop production improved in all regions. The Ohio and Upper Mississippi regions were the most efficient in 1949 and had the greatest improvement in efficiency. The Tennessee, Arkansas/Red, and Lower Mississippi Regions were least efficient in nitrogen use. The Lower Mississippi has doubled the excess nitrogen during the period studied. Although losses have increased in this region as well, there is only a modest improvement in the efficiency of the region in utilizing that nitrogen. Changes to agricultural systems in the Mississippi Basin will be needed to reduce hypoxia in the Gulf of Mexico. These research results will impact decisions affecting agriculture by providing information on the distribution of nitrogen in time and space. This information will help scientists and policy planners to isolate areas where research and policy changes may be implemented to effectively reduce excess nitrogen available to streams.

Technical Abstract: Agriculture is the major contributor to hypoxia in the Gulf of Mexico via nitrate loads in the Mississippi River. The distribution of excess agricultural nitrogen was calculated using dominant sources and losses of nitrogen. Sources include imported nitrogen such as fertilizer and manure, nitrogen fixed by legumes, and redeposition of ammonia. Losses include harvest, volatilization, plant senescence, and denitrification. Mineralization and immobilization were balanced with crop-residue nitrogen to estimate soil-nitrogen changes. The Upper Mississippi and Ohio hydrologic regions had the largest increases in sources since 1949 and the largest reductions in excess nitrogen. Only the Tennessee River showed little change in sources and excess nitrogen. All hydrologic regions improved agricultural nitrogen efficiency when excess nitrogen is measured against total sources. The Ohio and Upper Mississippi Regions were the most efficient in 1949 and had the greatest improvement in efficiency. These regions use a greater fraction of nitrogen sources for crops than do other regions. Excess nitrogen in the Tennessee, Arkansas/Red, and Lower Mississippi Regions are greatest when analyzed as a percent of the total sources. The Lower Mississippi, in particular, has doubled the excess nitrogen during the period. Although losses have increased in this region as well, there is only a modest improvement in the efficiency of the region in utilizing that nitrogen.

Last Modified: 9/21/2014
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