Submitted to: American Society of Agronomy Meetings
Publication Type: Abstract Only
Publication Acceptance Date: 11/9/2000
Publication Date: N/A
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 the excess nitrogen. Only the Tennessee River showed little change in sources and excess nitrogen. All hydrologic regions improved agriculture nitrogen efficiency when excess nitrogen is measured against total sources. The Ohio and Upper Mississippi iRegions were the most efficient in 1949 and had the greatest improvement i efficiency. These regions use a greater fraction of the 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.