Submitted to: Trans American Geophysical Union
Publication Type: Proceedings
Publication Acceptance Date: December 10, 1998
Publication Date: N/A
Nitrate is the principal nutrient transported through the Mississippi River basin that is associated with hypoxia in the Gulf of Mexico. Agricultural practices are major contributors to the nitrogen load of the River. This paper defines the geographic distribution of major agricultural sources and losses of nitrogen in the Mississippi River basin. The estimates of nitrogen residuals were developed by applying basic models of soil, plants, and atmospheric processes that cycle nitrogen to geographically referenced national and regional data bases. Sources include inorganic fertilizer, manure, atmospheric deposition, mineralizable nitrogen from soil organic matter, nitrogen fixed by crops, and redeposition of locally derived ammonia. Losses include crop harvests, losses to the atmosphere through volatilization of manure, inorganic fertilizer, plant senescence, and denitrification of soil nitrate. 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.