Nitrogen flux and sources in the Mississippi River Basin

Nitrogen from the Mississippi River Basin is believed to be at least partly responsible for the large zone of oxygen-depleted water that develops in t he Gulf of Mexico each summer. Historical data show that concentrations of nitrate in the Mississippi River and some of its tributaries have increased by factors of 2 to more than 5 since the early 1900s, We have used the his torical streamflow and concentration data in regression models to estimate the annual flux of nitrogen (N) to the Gulf of Mexico and to determine wher e the nitrogen originates within the Mississippi Basin, Results show that f ur 1980-1996 the mean annual total N flux to the Gulf of Mexico was 1 568 0 00 t/year. The flux was approximately 61% nitrate as N, 37% organic N, and 2% ammonium as N. The flux of nitrate to the Gulf has approximately tripled in the last 30 years with most of the increase occurring between 1970 and 1983. The mean annual N flux has changed little since the early 1980s, but large year-to-year variations in N flux occur because of variations in prec ipitation. During wet years the N flux can increase by 50% or more due to h ushing of nitrate that has accumulated in the soils and unsaturated zones i n the basin. The principal source areas of N are basins in southern Minneso ta, Iowa, Illinois, Indiana, and ohio that drain agricultural land. Basins in this region yield 800 to more than 3100 kg total N/km(2) per year to str eams, several times the N yield of basins outside this region. Assuming con servative transport of N in the Mississippi River, streams draining Iowa an d Illinois contribute on average approximately 35% of the total N discharge d by the Mississippi River to the Gulf of Mexico. In years with high precip itation they can contribute a larger percentage. (C) 2000 Elsevier Science B.V. All rights reserved.