Long-term changes in concentrations and flux of nitrogen in the Mississippi River Basin, USA

Current and historical data show that nitrogen concentrations and flux in t he Mississippi River Basin have increased significantly during the past 100 years. Most of the increase observed in the lower Mississippi River has oc curred since the early 1970s and is due almost entirely to an increase in n itrate. The current (1980-99) average annual nitrogen (N) flux from the Mis sissippi Basin to the Gulf of Mexico is about 1 555 500 t year(-1), of whic h about 62% is nitrate-N. The remaining 38% is organic nitrogen and a small amount of ammonium. The current (1980-99) average nitrate flux to the Gulf is almost three times larger than it was during 1955-70. This increased su pply of nitrogen to the Gulf is believed to be partly responsible for the i ncreasing size of a large hypoxic zone that develops along the Louisiana-Te xas shelf each summer. This zone of oxygen-depleted water has doubled in ar eal extent since it was first measured in 1985. The increase in annual nitr ate flux to the Gulf can be largely explained by three factors: increased f ertilizer use, annual variability in precipitation and increased streamflow , and the year-to-year variability in the amount of nitrogen available in t he soil-ground water system for leaching to streams. The predominant source areas for the nitrogen transported to the Gulf of Mexico are basins draini ng southern Minnesota, Iowa, Illinois, Indiana, and Ohio. Basins in this re gion yield 1801 to 3050 kg N km(-2) year(-1) to streams, several times the N yield of basins outside this region.