Agricultural-nitrogen contributions to hypoxia in the Gulf of Mexico

Nitrate (NO3) is the principal nutrient transported through the Mississippi River basin that is related to hypoxia in the Gulf of Mexico. Agriculture is a major contributor to the N load. Knowledge of the geographic distribut ion of NO3 sources and losses within the basin is critical to understanding the problem and identifying potential solutions. This paper defines the ge ographic distribution, by hydrologic unit, of major agricultural sources an d dominant losses of N in the basin. Sources include imported N such as ino rganic fertilizer, manure, and atmospheric deposition, and in situ sources such as mineralized ed N from soil organic matter, N-2 fixed by legumes, an d redeposition of locally derived ammonia (NH3). The dominant N losses incl ude crop harvests, losses to the atmosphere through volatilization of manur e and inorganic fertilizer, plant senescence, and denitrification of soil N O3. National data bases used in the analysis include the State Soils Geogra phic Database, 1992 Census of Agriculture, and the National Atmospheric Dep osition Program/National Trends Network. The hydrologic units with the larg est residual N contributions available to streams are located in the Upper Mississippi River and the Ohio River basins. Mineralizable soil N, inorgani c N fertilizer. legume fixation, and redeposition of locally derived NH3 co nstitute the major sources in this part of the basin, although manure is a minor source. However, these northern hydrologic regions use a greater frac tion of the sources to produce crop N than do the southern hydrologic regio ns. Residual contributions to the Tennessee, Arkansas/Red, and Lower Missis sippi hydrologic regions are greatest when analyzed as a percentage of the total sources.