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 distribution of NO3 sources and losses within the basin is critical to understanding the problem and identifying potential solutions. This paper defines the geographic distribution, by hydrologic unit, of major agricultural sources and dominant losses of N in the basin. Sources include imported N such as inorganic fertilizer, manure, and atmospheric deposition, and in situ sources such as mineralized ed N from soil organic matter, N-2 fixed by legumes, and redeposition of locally derived ammonia (NH3). The dominant N losses include crop harvests, losses to the atmosphere through volatilization of manure and inorganic fertilizer, plant senescence, and denitrification of soil NO3. National data bases used in the analysis include the State Soils Geographic Database, 1992 Census of Agriculture, and the National Atmospheric Deposition Program/National Trends Network. The hydrologic units with the largest residual N contributions available to streams are located in the Upper Mississippi River and the Ohio River basins. Mineralizable soil N, inorganic N fertilizer. legume fixation, and redeposition of locally derived NH3 constitute the major sources in this part of the basin, although manure is a minor source. However, these northern hydrologic regions use a greater fraction of the sources to produce crop N than do the southern hydrologic regions. Residual contributions to the Tennessee, Arkansas/Red, and Lower Mississippi hydrologic regions are greatest when analyzed as a percentage of the total sources.