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 N from soil organic matter, N2 fixed by legumes, and redeposition of locally derived ammonia (NH3). The dominant N losses [...]
