Nitrogen budget in the Upper Mississippi River watershed

Nitrogen budget calculations performed for high-flow and low-flow years in the major sub-basins of the Upper Mississippi River watershed show differen ces in nitrogen applications and discharges. Nitrogen budgets show that fer tilizer is the most important input of nitrogen to the basins, but also sho w that atmospheric input and animal manures can be significant inputs of ni trogen to the basins. The transport of nitrogen from the land to rivers var ies with the prevailing hydrologic conditions. The annual nitrogen budgets are not balanced. In years of high precipitation and river discharge, more nitrogen can be removed than had been applied that year, presumably from N stored in the soil or ground water. Storage of nitrogen in soils is a major unknown in the model, but calculations suggest that it is a significant re servoir of N.