Sources of Nitrate in Groundwater Understanding nitrate sources and how it reaches groundwater requires knowledge of the nitrogen cycle and groundwater recharge. The nitrogen cycle . Worldwide, nitrogen is the plant nutrient most limiting for crop production. Since early times, people have sought to add nitrogen to crops by using animal waste, human wastes, legumes or fertilizers. Nitrogen is naturally part of the environment.
Nitrogen accumulates in soil during the process of soil formation.
Virgin prairie soils contain as much as 6,000 to 10,000 pounds per acre of organically bound nitrogen. Once these soils are tilled to grow crops, organic matter content begins to decrease. As organic matter is oxidized, nitrogen is released primarily as nitrate. If not used by the growing crop, it is subject to leaching to groundwater. The illustration in Figure 1 shows that nitrogen enters the cycle from several sources.