No nitrate is naturally added to the soil during the late fall and winter because the cold weather prohibits mineralization and nitrification processes. During the spring and summer, the increased nitrogen-fixing activity of organisms and the addition of fertilizer causes the concentration of nitrates in the soil to steadily increase. Most of this nitrate is absorbed by plants. Thus, the removal of crops in the fall increases the chances for large flushes of nitrate from the soil to water bodies. Thus, methemoglobin formation may lead to asphyxia.
Water contaminated with nitrate is very difficult and costly to treat.
The growth of macrophytes and phytoplankton is stimulated principally by nutrients such as phosphorus and nitrogen. That is, if all phosphorous is used, plant growth will cease, no matter the amount of nitrogen available. Many bodies of freshwater are currently experiencing influxes of nitrogen and phosphorus from outside sources. The increasing concentration of available phosphorus allows plants to assimilate more nitrogen before the phosphorus is depleted. Thus, nitrogen levels control the rate of primary production.