PO 13. Describe how the following affect the fate of N in soil: the Nitrogen Cycle.

Nitrogen is an essential and often growth-limiting plant nutrient. Crops take up and release N through a series of processes known as the Nitrogen Cycle.  N availability limits the productivity of most cropping systems in the US, and a deficiency in nitrogen leads to yield declines or even complete crop failure.  Excessive applications however may contribute to acid rain, destruction of the ozone layer in the stratosphere, the greenhouse effect, eutrophication of surface waters, contamination of ground water, and fish and other marine life kills, as well as blue baby syndrome in infants and amphibian mortality and deformations. The nitrate concentration in ground and surface waters is an important water-quality index; the U.S. Environmental Protection Agency (EPA) has set the Federal Standard for the maximum permitted amount of nitrate N in drinking water at 10 mg N/L or 43 mg NO₃^-/L.

It is important from both an economic and an environmental standpoint to manage N optimally. Thus, the two primary objectives of N management are:

1. To have adequate inorganic N available during the growing season
2. To minimize the availability of inorganic N during the fall, winter, or early spring, when N may be transported to surface and groundwater.