The Nitrogen Cycle: What You Should Know

Nitrogen (N) makes up 78 percent of the air we breathe in the form of nitrogen gas (N2), but this form is unable to be used by plants. In fact, there are 34,000 tons of N in the air above an acre of land, but none of it can be used by crops. Nitrogen must be fixed in order to become available, which is done through the process of making industrial fertilizers or through nitrogen-fixing bacteria associated with the roots of legumes. A significant amount of nitrogen occurs in the soil naturally (2,000–4,000 pounds per acre, lbs/A), but 98 percent of that nitrogen is in the organic form and also cannot be used by plants. This organic nitrogen is found in all living and previously living material in the soil. Nitrogen naturally becomes available in soil as organic matter is mineralized, which results in around 60–80 pounds of nitrogen per acre per year for crop uptake. Two forms of inorganic nitrogen are plant available: ammonium N (NH4+) and nitrate N (NO3). Ammonium N is held on the soil particles and can be exchanged with other cations in order for plants to take it up, but it does not leach easily from the soil. Nitrate N, on the other hand, is found in the soil solution and can be leached from the profile. Nitrogen leaching needs to be managed properly to ensure that plants have access to the nitrate and also to minimize the nitrate pollution in waterways. Understanding the nitrogen cycle thoroughly allows us to do that. The processes involved in the cycle are summarized below.

The forms of nitrogen, the transformations that it undergoes in the soil, and the nitrogen loss pathways are summarized in the N cycle (Figure 1). Most of the transformations in the N cycle are the result of microbial activity. Because these are biological processes, they are very sensitive to the environment where they occur. Major factors influencing these processes are temperature and moisture and thus the weather. The challenge with managing N is to achieve maximum N availability when crops need N and to reduce loss of N to the environment. The goal is to minimize soluble forms of N when at times of little or no crop uptake. This can be achieved by understanding the N cycle and managing inputs. The three main pathways of N loss are nitrate leaching, denitrification, and volatilization, discussed below.

Nitrogen Cycle
Figure 1. Nitrogen cycle: Transformations between N forms.

SOURCE: Penn State Extension