The atmosphere holds the greatest reservoir of nitrogen, but, as gaseous, triple‐bonded N 2, it is chemically inert and unusable by plants and almost all other organisms. A few kinds of bacteria that possess the enzyme nitrogenase are the exceptions. They are able to convert (reduce) N 2 to ammonium ions (NH 4 +), which many organisms, including plants, are able to metabolize. The process is called nitrogen fixation and ranks equally with photosynthesis in significance to life. When organisms die, decay bacteria and fungi release the fixed N of organic compounds. New organisms then reformulate it into amino acids—hence proteins—together with nucleic acids, nucleotides, coenzymes, and vitamins all of which are essential to life. The denitrifying bacteria return some nitrogen to the atmosphere as N 2. This process— denitrification—is anaerobic and takes place in almost all types of soil. Microorganisms, therefore, control the major phases of the N cycle.
There is, in addition, an abiotic part to the cycle. A lesser amount of nitrogen is fixed from the atmosphere through reactions occurring in the gaseous emissions from volcanoes and from lightning discharges. This nitrogen is washed out of the atmosphere by rain and is added to the soil and water where it, too, can be used by organisms.
Human activities have a significant effect on nitrogen cycling. Production and use of nitrogen fertilizer, combustion of fossil fuels, and planting crops that fix nitrogen have unbalanced the previously stable relationship between fixation and denitrification. Gaseous industrial pollutants—abbreviated as NO x compounds—foul the air in many cities and wash out in sufficient amounts to constitute “acid rain” in some parts of the industrialized world.
The nitrogen cycle can be understood most easily by looking at its separate parts: nitrogen fixation, ammonification, nitrification, assimilation, and denitrification.