The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. Nitrogen makes up 78 percent of the earth’s atmosphere. The percentage of nitrogen in the atmosphere is maintained by nitrogen cycle.

Plants and animals cannot utilize atmospheric nitrogen readily. It has to be fixed by some organisms called as nitrogen fixers. Nitrogen-fixing bacteria like Rhizobium live in symbiotic association in the root nodules of certain leguminous plants. These bacteria convert atmospheric nitrogen into ammonia, which is utilized readily by plants. Nitrogen-fixing bacteria along with free living bacteria in the soil achieve 90 percent of nitrogen fixation. During lightening, the high temperatures and pressures created in the air convert nitrogen into oxides of nitrogen. These oxides dissolve in water to give nitric and nitrous acids and fall on land along with rain. Plants convert these nitrates and nitrites into amino acids. Ammonification is the process by which soil bacteria decompose dead organic matter and release ammonia into the soil. Nitrification is the process by which ammonia is converted into nitrites and nitrates. Denitrification is the process by which nitrates are converted into atmospheric nitrogen.