Nitrogen cycle is the circulation of nitrogen among the atmosphere, the soil and water, and the plants and animals of the earth. All living things require nitrogen, but most organisms cannot use the nitrogen gas that makes up about 78 percent of the atmosphere. They need nitrogen that has combined with certain other elements to form organic compounds. But the supply of this fixed nitrogen is limited, so complex methods of recycling nitrogen have developed in nature.
One part of the nitrogen cycle involves circulation of nitrogen between the soil and living things. After plants and animals die, they undergo decomposition by certain bacteria and fungi. These microorganisms produce ammonia from nitrogen compounds in dead organic matter and in body wastes excreted by animals. Plants absorb some of the ammonia and use it to make proteins and other substances essential to life. The rest of the ammonia is changed into nitrates by nitrifying bacteria. First, nitrifying bacteria called nitrite bacteria convert ammonia into nitrites. Then nitrate bacteria change nitrites into nitrates. Plants absorb most of the nitrates and use them in the same way as ammonia. Animals get nitrogen by eating plants or by feeding on animals that eat plants.
In another part of the cycle, a process called nitrogen fixation constantly puts additional nitrogen into biological circulation. In this process, nitrogen-fixing bacteria in the soil or water, or living within plants such as legumes, convert nitrogen from the atmosphere into nitrogen-containing organic substances.
While nitrogen fixation converts nitrogen from the atmosphere into organic compounds, a series of processes called denitrification returns an approximately equal amount of nitrogen to the atmosphere. Denitrifying bacteria convert nitrates and nitrites in soil into nitrogen gas or into gaseous compounds such as nitrous oxide or nitric oxide. However, fixed nitrogen may circulate many times between organisms and the soil before denitrification returns it to the atmosphere.
Some human activities influence the nitrogen cycle. Industry fixes vast quantities of nitrogen to produce fertilizer, much of which is washed off farmland and into waterways, polluting the water. The combustion of certain fuels produces nitrogen compounds that pollute the air. These compounds may also play a part in the warming of the earth's climate
