Nitrogen cycle

The nitrogen cycle describes how nitrogen moves between the air, soil, water and living organisms. Nitrogen gas makes up about 78 percent of the atmosphere, but its two atoms are held together by a strong triple bond, so plants and animals cannot use it directly, even though they need nitrogen to build proteins and nucleic acids. Almost every step of the cycle depends on microorganisms.

First, nitrogen fixation converts nitrogen gas into ammonia or ammonium. Free-living soil bacteria such as Azotobacter, cyanobacteria in water, and Rhizobium bacteria in the root nodules of legumes like peas and beans do most of this, and lightning fixes a small amount. Plants absorb the fixed nitrogen and build it into their molecules, and animals obtain it by eating plants. When organisms die or excrete wastes, bacteria and fungi release ammonium in a step called ammonification. Nitrifying bacteria such as Nitrosomonas then oxidize ammonium to nitrite and on to nitrate, and in low-oxygen conditions denitrifying bacteria turn nitrate back into nitrogen gas, closing the loop.

Humans have roughly doubled the amount of nitrogen entering the cycle, mainly by making fertilizer industrially and growing nitrogen-fixing crops. Runoff of this extra nitrogen feeds algal blooms and oxygen-starved dead zones, and nitrous oxide from soils and burning fuels is a powerful greenhouse gas.