Nitrogen cycle
Nitrogen is essential for proteins, nucleic acids and other biological molecules. Although the atmosphere contains about 78% nitrogen gas (N₂), most organisms cannot use atmospheric nitrogen directly.
Major stages
Nitrogen fixation: Converts atmospheric nitrogen into biologically usable compounds. It occurs through certain bacteria, lightning and industrial processes.
Nitrification: Microorganisms convert ammonia/ammonium into nitrite and then nitrate.
Assimilation: Plants absorb ammonium or nitrate and incorporate nitrogen into organic compounds.
Ammonification: Decomposers convert organic nitrogen in dead organisms and waste into ammonia/ammonium.
Denitrification: Microorganisms convert nitrate into gaseous nitrogen forms, returning nitrogen to the atmosphere.
Important examples
Rhizobium: Symbiotic nitrogen fixation in the root nodules of many legumes.
Nitrosomonas: Commonly associated with oxidation of ammonia to nitrite.
Nitrobacter: Commonly associated with oxidation of nitrite to nitrate.
Denitrifying bacteria: Return nitrogen to the atmosphere under suitable conditions.