Atmospheric nitrogen (N2)Nitrogen gas, N2, about 78 percent of the atmosphere. Its triple bond is so tight that plants cannot use it directly.
Nitrogen fixationBacteria convert nitrogen gas into forms living things can use. Rhizobium in legume root nodules and free-living Azotobacter both do it.
Legume (plant)A plant of the pea and bean family. Its roots hold nitrogen-fixing bacteria. Gardeners grow peas partly to add nitrogen to the soil.
Root nodulesSwellings on legume roots where Rhizobium bacteria live. The bacteria fix nitrogen for the plant.
AmmonificationCertain bacteria and fungi turn nitrogenous waste and dead organisms into ammonium.
NitrificationNitrifying bacteria such as Nitrosomonas turn ammonium into nitrites. Others such as Nitrobacter turn nitrites into nitrates.
AssimilationPlants absorb nitrate or ammonium through their root hairs. They build it into amino acids, nucleic acids and chlorophyll.
DenitrificationBacteria such as Pseudomonas and Clostridium turn nitrates back into nitrogen gas. The gas returns to the atmosphere.
Air, plant and remains: 5 2
LightningIts heat also fixes nitrogen. About 5 to 10 billion kilograms a year, far less than bacteria do.
Dead organisms and wasteRemains of dead plants and animals, and animal waste. Their nitrogen goes back to the soil.
In the soil: 3 3
Ammonium (NH4+)The form of nitrogen made by ammonification. Plants can take some of it up directly.
Nitrites (NO2-)The step between ammonium and nitrates in nitrification.
Nitrates (NO3-)A form plants take up through their roots. Also the form denitrifying bacteria return to the air as gas.
How this list was compiled: Read 2026-09-26. OpenStax Biology 2e, section 46.3 Biogeochemical Cycles, The Nitrogen Cycle, https://openstax.org/books/biology-2e/pages/46-3-biogeochemical-cycles : N2 about 78 percent of the atmosphere and its triple bond; nitrogen fixation by free-living and symbiotic bacteria, cyanobacteria, Rhizobium in the root nodules of legumes such as peas, beans and peanuts, and free-living Azotobacter; gardeners growing peas to add nitrogen to the soil; ammonification of nitrogenous waste and remains into ammonium by certain bacteria and fungi; nitrification by bacteria such as Nitrosomonas to nitrites and then to nitrates; denitrification by Pseudomonas and Clostridium back to nitrogen gas; people releasing nitrogen oxides by burning fossil fuels and fertilizer washing into lakes and rivers, causing eutrophication that depletes dissolved oxygen. Wikipedia, Nitrogen cycle, https://en.wikipedia.org/wiki/Nitrogen_cycle : 5 to 10 billion kg a year fixed by lightning, most fixation by bacteria; assimilation of nitrate or ammonium through root hairs into amino acids, nucleic acids and chlorophyll; Nitrobacter oxidising nitrites to nitrates; about 30 percent of total fixed nitrogen now produced industrially by the Haber-Bosch process. Licence: own work
The drawing: Drawn for this site by script, 2026-09-26. Own work, nothing traced. Licence: own work
How it works
A nitrogen cycle diagram labeled with 13 parts: the five processes as arrows, plus the air, a legume with its root nodules, dead matter and the three forms of nitrogen in the soil. Start with the 8 main parts.
Read the full guide
Follow the arrows in order
Fixation brings nitrogen gas into the roots. Ammonification turns waste and remains into ammonium, nitrification turns ammonium into nitrites and then nitrates, plants take nitrates up by assimilation, and denitrification returns nitrogen gas to the air.
Where people change the cycle
Burning fossil fuels releases nitrogen oxides, and fertilizer washes into lakes and rivers, where it can feed excess growth that uses up the water's oxygen. About 30 percent of fixed nitrogen is now made in factories.
Nitrification and denitrification sound alike: nitrification builds nitrates in the soil, denitrification sends them back to the air.
Questions and answers
Why can't plants use nitrogen from the air?
Nitrogen gas is held together by a triple bond that plants cannot break. Bacteria fix it first, some of them living in the root nodules of legumes.
What bacteria are in the nitrogen cycle?
Rhizobium and Azotobacter fix nitrogen, Nitrosomonas and Nitrobacter carry out nitrification, and Pseudomonas and Clostridium carry out denitrification.
Does lightning fix nitrogen?
Yes, about 5 to 10 billion kilograms a year, but most nitrogen is fixed by bacteria.