The nitrogen cycle moves nitrogen through five main stages: fixation, nitrification, assimilation, ammonification, and denitrification. Together these steps convert nitrogen from its inert atmospheric form into compounds that living things can use, shuttle it through food webs, and eventually return it to the atmosphere. The cycle is driven almost entirely by microorganisms, and understanding each stage helps explain everything from why farmers add fertilizer to why coastal waters sometimes turn green with algae blooms.
Stage 1: Nitrogen Fixation
Earth’s atmosphere is roughly 78 percent nitrogen gas, but almost no plant or animal can use nitrogen in that form. The triple bond holding the two atoms of N₂ together is extremely strong, and breaking it requires a specialized enzyme called nitrogenase. Biological nitrogen fixation is the most important route by which nitrogen becomes available to living organisms, and it demands a large amount of cellular energy to pull off.1PubMed. Mechanisms for protecting nitrogenase from inactivation by oxygen The end product is ammonia, a simple molecule that microbes and plants can actually work with.
The best-known nitrogen fixers are bacteria that live in partnership with legumes like soybeans, clover, and peanuts. These bacteria, collectively called rhizobia, take up residence inside specialized root structures called nodules. The symbiosis is elegant: the plant feeds the bacteria sugars, and the bacteria hand over ammonia in return. But there is a catch. Nitrogenase is irreversibly destroyed by even trace amounts of oxygen, yet the energy needed to run the enzyme comes from oxygen-dependent respiration.2PubMed. Oxygen and derived reactive species in legume-rhizobia interactions: paradoxes and dual roles The plant solves this contradiction by keeping oxygen concentrations inside the nodule extremely low, on the order of a few tens of nanomolar, while producing molecules called leghemoglobins. These proteins, chemically related to the hemoglobin in your blood, have a very high capacity for absorbing oxygen and deliver just enough to power respiration without poisoning the nitrogenase.3PubMed. Hemoglobins in the legume-Rhizobium symbiosis
Legume-rhizobia partnerships are not the only source of biological fixation. Free-living soil bacteria such as Azotobacter can fix nitrogen on their own, and cyanobacteria do the same in aquatic environments. Lightning also splits atmospheric nitrogen, though it contributes a much smaller share than biology. Today, the industrial Haber-Bosch process manufactures more fixed nitrogen than all natural sources combined, which has enormous consequences for ecosystems.
Stage 2: Nitrification
Once ammonia is in the soil or water, it does not stay as ammonia for long. Nitrification is the microbial process that converts ammonia first into nitrite and then into nitrate. Classically, scientists understood this as a strict two-team relay: one group of microbes (ammonia-oxidizing bacteria and archaea) handles the ammonia-to-nitrite step, and a second group (nitrite-oxidizing bacteria) finishes the job by turning nitrite into nitrate.4Nature. Complete nitrification by a single microorganism Nitrate is the form of nitrogen that most plants prefer to absorb through their roots, so nitrification is a critical bridge between fixation and the rest of the food web.
That two-step picture held for more than a century until 2015, when researchers discovered bacteria in the genus Nitrospira that can perform the entire oxidation from ammonia to nitrate by themselves. These “comammox” (complete ammonia-oxidizing) organisms carry genes for both steps in a single genome and turn out to be widespread in soils, freshwater, and engineered systems like wastewater treatment plants.5PubMed Central. Complete nitrification by Nitrospira bacteria The discovery challenged the long-standing assumption that ammonia and nitrite oxidation must be carried out by separate organisms.6PubMed Central. Wastewater-Derived Comammox Nitrospira for Next-Generation Wastewater Management
Nitrification matters beyond plant nutrition. The process releases hydrogen ions, gradually acidifying the soil, and it produces nitrate, which is highly mobile in water and easily leaches out of farm fields into streams and groundwater. Nitrification is also a source of nitrous oxide, a potent greenhouse gas. Microbial-driven nitrification and denitrification are the main biological routes by which nitrous oxide enters the atmosphere.7Agriculture. Impacts of Climate Change and Agricultural Practices on Nitrogen Processes, Genes, and Soil Nitrous Oxide Emissions: A Quantitative Review of Meta-Analyses
Stage 3: Assimilation
Assimilation is the stage where inorganic nitrogen finally becomes part of a living organism. Plants take up nitrate or ammonium from the soil through specialized transporters in their roots. Each transporter has a distinct structure that determines which form of nitrogen it grabs.8PubMed Central. Nitrogen Journey in Plants: From Uptake to Metabolism, Stress Response, and Microbe Interaction Once inside the plant, enzymes convert the nitrogen into amino acids, the building blocks of proteins. From there, nitrogen becomes embedded in DNA, chlorophyll, and virtually every other molecule that keeps the plant alive.
Animals get their nitrogen secondhand. You eat plants (or eat animals that ate plants), digest the proteins, and reassemble the amino acids into your own tissues. Every step of this food chain is assimilation in action. The nitrogen does not change its oxidation state during assimilation the way it does during fixation or nitrification; it simply moves from the non-living world into the living one, getting woven into organic molecules along the way.
Assimilation is the stage that most directly connects the nitrogen cycle to agriculture. When crop yields plateau despite good weather, nitrogen availability is often the bottleneck. That is why nitrogen fertilizers, whether synthetic or organic, exist: they supply extra ammonium or nitrate so plants can assimilate more nitrogen and grow faster.
Stage 4: Ammonification
Every organism that assimilates nitrogen eventually dies, and the nitrogen locked in its tissues needs to be recycled. Ammonification, sometimes called mineralization, is the stage where decomposing microbes break down organic nitrogen compounds like proteins and chitin and release ammonium back into the soil or water. Think of it as the reverse of assimilation: nitrogen leaves the organic world and re-enters the inorganic pool.
The process starts with extracellular enzymes that chop large protein and chitin molecules into smaller nitrogen-bearing fragments. This initial breakdown, called depolymerization, is often the rate-limiting step. How fast it happens depends on the chemistry of the organic material and the fertility of the soil.9Biology and Fertility of Soils. Differential response of chitinolytic and proteolytic depolymerization-mineralization pathways to organic nitrogen substrates Animal manure, fallen leaves, and dead microbes all go through ammonification, but at very different speeds. Materials that are low in carbon relative to nitrogen (a low carbon-to-nitrogen ratio) tend to release ammonium quickly. When the ratio is close to that of the microbes themselves, microbial growth and nitrogen release are both promoted.10Soil Use and Management. Carbon‐to‐nitrogen stoichiometry of organic amendments regulates microbial biomass growth and nitrogen mineralization in soil High-carbon materials like wood chips or straw, on the other hand, cause microbes to temporarily lock up available nitrogen rather than release it, which is why gardeners are warned not to mix fresh sawdust directly into planting beds.
Once ammonium is free in the soil, the cycle loops: it can be nitrified into nitrate, taken up again by plants, or volatilized as ammonia gas. Ammonification is what keeps nitrogen circulating rather than permanently locked away in dead organic matter.
Stage 5: Denitrification
Denitrification closes the loop by converting nitrate back into nitrogen gas, which escapes to the atmosphere. It is carried out by a range of bacteria that use nitrate instead of oxygen as an electron acceptor when oxygen is scarce, such as in waterlogged soils or the bottom of a lake. The sequence runs from nitrate to nitrite to nitric oxide to nitrous oxide and finally to dinitrogen gas. Each step is catalyzed by a different enzyme, and the process can stall at nitrous oxide if conditions are not right, which is one reason denitrification contributes to greenhouse gas emissions.7Agriculture. Impacts of Climate Change and Agricultural Practices on Nitrogen Processes, Genes, and Soil Nitrous Oxide Emissions: A Quantitative Review of Meta-Analyses
There is a related but distinct process called anammox (anaerobic ammonium oxidation) that also removes nitrogen from water. Anammox bacteria convert ammonium directly to dinitrogen gas using nitrite or nitric oxide as an electron acceptor, and they do this without oxygen.11Nature Communications. Extracellular electron transfer-dependent anaerobic oxidation of ammonium by anammox bacteria Anammox was only recognized in the 1990s and has since been found to play a major role in nitrogen removal in both ocean oxygen minimum zones and wastewater treatment facilities. Together, denitrification and anammox are the planet’s main mechanisms for pulling reactive nitrogen out of ecosystems and returning it to the atmosphere as harmless N₂.
Why the “Five Stages” Model Is a Simplification
Textbooks present the nitrogen cycle as a neat loop with five stations, but nature is messier. Several of the stages can happen simultaneously in the same handful of soil if oxygen levels vary over tiny distances. A well-aerated soil crumb might be running nitrification on its surface while denitrification proceeds in its oxygen-starved interior. Comammox bacteria, as mentioned earlier, blur the boundary between two classically separate steps. And dissimilatory nitrate reduction to ammonium (DNRA) short-circuits the pathway entirely by converting nitrate back to ammonium rather than sending it all the way to N₂ gas, keeping nitrogen available in the ecosystem instead of losing it to the atmosphere.
The cycle also looks different depending on where it is happening. In ocean oxygen minimum zones, where dissolved oxygen drops to near zero, nitrogen cycling is especially active. These zones support an almost complete nitrogen cycle, with anammox and denitrification responsible for large-scale nitrogen loss, while nitrate reduction to nitrite and DNRA serve as major remineralization routes.12PubMed. Microbial nitrogen cycling processes in oxygen minimum zones Nitrogen limits biological productivity in much of the surface ocean, so the balance between nitrogen fixation and nitrogen loss in these zones has cascading effects on carbon cycling and even atmospheric nitrous oxide levels.13Geoscientific Model Development. Formulation, optimization, and sensitivity of NitrOMZv1.0, a biogeochemical model of the nitrogen cycle in oceanic oxygen minimum zones
How Excess Nitrogen Causes Environmental Problems
When more nitrogen enters an ecosystem than the cycle can process, trouble follows. Agricultural fertilizer is the biggest culprit. Nitrate from cropland leaches into rivers and eventually reaches coastal waters, fueling explosive algae growth. When the algae die, decomposition consumes oxygen, creating “dead zones” where fish and shellfish cannot survive. River nitrogen pollution already drives eutrophication and dead zones in more than two-thirds of the world’s estuaries and bays.14Global Biogeochemical Cycles. Natural Isotope Constraints on River Nitrate Sources in Polluted Regions Agricultural catchments are especially vulnerable because nitrate is so mobile in water that it washes out of soils easily.15Sustainability. Assessment of the Validity of Introducing Nitrate Vulnerable Zones in Large Areas
Excess nitrogen is not just a water problem. Nitrous oxide released during nitrification and denitrification is a greenhouse gas roughly 270 times more potent than carbon dioxide per molecule over a century, and agricultural soils are a major source. The interplay between nitrate and nitrite, mediated by specialized enzymes, is central to how nitrogen moves through the cycle and how much leaks out as pollution or greenhouse gas along the way.16PubMed Central. Nitrate-Nitrite Interplay in the Nitrogen Biocycle
Slowing the Cycle Down on Purpose
Farmers face a dilemma: crops need nitrogen to grow, but much of the nitrogen applied as fertilizer is lost before plants can use it. Nitrification inhibitors are chemicals added to soil or mixed with fertilizer to slow down the ammonia-to-nitrate conversion, keeping nitrogen in the ammonium form longer. Because ammonium binds to soil particles and is less mobile than nitrate, this reduces both leaching into waterways and nitrous oxide emissions.
A large meta-analysis of nitrification inhibitors found that they boost crop yields by an average of about 5 percent and improve the proportion of applied nitrogen that plants actually recover by about 13 percent. Soil ammonium levels rose by roughly 42 percent (the whole point), while nitrate levels dropped by about 25 percent. The most striking effect was on nitrous oxide emissions, which fell by close to half compared to untreated fields.17Journal of Agriculture and Food Research. Effects of nitrification inhibitors on crop productivity, mineral nitrogen concentrations, and gaseous emissions under field conditions: A meta-analysis of recent decades The trade-off is that ammonia volatilization (ammonia gas escaping into the air) increased somewhat, so these tools shift the form of nitrogen loss rather than eliminating it entirely.
Different inhibitors perform differently. Common ones include dicyandiamide (DCD), 3,4-dimethylpyrazole phosphate (DMPP), and nitrapyrin. Each has strengths in certain climates, soil types, and application rates. The same analysis found that yield gains disappear or reverse when nitrogen application rates exceed a certain threshold, a reminder that inhibitors cannot overcome the consequences of simply dumping too much fertilizer.
Nitrogen Cycling in Unexpected Places
The nitrogen cycle is usually discussed in the context of fertile farmland or productive oceans, but it runs in some surprising places. In drylands, biological soil crusts, thin living layers of cyanobacteria, mosses, and lichens on the surface, are major nitrogen processors. Dark cyanobacteria-dominated crusts emit reactive nitrogen (as nitric oxide and nitrous acid) at rates roughly 20 times higher than the bare soil underneath them. Globally, biocrusts are estimated to emit about 1.7 teragrams of reactive nitrogen per year, accounting for around 20 percent of all nitrogen oxide emissions from soils under natural vegetation.18PubMed Central. Biological soil crusts accelerate the nitrogen cycle through large NO and HONO emissions in drylands Dryland ecosystems cover a large share of Earth’s land surface, so these tiny crust organisms play a disproportionate role in nitrogen chemistry.
How Scientists Trace Nitrogen Through Ecosystems
One reason our understanding of the nitrogen cycle has improved so much in recent decades is the development of stable isotope techniques. Nitrogen atoms come in two stable forms: the common nitrogen-14 and the rarer nitrogen-15. Different processes (fixation, nitrification, denitrification, plant uptake) each leave a distinct isotopic fingerprint, preferring one form over the other by slightly different amounts. By measuring the ratio of nitrogen-15 to nitrogen-14 in soil, water, or plant tissue, researchers can figure out where the nitrogen came from and which transformations it has been through.
Stable isotopes now allow detailed investigation of the sources, transformations, and deposition of reactive nitrogen species in the atmosphere and in precipitation, including nitrogen oxides, nitric acid, ammonia, and ammonium.19PubMed Central. Isotopic advances in understanding reactive nitrogen deposition and atmospheric processing In aquatic settings, isotope mixing models have been used to distinguish whether nitrogen in a wetland came from fertilizer runoff, atmospheric deposition, or microbial fixation within the wetland itself.20Water Research X. Use of stable nitrogen isotopes to track plant uptake of nitrogen in a nature-based treatment system This kind of detective work is essential for managing pollution, because the remedy for excess nitrogen depends entirely on identifying the source.
An Ancient Enzyme in a Modern World
Nitrogenase, the enzyme at the heart of nitrogen fixation, is extraordinarily old. Reconstructions of its evolutionary history suggest that the earliest versions used a molybdenum-based cofactor, and that this preference extends back to some of the deepest branches of the tree of life.21PubMed Central. Reconstructing the evolutionary history of nitrogenases: Evidence for ancestral molybdenum‐cofactor utilization Alternative forms that use vanadium or iron in place of molybdenum evolved later. The antiquity of nitrogenase means that biological nitrogen fixation was shaping Earth’s chemistry long before oxygen accumulated in the atmosphere, and the enzyme’s extreme sensitivity to oxygen may be a relic of those anoxic early conditions. Today, organisms that rely on nitrogenase still go to extraordinary lengths to protect it from oxygen, as the legume-nodule architecture illustrates. The fact that evolution never produced an oxygen-tolerant nitrogenase, despite billions of years of opportunity, hints at how deeply the enzyme’s chemistry is constrained by physics.