Nitrification is the microbial process that converts ammonia into nitrate, carried out in two main steps by specialized microorganisms in soil, water, and engineered systems. It sits at the heart of the global nitrogen cycle, quietly governing how nitrogen moves through ecosystems, how fertilizer is lost from farm fields, how drinking water stays safe, and even how greenhouse gases enter the atmosphere. The process is far more ecologically tangled and microbiologically diverse than textbooks once suggested, and recent discoveries have rewritten parts of the story entirely.
The Two-Step Chain
The classic understanding of nitrification divides it into two sequential reactions, each performed by a different group of microorganisms. In the first step, ammonia-oxidizing organisms convert ammonia to an intermediate called hydroxylamine, which is then further oxidized to nitrite. The enzymes doing the heavy lifting here are ammonia monooxygenase and hydroxylamine oxidoreductase.1PubMed. Molecular biology and biochemistry of ammonia oxidation by Nitrosomonas europaea In the second step, a separate set of organisms called nitrite-oxidizing bacteria take the nitrite and oxidize it to nitrate using an enzyme called nitrite oxidoreductase. The location of that enzyme within the bacterial cell, whether it sits in the outer cell membrane or deeper inside, affects how efficiently different species handle nitrite.2The ISME Journal. Kinetic plasticity of nitrite-oxidizing bacteria containing cytoplasmic nitrite oxidoreductase
For over a century, this two-organism relay was considered the only way nitrification happened. Then, in 2015, researchers reported something unexpected: a single bacterium from the genus Nitrospira that could perform both steps on its own, oxidizing ammonia all the way to nitrate without a partner. These organisms, dubbed “comammox” bacteria (short for complete ammonia oxidizers), carry the genetic machinery for both ammonia and nitrite oxidation and use both pathways simultaneously during growth.3PubMed Central. Complete nitrification by Nitrospira bacteria The discovery upended the assumption that nitrification always requires teamwork between two distinct microbial groups.
Who Does the Work
The first step, ammonia oxidation, was historically attributed to a handful of well-studied bacterial genera. But gene surveys over the past two decades have revealed that archaea, a domain of life distinct from bacteria, often outnumber ammonia-oxidizing bacteria in many environments. These ammonia-oxidizing archaea appear to be globally important players in the nitrogen cycle, and their discovery has forced a re-evaluation of how much of the world’s nitrification bacteria are actually responsible for.4PubMed. Relative contributions of archaea and bacteria to aerobic ammonia oxidation in the environment
The balance between archaea and bacteria shifts depending on the environment. In agricultural soils, for instance, nitrogen fertilization tends to boost bacterial ammonia oxidizers, while archaea often persist better during periods without added nitrogen. Seasonal shifts and cropping history both push the ratio back and forth.5The ISME Journal. Dynamics of ammonia-oxidizing archaea and bacteria populations and contributions to soil nitrification potentials Even in extremely dry soils, both groups have been found carrying the full genetic toolkit for ammonia oxidation, suggesting nitrification potential exists in environments once thought too harsh for it.6PubMed Central. Ammonia-oxidizing archaea and bacteria differentially contribute to ammonia oxidation in soil under precipitation gradients and land legacy
Temperature preferences also differ between the two groups. Modeling work on soil nitrification found that archaea favor substantially warmer conditions than bacteria, with an optimal temperature for activity roughly 13°C higher. Bacteria, by contrast, can remain active at much lower temperatures, with modeled minimums well below freezing.7The ISME Journal. Modeling of soil nitrification responses to temperature reveals thermodynamic differences between ammonia-oxidizing activity of archaea and bacteria This helps explain why different climates and seasons tend to be dominated by one group or the other.
Nitrification and Greenhouse Gas
Nitrification is not just a nitrogen-shuffling process. It is also a source of nitrous oxide, a greenhouse gas roughly 270 times more potent than carbon dioxide over a century. Ammonia oxidizers can produce nitrous oxide through several metabolic side routes, sometimes involving a hybrid pathway called nitrifier denitrification, where ammonia is first oxidized to nitrite and then partially reduced back through nitric oxide and nitrous oxide.8Soil Biology and Biochemistry. Role of nitrifier denitrification in the production of nitrous oxide
The amount of nitrous oxide produced varies among the different types of ammonia oxidizers. Ammonia-oxidizing bacteria generally yield more nitrous oxide than either archaea or comammox bacteria.9PubMed. Nitrous oxide production by ammonia oxidizers: Physiological diversity, niche differentiation and potential mitigation strategies Environmental stressors make things worse. In coastal waters where low oxygen and acidification co-occur, nitrification rates drop but the fraction of nitrogen converted to nitrous oxide increases, a troubling combination that could intensify as ocean conditions deteriorate.10PubMed Central. Hypoxia and acidification antagonistically suppress nitrification but enhance its N2O production in coastal waters
Losing Fertilizer From Farm Fields
For agriculture, nitrification is a double-edged process. Plants take up nitrogen as both ammonium and nitrate, but nitrate is far more mobile in soil. Once nitrification converts applied ammonium-based fertilizer into nitrate, that nitrate can leach into groundwater or wash off into rivers. It can also be lost as gas through a follow-on process called denitrification. The net result is that a significant fraction of the nitrogen farmers apply never reaches the crop.
One strategy to slow this loss is to apply synthetic nitrification inhibitors, chemicals that temporarily suppress the microbes performing the first step. In subtropical pasture trials, the inhibitors DCD and DMPP reduced fertilizer-induced nitrous oxide emissions by over 76% and 67% respectively, and cut nitrate in runoff and leachate as well. Interestingly, they did not significantly increase pasture yields, because the total nitrogen lost even without inhibitors was small relative to what the plants took up.11Geoderma. Nitrification inhibitors reduce nitrogen losses and improve soil health in a subtropical pastureland The value proposition for inhibitors, then, is less about boosting production and more about reducing environmental damage.
Plants themselves can fight back. Certain crops exude chemicals from their roots that naturally suppress ammonia-oxidizing bacteria, a trait called biological nitrification inhibition. Rice varieties, for example, produce a compound called 1,9-decanediol that blocks the ammonia monooxygenase enzyme.12PubMed. Biological nitrification inhibition by rice root exudates and its relationship with nitrogen-use efficiency Breeding crops for stronger biological nitrification inhibition is an active area of research, with the idea being that the plant’s own root chemistry could reduce the need for synthetic inhibitors.13Agriculture, Ecosystems & Environment. Harnessing biological nitrification inhibition to reduce soil nitrogen losses – Systematic quantification of plant and soil factors to maximise field-scale benefits
Soil Acidification
Nitrification also contributes to soil becoming more acidic over time. The process itself releases hydrogen ions, and when the nitrate produced leaches out of the root zone instead of being taken up by plants, those hydrogen ions stay behind without being neutralized. In cropped subtropical soils, nitrogen transformations including nitrification accounted for roughly 12% to 39% of total acid production, depending on conditions.14Geoderma. Contribution of different proton sources to the acidification of red soil with maize cropping in subtropical China This is a particular concern in regions with naturally acidic soils, where decades of heavy nitrogen fertilization have pushed pH to levels that impair crop growth and require costly lime applications to correct.
The Drinking Water Problem
Many water utilities use chloramines rather than free chlorine as a longer-lasting disinfectant in their pipe networks. The trouble is that chloramine slowly releases trace ammonia, and ammonia-oxidizing bacteria colonizing the pipe walls can feed on it. The resulting nitrification degrades the disinfectant residual, meaning the water reaching your tap may have less protection against pathogens than when it left the treatment plant.15PubMed Central. Review of Nitrification Monitoring and Control Strategies in Drinking Water System The nitrite produced can also create taste and odor issues and, if left unchecked, support the growth of other bacteria.
Utilities monitor for nitrification using a combination of chloramine residual measurements, nitrite levels, and more recently, spectroscopic indices that track the byproducts of microbial activity in the pipes.16PubMed. Developing a chloramine decay index to understand nitrification: A case study of two chloraminated drinking water distribution systems One complicating factor is the use of orthophosphate, which utilities add to control lead and copper corrosion. Evidence suggests orthophosphate may actually increase the rate of chloramine decay and enhance nitrification potential, meaning utilities managing lead risk and nitrification risk can find themselves pulling in opposite directions.17Journal of Water Process Engineering. Assessment of chloramine decay and nitrification potential following orthophosphate addition in a full-scale drinking water distribution system
Wastewater Treatment and an Unexpected Bonus
In wastewater treatment, nitrification is deliberately harnessed to remove ammonium from sewage before it is discharged. The conventional approach oxidizes ammonia all the way to nitrate, which is then reduced to nitrogen gas by denitrifying bacteria in a separate low-oxygen stage. A more energy-efficient alternative, called partial nitrification, stops the oxidation at nitrite and pairs it with a process called anammox, where specialized bacteria convert nitrite and ammonium directly to nitrogen gas. This shortcut saves aeration energy and requires fewer added carbon sources.18PubMed. A review of partial nitrification in biological nitrogen removal processes: from development to application
A less obvious benefit of nitrification in wastewater systems is the breakdown of pharmaceutical residues. Ammonia-oxidizing bacteria, while doing their primary job of converting ammonia, can co-metabolize certain drugs that pass through conventional treatment. Their ammonia monooxygenase enzyme happens to be reactive enough to attack some pharmaceutical molecules alongside its intended substrate. Studies have shown that compounds like ibuprofen and ketoprofen are completely transformed by nitrifying cultures, and that the rate of pharmaceutical degradation tracks closely with the rate of ammonia oxidation.19PubMed. Biotransformation of pharmaceuticals by ammonia oxidizing bacteria in wastewater treatment processes20Bioresource Technology. Co-metabolic oxidation of pharmaceutical compounds by a nitrifying bacterial enrichment The transformation products sometimes carry their own toxicity concerns, though, so this is not yet a clean win.21PubMed. Modeling of Pharmaceutical Biotransformation by Enriched Nitrifying Culture under Different Metabolic Conditions
Nitrification in the Ocean
In open water, nitrification operates under different constraints than in soil. Light is one of the most important. Ammonia oxidation in the ocean is typically suppressed in the sunlit surface layer and concentrated at deeper, darker depths. Work in the eastern tropical North Pacific found that although ammonia oxidation was inhibited at shallow depths and further suppressed by increasing light, it still remained the dominant source of nitrite production even at up to 25% of surface light levels.22Biogeosciences. Testing the influence of light on nitrite cycling in the eastern tropical North Pacific In the Arctic Ocean, a light threshold of about 0.11 mol photons per square meter per day was identified, above which nitrification was suppressed. That threshold extended deep enough to limit nitrification even in shelf bottom waters and the upper halocline.23Global Biogeochemical Cycles. Factors Regulating Nitrification in the Arctic Ocean: Potential Impact of Sea Ice Reduction and Ocean Acidification As Arctic sea ice retreats and allows more light into deeper water, nitrification in those layers could be further suppressed, with cascading effects on regional nutrient cycling.
Low-oxygen zones present their own dynamics. In the Baltic Sea’s hypoxic waters, archaeal ammonia oxidizers were found driving high nitrification rates right at the boundary where oxygen concentrations dropped off. These archaea even tolerated periodic exposure to hydrogen sulfide, a toxic compound associated with fully anoxic water, continuing to nitrify at reduced rates rather than shutting down entirely.24The ISME Journal. Significance of archaeal nitrification in hypoxic waters of the Baltic Sea
Aquariums and Biofilters
Anyone who has kept a fish tank knows about “cycling” the tank, the weeks-long process of establishing nitrifying microbes in the filter so that toxic ammonia from fish waste is converted to less harmful nitrate. The biofilters used in home aquariums and large-scale aquaculture systems work on the same principle as natural nitrification, just in a compact engineered space.25PubMed Central. “Candidatus Nitrosotenuis aquarius,” an Ammonia-Oxidizing Archaeon from a Freshwater Aquarium Biofilter
What surprised researchers examining aquarium biofilters is that the microbial communities inside them do not always match the textbook species. Archaea, not just bacteria, contribute to ammonia oxidation in freshwater aquarium filters. And comammox Nitrospira, the single-organism nitrifiers discovered in 2015, have been found among the dominant ammonia oxidizers in aquarium biofilter communities.26PubMed Central. Comammox Nitrospira among dominant ammonia oxidizers within aquarium biofilter microbial communities For hobbyists, this means the nitrogen cycle in your filter is more microbiologically sophisticated than the simplified guides suggest, though the practical takeaway, be patient during cycling and do not crash the filter, remains unchanged.
Nitrification Inside Living Animals
Some of the most unexpected nitrification happens not in soil or water but inside the bodies of marine sponges. Sponges are filter feeders that process enormous volumes of seawater, and many harbor dense communities of ammonia-oxidizing archaea and nitrite-oxidizing bacteria within their tissues. These symbionts convert the ammonia generated by the sponge’s own metabolism into nitrate, which is then released into the surrounding reef water. One Caribbean species, Chondrilla nucula, was measured releasing nitrate at about 600 nanomoles of nitrogen per gram of dry weight per hour, a substantial contribution to local nutrient availability on coral reefs.27Limnology and Oceanography. Nitrate release by Caribbean reef sponges
These nitrifying symbionts are not random colonizers. Research has shown that the particular lineages of ammonia-oxidizing archaea and nitrite-oxidizing bacteria found in sponges are vertically inherited, meaning they pass from parent to offspring rather than being picked up from the environment each generation. These lineages are widespread across marine sponges and corals globally, pointing to an ancient and tightly maintained partnership.28PubMed Central. Co-occurring nitrifying symbiont lineages are vertically inherited and widespread in marine sponges The sponge gets its waste ammonia detoxified, and the nitrifiers get a steady food supply and a protected habitat. On busy reef systems, the collective nitrate output from sponge-mediated nitrification can meaningfully influence the nutrient budget available to corals and algae.
Tracing Nitrogen With Isotopes
Scientists use the oxygen isotope signature of nitrate to trace where it came from, whether from nitrification, atmospheric deposition, or fertilizer. Nitrification incorporates oxygen atoms from both water and dissolved oxygen gas into the nitrate it produces, and the resulting isotopic ratio turns out to be surprisingly sensitive to the oxygen isotope composition of water. One study found that the oxygen isotope ratio of nitrate produced by nitrification tends to converge near the isotopic value of water, due to equilibration between water and the nitrite intermediate. Nitrate found in soils and groundwater often has higher oxygen isotope ratios than this baseline, which likely reflects additional processes like partial nitrate reduction layered on top of nitrification.29ACS Publications (Environmental Science & Technology). Constraining the Oxygen Isotopic Composition of Nitrate Produced by Nitrification This isotopic detective work helps environmental scientists figure out whether nitrate contamination in a well or stream came from fertilizer runoff, sewage, or natural soil processes, each of which leaves a different chemical fingerprint.