Nitrification

Nitrification is the biological oxidation of ammonia or ammonium to nitrite and subsequently to nitrate. It is a major component of the nitrogen cycle, occurring in natural environments and engineered treatment systems. In its familiar form, two microbial groups divide the process: ammonia oxidizers produce nitrite, and nitrite oxidizers produce nitrate. Certain microorganisms can perform both stages themselves. Conventional nitrification is an aerobic process, involving oxygen and microorganisms belonging to the Bacteria and Archaea. (nature.com)

Chemical transformations

Nitrification consists of two overall oxidation–reduction reactions. Written with ammonium as the starting material, and excluding incorporation of nitrogen into microbial biomass, they are:

Ammonia oxidation to nitrite

\mathrm{NH_4^+ + \tfrac{3}{2}O_2 \rightarrow NO_2^- + 2H^+ + H_2O}

Nitrite oxidation to nitrate

\mathrm{NO_2^- + \tfrac{1}{2}O_2 \rightarrow NO_3^-}

The combined reaction is therefore:

\mathrm{NH_4^+ + 2O_2 \rightarrow NO_3^- + 2H^+ + H_2O}

These equations describe net transformations rather than individual enzymatic steps. They show that nitrification consumes oxygen and releases hydrogen ions, reducing alkalinity and potentially lowering pH. The oxidation-only stoichiometry requires approximately 4.57 grams of oxygen per gram of ammonium nitrogen oxidized, with an alkalinity consumption equivalent to 7.14 grams of calcium carbonate per gram of nitrogen. These quantities refer to the mass of nitrogen, not the mass of the entire ammonium ion. Incorporation into biomass modifies the actual requirements. (epa.gov)

Ammonia and ammonium interconvert in aqueous solution, with their relative proportions dependent on pH. Consequently, the available substrate and the possibility of inhibition by un-ionized ammonia change with environmental conditions. Although ammonium is commonly used in overall equations and analytical reporting, this does not mean that every molecular step acts directly on the ammonium ion. (epa.gov)

Microorganisms and metabolism

Ammonia-oxidizing bacteria and archaea

Ammonia-oxidizing bacteria, abbreviated AOB, include members of genera such as Nitrosomonas, Nitrosospira, and Nitrosococcus. Ammonia-oxidizing archaea, abbreviated AOA, form a separate evolutionary group. Cultivation of the marine archaeon Nitrosopumilus maritimus, reported in 2005, demonstrated that aerobic ammonia oxidation was not restricted to bacteria. (epa.gov)

Many conventional nitrifiers obtain energy by oxidizing inorganic nitrogen compounds and use inorganic carbon for carbon fixation. This combination is called chemolithoautotrophy. Their growth therefore does not depend on light or on organic carbon as the principal carbon source. Ammonia oxidation involves specialized enzymes, including ammonia monooxygenase; the gene amoA, encoding one of its subunits, is widely used to investigate ammonia-oxidizing communities. (nature.com)

AOB and AOA are not ecologically interchangeable. Measurements of N. maritimus revealed a high affinity for ammonia, providing experimental evidence that substrate availability can help explain the separation of bacterial and archaeal ecological niches. However, the characteristics of one cultivated organism should not be treated as universal properties of all ammonia-oxidizing archaea. (nature.com)

Nitrite-oxidizing bacteria

Nitrite-oxidizing bacteria, abbreviated NOB, carry out the second stage. They include organisms in genera such as Nitrobacter, Nitrospira, Nitrospina, and Nitrococcus. Their activity connects nitrite production to nitrate formation. The balance between ammonia oxidation and nitrite oxidation is important: nitrite can accumulate when its production exceeds its consumption, including when nitrite oxidizers are inhibited. (epa.gov)

Complete ammonia oxidation

Complete ammonia oxidation, commonly called comammox, is the conversion of ammonia to nitrate by a single microorganism. Two independent studies published in 2015 demonstrated this ability in bacteria belonging to Nitrospira. Their genomes contained the machinery for both ammonia oxidation and nitrite oxidation, and cultivation confirmed the combined activity. (nature.com)

Comammox does not eliminate the chemical intermediate nitrite. Instead, it changes the biological organization of the pathway: both stages occur within one organism rather than being obligatorily divided between two microbial groups. Nor does identification of an organism as Nitrospira alone establish that it performs comammox, because the genus also contains conventional nitrite oxidizers. (nature.com)

Heterotrophic nitrification

The term heterotrophic nitrification describes nitrogen oxidation associated with organisms that depend on organic carbon, including some bacteria and fungi. It can involve inorganic or organic nitrogen substrates and is distinct from the classical chemolithoautotrophic pathway. Soil isotope-tracing experiments have demonstrated contributions from organic-nitrogen oxidation and have shown that plant activity and available organic substrates can influence these transformations. The pathways and their importance vary among organisms and environments. (nature.com)

Environmental controls

Nitrification depends on interacting physical, chemical, and biological conditions rather than on a single universal optimum. Important controls include:

Oxygen availability. Conventional nitrification requires oxygen; restricted oxygen supply can limit the overall process.
Substrate supply. Ammonia or ammonium availability controls ammonia oxidation, while nitrite availability controls the second stage.
pH and alkalinity. pH influences microbial activity and ammonia speciation. Insufficient buffering allows the acidity generated by nitrification to suppress further activity.
Temperature. Temperature affects growth and reaction rates, with consequences for seasonal performance.
Inhibitory substances. Elevated free ammonia and various industrial contaminants can inhibit particular nitrifying populations.
Microbial retention. In treatment systems, nitrifiers must remain in the system long enough to grow and maintain an active population. (epa.gov)

Ammonia oxidation and nitrite oxidation can respond differently to these controls. A system may therefore remove ammonium while accumulating nitrite instead of completing oxidation to nitrate. Soil experiments have also shown that ammonium sorption and free-ammonia exposure can influence nitrite accumulation and associated gaseous nitrogen production. (nature.com)

Role in the nitrogen cycle

Nitrification changes the chemical form of nitrogen; it does not itself convert that nitrogen to atmospheric nitrogen gas. It should be distinguished from several connected processes:

Process	Principal transformation
[[nitrogen-fixation	Nitrogen fixation]]
[[ammonification	Ammonification]]
Nitrification	Ammonia or ammonium is oxidized through nitrite to nitrate.
[[denitrification	Denitrification]]

These processes connect nitrogen supply, biological uptake, recycling, and loss across terrestrial and aquatic systems. (nature.com)

In soils, nitrification can turn nitrogen supplied by fertilizers or organic-matter breakdown into nitrate available for plant uptake. However, nitrate that exceeds biological demand can be lost through leaching or denitrification. Nitrate transported below the rooting zone can enter groundwater and subsequently reach rivers or other aquatic environments. Its fate depends on water movement, biological uptake, and further microbial transformations. (nature.com)

Nitrification is also connected to emissions of nitrous oxide, a greenhouse gas. Nitrous oxide can arise during ammonia oxidation and related reactions, while nitrate supplied by nitrification can support subsequent denitrification. The two stages of nitrification do not invariably proceed at matching rates, and nitrite accumulation can be associated with elevated nitrous oxide production. Consequently, a nitrification rate alone is insufficient to determine the resulting gas emissions. (nature.com)

Applications and management

Wastewater treatment

In wastewater treatment, nitrification converts ammonium into oxidized nitrogen forms. It is commonly coupled with denitrification to remove nitrogen from the liquid phase as nitrogen gas. Nitrification alone is therefore not equivalent to total nitrogen removal: most of the nitrogen remains in the water as nitrite or nitrate unless another process removes it. (epa.gov)

Nitrifiers can be retained in suspended-growth systems or attached biofilms. Treatment performance depends on adequate oxygen, suitable chemical conditions, and sufficient retention time. Because conventional nitrifying bacteria grow more slowly than many organic-matter-consuming bacteria, their growth requirements often govern the design of the aerobic stage of a treatment plant. (epa.gov)

Agricultural inhibition

Nitrification inhibitors are substances used to slow ammonium oxidation in agricultural soils. They are investigated and applied to delay nitrate formation and alter nitrogen losses. Their effectiveness is not uniform: experiments with the inhibitor 3,4-dimethylpyrazole phosphate, or DMPP, have found substantially different effects on nitrification and nitrous oxide emissions in different soils. Soil-specific responses prevent a single inhibition factor from describing all applications. (nature.com)

Drinking-water distribution

Nitrification can be undesirable in drinking-water distribution systems, particularly those using chloramines as a secondary disinfectant. Available ammonia can support nitrifying microorganisms, leading to nitrite and nitrate formation, accelerated disinfectant-residual degradation, and changes in microbial or biofilm growth. This makes nitrification an operational water-quality concern distinct from its beneficial role in wastewater treatment. (epa.gov)

Historical development and measurement

The biological nature of nitrification was established experimentally by Théophile Schloesing and Achille Müntz in 1877. Robert Warington subsequently investigated the process at Rothamsted. Around 1890, Sergei Winogradsky isolated organisms responsible for the separate oxidation stages, helping establish the classical model of two cooperating microbial groups. The discoveries of ammonia-oxidizing archaea in 2005 and comammox organisms in 2015 later expanded that model. (repository.rothamsted.ac.uk)

Nitrification is studied using chemical measurements, cultivation, isotope tracers, and molecular methods. Nitrogen-15 tracing can distinguish simultaneous nitrogen transformations, while surveys of genes such as amoA and nxrB characterize organisms with ammonia- or nitrite-oxidizing potential. Gene abundance is not itself a direct measurement of reaction rate. Likewise, nitrate accumulation represents a net change after concurrent production and consumption, rather than necessarily revealing gross nitrification. Combining process measurements with microbial identification is therefore important when assigning nitrification to particular organisms or pathways. (nature.com)