Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of these nitrites into nitrates. Nitrification is an important step in the nitrogen cycle in soil. This process was discovered by the Russian microbiologist, Sergei Winogradsky.

The oxidation of ammonia into nitrite, and the subsequent oxidation to nitrate is performed by two different bacteria (nitrifying bacteria). The first step is done by bacteria of (amongst others) the genus Nitrosomonas and Nitrosococcus. The second step (oxidation of nitrite into nitrate) is (mainly) done by bacteria of the genus Nitrobacter, with both steps producing energy to be coupled to ATP synthesis. Nitrifying organisms are chemoautotrophs, and use carbon dioxide as their carbon source for growth.

Nitrification also plays an important role in the removal of nitrogen from municipal wastewater. The conventional removal is nitrification, followed by denitrification. The cost of this process resides mainly in aeration (bringing oxygen in the reactor) and the addition of an external carbon source (e.g. methanol) for the denitrification.

In most environments both organisms are found together, yielding nitrate as the final product. It is possible however to design systems in which selectively nitrite is formed (the Sharon process).

Together with ammonification, nitrification forms a mineralization process which refers to the complete decomposition of organic material, with the release of available nitrogen compounds. This replenishes the nitrogen cycle.

Chemistry

Nitrification is a process of nitrogen compound oxidation (effectively, loss of electrons from the nitrogen atom to the oxygen atoms) :

1. NH₃ + O₂ → NO₂⁻ + 3H⁺ + 2e⁻
2. NO₂⁻ + H₂O → NO₃⁻ + 2H⁺ + 2e⁻

References

• Nitrification at the heart of filtration at fishdoc.co.uk
• Nitrification at University of Aberdeen · King's College
• Nitrification Basics for Aerated Lagoon Operators at lagoonsonline.com