Microorganisms drive major redox transformations among inorganic nitrogen compounds.

Nitrification is the aerobic oxidation of reduced nitrogen. In simplified form,

$$NH_4^+ \rightarrow NO_2^- \rightarrow NO_3^-.$$

Different microorganisms can carry out different steps. These reactions release energy that some chemoautotrophs capture for metabolism and carbon fixation.

Denitrification moves nitrogen in the opposite redox direction under low-oxygen conditions. Certain microorganisms use nitrate ($NO_3^-$) as an electron acceptor and reduce it through intermediate nitrogen oxides toward nitrogen gas:

$$NO_3^- \rightarrow NO_2^- \rightarrow \cdots \rightarrow N_2.$$

Because $N_2$ can escape to the atmosphere, denitrification removes biologically available fixed nitrogen from many ecosystems.

Nitrification and denitrification are therefore not merely labels in a cycle diagram. They are microbial energy metabolisms coupled to oxidation-reduction chemistry.

A useful contrast is

$$\boxed{\text{nitrification: reduced N} \rightarrow \text{more oxidized N}}$$

$$\boxed{\text{denitrification: oxidized N} \rightarrow N_2.}$$

Together with nitrogen fixation, assimilation and decomposition, these transformations control how nitrogen moves between atmosphere, inorganic pools and biomass.