How is fixed nitrogen recycled in the biosphere?

The nitrogen cycle describes how the ammonia created by the nitrogenase activity and converted into amino acids is ultimately recycled in the biosphere.

A process known as nitrification occurs when specific bacteria oxidize${\text{NH}}_{\text{3}}$ to (${\mathrm{NO}}_2^{-}$) and (${\mathrm{NO}}_3^{-}$) subsequently to produce nitrate. Denitrification is the process by which some organisms turn nitrate back into dinitrogen ${\mathrm{N}}_2$.

Plants, fungi, and several bacteria reduce nitrate to${\text{NH}}_{\text{3}}$ by a process known as ammonification, where nitrate reductase catalyzes the two-electron reduction of nitrate to nitrite (${\mathrm{NO}}_2^{-}$).

${\mathrm{NO}}_3^{-}+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{NO}}_2^{-}+{\mathrm{H}}_2\mathrm{O}$

Nitrite is converted to ammonia by nitrite reductase.

${\mathrm{NO}}_2^{-}+7{\mathrm{H}}^{+}+6{\mathrm{e}}^{-}\to {\mathrm{NH}}_3+2{\mathrm{H}}_2\mathrm{O}$

Certain bacteria perform the opposite of nitrogen fixation, the direct anaerobic oxidation of${\text{NH}}_{\text{3}}$ back to${\text{N}}_{\text{2}}$without the use of nitrate as an intermediary.