Question

How is fixed nitrogen recycled in the biosphere?

Short answer

The biosphere primarily recycles nitrogen fixed as ammonium ${\mathrm{NH}}_4^{+}$or successively oxidizes it to nitrite (${\mathrm{NO}}_2^{-}$) and nitrate (${\mathrm{NO}}_3^{-}$).

Step-by-step solution

Nitrogen Cycle.

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

Nitrification and Denitrification.  

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 ${\mathrm{N}}_2$.

Reduction of nitrate.  

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.