Explain the role of diazotrophs in nitrification.

Answer: Diazotrophs fix atmospheric N₂ into ammonia, which is the starting material that nitrifying bacteria oxidise to nitrite and nitrate.

Diazotrophs are prokaryotes that can fix free atmospheric nitrogen using the enzyme nitrogenase, e.g. free-living Azotobacter, Clostridium, cyanobacteria (Anabaena, Nostoc) and symbiotic Rhizobium in root nodules of legumes.

Nitrogen fixation: N₂ + 8e^– + 8H⁺ + 16 ATP → 2NH₃ + H₂ + 16 ADP + 16 Pi. The ammonia (NH₃/NH₄⁺) released into the soil is the substrate for nitrification.

Nitrification is the conversion of ammonia to nitrate by chemoautotrophic bacteria in two steps:

• 2NH₃ + 3O₂ → 2NO₂^– + 2H⁺ + 2H₂O (by Nitrosomonas, Nitrococcus)
• 2NO₂^– + O₂ → 2NO₃^– (by Nitrobacter)

Thus diazotrophs supply the ammonia that is nitrified; the nitrate formed is the main form of nitrogen absorbed by plants.