Which process in the terrestrial nitrogen cycle converts atmospheric inert nitrogen gas () into bioavailable ammonia (), performed biologically by symbiotic bacteria like Rhizobium in legume root nodules?

Answer and explanation

Correct answer: D. Nitrogen fixation

  1. Step 1: Atmospheric air is 78% nitrogen by volume, but the strong triple covalent bond () prevents plants and animals from absorbing it directly.
  2. Step 2: Biological nitrogen fixation breaks the triple bond using the enzyme nitrogenase, converting into reactive ammonia; this is carried out by free-living bacteria (Azotobacter) and symbiotic rhizobia inhabiting root nodules of leguminous plants.
Answer
Nitrogen fixation
Why not the others: 'Nitrification' Nitrification is the biological oxidation of ammonia into nitrites () and subsequently nitrates () by Nitrosomonas and Nitrobacter; 'Denitrification' Denitrification is the anaerobic microbial reduction of soil nitrates back into gaseous nitrogen () by bacteria such as Pseudomonas; 'Ammonification' Ammonification is the decomposition process where decomposers convert organic nitrogen from decaying biomass into ammonia.

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