Which process converts nitrogen gas into forms usable by plants?

• A. Nitrification
• B. Denitrification
• C. Nitrogen Fixation
• D. Nitrogen Assimilation

Answer: (C)

Converting atmospheric nitrogen gas into forms plants can use is nitrogen fixation. Nitrogen gas (N2) is very stable and not directly usable by most organisms. Certain bacteria and archaea carry the nitrogenase enzyme that splits N2 and combines it with hydrogen to form ammonia (NH3), which becomes ammonium (NH4+) in solution. This process requires energy (ATP) and occurs in two main ways: in symbiotic bacteria inside legume root nodules and in free-living soil microbes, with humans also producing a large-scale version through industrial fertilizer production.

Nitrification changes ammonium into nitrite and then nitrate, while denitrification reduces nitrate back to N2 or N2O. Nitrogen assimilation refers to plants taking up inorganic nitrogen (like ammonium or nitrate) and incorporating it into organic molecules, which happens after nitrogen is already in a usable form. So the step that directly converts N2 into usable forms is nitrogen fixation.