Which nitrogen process converts atmospheric nitrogen into usable forms for organisms?

• A. Denitrification
• B. Nitrification
• C. Nitrogen fixation
• D. Mineralization

Answer: (C)

Converting atmospheric nitrogen gas into forms that organisms can use is nitrogen fixation. Certain bacteria and cyanobacteria carry the enzyme nitrogenase that breaks the strong N≡N bond and adds hydrogen to create ammonia or ammonium, which plants and other organisms can assimilate. Denitrification, by contrast, returns fixed nitrogen back to the atmosphere as N2. Nitrification transforms ammonia into nitrite and then nitrate, but it starts from already fixed nitrogen rather than converting atmospheric N2 directly. Mineralization is the breakdown of organic nitrogen into ammonium. So the process that turns atmospheric nitrogen into usable forms is nitrogen fixation.