- A Denitrification converting nitrate back into atmospheric nitrogen gas
- B Nitrogen fixation (converting N2 to NH3 by bacteria)
- C Nitrification converting ammonia into nitrite and then nitrate
- D Ammonification converting organic nitrogen into ammonia
Correct answer: B. Nitrogen fixation (converting N2 to NH3 by bacteria)
Explanation: Nitrogen fixation: Rhizobium (symbiotic) and Azotobacter (free-living) convert N2 gas to NH3. This is the main biological entry of N2 into ecosystems.
Concept context
Ecosystems, food chains, energy flow, nutrient cycles, biodiversity, and conservation.