What is denitrification, and why is it important for nitrogen cycling?

• A. It fixes nitrogen.
• B. Denitrification converts nitrates back to N2 gas, returning nitrogen to the atmosphere and limiting nitrate pollution.
• C. It oxidizes ammonia to nitrites.
• D. It uses oxygen to convert nitrates to nitrogen oxides.

Answer: (B)

Denitrification is the microbial process that reduces nitrate (NO3−) to nitrogen gas (N2), which is released back into the atmosphere. This happens mainly in environments with little or no oxygen, where bacteria use nitrate as an alternative electron acceptor in respiration. By converting fixed nitrogen in soils or waters back to N2, denitrification helps complete the nitrogen cycle and reduces nitrate buildup that can pollute groundwater and cause nutrient pollution in ecosystems. It’s not about fixing atmospheric N2 into ammonia, nor about oxidizing ammonia to nitrite and nitrate, and it doesn’t primarily convert nitrates into nitrogen oxides using oxygen; those are different steps in nitrogen cycling.