Which process is primarily responsible for returning nitrogen to the atmosphere in the nitrogen cycle?

• A. Nitrogen fixation by legumes
• B. Assimilation by plants
• C. Nitrification by soil microbes
• D. Denitrification by bacteria

Answer: (D)

Verified step by step guidance

1. Understand the nitrogen cycle: The nitrogen cycle is a biogeochemical process that transforms nitrogen into various chemical forms as it circulates among the atmosphere, soil, and living organisms. Key processes include nitrogen fixation, assimilation, nitrification, and denitrification.
2. Identify the role of denitrification: Denitrification is the process by which certain bacteria convert nitrate (NO₃⁻) or nitrite (NO₂⁻) into nitrogen gas (N₂) or nitrous oxide (N₂O), releasing nitrogen back into the atmosphere.
3. Compare denitrification with other processes: Nitrogen fixation by legumes converts atmospheric nitrogen (N₂) into ammonia (NH₃), assimilation by plants incorporates nitrogen compounds into organic molecules, and nitrification by soil microbes converts ammonia into nitrate. None of these processes return nitrogen to the atmosphere.
4. Recognize the organisms involved: Denitrification is carried out by anaerobic bacteria, such as species of *Pseudomonas* and *Clostridium*, which thrive in oxygen-poor environments like waterlogged soils.
5. Conclude that denitrification is the correct process: Based on the nitrogen cycle and the role of denitrifying bacteria, denitrification is the primary process responsible for returning nitrogen to the atmosphere.