Which of the following processes converts nitrates in the soil to atmospheric nitrogen?

Step 1: Analyze the question The question asks which process changes nitrates (NO₃⁻) in the soil back into nitrogen gas (N₂) in the atmosphere.

Step 2: Concept: the nitrogen cycle

• Nitrogen fixation: N₂ is converted to nitrogen compounds. It is done by Rhizobium and Azotobacter, and also by lightning.
• Putrefaction (ammonification): Bacteria and fungi break down dead plants and animals and release ammonia.
• Nitrification: Ammonia is converted to nitrites (by Nitrosomonas) and then to nitrates (by Nitrobacter).
• Denitrification: Nitrates are converted to nitrogen gas (N₂) by denitrifying bacteria such as Pseudomonas denitrificans. This usually happens in waterlogged, oxygen-poor soils.

Step 3: Check each option

• A. Denitrification: Correct. Nitrates → N₂ gas, returned to the atmosphere.
• B. Lightning: Wrong. It helps fix atmospheric nitrogen into nitrates, which is the opposite direction.
• C. Nitrification: Wrong. It produces nitrates and does not remove them.
• D. Putrefaction: Wrong. It produces ammonia from dead organic matter.
• E. Rainfall: Wrong. It only carries dissolved nitrogen compounds into the soil.

Step 4: Answer The process that converts soil nitrates to atmospheric nitrogen is denitrification, which is option A.

Tip: Think of "de-" as "undoing." Denitrification undoes nitrification, so nitrates go back to nitrogen gas. Also remember that nitrification builds nitrates, while denitrification removes them.