Denitrifying bacteria reduce nitrates to nitrogen gas in anaerobic conditions

Steps:

• Identify the key process: bacteria use nitrates as an oxygen source in oxygen-poor, waterlogged soils.
• Note the waste product: nitrogen gas (N2) is released during nitrate breakdown.
• Recall bacterial roles in the nitrogen cycle: denitrification involves converting nitrates (NO3-) to N2.
• Match description to option B, as it fits anaerobic nitrate respiration producing N2.

Why B is correct:

• Denitrifying bacteria perform anaerobic respiration by reducing nitrates to nitrogen gas, defined as NO3- → NO2- → NO → N2O → N2.

Why the others are wrong:

• A: Decomposing bacteria break down organic matter into simpler compounds but do not specifically reduce nitrates to N2.
• C: Nitrogen-fixing bacteria convert atmospheric N2 into usable forms like ammonia, not the reverse.
• D: Root nodule bacteria fix nitrogen in plant roots, producing ammonia rather than releasing N2 from nitrates.

Final answer: B