In the nitrogen cycle, the nitrates in the soil are converted to atmospheric nitrogen by

Analyze the question: It asks which organisms convert soil nitrates (NO₃⁻) back into atmospheric nitrogen gas (N₂). This is the step that returns nitrogen to the air.

Concept: the roles of bacteria in the nitrogen cycle

| Process | Bacteria | What happens | |---|---|---| | Nitrogen fixation | Rhizobium, Azotobacter (nitrogen-fixing) | N₂ (air) → ammonia/nitrogen compounds | | Ammonification (decay) | Putrefying bacteria and fungi | Dead organic matter/proteins → ammonia | | Nitrification | Nitrosomonas, Nitrobacter (nitrifying) | Ammonia → nitrites → nitrates | | Denitrification | Pseudomonas denitrificans, Thiobacillus (denitrifying) | Nitrates → N₂ gas (returned to the atmosphere) |

Step-by-step solution:

1. The starting material is nitrates in the soil.
2. The end product is atmospheric nitrogen (N₂).
3. Nitrates → nitrogen gas is the definition of denitrification.
4. Eliminate the others: • A (putrefying bacteria) break down dead organisms into ammonia, not nitrogen gas. • B (nitrogen-fixing bacteria) do the opposite, turning atmospheric nitrogen into compounds. • C (nitrifying bacteria) convert ammonia to nitrites and nitrates, which makes nitrates rather than removing them. • D (denitrifying bacteria) convert nitrates to nitrogen gas. ✔

**Answer: D, denitrifying bacteria.

Exam tip: Note the prefix "de-", which means "remove" or "reverse". Denitrifying bacteria remove nitrates and return N₂ to the air. Fixing puts nitrogen in to the soil, and denitrifying takes it out.