Question
The correct sequence of steps in the nitrogen cycle is:
(A) Nitrogen fixation → Nitrification → Ammonification → Denitrification
(B) Nitrogen fixation → Ammonification → Nitrification → Denitrification
(C) Denitrification → Nitrification → Ammonification → Nitrogen fixation
(D) Nitrogen fixation → Nitrification → Denitrification → Ammonification
(NEET 2023 — this exact sequence question has appeared multiple times with shuffled options)
Solution — Step by Step
Atmospheric N₂ is inert — the triple bond (N≡N) makes it nearly impossible for most organisms to use directly. Nitrogen fixation converts N₂ → NH₃ (ammonia). This is done by Rhizobium (symbiotic, in legume root nodules) and free-living bacteria like Azotobacter and Anabaena.
When organisms die, decomposers break down proteins and nucleic acids. The nitrogen in these organic molecules is released as NH₃/NH₄⁺. This step is ammonification. Think of it as the “recycling” step — dead matter back to a simple form.
Nitrification is a two-stage oxidation:
- NH₄⁺ → NO₂⁻ (by Nitrosomonas)
- NO₂⁻ → NO₃⁻ (by Nitrobacter)
Plants absorb nitrates (NO₃⁻), so this step is what makes the fixed nitrogen actually useful to the food chain.
Denitrification by bacteria like Pseudomonas converts NO₃⁻ back to N₂ gas. This closes the cycle. Without this step, nitrates would accumulate in soil indefinitely.
The correct sequence is (B): Nitrogen fixation → Ammonification → Nitrification → Denitrification.
Why This Works
The nitrogen cycle is essentially a story of oxidation states. Atmospheric N₂ sits at oxidation state 0. Fixation brings it to −3 (NH₃). Nitrification pushes it up to +5 (NO₃⁻). Denitrification brings it back to 0. The cycle is thermodynamically driven — bacteria at each step extract energy from these redox reactions.
Why does ammonification come before nitrification in the natural cycle? Because nitrifying bacteria (Nitrosomonas, Nitrobacter) work on NH₄⁺ as their substrate. The NH₄⁺ must exist first — either from fixation or from decomposition of organic matter.
Rhizobium deserves special attention for NEET. It is mutualistic — the plant provides carbohydrates, Rhizobium provides fixed nitrogen. The enzyme nitrogenase does the actual fixation work, and it is inactivated by oxygen. This is why root nodules maintain a low-oxygen environment using leghaemoglobin (a pink pigment — another NEET favourite).
Alternative Method — Memory Trick
For sequence questions, use the mnemonic “FAN D”:
- Fixation
- Ammonification
- Nitrification
- Denitrification
The order FAN D also makes ecological sense: you first make usable nitrogen (F), then recycle dead organic nitrogen (A), then upgrade it for plants (N), then release it back to air (D). Follow the logic, not just the letters.
Common Mistake
The most common error: students place nitrification before ammonification. The logic trap is thinking “nitrogen fixation makes NH₃, so nitrification must come next.” But nitrification happens to NH₄⁺ from decomposing organic matter in the natural soil cycle, not directly from the fixed NH₃. In the full cycle, ammonification feeds nitrification — not fixation directly. If you get option (A) as your answer, you’ve fallen into this trap.
Also watch out for confusing the bacteria:
| Process | Bacteria | Substrate → Product |
|---|---|---|
| Fixation | Rhizobium, Azotobacter | N₂ → NH₃ |
| Nitrification (step 1) | Nitrosomonas | NH₄⁺ → NO₂⁻ |
| Nitrification (step 2) | Nitrobacter | NO₂⁻ → NO₃⁻ |
| Denitrification | Pseudomonas | NO₃⁻ → N₂ |
NEET frequently asks which bacterium does which specific conversion — not just which process it belongs to. Know the substrate and product for each organism.