Question
What is nitrogen fixation? Compare biological, industrial, and atmospheric methods of nitrogen fixation. Why is biological nitrogen fixation so important?
Solution — Step by Step
The atmosphere is 78% N₂, but most organisms cannot use molecular nitrogen directly because the N≡N triple bond is extremely strong (946 kJ/mol). Nitrogen fixation converts N₂ into usable forms — ammonia (NH₃), nitrates (NO₃⁻), or nitrites (NO₂⁻).
| Method | Process | Products | Scale |
|---|---|---|---|
| Biological | Nitrogenase enzyme in bacteria reduces N₂ to NH₃ | NH₃ | ~65% of total global fixation |
| Industrial (Haber process) | N₂ + 3H₂ → 2NH₃ at high T, P, catalyst | NH₃ (fertilisers) | ~30% |
| Atmospheric (Lightning) | Lightning energy breaks N₂, combines with O₂ | NO, NO₂ → washed down as HNO₃ | ~5% |
Free-living fixers: Azotobacter (aerobic), Clostridium (anaerobic), cyanobacteria (Nostoc, Anabaena).
Symbiotic fixers: Rhizobium (in root nodules of legumes — pea, soybean, groundnut). The bacterium provides NH₃ to the plant; the plant provides organic acids and shelter.
The enzyme nitrogenase catalyses the reaction:
Nitrogenase is extremely sensitive to oxygen — it is inactivated by O₂. In root nodules, leghaemoglobin (a pink pigment) scavenges oxygen to protect nitrogenase.
flowchart TD
A[Nitrogen Fixation Methods] --> B[Biological: ~65%]
A --> C[Industrial Haber: ~30%]
A --> D[Atmospheric Lightning: ~5%]
B --> B1[Free-living: Azotobacter, Nostoc]
B --> B2[Symbiotic: Rhizobium in legumes]
B2 --> B3[Nitrogenase enzyme]
B3 --> B4[Leghaemoglobin protects from O₂]
C --> C1[N₂ + 3H₂ at 200 atm, 500°C]
D --> D1[Lightning splits N₂]Why This Works
Biological nitrogen fixation is crucial because it adds bioavailable nitrogen to ecosystems naturally, without energy-intensive industrial processes. Legume crops (pulses) improve soil fertility through Rhizobium symbiosis — this is why crop rotation with legumes is a traditional agricultural practice in India.
Common Mistake
Students write “Rhizobium fixes nitrogen independently.” Rhizobium is a free-living aerobe in soil but can only fix nitrogen inside root nodules (where conditions are microaerobic). The symbiotic relationship is essential — the plant provides the low-oxygen environment that nitrogenase needs.
Leghaemoglobin is often confused with haemoglobin. Leghaemoglobin is found only in root nodules, has a pink colour, and its job is to bind O₂ to keep the nodule interior oxygen-free for nitrogenase activity. The globin part is coded by the plant, the haem by the bacterium.