Question
What are the different methods of nitrogen fixation — biological, industrial, and atmospheric — and how do they compare?
Solution — Step by Step
Atmospheric nitrogen () makes up 78% of air, but plants cannot use it directly. The triple bond in (, bond energy 946 kJ/mol) is extremely strong. Nitrogen must be “fixed” — converted to usable forms like , , or — before living organisms can incorporate it into proteins and nucleic acids.
Certain microorganisms possess the enzyme nitrogenase, which breaks the triple bond:
Key organisms:
- Symbiotic: Rhizobium (in root nodules of legumes), Frankia (in non-legumes like Alnus)
- Free-living aerobic: Azotobacter, Beijerinckia
- Free-living anaerobic: Clostridium
- Cyanobacteria: Anabaena, Nostoc (have heterocysts — specialized cells for N fixation)
Nitrogenase is oxygen-sensitive — it is inactivated by O. This is why Rhizobium root nodules contain leghemoglobin (a pink pigment that binds O to create a low-O environment for nitrogenase). NEET asks about leghemoglobin nearly every other year.
This process produces ammonia for fertilizers. It requires very high temperature and pressure to break the triple bond — consuming about 1% of the world’s total energy supply.
Lightning provides enough energy to combine N with O:
The NO is further oxidized to and dissolves in rain as nitric acid (), which reaches the soil as nitrate.
This natural process fixes about 5-8% of the total nitrogen fixed globally.
| Method | Conditions | Product | Contribution |
|---|---|---|---|
| Biological | Mild (body temp, 1 atm) | NH | ~60% of total N fixed |
| Industrial (Haber) | 450 degrees C, 200 atm | NH | ~25% (for fertilizers) |
| Atmospheric (lightning) | Extreme energy (lightning) | NO/NO | ~5-8% |
Biological fixation is the most efficient — it works at body temperature and atmospheric pressure, thanks to the enzyme nitrogenase.
flowchart TD
A["Nitrogen Fixation Methods"] --> B["Biological"]
A --> C["Industrial: Haber-Bosch"]
A --> D["Atmospheric: Lightning"]
B --> E["Symbiotic: Rhizobium in legume nodules"]
B --> F["Free-living: Azotobacter, Clostridium"]
B --> G["Cyanobacteria: Anabaena with heterocysts"]
C --> H["N2 + 3H2 at 450C, 200 atm, Fe cat gives 2NH3"]
D --> I["N2 + O2 gives NO by lightning energy"]
E --> J["Nitrogenase enzyme, leghemoglobin protects from O2"]Why This Works
The triple bond in N is one of the strongest bonds in chemistry. Breaking it requires either extreme conditions (high temperature in Haber process, lightning energy) or a highly specialized enzyme (nitrogenase with its iron-molybdenum cofactor). Evolution produced nitrogenase because nitrogen is essential for amino acids and nucleotides — organisms that could fix their own nitrogen had an enormous survival advantage.
The symbiotic relationship between Rhizobium and legumes is mutually beneficial: the bacteria fix nitrogen for the plant, and the plant provides carbohydrates and a protected low-oxygen environment for the bacteria.
Alternative Method
For NEET recall, organise the nitrogen cycle as a flowchart: Atmospheric N enters the cycle through fixation (biological/lightning/industrial), becomes NH or NO in soil, is taken up by plants, passes through the food chain, and returns to the atmosphere through denitrification by bacteria like Pseudomonas and Thiobacillus. Understanding this cycle as a loop makes individual questions easier.
Common Mistake
Students write that Rhizobium is a “nitrogen-fixing bacterium found in soil.” While Rhizobium does exist freely in soil, it fixes nitrogen only inside root nodules of leguminous plants — not as a free-living organism. Free-living nitrogen fixers are Azotobacter (aerobic) and Clostridium (anaerobic). This distinction between symbiotic and free-living fixers is a recurring NEET question.