Nitrogen cycle in ecosystem — nitrogen fixation, nitrification, denitrification

Question

Describe the nitrogen cycle. Explain the processes of nitrogen fixation (biological and non-biological), nitrification, ammonification, and denitrification. Name the key organisms involved in each step.

(NCERT Class 12 — frequently asked in NEET)

Solution �� Step by Step

Atmospheric $\text{N}_2$ (78% of air) is unusable by most organisms. It must be converted to a usable form.

Biological fixation:

Free-living bacteria: Azotobacter (aerobic), Clostridium (anaerobic), cyanobacteria like Anabaena and Nostoc
Symbiotic bacteria: Rhizobium in root nodules of leguminous plants
Enzyme involved: nitrogenase (highly sensitive to oxygen)

\text{N}_2 + 8\text{H}^+ + 8e^- + 16\text{ATP} \xrightarrow{\text{nitrogenase}} 2\text{NH}_3 + \text{H}_2 + 16\text{ADP}

Non-biological fixation:

Lightning converts $\text{N}_2$ to nitrogen oxides ( $\text{NO}_x$ ), which dissolve in rain as nitric acid
Industrial: Haber process ( $\text{N}_2 + 3\text{H}_2 \to 2\text{NH}_3$ ) for fertilisers

When organisms die or excrete waste, decomposer bacteria and fungi break down the nitrogen-containing organic matter (proteins, nucleic acids) into ammonia ( $\text{NH}_3$ ) or ammonium ( $\text{NH}_4^+$ ).

Key organisms: Bacillus, Pseudomonas, and various soil fungi.

A two-step process carried out by chemoautotrophic bacteria in soil:

Step 1: $\text{NH}_3 \to \text{NO}_2^-$ (nitrite) — by Nitrosomonas

Step 2: $\text{NO}_2^- \to \text{NO}_3^-$ (nitrate) — by Nitrobacter

Nitrate ( $\text{NO}_3^-$ ) is the form most plants absorb through their roots.

Certain anaerobic bacteria (Pseudomonas, Thiobacillus) convert nitrate back to molecular nitrogen:

\text{NO}_3^- \to \text{NO}_2^- \to \text{NO} \to \text{N}_2\text{O} \to \text{N}_2

This occurs in waterlogged, oxygen-depleted soils. It returns nitrogen to the atmosphere, completing the cycle. From an agricultural perspective, denitrification is undesirable because it removes usable nitrogen from soil.

Why This Works

The nitrogen cycle is nature’s recycling system for a critical element. Nitrogen is needed for amino acids, nucleic acids, and chlorophyll — yet atmospheric $\text{N}_2$ has a triple bond ( $\text{N}\equiv\text{N}$ ) that is extremely stable and hard to break. Biological nitrogen fixation by nitrogenase is one of the most energy-expensive reactions in biology (16 ATP per $\text{N}_2$ ), which is why nitrogen is often the limiting nutrient in ecosystems.

The cycle ensures that nitrogen flows between atmosphere, soil, organisms, and back — maintaining a dynamic equilibrium.

Alternative Method — Organism-Process Map

For NEET, map each organism to its process:

Rhizobium → Symbiotic N₂ fixation (legumes only)
Azotobacter → Free-living N₂ fixation (aerobic)
Clostridium → Free-living N₂ fixation (anaerobic)
Nitrosomonas → $\text{NH}_3 \to \text{NO}_2^-$ (nitrification step 1)
Nitrobacter → $\text{NO}_2^- \to \text{NO}_3^-$ (nitrification step 2)
Pseudomonas → $\text{NO}_3^- \to \text{N}_2$ (denitrification)

NEET loves matching organisms to processes — memorise this table cold.

Common Mistake

Students often say “Rhizobium fixes nitrogen independently.” Rhizobium can fix nitrogen only in symbiosis with legume root nodules — the plant provides the anaerobic environment (via leghemoglobin) and carbon sources that Rhizobium needs. Free-living Rhizobium in soil does NOT fix nitrogen. Also, don’t confuse nitrification (a useful process that makes nitrate available to plants) with denitrification (which removes nitrogen from soil).

Question

Solution �� Step by Step

Why This Works

Alternative Method — Organism-Process Map

Common Mistake

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