Nitrogen cycle in nature — fixation, nitrification, denitrification

Question

Describe the nitrogen cycle in nature. Explain the key processes: nitrogen fixation, ammonification, nitrification, and denitrification. Name the organisms involved in each step.

(NEET, CBSE Class 12 — Ecosystem / Ecology)

Solution — Step by Step

Atmospheric N $_2$ is converted to ammonia (NH $_3$ ) by nitrogen-fixing organisms. Biological fixation is done by Rhizobium (symbiotic, in legume root nodules), Azotobacter, and Nostoc (free-living). The enzyme nitrogenase catalyses this reaction. Lightning also fixes some nitrogen (abiotic fixation).

When organisms die, decomposers (bacteria and fungi) break down the nitrogen-containing organic matter (proteins, nucleic acids) into ammonia (NH $_3$ ) or ammonium ions (NH $_4^+$ ). This process returns nitrogen from dead organic matter back to the soil in usable form.

Nitrifying bacteria convert ammonia to nitrates in two steps. Nitrosomonas oxidises NH $_3$ to nitrite (NO $_2^-$ ). Then Nitrobacter oxidises NO $_2^-$ to nitrate (NO $_3^-$ ). Plants absorb nitrogen primarily as nitrates from the soil.

Plants take up NO $_3^-$ (and some NH $_4^+$ ) through roots and incorporate nitrogen into amino acids, proteins, and nucleic acids. Animals get their nitrogen by eating plants (or other animals).

Under anaerobic conditions, denitrifying bacteria (like Pseudomonas and Thiobacillus) convert nitrates back to atmospheric N $_2$ . This completes the cycle by returning nitrogen to the atmosphere.

graph TD
    A["Atmospheric N₂"] -->|"Nitrogen fixation<br/>Rhizobium, Azotobacter"| B["NH₃ / NH₄⁺"]
    C["Dead organic matter"] -->|"Ammonification<br/>Decomposers"| B
    B -->|"Nitrification<br/>Nitrosomonas"| D["NO₂⁻ (Nitrite)"]
    D -->|"Nitrification<br/>Nitrobacter"| E["NO₃⁻ (Nitrate)"]
    E -->|"Assimilation"| F["Plants → Animals"]
    F -->|"Death, excretion"| C
    E -->|"Denitrification<br/>Pseudomonas"| A

Why This Works

Nitrogen makes up 78% of the atmosphere, but most organisms cannot use N $_2$ directly because the triple bond (N $\equiv$ N) is extremely strong (946 kJ/mol). The nitrogen cycle converts this inert gas into biologically usable forms (NH $_3$ , NO $_3^-$ ) and back.

Without denitrification, all nitrogen would eventually accumulate as nitrates in the soil and water (causing eutrophication). Without fixation, the soil would run out of usable nitrogen. The cycle maintains a balance.

Alternative Method — Industrial Nitrogen Fixation

The Haber-Bosch process ( $N_2 + 3H_2 \to 2NH_3$ ) industrially fixes nitrogen for fertilisers. This has dramatically increased global food production but also disrupts the natural cycle — excess nitrates cause water pollution and algal blooms.

For NEET, remember the key organisms: Rhizobium (fixation in legumes), Nitrosomonas (NH $_3$ to NO $_2^-$ ), Nitrobacter (NO $_2^-$ to NO $_3^-$ ), Pseudomonas (denitrification). Knowing which bacterium does which step is a direct NEET question.

Common Mistake

Students confuse nitrification with nitrogen fixation. Fixation converts N $_2$ gas to NH $_3$ . Nitrification converts NH $_3$ to NO $_3^-$ . They are sequential but different processes done by completely different organisms. Also, nitrification is aerobic while denitrification is anaerobic.

Question

Solution — Step by Step

Why This Works

Alternative Method — Industrial Nitrogen Fixation

Common Mistake

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