Nitrogen cycle in nature — fixation, nitrification, denitrification

Question

Explain the nitrogen cycle in nature, covering nitrogen fixation, nitrification, ammonification, and denitrification with the organisms involved.

Solution — Step by Step

flowchart TD
    A[Atmospheric N2 78%] -->|Nitrogen Fixation| B[NH3/NH4+]
    B -->|Nitrification| C[NO2- Nitrite]
    C -->|Nitrification| D[NO3- Nitrate]
    D -->|Plant Uptake| E[Plant Proteins]
    E -->|Animal Consumption| F[Animal Proteins]
    F -->|Death/Excretion| G[Organic N in Soil]
    G -->|Ammonification| B
    D -->|Denitrification| A
    A -->|Lightning| B

Atmospheric N $_2$ is converted to ammonia (NH $_3$ ) or ammonium (NH $_4^+$ ). This happens through: (a) Biological fixation by bacteria like Rhizobium (symbiotic, in legume root nodules), Azotobacter and Clostridium (free-living), and cyanobacteria like Anabaena and Nostoc. The enzyme nitrogenase catalyses this reaction. (b) Atmospheric fixation by lightning, which provides energy to combine N $_2$ with O $_2$ .

Ammonia is oxidised to nitrate in two steps by chemoautotrophic bacteria: (a) NH $_4^+$ to NO $_2^-$ (nitrite) by Nitrosomonas. (b) NO $_2^-$ to NO $_3^-$ (nitrate) by Nitrobacter. Nitrate is the preferred form of nitrogen absorbed by most plants.

Plants absorb NO $_3^-$ (or NH $_4^+$ ) through roots and incorporate nitrogen into amino acids, proteins, nucleic acids, and other organic molecules. Animals obtain nitrogen by eating plants or other animals.

When organisms die or excrete waste, decomposer bacteria and fungi break down organic nitrogen compounds back to NH $_3$ /NH $_4^+$ . This is ammonification. The ammonia can then re-enter the nitrification pathway.

Under anaerobic conditions, denitrifying bacteria (like Pseudomonas and Thiobacillus) convert nitrate (NO $_3^-$ ) back to gaseous N $_2$ , returning nitrogen to the atmosphere. This completes the cycle.

Why This Works

Nitrogen is essential for amino acids, proteins, and nucleic acids — the molecules of life. Although 78% of the atmosphere is N $_2$ , most organisms cannot use it directly because the triple bond in N $_2$ is extremely strong. The nitrogen cycle solves this by converting N $_2$ to usable forms (fixation) and recycling nitrogen through ecosystems.

Common Mistake

Students confuse nitrification and nitrogen fixation. Nitrogen fixation converts N $_2$ to NH $_3$ (done by Rhizobium, Azotobacter). Nitrification converts NH $_3$ to NO $_3^-$ (done by Nitrosomonas and Nitrobacter). These are two different processes done by different organisms.

Remember the bacteria by their names: Nitrosomonas handles the first step (NH $_4^+$ to NO $_2^-$ ) — “nitroso” relates to nitrite. Nitrobacter handles the second step (NO $_2^-$ to NO $_3^-$ ) — “nitro” relates to nitrate.

Question

Solution — Step by Step

Why This Works

Common Mistake

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