Question
Explain the nitrogen cycle. Describe the specific roles of nitrifying bacteria and denitrifying bacteria in this cycle.
Solution — Step by Step
Nitrogen (N₂) makes up 78% of the atmosphere, yet most organisms cannot use atmospheric nitrogen directly. Proteins, nucleic acids (DNA, RNA), and other biomolecules require nitrogen in a usable (“fixed”) form — typically as ammonium () or nitrate ().
The nitrogen cycle is the series of processes by which nitrogen moves from the atmosphere through soil, water, and organisms, and returns to the atmosphere. It maintains a balance between atmospheric N₂ and biologically available nitrogen.
Nitrogen fixation: The conversion of atmospheric N₂ into ammonia (NH₃) or ammonium ().
Who does it:
- Free-living bacteria: Azotobacter (aerobic), Clostridium (anaerobic)
- Symbiotic bacteria: Rhizobium lives in root nodules of leguminous plants (peas, beans, groundnuts), fixing N₂ for the plant in exchange for carbohydrates — a mutualistic relationship
- Cyanobacteria (blue-green algae): Nostoc, Anabaena — important in paddy fields
- Lightning: High-energy electrical discharges fix small amounts of N₂ as nitrate
Ammonia produced enters the soil and can be taken up by plants.
Nitrification: The conversion of ammonia/ammonium to nitrite, then to nitrate. This is done in two steps by nitrifying bacteria — they are chemoautotrophs (get energy from oxidising inorganic compounds).
Step 1 — Ammonia oxidation:
Step 2 — Nitrite oxidation:
Why this matters: Nitrate () is the form most easily absorbed by plant roots. Without nitrifying bacteria, ammonia would accumulate and could be toxic, and plants would be nitrogen-starved.
Assimilation: Plants absorb or through roots and incorporate nitrogen into proteins, nucleic acids, and chlorophyll. Animals get nitrogen by eating plants (or other animals).
Ammonification: When plants and animals die, or when animals excrete waste, decomposers (bacteria and fungi) break down organic nitrogen back into ammonium ().
This returns fixed nitrogen to the soil for reuse.
Denitrification: The conversion of nitrate back to N₂ (and N₂O) and its return to the atmosphere. Carried out by denitrifying bacteria — they are anaerobes that use nitrate as an electron acceptor when oxygen is unavailable.
Why this matters: Denitrification completes the cycle by returning nitrogen to the atmosphere. Without it, all atmospheric N₂ would eventually end up as fixed nitrogen in the soil and ocean, and the atmospheric reservoir would be depleted. Denitrification also occurs in waterlogged or anaerobic soils — which is why poorly drained soil is low in available nitrogen.
Key bacteria: Pseudomonas denitrificans, Thiobacillus denitrificans.
Why This Works
The nitrogen cycle is a balanced system:
- Fixation adds nitrogen to the biologically available pool
- Nitrification converts it to the most plant-usable form (nitrate)
- Assimilation incorporates it into living organisms
- Ammonification returns it to the soil when organisms die
- Denitrification returns it to the atmosphere
Each step requires specific bacteria, and the cycle would break down without any one of them. This is why human activities (excessive fertiliser use, which adds artificial fixed nitrogen, or wetland drainage, which disrupts denitrification) can unbalance the nitrogen cycle.
NEET and CBSE Class 12 regularly ask: “What is the role of Rhizobium/Nitrosomonas/Pseudomonas in the nitrogen cycle?” Be specific: Rhizobium = fixes N₂ (nitrogen fixation). Nitrosomonas = NH₄⁺ → NO₂⁻ (nitrification step 1). Nitrobacter = NO₂⁻ → NO₃⁻ (nitrification step 2). Pseudomonas = NO₃⁻ → N₂ (denitrification).
Alternative Method
You can trace the nitrogen cycle as a numbered pathway:
- N₂ (atmosphere) → NH₃ [Nitrogen fixation — Rhizobium]
- NH₄⁺ in soil → NO₂⁻ [Nitrification I — Nitrosomonas]
- NO₂⁻ → NO₃⁻ [Nitrification II — Nitrobacter]
- NO₃⁻ → plant protein [Assimilation]
- Plant protein → animal protein [Consumption]
- Dead organic matter → NH₄⁺ [Ammonification — decomposers]
- NO₃⁻ → N₂ [Denitrification — Pseudomonas]
This step-by-step chain is perfect for a board exam diagram answer.
Common Mistake
Students often confuse nitrification with nitrogen fixation:
- Nitrogen fixation = N₂ (atmosphere) → NH₃ (usable form). Done by Rhizobium, Azotobacter.
- Nitrification = NH₄⁺ → NO₂⁻ → NO₃⁻ (converting ammonia to nitrate). Done by Nitrosomonas and Nitrobacter.
These are completely different steps. Also, do not say “nitrifying bacteria fix nitrogen” — they do not fix N₂. They only convert existing fixed nitrogen to a more oxidised form.