Describe the role of microorganisms in the nitrogen cycle.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Nitrogen makes up around 78% of the atmosphere, yet most organisms cannot use it directly. Microorganisms play key roles in making nitrogen available to living things and returning it to the atmosphere.
Model answer (4 marks)
Microorganisms are essential for the nitrogen cycle. Nitrogen‑fixing bacteria, such as Rhizobium in root nodules, convert atmospheric N₂ into ammonia or nitrate, which plants can absorb. In the soil, nitrifying bacteria (Nitrosomonas and Nitrobacter) oxidise the ammonia produced by fixation or decomposition into nitrite and then nitrate, the main form taken up by plants. Decomposers – bacteria and fungi – break down dead organic matter and urea, releasing ammonia back into the soil. Finally, denitrifying bacteria convert nitrate back into N₂ gas, returning nitrogen to the atmosphere.
Examiner tips
- Use the exact terms: nitrogen‑fixing, nitrifying, decomposers, denitrifying.
- Show the sequence: fixation → nitrification → plant uptake → decomposition → denitrification.
- Mention the key compounds (ammonia, nitrate).
- Keep the answer concise – 4–5 sentences is enough for full marks.
Common mistakes
- Confusing the roles of nitrifying and denitrifying bacteria.
- Forgetting to mention that decomposers release ammonia.
- Using vague language such as "bacteria help nitrogen" without specifying the processes.
Mark scheme (4 marks)
- Nitrogen-fixing bacteria convert nitrogen gas into ammonia / nitrates (compounds usable by plants)
- Nitrifying bacteria convert ammonia into nitrates in the soil
- Decomposers / bacteria / fungi break down dead organisms / protein / urea, releasing ammonia
- Denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the atmosphere
Key terms in this question
Related
- All Eduqas GCSE Biology revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →