Explain the role of nitrogen-fixing bacteria and denitrifying bacteria in the nitrogen cycle.

Edexcel GCSE Biology (1BI0) — 9.3 Material cycles · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Farmers growing legume crops such as peas notice that their soils remain fertile without the addition of nitrogen-containing fertilisers. However, in waterlogged fields, the concentration of nitrate ions in the soil can decrease significantly over time.

Model answer (4 marks)

Nitrogen‑fixing bacteria convert atmospheric N₂ into ammonia, providing plants with usable nitrogen.
They live in the soil or in root nodules of legumes, so legume crops can grow without added fertiliser.
Denitrifying bacteria convert nitrate ions (NO₃⁻) back into N₂ gas.
These bacteria are anaerobic and occur in waterlogged soils, which explains the drop in nitrate concentration in such conditions.

Examiner tips

Use the exact terms "nitrogen‑fixing bacteria", "denitrifying bacteria", "ammonia", "nitrate ions", "anaerobic".
Show the cause–effect link: fixing → ammonia for plants; denitrifying → loss of nitrate in waterlogged soils.
Keep the answer concise, matching the four points to the four marks.

Common mistakes

Confusing nitrogen‑fixing with nitrogen‑fixing bacteria; using "nitrogen fixation" instead of the bacteria’s role.
Forgetting that denitrifying bacteria are anaerobic and found in waterlogged soils.
Mixing up the direction of the reactions (e.g., saying denitrifying bacteria produce nitrate instead of consuming it).

Mark scheme (4 marks)

Nitrogen-fixing bacteria convert atmospheric nitrogen gas into ammonia (nitrogen-containing compounds)
Nitrogen-fixing bacteria are found in the soil or in root nodules of legumes, providing plants with usable nitrogen compounds
Denitrifying bacteria convert nitrate ions into nitrogen gas
Denitrifying bacteria are anaerobic and are found in waterlogged soils, which explains why nitrate levels fall in waterlogged conditions

Key terms in this question

nitrogen-fixing bacteria · denitrifying bacteria · nitrogen cycle