Describe the role of nitrifying bacteria and denitrifying bacteria in the nitrogen cycle.

Pearson Edexcel International GCSE Biology (4BI1) — 4.3 Cycles within ecosystems · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Nitrifying bacteria in the soil convert ammonia (NH₃) into nitrates (NO₃⁻), making nitrogen available for plant uptake.
Denitrifying bacteria in the soil convert these nitrates back into nitrogen gas (N₂), which is released into the atmosphere.
Thus nitrifying bacteria supply usable nitrogen to plants, while denitrifying bacteria return nitrogen to the air, completing the cycle.

Examiner tips

Use the exact terms 'nitrifying bacteria', 'denitrifying bacteria', 'ammonia', 'nitrates', 'nitrogen gas', and 'soil' as the scheme specifies.
Show the direction of conversion for each group to match the scheme points.

Common mistakes

Confusing the roles of nitrifying and denitrifying bacteria; e.g., saying nitrifying bacteria produce nitrogen gas.
Omitting the location (soil) or the specific forms of nitrogen (ammonia, nitrates, nitrogen gas).

Mark scheme (4 marks)

Nitrifying bacteria convert ammonia to nitrates (in the soil)
Nitrifying bacteria are found in the soil
Denitrifying bacteria convert nitrates (in the soil) into nitrogen gas
Denitrifying bacteria return nitrogen gas to the atmosphere

Key terms in this question

nitrifying bacteria · denitrifying bacteria · nitrogen cycle