Explain the role of decomposers and nitrifying bacteria in making nitrogen available for plant uptake.

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).

Nitrogen is an essential element for plant growth. Plants cannot use atmospheric nitrogen gas directly, so they depend on a series of processes in the nitrogen cycle to obtain nitrogen in a usable form.

Model answer (4 marks)

Decomposers break down dead plant and animal material, proteins and urea into ammonia (NH₄⁺). Nitrifying bacteria then oxidise this ammonium first to nitrite (NO₂⁻) and finally to nitrate (NO₃⁻). Nitrate ions are absorbed by plant roots and used to synthesise proteins. Thus decomposers provide the ammonia that nitrifying bacteria convert into nitrate, together returning nitrogen to a form usable by plants.

Examiner tips

Use the exact terms ‘decomposers’, ‘ammonia’, ‘nitrifying bacteria’, ‘nitrite’, ‘nitrate’, ‘plant uptake’.
Show the sequence: decomposers → ammonia → nitrifying bacteria → nitrite → nitrate → plant uptake.
Mention that both processes are linked – decomposers supply the substrate for nitrifying bacteria.

Common mistakes

Confusing ammonium with ammonia or using the wrong ion symbol (NH₃ vs NH₄⁺).
Omitting the step of nitrite or describing nitrification as a single step.
Failing to state that nitrate is the form absorbed by plants.

Mark scheme (4 marks)

Decomposers break down dead plant and animal material / proteins / urea into ammonia (ammonium ions)
Nitrifying bacteria convert ammonium ions into nitrite and then into nitrate ions
Nitrate ions are absorbed by plants (through roots) / used by plants to make proteins
Both processes are linked / decomposers produce the ammonia that nitrifying bacteria then act upon, so together they return nitrogen to a form usable by plants

Key terms in this question

decomposers · nitrifying bacteria