Exam Focus

🎯 Exam Focus

Frequently Examined

The nitrogen cycle appears regularly on AQA Paper 2 (Unit 4: Ecology). It is frequently combined with questions about crop rotation, soil fertility, and comparisons with the carbon cycle. Key AQA question patterns:

• Role of bacteria (2-3 marks): "Describe the role of bacteria in the nitrogen cycle" — name each type and what they convert. Do not just say "bacteria break things down" — be specific about which bacteria and what transformation they perform.
• Explain why legumes are used in crop rotation (3-4 marks): You must link root nodules → Rhizobium → nitrogen fixation → ammonia → nitrification → nitrates → improved plant growth for the following crop.
• Explain what happens to nitrogen when soil becomes waterlogged (3 marks): Anaerobic conditions → denitrifying bacteria activate → nitrates converted to N₂ → nitrogen lost from soil → reduced plant growth.
• Compare nitrogen and carbon cycles (4-6 marks): Focus on the differences: carbon enters via photosynthesis directly; nitrogen requires bacterial conversion. Bacteria are central to every stage of the nitrogen cycle; in the carbon cycle, bacteria only appear in decomposition.
• 6-mark extended response: Often asks you to describe the complete nitrogen cycle or explain why farmers add fertilisers. Examiners look for: all four bacterial types named, the correct transformations, correct conditions (aerobic/anaerobic), and a logical sequence.

Common mark losses: Saying plants absorb nitrogen gas. Confusing nitrifying and denitrifying bacteria. Not stating that denitrifying bacteria need anaerobic conditions. Forgetting that active transport is needed to absorb nitrates. Writing that the nitrogen cycle and carbon cycle work by the same processes.

Edexcel 1BI0 Paper 2 (T9 — Ecosystems and Material Cycles): Edexcel nitrogen cycle questions may present data comparing soil nitrate concentrations in fields with and without legume crop rotation, or data on the effects of industrial fertiliser use on river nitrate levels (eutrophication). "Evaluate" questions ask you to compare natural nitrogen fixation (Rhizobium in root nodules) with the Haber process for fertiliser production, assessing efficiency, cost, and environmental impact. "Suggest" questions may ask you to explain why nitrate levels drop after heavy rainfall — linking to leaching and denitrification under anaerobic waterlogged conditions. Edexcel mark schemes use "Accept…" and "Allow…" for equivalent bacterial transformation descriptions.