Nitrogen Cycle Diagram: Bacteria and Nutrient Flow
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key diagram
The nitrogen cycle shows how nitrogen moves between the atmosphere, soil and living organisms. Key processes are nitrogen fixation (N₂ to ammonia), nitrification (ammonia to nitrates), denitrification (nitrates to N₂) and decomposition. Bacteria drive most steps.
The nitrogen cycle is a challenging Extended tier diagram that requires students to name four types of bacteria and the processes they carry out. Every arrow must be labelled with both the process name and the type of bacteria involved.
Drawing the cycle
Draw a central area for the soil. Above it, draw nitrogen gas (N₂) in the atmosphere. In the soil, show ammonium compounds / ammonia and nitrates. Draw plants absorbing nitrates from the soil, and animals obtaining nitrogen by eating plants. Show dead organisms and excretory products returning to the soil.
Nitrogen fixation
Nitrogen-fixing bacteria convert atmospheric nitrogen gas (N₂) into ammonia / ammonium compounds in the soil. Some live free in the soil; others live in root nodules of leguminous plants (e.g. Peas, beans, clover). Lightning can also fix small amounts of nitrogen. Draw an arrow from "N₂ in atmosphere" to "ammonia in soil" labelled nitrogen fixation by nitrogen-fixing bacteria.
Nitrification and absorption
Nitrifying bacteria convert ammonia (NH₃) / ammonium compounds into nitrites, then into nitrates (NO₃⁻). This is nitrification: it requires oxygen (aerobic process). Plants absorb nitrates through their roots by active transport and use them to make amino acids and proteins. Draw arrows: ammonia → nitrates (labelled "nitrification by nitrifying bacteria") and nitrates → plants (labelled "absorption").
Decomposition and denitrification
Decomposers (saprophytic bacteria and fungi) break down dead organisms and animal waste, releasing ammonia back into the soil. This is decomposition (ammonification). Denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the atmosphere. This is denitrification: it occurs in anaerobic (waterlogged) conditions. Draw an arrow from nitrates to "N₂ in atmosphere" labelled "denitrification by denitrifying bacteria".
Label checklist
Check these labels are present and correctly positioned before finishing your drawing.
✓Nitrogen gas (N₂) in atmosphere
✓Ammonia / ammonium compounds in soil
✓Nitrates in soil
✓Plants
✓Animals
✓Dead organic matter
✓Nitrogen-fixing bacteria
✓Nitrifying bacteria
✓Denitrifying bacteria
✓Decomposers
✓Root nodules (on legumes)
Try a diagram question (3 marks)
Describe the role of nitrifying bacteria in the nitrogen cycle. [3]
Show the answer and marking guidance
Nitrifying bacteria convert ammonia / ammonium compounds into nitrites and then into nitrates.
This process is called nitrification.
Nitrates are the form of nitrogen that plants can absorb through their roots.
Original Cambridge-style practice written for this site.
Common diagram mistakes
Check these against your own drawing before moving on.
Confusing nitrogen-fixing bacteria with nitrifying bacteria.
Forgetting that denitrification occurs in anaerobic conditions.
Not mentioning that plants absorb nitrates (not ammonia) for making proteins.
Omitting decomposers, which convert dead matter back to ammonia.
Drawing the cycle without naming the types of bacteria at each step.