The nitrogen cycle at HL: This introduction to the nitrogen cycle is the same for SL and HL. As an HL student you go much further with it — there are extra HL-only pages on the detailed nitrogen cycle and on how human activity, like fertiliser use, disrupts it.
Get the basic nitrogen cycle solid here; it's the foundation for the HL-only nitrogen pages that follow.
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♻️ The Nitrogen Cycle
What's the big deal?: Nitrogen is like a VIP nutrient — plants need it to grow, but they can't just grab it from the air. The nitrogen cycle is nature's way of recycling this precious resource between air, soil, and living things.
🤔 Why Should You Care About Nitrogen?
Every protein in your body, every strand of your DNA — they all contain nitrogen. Without it, nothing grows.
The nitrogen paradox 🎭: The air is 78% nitrogen — we're swimming in it! But here's the catch: plants and animals can't use nitrogen gas (N₂). It's like being surrounded by locked treasure chests with no key.
📦 Where Is Nitrogen Stored?
- 🌫️ Atmosphere – the biggest store (nitrogen gas, N₂)
- 🌱 Soil – nitrates (NO₃⁻) and ammonium (NH₄⁺) that plants can absorb
- 🦌 Living things – locked up in proteins and DNA
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🔄 The 5 Steps of the Nitrogen Cycle
Think of it as a relay race where bacteria pass the nitrogen baton through different forms:
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1️⃣ Nitrogen Fixation — "Unlocking the treasure"
This is where N₂ gas gets converted into forms plants can actually use.
- ⚡ Lightning — zaps N₂ into nitrates (nature's spark!)
- 🦠 Nitrogen-fixing bacteria — live in soil OR in bumps (nodules) on legume roots like beans and peas
- 🏭 Factories — make fertilisers (this is how humans do it)
Real example 🌱: Farmers rotate crops with legumes (peas, beans, clover) because their root bacteria add free nitrogen to the soil!
2️⃣ Nitrification — "Upgrading the nitrogen"
Bacteria in soil convert ammonium → nitrites → nitrates (the form plants love most).
- Done by nitrifying bacteria
- NH₄⁺ → NO₂⁻ → NO₃⁻
- Nitrates dissolve in water so roots can slurp them up
3️⃣ Assimilation — "Eating the nitrogen"
Plants absorb nitrates through their roots and use them to build proteins and DNA.
- 🌿 Plants absorb nitrates from soil
- 🐰 Animals get nitrogen by eating plants (or eating other animals)
- This is how nitrogen enters food chains
4️⃣ Decomposition — "Recycling dead stuff"
When things die or poop, decomposers break down the nitrogen-containing molecules.
- 🍄 Bacteria and fungi are the recyclers
- Dead plants, animals, and waste → ammonium (NH₄⁺)
- Also called ammonification
5️⃣ Denitrification — "Releasing it back to the sky"
Some bacteria convert nitrates back into nitrogen gas — completing the cycle!
- Happens in waterlogged, low-oxygen soils
- NO₃⁻ → N₂ (back to the atmosphere!)
- Done by denitrifying bacteria
Memory trick 🧠: "Fix → Nitrify → Eat → Rot → Release" — that's the nitrogen cycle in 5 words!
💥 When Humans Add Too Much Nitrogen
Farmers add fertilisers to help crops grow, but too much nitrogen causes big problems:
- 1️⃣ Extra nitrates wash off fields into rivers and lakes
- 2️⃣ Algae go crazy and multiply (algal bloom) 🟢
- 3️⃣ Algae block sunlight → underwater plants die
- 4️⃣ Bacteria decompose all the dead stuff
- 5️⃣ Decomposition uses up oxygen → fish suffocate 🐟💀
Eutrophication: When excess nutrients (like nitrates) cause algae to explode, killing aquatic life. This is a super common exam topic!
🏭 Nitrogen Pollution from Burning Fuels
Burning fossil fuels releases nitrogen oxides (NOₓ) which cause:
- ☔ Acid rain — damages forests and lakes
- 🌡️ Nitrous oxide (N₂O) — a greenhouse gas 300× stronger than CO₂!
- 🌫️ Smog — harms health and reduces plant growth
✅ Quick Revision Checklist
- Biggest nitrogen store = atmosphere (but plants can't use N₂!)
- Bacteria run the whole show — they do fixing, nitrifying, decomposing, and denitrifying
- Nitrogen is often the limiting factor for plant growth
- Too much nitrogen → eutrophication → dead fish
- Nitrogen oxides from burning fuel → acid rain, smog, climate change
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📝 How this appears in exams
You'll rarely need to draw the full cycle. Instead, expect questions about: eutrophication, human impacts on nutrient cycling, or why nitrogen limits productivity. Focus on understanding the CONSEQUENCES, not memorising every arrow!
🌊 Eutrophication (common exam topic!)
Eutrophication: The enrichment of water bodies with nutrients (especially nitrogen and phosphorus), leading to excessive plant and algae growth.
The eutrophication sequence
- 1️⃣ Excess fertiliser runs off into lakes/rivers
- 2️⃣ Algae boom (algal bloom) due to abundant nutrients
- 3️⃣ Algae block sunlight → aquatic plants die
- 4️⃣ Algae die and sink to the bottom
- 5️⃣ Bacteria decompose dead matter → use up oxygen
- 6️⃣ Oxygen levels crash (hypoxia) → fish and other organisms die
- 7️⃣ 'Dead zones' form
🚜 Human impacts on the nitrogen cycle
- Fertiliser use — adds reactive nitrogen to ecosystems faster than natural processes
- Burning fossil fuels — releases NOₓ gases → acid rain, smog
- Sewage discharge — adds nitrogen directly to waterways
- Deforestation — removes plants that absorb nitrogen, increases runoff
- Intensive farming — concentrates animal waste (high in nitrogen)
Humans have more than doubled the rate of nitrogen entering ecosystems compared to pre-industrial times.
🌱 Why nitrogen limits productivity
Even though nitrogen is 78% of the atmosphere, most organisms can't use N₂ gas directly. This makes biologically available nitrogen the limiting factor for plant growth in many ecosystems.
- Plants need nitrogen for proteins, DNA, and chlorophyll
- Without enough nitrogen → stunted growth, yellow leaves
- Adding nitrogen fertiliser dramatically increases crop yields
- In natural ecosystems, nitrogen-fixing bacteria are the main source
Limiting factor = the resource in shortest supply that restricts growth. In most terrestrial ecosystems, this is nitrogen!
Exam Tips:
- Know the eutrophication sequence — it's a favourite exam question
- Be able to explain two human impacts on the nitrogen cycle with examples
- Understand why nitrogen is a limiting factor despite being abundant in air
- Link nitrogen pollution to acid rain (NOₓ) and climate change (N₂O is a greenhouse gas)
- Don't memorise every arrow — focus on processes and consequences
IB-style question — Nitrogen from atmosphere to protein in a decomposer
Outline the sequence of processes by which atmospheric nitrogen gas (N₂) becomes part of the protein in a decomposer organism. [4]
How to answer it, step by step
- Fixation → nitrification
• Nitrogen-fixing bacteria (e.g. Rhizobium) convert N₂ → ammonium (NH₄⁺).
• Nitrifying bacteria convert NH₄⁺ → nitrites → nitrates (NO₃⁻) in the soil. - Plant uptake → transfer to decomposer
• Plant roots absorb nitrates → incorporated into amino acids and proteins in the plant.
• Plant dies → decomposers consume dead organic matter and absorb the nitrogen into their own proteins.
Final answer
Name the bacteria types (nitrogen-fixing, nitrifying) and key compounds (N₂ → NH₄⁺ → NO₃⁻ → amino acids/protein). Vague answers like 'bacteria turn nitrogen into something plants can use' score 1 mark at most.
IB-style question — How nitrogen oxides (NOₓ) affect ecosystems
Outline two ways in which nitrogen oxide compounds (NOₓ) released by combustion can negatively affect natural ecosystems. [2]
How to answer it, step by step
- Acid rain
• NOₓ + water vapour → nitric acid (HNO₃) → acid rain → lowers soil/water pH → damages plant enzymes, kills acid-sensitive aquatic organisms. - Tropospheric ozone / photochemical smog
• NOₓ + sunlight + hydrocarbons → ground-level ozone → toxic to plants → inhibits photosynthesis, reduces primary productivity.
Final answer
One mark per pathway — state the pollutant formed (nitric acid / tropospheric ozone) AND its ecological effect. 'It causes pollution' with no mechanism scores 0.