3.9.9Additional lessons
Nutrient Cycling — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 27 of 101
Part 1 of 5Why Nutrients Must Be Recycled
Finite supply
- Elements such as nitrogen and phosphorus are available in limited amounts, so the same atoms must be used again and again.
Producers
- Plants take up nitrate and phosphate ions through their roots and use them to make proteins, DNA and ATP.
Decomposers
- Saprotrophs break down dead matter and waste and release nutrients as inorganic ions into the soil or water.
Phosphorus cycle
- Phosphorus has no gas phase, so it moves slowly from rock by weathering, through food chains, and back to sediments.
Part 2 of 5The Nitrogen Cycle
1
Nitrogen-fixing bacteria such as Rhizobium, living in root nodules of legumes, convert atmospheric nitrogen to ammonia.
2
Decomposers release ammonia from the breakdown of proteins and urea in a process called ammonification.
3
Nitrifying bacteria, such as Nitrosomonas and Nitrobacter, oxidise ammonium to nitrite and then to nitrate in the soil.
4
Plants absorb nitrate through their roots and use it to build amino acids and other nitrogen compounds.
5
Denitrifying bacteria in waterlogged, oxygen-poor soil convert nitrate back to nitrogen gas, which returns to the atmosphere.
Part 3 of 5Studying Cycles with Mesocosms
Studying Cycles with Mesocosms
Mesocosm
- A mesocosm is a small, enclosed and controlled model of an ecosystem, such as a sealed bottle garden or an aquarium.
Advantages
- Conditions can be controlled and varied one at a time, and the experiment can be repeated safely.
Light as input
- A sealed mesocosm can persist if it receives light, because nutrients are recycled inside it.
Limitations
- A small model cannot copy every feature of a natural ecosystem, so results should be applied with caution.
Part 4 of 5Human Effects on Nutrient Cycles
Fertiliser
- Adding nitrate and phosphate to crops raises yields but can run off into streams and lakes.
Eutrophication
- Excess nutrients cause rapid algal growth, and when the algae die, bacterial decomposition uses up oxygen.
Harvesting
- Removing crops takes nutrients away from the field, so they must be replaced by fertiliser or crop rotation.
Legume rotation
- Growing nitrogen-fixing legumes returns nitrogen compounds to the soil without synthetic fertiliser.
| Process | Organisms | Change |
|---|---|---|
| Nitrogen fixation | Rhizobium, Azotobacter | N₂ to ammonia |
| Nitrification | Nitrosomonas, Nitrobacter | Ammonium to nitrite to nitrate |
| Denitrification | Pseudomonas | Nitrate to N₂ |
| Ammonification | Saprotrophs | Organic N to ammonia |
Part 5 of 5Exam Checklist & Tips
3.9.9 – Key Points for the Exam
- Nitrogen fixation Conversion of nitrogen gas into ammonia by bacteria.
- Nitrification Oxidation of ammonium to nitrite and nitrate.
- Denitrification Conversion of nitrate to nitrogen gas by bacteria.
- Mesocosm A small, controlled model of an ecosystem.
- Eutrophication Nutrient enrichment that causes excessive algal growth.
Common Mistakes
- Saying plants take up nitrogen gas directly from the air.
- Confusing nitrification with nitrogen fixation.
- Forgetting that denitrification takes place under anaerobic conditions.
- Assuming nutrients are replaced from the Sun, which supplies only energy.
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