Level: JHS, Basic 9 | Subject: Integrated Science | Strand: Cycles | Topic: The Nitrogen Cycle (Earth Science)
Curriculum reference: B9.2.1.1 Understand environmental cycles and geological changes. Indicator B9.2.1.1.1: Examine the nitrogen cycle and its importance to living organisms. All the exemplars are merged into one set of notes.
1. Why Nitrogen Matters
About 78% of the air is nitrogen gas (N2). But plants and animals cannot use nitrogen gas directly. Living things need nitrogen to make proteins, DNA and chlorophyll. The nitrogen cycle shows how nitrogen moves between the air, the soil and living things.
2. The Main Processes
| Process | What happens |
|---|---|
| Nitrogen fixation | Nitrogen gas is changed into compounds plants can use (ammonia, nitrates) |
| Nitrification | Bacteria change ammonia into nitrites and then nitrates |
| Assimilation | Plants take in nitrates through the roots and build proteins. Animals get nitrogen by eating plants or other animals. |
| Ammonification (decay) | Decomposers break down dead plants, animals and waste, releasing ammonia |
| Denitrification | Denitrifying bacteria change nitrates back into nitrogen gas, which returns to the air |
3. Nitrogen Fixation in Detail
- Lightning: the heat and energy make nitrogen and oxygen in the air combine to form nitrogen oxides. Rain washes these into the soil as nitrates.
- Nitrogen-fixing bacteria: Rhizobium lives in swellings (root nodules) on legumes such as beans, groundnut and cowpea. It changes nitrogen gas to ammonia. The plant and the bacteria both benefit, so this is mutualism. Some bacteria, like Azotobacter, live freely in the soil.
- Industry: factories make fertiliser from nitrogen gas.
4. Role in Soil Fertility
- Lightning and bacteria add nitrates to the soil.
- Legumes enrich the soil, so farmers rotate cereals like maize with beans or groundnut.
- Manure, compost and decaying plants return nitrogen.
- Waterlogged soil encourages denitrification, so the soil loses nitrogen. Good drainage helps.
5. How Nitrogen Moves (Summary Path)
Air nitrogen (N2) to ammonia (fixation) to nitrites and nitrates (nitrification) to plants (assimilation) to animals (feeding) to dead matter and waste (death) to ammonia (ammonification) and back to the air (denitrification).
6. Human Effects
- Too much fertiliser washes into rivers and lakes and causes algae to grow too fast (eutrophication), which harms fish.
- Bush burning and cutting trees release nitrogen compounds and reduce organic matter.
- Good practices are crop rotation, planting legumes, using compost and applying fertiliser in the right amount.
Remember: Plants take nitrates, bacteria do most of the changing, and denitrification returns nitrogen to the air.
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Education Always (education.inspiringalways.com). Original notes written to follow the NaCCA Basic 9 Integrated Science curriculum.