How are carbon and nitrogen recycled through ecosystems, and what role do microorganisms play?
The carbon cycle and the nitrogen cycle, the role of decomposers and named bacteria, and how human activity affects these cycles and biodiversity.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on nutrient cycles, covering the carbon cycle, the nitrogen cycle and the bacteria involved, the role of decomposers, and how human activity affects the cycles and biodiversity.
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What this dot point is asking
WJEC Double Award Unit 1 wants you to describe the carbon and nitrogen cycles, explain the role of decomposers and named bacteria, and describe how human activity affects the cycles and biodiversity.
The carbon cycle
The carbon cycle keeps the amount of carbon dioxide in the air roughly constant, as long as the processes that remove it balance those that return it.
The nitrogen cycle
Plants need nitrogen to make proteins, but they cannot use nitrogen gas from the air directly; they absorb nitrates from the soil. Several types of bacteria recycle nitrogen:
- Nitrogen-fixing bacteria (in the soil and in root nodules of legumes) convert nitrogen gas into nitrogen compounds.
- Decomposers break down dead organisms and waste (such as urea), releasing ammonia.
- Nitrifying bacteria convert ammonia into nitrates, which plants can absorb.
- Denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the air.
Lightning also fixes some nitrogen by combining it with oxygen.
The role of decomposers
Without decomposers, nutrients would stay locked in dead material and the cycles would stop. Decomposers work fastest in warm, moist conditions with oxygen, which is why compost heaps are kept warm and turned to add air.
How fossil fuels store carbon
Some carbon is locked away for a very long time. When ancient plants and animals died and were buried without oxygen, decomposers could not break them down, so over millions of years they formed fossil fuels (coal, oil and natural gas), which store the carbon. When we burn these fuels, the stored carbon is suddenly released as carbon dioxide much faster than photosynthesis can remove it, which is why fossil fuel use raises the level of carbon dioxide in the atmosphere. This connects the carbon cycle to climate change.
Human impact and biodiversity
Human activity changes these cycles. Burning fossil fuels and deforestation increase the carbon dioxide in the atmosphere, contributing to climate change, and reduce the number of trees removing it. Habitat destruction, pollution and farming can lower biodiversity (the variety of living things). Maintaining biodiversity keeps ecosystems stable, so conservation measures such as protecting habitats and replanting forests are important.
Try this
Q1. Name the process that removes carbon dioxide from the atmosphere. [1 mark]
- Cue. Photosynthesis.
Q2. State the form of nitrogen that plants absorb from the soil. [1 mark]
- Cue. Nitrates.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe how carbon dioxide is removed from the atmosphere and how it is returned, naming the processes involved.Show worked answer →
A Unit 1 describe question worth 4 marks. Reward: carbon dioxide is removed from the air by photosynthesis in green plants (1); it is returned by respiration in plants, animals and microorganisms (1); by the combustion (burning) of fuels and wood (1); and by decomposition of dead organisms by microorganisms (1). Markers credit photosynthesis for removal and any of respiration, combustion or decomposition for return. A common error is to give only one return process.
WJEC style3 marksExplain the role of decomposers in recycling nutrients in an ecosystem.Show worked answer →
A Unit 1 explain question. Reward: decomposers (bacteria and fungi) break down dead organisms and waste (1); this releases nutrients such as carbon (as carbon dioxide) and nitrogen compounds back into the soil and air (1); these nutrients can then be reused by plants (for example nitrates absorbed by roots), so materials are recycled (1). Markers credit breakdown, release of nutrients and reuse by plants. A common error is to say decomposers make food for plants directly.
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