4c Cycles within Ecosystems¶
Part of 4 Ecology and the Environment.
Matter is recycled in an ecosystem, not used once and lost. The same atoms of carbon and nitrogen pass between the air, soil and living things, and decomposers keep the cycles turning.
What You Need to Learn¶
Further detail: Pearson Edexcel International GCSE Biology specification.
How carbon is cycled between the atmosphere, living organisms and fossil fuels. Biology-only students also learn how nitrogen is cycled by different groups of microorganism.
The Carbon Cycle¶
Carbon moves between the atmosphere, living organisms and the Earth in a continuous cycle. Only one process takes carbon dioxide out of the air in quantity, and the others put it back.
| Process | What happens to the carbon | Effect on atmospheric CO₂ |
|---|---|---|
| Photosynthesis | Plants and algae take in carbon dioxide and build it into glucose, and then into proteins, fats and other molecules | Removes it |
| Feeding | Carbon in organic molecules passes along the food chain as consumers eat producers and each other | None |
| Respiration | Plants, animals and decomposers release carbon dioxide as they respire | Adds it |
| Decomposition | Bacteria and fungi break down dead bodies and waste, and release carbon dioxide through their own respiration | Adds it |
| Combustion | Burning wood or fossil fuels releases stored carbon as carbon dioxide | Adds it |
Fossil fuels are the compressed remains of ancient dead organisms. Their carbon was locked away for millions of years, and burning them returns it to the atmosphere much faster than it was stored.
Exam technique
A carbon cycle diagram question usually asks you to name a process from the arrow. Arrows from the atmosphere into plants are photosynthesis. Arrows from plants, animals and decomposers to the atmosphere are respiration. An arrow from dead material to the atmosphere is decomposition, which is respiration by decomposers. Remember that plants respire as well as photosynthesise.
Biology-Only Content¶
This content is required for Biology-only students and is not required for Combined Science students.
The section below, on the nitrogen cycle, is Biology-only.
The Nitrogen Cycle¶
Nitrogen makes up about 78% of the atmosphere, but as N₂ it is chemically unreactive, and most living things cannot use it. Plants need nitrogen compounds, mainly nitrates, to make amino acids and therefore proteins. Animals get their nitrogen by eating plants or other animals. Four groups of bacteria move nitrogen between the air and the soil:
| Group | What it does | Conditions |
|---|---|---|
| Nitrogen-fixing bacteria | Convert nitrogen gas into ammonium ions and nitrates that plants can absorb. Some live freely in the soil, and others live in the root nodules of legumes such as peas and clover | Lightning also fixes small amounts of nitrogen |
| Decomposers (bacteria and fungi) | Break down proteins and urea in dead organisms and waste, releasing ammonia (ammonium ions) into the soil | Aerobic respiration needs oxygen |
| Nitrifying bacteria | Convert ammonia into nitrites and then into nitrates, in a process called nitrification | Need oxygen |
| Denitrifying bacteria | Convert nitrates back into nitrogen gas, which returns to the atmosphere | Work without oxygen, in waterlogged soil |
Plants take up nitrates through their roots by active transport and use them to build amino acids and proteins. Nitrogen then passes along the food chain, and returns to the soil as waste and dead material.
Worked example: tracing a nitrogen atom
A nitrogen atom in the protein of a dead clover plant is released as ammonium by decomposers. Nitrifying bacteria convert it to nitrate. A wheat plant absorbs the nitrate by active transport and builds it into an amino acid. A mouse eats the wheat, and the atom becomes part of the mouse's protein. When the mouse dies, decomposers return it to the soil as ammonium.
If the soil is waterlogged, denitrifying bacteria can instead convert the nitrate to nitrogen gas and the atom is lost to the air. This is why farmers who drain waterlogged fields, or who grow legumes in rotation, keep the soil fertile.
Why Cycles Matter¶
Without these cycles, the atoms that organisms need would become locked away in forms that nothing could reuse. Human activities that disrupt them, such as deforestation and adding excess fertiliser, can have serious consequences for ecosystems (see 4d Human Influences on the Environment).
Common Confusions¶
- Plants respire too: Plants take carbon dioxide in by photosynthesis and also release it by respiration.
- Decomposers and carbon: Decomposers return carbon dioxide to the air by respiration, as they break material down.
- Nitrogen fixation vs nitrification: Fixation turns nitrogen gas into nitrogen compounds. Nitrification turns ammonia into nitrates.
- Why legumes help: Legumes have nitrogen-fixing bacteria in their root nodules, which is why they add nitrogen to the soil.
Check Yourself¶
- Name the process that removes carbon dioxide from the atmosphere, and name two that add it.
- Explain why burning fossil fuels increases the carbon dioxide level in the atmosphere.
- Explain the role of decomposers in the carbon cycle.
- (Biology only) Explain why plants cannot use nitrogen gas directly from the air.
- (Biology only) State what nitrifying bacteria do, and what they need.
- (Biology only) A farmer drains a waterlogged field and the crop grows better. Suggest why.
Answers
- Photosynthesis removes it. Respiration and combustion (or decomposition) add it.
- Fossil fuels are the remains of ancient organisms, and their carbon has been locked away. Burning them releases that stored carbon as carbon dioxide.
- Decomposers break down dead organisms and waste, and they release carbon dioxide into the air through their respiration, so the carbon becomes available again to producers.
- Nitrogen gas is chemically unreactive. Plants need nitrogen in compounds such as nitrates, and only nitrogen-fixing bacteria and lightning can convert nitrogen gas into those compounds.
- Nitrifying bacteria convert ammonia into nitrites and then nitrates, and they need oxygen.
- Waterlogged soil has little oxygen, so denitrifying bacteria thrive and convert nitrates to nitrogen gas, which is lost. Draining the soil allows nitrifying bacteria to work, so more nitrate is available for the crop.
Key Terms¶
- Carbon cycle: the cycling of carbon atoms through organisms and the environment.
- Nitrogen cycle: the cycling of nitrogen through the atmosphere, soil and organisms.
- Decomposition: the breakdown of dead organic material by bacteria and fungi.
- Combustion: the burning of organic material or fossil fuels, releasing carbon dioxide.
- Nitrogen fixation: the conversion of atmospheric nitrogen gas into nitrogen compounds usable by plants.
- Nitrogen-fixing bacteria: bacteria that convert atmospheric nitrogen into usable nitrogen compounds.
- Nitrifying bacteria: bacteria that convert ammonia into nitrates in the soil.
- Denitrifying bacteria: bacteria that convert nitrates back into nitrogen gas.
- Decomposer: an organism that breaks down dead organic matter.