How are the nutrients that living things need recycled through an ecosystem?
Nutrient cycling in ecosystems: the role of decomposers, and the cycling of carbon and nitrogen between organisms, the soil, water and the atmosphere.
An SQA National 5 Environmental Science answer on nutrient cycling, covering the role of decomposers, the carbon cycle through photosynthesis, respiration, decay and combustion, and the nitrogen cycle through fixation, nitrification, uptake and denitrification.
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What this dot point is asking
The SQA wants you to explain that the nutrients living things need are recycled (unlike energy, which flows one way and is lost), to describe the role of decomposers, and to outline how carbon and nitrogen move between organisms, the soil, water and the atmosphere.
Why nutrients are recycled but energy is not
Living things need elements such as carbon and nitrogen to build their bodies. The Earth has a fixed amount of these, so they must be recycled and used over and over. This is the key contrast with energy: energy flows through an ecosystem in one direction and is lost as heat at each trophic level, but matter (nutrients) is cycled round and reused.
Decomposers: the engine of recycling
Decomposers are essential to every nutrient cycle. When plants and animals die, and when animals produce waste, the carbon, nitrogen and other nutrients are locked inside that material. Decomposers break it down, returning carbon dioxide to the air and mineral nutrients to the soil, where plants can take them up again. Without decomposers, dead material would pile up and the cycles would stop.
The carbon cycle
Carbon moves between the atmosphere (as carbon dioxide), living organisms and fossil fuels. The processes are:
- Photosynthesis removes carbon dioxide from the atmosphere: green plants fix it into glucose using light energy. This is the main process that takes carbon out of the air.
- Respiration returns carbon dioxide to the atmosphere: plants, animals and decomposers release it as they release energy from food.
- Decay (decomposition) returns carbon dioxide as decomposers break down dead material.
- Combustion (burning) returns carbon dioxide when fossil fuels (coal, oil, gas) and wood are burned.
When plants and animals die and are buried for millions of years, their carbon can form fossil fuels, which is a long-term store. Burning these fuels releases that stored carbon quickly, which is why combustion of fossil fuels raises atmospheric carbon dioxide.
The nitrogen cycle
Plants and animals need nitrogen to make proteins. The air is about 78 per cent nitrogen gas, but most organisms cannot use nitrogen gas directly. The cycle converts it into usable forms and back again:
- Nitrogen fixation. Nitrogen-fixing bacteria (in the soil and in root nodules of plants such as peas, beans and clover) convert nitrogen gas into nitrates. Lightning also fixes some nitrogen.
- Uptake and use. Plants absorb nitrates from the soil through their roots and use them to build proteins. Animals get their nitrogen by eating plants (or by eating other animals).
- Decomposition. When organisms die or produce waste, decomposers break down the nitrogen compounds and release ammonia, which other soil bacteria convert into nitrates (nitrification), returning nitrogen to the soil.
- Denitrification. Denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the atmosphere and completing the cycle.
Examples in context
Example 1. Legumes and natural fertiliser. Farmers grow clover or beans in a field because the nitrogen-fixing bacteria in their root nodules add nitrates to the soil. This is a natural way to enrich the soil with nitrogen and reduce the need for manufactured fertiliser, linking the nitrogen cycle to sustainable farming.
Example 2. Fossil fuels as a slow part of the carbon cycle. Coal formed from plants that lived hundreds of millions of years ago. The carbon they fixed by photosynthesis stayed locked away until it was mined and burned. Combustion releases that ancient carbon as carbon dioxide far faster than photosynthesis can remove it, which is why burning fossil fuels raises atmospheric carbon dioxide.
Try this
Q1. Name the two processes that return carbon dioxide to the atmosphere from living organisms. [1 mark]
- Cue. Respiration and decay (decomposition). Combustion also returns carbon dioxide but is not from living organisms.
Q2. State why animals depend on plants and bacteria to obtain the nitrogen they need. [2 marks]
- Cue. Animals cannot use nitrogen gas directly; bacteria fix it into nitrates, plants build it into proteins, and animals get nitrogen by eating plants.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style4 marksDescribe how carbon is added to and removed from the atmosphere, naming the process in each case.Show worked answer →
A 4-mark describe answer needs processes that add carbon and processes that remove it, so plan two adding marks and two removing marks.
Carbon is removed from the atmosphere by photosynthesis, in which green plants take in carbon dioxide and fix it into glucose.
Carbon is added to the atmosphere by respiration, in which plants, animals and decomposers release carbon dioxide. It is also added by combustion (burning) of fossil fuels and wood, and by the decay of dead material by decomposers, which releases carbon dioxide.
Markers reward each correctly named process linked to adding or removing carbon dioxide. Naming a process without saying whether it adds or removes carbon may lose the mark.
SQA N5 style3 marksExplain the role of decomposers in the recycling of nutrients in an ecosystem.Show worked answer →
This asks for the function of decomposers, so each point must explain what they do and why it matters.
Decomposers, which are bacteria and fungi, break down dead organisms and waste material such as leaves, faeces and dead bodies.
As they break this material down, they release the nutrients it contains, such as nitrogen and carbon compounds, back into the soil and the atmosphere.
This makes the nutrients available again so that plants can take them up and reuse them, which keeps the nutrient cycle going. Without decomposers, nutrients would stay locked in dead material and the cycle would stop.
Markers reward the breakdown of dead material, the release of nutrients, and making them available for reuse by plants.
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