B41: Nutrient Cycles
The carbon cycle and the water cycle
Key Definitions
Carbon cycle: The continuous process by which carbon is exchanged between the atmosphere, oceans, living organisms and the Earth's crust through processes such as photosynthesis, respiration, combustion and decay.
Water cycle: The continuous movement of water between the Earth's surface and the atmosphere through evaporation, condensation, precipitation, transpiration, runoff and infiltration.
Decomposition: The breakdown of dead organisms and waste material by bacteria and fungi (decomposers), returning nutrients to the soil and carbon dioxide to the atmosphere.
Composting: The controlled decomposition of organic waste (food scraps, garden waste) to produce a nutrient-rich material (compost) used as a natural fertiliser.
Biogas generator: A system that uses anaerobic decomposition of organic waste by microorganisms to produce biogas (mainly methane), which can be used as a fuel for heating and cooking.
The Carbon Cycle
Carbon is constantly recycled through the environment. The main processes are:
| Process | Effect on Atmospheric CO₂ | Detail |
|---|---|---|
| Photosynthesis | Removes CO₂ | Plants and algae absorb CO₂ from the air and use light energy to convert it into glucose and oxygen |
| Respiration | Adds CO₂ | All living organisms respire, breaking down glucose to release energy and producing CO₂ as a waste product |
| Combustion | Adds CO₂ | Burning fossil fuels (coal, oil, natural gas) and wood releases CO₂ that was locked up in these carbon stores |
| Decay (decomposition) | Adds CO₂ | Decomposers (bacteria and fungi) break down dead organisms and waste, releasing CO₂ through their own respiration |
| Fossilisation | Removes CO₂ (long-term) | Over millions of years, dead organisms that are not fully decomposed form fossil fuels, trapping carbon underground |
The carbon cycle is balanced in nature: The amount of CO₂ removed by photosynthesis roughly equals the amount added by respiration and decay. Human activities – especially burning fossil fuels and deforestation – are adding more CO₂ to the atmosphere than is being removed, causing atmospheric CO₂ levels to rise.
Example 1: Carbon Cycle in a Forest
Trees absorb CO₂ through photosynthesis and store carbon in their wood, leaves and roots. When leaves fall and decompose, decomposers release CO₂ back into the atmosphere through respiration. If the tree dies and is buried under sediment before fully decomposing, its carbon may eventually form coal over millions of years. When humans burn this coal, the ancient carbon re-enters the atmosphere as CO₂.
Example 2: Ocean and the Carbon Cycle
The oceans absorb large amounts of CO₂ from the atmosphere (CO₂ dissolves in seawater). Marine organisms such as phytoplankton photosynthesise underwater, removing dissolved CO₂. When marine organisms die, their shells and skeletons sink to the ocean floor, eventually forming limestone – a long-term carbon store. This is why the oceans are sometimes called a "carbon sink".
The Water Cycle
Water is constantly recycled between the Earth's surface and the atmosphere:
| Process | Description |
|---|---|
| Evaporation | Heat energy from the Sun causes water in oceans, lakes and rivers to turn into water vapour and rise into the atmosphere |
| Transpiration | Water absorbed by plant roots passes through the plant and evaporates from the leaves via stomata, adding water vapour to the atmosphere |
| Condensation | As water vapour rises and cools, it turns back into liquid water droplets, forming clouds |
| Precipitation | When water droplets in clouds become heavy enough, they fall as rain, snow, sleet or hail |
| Runoff | Water flows over the ground surface into rivers, lakes and eventually back to the sea |
| Infiltration | Water soaks into the ground and is stored in soil and porous rock (groundwater/aquifers) |
Transpiration vs Evaporation: Evaporation is water turning to vapour from any surface (lakes, rivers, puddles). Transpiration is specifically water loss from plant leaves through stomata. Both add water vapour to the atmosphere but transpiration is a biological process.
Example 3: Water Cycle in a Rainforest
In the Amazon rainforest, transpiration from trees is so significant that it creates a local water cycle. Water evaporates from the ocean, falls as rain over the forest, is absorbed by tree roots, and is transpired back into the atmosphere. About 50–80% of rainforest moisture comes from transpiration, not from the ocean directly. Deforestation disrupts this local cycle, reducing rainfall in the region.
Decomposition
Decomposers (bacteria and fungi) break down dead organic matter and waste. They are essential for nutrient recycling because they release locked-up minerals and elements back into the soil, making them available again for plants.
Factors Affecting the Rate of Decomposition
| Factor | Optimal Condition | Explanation |
|---|---|---|
| Temperature | Warm (around 30–40°C) | Decomposers are microorganisms – their enzymes work faster in warm conditions. Too cold = slow enzyme activity; too hot = enzymes denature |
| Moisture | Moist (but not waterlogged) | Decomposers need water for cellular processes and to dissolve nutrients. Dry conditions slow decomposition; waterlogged conditions reduce oxygen |
| Oxygen availability | Aerobic (plenty of oxygen) | Most decomposers respire aerobically and need oxygen. Anaerobic conditions slow decomposition and produce different products (e.g. methane) |
Example 4: Why Bodies Preserve in Peat Bogs
Peat bogs are cold, acidic and waterlogged (low oxygen). These conditions are very poor for decomposition, which is why ancient human bodies such as "Tollund Man" (preserved for over 2,000 years in a Danish peat bog) have been found with skin, hair and clothing still intact. The cold, acidic and anaerobic conditions prevented decomposers from breaking down the tissue.
Composting and Biogas Generators
Composting
Composting uses decomposers to break down kitchen and garden waste in controlled conditions. The key is providing optimal conditions for aerobic decomposition:
- Adding brown material (dry leaves, cardboard) provides carbon and creates air spaces for oxygen
- Adding green material (grass clippings, food scraps) provides nitrogen and moisture
- Turning the compost heap regularly introduces oxygen and mixes the materials
- Keeping it warm (insulated bin or sunny spot) maintains enzyme activity
The end product is compost – a dark, crumbly, nutrient-rich material that improves soil structure and fertility without the need for artificial fertilisers.
Biogas Generators
Biogas generators use anaerobic decomposition to produce methane:
- Organic waste (animal manure, food waste, crop residues) is placed in an anaerobic digester
- Decomposers break down the waste without oxygen, producing biogas (approximately 60–70% methane, 30–40% carbon dioxide)
- The methane is collected and burned as a fuel for heating, cooking or electricity generation
- The remaining material (digestate) is a nutrient-rich fertiliser
Example 5: Biogas in Rural India
In rural India, small-scale biogas generators are common. Animal dung (from cows and buffalo) is fed into an anaerobic digester. The methane produced is piped directly to the kitchen for cooking, replacing wood fires. This reduces deforestation, reduces smoke inhalation (improving health), and produces a fertiliser by-product for crops. It is an example of sustainable development because it uses waste as a resource without depleting natural capital.
Example 6: Comparing Composting and Biogas
| Feature | Composting | Biogas Generator |
|---|---|---|
| Conditions | Aerobic (needs oxygen) | Anaerobic (no oxygen) |
| Products | Compost (fertiliser) | Methane (fuel) + digestate (fertiliser) |
| Temperature | Ambient to warm | Warm (30–40°C optimal) |
| Speed | Weeks to months | Days to weeks |
| Gas produced | CO₂ (released to air) | Methane + CO₂ (captured as fuel) |
Practice Questions
Q1: Foundation Name the process that removes CO₂ from the atmosphere and the process that adds CO₂ to the atmosphere in the carbon cycle.
Q2: Foundation Describe the water cycle. Include the processes of evaporation, condensation, precipitation and transpiration.
Q3: Higher Explain how the combustion of fossil fuels affects the balance of the carbon cycle.
Q4: Higher Explain why decomposition is slower in a waterlogged peat bog than in a well-drained forest soil.
Q5: Foundation Compare composting with biogas production. Include the conditions required and the products formed in each.
Answers
- Photosynthesis removes CO₂ from the atmosphere (plants absorb CO₂ and convert it to glucose using light energy). Respiration adds CO₂ to the atmosphere (all living organisms break down glucose to release energy, producing CO₂ as a waste product). Other processes that add CO₂ include combustion and decay.
- Heat from the Sun causes water in oceans, lakes and rivers to evaporate, turning into water vapour. Plants also release water vapour through transpiration from their leaves. The water vapour rises, cools and condenses to form clouds of liquid water droplets. When the droplets become heavy enough, they fall as precipitation (rain, snow, etc.). Some water flows over the ground as runoff into rivers, and some infiltrates into the soil and rock. Water eventually returns to the oceans and the cycle continues.
- In the natural carbon cycle, the CO₂ added by respiration and decay is roughly balanced by the CO₂ removed by photosynthesis. However, combustion of fossil fuels releases CO₂ that was locked underground for millions of years. This adds CO₂ to the atmosphere much faster than photosynthesis can remove it. Deforestation also reduces the number of plants available to absorb CO₂ through photosynthesis. The result is a net increase in atmospheric CO₂, which is causing global warming.
- Decomposition is slower in a waterlogged peat bog because: (1) The lack of oxygen – water fills the air spaces in the soil, meaning aerobic decomposers (bacteria and fungi) cannot respire efficiently. Only slow anaerobic decomposition can occur. (2) The acidic conditions – peat bogs are acidic, which inhibits many decomposer enzymes. (3) The low temperature – peat bogs are often cold, further slowing enzyme activity. In a well-drained forest soil, there is plenty of oxygen in the air spaces, a neutral pH and typically warmer conditions, all of which favour rapid aerobic decomposition.
- Composting requires aerobic conditions (oxygen is needed for decomposers to respire efficiently). The organic waste is mixed and turned regularly to introduce oxygen. The main product is compost, a nutrient-rich material used as a fertiliser. CO₂ is released as a by-product of respiration. Biogas production requires anaerobic conditions (no oxygen). Organic waste is placed in a sealed digester where anaerobic microorganisms break it down. The main product is biogas (methane + CO₂), which is captured and burned as fuel. A nutrient-rich digestate is also produced, which can be used as fertiliser. Composting is slower but simpler; biogas production is faster and produces a useful fuel but requires a sealed generator.
Exam Tips
- When drawing or describing the carbon cycle, always include photosynthesis, respiration, combustion and decay – these are the four key processes examiners look for
- Be clear about the direction of CO₂ movement: photosynthesis removes it, respiration/combustion/decay add it
- Don't confuse transpiration (water loss from leaves) with respiration (energy release from glucose) – they sound similar but are completely different processes
- When explaining factors affecting decomposition, link each factor to enzyme activity – this shows deeper understanding
- For biogas questions, always mention that it is anaerobic decomposition and that the methane is the useful product
- Remember: combustion of fossil fuels returns carbon that was locked up for millions of years back to the atmosphere – this is why it unbalances the carbon cycle
🔬 Required Practical
Effect of Temperature on Decay Rate
Investigate the effect of temperature on the rate of decay by measuring mass loss in organic material (e.g. milk) at different temperatures.
Method: 1) Fill identical containers with the same mass of milk. 2) Add the same mass of lipase solution to each. 3) Place containers at different temperatures (e.g. 5°C, 20°C, 37°C, 60°C). 4) Measure the pH change (using pH sensor or indicator) at regular intervals over the same time period. 5) The faster the pH change, the faster the rate of decay (lipase breaks down lipids, producing acidic products).
Variables: Independent = temperature; Dependent = rate of decay (pH change per unit time); Control = volume of milk, concentration of lipase, container size.
Expected results: Decay rate increases with temperature up to an optimum (around 37°C for most decomposers), then decreases at higher temperatures as enzymes denature.
🔢 Maths Skills
Mathematical Skills
Calculating decay rates from mass loss data: rate of decay = mass lost / time. For example, if a 50 g sample of leaf litter loses 15 g over 30 days, the decay rate = 15/30 = 0.5 g/day. Percentage mass loss = 15/50 x 100 = 30%.
Plotting decay rate against temperature: you may need to read values from a graph or calculate the gradient of a line to determine the rate.
⚠️ Common Misconceptions
Watch Out!
Students often think decomposition only happens in soil. Wrong: Decomposition only happens in soil Correct: Decomposition happens wherever decomposers (bacteria and fungi) are present — in water, on dead animals, in compost heaps, and even in your gut
Students often think the carbon cycle and water cycle are completely separate. Wrong: The carbon cycle and water cycle are independent Correct: They interact — plants need water to photosynthesise (removing CO₂); transpiration returns water to the atmosphere while simultaneously the plant is absorbing CO₂; decomposition adds both CO₂ and water vapour to the atmosphere
✍️ 6-Mark Question
Extended Answer
6 marks: Describe the carbon cycle and explain the role of decomposers in it.
Carbon is continuously recycled through the environment. Photosynthesis by plants and algae removes CO₂ from the atmosphere, converting it into glucose and oxygen. Respiration by all living organisms adds CO₂ back to the atmosphere as they break down glucose to release energy. Combustion of fossil fuels and wood also adds CO₂. When organisms die, decomposers (bacteria and fungi) break down their dead bodies and waste material. Decomposers respire as they do this, releasing CO₂ back into the atmosphere. Over millions of years, organisms that are not fully decomposed may form fossil fuels, trapping carbon underground. Decomposers play a crucial role by unlocking the carbon stored in dead organic matter and returning it to the atmosphere as CO₂ through their respiration. Without decomposers, carbon would remain locked in dead organisms and nutrients would not be recycled, preventing new plant growth.
Mark scheme: 1 mark for photosynthesis removing CO₂, 1 mark for respiration adding CO₂, 1 mark for combustion, 1 mark for decomposition adding CO₂, 1 mark for fossilisation, 1 mark for explaining decomposer role
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
An experiment measures mass loss from identical leaf litter samples at different temperatures: 5°C = 2 g lost in 30 days; 20°C = 8 g lost; 37°C = 14 g lost; 60°C = 3 g lost. Calculate the decay rate at each temperature (g/day). Explain why decay is slowest at 5°C and 60°C, even though 60°C is warm. Evaluate how reliable these results would be if only one sample was used per temperature.
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🎬 Video Resources
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