Cycles: Water, Carbon, and Decomposition

Recycling Materials

All materials in the living world are recycled — they pass between the living (biotic) and non-living (abiotic) components of ecosystems. This ensures that elements like carbon, water, and nitrogen are continuously available.

Decomposers play a vital role in this recycling.

Decomposition

Decomposers (mainly bacteria and fungi) break down dead organisms and waste products. They secrete enzymes onto the dead material, digest it externally, and absorb the soluble products. This process:

• Returns mineral ions and nutrients to the soil for plants to absorb
• Releases carbon dioxide back into the atmosphere (through respiration)

Factors Affecting Rate of Decomposition

Factor	Effect
Temperature	Higher temperature (up to about 50°C) increases enzyme activity and decomposer metabolism → faster decomposition. Above this, enzymes denature. Very cold temperatures slow decomposition greatly.
Water (moisture)	Decomposers need water for chemical reactions and to survive → moist conditions speed up decomposition. Very dry conditions slow it.
Oxygen availability	Most decomposers are aerobic — they need oxygen for respiration. Decomposition is faster in well-aerated conditions. Waterlogged or compacted soil slows decomposition (anaerobic conditions).
Number of decomposers	More decomposers = faster breakdown

Required Practical: Decomposition

Aim: Investigate the effect of temperature on the rate of decomposition of milk by lipase (as a model for decomposition).

Method:

1. Add lipase solution to milk containing phenolphthalein indicator and a small amount of sodium carbonate (to make it alkaline — the indicator turns pink)

2. As lipase breaks down the fat in milk, fatty acids are produced → the pH drops → the indicator turns colourless (goes from pink to colourless)

3. Time how long it takes for the colour change at different temperatures (e.g. 10°C, 20°C, 30°C, 40°C, 50°C, 60°C)

4. Calculate the rate of decomposition: rate = 1 ÷ time

Expected results: Rate increases with temperature up to the optimum (~40°C for lipase), then drops sharply as the enzyme denatures.

Composting

Composting is the process of using decomposers to break down garden and kitchen waste into compost (nutrient-rich material for soil).

Conditions for efficient composting:

• Warm temperature (speeds up decomposer activity)
• Moist (not waterlogged)
• Good oxygen supply — turning the compost adds air
• Small pieces of waste (larger surface area for decomposers)

The Carbon Cycle

Carbon is constantly recycled between the atmosphere, living organisms, and the Earth.

Carbon enters the atmosphere as CO₂ by:

• Respiration — all living organisms (plants, animals, decomposers) respire, releasing CO₂
• Combustion — burning fossil fuels (coal, oil, gas) and wood releases CO₂
• Decomposition — decomposers respire as they break down dead material, releasing CO₂

Carbon is removed from the atmosphere by:

• Photosynthesis — plants absorb CO₂ and convert it into glucose (organic compounds)
• Dissolving in oceans — CO₂ dissolves in seawater

Carbon passes through the food chain:

• Plants convert CO₂ into organic molecules (glucose, starch, cellulose)
• Animals eat plants → carbon in plant molecules becomes part of animal molecules
• When organisms die, decomposers break down the carbon compounds
• Some dead organisms are buried and compressed over millions of years → form fossil fuels (coal, oil, natural gas)

The Water Cycle

Water is continuously cycled between the atmosphere, land, and oceans:

1. Evaporation — heat from the Sun causes water to evaporate from oceans, lakes, rivers, and soil

2. Transpiration — water vapour is released from plant leaves through stomata

3. Condensation — water vapour rises, cools, and condenses to form clouds

4. Precipitation — water falls as rain, snow, sleet, or hail

5. Water flows over the surface (surface run-off) or through the ground (groundwater flow) back to rivers, lakes, and the sea

6. Plants absorb water through their roots — the cycle continues

The water cycle is driven by energy from the Sun.

The Nitrogen Cycle (Higher Tier)

Nitrogen is essential for making amino acids and proteins. Although 78% of the atmosphere is nitrogen gas (N₂), organisms cannot use it in this form.

Key processes:

• Nitrogen-fixing bacteria (in soil or in root nodules of leguminous plants like peas and clover) convert N₂ → ammonia/ammonium compounds
• Nitrifying bacteria convert ammonia → nitrites → nitrates (which plants can absorb)
• Denitrifying bacteria convert nitrates → N₂ gas (released back to the atmosphere) — this happens in anaerobic (waterlogged) conditions
• Decomposers break down dead organisms and waste → release ammonia into the soil
• Plants absorb nitrates through their roots by active transport and use them to make amino acids → proteins
• Lightning can also fix nitrogen — the energy converts N₂ into nitrogen oxides that dissolve in rain

Exam Tips

• In the carbon cycle, know the processes that release CO₂ (respiration, combustion, decomposition) and those that remove it (photosynthesis)
• Decomposers release CO₂ through their own respiration — they do not just break things down
• The water cycle is driven by the Sun's energy
• For the decomposition practical, rate = 1 ÷ time