Nutrient Cycles: Carbon

This lesson covers the carbon cycle, including the processes of photosynthesis, respiration, decomposition, combustion, and fossilisation, as well as carbon sinks and sources, as required by the Edexcel A-Level Biology specification (9BI0), Topic 10 -- Ecosystems.

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Why Do Nutrients Cycle?

Unlike energy (which flows through ecosystems and is lost as heat), nutrients (chemical elements such as carbon and nitrogen) are recycled within ecosystems. The same atoms of carbon that were once part of a dinosaur could now be part of the CO2 in the atmosphere or the glucose in a plant.

Nutrient cycling involves:

• Living organisms (biotic processes)
• The abiotic environment (atmosphere, water, soil, rocks)
• Decomposers (bacteria and fungi that break down dead organic matter)

Key Link: This concept connects directly to the first lesson on ecosystems: energy flows (one-way) but nutrients cycle (recycled). You should be able to explain why energy cannot be recycled (it is lost as heat at each trophic level through respiration, as covered in the second law of thermodynamics), whereas atoms are conserved and can be reused indefinitely.

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The Carbon Cycle

Carbon is a fundamental element of life -- it forms the backbone of all organic molecules (carbohydrates, lipids, proteins, nucleic acids). The carbon cycle describes how carbon moves between the atmosphere, biosphere, hydrosphere, and lithosphere.

flowchart TB
    A["CO2 in\natmosphere\n(~420 ppm)"] -->|"Photosynthesis\n(plants, algae)"| B["Carbon in\nproducers\n(organic molecules)"]
    B -->|"Respiration\n(producers)"| A
    B -->|"Feeding"| C["Carbon in\nconsumers"]
    C -->|"Respiration\n(consumers)"| A
    B -->|"Death and\ndecomposition"| D["Carbon in\ndecomposers\n(bacteria, fungi)"]
    C -->|"Death and\ndecomposition"| D
    D -->|"Respiration\n(decomposers)"| A
    B -->|"Fossilisation\n(millions of years)"| E["Fossil fuels\n(coal, oil, gas)"]
    C -->|"Fossilisation"| E
    E -->|"Combustion\n(burning fossil\nfuels)"| A
    A -->|"Dissolves in\noceans"| F["Carbon in\noceans\n(dissolved CO2,\ncarbonates)"]
    F -->|"Release"| A
    F -->|"Sedimentation"| G["Carbonate rocks\n(limestone)"]
    G -->|"Weathering /\nvolcanic activity"| A

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Key Processes in the Carbon Cycle

1. Photosynthesis

Photosynthesis removes CO2 from the atmosphere and fixes it into organic molecules (glucose):

$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$6CO2​+6H2​Olight​C6​H12​O6​+6O2​

• Carried out by plants, algae, and cyanobacteria
• The carbon in CO2 becomes the carbon in glucose and other organic molecules
• This is the main process that removes CO2 from the atmosphere
• Globally, photosynthesis fixes approximately 120 Gt (gigatonnes) of carbon per year on land and a similar amount in the oceans

2. Respiration

Respiration releases CO2 back into the atmosphere:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$C6​H12​O6​+6O2​→6CO2​+6H2​O+ATP

• Carried out by all living organisms (producers, consumers, and decomposers)
• Carbon in organic molecules is oxidised to CO2
• This is the main biological process that returns CO2 to the atmosphere

3. Decomposition

When organisms die, decomposers (bacteria and fungi) break down the dead organic matter:

• Decomposers secrete extracellular enzymes onto the dead material (saprophytic nutrition)
• Complex organic molecules are hydrolysed into simpler molecules
• Decomposers respire these molecules, releasing CO2 into the atmosphere

Factors Affecting the Rate of Decomposition

Factor	Effect on Decomposition Rate
Temperature	Higher temperature increases enzyme activity and metabolic rate (up to optimum); very high temperatures denature enzymes
Moisture	Water is needed for enzyme activity and microbial growth; too dry = slow decomposition
Oxygen availability	Aerobic decomposition is faster than anaerobic; waterlogged soils have slow decomposition
pH	Most decomposers work best at neutral pH; very acidic or alkaline conditions slow decomposition
Type of material	Soft tissue decomposes faster than woody material (lignin is resistant to decomposition)

Exam Tip: Peat bogs are acidic, waterlogged (anaerobic), and cold -- all conditions that slow decomposition. This is why peat accumulates as a massive carbon store. When peat bogs are drained for agriculture or fuel, the peat decomposes rapidly, releasing vast amounts of stored CO2. The UK's peat bogs store an estimated 3.2 billion tonnes of carbon.

4. Combustion

Combustion (burning) of organic matter releases CO2:

• Fossil fuel combustion -- burning coal, oil, and natural gas for energy
• Biomass combustion -- forest fires, burning wood, crop residues
• Combustion rapidly converts organic carbon back to atmospheric CO2
• Human fossil fuel burning currently releases approximately 9.5 Gt of carbon per year -- far more than natural processes can absorb

5. Fossilisation

Under certain conditions, dead organisms are not fully decomposed and instead become preserved as fossil fuels over millions of years:

• Coal -- formed from plant material in swamps that was buried under sediment (prevented decomposition due to anaerobic conditions)
• Oil and natural gas -- formed from marine organisms (plankton) buried under sediment on the ocean floor
• This process is extremely slow (millions of years) and acts as a long-term carbon sink

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Carbon Sinks and Sources

A carbon sink absorbs more carbon than it releases. A carbon source releases more carbon than it absorbs.

Carbon Sink	Mechanism	Estimated Carbon Stored
Forests	Trees absorb CO2 through photosynthesis and store carbon in biomass	~450 Gt C globally
Oceans	CO2 dissolves in seawater; phytoplankton fix CO2; carbon incorporated into shells	~38,000 Gt C (dissolved)
Soil	Organic matter (humus) from decomposition stores large amounts of carbon	~2,500 Gt C (top 1m)
Fossil fuel deposits	Carbon locked up underground in coal, oil, and gas	~4,000 Gt C
Carbonate rocks	Limestone (CaCO3) stores carbon for geological timescales	~60,000,000 Gt C

Carbon Source	Mechanism
Combustion of fossil fuels	Burning coal, oil, and gas releases stored carbon as CO2
Deforestation	Cutting and burning trees releases stored carbon; reduces photosynthetic capacity
Respiration	All organisms release CO2 through respiration
Volcanic eruptions	Release CO2 from carbonate rocks deep in the Earth's crust
Peat extraction and drainage	Exposes stored carbon to aerobic decomposition

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The Role of the Oceans

The oceans are a major component of the carbon cycle:

• CO2 dissolves in seawater, forming carbonic acid (H2CO3), which dissociates into bicarbonate (HCO3-) and carbonate (CO32-) ions
• Marine organisms use calcium and carbonate ions to build shells (CaCO3)
• When these organisms die, their shells sink and form sedimentary carbonate rocks (limestone) on the ocean floor
• Phytoplankton carry out approximately 50% of all photosynthesis on Earth
• The deep ocean acts as a long-term carbon store, but carbon exchange between the surface and deep ocean is slow (hundreds to thousands of years)

Ocean Acidification

As atmospheric CO2 increases, more dissolves in the oceans, lowering the pH:

$\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$CO2​+H2​O⇌H2​CO3​⇌H++HCO3−​

Ocean pH has decreased by approximately 0.1 units since pre-industrial times (from 8.2 to 8.1). While this seems small, pH is a logarithmic scale, so this represents a 26% increase in hydrogen ion concentration. This reduces the availability of carbonate ions, making it harder for corals and molluscs to build calcium carbonate structures.

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Human Impact on the Carbon Cycle

Human activities have significantly altered the carbon cycle, primarily by:

1. Burning fossil fuels -- releases ancient carbon that was locked underground for millions of years
2. Deforestation -- reduces the capacity of terrestrial ecosystems to absorb CO2
3. Agriculture -- rice paddies and livestock produce methane (CH4, another carbon-containing greenhouse gas)
4. Cement production -- heating limestone (CaCO3) releases CO2

These activities have increased atmospheric CO2 concentration from approximately 280 ppm (pre-industrial) to over 420 ppm today.

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Worked Example 1: Carbon Cycle Processes

Question: In a woodland ecosystem, the trees fix 50,000 kg of carbon per year through photosynthesis. The trees lose 30,000 kg of carbon per year through respiration. Calculate the net carbon fixation and explain what happens to this carbon.

Answer:

$\text{Net carbon fixation} = 50{,}000 - 30{,}000 = 20{,}000 \text{ kg C year}^{-1}$Net carbon fixation=50,000−30,000=20,000 kg C year−1

This 20,000 kg of carbon is used for growth (new leaves, wood, roots) and represents the net primary productivity in terms of carbon. This carbon is stored in the biomass of the trees and is available to consumers (herbivores) and decomposers.

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Worked Example 2: Interpreting Carbon Flux Data

Question: A student monitors the CO2 concentration above a UK forest over 24 hours in July. At dawn (05:00), the CO2 concentration is 415 ppm. By mid-afternoon (15:00), it falls to 400 ppm. By the following dawn, it returns to 415 ppm. Explain these changes.

Answer:

During the day, the rate of photosynthesis exceeds the rate of respiration (because light drives the light-dependent reactions). The forest is acting as a net carbon sink, removing CO2 from the air faster than it produces it through respiration. This reduces the local CO2 concentration from 415 to 400 ppm.

At night, there is no photosynthesis (no light), but respiration continues in all organisms (trees, animals, decomposers in the soil). The forest becomes a net carbon source, releasing CO2 faster than it absorbs it (absorption is zero). This causes CO2 concentration to rise back to 415 ppm by dawn.

On an annual basis, a healthy growing forest is a net carbon sink because the total annual photosynthesis exceeds total annual respiration.

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Common Exam Mistakes

1. Saying plants only respire at night. Plants respire all the time (24 hours). During the day, the rate of photosynthesis exceeds respiration, so there is a net uptake of CO2. This does not mean respiration stops.
2. Confusing carbon cycle with energy flow. Carbon is recycled (the same carbon atoms move between atmosphere, organisms, and soil). Energy is not recycled -- it flows through the ecosystem and is eventually lost as heat.
3. Forgetting decomposers. Decomposers are essential to the carbon cycle. Without them, dead organic matter would accumulate and carbon would not be returned to the atmosphere.

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Summary

• Carbon cycles between the atmosphere, biosphere, hydrosphere, and lithosphere.
• Photosynthesis removes CO2 from the atmosphere; respiration and combustion return it.
• Decomposition by bacteria and fungi recycles carbon from dead organisms.
• Fossil fuels represent ancient carbon stored underground for millions of years.
• The oceans are a major carbon sink (dissolved CO2, shells, carbonate rocks).
• Peat bogs are significant carbon stores where decomposition is inhibited by acidic, anaerobic, cold conditions.
• Human activities (fossil fuel burning, deforestation) have increased atmospheric CO2 from ~280 to ~420 ppm and disrupted the natural carbon cycle.

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A-Level Deep Dive: The Carbon Cycle

Spec mapping

The Edexcel 9BI0 specification places the carbon cycle in Topic 5: On the Wild Side — Photosynthesis, Energy and Ecosystems, on Paper 2 (Energy, Exercise and Coordination). This lesson is the matter-cycling counterpart of the energy-flow lesson (lesson 2): where energy flows through an ecosystem and is dissipated as heat, carbon cycles between four reservoirs — atmosphere, biosphere, hydrosphere and lithosphere — driven by the same photosynthetic and respiratory machinery that sets GPP and NPP. Statements concern: the role of photosynthesis in fixing atmospheric $\text{CO}_2$CO2​ into organic compounds at the producer level; the role of respiration in returning $\text{CO}_2$CO2​ to the atmosphere at every trophic level (producers, consumers, decomposers); the role of decomposition by saprotrophic bacteria and fungi in returning carbon from dead organic matter to the atmosphere; the role of fossilisation in locking carbon into geological reservoirs (coal, oil, gas, peat, carbonate rocks) over millions of years; the role of combustion of fossil fuels and biomass in releasing stored carbon back to the atmosphere; and the anthropogenic perturbation of the carbon cycle — industrial $\text{CO}_2$CO2​ emissions of ~36 Gt/yr have raised atmospheric $\text{CO}_2$CO2​ from ~280 ppm pre-industrial to >420 ppm today, driving climate change (refer to the official Pearson Edexcel 9BI0 specification document for exact wording). Synoptic links radiate to Topic 5 — Photosynthesis (the carbon-fixation step at the chloroplast) and Topic 5 — Respiration (the dominant carbon-return flux at every trophic level), to lesson 2 — Energy Transfer (carbon flow tracks energy flow through the same trophic transfers), to lesson 4 — Nitrogen Cycle (the parallel matter cycle), and to Topic 4 — Biodiversity (deforestation as a biodiversity-loss driver that also perturbs the carbon cycle).

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Worked example with full mark scheme

Question (8 marks):

A managed temperate woodland of area $1\,\text{km}^2$1km2 is monitored for one year. Annual measurements show: photosynthetic $\text{CO}_2$CO2​ uptake $= 12\,\text{Gg}\,\text{C}\,\text{yr}^{-1}$=12GgCyr−1 (gigagrams of carbon); ecosystem respiration (producers + consumers + decomposers) $= 9\,\text{Gg}\,\text{C}\,\text{yr}^{-1}$=9GgCyr−1; net loss of carbon to soil organic matter $= 0.5\,\text{Gg}\,\text{C}\,\text{yr}^{-1}$=0.5GgCyr−1.

(a) Define carbon flux and carbon stock and identify which two reservoirs exchange carbon via photosynthesis and respiration. (2)

(b) Calculate the woodland's net ecosystem exchange (NEE) of carbon and state whether it is a net carbon sink or source. (2)

(c) Explain three processes by which carbon stored in this woodland could be returned to the atmosphere, and identify the timescale of each. (4)

Solution with mark scheme:

(a) M1 (AO1.1) — a carbon flux is the rate of transfer of carbon between two reservoirs (e.g. Gg$\,$C$\,$yr$^{-1}$−1); a carbon stock is the amount of carbon held within a single reservoir at a point in time (e.g. Gg$\,$C). A1 (AO1.2) — the atmosphere and biosphere exchange carbon via photosynthesis (atmosphere → biosphere as $\text{CO}_2$CO2​ is fixed into organic compounds) and respiration (biosphere → atmosphere as organic compounds are oxidised back to $\text{CO}_2$CO2​).

(b) M1 (AO2.2) — $\text{NEE} = \text{photosynthetic uptake} - \text{ecosystem respiration} = 12 - 9 = 3\,\text{Gg}\,\text{C}\,\text{yr}^{-1}$NEE=photosynthetic uptake−ecosystem respiration=12−9=3GgCyr−1 (net uptake from the atmosphere). A1 (AO3.1a) — this is a net carbon sink: the woodland fixes more $\text{CO}_2$CO2​ than it releases, so atmospheric carbon is being transferred to biosphere and soil reservoirs. The 0.5 Gg/yr accumulating in soil organic matter is the long-term sequestered fraction.

(c) M1 (AO1.2) — respiration by producers, consumers and decomposers oxidises organic compounds to $\text{CO}_2$CO2​; timescale = continuous (operates 24 hours a day, year-round). M1 (AO1.2) — decomposition by saprotrophic bacteria and fungi processes leaf litter, dead wood and dead organisms; timescale = months to decades depending on substrate (leaves decompose in months, lignified wood in decades). M1 (AO1.2) — combustion during forest fires (or harvested timber being burned) releases stored carbon rapidly; timescale = minutes to hours for the fire itself, but the carbon released represents decades-to-centuries of accumulated growth. A1 (AO3.1a) — under undisturbed conditions a fraction of carbon escapes the fast cycle by entering fossilisation pathways (peat formation in waterlogged anaerobic soils, carbonate-rock formation in marine systems); timescale = millions of years, transferring carbon from the fast biological cycle to the slow geological cycle.

Total: 8 marks.

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Explain how the carbon cycle links the atmosphere and the biosphere, and evaluate why the anthropogenic perturbation of this cycle differs in kind from natural disturbances.

Mark scheme decomposition by AO: