Carbon Sequestration and Release

Carbon sequestration is the process by which carbon is removed from the atmosphere and stored in a carbon sink. Carbon release is the reverse — the transfer of stored carbon back to the atmosphere. The balance between these two processes determines whether atmospheric CO₂ concentrations rise, fall, or remain stable. For AQA A-Level Geography, understanding the natural and anthropogenic factors controlling this balance is essential for evaluating climate change causes and management responses.

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Photosynthesis, Respiration, and the Carbon Balance

Gross Primary Productivity (GPP)

GPP is the total amount of carbon fixed by photosynthesis in an ecosystem per unit time, typically expressed in gC/m²/yr or GtC/yr.

• Global terrestrial GPP is estimated at approximately 120 GtC/yr (Beer et al., 2010).
• Tropical forests are the most productive biome, contributing approximately 34% of global terrestrial GPP.

Net Primary Productivity (NPP)

NPP is the carbon remaining after plant respiration (autotrophic respiration, Ra) is subtracted:

NPP = GPP − Ra

Approximately 50% of GPP is consumed by plant respiration, so global terrestrial NPP is roughly 60 GtC/yr.

Net Ecosystem Productivity (NEP)

NEP accounts for heterotrophic respiration (Rh) — decomposition by fungi, bacteria, and other organisms:

NEP = NPP − Rh = GPP − Ra − Rh

If NEP > 0, the ecosystem is a net carbon sink (sequestering more carbon than it releases). If NEP < 0, the ecosystem is a net carbon source (releasing more carbon than it absorbs).

Disturbance and Net Biome Productivity (NBP)

When disturbances such as fire, logging, and storm damage are included, the remaining flux is Net Biome Productivity (NBP) — the true net carbon exchange between an ecosystem and the atmosphere over large spatial and temporal scales.

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Soil Carbon

Soils are the largest terrestrial carbon store, containing approximately 1,500–2,400 GtC in the top 2 metres — roughly three times more carbon than the atmosphere.

How Carbon Enters Soils

• Litterfall: Dead leaves, branches, roots, and other organic matter decompose on and within the soil.
• Root exudates: Living roots release carbon-rich compounds into the surrounding soil.
• Root turnover: Fine roots die and decompose, contributing carbon at depth.

How Carbon Leaves Soils

• Decomposition (microbial respiration): Soil microorganisms break down organic matter, releasing CO₂. The rate depends on temperature, moisture, oxygen availability, and the chemical composition of the organic matter.
• Erosion: Physical removal of carbon-rich topsoil by water or wind.
• Leaching: Dissolved organic carbon (DOC) carried by water through the soil profile and into rivers.

Temperature Sensitivity

Decomposition rates roughly double for every 10°C increase in temperature (Q10 relationship). This means that warming accelerates the release of soil carbon, creating a positive feedback with climate change.

Exam Application: The temperature sensitivity of soil carbon is a critical argument in discussions of feedback mechanisms. Warmer temperatures → faster decomposition → more CO₂ → more warming.

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Peat Formation and Peatlands

Formation

Peat forms under waterlogged, acidic, oxygen-poor conditions where the rate of organic matter production exceeds the rate of decomposition.

• Accumulation rate: Approximately 0.5–1 mm per year in UK blanket bogs.
• Global peatland carbon store: Approximately 600 GtC, covering only about 3% of the global land surface but storing roughly 30% of all soil carbon.

UK Peatlands

The UK contains extensive blanket bogs and raised bogs, particularly in Scotland, northern England, and Wales:

• UK peatlands store approximately 3.2 billion tonnes of carbon — more than all the forests of the UK, France, and Germany combined.
• When in good condition (waterlogged, with active Sphagnum moss growth), peatlands act as carbon sinks.
• When degraded (drained, eroded, burned), peatlands become significant carbon sources.

Threats to Peatlands