Soil Carbon Release (HL) (DP IB Environmental Systems & Societies (ESS))

Soil Carbon Release

• Soil is a major carbon store

  • Soils hold approximately twice as much carbon as the atmosphere

• Soils store a significant amount of carbon in the form of organic material and carbon compounds

  • When soils break down organic matter, carbon is released into the atmosphere as:

    • Carbon dioxide (CO₂)

    • Methane (CH₄)

  • This carbon release can contribute to global greenhouse gas levels and climate change

Factors that lead to soil carbon release

• Decomposition:

  • Microorganisms in the soil decompose organic matter

  • This releases CO₂ when oxygen is available

  • In waterlogged, low-oxygen conditions (such as wetlands), decomposition produces CH₄ instead

• Impacts of global warming:

  • Rising temperatures accelerate decomposition

  • This leads to faster release of CO₂ and CH₄

  • This creates a feedback loop where more carbon release leads to more warming, which then increases soil carbon release even further

• Agricultural practices:

  • Tilling (turning over the soil) exposes deeper layers to air, increasing decomposition rates and CO₂ release

  • Use of fertilisers and other agricultural chemicals can also increase microbial activity, which increases CO₂ emissions from soil

• Drainage of wetlands:

  • Wetlands are important carbon stores

  • Draining wetlands for agriculture or development introduces oxygen to previously waterlogged soils

  • This exposure speeds up decomposition, releasing CO₂ and CH₄ into the atmosphere

Tipping points in carbon release

• A tipping point in soil carbon release may be reached if rising temperatures lead to rapid and uncontrollable carbon emissions

• Once past this point, the release of carbon can further increase temperatures

  • This could create a self-reinforcing cycle of warming

• For example, permafrost thawing in the Arctic:

  • As Arctic temperatures rise, permafrost thaws, releasing both CO₂ and CH₄, which were previously trapped in frozen soil

  • Thawing of permafrost is a major concern because it contains large amounts of carbon

  • This amount of carbon could speed up global warming if it enters the atmosphere

Methane clathrates

• Methane clathrates are ice-like structures containing trapped methane

  • They are found in permafrost and under the ocean floor

• If temperatures rise enough to melt these clathrates, they release large amounts of methane

  • As methane is a very potent greenhouse gas, this could accelerate climate change