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Soil carbon sequestration

Also known as
soil carbon, carbon sequestration

Soil carbon sequestration is the removal of carbon dioxide from the atmosphere by plants and its storage in soil as organic carbon. Farming practices that add more plant carbon to the soil and reduce its loss can increase soil carbon over time.

What is soil carbon sequestration?

Plants take carbon dioxide from the air during photosynthesis. When roots, residues and root exudates enter the soil, soil organisms break them down; some carbon returns to the air as carbon dioxide, and some remains in the soil as soil organic matter. Soil carbon sequestration refers to a net increase in that stored carbon, which removes carbon dioxide from the atmosphere.

Research has shown that soil microorganisms play a central role in how much plant carbon is converted into stable soil carbon rather than released.

Practices that can build soil carbon

  • Cover crops, which add roots and residue between cash crops.
  • Reduced tillage and no-till farming, which slow decomposition, although their effect on carbon deeper in the soil is debated.
  • Diverse rotations and perennial crops.
  • Improved grazing management.
  • Organic amendments, such as manure and compost.

Limits and challenges

  • Slow change. Soil carbon changes over years to decades.
  • Saturation. Soils approach a new, higher equilibrium, after which gains slow.
  • Permanence. Carbon can be lost again if practices change.
  • Measurement. Soil carbon varies widely across short distances and with depth, making changes costly to measure accurately.

These challenges matter especially for soil carbon credits, which also require showing additionality, that the carbon would not have been stored anyway.

Frequently asked questions

It depends on soil type, climate, current carbon levels and management. Soils depleted by past management often have the greatest capacity to gain carbon.

Soil organic carbon is the carbon contained in soil organic matter. Organic matter includes carbon and other elements.

  • Soil organic matter SOM

    Soil organic matter (SOM) is the fraction of soil made of living organisms, fresh and decomposing plant and animal residues, and stable, well-decomposed material called humus. Although it usually makes up only a few percent of soil by weight, it strongly affects fertility, structure and water holding.

  • Additionality

    Additionality is the principle that a carbon credit should represent emission reductions or carbon removals that would not have happened without the project the credit pays for. If the practice would have been adopted anyway, the credit is not additional.

  • Regenerative agriculture

    Regenerative agriculture is an approach to farming that aims to improve soil health and the wider environment over time, rather than simply sustaining them, through practices such as minimal tillage, cover crops, diverse rotations and integrating livestock. It has no single legal definition.

  • Soil health

    Soil health is the continued capacity of a soil to function as a living ecosystem that sustains plants, animals and people. A healthy soil holds and supplies water and nutrients, supports roots and soil life, cycles organic matter and resists erosion.

Sources

  1. Soil carbon and soil health, USDA Natural Resources Conservation Service
  2. Climate Change and Land, Intergovernmental Panel on Climate Change (IPCC), 2019