Soil Organic Carbon Sequestration

Soil organic carbon sequestration is the net accumulation of carbon in soil organic matter, mainly after plants remove carbon dioxide from the atmosphere through photosynthesis and transfer part of it to soil through roots and residues. Agricultural soil acts as a carbon sink when organic-carbon inputs exceed losses through decomposition, erosion and other pathways; it acts as a source when losses exceed inputs. Sequestration therefore means an increase in soil organic carbon stock over time, not merely the presence of a large existing stock.

The soil-carbon balance

The direction and duration of change depend on both the quantity of carbon entering the soil and the extent to which that carbon is protected from rapid decomposition.

• Carbon enters agricultural soil through roots, root exudates, crop residues, manures, composts and other organic amendments.
• Microorganisms decompose organic matter and return much of its carbon to the atmosphere as carbon dioxide.
• Some organic carbon persists because it is enclosed within soil aggregates, chemically associated with mineral particles or present in forms resistant to decomposition.
• Erosion can remove carbon-rich topsoil, while fire and residue removal reduce the amount of plant carbon returned to the soil.
• Soil-carbon gains are finite and reversible: stocks may approach a new equilibrium and can decline again after a change in management.

Tillage and crop-residue management

• Frequent or intensive tillage breaks soil aggregates, increases aeration and exposes previously protected organic matter to microbial decomposition.
• Tillage can also accelerate erosion of carbon-rich surface soil, especially where the soil remains bare.
• Reduced tillage or no-till generally lowers soil disturbance, protects aggregates and can increase organic carbon near the surface.
• A surface-soil increase under no-till does not always represent an equal increase through the entire soil profile; assessment must use a consistent depth and soil mass.
• Retaining crop residues supplies carbon and nutrients, protects the surface from erosion and moderates soil temperature and moisture.
• Removing or burning residues reduces carbon returned to the soil; burning also releases much of the residue carbon directly to the atmosphere.
• Residue incorporation adds carbon but also places it in contact with microbes, so the amount retained depends on climate, soil properties, aeration and subsequent management.

Role of soil moisture

Moisture affects both plant production, which determines carbon inputs, and microbial activity, which determines decomposition losses.

• Adequate moisture can raise plant growth and root or residue inputs, thereby favouring soil-carbon accumulation.
• Warm, moist and well-aerated conditions generally favour rapid microbial decomposition and carbon-dioxide release.
• Severe dryness suppresses decomposition but can also reduce plant growth and carbon inputs, so drought does not necessarily increase soil carbon.
• Prolonged saturation restricts oxygen and slows aerobic decomposition, but anaerobic conditions can generate methane; therefore, an increase in soil carbon need not imply a lower total greenhouse-gas impact.
• Drainage and irrigation can alter soil-carbon stocks by changing vegetation growth, aeration and decomposition rates.

Land use and the sink-source outcome

• Conversion of forests, grasslands or other long-established vegetation to frequently tilled cropland commonly reduces soil organic carbon through lower inputs, disturbance and erosion.
• Cover crops, diverse crop rotations and the inclusion of deep-rooted or perennial species increase the duration and diversity of carbon inputs to soil.
• Agroforestry and perennial vegetation can add carbon through both above-ground litter and extensive root systems.
• Manure and compost can increase carbon stocks in the receiving field, although system-level accounting must distinguish newly sequestered carbon from carbon merely transferred from another location.
• Contour farming, vegetative cover and other erosion-control measures help retain carbon-rich topsoil.
• Restoration of degraded cropland or grassland can make soil a sink, whereas residue removal, bare fallows, erosion and repeated intensive disturbance can make it a source.
• Sink or source status must be evaluated over a specified period, soil depth and land area; the complete climate assessment should also include methane and nitrous-oxide emissions.

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