What is soil carbon?
The sequestration mechanism, from photosynthesis to stable organic matter
Soil is the second largest terrestrial carbon sink after oceans. Plants capture CO₂, microbes turn it into stable organic matter. Regenerative agriculture accelerates this natural cycle.
Before discussing credits, you need to understand what soil carbon is and how it gets there. The mechanism rests on three actors: plants, soil microbes and the chemistry of stable organic matter. Understanding these three links clarifies both the potential and the limits of sequestration.
Photosynthesis: plants fix atmospheric CO₂ into sugars and root exudates.
Microbes: bacteria and fungi turn these compounds into biomass, then into humus.
Stable organic matter: bound to soil minerals, it can hold 100 to 1,000 years.
Global potential: 0.4 to 1.2 Gt of carbon per year according to Lal (2004) and the IPCC.
Soil as a carbon sink
Global soils hold about 1,500 gigatonnes of organic carbon in the top metre, two to three times more than the atmosphere. It is the second largest terrestrial carbon sink after oceans, and by far a major sink among continental sinks. That mass is not static: it constantly exchanges with the atmosphere via photosynthesis (input) and microbial respiration (output). Agricultural practices modulate that balance. Land that is intensively tilled and bare for months loses carbon over time. Land that is covered, sensibly grazed and fed organic residues gains it. The global potential for additional sequestration, per Rattan Lal's work (Science 2004) and confirmed by the 4 per 1,000 initiative launched at COP21 Paris, sits between 0.4 and 1.2 gigatonnes of carbon per year, roughly 1.5 to 4.4 Gt CO₂eq per year. That is a significant fraction of current global emissions.
Photosynthesis: the plant carbon pump
The mechanism starts in the leaves. A growing plant captures atmospheric CO₂ via photosynthesis and turns it into simple sugars (glucose, sucrose). Part fuels above-ground tissue growth. Another part, often under-appreciated, flows down to the roots: root exudation. A healthy plant can devote 20 to 40 % of fixed carbon to feeding the rhizosphere, the soil zone immediately around the roots. Cover crops, agroforestry and perennial pastures significantly increase this flow: more leaves photosynthesise, more roots exude, more carbon flows into the soil. For soil credits, this exudate flow is the engine.
The role of mycorrhizal fungi
In the rhizosphere, exudates feed a wealth of organisms, but one actor plays an outsized role: arbuscular mycorrhizal fungi (AMF). These fungi form a symbiotic association with about 80 % of land plants: they extend their hyphae through the soil and supply the plant with water and nutrients (phosphorus, nitrogen) in exchange for carbon. A plant in symbiosis can transfer up to 20 % of fixed carbon directly to AMF. These fungi produce a specific glycoprotein, glomalin, which aggregates soil particles and resists decomposition for decades. The denser the mycorrhizal network, the more glomalin accumulates, the more stable carbon becomes. Regenerative practices favour this symbiosis: limited tillage, plant diversity, permanent living roots.
Microbes: turning carbon into humus
Exudates and plant residues are only a starting point. Once in the soil, carbon is metabolised by bacteria and saprophytic fungi that decompose fresh organic matter. These microbes incorporate part of the carbon into their own biomass. Upon their death (rapid turnover, days to months), their biomass yields heavily transformed carbon that binds to soil mineral particles to form mineral-associated organic matter (MAOM). MAOM is the stable soil carbon pool: half-life can exceed 100 years. The labile pool, by contrast (fresh particulate organic matter), turns over in a few years. For credits, what matters is MAOM dynamics, not the short-term fluctuation of particulate organic matter.
Soil carbon pools and their stability
Soil scientists classically distinguish three pools. The labile pool (5 to 15 % of total carbon) covers fresh organic matter and microbial biomass, with short residence time. The intermediate pool (degraded particulate organic matter) holds for decades. The stable pool, dominated by MAOM and highly condensed molecules (humus dark matter), can hold 100 to 1,000 years, even longer in deep clayey soils. Robust soil credits target a net transfer from the labile pool to the stable pool. That is why modern methodologies measure carbon at depths reaching 30 to 100 cm: the deeper layer is more stable, because clay minerals there durably immobilise organic matter. A measurement limited to the top 10 cm over-weighs the labile pool and under-estimates durable sequestration.
Saturation: how much carbon can a soil store?
No soil can store carbon indefinitely. Each soil has a saturation threshold set by texture (clayey soils saturate higher than sandy soils), climate (cold and wet soils saturate higher), structure and management history. When regenerative practices are adopted on a degraded soil, sequestration rates are fast in the first 10 to 20 years, then slow as the soil approaches a new equilibrium plateau. This has two consequences for credits. First, soil projects issue more tonnes in the early years. Second, a soil returned to intensive tillage rapidly loses the gains, because it shifts to a lower equilibrium plateau. This saturation logic is why methodologies impose a long monitoring horizon (20 to 40 years): the project must be tracked until stabilisation for credits to remain credible.
Measuring soil carbon: methods and precision
Measuring soil carbon is technically demanding. The reference method remains core sampling followed by laboratory dry-combustion analysis (Dumas method). Reaching sufficient precision (around ±0.1 to 0.2 t C/ha) requires dozens of cores per plot, with corrections for soil bulk density variations. Modern approaches complement this with modelling (RothC, Century, DayCent) calibrated on measured data, and satellite remote sensing tracking practices (vegetation cover, biomass, intervention dates). This 'ground-truth + model + satellite' combination is today the standard required by Verra VM0042 v2.2 and Gold Standard SOC modules. MRV cost remains a major item, typically 5 to 15 % of a project's carbon revenue.
Why soil carbon is a unique sink
Soil carbon offers a rare combination. It is massive (1,500 Gt globally, more than the atmosphere). It is directly measurable by coring, unlike atmospheric fluxes. It produces immediate co-benefits (water retention, biodiversity, fertility, drought resilience). It leverages an existing economic activity (agriculture) rather than building dedicated infrastructure. And it is diffuse, present wherever cropland exists, so addressable at very large scale. These properties make it the cornerstone of nature-based climate strategies, whether under the European CRCF, the SBTi BVCM, or CAP eco-schemes.
A healthy plant sends 20 to 40 % of fixed carbon to its roots, and up to 20 % directly to its mycorrhizal fungal partners. That flow fuels durable storage.
Frequently asked questions
Sources & official references
- Lal R. (2004) Soil Carbon Sequestration Impacts on Global Climate Change and Food Security, Science 304:1623
- 4 per 1,000 Initiative (launched at COP21, Paris)
- Minasny B. et al. (2017) Soil carbon 4 per mille, Geoderma
- IPCC AR6 WG3 : Chapter 7: Agriculture, Forestry and Other Land Uses
- FAO : Soil Organic Carbon: the hidden potential
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