What is Soil Carbon Sequestration?

What is Soil Carbon Sequestration?

Soil carbon sequestration is the process by which atmospheric carbon dioxide is captured by plants through photosynthesis and transferred to the soil as organic matter, where it can be stored for decades to centuries. Soils are the largest terrestrial carbon pool, containing approximately 2,500 gigatonnes of carbon — more than three times the amount in the atmosphere and four times the amount in all living vegetation. Enhancing soil carbon storage through land management practices is both a climate mitigation strategy and a soil health intervention.

Why It Matters

The world's soils have lost an estimated 133 gigatonnes of carbon since the onset of agriculture, primarily through tillage, deforestation, and land degradation. This represents both a historical emissions source and a restoration opportunity. The "4 per 1000" initiative, launched at COP21 in 2015, calculated that increasing soil organic carbon stocks by just 0.4% annually in the top 30-40 cm of agricultural soils could theoretically offset a significant portion of annual anthropogenic CO₂ emissions.

The co-benefits are substantial and immediate. Soils with higher organic carbon content retain more water (each 1% increase in soil organic matter increases water-holding capacity by approximately 75,000 liters per hectare), resist erosion more effectively, support greater microbial biodiversity, and cycle nutrients more efficiently. For farmers, higher soil carbon translates directly to improved productivity, lower input costs, and greater resilience to drought and extreme rainfall.

Carbon markets are creating financial incentives. Voluntary carbon markets have developed methodologies — through registries like Verra, Gold Standard, and the Climate Action Reserve — for quantifying and crediting soil carbon sequestration from improved agricultural practices. While prices per tonne vary ($15-50 for soil carbon credits as of 2025), the revenue potential is meaningful for farmers managing large acreages, and corporate demand for nature-based removal credits continues to grow.

The policy landscape is supportive. The EU Carbon Farming initiative, the US Growing Climate Solutions Act (2022), and Australia's Emissions Reduction Fund all include mechanisms for rewarding soil carbon increases. These policy signals, combined with private sector demand, are creating a convergence of incentives for soil carbon management.

How It Works / Key Components

Carbon enters soil through three primary pathways: root exudates (sugars and organic acids secreted by living roots that feed soil microbiota), root turnover (dead roots decomposing into soil organic matter), and surface litter incorporation (crop residues, cover crop biomass, and organic amendments decomposing on and into the soil surface). The stability of stored carbon depends on soil mineralogy, microbial processing, and physical protection within soil aggregates.

Management practices that increase soil carbon inputs include cover cropping (adding 0.5-1.0 tonnes of carbon per hectare annually), reduced or no tillage (reducing oxidative carbon loss), organic amendments (compost, manure, biochar), perennial crop systems, and agroforestry. Each practice influences different carbon pools — labile (active, cycling within years), slow (intermediate, cycling over decades), and passive (stable, persisting for centuries). The most durable sequestration strategies target the slow and passive pools.

Measurement, reporting, and verification (MRV) is the critical bottleneck. Direct soil sampling is accurate but expensive and spatially variable — carbon content can differ by 30% between samples taken 10 meters apart in the same field. Emerging technologies including spectroscopic sensors, remote sensing-based models, and machine learning algorithms are improving cost-effectiveness and spatial resolution. The USDA's COMET-Farm tool and similar platforms provide practice-based carbon modeling for farmers.

Permanence is the key challenge for soil carbon as a climate mitigation strategy. Unlike geological storage, soil carbon can be re-released if management practices change — a farmer who builds soil carbon through no-till and cover cropping could release it by reverting to conventional tillage. Carbon credit methodologies address this through buffer pools, monitoring requirements, and contractual permanence commitments, but the fundamental reversibility distinguishes soil carbon from more permanent removal approaches.

Council Fire's Approach

Council Fire advises on soil carbon sequestration within integrated climate and agricultural strategy. We help landowners, food companies, and investors navigate carbon market participation, design MRV systems that meet registry requirements, and develop landscape-scale programs that stack soil carbon benefits with biodiversity, water quality, and supply chain resilience outcomes.

Frequently Asked Questions

How much carbon can soil actually store?

Technical potential estimates range from 2-5 gigatonnes of CO₂ equivalent per year globally, though realizable potential is lower given economic, behavioral, and biophysical constraints. Individual field-level sequestration rates typically range from 0.5-2.0 tonnes of CO₂ per hectare per year, varying with climate, soil type, baseline carbon levels, and management intensity. Depleted soils have the highest sequestration potential.

Are soil carbon credits credible?

Credibility depends on methodology rigor and MRV quality. Early soil carbon credits faced legitimate criticism for over-crediting and weak permanence guarantees. Current methodologies from established registries have improved significantly, incorporating direct sampling requirements, conservative baselines, and buffer pools. Buyers should evaluate credits based on the specific methodology, verification body, and monitoring protocols rather than dismissing the category wholesale.

How long does it take to build soil carbon?

Measurable increases in soil organic carbon typically appear within 3-5 years of adopting regenerative practices, with accumulation rates highest in the first decade and gradually declining as soils approach a new equilibrium. The timeline depends on starting conditions — severely degraded soils with low baseline carbon can accumulate faster than already-healthy soils. Building a full centimeter of topsoil, however, takes decades to centuries under natural processes.