As global warming intensifies, reducing atmospheric carbon dioxide (CO₂) has become a central challenge. In addition to mitigating emissions at their source, society must actively remove existing CO₂ from the atmosphere and store it in stable reservoirs — an approach known as carbon sequestration. In recent years, emerging strategies such as biochar-based carbon sequestration have gained increasing attention due to their long-term stability and environmental co-benefits. This review examines the mechanisms of biochar-based sequestration and compares it with other major carbon removal pathways.
Introduction to Biochar: How It Captures and Stores Carbon
Biochar is a carbon-rich solid formed by pyrolyzing biomass at ~350–700°C under low-oxygen conditions. Its aromatic carbon structure resists microbial degradation, enabling long-term carbon storage in soils. Typically, 30%–60% of original biomass carbon is retained in biochar after pyrolysis—much higher than natural decomposition.
Biochar Carbon Storage Process
- Carbon capture: Plants absorb CO₂ through photosynthesis and store it in biomass.
- Carbon stabilization: Pyrolysis converts a portion of this biomass carbon into stable biochar that can persist in soils for hundreds to thousands of years.
Biochar vs Natural Carbon Cycle
| Biochar vs Natural Carbon Cycle | |||
|---|---|---|---|
| Feature | Plant Residues | Soil Organic Matter | Biochar |
| Source | Plant residues after harvest or litter | Decomposed residues and microbial products | Biomass converted through pyrolysis |
| Decomposition | Rapid microbial breakdown | Slow microbial breakdown | Highly resistant to decomposition |
| Carbon Retention | 1–20 years | 10–100 years | ~100–1000 years |
| Cycle Type | Short-term natural cycle | Medium-term natural cycle | Human-designed long-term storage |
| Return to Atmosphere | Fast | Moderate | Very slow |
Biochar vs Forest-Based Carbon Sequestration
Forest-based carbon sequestration means forests absorb CO₂ from the atmosphere through photosynthesis and store carbon in woody biomass, roots, leaf litter, and soils. This approach is well-established and provides multiple ecosystem benefits. However, the stored carbon is vulnerable to disturbances such as wildfires, pests, disease, and harvesting, which reduce long-term permanence. Large-scale forest expansion may compete with other land uses.
| Dimension | Biochar | Afforestation |
|---|---|---|
| Source | Waste biomass | New plant growth |
| Carbon Stability | High | Medium–High (released when trees die) |
| Land Dependence | Medium (requires soil application) | High (requires large-scale land) |
| Co-benefits | Soil improvement, yield increase | Biodiversity, habitat restoration |
| Risks | Cost, feedstock supply | Fire, pests, logging |
Biochar vs Soil Carbon Sequestration
Soil carbon sequestration includes various ways of managing land, especially farmland, so that soils absorb and hold more carbon. Increasing soil carbon is accomplished in various ways, including: (1) reducing soil disturbance by switching to low-till or no-till practices or planting perennial crops; (2) changing planting schedules or rotations, such as by planting cover crops or double crops instead of leaving fields fallow; (3) managed grazing of livestock; and (4) applying compost or crop residues to fields.
This approach is cost-effective, compatible with existing farming systems. However, soil carbon is relatively dynamic and can be released back into the atmosphere under changes in management or climate. Additionally, soil carbon stocks may eventually reach a saturation point.
| Feature | Biochar | Soil Carbon Sequestration |
|---|---|---|
| Carbon Source | Waste biomass | Crop residues, organic amendments, and soil management practices |
| Carbon Stability | High | Medium, easily re-emitted |
| Cost | Medium | Low |
| Risks | Feedstock availability, production cost | Poor management can lead to carbon loss |
Biochar vs Ocean Carbon Sequestration
Ocean Carbon Sequestration occurs as oceans store carbon through physical processes like physical solubility (CO₂ dissolution), biological pumps (phytoplankton uptake and sinking), and carbonate chemistry. Human interventions, such as iron fertilization or alkalinity enhancement, are being explored to enhance this natural carbon storage.
| Feature | Biochar | Ocean Carbon Sinks |
|---|---|---|
| Carbon Source | Plant residues or organic waste converted through pyrolysis | Atmospheric CO₂ absorbed by seawater or captured by marine life |
| Carbon Stability | High; long-term fixed in soil | High; but human interventions carry ecological risks |
| Controllability | Production and soil application are controllable | Difficult to control; monitoring and verification are challenging |
| Additional Benefits | Improves soil and agricultural productivity | Large-scale natural carbon reservoir, ecosystem services |
| Risks | Feedstock, production cost | Ecological impact, uncertainty of enhanced sequestration |
Biochar vs Direct Air Capture and Carbon Storage (DACCS)
Direct Air Carbon Capture and Storage (DACCS) systems are technologies designed to remove carbon dioxide (CO2) directly from the atmosphere and then store it in geological formations underground or in other long-term storage facilities. DACCS typically involves several steps:
- Air Capture: DACCS systems use chemical processes or sorbents to capture CO2 directly from the air. This captured CO2 is then concentrated for storage.
- Carbon Dioxide Separation: The captured CO2 is separated from the air and concentrated into a pure stream of CO2.
- Storage: The concentrated CO2 is then transported to storage sites, typically deep underground, where it is injected into geological formations such as depleted oil and gas reservoirs, saline aquifers, or other suitable geological formations. The CO2 is stored securely to prevent its release into the atmosphere.
| Feature | Biochar | DACCS |
|---|---|---|
| Carbon Source | Plant residues or organic waste converted through pyrolysis | Directly captured from ambient air |
| Carbon Stability | High; long-term fixed | High; can be stored permanently |
| Land Requirement | Moderate; applied to soil | Low; flexible deployment |
| Cost and Energy | Medium; dependent on feedstock and process | Very high; requires large amounts of low-carbon energy |
| Additional Benefits | Improves soil and crop yield | Achieves negative emissions; limited ecological co-benefits |
| Scalability | Limited by biomass availability | Theoretically unlimited, but constrained by cost and energy |
Comparative Analysis: Biochar vs. Other Carbon Sinks
| Carbon Sink Method | Stability | Cost | Co-benefits | Controllability | Scalability |
|---|---|---|---|---|---|
| Biochar Carbon Sink | ★★★★★ (long-term stable) | Medium | Improves soil, utilizes waste biomass | High | Medium |
| Forestry Carbon Sink | ★★★ | Low | Significant ecological benefits | Medium | Medium–High |
| Soil Carbon Sink | ★★ ~ ★★★ | Low | Enhances soil fertility | Medium | Medium |
| Ocean Carbon Sink | ★★★★ (naturally large capacity) | Low–High | Technology-dependent | Low | Very High |
| DACCS | ★★★★★ | High | Strong controllability | Very High | Medium |
Overall, biochar sits in a favorable niche: it provides durable carbon storage while delivering tangible agricultural and environmental co-benefits. When feedstock is a waste stream and pyrolysis energy is utilized, the net climate and economic case strengthens.
Conclusion
Biochar represents a multifunctional carbon sequestration pathway that combines durable carbon storage with meaningful co-benefits for soil health, agricultural productivity, and pollution mitigation. Compared with traditional sinks such as forests and soils, biochar offers higher carbon permanence and distinct agricultural advantages, while remaining more affordable than technologically intensive options like direct air capture.
Challenges remain—cost, standardization, and consistent agronomic performance across contexts—but biochar’s potential to integrate waste valorization, climate mitigation, and sustainable agriculture makes it a compelling component of diversified net-zero strategies. In practice, biochar will likely be most effective when deployed alongside forests, soil-management practices, ocean-based strategies, and engineered removal technologies as part of a portfolio approach to climate mitigation.