What is biochar, and does it lock carbon away?

Biochar is a charcoal-like solid made by heating plant material, such as crop residues or wood, with little or no oxygen. It can be mixed into soil, where much of its carbon may stay for a long time. That is why it is studied both as a way to take carbon out of the air and keep it stored, and as a soil amendment that might change crop yields.

The idea is not new. In a 2010 paper in Nature Communications, Woolf and colleagues estimated the maximum sustainable potential of storing biochar in soil to reduce climate change, an idea that others had already suggested. They also wrote that substantial uncertainties exist, however, regarding the impact, capacity and sustainability of biochar at the global level. That caveat still frames the debate.

How biochar is made and how it stores carbon

Biochar is made by pyrolysis, which heats biomass with little or no oxygen. Woolf and colleagues describe biochar as the carbon-rich solid that pyrolysis produces. Most of its carbon is in a form that decays very slowly once it is in soil.

How long it lasts is measured with decomposition studies that use isotope-labelled carbon. The labelling lets researchers tell biochar carbon apart from the soil's own carbon. Wang, Xiong and Kuzyakov (2015) pooled 128 observations from 24 studies and fitted a two-pool model. In that model, about 3% of biochar carbon has a mean residence time of roughly 108 days, and about 97% has a residence time of roughly 556 years. The long figure is a model output, not a direct measurement, and it rests on a limited set of studies.

What has been demonstrated

Here, "demonstrated" means measured in peer-reviewed studies, with the caveats given.

Slow decay in soil. The isotope studies pooled by Wang and colleagues show that most biochar carbon decays very slowly. A second analysis in the same paper found that biochar slightly slowed the breakdown of the soil's existing organic matter overall, although in sandy soils it sped that breakdown up.

Average yield gains, with wide spread. Jeffery and colleagues (2011), in Agriculture, Ecosystems and Environment, reported a grand mean increase in crop productivity of 10%. Individual analyses ranged from minus 28% to 39%. The gains were largest on acidic and neutral soils and on coarse or medium-textured soils. Biosolids were the only feedstock with a statistically significant negative effect. The authors note that the available studies were short, limited to one to two years.

Regional differences. Jeffery and colleagues (2017), in Environmental Research Letters, found that biochar has, on average, no effect on crop yield in temperate latitudes. They found an average increase of about 25% in the tropics, which they attribute to liming and fertilisation.

What is only reported

Here, "reported" means figures from a certification body, a market tracker or a press release. None of these have been independently verified in this article.

Certificates. ESG Today, republishing a Puro.earth press release, dated 20 March 2025, says the registry reached 1 million carbon removal certificates in the first quarter of 2025, with biochar accounting for 34.1% of issuances. Issuing a certificate is not the same as the carbon being stored.

Contracted volumes. CDR.fyi's 2025 market snapshot reports 3.04 million tonnes of biochar carbon removal credits contracted from the start of 2022 to the end of the second quarter of 2025. It says 1.6 million tonnes were contracted in the first half of 2025 alone. Contracts are commitments to buy, not confirmed removals.

Delivered and retired volumes. The same tracker is inconsistent. It gives delivered totals of 658 thousand tonnes in one place and 683 thousand in another. Retired totals are 302 thousand and 330 thousand tonnes. It does not explain the gap. On its own figures, delivered tonnes were about 22% of contracted tonnes at the end of Q2 2025. The snapshot, published in September 2025, stops at that point, and this article has not checked later figures.

What is speculative

Several things are plausible but not shown by the evidence reviewed here.

Scale. Woolf and colleagues estimated a sustainable potential for soil biochar, but their own abstract says substantial uncertainties remain. Moving from pilot-scale projects to volumes that would matter for the climate is not demonstrated.

Mainstream carbon removal. Biochar is sold as carbon removal, but whether it becomes a standard part of climate policy or corporate offsetting depends on factors that the sources do not cover, including how credits are priced, verified and accepted.

Niche uses. Some proposed uses are untested in the sources reviewed. These include blending biochar into growing media for indoor farms, making biochar on farms from crop residues to recycle nutrients. Jeffery and colleagues (2017) suggest focusing yield-related use on low-nutrient, acidic tropical soils, but whether such targeting works at scale is not tested in these sources. Each is a hypothesis, not a result.

Limits and open problems

Permanence. The long residence time is a model result based on a limited number of studies. Whether the carbon stays stored for centuries in real fields, under real climates and management, is not measured directly.

Measurement and verification. Credits are issued under registry methodologies. This article did not review those methodologies, so it cannot say how permanence or the amount of carbon removed is checked.

Soil effects vary. Yield results depend on soil type, feedstock and application rate. In the 2011 review, the greatest positive result came from applications of 100 tonnes per hectare. Biosolids gave a clear negative result. Results from one to two years may not hold for longer periods.

Reporting. Credit volumes, deliveries and retirements are reported inconsistently, which makes it hard to check how much carbon has actually been stored.

What to watch next

• Longer field trials that test whether yield patterns hold beyond one to two years, a gap the 2011 review pointed to.
• Full-year 2025 figures from CDR.fyi or registries, and whether they reconcile the delivered and retired totals.
• Whether Puro.earth reached a second million certificates by the end of the first half of 2026, as its March 2025 release projected. This article has not checked registry data on this.
• Published methodology and verification rules for biochar credits, which were outside the scope of this article.