A clarification statement from researchers
June 10th, 2023
Est. reading time: 5 min
The modern biochar industry can be traced back to the early 2000s and is now growing rapidly. This scaling is supported by financial mechanisms that mainly build upon the expected climate benefits of carbon dioxide removal (CDR) through biochar. The durability of carbon storage is an essential aspect of any CDR method.
We - the authors of and contributors to this statement - are researchers who study biochar durability and have realized that policy and industry need a working answer to the question of “how much carbon is stored in biochar and for how long”, so that biochar deployment can contribute to urgently-needed climate action now, despite remaining knowledge gaps.
This statement was prepared with the aim to synthesize current understanding of the durability of biochar carbon storage, and provide a base for factual discussion. As a consequence, the statement does not provide a definite quantitative answer to “how much, for how long”.
 There is a clear consensus that biochar is much more persistent in soils than fresh biomass.
 There is a clear consensus that biochar produced and used following best practice is a relevant climate change mitigation option. Long-term carbon storage in biochar is one of several reasons for this. The remaining uncertainty about the durability of biochar carbon storage should not delay support for biochar deployment.
 Established quantification methods of biochar persistence (e.g. referenced in IPCC inventory guidelines and used in voluntary carbon markets, to date) provide that on average about 80% of biochar carbon remains in storage after 100 years. Various research efforts are ongoing to elucidate when and how millennia durabilities can be guaranteed, as well as what biochars and environmental conditions may result in lower durabilities.
 The main reason for the high durability of biochar carbon storage is the formation of fused aromatic structures during biomass pyrolysis. A high degree of fused aromatic structures makes biochar much less prone to microbial decomposition than fresh biomass.
 A second reason for the high durability of biochar carbon storage is the physical disintegration of biochar during its aging in soils. As a result, smaller biochar particles are subject to downward movement in the soil profile, ultimately reaching compartments that can guarantee millennial persistence.
 Other processes also play a role in explaining biochar persistence in soils. These include protection mechanisms in soils, such as adsorption to soil particles and organic matter.
 A given biochar sample has a heterogeneous molecular structure, with portions that have different degrees of aromaticity. The less aromatized portion of biochar is subject to faster decomposition. This portion is primarily what decomposes in the short-term and what is measured in laboratory experiments.
 In the field, certain biological and chemical processes can enhance degradation, presumably of the less durable fractions, relative to laboratory conditions.
 This said, there is consensus that once biochar has been applied to soil there is little to no risk of sudden and uncontrolled complete reversal of biochar carbon storage (neither from human activities nor natural events like fires).
 There is a clear consensus that not all biochars have the same properties and that this affects durability. The process conditions needed to obtain biochars with high durability are known. Biochars shall be produced at higher temperatures and with long enough residence time to ensure that carbonisation is complete and fused aromatic structures are formed.
 Biochar characterisation is important. This includes knowing the feedstock type, the pyrolysis conditions, and biochar properties that indicate its degree of aromaticity (e.g. hydrogen to carbon ratio, electrical conductivity).
 Established quantification methods of 100-year biochar persistence (e.g. referenced in IPCC inventory guidelines and used in voluntary carbon markets, to date) extrapolate short-term soil decomposition processes, and do not fully consider the processes that may explain millennial persistence.
 These calculation methods are based on the results of more than 100 individual biochar incubation experiments, of which two have been performed under field conditions.
 An active area of research relevant for biochar carbon storage durability is the development of advanced analytical characterisation methods of biochar that will enable measurement of the physicochemical heterogeneity in carbon structures present in biochar.
 Another area of continued research is biochar incubation, with a focus on field conditions, to elucidate both differences from laboratory conditions, and how transport processes affect biochar in the field.
 Relative to soil and forestry carbon storage, biochar carbon storage is much less subject to risks of reversal (human activities and climatic events). Hence, biochar does not require extensive in-field monitoring.
 Relative to geological storage of carbon dioxide (e.g. direct air carbon capture and storage (DACCS), bioenergy with CCS), biochar carbon storage depends on more parameters like production conditions, soil quality and climate, but is associated with more socio-environmental co-benefits.
 The aim of this text is not to close the scientific discussion. Even though some aspects are generally accepted by the scientific community and sources of variability are largely well understood; other aspects are more uncertain or still unknown. In parallel to biochar production scaling, the research is continuing with multiple groups investigating the remaining uncertainties and unknowns.
 The climate impact of biochar does not only depend on the carbon stored in the biochar and its durability. Other factors such as supply-chain emissions, biomass feedstock source, land use change, effects of biochar on greenhouse gas emissions from soil, biochar effects on plant productivity, and baseline situation are important for a full assessment of biochar as a climate change mitigation option. Likewise, biochar is not just a climate technology, but also has various potential socio-environmental co-benefits and risks to be managed. This being said, this text is limited to discussion of the durability of biochar carbon storage.
 Different terms have been used to describe the durability of biochar carbon storage, but also the physical presence of biochar in soils, e.g. persistence, permanence, recalcitrance, residence times, stability. Today, the term “durability of carbon storage” is preferred in policy contexts, but various academic disciplines such as soil science have other established terms like “persistence”. Here, both durability and persistence are used, rather interchangeably. It is important to be aware of differences in meaning that exist between disciplines.
The statement was finalized by Elias S. Azzi# & Cecilia Sundberg# to whom correspondence can be addressed, with contributions of a wide group of researchers that study the durability of biochar carbon storage. The statement is the result of discussions and collaborative writing within that group, including a series of online meetings held in the two months preceding the 2023 Biochar Summit (Helsingborg, Sweden).
While the statement reflects views that are shared by many contributors, some points of disagreement remain. Thereby, the contributors to the statement are fully acknowledged for their contributions; however, the responsibility for the final statement rests upon the authors.
Contributors, alphabetically sorted:
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