The Biochar Persistance Evaluation
The new biochar persistence evaluation is an innovation from Carbon Standards International.
The new biochar persistence evaluation is an innovation from Carbon Standards International. The content has been coordinated from Ithaka Institute with the scientific community and is based on different scientific publications. The new Upper Persistence Class of Biochar presents a geologically persistent carbon (GPC) of 90%.
The structure of biochar
The principal structure of biochar consists predominantly of aromatic rings, which are structural units composed of six carbon atoms. Hydrogen is bound to those carbon atoms at the outer fringes. These rings are fused into clusters, such that adjacent rings share carbon atoms along edges and hydrogen content is reduced with cluster size. Clusters of fused aromatic rings vary in size, complexity, and degree of structural organization. The biological and chemical stability of biochar increases with increasing aromatization (aromaticity), increasing aromatic ring condensation, and increasing structural ordering of aromatic domains.
| While no biochar carbon fraction can be regarded as completely inert, increasing aromatic condensation and ordering substantially increases the stability of the compounds (Schmidt et al., 2025). |
Biochar carbon fractions dominated by such highly condensed aromatic structures exhibit a markedly increased likelihood of surviving for more than 1000 years after application to soil, as supported by observations of ancient charcoal and pyrogenic carbon persisting over millennial timescales in soils and sediments (Howell et al., 2022). Persistence beyond this timescale is commonly interpreted as a transition from the fast and intermediate carbon cycle into the geological carbon cycle (Schmidt and Hagemann, 2024; Schmidt and Noack, 2000). Biochar carbon that reaches millennial persistence can therefore be considered a geological carbon sink, acknowledging that this classification reflects a probabilistic assessment of long-term stability rather than absolute permanence (see also Chapter 2.2 of the Global Biochar C-Sink Standard).
The Probabilistic approach
The Global Biochar C-Sink Standard does not define a discrete physical biochar fraction that is assumed to persist without degradation and thus be inert for more than 1000 years. Instead, it distinguishes between two conceptual carbon-sink pools based on persistence probabilities following biochar application to soil. In Version 3.2 of the Global Biochar C-Sink Standard, these pools were referred to as persistent aromatic carbon (PAC) and semi-persistent carbon (SPC). To avoid misunderstandings arising from the structural connotation of the term “aromatic”, this annex and the forthcoming Version 4.0 of the Standard replace PAC with the term geologically persistent carbon (GPC).
| GPC denotes the allocated fraction of biochar carbon expected, with high probability, to persist beyond 1000 years after soil application and thus enter the geological carbon cycle. The non-GPC carbon that provides temporary carbon storage and is expected to be oxidized to CO2 within 1000 years after soil application is still referred to as SPC and its degradation function remains unchanged. |
Analytical Methods
While any fraction of biochar carbon may become physico-chemically protected in soil, the probability that a given polyaromatic cluster persists for more than 1000 years increases with cluster size and structural ordering. GPC and SPC do not represent chemically distinct or sharply separable fractions, but probabilistic pools reflecting different likelihoods of long-term persistence.
| To estimate the relative sizes of the GPC and SPC pools for a given biochar, analytical proxies based on readily quantifiable properties are required. The two methods, random reflectance (Ro) and hydropyrolysis (HyPy), can be used to reliably determine carbon pools and persistence classes. |
Our accredited laboratories are ready to perform the analyses.
The Upper Persistance Class of Biochar
Over the past years, additional analytical methods—most notably random reflectance (Ro) and hydropyrolysis (HyPy) have generated a substantial and consistent body of data demonstrating higher discriminatory power for identifying biochars with very high degrees of aromatic condensation and structural ordering. These methods enable a more precise identification of biochars in which the reactive carbon fraction is exceptionally low and the probability of millennial-scale persistence is correspondingly high.
| Based on the expanded analytical data, we introduce in an ongoing pilot project a new “Upper Persistence Class of Biochar”. This is a class for particularly highly condensed biochar in which a 90 percent pool of geologically persistent carbon (GPC) can be certified. |
Following public consultation in the first quarter of 2026, the Global Biochar C-Sink Standard Version 4 is expected to formally integrate a new persistence classification based in the new analytical methods.
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