Tracking to the C-Sink

Good day ,

We are pleased to share with you the updated rules for tracking biochar C-sinks.
These rules apply to C-sinks submitted for verification in the C-Sink Tool on or after
October 15, 2025.

Key updates at a glance

• From October 15, 2025, biochar no longer needs to be tracked all the way to the end-point if it has been blended into an approved C-sink matrix. All matrices for which end-point tracking becomes optional are marked as “Diffuse C-sink authorized” in the positive list of matrices.
• For all matrices in the category “Biological Matrix”, the contained biochar can be registered as a diffuse C-sink. In these cases, it must be proven that the biochar product is suitable only for agricultural or urban use.
• End-point tracking remains possible at any time – and is strongly recommended.

Issuance of secure C-sinks

For CSI to issue a carbon sink, two essential conditions must be met:

1. Safe embedding of the biochar

The biochar must be incorporated into a matrix that prevents it from being burned or otherwise rapidly degraded, which would release the CO₂ back into the atmosphere within a short time. The risk of reversal must be minimal. The goal is to ensure long-term carbon storage.

2. Scientific accessibility of the matrix

There must be sufficient knowledge about the matrix available to reliably model the degradation of the semi-permanent carbon fraction in biochar. These models, expressed as persistence curves, form the basis for calculating long-term storage.

Ideally, both CSI and the buyer of a C-sink know exactly where the biochar is located. This can enhance the credibility and traceability of a C-sink.

Why spatial distribution matters

When large amounts of biochar are concentrated in a single location, the risks increase:

• Reversal risk: Fire or soil erosion could release a substantial portion of the stored carbon back into the atmosphere.
• Modelling accuracy: Local soil conditions may differ from the assumptions used in persistence models, which could affect reliability. Although this is rare, it must be taken into account when large amounts are concentrated in a single location.

In contrast, if biochar is applied in small, well-distributed amounts – as a rule of thumb, one tonne CO₂e per hectare per year – these risks become negligible, since small deviations average out. To qualify as a diffuse sink, it is sufficient to include a clear dosage recommendation on the product label (e.g. converted into fertilizer amount per hectare).

Examples for biological matrices:

• Slurry - When about 0.5 % biochar is mixed into slurry, and applied at 40 m³ per hectare per year, this equals approximately 0.2 tonnes of biochar per hectare annually – corresponding to roughly 0.6 t CO₂e.
• Animal feed – If one cow consumes about 12 t of feed per year, with 0.1 % biochar added. The cow produces about 20 m³ of slurry annually, of which roughly 40 m³ per hectare and year is applied to fields. This corresponds to the slurry of two cows per hectare, i.e. about 240 kg biochar per hectare annually, which fulfills the conditions for a diffuse sink.

Voluntary tracking adds value

For many matrices, persistence can be evaluated at the time of incorporation, and the risk of reversal is minimal. However, although not mandatory, geo-tracking can still enhance the overall quality and credibility of the sink by adding an extra layer of transparency and assurance.

Voluntary full tracking provides:

• More information for C-sink storytelling.
• Increased legal certainty regarding local regulations or marketplace requirements.
• Improved traceability and long-term transparency.

The Global C-Sink Registry will soon display at a glance whether a sink has been tracked all the way to its final end-point or only up to product (or matrix) incorporation — providing a clear and visible quality marker.

We look forward to presenting the new rules in more detail during our webinar.

For further questions, please don’t hesitate to contact us:

Kind regards,

Carbon Standards International AG
Patrizia Pschera
Product Manager Global Biochar C-sink

E-mail