Comparative assessment of novel and traditional biochar stability proxies

As biochar gains momentum as a carbon dioxide removal (CDR) strategy, robust, reproducible, and comparable stability metrics are increasingly needed. A growing number of novel biochar stability assessment methods are being adopted by the carbon crediting industry, however, comparative assessments between these novel approaches and with more established standards, such as molar H:C and O:C ratios, remain limited. Furthermore, there is ongoing dissensus among researchers and producers regarding which proxies most reliably capture long-term biochar stability.

This study aims to evaluate the relationships, strengths, and limitations of multiple novel and traditional biochar stability proxies. Biochars were produced from two common feedstocks, barley straw and chestnut wood, pyrolyzed at 400, 600, and 700 °C under an inert N₂ atmosphere. Biotic incubations were conducted using a soil microbial inoculum to quantify mineralizable carbon using gas chromatography. Borrowing from organic petrography, random reflectance microscopy was conducted as a measure of thermal maturity and Rock-Eval 6 was used to determine thermal stability and organic matter transformation. Lastly, chemical oxidation methods were explored as rapid proxies for biochar reactivity, including comparison of different oxidizing agents: bleach, nitric acid, and hydrogen peroxide (Edinburgh stability tool). Elemental analysis was used to calculate H:C and O:C molar ratios in order to benchmark results against established stability criteria.

Correlation and statistical analysis are used to explore how these techniques relate to each other, as well as the influence of feedstock and production temperature. Preliminary results suggest that while many stability metrics correlate well, some exhibit greater reproducibility across replicates and others demonstrated more sensitivity to methodological or user-related variability. The comparison highlights the importance of method standardization, particularly for emerging stability assessment techniques such as reflectance microscopy. Overall, the results provide insight into how stability metrics align and diverge, informing biochar producers and researchers on selecting tools or multi-proxy approaches to accurately evaluate biochar stability.