The H/C molar ratio and its potential pitfalls for determining biochar's permanence

Biochar carbon removal (BCR) is widely recognized as a globally feasible technique for removing CO₂ from the atmosphere and storing carbon in a stable form within the environment. The hydrogen-to-carbon (H/C) molar ratio serves as the primary proxy for classifying biochar into different quality categories and is a key parameter in decay models used to estimate its long-term stability. In the context of climate credit systems that rely on biochar for carbon sequestration, an accurate assessment of biochar's carbon pools and permanence is crucial. The results of this study confirm that the H/C molar ratio is a robust bulk geochemical proxy for biochar carbonization. However, its use as a standalone benchmark for biochar permanence should be approached with caution. To ensure a more comprehensive assessment, the H/C molar ratio should be combined with the random reflectance (R_o) method, which provides spatially resolved insights into the degree of carbonization within a biochar sample. Relying exclusively on a single bulk H/C molar ratio may, in some cases, lead to inaccurate determinations of biochar's carbon storage security. Such limitations could undermine the credibility of climate credit systems that depend on biochar for permanent carbon dioxide removal. Therefore, integrating both H/C ratio and R_o analysis is essential for accurately evaluating biochar stability and its long-term carbon sequestration potential.