Evaluating the two-pool decay model for biochar carbon permanence

Accurate estimation of biochar carbon permanence is essential for assessing its effectiveness as a carbon dioxide removal (CDR) strategy. The widely adopted framework, based on the two-pool carbon exponential decay model, forms the basis of policy guidelines and national CDR accounting. However, our re-analysis of the meta-data used in this model reveals significant deficiencies in its parameterization, leading to two critical issues. First, the current parameterization assigns a disproportionally low percentage of the labile carbon fraction (C1) relative to the recalcitrant fraction (C2), effectively reducing the model to a single-pool approach. Due to the limited duration of incubation experiments, the decay constant of the labile fraction is incorrectly applied to the entire biochar mass, resulting in a considerable overestimation of the biochar decay rate. Second, our analysis reveals a lack of causal correlation between the assigned proportions of C1 and C2 and key carbonization parameters such as production temperature and hydrogen-to-carbon (H/C) ratios, suggesting that the model does not accurately represent the underlying chemistry. This misalignment contradicts the established relationship between increased biochar stability and a higher degree of carbonization. Consequently, the the parameterization of current model may not adequately reflect the carbon sequestration potential of biochar. While a multi-pool decay model is suitable for predicting the permanence of biochar, the primary issue with the current model lies in its parameterization rather than its structure. To address these limitations, we recommend that future research prioritize the development of a revised multi-pool decay model with improved parameterization, supported by empirical decomposition data from a variety of experimental methods, including incubation studies, accelerated aging experiments, and comprehensive physicochemical characterization. This refined approach will improve the accuracy of biochar permanence estimations, strengthening its role in global carbon management strategies.