A unified framework for biochar carbon permanence

Biochar carbon permanence remains difficult to quantify because multiyear soil incubation experiments reproduce biological decay but commonly measure only 1–2% carbon loss from highly carbonized biochars, leaving long-term decay constants poorly constrained. Accelerated thermochemical oxidation resolves the full carbon reactivity spectrum and supports kinetic extrapolation, but it does not directly represent biological decay in soil. This study links the two approaches using raw biomass as a common reference. Multipool kinetic models were fitted to accelerated oxidation data for raw wood, two intermediate biochars, inertinite-rich biochar, and synthetic graphite, and to five published soil incubation datasets for raw biomass. The biological reference rate, defined as the geometric mean of the effective rate constants derived from the incubation datasets, was 51.3-fold, or 1.71 orders of magnitude, faster than the thermochemical decay rate of raw wood. Thermochemical rate constants were therefore uniformly increased by this factor to generate a conservative biologically anchored persistence scenario. After calibration, effective half-lives were 2.06 years for raw wood, 4.38 × 10 3 and 9.15 × 10 3 years for the two intermediate biochars, and 1.25 × 10 5 years for inertinite-rich biochar. At 1000 years, the corresponding modeled fractions of carbon remaining were 0%, 76%, 78%, and 99%, respectively. Permanence increased strongly and nonlinearly with mean random reflectance and was best represented by a double logistic function, consistent with two transitions in carbon stability during progressive structural condensation. This method bridges the limitations of soil incubation and thermochemical oxidation and provides conservative lower-bound estimates of long-term biochar carbon persistence across the carbonization continuum.