Quantifying pyrogenic carbon from thermosequences of wood and grass using hydrogen pyrolysis

Previously studied thermosequences of wood (chestnut) and grass (rice straw) biochar were subjected to hydrogen pyrolysis (hypy) to evaluate the efficacy of the technique for determining pyrogenic carbon (CP) abundance. As expected, biochar from both wood and grass produced at higher temperature had higher CP amount. However, the trend was not linear, but more sigmoidal. CP/CT ratio values (CT = total organic carbon) for the wood thermosequence were ⩽0.03 at biochar production temperature (TCHAR) ⩽ 300 °C. They increased dramatically until 600 °C and remained relatively constant and near unity at higher biochar production temperature. Grass biochar was similar in profile, but CP/CT values rose dramatically after 400 °C. The findings are consistent with the hypothesis that hypy residues contain polycyclic aromatic hydrocarbons (PAHs) with a degree of condensation above at least 7–14 fused rings, with labile organic matter and pyrogenic PAHs below this degree of condensation removed by hypy. ood and grass thermosequences displayed δ13CP values that decreased with increased TCHAR, indicating that recalcitrant carbon compounds (pyrogenic aromatic PAHs with a relatively high degree of condensation) were first formed from structural components with relatively high δ13C values (e.g. cellulose). Relatively constant δ13C values at TCHAR ⩾ 500 °C suggested the dominant pyrolysis reaction was condensation of PAHs with no additional fractionation. Comparison of hypy with benzene polycarboxylic acid (BPCA), ‘ring current’ NMR and pyrolysis gas chromatography–mass spectrometry (GC–MS) results from the same suite of samples indicated a consistent overview of the structure of CP, but provided unique and complimentary information.