Abstract

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 (C-P) abundance. As expected, biochar from both wood and grass produced at higher temperature had higher C-P amount. However, the trend was not linear, but more sigmoidal. C-P/C-T ratio values (C-T = total organic carbon) for the wood thermosequence were = 0.03 at biochar production temperature (T-CHAR) = 300 degrees C. They increased dramatically until 600 degrees C and remained relatively constant and near unity at higher biochar production temperature. Grass biochar was similar in profile, but C-P/C-T values rose dramatically after 400 degrees 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. Both wood and grass thermosequences displayed delta C-13(P) values that decreased with increased T-CHAR, indicating that recalcitrant carbon compounds (pyrogenic aromatic PAHs with a relatively high degree of condensation) were first formed from structural components with relatively high delta C-13 values (e. g. cellulose). Relatively constant delta C-13 values at T-CHAR >= 500 degrees 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 C-P, but provided unique and complimentary information. (C) 2013 Elsevier Ltd. All rights reserved.