TOWARD AN EVALUATION OF BIOCHAR RECALCITRANCE

Black carbon accounts for a significant portion of soil organic matter, its stability is of great relevance to carbon sequestration and carbon cycle modeling. Therefore, it is important to quantitatively evaluate and predict the stability of biochar from anthropological and natural sources. In this study, biochars were synthesized by pyrolysis using three types of biomass as feedstocks at temperature ranging from 250 to 600 °C. We characterized the biochars by ¹³C solid-state nuclear magnetic resonance spectroscopy (NMR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). By correlating the thermal patterns with NMR spectra, we found that the labile and stable fractions quantified by thermal analysis were equivalent to sp3-hybridized and sp2-hybridized carbon fraction measured by NMR, respectively. Recalcitrance indices (50% burn off temperature) calculated for the respective two fractions behaved differently, which decreased with increasing labile fraction content, while remained relatively consistent for the stable fractions. The results suggest that parameters characterizing the whole sample may not accurately reflect the recalcitrance of biochar, due to the Presence of at least 2 pools of differing thermal stability and heterogeneous composition of biochar. In accordance with findings from the environmental stability of biochars, in which there is a concurrence of initial fast decomposition and long term stability, the labile fraction is likely to be responsible for the initial fast decomposition while the stable fraction contributes to the long term stability of biochar. Therefore, a new conceptual model with separate evaluation on the recalcitrance of the two C fractions seems to be more reliable and relevant. Validation with charcoal samples from a prescribed fire showed that the above analysis remains valid, despite the more complex pyrolysis conditions.