Chemical Structure and Molecular Dimension As Controls on the Inherent Stability of Charcoal in Boreal Forest Soil

In this study, we analyzed the molecular structure of charcoals from historic wildfires in the boreal forests of Alaska and Scandanavia. We deliberately selected charcoals from organic soil horizons, to investigate the inherent biological and chemical stability of charcoal C without the protective influence of soil minerals. We use ¹⁴C radiocarbon dating to determine the age of the charcoals, differential scanning calorimetry to assess thermal stability, and solid-state ¹³C NMR to assess the chemical structure. Specifically, we employ C-H dipolar-dephasing NMR experiments to estimate the relative abundance and molecular dimensions of condensed aromatic domains and aliphatic structures (see figure). We tested the hypothesis that the stability, quantified as 14C age and thermal stability, is related to the dimension of condensed aromatic ring structures in the charcoal. Our results suggest that the dimension of the condensed aromatic ring clusters may be an important molecular parameter to include in algorithms used to model/predict the residence time of charcoal C in soil. Our findings also have implications for the design of “biochars” for soil C sequestration.