Isolation and Characterization of the Most “Recalcitrant” Organic Matter Fraction in a Chromosol and a Vertosol.

The methods used to isolate and characterize so-called char (pyrolyzed organic materials) from soils vary widely, and there is little agreement in the literature as to which method truly isolates the most chemically recalcitrant and persistent fraction of soil organic matter. Additionally, the relative importance of chemical/structural recalcitrance of organics versus protection by the mineral matrix in the preservation of soil organic matter has yet to be resolved. In an attempt to elucidate the importance of structural recalcitrance to the preservation of soil organic matter, we examined two soils of strongly contrasting morphology using a variety of char isolation techniques coupled with quantifications of the molecular structure and mean residence time of the isolated organic materials. Surface and subsurface soil samples were examined from a Chromosol and a Vertosol. δ¹³C values suggest that charcoal in the Chromosol was sourced from eucalyptus, whereas charcoal in the Vertosol was formed from grass. Soils were sieved at 53 µm, treated with HF, then subjected to three common “char isolation” treatments: chromic acid, photo-oxidation, and chromic acid followed by photo-oxidation. Molecular structure of the organic residues left after each treatment were quantified by solid-state ¹³C CP/MAS NMR and NEXAFS, and the mean age of the organic residues and macroscopic char particles (>53 µm) were estimated based on radiocarbon abundance. In all cases, treatment with chromic acid followed by photo-oxidation isolated the smallest portion of organic matter (5-10% of <53 µm C) which also had the longest mean residence time (estimated 600-3,460 yrs), but not necessarily the highest proportion of aromatic C. Interestingly, in the Chromosol macroscopic char particles were actually depleted in radiocarbon in comparison to post-treatment organic residues, but this was not the case in the Vertosol where macroscopic char was much less abundant. Molecular structure measurements suggest strong differences in the composition of the organics among treatments, and indicate that not all residues were enriched in aromatic compounds.