Using the BPCA Molecular Marker Fingerprint to Reconstruct the Formation Temperature of Wood Char.

Wood char is an important source of environmental black carbon that contributes to the long term C cycle. Increasing formation temperature may reduce the degradability of wood chars significantly. Information about the formation temperature of natural chars in the environment is difficult to obtain. However, the maximum temperatures experienced by chars might be reflected in their chemical properties.
The analysis of benzene polycarboxylic acids (BPCA) as a quantitative measure for black carbon in soil samples is a well-established method. The oxidation of polycondensated black carbon molecules leads to the formation of BPCA, which subsequently can be quantified by GC-FID. Further, the samples were analysed for BET-N2 specific surface area, char colour and elemental composition (CHNO). The relative contribution of the individual acids to total BPCA depends on the cluster sizes of black carbon (i.e. degree of condensation and aromaticity of the chars) and therefore may represent changes in chemical quality, including the source and the formation conditions of the char analysed. Here, we tested if the BPCA patterns reflect the maximum temperature experienced by laboratory produced wood char. We expected that molecules that indicate a higher degree of condensation of the char like (mellitic acid) to increase with increasing temperature of formation. In fact, results indicate that at higher temperatures the proportion of BPCA reflecting a higher degree of condensation is increasing.
In a next step, the results will be compared to the properties of natural wood chars from slash-and-burn experiments. Our results will provide data to test the suitability of BPCA molecular markers as an explicit measure for the formation temperatures of natural wood chars in order to characterize chars from different environments and to improve our understanding of the fire history of ecosystems.