Fractionation of fresh natural charcoal on a size basis allows obtaining materials with different fuels source, composition and recalcitrance

Charcoal, the black particulate residue from incomplete burning of biomass, is a heterogeneous material showing a quite large range of properties. This range can be even greater for charcoal produced during wildfires. This variability represents a serious limitation to those studies based on natural charcoal, which, on the other hand, are necessary for a meaningful representation of natural environments. In this study, we tested the hypothesis that particle size is suitable to separate charcoal into fractions with distinct properties. For this purpose, at two forests involved in quite different wildfires in terms of severity, we collected and fractionated the charcoal into four size fractions (>2, 2–1, 1–0.5, <0.5 mm) by sieving. Then, we analysed all the fractions for ash content, elemental composition, chemical structure (by IR spectroscopy), fuel source and charcoal reflectance (by reflected-light microscopy), and chemical and thermal recalcitrance (by chemical and thermal oxidation). At both sites, the finest fraction (<0.5 mm) was by far the most abundant. C concentration and C/N ratio decreased with decreasing size fraction, while pH and the ash content showed the opposite trend. No clear trend was observed for N concentration or δ13C. The coarse fractions showed a higher contribution of amorphous carbon and a higher recalcitrance. We also observed that certain fuel types were preferentially sorted into particular size fractions. We conclude that the differences occurring between charcoal size fractions are most likely primarily imposed by fuel source and secondarily by fire conditions. Size fractionation, therefore, is a valuable tool to characterise charcoal, as each fraction displays a narrower range of properties than the sample as a whole. The most suitable number of fractions to be differentiated should be defined on case by case basis. We propose the ash content of charcoal fractions as a reliable indicator of substantial similarity of two or more fractions