Thermal alteration of organic matter during a shrubland fire: A field study

Abstract

Vegetation fires profoundly alter the C cycle of terrestrial ecosystems, notably through the potential formation of highly stable pyrogenic structures. Fire-induced changes in the structure of organic matter (OM) have been studied mainly under controlled laboratory conditions. The objective of this work was to characterise changes in OM chemistry occurring in the litter layer of a scrub–oak ecosystem subjected to a prescribed fire. Maximum temperatures reached during the fire were monitored with thermo-sensitive paint. Litter samples collected before and after the fire were subjected to size fractionation, each size fraction being divided on the basis of visual observation into burnt and unburnt components, i.e. black and brown, respectively. All fractions were analysed for C and N contents and stable carbon isotopic composition. Shifts in the composition of >2 mm fraction components were evaluated using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and differential scanning calorimetry (DSC). The effects of charring were evaluated through comparisons of black vs. brown post-fire litter in >370 °C plots. Comparison of bulk pre- vs. post-fire litter proved unreliable because of the complicating effect of fresh litter fall during the fire event. Charring significantly increased the litter C content by 115–142 mg g −1 and significantly decreased the δ 13C value by an average of 0.8‰. The NMR and DSC analyses indicated that O-alkyl compounds were preferentially lost vs. aryl and alkyl compounds. This suggests a preferential loss of cellulose components and a relative preservation of lignin and lipids. However, the charred litter samples had a low degree of condensation vs. a graphitic-like model. The findings suggest that leaf-derived charcoal produced during natural vegetation fires does not contribute much to the highly stable fraction of pyrogenic OM.