A demonstration of the high variability of chars produced from wood in bushfires

Vegetation fires play a major role in global C cycling through the addition of inert carbon (char) to the environment. The objective of this study was to compare and contrast the chemical composition of 53 natural chars collected from the soil surface 6-32. years after a natural fire. In order to achieve this, we applied a recently developed nuclear magnetic resonance (NMR) technique that gauges the degree of condensation of aromatic structures within chars. Our results show that the degree of condensation varied considerably among char samples (n=4-5), collected from burnt-out tree stumps at each of the 11 fire sites. This demonstrates that there is a great degree of variability in the composition of the char produced in natural fires, which is likely to be reflected in widely varying rates of char decomposition. This highlights a major difficulty in quantifying the effects of vegetation fires on global C cycling. Importantly, no differences could be discerned between chars of different ages, indicating that ageing of this type of char in this environment is slow on the decadal timescale. Finally, this study demonstrates that although char samples appear to preserve a record of fire conditions, great care must be taken when interpreting this record to account for the high degree of heterogeneity in char composition. Bulking of char samples would alleviate this problem to some extent; however, bulking would mask the inter-particle heterogeneity clearly evident in this study. © 2012 Elsevier Ltd.Anna V. McBeath, Ronald J. Smernik, Evelyn S. Krul