Charcoal re-combustion efficiency in tropical savannas

Abstract Mass balance considerations suggest a significant gap in our understanding of the processes by which pyrogenic carbon (PC) is re-mineralized in the environment. Re-combustion by subsequent fires has been evoked as a plausible mechanism explaining significant losses of PC in fire-prone ecosystems, a claim not yet backed up with experimental data. In this study, four burning experiments were conducted in two northern Australian open savanna woodlands subject to regular prescribed fires to specifically assess charcoal re-combustion. The experimental burns were designed to provide a set of biotic and abiotic conditions capable of maximizing the re-ignition potential of surface charcoal fragments. We also tested whether the size of charcoal fragments had a significant effect on their combustion potential. Although temperature profiles for each burn indicated that the conditions for potential combustion of the charcoal were met, out of the 264 charcoal pieces being monitored only five were totally combusted. Charcoal mass losses were independent of particle size, and averaged less than 8%. The results suggest that turnover times for charcoal in tropical savannas as a result of re-combustion alone are on the order of one century. The comparatively long turnover time ascertained under experimental conditions designed to maximize charcoal re-burning potential shows that re-combustion is not an efficient sink for pyrogenic carbon in tropical savannas. This finding strongly suggests that other processes must play a more substantial role in the re-mineralization of charcoal in order to balance the PC budget in these ecosystems.