Effect of repeated burnings, fire and charcoal characteristics on natural charcoal re-combustion in a Mediterranean environment as evaluated in laboratory burning experiments

Pyrogenic carbon (PyC) is one of the most refractory compounds in the soil organic carbon pool. A gap between the hypothetical amount of PyC in a certain soil and its presence has been frequently reported. This gap has been thought to be the consequence of PyC loss through different pathways: erosion, leaching, biotic and abiotic degradation. The oxidation of PyC from subsequent wildfires is frequently highlighted as an important driver of its loss in many environments, but empirical evidences from Mediterranean ones are completely lacking. To address this gap, we have simulated different fires in laboratory conditions with two litter types, Mediterranean maquis and pine forest, by placing natural charcoal samples of known origin and composition below the litter to be burned. Three fire simulations were carried out for both litter types using 45 charcoal samples of different sizes and recalcitrance. We therefore evaluated the effect of these latter variables on charcoal mass loss and composition through several subsequent combustions. Results showed that charcoal mass loss mostly depended on the fuel characteristics of the two environments and, ultimately, on fire characteristics, as the charcoal mass loss was much higher in pine litter (50%) than in maquis litter (20%). However, in the latter, charcoal mass loss significantly increased after a second and a third consecutive fire, suggesting that the number of fire events to which charcoal particles are subjected to could be an important variable in the Mediterranean maquis environment. Size and recalcitrance of charcoal did not show any significant relationship with charcoal mass loss. After three fires, the less-recalcitrant charcoal fraction showed a significantly higher consumption, suggesting a role of charcoal recalcitrance in counteracting charcoal consumption after multiple burnings. This study presents the first empirical results suggesting that fire can act as a major driver for charcoal degradation in the Mediterranean forests where the fire-return interval is low, and charcoal particles on the ground might be subjected to repeated burnings.