Differential thermogravimetry and differential scanning calorimetry of soil organic matter in mineral horizons: Effect of wildfires and land use

Wildfires strongly affect the biochemical quality (q) of soil organic matter (SOM), but whether these effects are relevant in the long term is less clear. In Mediterranean ecosystems, the post-fire recovery of vegetation may occur quickly and the new and fresh organic inputs to the soil could mask the effects of wildfires. To verify this hypothesis, we studied a series of plots in Vall de Gallinera (Alacant, E Spain), affected by 0, 1 or 2 wildfires in the last decades and including never cropped plots (NC), old crops early abandoned, or old crops recently abandoned. The study focused on the uppermost 5 cm of mineral soil, where an effect of wildfires is likely to be detected. SOM was studied by differential thermogravimetry (DTG) and differential scanning calorimetry (DSC). The study focused on several indicators of SOM q: (i) T50, i.e. the temperature at which 50% of SOM is lost (in DTG), or at which 50% of the energy content of SOM has been released (in DSC); (ii) the energetic yield, in J/mg OC; (iii) the area of the recalcitrant (thermoresistant) peak, in % of total SOM; and (iv) the exact position of peaks. T50 for DTG was about 348 °C and for DSC about 378 °C. The recalcitrant pool was about 45% of SOM (in DTG), and about 53% of total energy yield (in DSC). The energetic yield was ∼2 J/mg OC. All these parameters were little or not affected by wildfires. Some results suggested a small increase in q with wildfires: the position of the recalcitrant peak shifted to lower temperatures and the energetic yield slightly increased. We did not detect a charcoal signature (either a shift of the recalcitrant peak towards higher temperatures or the presence of an extra-recalcitrant peak). In summary, we did not detect a shift towards a higher dominance of recalcitrant forms in SOM due to wildfires. The post-fire recovery of the vegetation and the inputs of fresh debris to the soil seem to be enough to mask the immediate effects of wildfires on SOM biochemical quality, up to the point that it may even be difficult to detect them in the medium or long term.