APPLICATION OF THERMAL AND SPECTROSCOPIC TECHNIQUES TO ASSESS THE STABILITY OF PYROGENIC ORGANIC MATTER IN A BURNT FOREST

Fire events generally are considered as a major disturbance factor in forest ecosystems. The high temperatures may irreversibly alter physical, chemical, and biological soil properties. Fire can totally oxidise soil organic matter (SOM), however, under environmental conditions, the combustion is often incomplete, leading to formation of rearranged macromolecules, with prevailing aromatic nature and characterised by marked recalcitrance to degradation. In a burnt pine forest in Tuscany, Central Italy, we sampled the charcoal layer formed onto the ground and the top ten cm of mineral soil. The latter and the litter layer were sampled in an adjacent unburnt area similar to the burnt one prior to the wildfire occurrence. The aim of our work was to evaluate, combining thermal and spectroscopic analyses, how much fire may influence the biogeochemical stability of SOM. The composition of the intact litter, the charcoal on the ground and the SOM from the mineral soil were analysed by attenuated total reflectance/Fourier transform infrared (ATR/FT-IR), solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, thermogravimetric (TG-DTG), and differential scanning calorimetry (DSC) analysis. Such analytical techniques were used to assess the effect of fire at molecular level, so as to infer the thermal and biochemical stability of the investigated organic pools. Overall, our findings showed that the wildfire, although of moderate intensity, had modified tremendously the chemical structure of the residual SOM, making it little appetizing to the heterotrophic organisms and, hence, able to stay for a long time in soil.