Application of thermal and spectroscopic techniques to assess fire-induced changes to soil organic matter in a Mediterranean forest

• Fire markedly impacted soil organic matter in a Mediterranean forest.
• Fire caused preferential loss of labile soil organic matter.
• Fire apparently induced formation of stable heterocyclic N containing molecules.

Organic matter is the soil component most sensitive to fire, being the fuel on the ground and below. It is affected by fire in terms of both abundance and composition. Incomplete combustion yields condensed carbon forms characterized by higher recalcitrance to biological and chemical attack compared to the parent material. In this study, thermogravimetry (TG-DTG), differential scanning calorimetry (DSC), attenuated total reflectance/Fourier transform infrared spectroscopy (ATR/FT-IR), and Raman spectroscopy were applied to (1) charcoal particles accumulated on the ground, (2) the top 10 cm of soil, and (3) the organic matter extracted using an alkaline solution, to investigate the wildfire effect on the organic pool of the loamy and sub-acid soil of a maritime pine (Pinus pinaster Ait.) forest. The same investigations were performed on the litter layer and the mineral soil of an adjacent unburnt area, similar to the burnt one before fire occurrence. TG-DTG and DSC measurements highlighted an accumulation of soil organic matter (SOM) in the burnt soil, which suggests an incorporation of partly charred necromass into soil and the formation of pyrogenic highly refractory organic matter, i.e. black carbon. The ATR/FT-IR showed changes in the fire affected extractable SOM compatible with the formation of stable N containing compounds, which was well confirmed also by the presence of CN group vibration in the Raman spectrum. The combined application of thermal and spectroscopic techniques resulted to be highly useful to rapidly estimate many of the effects of fire on soil organic matter.