Heating response patterns of Alpine soils: from a plot-scale to lab experiments

Wildfires play the role of ecosystem shapers in the majority of terrestrial biomes. Nowadays, their regimes are changing as a consequence of land abandonment and climate change. After-fire dynamics are widely studied in North America and Mediterranean environments. However, soils developed in different biomes might not unequivocally respond to fire-induced heating, and forests of the Western Italian Alps are not unfamiliar to fire occurrence. For these reasons, we conducted several experiments (at plot and lab scale) at environmentally realistic conditions to systematically assess the impacts of fire on the physico-chemical properties of soils belonging to the Italian Alpine ecological region. A homogenous pine forest (Pinus sylvestris L.) located in a mountain region near Torino experienced the passage of a severe and large wildfire in fall 2017. The field survey carried out in 2020 revealed that lower organic carbon (OC) contents and higher bulk density (BD) values were associated to a greater fire severity. Abundance of pyrogenic carbon was related to the steepness degree, as a consequence of erosion. In the superficial horizons, the naturally high WR expected from soils developed under a conifer stand was not present. To elucidate mechanisms regulating WR occurrence and evolution, the thermal transformations borne by Alpine soils were investigated at controlled laboratory conditions. Topsoil samples displayed extremely different wettable behaviors upon increasing temperatures (Ts), with or without WR build-up. This occurred mainly in relation to content and composition of organic matter (OM), particle size distribution and abundance of iron (Fe) oxides. Notwithstanding the initial sample hydrophobicity, WR was dramatically lost above 200 °C due to increasing pH values, inducing OM de-sorption from the negatively charged mineral surfaces. In the same T range, the thermal transformation of soil Fe oxides were found to be primarily directed towards oxidative processes (hematite formation). Ts up to 300 °C could have potentially promoted the stabilization of the remaining (non-combusted) OM, with the synthesis of defect-rich Fe oxides and enrichment in condensed and aromatic compounds, and yet OM was highly dispersible at the high pH values resulting from the thermal treatment, such that OC might be weakly retained on mineral phases in an after-fire scenario.