Simulating wildfires with lab-heating experiments: Drivers and mechanisms of water repellency in alpine soils

Wildfires induce deep modifications in soils. Water Repellency (WR) is one of the prime edaphic properties experiencing alterations upon heating. Despite occurrence, extent and persistence of burning-induced soil WR has been extensively discussed, the dynamics at the basis of its formation (and loss) are still widely unclear. The vast majority of research on this topic has been conducted in the Mediterranean area, even if alpine environments are far from being untouched by fires. Alpine soils are less developed than Mediterranean ones, and differ in mineralogy and composition of organic matter (and thereby present different interactions). We thus wanted to clarify some key mechanisms regulating WR thermal alterations in an understudied environment. Our sampling design aimed at collecting soils representative of the Western Alps. Sample heating was performed in the lab at fixed temperatures (up to 300 °C), on a set of A horizons developed under pine and beech forest covers. Water Drop Penetration Time (WDPT) and Sessile Drop Contact Angle (CA) were used to test WR. Soils were analyzed in terms of organic matter (OM), pH, serpentine relative abundance, texture and DCB-extractable iron (Fe). WR was found to be extremely variable. Soils rich in OM, especially if containing abundant aromatic compounds, were found to be the most hydrophobic at room temperature. In samples exhibiting an increase in WR upon burning, repellency was maximized in correspondence of 200 °C. WR was drastically lost when samples were heated at temperatures greater than 200 °C. Above this threshold, pH systematically increased and the percentage of organic carbon (OC) sharply decreased. The change in pH is likely to have triggered an increase in the negative charge of mineral surfaces, resulting in a significant OM desorption and OC oxidation, eventually leading to a super-hydrophilic behavior in soil. The present investigation evidenced the susceptibility of Alpine soils towards thermal alteration, addressing the factors (organic matter composition and mineralogy) that mostly influence the hydrophobic behavior of these soils.