Response of severity and persistence of water repellency to thermal heating parameters in eucalyptus and pine forest soils in Sri Lanka

Soil water repellency (SWR) relates to plant species such as conifers and Eucalyptus with high amounts of hydrophobic substances such as resins, waxes, and other oily substances, and are highly vulnerable to wildfires. The heat generated through fire can influence SWR, although the changes can deviate based on soil, climatic, and environmental conditions. This study aims to examine the impact of heating temperature and duration of exposure to heat on the severity and persistence of SWR in eucalyptus and pine forest soils in Sri Lankan highlands. Soils collected from three depths (0–5, 5–10, 10–15 cm) were air dried, passed through a 2 mm sieve, and exposed to six heating temperatures (50, 70, 100, 150, 200, 250°C) and five durations of exposure (20, 40, 60, 90, 120 min). SWR was determined using water drop penetration time (WDPT) and soil-water contact angle. Both eucalyptus and pine soils showed water repellent conditions at the field and at air-dried conditions. Both soils reached completely wettable level (WDPT < 5 s) with very low contact angles when exposed to 200–250°C for 90–120 min, except for the contact angle of the eucalyptus 0–5 cm layer that decreased to a minimum of 40°. SWR showed fluctuations with increasing temperature between 28 and 150°C at all durations of exposure. SWR showed significant positive linear correlation with soil organic matter (SOM) content, of which the strength was moderate (R² = 0.64 and 0.50, respectively, for eucalyptus and pine), indicating that the heat-induced change in SWR was not strongly dependent on SOM content. Contrasting transitions in SOM due to removal via various processes and structural and molecular rearrangements might explain fluctuations of SWR at lower temperatures (28–150°C). The findings would be useful for understanding the impacts of various heating dynamics that might occur during forest fires. Further research considering the other important factors, such as molecular-level variations in organic substances as well as mineralogical and climatic aspects, would be important for a better understanding of hydrological responses on burnt forest floors.