Alterations in aggregate characteristics of thermally heated water-repellent soil aggregates under laboratory conditions

The heat generated during wildfires modifies soil characteristics, including soil water repellency (SWR) and the water stability of aggregates, which are known to be interrelated. SWR lowers the rate of water entry into aggregates, minimizing aggregate disruption and subsequent erosion. This study aimed to examine these aggregate characteristics (SWR, water stability of aggregates) of thermally heated water-repellent soil aggregates under laboratory conditions. Water-repellent aggregates were collected from Eucalyptus grandis forest soil separately from four soil depths (0–5, 5–10, 10–15, and 15–20 cm) with varying initial repellency levels. Using an automated programmable muffle furnace, aggregates were separately exposed to three heating temperatures, T_H (150, 200, 250 °C), three rates of heating (speed of rising temperature to reach relevant T_H), R_H (200, 400, 800 °C h⁻¹), and three durations of exposure to relevant T_H, E_D (30, 60, 120 min). The molarity of an ethanol droplet test was used to measure the contact angle (contact angle>90°). The water drop penetration time (WDPT) was also measured. The SWR of aggregates declined with the increasing T_H and E_D. All aggregates were wettable once exposed to 250 °C. At the lowest T_H and E_D (150 °C, 30 min), the contact angle was <90° only in the least repellent aggregates collected from 10–15 and 15–20 cm depths. Although R_H indicated the least influence on the measured parameters, the slowest R_H (200 °C h⁻¹) caused a comparatively greater decline in SWR. Water stability of aggregates increased with heating irrespective of decreasing SWR. Further investigations on heat-induced changes in organic compounds at molecular levels would be necessary to understand the theories for the behavior of aggregates.