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

Abstract

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.

The Effect of Heating on Properties of Sandy Soils

“…Negri et al [36] observed an increase in WDPT with increasing heating temperatures in an interval from 50 • C to 200 • C and a dramatic drop to 0 for the heating temperature of 250 • C in Regosols and Cambisols. Perera et al [37] found that both the persistence and severity (estimated using the Molarity of an Ethanol Droplet test) of SWR of aggregates from sandy soil decreased with the heating temperature and all aggregates were wettable once exposed to 250 • C.…”

Heat-induced alterations in moisture-dependent repellency of water-repellent forest soils: A laboratory approach with Japanese Andosols

“…The average value of three replicates was considered as the WDPT value of a particular sample. The measurement was terminated after 3600 s, and the penetration times exceeding 3600 s were considered as 3600 s. The SWR of samples can be categorized based on the WDPT values, as wettable (WDPT ≤ 1 s), non-repellent (WDPT = 1-5 s), slightly (WDPT = 5-60 s), strongly (WDPT = 60-600 s), severely (WDPT = 600-3600 s), and extremely water-repellent (WDPT = ≥3600 s) (Bisdom et al, 1993;Perera et al, 2023).…”