Abstract:
Water repellency (WR) is a phenomenon where soil resists wetting due to the presence of
organic matter (OM). Compositional and structural changes in this OM can alter the degree
of WR, subsequently affecting the bio physicochemical properties of soils. Forest fires can
transform biomass into ash with varying hydrophobicity levels, which in turn affect the soil
hydrology by reducing water infiltration and disrupting moisture distribution. However,
limited information exists on how temperature variations affect WR and functional groups
of litter ash during heating. This study investigated the temperature influences on WR and
functional groups in leaf litter ash of Japanese cedar (CED) and Japanese cypress (CYP). The
litter of these two coniferous species are rich in hydrophobic resins and waxes to induce
WR and are susceptible to wildfires. Leaf litter was heated at 100, 200, 300, 400, 500, and
600 °C for 20 min in a muffle furnace, with a control sample at 28 °C. The degree of WR
(contact angle; θ) and functional group changes were measured with the molarity of
ethanol droplet test and Fourier transform infrared spectroscopy (FTIR), respectively. The
control samples of both species were highly water-repellent (θ = 125° and 119°) for CED
and CYP, respectively. Litter WR decreased with increasing temperature to become
wettable at 600 °C (θ = 90°). In FTIR analysis, three absorbance peaks of B, C, and E were
recorded (wavenumbers = 3020–2800 cm–1, 1640–1600 cm–1, 1170–950 cm–1,
respectively), which consist of hydrophobic functional groups causing ash WR. These peaks
disappeared at 300–400°C, parallel to the disappearance of ash WR. Simultaneously, peaks
D and F (wavenumbers = 1400 cm–1 and 872 cm–1, respectively) appeared, which may not
cause ash WR. These findings highlighted the heating effect on WR and the functional
groups of litter ash, providing insights into post-fire soil hydrology. To reflect real field
conditions, further studies should focus on variations in oxygen availability, heating rates,
and moisture conditions.
