Abstract:
Soil water repellency (WR) is a global phenomenon that limits spontaneous water entry into soils.
Forest soils under specific vegetation types, which are rich in hydrophobic materials often show
WR. Forest fires burn organic litter materials in forest floors into ash affecting WR. Factors like
fire intensity, litter thickness, and heating conditions can affect ash properties. Available findings
on changes in carbon (C) contents in forest soils that are water-repellent (WRS) and non-repellent
soils (NRS), and litter material under different heating temperatures in and soils remain
uncertain. This study aimed to explore how different heat parameters and litter thickness
influence hydrophobic conditions of litter ash and underlying soils, considering the C loss. Soil
and litter samples were collected from Casuarina (Hambantota) and Mahogany (Mapalana)
forests, representing WRS and NRS, respectively. Soil samples were placed in containers, the litter
was added in three thickness levels (3, 6, and 10 cm), and then subjected to laboratory burning
at eight temperatures (150, 200, 250, 300, 350, 400, 500, and 600°C) under two exposure
durations (20 and 40 min). After heat treatments, WR was measured on both litter ash and soil
surface using water drop penetration time test. Organic matter contents of soil and litter were
determined by Walkley-Black and loss on ignition methods, respectively. Both Mahogani litter
(ML) and Casuarina litter (CL) initially showed WR. On both litter ash surfaces, WR increased with
increasing temperature up to 350°C and then decreased to disappear as the litter ash reached
high burning level. CL showed a consistent surface WR was under all litter thicknesses. This might
be because the narrow regular morphology of CL can facilitate homogeneous burning. ML showed
the highest surface WR when the litter layer was the thickest. The broad irregular morphology of
ML might influence relocation of hydrophobic volatile substances influencing WR at different
litter thicknesses. Top layer of litter showed the lowest organic matter content for all three litter
thicknesses, where the loss of organic matter decreased with increasing thicknesses. Weak to
moderate correlations existed between SWR and organic matter contents. Organic matter
contents showed strong negative correlations with temperature in both Mahogany and Casuarina
litters. Results revealed that the litter thickness highly influence the surface WR in burned forest
soils as well as the loss of terrestrial C to the atmosphere, and the increasing temperature and
duration of exposure intensifies the C loss.
