Effects of heat and litter thickness on water repellency of burned forest soils.

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.