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Water repellency in Japanese coniferous forest soils as affected by drying temperature and moisture

Soil water repellency occurs where the attraction between solid and liquid phases is weak. The objective of this study was to examine the occurrence and behavior of water repellency in Japanese coniferous forest soils as affected by drying temperature and moisture. Soil samples were obtained from forest soils under Chamaecyparis obtusa (CYP) and Cryptomeria japonica (CED), and an uncultivated land (weed and grass cover) in the Field Science Center of Ibaraki University (FSC) in Ibaraki prefecture, Japan. The persistence of soil water repellency (SWR) was estimated using the water drop penetration time (WDPT) test and the severity of SWR using the molarity of ethanol droplet (MED) test. In Experiment 1, soil samples were exposed to 25 °C and 35 °C temperatures for a period of 30 d. SWR and moisture contents were measured at weekly intervals. In Experiment 2, moisture-dependent repellency was examined in a drying process. CED soil was extremely repellent (WDPT>3600 s; contact angle>106°) and FSC soil was non-repellent (WDPT<1 s). The CYP soils were strongly (35 °C) to severely (25 °C) repellent compared with CED soils, and more prone to significant changes caused by temperature and duration. In CED soil, the changes in contact angles were slight while moisture content variations were significant. In CYP soils, the changes in repellency were significant while moisture content variations were slight. The integrated area below the water-dependent repellency curve (SWR) was greater in CED soil (~140 log s g 100 g−1) than in the CYP soils (~60 log s g 100 g−1). Slight changes in moisture content may or may not cause severe changes in SWR depending on the level of moisture content. These alterations in moisture content in the surface soils may occur as a result seasonal environmental changes and contribute to topsoil loss in mountain areas with high slopes.