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SOIL PHYSICS

Water Repellency of Aggregate Size Fractions of a Volcanic Ash Soil

^a Graduate School of Science and Engineering, Saitama Univ., 255 Shimo-okubo, Sakura-ku, Saitama, 338-8570, Japan
^b Environmental Engineering Section, Dep. of Biotechnology Chemistry, and Environmental Engineering, Aalborg Univ., Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
^c Dep. of Agroecology, Univ. of Aarhus, P.O. Box 50, DK-8830 Tjele, Denmark

^* Corresponding author (kawamoto{at}post.saitama-u.ac.jp).

Water repellency (WR) of soils can induce hydrological problems such as reduced water infiltration, enhanced surface runoff and erosion, and the forming of preferential flow patterns in soil. Although soil organic matter (SOM) may cause both soil aggregation and a hydrophobic-material-coating of aggregates, little is known about WR in aggregated soils. We investigated the degree of WR as functions of volumetric water content () and pF [= log (-; soil-water potential)] for sieved fractions of a volcanic ash soil samples from different depths with varying soil organic carbon (SOC) contents of between 1.1 and 12.3%. Water repellency was determined by the molarity of ethanol droplet (MED) test. Water repellency was observed in the samples with SOC 4.6%, and the effects of sample pretreatments (pressure chamber desorption, air-drying at 20°C, and oven-drying at 60°C) on the degree of WR were small. The degree of WR varied greatly with both SOC content, , and pF. Peaks of WR were observed when the water retained in intra-aggregate pores was drained to a moderate extent with the corresponding pF values located in a relatively narrow range from 3.2 to 3.6. This indicates that the hydrophobicity of high-SOC aggregate surfaces might be enhanced the most at a specific soil-water potential. Examining relations between water repellency parameters, the integrated areas below the WR- and WR-pF curves were useful indexes for characterizing WR, and linear relationships between the integrated areas and both SOC and water contents at maximum repellency were found.

Abbreviations: MED, molarity of ethanol droplet • SOC, soil organic carbon • SOM, soil organic matter • WR, water repellency • VG, van Genuchten

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