Effects of moisture conditions on potential soil water repellency in a tropical forest regenerated after fire

Potential water repellency (PWR) is a common index to indicate the degree to which soils repel water. Soil organic matter (SOM) is a requisite substance for the water repellency but many studies have shown that SOM content alone could not fully account for the observed variation in PWR. We investigated potential factors responsible for PWR of soils in a tropical forest in East Kalimantan, Indonesia. In addition to the well-investigated soil properties (e.g., total SOM content, pH, and the amount of iron or aluminum oxides), we also focused on soil moisture content at the time of sampling and water-extractable organic matter (WEOM) content based on the hypothesis that PWR may depend on amphiphilic fractions, including WEOM, in the outermost layer of SOM adsorbed on soil particles — soil water may change the amount and/or the conformation of the amphiphilic fractions. Results showed that the degree of PWR had the highest correlation with the amount of WEOM, not with SOM, among the factors investigated (R2 = 0.29). The WEOM content (R2 = 0.65) better explained the variation in water repellency than SOM content (R2 = 0.47) even after soils were soaked in n-hexane (a non-polar solvent) and expected to have uniform SOM conformation (hydrophobic components dominant on the surfaces). The combination of soil moisture and SOM contents better explained the PWR than SOM content alone. These results suggest that soil water content can have substantial effects on PWR by changing the availability and/or conformation of the amphiphilic SOM, including WEOM.