Application of Infrared Spectroscopy and Thermal Analysis in Explaining the Variability of Soil Water Repellency

Forests play important role in hydrological processes such as evapotranspiration, infiltration, surface runoff, and distribution of precipitation waters. This study evaluates soil water repellency (SWR) in a mountain forest area of Slovakia (Central Europe). Findings of previous studies suggest that the variability of SWR is associated mainly with differences in soil moisture. On the other hand, the role of soil organic matter (SOM) quality in spatial and/or temporal WR changes is less clear, particularly at the plot scale. To measure SOM quality, we used Fourier-transform infrared (FTIR) spectroscopy and thermogravimetry (TG). It was found that FTIR data and the results of thermal analysis are linked to dissimilar wettability of the studied soils. WR samples contained more aliphatic structural units in comparison to wettable soils, which showed a higher relative amount of polar functional groups. Thermogravimetric data suggest that SOM in all 45 samples is relatively labile. This is in accordance with strongly acidic pH and high C/N ratio. The amount of SOM degraded at around 250 °C was significantly correlated with SWR data and at the same time with FTIR peak areas characteristic for aliphatic structural units. This suggests that the accumulation of raw (labile) OM, containing nonpolar functional groups, supports the susceptibility of soils to WR. A significant portion of the variability in WR data was explained by multiple regression analysis, using field water content, FTIR peak areas, and SOM thermal characteristics as predictors. The results confirmed that even the soils occurring in a relatively humid and cold climate may show considerable WR during summer.