Influence of drying conditions on wettability and DRIFT spectroscopic C-H band of soil samples

This study assesses the effect of various drying procedures on water repellency measured by water drop penetration time (WDPT) and spectroscopic parameters gauged by Diffuse Reflectance Infrared Fourier Transform spectroscopy (DRIFT) of two anthropogenically-influenced soils at sites in Berlin. Wettable and water-repellent samples were dried at various temperatures and at prescribed relative humidity. WDPT and DRIFT spectroscopic characteristics were obtained from both dried and field-moist subsamples. Normalization of DRIFT spectral C-H band intensity ('surface hydrophobicity') against the integral absorption intensity over the wave number range 4000-400 cm⁻¹ resulted in an apparent resolution between the effects of water content and changes in C-H absorption at the surface. To our current knowledge, the latter could be best explained with changes in the three-dimensional rearrangement of organic molecules or moieties on inner and outer soil organic matter (SOM) surfaces, whereas the former could be a direct consequence of the fraction of inner and outer soil surfaces covered with water and of the mean thickness of the respective water films. Further evidence for this model is required from other investigations focusing on the surfaces in soil, before drawing final conclusions. The results show that the method of drying affects WDPT to a greater extent than soil water content after drying. DRIFT spectra suggest that exposure to high temperature results in some reorganization of SOM in the outer layer. It is further suggested that short exposure may result in a heterogeneous distribution of water leading to localized variation and inconsistency in WDPTs. Drying for 4 weeks under controlled relative humidity at 20°C is suggested as a reference preparation method combining the benefits of an almost unchanged SOM surface compared with field-moist samples with homogeneous moisture distribution.