Hydrophobic dissolved organic matter: Preferential retention in soil due to wet-dry cycles

Dissolved organic matter (DOM) is the most mobile and reactive component of the soil carbon pool, yet how changes in water content controls DOM retention in soils is not well understood. Transport and retention of DOM at two comparatively low and high loadings on a homogenized subsoil sample were determined via column breakthrough experiments, where the role of water saturation and two consecutive wet-dry cycles were systematically compared. In the column effluent, dissolved organic carbon (DOC) and dissolved total nitrogen (TN) as well as aromaticity by the specific UV absorption at 280 nm (SUVA) were determined. The total retention of DOC following wet-dry cycles was significantly higher than under water saturated condition. Rewetting particularly led to stronger depletion of hydrophobic fractions in the leached DOM. This was especially evident when the DOM loading before drying was lower. Nevertheless, a consecutive second wet-dry cycle led to comparatively less depletion of hydrophobic fractions in the leachate in both DOM loadings. Significant decreases in soil wettability and surface free energy were also evident after DOM percolation, particularly after drying, as determined by analyzing the sessile drop contact angle. We conclude that wet-dry cycles increase DOM retention in soils. This is generally due to more DOM interactions with soil mineral surfaces amplifying the retention of hydrophobic fractions. The retention of less hydrophilic DOM in the consecutive second wet-dry cycle indicates the formation of layered coatings of higher soil water repellency on mineral surfaces, which can also influence other soil properties.