Relating soil organic matter composition to soil water repellency for soil biopore surfaces different in history from two Bt horizons of a Haplic Luvisol

The deposition of organic matter (OM), which is known for its high potential water repellency, on biopore walls can enhance preferential flow through these pores. In this study, OM composition determined with diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was related to soil water repellency (SWR) determined with sessile drop method, Wilhelmy plate method, and sorptivity tests. We hypothesized that the chemical composition (in terms of potential wettability index; a) is related to the physical properties (i.e., contact angle) of biopore walls, (b) depends on the history of the biopore, and (c) differs from the bulk soil matrix. Thus, the main objective was to identify the relation between OM composition determined with DRIFT spectroscopy and SWR in structured soils. The experiments were carried out on biopores and their surrounding soil matrices, excavated from 2 depths of a haplic Luvisol, with 3 different biopore histories (i.e., root channels, earthworm burrows, and root channels that were short-term colonized by an earthworm). All measurements at intact biopore surfaces indicated a larger SWR at the surface of biopore walls as compared with the surrounding matrices and showed a higher proportion of hydrophobic functional groups. The OM composition determined with DRIFT spectroscopy correlated (R² > .7) with contact angles (sessile drop method) that is in line with results of both water sorptivity and Wilhelmy plate method for soils with reduced wettability. The surfaces of short-term colonized earthworm burrows had the most varying hydrophobic to hydrophilic components (A/B)-ratio of all investigated biopore surfaces depending on soil depth. For biopore surfaces at this depth, contact angles >90° were frequently observed. The results also indicate that earthworms can lower SWR by aggregate disruption.