In Situ DRIFT Characterization of Organic Matter Composition on Soil Structural Surfaces

Abstract

The properties of preferential flow path surfaces may affect transport processes in structured soil. Wetting and sorption depends on the organic matter (OM) at the effective outermost surface of flow paths, which is largely unknown locally with respect to OM composition and distribution. The objectives are to adapt Diffuse Reflectance infrared Fourier Transform (DRIFT) spectral analysis for the in situ characterization of coated crack surfaces and linings of earthworm burrow walls. The Kubelka‐Munk (KM) transformed DRIFT spectra are used to analyze OM composition in terms of the ratio between CH‐ (Band A) and C=O groups (Band B). The A/B ratio indicates a relation between hydrophobic and hydrophilic groups in OM. Disturbed soil samples are arranged (i) in standard cup (CUP) and (ii) larger box (BOX) devices; intact aggregate and burrow surfaces are prepared from clods with cracks and earthworm burrows. The disturbed samples with relatively homogeneous surfaces are used to compare data obtained from CUP with those from the BOX device. The A/B ratios are determined in a stepwise mapping‐type procedure and compared with pictures of the surfaces for inferring a spatial distribution of OM composition along transects. The DRIFT mapping results indicate negligibly small texture effects for the finer‐textural subsoil samples for which the applicability of KM is not restricted. For intact surfaces like soil aggregates, cracks, and earthworm burrow walls, the A/B ratios are relatively higher near a root residue and relatively lower near a quartz grain. The millimeter‐scale variability of OM composition obtained here by an in situ analysis of A/B ratios at structural surfaces seems promising for determination of locally distributed OM properties. The DRIFT mapping may improve understanding the development of soil structural properties, from which descriptions of preferential flow and transport may benefit.