Millimetre-scale distribution of organic matter composition at intact biopore and crack surfaces

摘要

Structured subsoil horizons are characterized by biopores and shrinkage cracks, which may serve as preferential flow paths. The surfaces of cracks and biopores may be coated by clay-organic material. The spatially-distributed organic matter (OM) composition at such structural surfaces was studied at the millimetre scale using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy in the mid-infrared range (MIR). Intact biopores such as earthworm burrows and root channels, and crack surfaces of nine subsoil horizons were analysed. The samples were from arable and forest Luvisols, one Regosol, one Stagnosol and Cambisols developed from loess, till, mudstone and limestone. For better comparison between soils, the DRIFT signal intensities were corrected for the particle-size effects. The OM was characterized by the ratio between alkyl- (C-H) and carbonyl (C=O) functional groups (C-H/C=O), which represent an index of the potential wettability (PWI) of the OM. The PWI was larger for biopores than for crack surfaces and the soil matrix, indicating a smaller potential wettability of OM at biopore surfaces. The millimetre-scale spatial variability of OM was especially large for the surfaces of root channels. Samples from till-derived Luvisols had smaller PWI (with greater potential wettability than surfaces from loess-derived Luvisols) than other soil types. The mean PWI of the arable Luvisol crack surfaces was less than that of the forest Luvisol samples. The results suggest that the spatial distribution of OM properties at intact structural surfaces may be important for describing sorption and mass transfer processes during preferential flow.