Spatial Distribution of Organic Matter Compounds at Intact Macropore Surfaces Predicted by DRIFT Spectroscopy

Abstract

Core Ideas Clay coatings in Bt horizons are enriched in heterocyclic organic matter compounds. The enrichment results from preferential transport in the soil macropore network. These compounds probably originate from long‐term humification and biomass combustion. Py‐FIMS and DRIFT spectroscopy approach enables millimeter‐scale spatial characterization. The illuvial Bt horizon of Luvisols is characterized by coatings of clay and organic matter (OM) at the surfaces of cracks and biopore walls. The occurrence and distribution of OM compounds such as heterocyclic N compounds (NCOMP) and benzonitrile and naphthalene (BN+NA) in macropore coatings could have important effects on preferential transport processes in structured soil. The objective of this study was to estimate the millimeter‐scale distribution of NCOMP and BN+NA at intact surfaces of coated and uncoated cracks, pinholes, and burrow walls. The proportions of NCOMP and BN+NA in OM were determined by pyrolysis–field ionization mass spectrometry (Py‐FIMS) from disturbed material that was manually separated from the intact sample surfaces. Fourier‐transform infrared spectroscopy in diffuse reflectance mode (DRIFT) and partial least squares regression (PLSR) between DRIFT signals and Py‐FIMS data were used to generate millimeter‐scale maps of predicted NCOMP and BN+NA proportions at intact macropore surfaces. The NCOMP and BN+NA proportions were linearly related to C=O and C=C groups from OM and to O–H groups from clay minerals in DRIFT spectra. The millimeter‐scale distribution of NCOMP and BN+NA at intact macropore surfaces could be predicted in terms of relative differences among the macropore types. Increased NCOMP and BN+NA proportions corresponded to the spatial distribution of crack coatings and pinholes. The relative enrichment suggested that pyrogenic OM has been preferentially transported in cracks rather than in biopores and that it was stabilized by clay minerals and prevented from decomposition. The results indicated that preferential transport affects OM turnover processes in the subsoil.