Two-dimensional distribution of soil organic carbon at intact macropore surfaces in BT-horizons

Abstract

In structured soils, the surfaces of biopores, shrinkage cracks, and inter-aggregate spaces are often enriched in organic matter (OM) as compared to the matrix. The small-scale distribution of OM at macropore surfaces is affecting preferential flow, mass transfer, and OM turnover in macropores. While the mm-scale spatial distribution of the OM composition at intact surfaces could be observed, the distribution of the organic carbon (OC) content remained limited to date due to a suitable approach. The objective was to develop and test a method for quantifying the two-dimensional (2D) OC distribution at intact macropore surfaces at the mm-to-cm scale. We combined diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and OC analyses of reference samples of few milligrams manually separated from macropore surfaces. DRIFT spectra were measured in a mapping approach at intact surfaces from earthworm burrows, cracks with and without clay-organic coatings, and single pore fillings (‘pinholes’) from Bt-horizons of Luvisols developed on loess and glacial till. The sample’s OC contents were determined with elemental isotope ratio mass spectrometry (EA-IRMS) and related to the DRIFT spectra by partial least squares regression (PLSR). The DRIFT mapping spectra were used to predict the 2D spatial distribution of the OC content at intact macropore surfaces via the PLSR models. The maps revealed mm-scale differences in the OC contents of the macropores which were often related to the 2D distribution of the macropore type surfaces. The validation of the PLSR showed R2 values of 0.77 (loess) and 0.83 (till) between measured and predicted OC contents. The prediction accuracy of the OC values was reduced by the micro-topography of the intact surfaces and by a variable thickness of the clay-organic surface layer. DRIFT mapping spectra from intact soil macropore surfaces were suitable to quantify the 2D OC distribution of the outermost surface layer. The heterogeneous small-scale spatial distribution of the OC contents at macropore surfaces implies a more specific consideration of these macropore walls when studying inter-domain mass transfer and OM turnover in structured soils.