Combination of Imaging Infrared Spectroscopy and X-ray Computed Microtomography for the Investigation of Bio- and Physicochemical Processes in Structured Soils

Abstract

Soil is a heterogeneous mixture of various organic and inorganic parent materials. Major soil functions are driven by their quality, quantity and spatial arrangement, resulting in soil structure. Physical protection of organic matter (OM) in this soil structure is considered as a vital mechanism for stabilizing organic carbon turnover, an important soil function in times of climate change. Herein, we present a technique for the correlative analysis of 2D imaging visible light near-infrared spectroscopy and 3D X-ray computed microtomography (µCT) to investigate the interplay of biogeochemical properties and soil structure in undisturbed soil samples. Samples from the same substrate but different soil management and depth (no-tilled topsoil, tilled topsoil and subsoil) were compared in order to evaluate this method in a diversely structured soil. Imaging spectroscopy is generally used to qualitatively and quantitatively identify OM with high spatial resolution, whereas 3D X-ray µCT provides high-resolution information on pore characteristics. The unique combination of these techniques revealed that, in undisturbed samples, OM can be found mainly at greater distances from macropores and close to biopores. However, alterations were observed because of disturbances by tillage. The correlative application of imaging infrared spectroscopic and X-ray µCT analysis provided new insights into the biochemical processes affected by soil structural changes.

Accounting for soil architecture and microbial dynamics in microscale models: Current practices in soil science and the path ahead

“…The description of soil architecture generally relies on cutting-edge imaging techniques (e.g., Baveye et al, 2018;Lucas, Pihlap, Steffens, Vetterlein, & Kögel-Knabner, 2020), such as through the use of X-ray computed tomography (CT), either at synchrotron facilities or using table-top laboratory equipment. The quality and resolution of the images determine the extent to which they may provide 3D spatial information about, for example, the geometry of the pore space, or of water and air interfaces, and about the location of solid particulate organic matter (POM).…”

Perspectives from the Fritz‐Scheffer Awardee 2020—The mutual interactions between roots and soil structure and how these affect rhizosphere processes#

“…Left, 2D slices of 10 cm, 3 cm, and 0.7 cm samples, which show the appearance of small image features visible in small samples. Right, respective pore size distributions of the three image sizes in blue (darker color for decreasing sample size) and their joint distribution in black (Lucas et al., 2020)…”

“…Changes in the physical protection of organic matter due to differences in the porosity and connectivity of rhizosphere may also change the relative distribution of organic matter in biopore/rhizosphere walls (Lucas et al., 2020; Rabbi et al., 2016). Recently, it was shown that macropores between 30 and 150 µm favor carbon storage (Kravchenko et al., 2019).…”

Spatial molecular heterogeneity of POM during decomposition at different soil depths resolved by VNIR hyperspectral imaging

“…Laboratory visible-near-infrared (VNIR) imaging spectroscopy applied to Histosols has been shown to enable the identification of OM with different compositions (Granlund et al, 2021;Steffens et al, 2014). This method was also successfully used for the classification of diagnostic soil horizons and the mapping of several elements (C, N, Fe) at the pedon scale (Hobley et al, 2018;Sorenson et al, 2020;Steffens & Buddenbaum, 2013), as well as for the determination of soil structure arrangement and mapping of soil components (POM, Fe oxides, mineral matrix) at a submillimetre scale (Lucas et al, 2020;Mueller et al, 2021). Combined with modern prediction methods based upon machine learning and artificial intelligence (e.g., random forest or artificial neural networks), this high spatialresolution analytical technique enables mapping of chemical composition in soil profiles at a sub-millimetre scale.…”