Mass‐balance approach to quantify water distribution in soils based on X‐ray computed tomography images

Abstract

X‐ray computed tomography (CT) is commonly used in soil science to quantify distributions of pores, water and solids within soil samples. Particularly promising for liquid visualization is dual‐energy scanning of samples with added dopant solutions. The approach uses solutions of heavy elements as a proxy for the soil water and utilizes the abrupt increase of the mass attenuation coefficient of heavy elements when crossing an X‐ray absorption edge, detectable by dual‐energy CT scanning. However, a key step in visualization of the added liquid is image segmentation. In partially saturated soils, the X‐ray CT images consist of voxels with the presence of solid, liquid and gaseous phases in different ratios. These ratios are generally ignored by the traditional image segmentation methods, which separate image voxels into groups with either zero or complete saturation by either prevailing phase. The objective of this study was to develop and test a new physically based segmentation approach that accounts for partial saturation of image voxels by liquids. We combined a theoretical equation for detected photon intensity with the mass conservation equation to compute saturation of image voxels by dopant solution applied to the soil. This approach was tested on soil micro‐columns with potassium iodide (KI) solutions. The proposed approach outperformed the traditional segmentation methods, particularly at low and intermediate water contents. Tests conducted for KI solutions in the concentration ranges from 2 to 20% revealed deterioration of the method accuracy with decreasing dopant concentration in the soil. Based on the results of this study we recommend 10 wt% KI solution for quantifying spatial distribution of liquids in soils using dual‐energy X‐ray microtomography. Highlights We developed a new approach to quantify soil water distribution based on X‐ray CT. This approach accounts for partial saturation of image voxels by liquids. This approach outperforms the standard image segmentation methods. Its application is limited to using dual energy monochromatic X‐ray beams.

Composition and metabolism of microbial communities in soil pores

“…We employed multi-energy X-ray computed tomography (µCT) scanning to visualize the specific locations of and volumes occupied by the liquids added to pores of different sizes. Specifically, to characterize the distributions of glucose solutions when added to the cores, three intact mini-cores (8 mm Ø and 5 mm height) were subjected to multi-energy X-ray µCT scanning after adding the dopant solutions (10% KI and 10% BaCl 2 ) to the small and large soil pores 104 . Since glucose at the concentrations used in this study does not precipitate and the water-glucose solution has a similar viscosity to the KI and BaCl 2 solutions, we assume that the distribution of glucose in soil pores does not differ significantly from those for the dopant solutions when applied in the same volumes and at the same matrix potentials.…”

Soil pores created by diverse plant communities drive microbial community composition and processes with ecosystem consequences

“…We employed multi-energy X-ray computed tomography (µCT) scanning to visualize the specific locations of and volumes occupied by the liquids added to pores of different sizes. Specifically, to characterize the distributions of glucose solutions when added to the cores, three intact mini-cores (8 mm Ø and 5 mm height) were subjected to multi-energy X-ray µCT scanning after adding the dopant solutions (10% KI and 10% BaCl2) to the small and large soil pores 68 . Since glucose at the concentrations used in this study does not precipitate and the waterglucose solution has a similar viscosity to the KI and BaCl2 solutions, we assume that the distribution of glucose in soil pores does not differ significantly from those for the dopant solutions when applied in the same volumes and at the same matrix potentials.…”

Editorial for the special issue on “Advances in soil scaling: Theories, techniques and applications”

“…Segmentation of images (i.e., separation of pore space from solids) has been and remains a necessary preprocessing operation before any study of scale dependencies at the pore or aggregate scale. This operation with X‐ray CT imagery can lead to additional uncertainty, especially if parts of the pore space are filled with loose organic material (Gerke & Karsanina, 2020) or include incompletely saturated pores (Guber, Kutlu, Rivers, & Kravchenko, 2020). The latter authors proposed an elegant solution for the segmentation problem based on saturating the soil sample with a salt solution that exhibits an abrupt and notable change of the mass attenuation coefficient at the threshold level of the beam energy.…”