Micro-Scale Variability of Soil Porosity as Described by CT Scanning, and Relationship with Soil Gas Diffusion.

Gas movement in porous media is important for many environmental, engineering, ecological, and agricultural questions, including soil-atmosphere gas exchange, respiration of plants and micro-organisms, and various chemical reactions. The number, size, geometry, connectivity, and tortuosity of pores influence gas movement. The complexity of soil pore systems can be described by multi-fractal analysis at the microscopic scale, while soil gas diffusion coefficients (D_s/D_o) are usually measured at the core scale. The goals of this study were therefore to 1) quantitatively describe air-macroporosity from high-resolution X-ray computed tomography (CT) scanning images of ten intact soil cores, 2) analyze the frequency distribution of this air-macroporosity among columns using multi-fractals, and 3) assess the relationships, based on Spearman’s correlation coefficients, between D_s/D_o measured in laboratory and the multi-fractal parameter estimates describing pore structural variability. Depending on the threshold used to define air voxels in CT images, air-macroporosity varied from 0.001 to 0.295 air_voxels.total^-1_voxels, while measured columns air-filled porosity ranged from 0.186 to 0.305 m³_air.m^-3_{dry soil}. The multi-fractal behavior of the air-macroporosity distribution differed between groups of soil columns, based on the shape and symmetry of their singularity spectrum and the degree of curvilinearity of their Rényi spectrum. Correlations found between D_s/D_o and some parameter estimates of the singularity spectra suggest that the air-macropore concentration, lower or higher depending on the image thresholding, could influence gas diffusion. A strong correlation between D_s/D_o and the entropy dimension could mean that the degree of heterogeneity of the air-macroporosity distribution within a column was crucial for gas diffusion. The correlation dimension (D_q=2) was strongly linked to D_s/D_o, which suggests that a second-order power law could describe the scaling relation between the air-macroporosity distribution and D_s/D_o. Thus, the micro-scale description of soil pore variability may be regarded as a way to improve our understanding of gas movement in soils.