Linking Air and Water Transport In Intact Soils to Macro-Porosity by Combining Laboratory Measurements and X-Ray Computed Tomography.

Soil macropores often control fluid flow and solute transport, and quantification of macropore characteristics including their variability in space and time are essential to predict soil hydraulic and hydrogeochemical functions. In this study, measurements of air and solute transport properties and direct macropore visualization by X-ray CT scanning were carried out on 17 large (19-cm diam.; 20-cm length) undisturbed soil columns sampled across a field site (Silstrup, Denmark) with natural gradients in texture and density. Air permeability (k_a) at in-situ water content and -20 hPa of matric potential, and 5% arrival time of tracer (time taken to recover 5% of the applied tracer in the outflow) were measured and linked to macropore characteristics (macro-porosity, macro-porosity of the restricting layer, number of macropores, branches, node density, and length density) quantified via X-ray CT. There was a high positive correlation between the CT-inferred macro-porosity carefully corrected for rock artifacts and the other macropore characteristics. High variability in k_a (4.66 to 78.10 µm² at -20 hPa), and 5% arrival time (0.07 to 2.36 h) were observed. Both k_a and 5% arrival time were strongly positively correlated with macro-porosity (R² = 0.77 for k_a at -20 hPa; R² = 0.59 for 5% arrival time) and macro-porosity of the restricting layer (R² = 0.80 for k_a at -20 hPa; R² = 0.70 for 5% arrival time). High negative correlations with soil bulk density were found for k_a, 5% arrival time, and macro-porosity, suggesting that density is the main control of functional soil structure and gas and solute transport at the Silstrup site. Linking gas transport and chemical tracer experiments with X-ray CT based visualization and quantification of macro-porosity was found to be a powerful method to understand field scale variations in gas and solute transport properties as influenced by soil structure and density.