Coupling gas transport measurements and X-ray tomography scans for multiscale analysis in silty soils

Abstract

Scale is a key issue in soil studies. The idea that a sample must be of adequate size to embody a specific physical property comes from the representative elementary volume (REV) concept that defines minimum sample size for consistent results. Other approaches to describe soil spatial heterogeneity rely on the fractal dimension concept (FDC), which assumes structure changes with scale continuously, and the discrete hierarchy concept (DHC), which assumes change occurs discretely. This study considered using gas transport measurements combined with X-ray computed micro-tomography (μCT) for multi-scale analyses. Specifically, 24 large (“L cores”, 628.3 cm3) core volume samples were collected from two farms and two soil depths (3–11 cm and 20–28 cm) in northeast Italy. Gas transport parameters, such as air-filled porosity, air permeability, and gas diffusivity, were measured on the original cores and on successively sub-sampled medium (“M cores”, 100.4 cm3) and small (“S cores”, 4.7 cm3) cores. X-ray μCT–derived porosity indices were calculated for the two smaller scales. Soil core sub-sampling resulted in reduced soil gas transport property measurements, especially in the deepest depth when related to large and continuous bio-pore decreases in root channels and wormholes. In small core volumes, the pore network was dominated by small isolated pores, which might obstruct gas diffusion at that scale. All three concepts named above could be reconciled with our data. The limited numbers of samples and observation scales hindered identifying which model described soil spatial heterogeneity best. Finally, our results suggested the importance of considering scale effects on soil physical properties and their measurement consistency.