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ergy-level scanning, which is characterized by scanning with two different energy levels simultaneously. Impor-Numerous soil ecological functions are influenced by soil structure tant results from dual energy-level scanning with x-rays through its impact on spatial and temporal distributions of soil particles, water, and air within the soil profile. The nondestructive tech-and ␥-rays were presented by Phogat et al. (1991) and nique of x-ray computed tomography (CT) was used for studying soil DiCarlo et al. (1997). We apply dual energy x-ray CT structure. X-ray attenuation determined for two energy levels (80 kV using a commercial scanner (i) to determine the variaand 120 kV) was used to calculate distributions of water, air, and tions of water content, dry bulk density, and phase comsolids, as well as the voxel dry bulk density for two silt loam subsoils. position at the microscale and (ii) to measure their spa-The spatial resolution during scanning was 0.25 mm in the horizontal tial distribution in naturally structured soils. These and 1 mm in the vertical direction. For different voxel sizes, the measurements for the characterization of soil structure weighted mean of the derived volumetric water, air, and solid contents, should enhance our knowledge regarding the impact of and the dry bulk densities agreed with the sample's phase composition soil structure on water and gas transport processes. and dry bulk density obtained by weighing. The use of dual energy scanning to study the heterogeneity of soil structure and the spatial distribution of water, air, and solids is discussed. THEORY Soil Phases, the Linear Attenuation Coefficient, and Hounsfield Unit A n important limitation in soil science is the lack The three phases of soil (solid, water, and air) contribute of knowledge concerning the effect of soil structure specifically to the attenuation of x-rays depending on their on functional processes. The application of x-ray comvolumetric fractions. A linear relationship exists between the puted tomography (CT) provides the possibility for a linear attenuation coefficient, soil , as measured for defined

The dependence of macroscopic soil parameters on sampling volume is currently the object of renewed research focus. In this paper, x-ray computed tomography data related to cores obtained in two different locations in a field soil are used to simulate this dependence. Several integration methods are adopted, to mimic different measuring devices. Calculation results, relative to the volumetric water content, volumetric air content, gravimetric water content and dry bulk density, demonstrate that the size (up to 60×60×30 mm3), shape and positioning of sampling volumes influence significantly the measured values of soil parameters. In some cases, the instrumental dependence disappears within a range of sampling volumes, in agreement with a hypothesis underlying the so-called representative elementary volume concept. However, some parameters, like the soil bulk density, do not level off with increasing sampling volumes. These observations open new avenues for research on measurement processes in soils and other heterogeneous media.

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