Molecular diffusion dominates the transport of gases in the soil. Diffusion through the air-filled pores maintains the gaseous exchange between the atmosphere and the soil, and diffusion through water films of varying thickness maintains the exchange of gases with soil organisms. Diffusion through both pathways can be described by Fick's law,
J = -D dc/dx, where J is the rate of gas diffusion (g cm 2 sec1), D is the diffusion coefficient (cm2 sec-1), c is the gas concentration (g cm-3), x is the distance (cm), and dc/dx is the concentration gradient.
The diffusion coefficient in soil is much smaller than that in air because of the limited fraction of total pore volume occupied by continuous air-filled pores and pore tortuosity, soil particles, and water, reducing the cross-sectional area and increasing the mean path length for diffusion. It is referred to as the effective diffusion coefficient, De, and is a function of the air-filled porosity. In addition to the diffusive path in the air phase of the soil, diffusion of gases in water is ~ 1/10,000 of that in air (Table 2.4). Thus, gaseous diffusion through a 10-pm water film offers the same resistance as diffusion through a10-cm air-filled pore.
The work of Kubiena in the 1930s contributed significantly to our understanding of the nature of soil solid and pore space at the microscopic scale. Much of this early work was based on the examination of thin sections (25 pm thick) of intact blocks of soil. Adaptation of advancements in the acquisition and computer-assisted analysis of digital imagery during the past quarter century have led to the quantitative spatial analysis of soil components. Thin sections represent only a single slice of soil so it is practically impossible to extrapolate observations accurately into three dimensions. Recent developments in microcomputerized X-ray tomography (CT scanning) allow study of the properties of the soil's intact three-dimensional structure. These systems have resolution capabilities down to 10 pm, which allow differentiation of solids, and are able to quantify the distribution of organic and mineral materials. The technology is also able to readily distinguish air-filled and water-filled pore space. Distinguishing microbes from soil particles with this technology, however, is still not possible. A CT image of a soil core in three orthogonal planes is shown in Fig. 2.6. Highly attenuating features like iron oxide nodules appear bright in the imagery, while features with low attenuation capability such as pore space appear dark.
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