Publication Date:
2011-12-17
Description:
Evidence of anisotropy is reported for advective air and water permeabilities in soils. Thus, anisotropy is likely to exist also for diffusive gas fluxes. Information about direction-dependent soil gas diffusivity is scarce and most modeling approaches assume isotropy. At hundreds of closely lying positions in a compacted and adjacent undisturbed forest soil, gas diffusivity ( D s / D 0 ) was measured either in vertical or horizontal direction. The volume-independent diffusion efficiency ( i.e., diffusivity divided by air-filled porosity) was fitted by a generalized additive model (GAM). Significant regressors were air-filled porosity (ϵ), soil depth, and the discrete diffusion direction. The model yields in all cases higher vertical diffusion efficiencies. The compaction factor did not yield a significant regressor of its own, i.e., the reduction of diffusivity in the compacted soil was the same as in low-porosity samples of the undisturbed profile. To elucidate the role of sharing vertically and horizontally orientated pore space and a potential competition between diffusivity in different spatial directions, simple geometric models consisting of 3-dimensionally crossed pores have been parameterized. These models provided a good explanation of the typical nonlinear D s / D 0 (ϵ) relationship. By simple one-parameter correction (linear or power function), this mechanistic model could be fitted to the data. The one-parameter correction of the geometric model could be a straightforward approach to consider direction dependence of measured diffusivities. However, by applying this approach to the observations the anisotropy effect was not clearly evident, which could be attributed to a changing D s / D 0 (ϵ) relationship with depth. As a reason for the preference of the vertical gas diffusion the dominance of vertical stresses and the activity of anecic earthworms are discussed. Direction dependency of gas diffusivity seems to be a basic feature of natural pore systems and has to be considered for modeling gas fluxes in soils. Generally, a preferential vertical diffusion direction reduces horizontal balancing and increases the heterogeneity of gas concentrations in the soil air.
Print ISSN:
1436-8730
Electronic ISSN:
1522-2624
Topics:
Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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