Saturday, 15 July 2006

Topographic Controls on Soil Thickness in the Ouachita Mountains.

Jonathan D. Phillips, Tobacco Road Research Team, Dept of Geography, 1457 Patterson Office Tower, Univ of Kentucky, Lexington, KY 40506-0027, Daniel A. Marion, Southern Research Station, USDA Forest Service, P.O. Box 1270, Hot Springs, AR 71902, and Kenneth Luckow, USDA Forest Service, Ouachita National Forest, Hot Springs, AR 71902.

Theoretical models of landscape, hillslope, and soil/regolith evolution are often based on a notion of steady-state equilibrium soil thickness, whereby soil thickness is closely related to slope curvature, and rates of soil production are approximately balanced by rates of surface removal. While systematic broad, landscape-scale correlations exist between topography or geomorphic setting and soil thickness in the Ouachita Mountains, Arkansas, at the local scale soils on hillslopes show very little relationship between depth or thickness and topography. At 16 0.13-ha study plots, soil thickness and morphology in more than 400 pits and auger holes was compared to topographic variables, based on meter-scale field mapping. Thickness was poorly related to slope gradient, slope curvature, elevation, and to qualitative characterizations of topographic position. Biomechanical effects of trees on soil thickness, and local structural and lithological variations in the underlying rock appear to account for a majority of the variability in soil thickness, obscuring any topographic effects. The concept of equilibrium soil or regolith thickness cannot be uncritically assumed or applied, particularly in forested environments. The equilibrium model, based on a balance between weathering at the base of the regolith and surficial removals, should be viewed as a special case of a more general model of soil thickness that includes upbuilding and deepening by a variety of geomorphological, pedological, and biological processes.

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