Monday, 10 July 2006

Geomorphic Influence on Southern Driftless Area Soilscapes.

Krista Stensvold, Dept of Soil Science, Univ of Wisconsin-Madison, 809 East Johnson Street, Apt. E, Madison, WI 53703 and Cynthia Stiles, Univ of Wisconsin, Dept of Soil Science, 1525 Observatory Drive, Madison, WI 53706.

A toposequence of 12 upland soil pedons within a first-order watershed in the southern Driftless Area of Wisconsin was studied to determine the influence of aspect on soil development. The pedons were located on summit, shoulder, backslope, and footslope positions on contrasting north and south aspects (1-20% slope). Physical, chemical, and mineralogical analyses of each profile were conducted to give complete characterizations of each profile. The southern Driftless Area landscape and soils have been subjected to both periglacial conditions during the Pleistocene and temperate climates during the Holocence. The soils of this soilscape are derived from late Pleistocene loess and dolomite residuum. The clay mineralogy at these sites reflects the contributions to these soils from late Pleistocene loess. Each pedon is made up of silty Ap and Bt horizons overlying a clay rich 2Bt horizon. Below the 2Bt is a fine sandy loam saprolite that has developed from the underlying dolostone bedrock. Differences in soil properties on contrasting slope aspects over a range of slope gradients are attributed to variations in energy inputs (radiant and gravitational) into the system over time. Percent increase of clay content in the subsurface horizon (2Bt), the zone of maximum accumulation, was significantly higher on south slopes (41.6%) than on north slopes (36.1%). Maximum accumulation of both iron and aluminum oxides occurred in conjunction with the zone of maximum clay accumulation and was greater on south slopes. Translocation processes were determined to be the dominant soil forming process in the soilscape and are thus assumed to be sensitive to variations in solar insolation on slopes of different aspects. South-facing slopes in the southern Driftless Area receive 51.5% more solar radiation than north-facing slopes at the winter solstice, the time of maximum difference. In addition to the amount of solar radiation received by each slope, the duration of which each slope is susceptible to pedogenic processes is also an important factor. South-facing slopes thaw earlier in the season than north-facing slopes and stay exposed later into the season. These early and late season periods of pedogenesis are especially crucial due to the lack of vegetation and increased amounts of precipitation common in this climate regime. The difference in duration of pedogenesis on each slope may only be minor factor annually but over pedogenic time has resulted in a noticeable difference in levels of development between slopes. Hillslope erosion processes were predictably sensitive to slope gradient. We used the thicknesses of individual horizons to tell us about the temporal differences in erosion rates in these polygenetic soils. Surface soils, formed from modern day pedogenic processes, show uniformity in thickness over the entire landscape while subsurface secondary argillic (2Bt) horizons that began forming during the early Pleistocene showed decreasing thickness with increasing slope gradient. This indicates that modern day erosional processes are not influenced by slope gradient on slopes <20% but that Pleistocene erosional rates were more sensitive to difference in slope gradient.

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