THE IMPACT OF VEGETATION ON SOIL LABILE SILICA: A CASE STUDY USING LOESS-DERIVED SOILS OF THE UPPER MISSISSIPPI VALLEY

Plants can have profound impacts on cycling of silica in soils through weathering processes and biomineralization. It has also been shown that soils under different plant communities, i.e. grassland and forest, can exhibit drastically different profiles of labile silica (Si_labile, defined as all relatively soluble and mobile forms of silica). However, other factors can influence soil Si_labile patterns, including variable inheritance of Si_labile from parent materials. The goal of this study was to test for differences in soil Si_labile across the late Holocene prairie-forest ecotone on broad upland summits in southeastern Minnesota, USA, a landscape very similar to the Wisconsin Driftless Area that Jim Knox has studied for decades. As in the Driftless Area, thick late Quaternary loess mantles the study area, providing a relatively uniform parent material with fairly high inherited Si_labile (probably ultimately derived from Cretaceous shale and delivered to the loess source area by the Laurentide Ice Sheet). Thus, effects of vegetation should not be obscured by large contrasts in inherited Si_labile availability. Soil orders (Alfisols and Mollisols) were used as proxies for late Holocene vegetation, given a relatively close correspondence between their distribution and the pattern of forest and prairie as recorded in the Public Land Survey (1850s). Selective dissolution (SD) was employed to quantify total Si_labile for each soil profile and for each horizon. Floatation techniques (which extract silt-sized Si_labile particulates) were also used to facilitate identification of silica forms. Whole-soil SD-extractable Si_labile amounts were remarkably similar for both soil orders. Likewise, the vertical distribution of Si_labile silt-sized particulate forms and quantities were not statistically different. Taken together, these findings suggest that the impact of grassland and forest has been minimal or similar in this region, possibly due to ecotonal shifts throughout the Holocene. However, the greatest quantity of SD-extractable Si_labile was found in A horizons for Mollisols and B horizons for Alfisols, indicating that the plant communities did have an impact on Si_labile mobility within the solum.