Human impacts on soils and geomorphic processes on the Des Moines Lobe, Iowa
A watershed scale approach to examining post-European settlement erosion-sedimentation was undertaken on the Des Moines Lobe in northcentral Iowa. Two closed basins comprised of glacigenic diamicton were selected for detailed field examination. The objectives of the study were to quantify post-settlement erosion-sedimentation rates and to characterize post-settlement hillslope sediments (PSHS) and the underlying buried soils. Soil morphology, particle-size distribution, mass magnetic susceptibility measurements, organic carbon depth distributions, quantitative soil color measurements, and cesium-137 depth distributions were utilized to identify PSHS and quantify erosion-sedimentation rates. Maximum thicknesses of 100 and 93 cm of PSHS were described in the two basins. Average post-settlement erosion rates were calculated to be 6.3 and 12 Mg/ha/yr for the two basins. These rates are approximately 10 times greater than pre-settlement rates for similar basins and geologic materials in northcentral Iowa reported in the literature. Post-settlement erosion-sedimentation resulted in the loss of at least 1.0 m of total relief in both basins since the time when land clearing and cultivation were initiated (approximately 1870). PSHS were coarser, contained less organic carbon, exhibited higher mass magnetic susceptibility values, and had higher bulk densities than the underlying buried soils. Difficulties utilizing Soil Taxonomy and master horizon nomenclature were encountered during the classification and description of PSHS;Quantitative relationships between soil color, soil texture, and organic matter content of three small drainage basins on the Des Moines Lobe were also examined. Quantitative soil color measurements were made on 130 Ap horizons using a tristimulus colorimeter. Strong relationships were observed between organic matter content and percent moist reflectance (r2 = 0.77), moist Munsell value (r2 = 0.77), moist Munsell chroma (r2 = 0.68), percent sand (r2 = 0.74), percent clay (r 2 = 0.71), and geometric mean particle diameter (r2 = 0.74). Logarithmic relationships existed for reflectance, Munsell value and chroma, and geometric mean particle diameter while linear relationships were observed for sand and clay contents. The rapid quantification of soil color properties with tristimulus colorimeter technology should prove to be a useful applied tool in the prediction of organic matter contents for soils in the study region.