Friday, 14 July 2006

Pedogenesis of the Serpentinitic Soils along a Toposequence in Eastern Taiwan.

Zeng-Yei Hseu, Department of Environmental Science and Engineering, National Pingtung University of Science and Technology, 1 Hseuh-Fu Road, Nei-Pu, Pingtung, 91201, Taiwan

Soils derived from serpentinitic rocks may show strong chemical fertility limitations due to low Ca/Mg ratios, lithiogenic Cr and Ni enrichments, and serpentine mineral transformation. In this study, five soil pedons were selected on the summit, shoulder, upper backslope, lower backslope and footslope positions (TA-1 to TA-5) along a serpentinitic toposequence in eastern Taiwan and characterized soil chemical properties, mineral weathering sequences and genetic processes. It is surprising that the toposequence is only about 200 m long, but five Soil Orders of the Soil Taxonomy were represented, including Entisols (TA-1), Vertisols (TA-2), Alfisols (TA-3), Ultisols (TA-4) and Inceptisols (TA-5). As expected in pedons TA-3 and TA-4 with relatively flat landscape, the soils in general have undergone intensive weathering, extensive leaching, resulting in metal and clay translocation. However, high clay contents (>40%) are found in all the pedons. The soils derivered from ultramafic rocks are reflected in relatively high cation exchange capacity in this study, which ranged approximately in 26-74 cmol/kg. Exchangeable Ca/Mg ratios in all the soils are lower than 1.0 except for a few surface horizons. During the weathering serpentinitic minerals are unstable in soil environments, so that serpentine is only found in the soils of pedons TA-1 and TA-2 where kaolinite and gibbsite could not be identified. Trace kaolinite, gibbsite and quartz are found in TA-3 and TA-4 pedons. Serpentine is easily transformed as smectite, especially in TA-1 and TA-2 pedons associated with clear surface cracks and slickensides. However, vermicullite is progressively transformed from smectite or mixed layer mineral of chlorite-vermiculite with respect to Mg leaching under well drainage conditions. The weathering sequence of mineral in the study area is serpentine > chlorite > smectite > vermiculite > kaolinite > quartz, respectively. This weathering sequence coincided well with the Cr and Ni levels in the soils along the serpentinitic toposequence.

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