Saturday, 15 July 2006

Clay mineralogical characterization of a toposequence of perhumid subalpine forest soils in northeastern Taiwan.

Chuangwen Pai, The experimental forest, college of bioresource and agriculture, National Taiwan University, No.12, Section 1, Chien-Shan Road, Chu-Shan, 55743 Nan-Tou Hsien,, Taiwan, Ming Wang, National Taiwan University, Dept. of Agric. Chemistry, National Taiwan University, Taipei, 10764, Taiwan, and Chih-Yu Chiu, Research Center for Biodiversity, Academia Sinica, Nankang, Taipei, Taiwan.

This study conducted a detailed characterization of the clay mineralogy along a toposequence of preumid foest soils facing a subalpine lake. Toposequence of transect was investigated from summit to footslope, and lakeshore. Their natural drainage varies from well drained to very poorly drained soils. Three pedons were investigated by horizon and clay minerals were examined by X-ray diffraction (XRD). The magnitude of the layer charge of the 2:1 phyllosilicates was estimated using the alkylammonium exchange method (nC = 12). The mineralogical composition of soils at summit site is different compared to soils at footslope and lakeshore sites. The major mineral in the well drained soils at summit site were illite and kaolinite, while in the poorly drained soil illite and vermiculite is dominant. In the very poorly drained soils of lakeshore site, almost permanent saturation inhibited weathering of minerals. In these soil vermiculite, illite and vermiculite-illite are the dominant minerals. Toposequence may give insight into the influence of individual factors on the weathering rates. The mean layer charge of vermiculite ranged between 0.63 and 0.78 cmolc/(O10(OH)2). The distribution of clay layer charge varied with location and soil depth. Vermiculite in the lower horizons displayed higher mean layer charge, indicating stronger weathering intensity in the surface horizons. Acidification by organic acids near the surface may be resulted in the reduction of layer charge in vermiculite. Differences in layer charge with landscape position may be due to differences in weathering processes. Drainage condition caused by topography is a critical factor for the transformation and redistribution of clay minerals. High precipitation in combination with surface run off may enhance the variation of the type and layer charge of clay minerals along this toposequence.

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