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

We have investigated the clay mineralogy of 3 pedons along a toposequence of perhumid forest soils facing a subalpine lake in northeastern Taiwan to gain insight into the individual factors that affect weathering rates. Toposequence of the transect was investigated from summit to footslope and at the lakeshore. The natural drainage of soils varies from well drained to poorly drained. The major minerals in the well-drained soils at the summit site were illite and kaolinite, whereas illite and vermiculite were dominant in the poorly drained soil at the footslope. In the poorly drained soils of the lakeshore site, almost permanent saturation inhibited weathering of minerals, and vermiculite, illite and illite–vermiculite were the dominant minerals. The magnitude of the mean layer charge of vermiculite was estimated by use of the alkylammonium exchange method (nC = 12). 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 surface horizons displayed lower mean layer charges, indicating stronger weathering intensity in the surface horizons. Acidification by organic acids may result in the reduction of layer charge in vermiculite, particularly at the surface. Differences in layer charge with landscape position may be due to differences in weathering processes. The drainage condition caused by topography is a critical factor in the transformation and redistribution of clay minerals. High precipitation, combined with surface runoff, enhance the variation of the type and mean layer charge of clay minerals along this toposequence.