| Summary: | 碩士 === 國立臺灣大學 === 農業化學研究所 === 104 === Clay illuviation associated with podzolization or with podzolic features has been found in the subalpine forest soils in Taiwan. It was actually occured in the fine-textured soils derived from sandstone or shale. Clay illuviation is not generally involved in the mechanism of Podzol soil formation process. Three soil pedons were separately sampled at Wansueishan, Ogasawara and Chushan Mountains. The study area is located at near 2400 meters elevation with annual rainfall near 4000 mm. The soil temperature regime is mesic and the soil moisture regime is udic. Coniferous vegetations are the dominant with Taiwan red cypress (Chamaecyparis formosensis), Chinese hemlock (Tsuga chinensis) and Taiwan cypress (Chamaecyparis obtuse) in this area. The minor shrub communities are pine (Pinus armandii) and Rhododendron (Rhododendron simsii). The ground is covered mainly by Yushan bamboo (Yushania niitakayamensis), miscanthus (Miscanthus floridulus) and some mosses. The objectives of this study were performed by soil morphological characteristics, physical and chemical properties, and mineral composition to study the unique soil formation processes in podzolic soil combined with clay illuviation in the subalpine forests.
The results indicate that the soil texture of three pedons can be distinguished into two groups, one is sandy clay loam and the other one is clay loam. Clay contents were the lowest in the albic horizon. The clay illuviation is significantly increased in the upper part of B horizon of the Podzols, and then decreased in the C horizon. We also observed clay was moved downward and accumulated both in argillic horizon and spodic horizon. Soil chemical properties show that soil is strong acidity (pH 3.3-4.9) and very low bases saturating percentage (<10%). The soil organic carbon is mainly distributed in topsoil because the higher humus is not easily decomposed by microorganism, low temperature and coniferous vegetation. The content of organic carbon and different forms of Fe and Al were significantly higher in spodic and argillic horizon than those of albic horizon, which is indicated that large amount of active amorphous and organic-sesquioxide materials were accumulated in the soil. Podzolization processes were characterized by the accumulated organic carbon, Fe, and Al in spodic horizon, therefore we can find the significant correlations among organic carbon, Fe and Al content in the E and B horizon (p<0.01).
Distinct organo-iron complexes materials and clay coatings were accumulated on the coarse grains of the Bhs horizon by polarized microscopy. Except of the dark pellets of organic compounds, the slightly oriented clay coating also appeared along the soil pores and channels. Therefore, pedogenic processes in the study area are mainly characterized with the illuviation of organo-metallic (Fe and/or Al) complex materials and fine clay particle.
The clay mineral compositions of three pedons in study area can be divided into two parts due to the horizon differentiation. The albic horizon was prominent with illite, vermiculite and vermiculite-illite interstratified minerals. The other part is mainly consisted of much more vermiculite, vermiculite-illite interstratified minerals and hydroxy-interlayered vermiculite (HIV) in the spodic and argillic horizons. The weathering sequence of clay mineral of Podzols with fine textures in the study area is proposed as: from illite to vermiculite, then to HIV and vermiculite.
As the results, two of three studied soil pedons can be classified as Ultic Haplorthod or Typic Haplorthod and the other one pedon can be classified as Typic Hapludult based on USDA Soil Taxonomy. We proposed that clay illuviation was one of the pedogenic processes in the study area. However, podzolization was only formed at the area with gentle slope or flat landscape to promote the strong leaching process with very high precipitation in the study area. To consider the environment condition with the dense coniferous vegetation dominated at the study area, the weathering time of podzolization is the main pedogenic process factor that leads to the soil transition. Overall, the phenomena of pedogenic processes both podzolization and clay illuviation could be observed together in flat landscape region of Ali Mountain.
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