Differentiation of fine-textured podzolic soils controlled by climate and landscape in Taiwan

Podzolization is considered to be facilitated by factors such as cool and humid climate, sufficient supply of certain types of organic acids, and coarse-textured parent materials. However, podzolic soils in subalpine areas in Taiwan are characterized by clay content higher than 30%, which differed from that in typical podzolic soils in temperate and boreal regions. This study examined the importance of factors controlling the differentiation of fine-textured podzolic soils in Taiwan. Samples from 31 pedons were examined in this study, and the pedons were grouped into four types, namely Spodosols, Ultisols, Inceptisols, and Inceptisols with placic horizons. Soil colors with 7.5YR 5/6 (the accumulation of spodic materials) and micromorphological observation of oriented clay coatings in the Spodosols confirmed the occurrence of podzolization and clay illuviation, and the selective extraction of Fe and Al confirmed that the Spodosols (Type I) had a higher accumulation of organometallic complexes than the other podzolic soils. A two-step mechanism, namely (1) clay illuviation at an early stage of pedogenesis and (2) podzolization after the canopy vegetation formed, was proposed for the formation of fine-textured Spodosols. Photomicrographs of the Ultisols (Type II) displayed clay hypocoatings masked by spodic materials, and argillic horizons were characterized by a high amount of nanocrystalline iron and high optical density of the oxalate extract. However, the accumulated nanocrystalline iron oxides in the Ultisols were due to the degradation of organic matter under high summer temperatures. Moreover, the leaching of iron in the albic horizons of the Ultisols was attributed to episaturation caused by high clay contents. The Inceptisols (Type III) were transitional soils before forming the Spodosols or the Ultisols, and high clay contents further induced episaturation and formed placic horizons in the Inceptisols (Type IV). Principal component analysis revealed that soil texture and precipitation were the main factors controlling the differentiation of podzolic soils in Taiwan. These factors determined the penetrability and leaching potential of soils, and only the coarse-textured soils with higher leaching potential had the chance of developing Spodosols. However, the developments of Ultisols and Inceptisols with placic horizons were controlled by air temperature and landscape, namely elevation and slope. The bleached E horizons in podzolic Ultisols were attributed to their flat landscape, which favored episaturation and caused the reduction of iron. Low elevation and high temperature in these regions also facilitated the degradation of organic matter and the accumulation of nanominerals.