公告日期 : 2013-12-27
Morphology, Geochemistry, and Mineralogy of Serpentine Soils under a Tropical Forest in Southeastern Taiwan
Authors：Yao-Tsung Chang, Zeng-Yei Hseu, Yoshiyuki Iizuka, Chun-Der Yu
Key words：chromium, clay mineral, geochemistry, nickel, serpentine soil.
Serpentine soils pose ecological or environmental risks because of high levels of Cr and Ni and low Ca/Mg ratios. Studies on the morphological characteristics, geochemical processes, and the mineralogical composition of serpentine soils in tropical forest ecosystems are limited. Two pedons (TK-1 and TK-2) on the ophiolite complex in the Chishang area, southeastern Taiwan, were chosen to represent the degree of soil development. The TK-1 pedon is an Entisol, whereas the TK-2 pedon is a Vertisol based on the US soil classification system. This study explored the soil morphology to establish a regional baseline for naturally occurring Cr and Ni to link to related soil geochemical properties and mineral features in this tropical forest ecosystem. The results indicated that weathering of primary minerals (i.e., serpentine and olivine) has occurred, and pedogenic Fe oxides have accumulated in the soils. However, both pedons had pH values of < 7.0, and amounts of organic carbon were low. Their cation exchange capacities were high; however, the deficiency of K in these soils may be exacerbated by the presence of clay minerals, such as mica, vermiculite, and smectite. At the exchange sites of the soils, Mg was higher than Ca, and the difference was obvious toward the bottom of the soil profile. Total contents of Cr and Ni greatly exceeded background levels in other parts of Taiwan. Clear amounts of labile Cr and Ni may have been fixed by pedogenic Fe oxides, which corresponds to the observation of serpentine mineral weathering. Concentrations of dithionite-citrate-bicarbonate-extractable Cr and Ni increased with soil development because of sorption/co-precipitation by pedogenic Fe oxides, which accumulated more in the TK-2 pedon than in the TK-1 pedon. Moreover, differences in the clay mineral composition between pedons indicate the importance of understanding the localized mineralogy when developing effective site-specific forest management strategies.