Micromorphological and chemical characteristics of placic and ortstein horizons in subtropical subalpine forests

摘要

Pedons with a placic or ortstein horizon are commonly occurred in the subalpine and alpine forests in Taiwan. We selected a representative pedon with well-developed placic horizon in subalpine forests in northeastern Taiwan and another with a well-developed ortstein horizon in subalpine forests in central Taiwan to compare the differences between these two horizons. Micromorphological observations indicated that the placic horizon was characterized by vughy microstructure and was composed of iron oxides, whereas ortstein horizon was characterized by granular and bridge microstractures between quartz grains. Analytical results of cementing materials between skeleton grains in these two horizons by using energy dispersive spectrometry (EDS) further indicated that Fe, O and C were predominant elements in the placic horizon, whereas Al, Si and C were major elements of the cementing materials In the ortstein horizon. The elemental mapping determining by electron probe micro-analyses (EPMA) also indicated that organically complexed-Al was the dominant material, which was present between soil aggregates and coated on quartz grains in the ortstein horizon as compared with those of the placic horizon. Chemical analytical results showed that pedogenic Fe (Fe_d) was the dominant Fe fraction in both placic and ortstein horizons; however, the Fe_d contents were much higher in the placic horizon than in the ortstein horizon. Dominant Al fractions were different between the two horizons, organically complexed-Al (Alp) was predominant in the ortstein horizon, and pedogenic Al (Al_d) was dominant in the placic horizon. We concluded that different formation mechanisms occurred between these two horizons; the placic horizon was related to redox processes, and theortstein horizons related to podzolization.