Ca Depletion in the Soil Column on a Granite Substrate on the Island of Yakushima, a World Natural Heritage Site

Abstract

We investigated the vertical profile of element distributions in the soil column on a granite substrate at three sites on the island of Yakushima, a world natural heritage site in southwestern Japan, where stream acidification due to acid rain is of increasing concern. Statistical analysis shows that the elements in the granitic soil can be classified into three groups according to their mineralogical derivations: Group 1 (K, Mg, Mn, Rb, and Nb), group 2 (Fe, Ti, Zr, Cr, and Pb), and group 3 (Ca, Na, Sr, and P), which are related to the presence of K-feldspar and K-mica, Fe-Ti oxides, and plagioclase and apatite, respectively. The pattern of distribution of group-1 elements in the soil column differs slightly among the sites because of differences in the proportions of K-feldspar and K-mica, The organic surface soil is enriched in group-2 elements and shows a strong magnetic susceptibility, indicating the presence of biogenetic magnetite. At all sites, Ca and Sr are intensely depleted due to the selective dissolution of plagioclase. Ca depletion compared to the original granite reaches more than 90 % in the C horizon, resulting in a decreasing capacity for the neutralization of acid rain and an increasing contribution of atmospheric Ca to the surface soil and vegetation.