Initial Soil Formation Processes of Volcanogenous Regosols (Scoriacious) from Miyake-jima Island, Japan

Abstract

In the initial soil formation processes of the Volcanogenous Regosols (Scoriacious) from Miyake-jima Island, Japan, the soil profile morphology showed a distinct formation of the A horizon, about 10 cm thick, over C horizons during a period of 125 years. Along with these soil formation processes under the primary succession of vegetation from bare land to mixed forest, the following changes in the soil characteristics were observed of four study sites where soil formation had occurred over a period of 16, 38, 60 and 125 years in scoriacious volcanic materials ejected during each eruption. Whole soil samples of the surface horizon showed that the proportion of the > 2mm fraction decreased and the clay fraction increased. Electron microscopic observation revealed that pores on the scoria surface were newly formed and had expanded with advanced weathering, and that the formation of a weathering zone on the scoria surface (polygonal shape) was more recognized with the passage of time. Soil pH of the surface horizon decreased, and the amount of carbon in the surface samples showed a linear increase with the passage of time. The amounts of exchangeable bases and the CEC value of the surface samples showed comparable increase rates with time. Consequently, the base saturation percentage was kept at about 100% during the 125-year period. It was suggested that the increase in the amounts of exchangeable bases was controlled by the increase in the amounts of soil organic matter accumulated during the initial soil formation processes of Volcanogenous Regosols (Scoriacious).