We investigated the morphological features and physicochemical properties of soils under vegetations dominated by Miscanthus sinensis (Site 1) or Phragmites australis (Site 2) in a semi-natural grassland, Asagiri Highland, central Japan. We considered factors that contribute differences between the distributions of the two grasses species. The soil of site 1 was generated from secondary sediments of the volcanic ejecta of Mt. Fuji. At site 2, buried horizons (39–65 cm) were observed in the soil profile. In addition, Al, Fe, and Si contents extracted by acid oxalate solution and phosphate absorption coefficient of site 2 were lower than those of site 1. These results suggest that the soil of site 2 is generated from mixed sediments, which are secondary sediments of the volcanic ejecta from Mt. Fuji and alluvial deposit from Tenshu Mountains. According to the Soil Classification System of Japan (2017), these pedons were classified as Cumulic Allophanic Andosol (site 1) and Thapto-humic Allophanic Andosol (site 2). In addition, abundance of rock and coarse sand, saturated hydraulic conductivity, and inorganic nitrogen concentration were higher in the surface horizons of site 1 than those of site 2, whereas exchangeable K and available P (Bray 2) concentrations of site 2 tended to be higher than those of site 1. These results suggest that the factors that contribute to the distributions of the two grass species are (1) abundance of rock and coarse sand, (2) permeability of topsoil, and (3) available nutrients (P, K, and N). Additionally, these differences are due to the deposition patterns of the soil parent materials.

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We collected soil samples (0 – ca. 30 cm) at 15 sites in the area from Towada caldera (Lake Towada) to Oga Peninsula at the Japan Sea coastal area, the northern part of Akita Prefecture. The soil samples were used for chemical analyses including acid oxalate-extractable aluminum, iron and silicon, and P retention for the determination of the andic soil properties. We also performed the elementary analysis of volcanic glasses of 0.05 – 0.2 mm fraction for each grain by the scanning electron microscope-energy dispersive X-ray spectroscopy and determined the origin of each glass grain by the method of the cryptotephrology for 9 sites. All the soil samples satisfied the requirements for the properties of andic, regosolic andic of Andosols or andic of Brown Forest soils in the Japanese Soil Classification System (2017). All the soil samples contained volcanic glasses that were identified as Towada tephras, such as Towada-a tephra (AD 915), the effect of the tephra was greater in the soil closer to the source. Soil samples in about half of the sites also contained glasses that were identified as Baitoushan-Tomakomai tephra (AD 946-947). The soil samples in Oga Peninsula were strongly influenced also by volcanic glasses, which appear to be Toga pumice (420 ka) erupted from Toga caldera located in the western part of the peninsula. The results obtained in this study are well harmonized with the information on tephrochronology in the previous studies, indicating that the method of the cryptotephrology will be a useful tool for the pedology of volcanic ash soils.

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