Possible transition from silandic to aluandic andosols with acidification as evidenced by detailed profile investigation of two well-preserved forest andosols in Japan

Alam, M. M.*; Yamakita, Eri*; Inoue, Yuzuru*; Koarashi, Jun   ; Atarashi-Andoh, Mariko  ; Abe, Yukiko; Nakayama, Hiromasa   ; Mori, Yuki*; Hiradate, Shuntaro*

Andosols are one of the major soil types in Japan. Andosols have been grouped into two distinct categories: Silandic and Aluandic, but their formation has not been well investigated. To clarify the formation process of the Andosols, two soil profiles were collected from natural forest sites at Aya (Sil-A) and Shiiba (Alu-A) in Miyazaki. The soil samples were analyzed for carbon stock, and chemical and mineralogical properties. Based on the analytical data, Sil-A was classified as a Silandic Andosol and an Allophanic Andosol, whereas Alu-A was classified as an Aluandic/Silandic Andosol and a Non-allophanic Andosol. The changes in the chemical and mineralogical properties with depth suggested that acidification had happened from upper layers, resulting in dissolution of the allophanic materials and formation of Al-humus complexes in both soils, and Alu-A would have been more deeply affected by this process than Sil-A. Therefore, we conclude that Silandic Andosols can be transformed into Aluandic Andosols with acidification.