Possible transition from Silandic to Aluandic Andosols with acidification as evidenced by detailed profile investigation of two well-preserved forest Andosols in Japan

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ABSTRACT In Japan, Andosols are one of the major soils and they occupy about 30% of the total land surface. Andosols with andic properties have been grouped into two distinct categories: Silandic and Aluandic, but their formation has not been well investigated with reliable soil analytical data and needs to be studied for systematic understanding. To clarify the formation process of Silandic and Aluandic Andosols, two soil profiles, collected from natural forest sites at Aya (Sil-A) and Shiiba (Alu-A) in Miyazaki Prefecture, Japan, were investigated in detail. The soil samples were taken from the soil profiles at a 5 cm interval to reveal the changes in chemical and mineralogical properties with depth. The smooth changes in soil acidity (pH(H2O), pH(KCl), and y1), carbon stock (total carbon and δ13C), cation retention properties (cation exchange capacity and base saturation), and mineralogical properties (pyrophosphate and acid-oxalate extractable iron, aluminum, and silicon) indicated that both soil profiles had been formed under well-preserved natural forests. Based on the analytical data, Sil-A was classified as a Silandic Andosol (World Reference Base for Soil Resources 2022: WRB 2022) and an Allophanic Andosol (The Fifth Committee for Soil Classification and Nomenclature 2017: FCSCN 2017), whereas Alu-A was classified as an Aluandic (0–30 cm)/Silandic (30–90 cm) Andosol (WRB 2022) and a Non-allophanic Andosol (FCSCN 2017). The changes in the chemical and mineralogical properties with high depth-resolution suggested that the allophanic material contents were increased with depth within 0 to 50 cm, and they were related with soil pH(H2O) in both soils. It was 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, the authors conclude that Silandic/Allophanic Andosols can be transformed into Aluandic/Non-allophanic Andosols with acidification, and the phenomenon is typically observed in upper layers and progresses to deeper layers.