Relationships among non-allophanic/allophanic indicators in six Japanese forest Andosol profiles: implications for soil classification and genesis

Andic Andosols, which are widespread across the Japanese terrestrial surface, have been classified into two groups: Allophanic and Non-allophanic, based on the non-allophanic/allophanic indicators: oxalate-extractable silicon (Sio), the ratio of pyrophosphate- to acid-oxalate-extractable aluminum (Alp/Alo), and exchangeable acidity (y1), as defined by the Fifth Committee for Soil Classification and Nomenclature 2017 (FCSCN 2017). However, the consistency among these indicators has not yet been investigated with soil analytical data. Six Andosol profiles from forested sites across Japan, sampled at 5 cm intervals, were investigated in detail to clarify the consistency among the non-allophanic/allophanic indicators and discuss the changes in these parameters with soil genesis. All profiles exhibited smooth changes in chemical (pH(H2O), pH(KCl), total carbon (C) content, stable C isotope ratio (δ13C), cation exchange capacity, and base saturation) and mineralogical properties (Alo +1/2 Feo, Sio, Alp/Alo, and y1), indicating that they were well-preserved and formed under natural and semi-natural soil formation processes. According to FCSCN 2017, two of the six profiles were classified as Allophanic Andosols, while the other four were classified as Non-allophanic Andosols. In identifying non-allophanic/allophanic diagnostic properties by FCSCN 2017, the authors found inconsistencies among the three non-allophanic indicators in three of the six soil profiles. The presence of highly acidic allophanic material-rich layers and low-acidic allophanic material-poor layers was confirmed. The authors suggest that Sio content would be a better indicator to diagnose Andic Andosols as allophanic material-rich or allophanic material-poor, and that the Alp/Alo ratio would not be a reliable indicator of the richness of allophanic materials, because the Alp/Alo ratio can be deeply influenced by soil organic matter content. Moreover, Sio and y1 values would be informative parameters for diagnosing different characteristics, i.e. allophanic material content and soil acidity, respectively, for improved Andosol classification. It was also confirmed that acidification from the surface layers dissolves the allophanic materials and forms Al-humus complexes, indicating the transition from Allophanic to Non-allophanic Andosols. The findings of this study would contribute to the refinement of Andosol classification and enhance the understanding of Andosol genesis under forests.