Changes in aluminum pools of andisols due to soil acidification

Abstract

Aluminum concentrations in soil solutions are not only controlled by inorganic clay minerals but also by organically bound aluminum. The objective of this study was to determine which pools contribute to Al dissolution. Soil samples were taken at various distances from tree trunks and at various depths at the Rolling Land Laboratory (RLL), Hachioji, Tokyo. Selective dissolution techniques were used to analyze the changes in pools of solid-phase aluminum. Soil pH values around Hinoki cypresses were in the aluminum buffer range. Exchangeable aluminum contents in soils under Hinoki cypresses were 104 mmol_c kg-⁻¹ on the average. This value was similar to that of the cation exchange capacity (CEC) of Andisols at RLL at a soil pH of 4. The relationship between the soil pH and exchangeable, organically bound, and amorphous aluminum pools showed that dissolved aluminum ions in the soil solution were primarily derived from the amorphous Al pool. Dissolved aluminum ions were substituted with base cations of soils, resulting in the increase of the content of exchangeable Al and/or the formation of complexes with organic matter which increased the proportion of organically bound Al pools. Increase in the proportion of organically bound Al pools indicated the importance of complexation with soil organic matter for controlling the aluminum concentration in the soil solution.

Key Words:

• amorphous aluminum
• Andisols
• organically bound aluminum
• selective dissolution techniques
• spatial variability