Soluble silica in a tephra-derived spodosol in the mountainous region of Chichibu, Japan

Abstract Si sorption and release characteristics of a tephra-derived Spodosol in the mountainous region of Chichibu, Japan, were studied, and cation-exchange resin extraction of protoimogolite was tested. The upper compartment and the lower compartment of the Bhs/Bs interface differed considerably in their reactivity for H4SiO4. The upper compartment (Oa, E, Bhs1, and Bhs2 horizons) showed a weak affinity for H4SiO4. These horizons gave zero-order H4SiO4 release, and the initial stage of H4SiO4 release was well described by the parabolic diffusion law. These results suggest that the upper profile contains a large amount of free Si and acts as a Si pool. Especially, the Oa horizon did not show any H4SiO4 adsorption in a 0–3 mmol L-1 H4SiO4 solution, and gave a much larger H4SiO4 release rate. The lower compartment (Bs1 and Bs2 horizons) which contains allophane/imogolite showed a strong affinity for H4SiO4. The H4SiO4 release rate of these horizons was described by the Elovich equation. These findings sugg...