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

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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 suggest that in the lower profile, the H4SiO4 concentration is well controlled by the reaction with surface Al-OH groups of allophane/imogolite and hydroxy-interlayer minerals. Cation-exchange resin, Chelex 100, extracted a very small amount of Si from only the Bhsl and Bhs2 horizons, suggesting that a trace amount of protoimogolite accumulated in these horizons. The protoimogolite was probably complexed with humic substances.