Development of a method for sequential extraction of Si-pools from soils

For the first time, a method for sequential extraction of silicon (Si) from soils has been developed. This new method enables investigating different Si pools in soils and thus represents an important contribution to improve our understanding of biogeochemical Si-processes and global Si-cycling. The method fractionates and analyses various Si-pools, from the most mobile to immobile phases in soils. The sequential extraction allows differentiation of soluble and biogenic as well as pedogenic amorphous and crystalline silicon, so that the different Si-pools can be analysed. Firstly, the most mobile phase is separated by extraction with CaCl2, followed by extraction of the adsorbed Si-fraction with acetic acid. Thereafter, the fraction of silicon occluded in sesquioxides is isolated by treatment with ammonium oxalate in the light. Oxidation with hydrogen peroxide releases silicon bound in organic matter. Bio-opals (mainly phytoliths) are separated with sodium polytungstate, while minerogenic (chemically precipitated) amorphous silica is extracted with sodium hydroxide. The total silica content is determined by fusion with lithium borate and subsequent dissolving in nitric acid. Si contents in the extracts are measured by ICP-OES. The share of crystalline silicates is calculated. The new method was applied to a soil catena in the Black Forest in SW-Germany. The results of the sequential extraction indicate that most of the Si (9899%) is bound in primary and secondary silicates. Up to 1% of Si originates from the abiotic amorphous phase or from organic matter.