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The importance of silicon (Si) compounds in agriculture and geochemical cycles has received increasing attention over the last decade; however, quantitative data on non-crystalline pedogenic Si phases in soils are still rare. Recently, the authors developed a method for sequential Si extraction from soils, in order to improve the quantification of different Si compounds in soils. The method has been tested on samples of known composition. Here, the method is applied for the first time to complete soil profiles. Six different soil types from south-west Germany that have developed since the end of the last glacial period were selected. Most of the Si in these soils was bound in primary and secondary silicates. In mineral soil horizons, the second-highest proportion of Si was in precipitates of amorphous silica (minerogenic amorphous silica), whereas in some O horizons, the second-most important Si fraction was in biogenic amorphous silica. Topsoil horizons and clayey subsoil horizons of a Luvisol and a Stagnosol especially accumulate amorphous silica. Silicon from bio-opal contributed up to 14% to the total Si in Oa horizons of the studied soils. The smallest amounts of Si were found in the mobile and adsorbed Si fractions. Some methodological limitations are identified and discussed; however, the new sequential method of Si extraction enabled separation of different Si fractions in typical soils of a temperate-humid climate.

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This study focuses on optimising amorphous silica extraction from soils by using NaOH. Based on the results of this test series, a method for quantifying amorphous silica in soils from temperate-humid climate is proposed. All tests were carried out on materials of known composition (well-defined samples of feldspar, clay minerals, bio-opal, silica gel, and mixtures of these components) and on soil samples, to determine the optimum set of conditions, in terms of solid : solution ratio, temperature and extraction time, for dissolving amorphous silica without considerably attacking other solid silicon compounds. A solution of 0.2 m NaOH almost completely extracted amorphous silica, and when applied at room temperature and a solid : solution ratio of 1 : 400, only slightly broke down crystalline Si compounds. The predictable and reproducible underestimation was considered more acceptable than the variable partial dissolution of silicates that occurs during extraction at higher temperatures. We therefore recommend using this method on soils from temperate-humid climate to estimate the amorphous Si fraction.

Quantification of Si in its different forms in soil is a prerequisite to understand the geochemical distribution and fate of Si along with their driving biogeochemical processes. However, different Si fractions in floodplain soils have not been quantified yet, and little is known about the processes driving Si fractionation in these soils. The aim of this study was to clarify the processes that drive formation and distribution of Si among fractions in floodplain soils. We obtained and quantified these fractions using a sequential Si extraction method (Georgiadis et al., 2013) in three Mollic Fluvisols along the Central Elbe River. The highest Si proportion apart from the residual fraction was found in minerogenic amorphous silica (up to 5.6% of total Si), followed by Si occluded in pedogenic oxides and hydroxides (up to 0.7% of total Si). Silicon from biogenic amorphous silica amounted to 0.02-0.6% of total Si. The smallest proportion of Si was found in the mobile Si fraction and made up about 0.01% of the total Si. The results of this study demonstrate the importance of the soil water budget on the accumulation of easy-to-mobilise Si, Si occluded in pedogenic oxides and hydroxides and amorphous silica. Reductive dissolution of Fe and Mn oxides may induce Si release into the soil solution, subsequent oxidizing conditions may induce Si accumulation by adsorption, co-precipitation and occlusion of Si on/with newly formed Fe and Mn oxides. Accumulation of bio-opal after flooding may induce larger amounts of biogenic amorphous silica in floodplain soils than in terrestrial soils. Finally, floodplain soils may accumulate larger amounts of Si bound to occluded particulate SOM than terrestrial soils, which experience less input of particulate SOM than floodplain soils. Literatur