Development of a method for sequential Si extraction from soils
Introduction
Amounts of Si in soils depend on parent material and soil type, reflecting pedogenic processes at pedon and landscape scale. They vary from less than 1 mass% Si in Histosols to up to 45 mass% Si in Podzols (Sommer et al., 2006). Si in soils most commonly occurs in quartz (Drees et al., 1989) followed by Si in primary silicates and clay minerals. In addition, various other Si compounds occur in soils that have attracted less attention in the past (Sauer et al., 2006). In general, Si released during weathering of primary silicates can be leached or consumed in the formation of secondary silicates, or it may precipitate as pedogenic silica from the soil solution (McKeague and Cline, 1963a). Silica precipitates may form amorphous coatings on mineral grains (Veerhoff and Brümmer, 1993) or mixtures with crystalline substances (Follett et al., 1965).
Si concentration in soil solution varies between 0.4 and 2000 μmol l− 1, depending on parent material, soil development, depth and temperature (Conley et al., 2006). Dissolved silica is usually present in marine and terrestrial environments as monomeric silicic acid (Shell, 1962). Silicic acid starts deprotonating above pH 9 (Dietzel, 2000, Wonisch et al., 2008). Dissolved silica in soil solution is part of a dynamic system; its amounts depend on mineral composition, adsorption effects, water balance, temperature and biochemical activity (Milnes and Twidale, 1983). Various organic compounds may increase or decrease silica solubility (Iler, 1979, Summerfield, 1983). Organic compounds such as low molecular weight organic acids contribute to the weathering of soil minerals through acidification and complexation (Bennett et al., 1991, Bennett et al., 2001, Crook, 1968, Drever, 1994). Complexes of organic compounds and silica in soil solution have been identified (Matichenkov and Snyder, 1996). Microorganisms (bacteria, fungi, protozoa) may also affect Si-release from litter or minerals. Some microorganisms have developed mechanisms for Si-storage (biomineralization) in their bodies (Sommer et al., 2006). Thus, together with plants they are an important source of mobile Si in soils.
Silicic acid can be adsorbed to the surfaces of various soil components (Beckwith and Reeve, 1963, Dietzel and Böhme, 1997, Hansen et al., 1994 Iler, 1979, Jones and Handreck, 1963, McKeague and Cline, 1963b, McKeague and Cline, 1963c). Adsorption of monosilicic acid in soils depends on soil reaction, soil composition and the specific surface of sorbents and increases with increasing pH and specific surface area of the soil particles (Beckwith and Reeve, 1963, Beckwith and Reeve, 1964, Cornell and Schwertmann, 1996 Gehlen and van Raaphorst, 2002, Jones and Handreck, 1963, McKeague and Cline, 1963b). Pedogenic oxides and hydroxides that are abundant in soils (Schwertmann and Taylor, 1989) play an important role in adsorption, occlusion and release of silicic acid in soils (Cornelis et al., 2010, McKeague and Cline, 1963a, Opfergelt et al., 2009).
The largest proportion of amorphous silica in most soils is represented by bio-opal (Drees et al., 1989) in form of phytoliths, siliceous sponges, diatoms and radiolarians. Some authors assume that biogenic silica is a major source of dissolved Si in soil solution (Cornelis et al., 2010, Derry et al., 2005, Miretzky et al., 2001). Alexandre et al. (1997) estimate that Si release from bio-opal is two to three times as great as that from silicate weathering. This implicates a high turnover of this fraction.
Quantification of Si pools in soils is needed for improving our understanding of biogeochemical processes involving Si in the plant–soil–soil water–groundwater system which represent a black box in global Si-cycling models so far. Such quantification requires a sequential extraction method for the various Si fractions in soils, analogous to e.g. sequential heavy metal extraction. However, a sequential extraction method has not been available so far for Si. The objective of this work was to develop such a method. For this purpose, quantitative chemical extraction methods for extracting silicon from different soil fractions were tested. Those methods that turned out most suitable were optimized and implemented as extraction steps in a sequential extraction procedure. The aim was to develop a procedure that is applicable in any ordinarily equipped soil science laboratory.
Section snippets
Well-characterized soil compounds and soils used for the tests and optimization procedures
The effects of various extractants were tested on selected well-defined soil compounds (Table 1). Seven soil profiles from four different areas of SW-Germany were described and sampled to cover a range of different soils. Soils include a Cambisol and two Podzols on Triassic sandstone under forest of silver fir and Norway spruce in the northern Black Forest (Seebach area), one of the Podzols with thick eluvial horizon and one with thick illuvial horizon (on the upper and lower parts of the same
Mobile Si
The highest Si concentrations in the CaCl2 extracts are reached between 3 and 21 days (Fig. 1). After three weeks of extraction the Si concentrations are about 3–6 times higher than after 1 day. Si concentration in extracts of organic layers and topsoil horizons continues to increase considerably over several days, whereas Si concentration in extracts of subsoil horizons tends to reach its maximum already after 3 days. This is especially true for the footslope Podzol and the Cambisol in the
Conclusions
A method for sequential extraction of Si fractions from soils has been developed, based on numerous series of extraction experiments using well-characterized isolated soil compounds and selected soil samples. The procedure has been developed by first improving and further developing existing specific extraction methods before implementing them as subsequent steps into the sequential procedure. The sequential extraction of Si introduced here extracts step by step various Si-fractions, from the
Acknowledgments
This study was funded by the German Research Foundation (DFG STA 146/48-1) — “Multiscale analysis of Si cycling in terrestrial biogeosystems”. We also thank the Alfred Toepfer Stiftung F.V.S. for financial support.
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