Occurrence of polymerized silicic acid and aluminum species in two forest soil solutions with different acidity

Silica species were determined in soil solutions from different soil horizons of acidic Dystric Cambisols in Austria and France. In general, more acid pH and elevated concentrations of Al, Si, and DOC were found in solutions of the upper soil horizons and in particular in the Austrian soil solutions. Aqueous Al speciation was dominated by complexes with respect to DOC and sulphate and by free Al3+, whereas dissolved Si was dominated by monosilicic acid. The average proportion of low molecular weight polymerized silicic acid (PS) is about 10 mol% of total dissolved Si. However, PS proportions sporadically reached up to 23 mol%. The origin of polymers is most likely due to weathering of soil silicates and polymerization of silicic acid by freezing or evaporation events. We found a positive relationship between the occurrence of PS and the saturation degree with respect to secondary Al-silicates. Thus, we suggest that PS may even indicate precursors for the formation of secondary Al-silicates. In accordance with this concept, our results show that PS preferentially occurred in the upper B1 horizons, where low pH and high Al content may prevent its destruction.