Assessing the extraction and quantification of amorphous silica in soils of forest and grassland ecosystems

The objective of this study was to evaluate different methodologies with regard to their ability to extract the amorphous silica (ASi) content of soils from diverse geochemical settings. The methods used in this work included three acid extraction techniques (oxalate, citrate and acetate) and two alkaline extraction techniques (sodium carbonate and sodium hydroxide) which are more commonly used for the measurement of ASi in aquatic sediments. Our results indicate that the amount of Si extracted from phytolith samples with the acid methods was an order of magnitude lower than the amount of Si extracted using alkaline extractions. When applied to natural soil samples, these extractions show that the acid techniques are only able to extract loosely bound components such as adsorbed Si and Si bound in amorphous matrices with Al and Fe. While sodium carbonate or sodium hydroxide extracted the same amount of ASi in Podzols, sodium carbonate was able to extract only part of the ASi extracted with sodium hydroxide in Chernozems. Pre-treatment of the samples with hydrochloric acid before the sodium carbonate extraction did not increase amounts of ASi extracted. The present work suggests that alkaline methods used commonly for ASi on aquatic sediment samples can be used on a wide variety of soils, even if in some cases a stronger base is required to completely dissolve ASi from certain soil types where older and less reactive ASi fractions are present