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

Abstract

Many studies have highlighted the importance of the Amorphous Silica (ASi) pool to the overall mass balance in the biogeosphere. In order to advance our knowledge of measurements and quantification of this pool, it is necessary to compare the ability of different extractants to dissolve ASi in soils and to test methods developed in the aquatic sciences to soils systems. The methods used in this work included three acid extraction techniques (0.2 m NH 4-oxalate, 0.1 m NH4-citrate and 0.5 m NH 4-acetate) and two alkaline extraction techniques (0.094 m Na 2CO3 and 0.5 m NaOH), 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 two orders of magnitude less than the amount of extracted by 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 Na2CO3 or NaOH extracted the same amount of ASi in Podzols, Na2CO 3 was able to extract only part of the ASi extracted with NaOH in Chernozems. Pre-treatment of the samples with 0.1 m HCl before the Na 2CO3 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. © 2007 British Society of Soil Science.