Assessment of the 1% Na2CO3 technique to quantify the phytolith pool

Amorphous silica (ASi) pool and fluxes have become parameters of a growing interest in the understanding of the biogeochemical cycle of Si and the modeling of anthropogenic impacts. Extraction by 1% Na2CO3 followed by a correction for crystalline Si (DeMaster, 1981) has recently become widely used and is proposed as the standard technique for quantifying amorphous silica (ASi) pools in soil and continental aquatic environments. However, the 1% Na2CO3 method was developed to quantify diatom frustules and not phytoliths (PhSi), which constitute the most common fraction of ASi in soils. The aim of this study was to assess the 1% Na2CO3 method for fresh and aged phytoliths. We founded a significant positive correlation between Si extracted by the 1% Na2CO3 method and Si extracted by other standard procedures (Guntzer et al., 2010) for various shoot samples (elm, horsetail, fern and larch). Because the Si shoot samples are mainly composed of phytoliths, we concluded that the 1% Na2CO3 method was well adapted for the determination of PhSi of fresh phytoliths containing less than 70 mg Si g(-1). To assess the 1% Na2CO3 method for PhSi of aged phytoliths, we used artificial mixtures of soil phytoliths from La Reunion (Meunier et at, 1999) and quartz. Our results showed that the 1% Na2CO3 method underestimated PhSi for aged phytoliths by a factor of 3.7. Combining the 1% Na2CO3 method and a physical method of extraction using samples of various origins showed a dispersion of the data that can be explained by a combination of several factors including loss during physical extraction and the presence of resistant phytoliths. In the absence of standard technique, we recommend to check the nature of the particles using physical extraction in combination with the 1% Na2CO3 in order to provide a more careful analysis of ASi