Assessment of the 1% Na2CO3 technique to quantify the phytolith pool
Introduction
In any study dealing with continental weathering processes and the biogeochemical cycle of Si (Dürr et al., 2011, Struyf and Conley, 2012, Tréguer and De La Rocha, 2013) it is common practice to determine two parameters: dissolved silica (DSi) and particulate silica (PSi). Since Conley (1997), the biogenic silica fraction (BSi) from the rivers and the estuary PSi is taken into account in the global cycle of Si because it is mostly composed of diatom frustules that dissolve faster than the crystalline silicates and constitute a pool of bio-available Si for marine diatoms. In soils, BSi that is dominantly composed of phytoliths (PhSi), is useful for assessing the role of vegetation in the biogeochemical cycle at the ecosystem level (Alexandre et al., 1997, Bartoli, 1983, Cornelis et al., 2011a). Besides phytoliths the soil amorphous silica pool (ASi) may also contain other biogenic silica particles i.e. diatoms, sponges, amoebae (Cary et al., 2005, Clarke, 2003, Sommer et al., 2006) as well as non-biogenic silica material originating from the bed rock (i.e. glass shard) or from pedogenic processes (ISi, Saccone et al., 2007). Pedogenic processes lead to the precipitation of particles with shapes varying from spheres attached to the surface of the detrital grains to cements and duricrusts (Alexandre et al., 1997, Charwick et al., 1989, Drees et al., 1989, Sommer et al., 2006).
ASi combined with poorly crystallized aluminosilicates and Si adsorbed on the surface of iron oxides constitute a complex pool of plant available Si for crops (Cornelis et al., 2011b, Guntzer et al., 2012a, Ma and Takahashi, 2002, Sauer et al., 2006). Recent studies have suggested that DSi and ASi may be impacted by human activities (Carey and Fulweiler, 2012, Clymans et al., 2011a, Conley et al., 2008, Guntzer et al., 2012b, Metzer et al., 2010, Saccone et al., 2008, Struyf et al., 2010a). The global impact of anthropogenic effects (deforestation, cultivation, urbanization) on the global Si cycle is therefore a topic of utmost interest that requires more data and modeling (Laruelle et al., 2009, Struyf and Conley, 2012).
Although DSi and PSi are easily measured with a high accuracy by standard techniques, the quantification of ASi and its sub fraction PhSi is currently performed by several techniques that have not yet been calibrated and assessed in detail (Sauer et al., 2006). Two approaches are currently used to quantify ASi/PhSi. The first approach is based on a physical extraction using heavy liquid flotation (Alexandre et al., 1997, Cornelis et al., 2010, Cornelis et al., 2011b, Kelly, 1990, Piperno, 1988). This non-destructive method was first applied for analyzing phytolith morphology that can be a useful tool in archaeology or palaeoenvironmental studies. Several drawbacks have been pointed out when using physical extraction: poor reproducibility (Herbauts et al., 1994); ASi dissolution when using dispersing agent (Zhao and Pearsall, 1998) and underestimation following clay separation that may contain a significant ASi fraction (Saccone et al., 2007).
The second approach is based on a wet-alkaline digestion. Two extractants are generally used: NaOH (Koning et al., 2002) and Na2CO3 (see review in Sauer et al., 2006). According to Saccone et al. (2007), the extraction by NaOH provides either similar or higher ASi values than extraction by Na2CO3 for reasons not fully understood. The NaOH procedure using an Al correction for subtracting the contributions of crystalline silicates is difficult to implement in routine (Sauer et al., 2006). The method of DeMaster (1981) therein referred to as the 1% Na2CO3 method, consists of a Na2CO3 kinetic extraction of ASi, that takes into account the crystalline contribution (DeMaster, 1981), is easy to implement and is now widely used (e.g., Clymans et al., 2011a, Clymans et al., 2011b, Conley et al., 2008, Metzer et al., 2010, Saccone et al., 2007, Struyf et al., 2010a, Struyf et al., 2010b). The 1% Na2CO3 method was originally proposed for the quantification of amorphous silica pools in marine sediments that were mostly made of diatom frustules (Conley, 1998, DeMaster, 1981).
According to Clymans et al. (2011b), “over the recent years, the wet alkaline…has become the standard technique for ASi in marine as well as in soil research” and “the method of DeMaster (1981) is now the de facto standard for the analysis in aquatic environment.” As suggested by Cornelis et al. (2011b) the 1% Na2CO3 method has nonetheless some limitations as it may also dissolved poorly crystallized aluminosilicates present in volcanic rocks. Notwithstanding the advantages of the 1% Na2CO3 method, it is yet to be validated for phytoliths. Indeed, it is assumed that phytoliths are dissolving at the same rate as the diatoms for which the method was originally developed, but there is a lack of research to support this assumption. Like sponge spicules or radiolarian tests (Conley, 1998), phytoliths may be more difficult to digest. The solubility of phytoliths at acidic to neutral pH is similar to others forms of amorphous silica (Fraysse et al., 2009). However at the alkaline condition, Cabanes et al. (2011) showed that the solubility of fossil (from soil) phytoliths was found to be 1.8 to 2 times lower than that of modern phytoliths (from fresh plant).
In the present study, we tested the efficiency of the 1% Na2CO3 method to extract PhSi from soils and fresh water sediments. First, the 1% Na2CO3 method was applied to fresh phytoliths and compared with other standard methods (Guntzer et al., 2010). Second, we evaluated the ability of the 1% Na2CO3 method to dissolve aged (or fossil) phytoliths by using artificial mixtures of ground quartz and soil phytoliths that have previously been well characterized. Finally, we compared 1% Na2CO3 and physical extraction methods for quantifying ASi in various soils and sediments.
Section snippets
The 1% Na2CO3 method
The principle is based on the chemical ability of ASi to dissolve at a faster rate than crystalline silicate particles. At 85 °C in a 1% Na2CO3 solution (pH = 11.2), approximately 30 mg of diatoms are totally dissolved within 3 h (DeMaster, 1981). Forty milliliters of 1% Na2CO3 solution were added to 30 mg ± 0.2 (using a Sartorius BP121S balance) of dried material (soil or sediment) in polypropylene tubes and placed for digestion in a shaker bath at 85 °C with caps slightly loosened to vent gases. After
Results
For samples containing only fresh phytoliths, PhSi extracted by the 1% Na2CO3 method was significantly well correlated (R2 = 0.99) with the PhSi extracted by the calibrated Tiron method (Table 2 and Fig. 1). The concentration of Si determined for aged phytoliths was 258.2 mg g− 1 ± 0.6%. This concentration was used to define the expected 100% PhSi pole if the 1% Na2CO3 technique was able to dissolve all the aged phytoliths. Therefore, PhSi amounts for the samples containing a mixture of aged
Validation of the 1% Na2CO3 method for fresh and aged phytoliths
Our present data showed that the 1% Na2CO3 method is well correlated with standard methods for the quantification of fresh phytoliths. We did not measure PhSi for samples containing values higher that 70 mg g− 1, which is a value rarely achieved in soils (Clarke, 2003). Saccone et al. (2007) previously applied the 1% Na2CO3 method to a horsetail sample containing a larger amount of ASi i.e. 336.2 mg SiO2 g− 1 (157 mg Si g− 1). The authors isolated phytoliths from plants by digestion in H2O2 and HNO3
Conclusion
The 1% Na2CO3 technique has been assessed for fresh phytoliths and was found to be well adapted for PhSi values below approximately 70 mg g− 1. We showed that the grain size and age of the phytoliths in soils and sediments can affect the interpretation of PhSi concentration obtained using the 1% Na2CO3 method. Comparing physical extraction and the 1% Na2CO3 method did not show a statistically significant correlation for values below 10 mg g− 1 but gave a reasonable estimation range of ASi/PSi taking
Acknowledgments
This study has benefited from the support of the French National Program EC2CO and from the Institut de la Recherche pour le Développement. The South Indian samples were analyzed in the framework of ORE–BVET project (Observatoire de Recherche en Environnement–Bassin Versant Expérimentaux Tropicaux, www.orebvet.omp.obf-mip.fr). We thank Karnataka Forest Department and the staff of Bandipur National Park for all the facilities and support they provided. We also thank Jérôme Labille, Doris
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2021, GeodermaCitation Excerpt :We propose that, in the studied soils, ox-Na2CO3-Si and XRD-hlPhSi contents do not represent the same phytolith pool. Phytoliths with different dissolution rates coexist in soil: fresh phytoliths quickly dissolve whereas aged ones dissolve slowly (Alexandre et al., 2011, 1997; Meunier et al., 2014). Strömberg et al. (2018) have distinguished labile from stable phytoliths, and suggested some preservation modes of fossil phytoliths.