Elsevier

Geoderma

Volume 216, March 2014, Pages 30-35
Geoderma

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

https://doi.org/10.1016/j.geoderma.2013.10.014Get rights and content

Highlights

  • The 1% Na2CO3 method is able to quantify fresh phytoliths

  • Aged phytoliths are only partially dissolved by the 1% Na2CO3 method.

  • Physical extraction is a must to identify the proportion of phytoliths while the 1% Na2CO3 method can underestimate (up to a factor of 3.7) the ASi measurements

Abstract

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 Réunion (Meunier et al., 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.

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

References (48)

  • F. Fraysse et al.

    Surface chemistry and reactivity of plant phytoliths in aqueous solutions

    Chem. Geol.

    (2009)
  • J.D. Rimstidt et al.

    The kinetics of silica–water reactions

    Geochim. Cosmochim. Acta

    (1980)
  • Z. Zhao et al.

    Experiments for improving phytolith extraction from soils

    J. Archaeol. Sci.

    (1998)
  • F. Bartoli

    The Biogeochemical cycle of silicon in two temperate forest ecosystems

  • F. Bartoli et al.

    Dissolution of biogenic opal as a function of its physical and chemical properties

    Soil Sci. Soc. Am. J.

    (1980)
  • J.C. Carey et al.

    Human activities directly alter watershed dissolved silica fluxes

    Biogeochemistry

    (2012)
  • L. Cary et al.

    Contribution of phytoliths to the suspended load of biogenic silica in the Nyong basin rivers (Cameroon)

    Biogeochemistry

    (2005)
  • O.A. Charwick et al.

    Silicification of Holocene soils in Northern Monitor Valley, Nevada

    Soil Sci. Soc. Am. J.

    (1989)
  • W. Clymans et al.

    Anthropogenic impact on amorphous silica pools in temperate soils

    Biogeosciences

    (2011)
  • D.J. Conley

    Riverine contribution of biogenic silica to the oceanic silica budget

    Limnol. Oceanogr.

    (1997)
  • D.J. Conley et al.

    Deforestation causes increased dissolved silicate losses in the Hubbard Brook Experimental Forest

    Glob. Chang. Biol.

    (2008)
  • J.T. Cornelis et al.

    Tree species impact the terrestrial cycle of silicon through various uptakes

    Biogeochemistry

    (2010)
  • J.T. Cornelis et al.

    Tracing the origin of disolved silicon transferred from various soil-plant systems towards rivers: a review

    Biogeosciences

    (2011)
  • J.T. Cornelis et al.

    Identification and distribution of the readily soluble silicon pool in a temperate forest soil below three distinct tree species

    Plant Soil

    (2011)
  • Cited by (58)

    • Dissolution does not affect grass phytolith assemblages

      2023, Palaeogeography, Palaeoclimatology, Palaeoecology
    • Rainfall is the major driver of plant Si availability in perudic gibbsitic Andosols

      2021, Geoderma
      Citation 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.

    View all citing articles on Scopus
    View full text