Abstract
Ge/Si ratios of plant phytoliths have been widely used to trace biogeochemical cycling of Si. However, until recently, information on how much of the Ge and Si transferred from soil to plants is actually stored in phytoliths was lacking. The aim of the present study is to (i) compare the uptake of Si and Ge in three grass species, (ii) localize Ge and Si stored in above-ground plant parts and (iii) evaluate the amounts of Ge and Si sequestrated in phytoliths and plant tissues. Mays (Zea mays), oat (Avena sativa) and reed canary grass (Phalaris arundinacea) were cultivated in the greenhouse on soil and sand to control element supply. Leaf phytoliths were extracted by dry ashing. Total elemental composition of leaves, phytoliths, stems and roots were measured by ICP-MS. For the localization of phytoliths and the determination of Ge and Si within leaf tissues and phytoliths scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) was used. The amounts of Si and Ge taken up by the species corresponded with biomass formation and decreased in the order Z. mays > P. arundinacea, A. sativa. Results from LA-ICP-MS revealed that Si was mostly localized in phytoliths, while Ge was disorderly distributed within the leaf tissue. In fact, from the total amounts of Ge accumulated in leaves only 10% was present in phytoliths highlighting the role of organic matter on biogeochemical cycling of Ge and the necessity for using bulk Ge/Si instead of Ge/Si in phytoliths to trace biogeochemical cycling of Si.
Highlights
Germanium (Ge) is widespread in most rock forming minerals and soil minerals, but with quantities that are a factor of roughly 10,000 lower than silicon (Si)
A comparison of plants cultivated on soil with those on sand revealed significantly lower concentrations in soil than on sand, except in the reference plants where Ge was not artificially added (Fig. 1, Online Resource 1). This can be largely explained by interactions of the Ge in the watering solution with Fe-oxyhydroxides and soil organic matter lowering its solubility and availability to plants (Bernstein 1985; Kabata-Pendias 2010; Wiche et al 2018). Overall these results suggest that the mobilization of Ge together with Si in the rhizosphere is a common mechanism among grasses to maintain Si supply.differences in the capability to take up the mobilized elements and transfer it into the shoots seems to be the critical step determining the distribution/allocation/accumulation of Ge in different plant tissues
Based on the chemical similarities between Ge and Si for a long time it was assumed that Ge taken up by plants behaves in the plant and is stored in phytoliths together with Si justifying the use of phytolith Ge/Si ratios to trace biogeochemical cycling of Si
Summary
Germanium (Ge) is widespread in most rock forming minerals and soil minerals, but with quantities that are a factor of roughly 10,000 lower than silicon (Si). As a result of laboratory and field experiments, there is evidence that the biogeochemical cycling of Ge is not analogous to that of Si (Pokrovski and Schott 1998). The rationale behind this argument is that, unlike Si, Ge exhibits distinct chalcophilic, lithophilic, siderophilic and organophilic behaviour in the environment. Under most biogeochemically relevant conditions Ge is able to form stable complexes with soil organic matter while for Si this organophilic behaviour seems to be much less pronounced (Pokrovski and Schott 1998)
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