The ratio of germanium to silicon in plant phytoliths: quantification of biological discrimination under controlled experimental conditions

Abstract

Slight differences in the chemical behavior of germanium (Ge) and silicon (Si) during soil weathering enable Ge/Si ratios to be used as a tracer of Si pathways. Mineral weathering and biogenic silicon cycling are the primary modifiers of Ge/Si ratios, but knowledge of the biogenic cycling component is based on relatively few studies. We conducted two sets of greenhouse experiments in order to better quantify the range and variability in Ge discrimination by plants. Graminoid species commonly found in North American grassland systems, Agropyron smithii, Schizachyrium scoparium, and Andropogon gerardii were grown under controlled hydroponic environmental conditions. Silicon leaf contents were positively correlated with solution Si and ambient temperature but not with nutrient solution pH, electrical conductivity, or species. The Ge/Si ratio incorporated into phytoliths shows a distribution coefficient [(Ge/Si) phytolith /(Ge/Si) solution ] of about 0.2 and is remarkably invariant between species, photosynthetic pathway, and solution temperature. Ge seems to be discriminated against during the uptake and translocation of Si to the opal deposition sites by about a factor of five. In the second experiment, a wider range of graminoid species (Agropyron smithii, Bouteloua gracilis, Buchloe dactyloides, Oryzopsis hymenoides, Schizachyrium scoparium and Andropogon gerardii) were grown in two different soil mediums. Plant phytoliths showed a distribution factor of about 0.4 for field grown grasses, and 0.6 for potting soil grown grasses with no clear trends among the species. Evidence of the direction and degree of biological Ge discrimination during plant uptake provides a geochemical finger print for plants and improves the utility of Ge/Si ratios in studies of terrestrial weathering and links between Si cycles in terrestrial and marine systems.

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

“…Studying Ge/Si fractionation allows for tracing the weathering of Sicontaining phases and the dynamics of DSi produced in soilplant systems (Kurtz et al, 2002;Derry et al, 2005;Scribner et al, 2006). Secondary clay minerals and biogenic opal display contrasting Ge/Si ratios since clay-sized weathering products are enriched in Ge (Murnane and Stallard, 1990;Kurtz et al, 2002;Kurtz and Derry, 2004;Delvigne et al, 2009;Opfergelt et al, 2010;Lugolobi et al, 2010;Cornelis et al, 2010a) and biogenic silica (BSi) polymerized in plants as phytoliths is depleted in Ge (Derry et al, 2005;Blecker et al, 2007;Delvigne et al, 2009;Cornelis et al, 2010a;Lugolobi et al, 2010). The current interpretation of Ge/Si ratios of soil solutions relies on the relative importance of a low-Ge end member derived from incongruent dissolution of fresh rock minerals and biomineralization and a high-Ge end member produced by congruent dissolution of pedogenic minerals (Kurtz et al, 2002).…”

Subcellular localization of silicon and germanium in grass root and leaf tissues by SIMS: evidence for differential and active transport

“…Conley (2002) estimated biological Si cycling in the terrestrial environment to be 60-200 Tmol year -1 ; compared to the 240 Tmol year -1 estimated to be cycled in the marine environment (Conley 2002;Treguer et al 1995). The annual biological turnover of Si in terrestrial environments is at least an order of magnitude higher than the global rate of silica release from weathering of silicate minerals [5-8 Tmol year -1 (Berner and Berner 1996)]. Understanding the role of biological cycling of silica in soil-forming processes and in controlling stream and soil water chemistry is a significant challenge in biogeochemistry.…”

Review of typologic and morphometric analysis of phytoliths produced by wheat and barley

“…More recent research has focused on phytolith chemistry and composition (e.g. Blecker et al 2007, Elbaum et al 2009, Fraysse et al 2009) and the application of phytolith analysis in archaeobotanical, paleoecological and phytogeographical research.…”