Ca isotopes in ecosystem research

Anne-Désirée Schmitt1, Peter Stille1, Chabaux François1, Marie-Laure Bagard1, François Labolle2, Sophie Gangloff1, Florian Cobert1
1 LHyGes-EOST/University of Strasbourg
2 Université de Strasbourg, Institut de Zoologie et de Biologie générale, 12, rue de l’université, F-67000 Strasbourg

Invited Talk 8 in Trace element and metal biogeochemistry

14.07.2014, 10:45-11:15, H20

Recent studies have shown that Ca isotope ratios have the potential to be important tracers of biological activities in forested ecosystems or more generally in plant physiology and in biogeochemistry.

Hydroponic experiments, performed on rapid growing bean plants, that allow to have a complete growth cycle, helped to identify the mecanisms responsible for these Ca isotope fractionations (Cobert et al., 2011; Schmitt et al., 2013). Indeed, the adsorption of Ca by lateral roots, that are enriched in 40Ca compared to the nutritive medium, follows a closed-system equilibrium fractionation with a fractionation factor of 0.9988, suggesting that Ca forms exchangeable bonds with the RCOO- groups in the cell wall structure of the lateral roots. Two other fractionation levels have been identified within the plant during the Ca transfer from roots to shoots. When the xylem sap goes to the shoots, 40Ca is preferentially bound to the polygalacturonic acids (pectins) of the middle lamella of the xylem cell wall. Finally, a third fractionation occurs in the reproductive organs also caused by cation-exchange processes with pectins. The fractionation mechanisms are the same whatever the Ca content and pH of the nutritive solution. Only the bean plants average signature as well as the amplitude of the Ca isotopic fractionation within plant organs are highly dependent on the composition of the nutritive solution. A comparative field study is installed to examine the Ca isotopic fractionation in trees of a forested watershed.

Field studies performed in forested watersheds point for their part to the importance of Ca isotope fractionations in soil solutions due to biological activity in the surficial soil horizons.  Two studies performed in two distinct environments suggest different mechanisms. One study, performed in the temperate forested Strengbach watershed points to the importance of vegetation nutrient uptake (Cenki-Tok et al., 2009), whereas the other one, performed in the boreal permafrost-dominated siberian Kulingdakan watershed, rather hints on the importance of biological degradation and the presence of organo-mineral colloïds (Bagard et al., 2013). These studies emphasise that Ca isotopes are important tracers of the Ca nutrient availability in soils and may be used as a tool to identify and quantify Ca uptake/recycling in the critical zone.

Cenki-Tok et al., 2009. GCA 73, 2215-2228 ; Cobert et al. 2011, GCA 75, 5467-5482; Bagard et al., 2013. GCA 114, 169-187 ; Schmitt et al., 2013. GCA 110, 70-83. 

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last modified 2014-06-19