Experimental identification of Ca isotopic fractionations in higher plants Florian Cobert a , Anne-De´sire´e Schmitt b,⇑ , Pascale Bourgeade b , Franc ßois Labolle c , Pierre-Marie Badot b , Franc ßois Chabaux a , Peter Stille a a Laboratoire d’Hydrologie et de Ge´ochimie de Strasbourg, Universite´ de Strasbourg/EOST, CNRS, 1 rue Blessig, F-67000 Strasbourg, France b Universite´ de Franche-Comte´ et CNRS-UMR 6249, Chrono-environnement, 16, Route de Gray, 25030 Besanc ßon Cedex, France c Universite´ de Strasbourg, Institut de Zoologie et de Biologie ge´ne´rale, 12, rue de l’Universite´, 67000 Strasbourg, France Received 22 November 2010; accepted in revised form 20 June 2011; available online 29 June 2011 Abstract Hydroponic experiments have been performed in order to identify the co-occurring geochemical and biological processes affecting the Ca isotopic compositions within plants. To test the influence of the Ca concentration and pH of the nutritive solution on the Ca isotopic composition of the different plant organs, four experimental conditions were chosen combining two different Ca concentrations (5 and 60 ppm) and two pHs (4 and 6). The study was performed on rapid growing bean plants in order to have a complete growth cycle. Several organs (root, stem, leaf, reproductive) were sampled at two different growth stages (10 days and 6 weeks of culture) and prepared for Ca isotopic measurements. The results allow to identify three Ca isotopic fractionation levels. The first one takes place when Ca enters the lateral roots, during Ca adsorption on cation-exchange binding sites in the apoplasm. The second one takes place when Ca is bound to the polygalacturonic acids (pectins) of the middle lamella of the xylem cell wall. Finally, the last fractionation occurs in the reproductive organs, also caused by cation-exchange processes with pectins. However, the cell wall structures of these organs and/or number of available exchange sites seem to be different to those of the xylem wall. These three physico-chemical frac- tionation mechanisms allow to enrich the organs in the light 40 Ca isotope. The amplitude of the Ca isotopic fractionation within plant organs is highly dependent on the composition of the nutritive solution: low pH (4) and Ca concentrations (5 ppm) have no effect on the biomass increase of the plants but induce smaller fractionation amplitudes compared to those obtained from other experimental conditions. Thus, Ca isotopic signatures of bean plants are controlled by the external nutritive medium. This study highlights the potential of Ca isotopes to be applied in plant physiology (to identify Ca uptake, circulation and storage mechanisms within plants) and in biogeochemistry (to identify Ca recycling, Ca content and pH evolutions in soil solutions through time). Ó 2011 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Sustainable development of forested ecosystems includ- ing understanding of nutrient element fluxes at the watershed scale require a better comprehension of the dynamics of nutritive elements at the rock–water–soil–plant interfaces and the unravelling of the multiple transfer pro- cesses occurring between these different compartments. While chemical weathering of soil minerals has intensively been studied at various scales, the role of vegetation on con- tinental weathering processes and the contribution of plants to the dissolved loads of rivers have been less thoroughly investigated. However, plants are known to accelerate chemical weathering by mobilising metals from soils by a factor of 2–5 and to retard their release to drainage waters by recycling metals (Berner et al., 2004). 0016-7037/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.06.032 ⇑ Corresponding author. Tel.: +33 3 68 85 04 43; fax: +33 3 68 85 04 02. E-mail address: adschmit@univ-fcomte.fr (A.-D. Schmitt). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 75 (2011) 5467–5482