Cette thèse a pour but d’identifier et de préciser les processus biotiques et abiotiques qui contrôlent le comportement du Ca et plus spécifiquement le fractionnement des isotopes du Ca à l’interface géosphère-biosphère-hydrosphère en combinant différentes études expérimentales (hydroponiques et microcosmes). Les résultats obtenus lors des expérimentations hydroponiques ont permis d’identifier 3 niveaux de fractionnement des isotopes du Ca au sein des végétaux, ces 3 niveaux de fractionnement sont induits par des mécanismes physico-chimiques qui enrichissent les organes de végétaux en 40Ca.Les résultats des expérimentations hydroponiques montrent également que l’évolution de la composition isotopique du Ca de la solution nutritive et des plantes suit une loi de fractionnement des isotopes du Ca à l’équilibre (αplantes/solution nutritive = 0,99858). Les expérimentations abiotiques en microcosmes, quant à elles, indiquent que la dissolution de l’apatite par des acides organiques ou inorganiques n’influence pas la signature isotopique en Ca de la phase dissoute résultante. À l’inverse, lors des expérimentations biotiques en microcosmes, seule l’action combinée des racines de pins et des bactéries sur un substratum d’apatite enrichit la solution qui percole en 44Ca de 0,22‰. / The aim of this thesis is to identify and to specify biotic and abiotic processes affecting the Ca behavior and specifically the Ca isotope fractionation at the geosphere/biosphere/hydrosphere interface by combining different experimental studies (hydroponic and microcosm).Results of hydroponic experiments allow to identify three Ca isotopic fractionation levels, which are driven by physico-chemical mechanisms and enrich the plant organs in the light 40Ca isotope. Moreover, the evolution of Ca isotopic composition of nutrient solution and plants follow an equilibrium law (αplants/nutrient solution = 0.99858).Abiotic column experiments show no Ca isotopic fractonation during apatite disolution, whatever the nature of the acid (mineral or organic). At the opposite, during biotic microcosm experiments, only the combined action of roots of scots pines and bacteria on apatite substratum enrich the percolate solution in heavy 44Ca of 0.22‰.

The stable isotope ratio of non-exchangeable hydrogen (δ2Hn) emerged as a promising new tool for source attribution in aquatic ecology, which can better discriminate between the organic matter sources. However, determining the absolute isotopic values is problematic due to the functional groups with hydrogen (H) that can easily exchange with the ambient water. This can lead to significant uncertainties in the absolute isotopic values, which eventually translates into errors in source attribution. However, controlled H exchange experiments with dual water equilibrations can alleviate this problem. However, current methods report significant variation in the absolute values, likely originating in partial H exchange, uncertainty in fractionation factors between exchangeable H and ambient water, and residual moisture. Here I used two methods for H exchange based on steam and liquid water using novel equipment for sample preparation called the Isobox. I evaluated the exchangeable H fraction in materials commonly analysed in ecology. Furthermore, I have investigated how these methods and associated analytical uncertainties can influence the mathematical mixing models used to resolve the source attribution using known sample mixtures made with soils and algae. The results show that 1) H exchange experiments using liquid water equilibration provide a higher exchangeable H fraction than the steam-based method, which was approximately three times higher. 2) The two-source mixing model proved to effectively determine the source attribution with known soil and algae mixtures, as evidenced by both water and steam equilibration. However, prior exposure to isotopically divergent waters can lead to source attribution errors, particularly with steam-based methods that provide low exchangeable H fractions. 3) When labile H is fully exchanged, source attribution does not depend on absolute δ2Hn determination, and simple one-water equilibration is sufficient. 4) Additional uncertainties in source attribution could originate from fractionation factors and δ2H measurement variability. The findings of this study conclude that the variations in fractionation factors did not significantly alter the mixing model as the error was below 5%. 5) Based on these experiments, I recommend keeping the analytical uncertainty of δ2H below ±5‰, which amounts to about a 6% error factor in source attribution. Finally, these experiments and analyses show how the methods can generate reliable data, depending on the research questions and whether absolute or relative isotopic values are required. This study provides different analytical pathways.