Etude du rôle de la végétation dans le cycle biogéochimique du magnésium : Approche isotopique

Abstract : The knowledge of biogeochemical cycles in ecosystems and the biological processes responsible for the mobility of elements in soil is still a challenge. Some studies show the important role of plants on mineral weathering and mobilization of elements in the environment. However, identification and quantification of biological mechanisms behind the dynamics of the elements are still improving. On a global scale, or even a smaller scale such as a watershed, the impact of plants on dissolved elements carried by rivers is difficult to quantify. Therefore, the objective of this thesis was to develop and use a new isotopic tracer to allow identification of altered mineral phases and biological processes involved. Among the elements for which it is now possible to characterize accurately the isotopic signatures, magnesium (Mg) has been chosen (1) because of its link with the carbon cycle, and (2) because it represents a macronutrient essential for living organisms: it is particularly important for plant growth. Thus, the dynamics of Mg will depend on the biological and physico?chemical mechanisms implemented to its mobilization and/or assimilation. The first part of this thesis has been devoted to the development of a protocol for chemical and analytical analysis of Mg isotopes in a wide range of samples. This protocol was validated by measuring the Mg isotope compositions of various reference materials including typical continental crust rocks, plants and waters. It was then applied to study the role of vegetation in the biogeochemical cycling of Mg. A first study was carried out under controlled conditions to quantify the Mg isotope fractionation during plant growth, using two plant species and two different sources of Mg (directly available for plant nutrition (in solution) or not (in the structure of minerals). Both plant species are systematically enriched in heavy isotopes relative to Mg initial sources. Within the plant, the light isotopes preferentially migrate from roots to leaves. This fractionation during the translocation of Mg within the plant seems even stronger the source is poor in Mg, suggesting a key role in the growth environment. For the first time, Mg isotope compositions of natural plants were analyzed, and show a range not significantly different from that of the continental rocks and silicate rivers. A study conducted on a small silicate watershed in Vosges (Northeast of France) shows that the Mg isotope compositions of the herbaceous plant or young spruce reflects the same sense of fractionation than those identified experimentally. During the growing season, the Mg isotopic composition of soil solutions in the surface horizons cannot be explained by a simple mixture and reflect the essential role of Mg uptake by plant and biological recycling via litter degradation. The isotope composition of Mg of the stream varies with the discharge. The streamwater results from either soil surface runoff or water, with a long residence time in the saprolith reflecting water-rock, precipitation-dissolution, interactions. All these studies demonstrate the interest to exploit this new tracer isotope at a soil or ecosystem scale, but also highlight its limits on a larger (continental) scale.
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Bolou Emile Bolou Bi. Etude du rôle de la végétation dans le cycle biogéochimique du magnésium : Approche isotopique. Sciences de la Terre. Université Henri Poincaré - Nancy 1, 2009. Français. ⟨NNT : 2009NAN10077⟩. ⟨tel-01748320⟩

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