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Biodisponibilité du calcium, magnésium et potassium dans les sols forestiers : Quantification, traçage et caractérisation des sources biodisponibles par dilution isotopique stable (⁴⁴Ca, ²⁶Mg et ⁴¹K)

Abstract : The plant available pools of calcium, magnesium and potassium are commonly assumed to be stored in the soil as exchangeable cations adsorbed on the cationic exchange capacity and extractible with a concentrated salt reagent (e.g. NH₄⁺, Ba²⁺). Many forest ecosystems have low to very low exchangeable pools of nutrient cations in the soil yet they show no nutrient deficiency symptoms. The discrepancies between different soil chemical fertility diagnosis approaches suggest that trees access nutrient resources that are not currently taken into account by conventional soil analysis methods. In the context of global change, the degradation of the chemical fertility of forest soils is a major concern. Identifying and quantifying the plant-available resources of nutrients in the soil is thus essential to better understand the biogeochemical functioning of forest ecosystems and ensure their sustainable management. The main objective of this PhD research is to identify and quantify the pools of Ca, Mg and K that contribute directly to the geochemical equilibrium processes between the soil and the solution with an innovative approach: ⁴⁴Ca, ²⁶Mg and ⁴¹K stable isotopic dilution. The isotopically exchangeable pools of Ca, Mg and K were measured by isotopic dilution in 177 individual soil samples from 26 sites covering a wide range of climatic, soil and forest cover contexts. The results demonstrate that the source/sink pool of the soil (pool in geochemical equilibrium with the soil solution, i.e. isotopically exchangeable pool) was not solely composed of the exchangeable pools measured by conventional soil analysis methods. The isotopically exchangeable pools were greater than the conventional exchangeable pools for 81%, 87% and 90% of the analyzed samples respectively for Ca, Mg and K. The additional pools of Ca, Mg and K in many cases were greater than 50% of their respective conventional exchangeable pool. Although the isotopically exchangeable pools are mainly composed of Ca, Mg and K extractible with an NH₄⁺ reagent, results show that Ca, Mg and K contained in soil phases extractible with a strong acid (HNO₃) also contribute directly to equilibrium processes between the soil and the solution. These available pools that are not currently accounted for by conventional soil analysis methods, are likely associated to amorphous and poorly crystalline secondary minerals in interaction with organic compounds for Ca and to K-specific sites in phyllosilicates and/or amorphous aluminosilicates for K. Comparing the changes in conventional physical and chemical soil properties and isotopically exchangeable pools of Mg, Ca and K at 11 sites of the RENECOFOR network between two sampling dates 11 years apart enabled to demonstrate that the isotopically exchangeable pools are very dynamic and play an active role in buffering the plant-availability of nutrient cations in the soil.
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Submitted on : Friday, September 24, 2021 - 5:40:03 PM
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Jérémie Bel. Biodisponibilité du calcium, magnésium et potassium dans les sols forestiers : Quantification, traçage et caractérisation des sources biodisponibles par dilution isotopique stable (⁴⁴Ca, ²⁶Mg et ⁴¹K). Ecosystèmes. Université de Lorraine, 2021. Français. ⟨NNT : 2021LORR0078⟩. ⟨tel-03354259⟩

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