Role of exogenous organic matter in the transfer of trace elements in the soil-plant system: contribution of metal isotopes, rare earth elements, and organic matter characterization by Bryan Arbalestrie

Trace elements are chemical elements present in low concentrations in the environment. While some are essential for organisms, all trace elements become toxic above a certain threshold. Imbalanced levels—either elevated or deficient—pose health risks, raising global concerns. In soils, trace elements primarily derive from parent materials, but human activities, such as industrial and agricultural activities, have substantially increased soil contents. Their dynamics within the soil–plant system, crucial to human nutrition, are not well understood, especially regarding the factors influencing retention and transfer in agroecosystems. Agricultural practices like adding exogenous organic matter to soils can affect trace element dynamics, influencing both environmental and health outcomes. This thesis aimed to assess the influence of exogenous organic matter on trace element behavior in the soil-plant system, focusing on processes that control trace element availability and mobility. Results indicated that the application of exogenous organic matter increased or decreased trace element transfer to soil solutions and plant parts, suggesting that organic matter served both as a source and retention surface for trace elements. Soil properties, however, were identified as key factors controlling the impact of organic matter on trace element transfer. Comprehensive chemical characterizations (13C solid-state NMR, Rock-Eval, and Py-GCMS) of both soil and exogenous organic matters revealed that organic matter quality mostly controlled trace element transfer to soil solution. These findings underline the importance of intrinsic soil characteristics in optimizing trace element management through organic matter inputs. Finally, to address the low natural concentrations and time limitations of the experimental setups, rare earth elements and stable metal isotopes were used as geochemical tracers to better understand trace element transfer processes.