Hétérogénéité spatiale des processus biogéochimiques contrôlant le cycle du fer et du selenium dans les sols

Understanding and predicting the fate and transport of nutrients and contaminants in natural systems is a continuing challenge in environmental geochemistry. Within a soil, biogeochemical processes controlling elemental cycling are heterogeneously distributed due to its complex physical structure. The aggregate scale (mm-cm) is of particular interest due to the sharp transition in pore size between the aggregates themselves and the macropores surrounding them. The objective of this study is to investigate how the coupled physical (transport) and biogeochemical processes that occur at the soil-aggregate scale affect redox sensitive elements within soils. We will present a combined experimental and modelling study on single artificial soil aggregates assessing the biogeochemical processes governing transformations of iron and immobilization of selenium in a complex, but controlled, setting representative of natural systems. Circumventing byproduct accumulation and substrate exhaustion common in batch systems and avoiding the poor physical analogy to aggregated soils of homogenously packed columns, our novel experiments mimic soils using constructed cm-scale aggregates in flow-through reactors, which results in diffusively and advectively controlled regions. A reactive transport model is used to delineate transport regimes, identify reaction zones, and estimate kinetic parameters and reaction rates at the aggregate scale. Our findings demonstrate significant aggregate-scale variations in biogeochemical processes and consequent distribution patterns of solid phases within soils. We show that those chemical gradients are mainly controlled by diffusive mass-transfer limitations of both solute delivery to the aggregates and metabolite removal from the aggregates. This highlights the importance of appreciating the spatial connection between reaction and transport fronts and of obtaining information on transport-limited, intra-aggregate biogeochemical dynamics to better understand reactive transport of redox-sensitive species in structured soils.