Effects of Coupled Biogeochemical and Transport Processes on Soil Aggregate-Scale Selenium Speciation

Abstract:

Biogeochemical processes controlling elemental cycling in soil 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, which can lead to mass-transfer limitations and to chemical gradients over short distances. The objective of this study is to investigate how the coupled transport and biogeochemical processes that occur at the soil-aggregate scale affect selenium speciation and immobilization within soils.

We present a combined experimental and modelling study on artificial soil aggregates using a complex, but controlled, setting representative of natural systems. Circumventing byproduct accumulation and substrate exhaustion common in batchs and avoiding the poor physical analogy to soils of homogenously packed columns, our 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.

Model simulations showed extensive intra-aggregate, mm-scale radial variations in selenium distribution, reproducing the trends observed experimentally. Anoxic microzones developed over time within soil aggregates, both under oxic and anoxic conditions. We showed that those chemical gradients are mainly controlled by the coupling and respective importance of transport and microbial selenium reduction. Furthermore, we found that solid-phase concentrations of reduced selenium increased from the advection boundary (macropore) toward the aggregate cores, which would imply that more selenium can be sequestered in soils with larger aggregates. Simulations predict that selenium retention is positively correlated with aggregate size. Overall, this work highlights the importance of the spatial connection between reaction and transport fronts. It points out to the importance of obtaining information on transport-limited, intra-aggregate biogeochemical dynamics to better understand reactive transport of redox-sensitive species in structured soils.