Effects of aggregate-scale heterogeneity on selenium immobilization in soil

Background/Question/Methods

Understanding and predicting the fate and transport of nutrients and contaminants in natural systems is a continuing challenge in environmental science. Within soils, 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 selenium reduction and immobilization within soils.

We present a combined experimental and modelling study on single artificial soil aggregates assessing the transport and biogeochemical processes governing selenium mobility 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.

Results/Conclusions

Our findings demonstrate significant aggregate-scale variations in biogeochemical processes and consequent distribution patterns of selenium within soils. Anoxic microzones develop over time within soil aggregates, both under oxic and anoxic conditions. We show that those chemical gradients are mainly controlled by the coupling and respective importance of aggregate-scale transport and microbial selenium reduction. Furthermore, we found that under all conditions investigated solid-phase concentrations of reduced selenium increased toward the aggregate cores and double within the first mm away from the advection boundary (macropore). Simulations predict that selenium retention is positively correlated with aggregate size and that under oxic conditions aggregate size and electron donor concentrations have a positive synergistic effect on selenium retention.

Overall, this work 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.