Given the role of Se as both an environmental contaminant and a micronutrient, the microbial reduction and subsequent sequestration of bioavailable Se in soils are of great ecological interest. Primary particles in surface soils are typically bound into loosely packed, microporous aggregates, which may be critical spatial units in determining the fate of Se in soils. Surrounded by macropores where preferential flow rapidly advects dissolved compounds, soil aggregates are domains of slow diffusive transport where spatial variations in chemical concentrations and biogeochemical reactions can prevail. We conducted a series of controlled flow‐through experiments utilizing three‐dimensional, artificial soil aggregates (2.5‐cm i.d.) surrounded by a macropore. Aggregates were composed of either quartz sand or ferrihydrite‐coated sand inoculated with one of two Se‐reducing bacteria ( Thauera selenatis or Enterobacter cloacae SLD1a‐1). Selenite export rates varied between 0.02 ± 0.01 and 3.4 ± 0.2 nmol h ⁻¹ g ⁻¹ as a function of aeration condition and input solution composition (higher SeO ₄ ²⁻ or C‐source concentrations led to higher SeO ₃ ²⁻ export). Oxic input conditions significantly decreased Se reduction; however, the detection of SeO ₃ ²⁻ in effluent samples indicates the occurrence of anoxic microzones within aggregates. Furthermore, we found that solid‐phase concentrations of reduced Se increased toward the core of aggregates and are estimated to at least double within the first millimeter into the aggregate under all conditions investigated. This indicates that concentrations of reduced Se may generally be expected to increase with distance from the advection boundary (macropore) inside aggregates, which would imply that soils with larger aggregates retain more Se.

Plant-soil interactions affect arsenic and nutrient availability in arsenic-contaminated soils, with implications for arsenic uptake and tolerance in plants, and leaching from soil. In 22-week column experiments, we grew the arsenic hyperaccumulating fern Pteris vittata in a coarse- and a medium-textured soil to determine the effects of phosphorus fertilization and mycorrhizal fungi inoculation on P. vittata arsenic uptake and arsenic leaching. We investigated soil arsenic speciation using synchrotron-based spectromicroscopy. Greater soil arsenic availability and lower nutrient content in the coarse-textured soil were associated with greater fern arsenic uptake, lower biomass (apparently a metabolic cost of tolerance), and arsenic leaching from soil, due to lower transpiration. P. vittata hyperaccumulated arsenic from coarse- but not medium-textured soil. Mass of plant-accumulated arsenic was 1.2 to 2.4 times greater, but aboveground biomass was 74% smaller, in ferns growing in coarse-textured soil. In the presence of ferns, mean arsenic loss by leaching was 195% greater from coarse- compared to the medium-textured soil, and lower across both soils compared to the absence of ferns. In the medium-textured soil arsenic concentrations in leachate were higher in the presence of ferns. Fern arsenic uptake was always greater than loss by leaching. Most arsenic (>66%) accumulated in P. vittata appeared of rhizosphere origin. In the medium-textured soil with more clay and higher nutrient content, successful iron scavenging increased arsenic release from soil for leaching, but transpiration curtailed leaching.

Soil aggregates are mm- to cm-sized microporous structures separated by macropores. Whereas fast advective transport prevails in macropores, advection is inhibited by the low permeability of intra-aggregate micropores. This can lead to mass transfer limitations and the formation of aggregate scale concentration gradients affecting the distribution and transport of redox sensitive elements. Selenium (Se) mobilized through irrigation of seleniferous soils has emerged as a major aquatic contaminant. In the absence of oxygen, the bioavailable oxyanions selenate, Se(VI), and selenite, Se(IV), can be microbially reduced to solid, elemental Se, Se(0), and anoxic microzones within soil aggregates are thought to promote this process in otherwise well-aerated soils. To evaluate the impact of soil aggregate size on selenium retention, we developed a dynamic 2-D reactive transport model of selenium cycling in a single idealized aggregate surrounded by a macropore. The model was developed based on flow-through-reactor experiments involving artificial soil aggregates (diameter: 2.5 cm) made of sand and containing Enterobacter cloacae SLD1a-1 that reduces Se(VI) via Se(IV) to Se(0). Aggregates were surrounded by a constant flow providing Se(VI) and pyruvate under oxic or anoxic conditions. In the model, reactions were implemented with double-Monod rate equations coupled to the transport of pyruvate, O₂, and Se species. The spatial and temporal dynamics of the model were validated with data from experiments, and predictive simulations were performed covering aggregate sizes 1–2.5 cm in diameter. Simulations predict that selenium retention scales with aggregate size. Depending on O₂, Se(VI), and pyruvate concentrations, selenium retention was 4–23 times higher in 2.5 cm aggregates compared to 1 cm aggregates. Under oxic conditions, aggregate size and pyruvate concentrations were found to have a positive synergistic effect on selenium retention. Promoting soil aggregation on seleniferous agricultural soils, through organic matter amendments and conservation tillage, may thus help decrease the impacts of selenium contaminated drainage water on downstream aquatic ecosystems.