Soil‐Aggregate‐Scale Heterogeneity in Microbial Selenium Reduction

Abstract

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 SeO42− or C‐source concentrations led to higher SeO32− export). Oxic input conditions significantly decreased Se reduction; however, the detection of SeO32− 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.

The microbial impact on the sorption behaviour of selenite in an acidic, nutrient-poor boreal bog

“…In the environment selenium occurs with different oxidation states forming selenide (Se 2À ), elemental Se (Se 0 ), selenite (SeO 3 2À ), selenate (SeO 4 2À ), and organic Se (Kausch et al, 2012;Pezzarossa et al, 1999). At high redox potential selenate dominates and at intermediate redox conditions selenite becomes more prevailing (Nakamaru and Altansuvd, 2014;Pezzarossa et al, 1999).…”

“…The actual mechanism of selenium association with organic matter is still not explained, but organic fractions have been shown to be the major carriers of selenium (Coppin et al, 2009). It has been suggested that selenium sorption on organic particles could only be indirect, mainly resulting from association with Fe oxides or clays residing either on the organic matter surface or fixed within its matrix (Coppin et al, 2009) and is linked to the microbial reduction, which in turn is affected by the local chemical conditions of the soil (Kausch et al, 2012).…”

“…Microbial reduction is one of the major factors influencing the speciation of selenium oxyanions (SeO 4 2À and SeO 3 2À ) and affects the environmental mobility and migration of selenium by removing soluble selenium from solution. In organic soils selenium is often bound to organic substances (Kausch et al, 2012;Pezzarossa et al, 1999) although the association mechanism of selenium with organic matter has so far not been explained. In the organic peatland explored in this study, elevated selenium retention compared to mineral soils, was expected as organic matter has shown to act as the main carrier of selenium (e.g.…”

“…Bruggerman et al, 2007;Kamei-Ishikawa et al, 2008), although the actual mechanism of selenium association with organic matter is still not explained (Coppin et al, 2009). It has been suggested that selenium sorption on organic particles could only be indirect, mainly resulting from association with Fe oxides or clay minerals residing either on the organic matter surface or fixed within its matrix (Coppin et al, 2009) and is linked to the microbial reduction, which in turn is affected by the local chemical conditions of the soil (Kausch et al, 2012). In addition, nitrogen-containing groups (eNH 2 ) found in organic matter are protonated under acidic conditions and selenite sorption onto the resulting positively charged groups through electrostatic interactions is possible.…”

Dependence of Arsenic Fate and Transport on Biogeochemical Heterogeneity Arising from the Physical Structure of Soils and Sediments

“…Physical structure common to soils and sediments results in chemical gradients conducive to submicrometer‐scale variation in biogeochemical processes (Tokunaga et al, 2003; Pallud et al, 2010; Masue‐Slowey et al, 2011a; Kausch et al, 2012), which may influence the fate of As. Soils are structured media, having a multitude of pore domains that result in advective and diffusively controlled solute transport.…”