Mn/Fe reduction in sandy soil during degradation of de-icing chemical: Respiration kinetics and prokaryotic community composition
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At Oslo airport, Gardermoen, Norway, large quantities of propylene glycol (PG) are used as de-icing fluid during winter, causing high loads of this chemical to infiltrate in surrounding soil during snow melt and increasing concentrations of soluble manganese (Mn2+) and ferrous iron (Fe2+) in the groundwater. Previous studies have suggested that anaerobic microbial Mn and Fe reduction fuelled by PG in deeper soil layers is the primary reason for the observed increase of Mn2+ and Fe2+ and proposed nitrate (NO3-) fertilization as a mitigation measure. However, laboratory and field experiments with NO3- addition have yielded inconsistent, partly adverse results. To better understand the effect PG has on Mn2+ and Fe2+ release in the Gardermoen soil system, in the presence or absence of moderate NO3- concentrations, a series of batch incubation experiments was carried out with non-contaminated top and subsoil sampled at a research site close to Oslo airport. Microbial activity was measured as O2, CO2, NO, N2O and N2 kinetics, while the release of Fe2+ and Mn2+ was monitored by subsampling the soil solution through microrhizones. After 26 days of incubation, 16S-rDNA was extracted and sequenced to study the effect of PG and N on microbial community composition. Both, top- and subsoil released Mn2+ and Fe2+ in untreated controls but release rates were larger in the presence of PG. Top soil released on average 100 to 1000 times more Mn2+ than subsoil, which was attributed to a larger abundance of Mn(IV) reducing bacteria in the topsoil. Unlike in top soil, moderate NH4NO3 addition to the subsoil triggered additional Mn2+ release, apparently by relieving N limitation of microbial growth. The comparison of 16S rRNA-based taxonomic abundances before and after incubation of subsoil with PG and NO3- revealed that the metabolically versatile families Comamonadaceae, Oxalobacteraceae and Pseudomonadaceae increased in relative abundance, likely dominating PG metabolism and Mn and Fe reduction observed at the end of the incubation. Therefore, NO3- addition, although providing an alternative electron acceptor, cannot be recommended for mitigation of metal release, as it poses the risk to increase microbial Mn and Fe reduction in the subsoil. In contrast, measures that lead to a better aeration of the top layer, particularly during wet periods, appear to be the more promising approach to avoid Mn2+ and Fe2+ release.