The biogeochemical cycling of sulfur influences trace-metal redox behavior, nutrient cycling, microbiological energetics, and plant growth. Under anaerobic soil conditions, sulfate is reduced to sulfide, which subsequently reacts with Iron (II or III) to form insoluble, black-colored iron monosulfides (FeS). FeS are a precursor to pyrite formation, potential acid sulfate soil materials, and trace metal scavengers. While FeS concentrations are stable under anaerobic conditions, they disappear upon oxidation and consequentially lower soil pH. Although the exact mechanism for FeS formation remains unclear, it is understood that this process requires the presence of a sulfur and iron source, organic carbon, sulfate reducing bacteria, and strongly reducing conditions to occur. This study investigated the limitations associated with the formation and expression of FeS in soils. Eighty laboratory mesocosms were constructed, representing five different levels of Fe and S added to autoclaved sands, each treated with or without organic carbon and sulfate reducing bacteria. The percent of FeS formed in the soil morphology was assessed once a week across a 120-day wet incubation period. After the wet incubation period was complete, soils were re-aerated for two weeks to assess the percent of FeS lost from the soil morphology and the potential acidity produced.
Results/Conclusions
Initial evidence of FeS formation was observed after 90 days. These preliminary results suggest that it took a substantial amount of time to establish highly reducing conditions (i.e. relatively low oxidation-reduction potential) in the mesocosm soils. Once highly reducing conditions were established, FeS concentrations became evident in the soil morphology, ranging from 2 to 30% of the soil matrix. The largest amount of FeS was observed in mesocosms treated with organic carbon and sulfate reducing bacteria, regardless of iron and sulfur levels. The FeS concentrations also became more distinct as the soils reached 120-days of wet incubation. Upon re-oxidation, all morphological evidence of FeS disappeared from the soil morphology within two days. Additional analyses will reveal the impact of varying levels of Fe and S on FeS formation and the impact of oxidized FeS on soil pH. Results from this study will provide a better understanding of the biogeochemical cycling of iron and sulfur, particularly the formation and transformation of FeS features under variable soil conditions. These results are valuable to soil and wetland managers for the improvement of wetland identification and function, and will inform the management of potential acid sulfate soil materials under wet-dry cycles.