Visual Assessment of Sulfate Reduction to Identify Hydric Soils

Abstract

Core Ideas Iron monosulfides determine the presence of highly reduced conditions. Visual sulfate reduction on IRIS surfaces can be used to identify hydric soils. Greater S content in saturated soils results in greater monosulfide formation. Some hydric soils present a unique set of problems associated with their identification, including morphological features not reflective of current hydrology and/or masked redoximorphic features. A simple, reliable tool to identify reduced soil environments is IRIS (Indicator of Reduction in Soil) tubes and panels. In saturated, anaerobic soils with substantial sulfur (S) contents, a commonly observed phenomenon is the reduction of S to form black iron monosulfides (FeS). The objective of this experiment was to document S reduction on polyvinylchloride surfaces coated with synthesized iron (Fe) oxides to simplify the identification of hydric soils wetlands containing moderate to high S concentrations. The IRIS panels were installed in soil mesocosms containing varying concentrations of sulfate and saturated for 11 wk. Oxidation–reduction potential and pH were measured weekly throughout the study. At the termination of the experiment, the IRIS panels were removed and analyzed quantitatively for percentage of oxidized iron (Fe3+) remaining, reduced iron (Fe2+) removed, and reduced S (FeS) precipitated onto the panel. When IRIS panels displayed 2% FeS precipitate, 77% of the panels met the National Technical Committee for Hydric Soil criteria using reduced Fe removal from IRIS panels, and 100% of the panels met the criteria established using measured oxidation–reduction potential. The strong visual observation of FeS is a simple, quick determination of highly reduced conditions. Wetlands are a valuable natural resource that can be challenging to delineate, and the incorporation of a visual S reduction criteria on IRIS surfaces is beneficial for the timely and accurate identification of hydric soils.