Visual Assessment of Sulfate Reduction to Identify Hydric Soils

Abstract

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 docu-ment S reduction on polyvinylchloride surfaces coated with synthesized iron (Fe) oxides to simplify the identification of hydric soils wetlands contain-ing 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 through-out the study. At the termination of the experiment, the IrIS panels were removed and analyzed quantitatively for percentage of oxidized iron (Fe 3+) remaining, reduced iron (Fe 2+) 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 cri-teria 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.