Development and Documentation of a Standard for Visual Sulfur Reduction to Identify Seasonally Inundated Wetlands.

Hydric soils present a unique set of problems associated with their identification, including morphological features not reflective of current hydrology and masked redoximorphic features. A novel tool to identify reduced soil environments is Indicator of Reduction in Soil (IRIS) tubes. This concept was developed to demonstrate whether or not Fe is oxidized (colored and solid) or reduced (colorless and soluble) and therefore anaerobic in the soil environment. Another commonly observed phenomenon in soils is the reduction of S to form black monosulfides (FeS) under anaerobic conditions. The objective of this experiment was to document and develop a visual standard for S reduction on PVC surfaces coated in Fe oxide paint in order to simplify the identification of hydric soils in some settings. IRIS panels were installed in soil mesocosms containing varying concentrations of sulfate and saturated for 11 weeks. Throughout the duration of the study, oxidation-reduction potential and pH were measured weekly. When the experiment concluded the IRIS plates were removed and analyzed quantitatively for percent of oxidized iron (Fe³⁺), percent of reduced iron (Fe²⁺), and percent of reduced S (FeS). When at least 30% Fe was reduced from the IRIS plates, more than 2% prominent black FeS stains were observed. The strong visual observation of monosulfides is a simple, quick determination of highly reduced conditions. Wetlands are a valuable natural resource that can be challenging to delineate; the incorporation of a visual S reduction standard on IRIS surfaces will be beneficial for timely and accurate identification of hydric soils.