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 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 (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 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.Abbreviations: B, blue; Eh, oxidation-reduction potential; G, green; IRIS, Indicator of Reduction in Soil; NTCHS, National Technical Committee for Hydric Soils; R, red; redox, oxidation-reduction. W etlands are recognized as an important source of biodiversity, recreation, and ecosystem services. The protection and delineation of wetlands is not only ecologically important, but it is also mandated by federal law under Section 404 of the Clean Water Act (USEPA, 1972). Wetland delineations encompass three factors: hydrophytic vegetation, hydric soils, and wetland hydrology (United States Army Corps of Engineers, 1987). Seasonal vegetation and fluctuating water tables can alter the presence of vegetative and hydrologic wetland indicators throughout the year. Alternatively, the biogeochemical conditions that result in hydric soil morphologies persist in the soil throughout seasonal variation and environmental modifications, making hydric soils particularly useful in wetland delineation (USDA-NRCS, 2010).A hydric soil is defined as a "soil that formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part" (NTCHS, 2001 Core Ideas• Iron monosulfides determine the presence of highly reduced conditions.• Visual sulfate reduction on IrIS su...

Macro and microscopic visual imaging tools to investigate metal reducing bacteria in soils

“…An important limitation of this technology is the oxygen permeability of cellulose acetate butyrate (Quintero, Galotto, Rodriguez, & Guarda, 2014), which in some cases resulted in reprecipitation of iron-oxides on the clear-IRIS tubes surface. Another limitation is precipitation of opaque iron-sulfides where sulfur is present (Vaughan, Miller, Navarro, & Appel, 2016). Using an approach such as ours, clear-IRIS tubes may provide an opportunity for remote sensing and time-series redox data collection.…”

Development and application of the Hydric Soil Technical Standard

“…Field and laboratory studies have evaluated anaerobic conditions using multiple techniques, and determinations of anaerobic conditions made using the HSTS Eh‐pH threshold have shown general agreement with determinations obtained using α α’‐dipyridyl dye and IRIS methodologies which identify Fe reduction (Jenkinson & Franzmeier, 2006; Vaughan, Miller, Navarro, & Appel, 2016). For example, Berkowitz et al.…”

Rapid formation of iron sulfides alters soil morphology and chemistry following simulated marsh restoration

“…The presence of FeS has been well documented in marsh soils and coastal sediments (Vaughan et al, 2016), and the negative impacts of FeS oxidation and soil acidity have been observed following improper Fig. 1.…”

“…Field observations documented the presence of a black FeS horizon following sediment placement (top left panel) in several restoration scenarios. At restoration field sites, FeS was verified using treatment 3% H 2 O 2 resulting in oxidation and removal of the black color (lower left panel; soil ped on left is untreated, ped on right is treated); rapid (< 1 h) precipitation of FeS on the surface of Indicator of Reduction in Soil (IRIS) tubes (center panel; after Vaughan et al, 2016); and laboratory aerobic incubation of three replicate soil samples resulting in a decrease of soil pH below 4.0 (adapted from Berkowitz and VanZomeren, in press). Note that these field observations led to the initiation of the current study, which reports data from simulated restoration conditions occurring in laboratory microcosms.…”