Indicator of Reduction in Soil (IRIS): Evaluation of a New Approach for Assessing Reduced Conditions in Soil
By Karen L. Castenson and Martin C. Rabenhorst
In Soil ©, Soil Sci. Soc. Am. J. 70:1222–1226 (2006)
ABSTRACT: Research and management of wetlands often requires the documentation of reducing soil conditions. Documentation of reduction in soils by measuring oxidation–reduction (redox) potentials using Pt electrodes is often difficult because of the time and cost involved in employing these techniques. This study evaluated a new procedure called Indicator of Reduction in Soil (IRIS) that has been recently developed to assist in the detection of reduced soil conditions. Polyvinyl-chloride (PVC) tubes coated with a ferrihydrite paint were inserted into the upper 50 cm of the soil for periods of 12 to 32 d. Soil redox potentials, water table height, and soil temperature were measured concurrently. Upon removal, the tubes were analyzed to assess the loss of ferrihydrite paint from the tube surface. When ferrhydrite paint was substantially depleted from 20% of the area of the IRIS tube, 87% of the observations at the corresponding depth indicated the soil was reduced. When ferrhydrite paint was substantially depleted from 30% of the area of the IRIS tube, essentially all of the soil observations at corresponding depths showed that the soil was reduced. Although not without com- plications, IRIS tubes appear to be a promising new alternative to traditional methods used to identify reducing conditions in soil.
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Rabenhorst and Burch
Synthetic Iron Oxides as an Indicator of Reduction in Soils (IRIS) ©, Soil Sci. Soc. Am. J. 70:1227–1236 (2006)
ABSTRACT: Various circumstances in wetland delineation or mitigation strategies require the documentation of reducing conditions in soils. An in- novative approach for assessing reduction in soils known as Indicator of Reduction in Soils (IRIS) has been recently introduced. IRIS tubes are manufactured by coating polyvinylchloride (PVC) pipe with a paint prepared from an Fe oxide (mainly ferrihydrite) suspension. When the iron oxide is synthesized by adding KOH to a FeCl3 solution, the min- eral phase is essentially pure ferrihydrite. Over time, and under par- ticular conditions, the ferrihydrite is transformed to more crystalline forms such as goethite and hematite. The objectives of this study were (i) to assess the mineralogical composition of iron oxide paint synthesized for the manufacture of IRIS tubes and (ii) to assess the significance of mineralogical composition of the Fe oxide paint on desirable qualities for the construction of IRIS tubes. The effects of pH and storage time on the mineralogical alteration of ferrihydrite are exam- ined and compared with performance of the paint by a scale for adhesion and durability. Paint composed of nearly pure ferrihydrite shows poor adhesion and durability. Data suggest that for paint to adhere well to the PVC tubing, the Fe oxide suspension must contain a minimum of 30 to 40% goethite. This desired paint composition can be facilitated during synthesis by titrating the Fe oxide suspension to pH 11 or 12 rather than pH 7.5, which is typically prescribed f0r the formation of synthetic ferrihydrite.
Jenkinson, Franzmeier, and Lynn
Soil Hydrology on an End Moraine and a Dissected Till Plain in West-Central Indiana ©, Copy JES , Soil Sci. Soc. Am. J. 66:1367–1376 (2002).
ABSTRACT: Soil hydrologic properties are a function of precipitation (P)-evapotranspiration relations, stratigraphy, and geomorphology. An understanding of soil hydrology helps us predict many soil and ecosystem properties. We studied soil hydrology on an end moraine and on a dissected till plain in west-central Indiana. We measured hydraulic head, water table level, redox potential (EH ), and temperature, with piezometers (0.25-, 0.50-, 1.0-, 2.0-, 4.5-m depth), observation wells, platinum electrodes (0.25-, 0.50-, and 1.0-m), and thermocouples (0.25-, 0.50-, 1.0-, and 2.0-m), respectively, in soils along two soil toposequences for 9 yr. Water table levels drop rapidly when hardwood trees first leaf out in the spring, and rise rapidly after the trees go dormant in the fall. The Thornthwaite model underestimates evapotranspiration in the forest in the spring. In the dissected plain underlain with dense till, water is held up by the slowly permeable till. Water moves from the interior of the till plain to the dissected bevel where it periodically rises within 1 m of the surface but does not cause redoximorphic features. Soils on the crest of a moraine are similar in morphology to those on the till plain bevel, but have essentially no high water table because there is no upslope contributing area to serve as a water source. In the wetter soils, reduction begins when a soil horizon becomes wet but not saturated, and proceeds more rapidly when the horizon is saturated. There is a lag period of 2 to 8 wk between initial saturation of the soil at 25 cm and minimum EH.
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