Development and Evaluation of Iron‐Coated Tubes that Indicate Reduction in Soils

Abstract

Soil drainage conditions are important to land use decisions. Traditionally, anaerobic conditions induced by poor drainage have been evaluated by observing soil color related to Fe and Mn oxides, using α, α‐dipyridyl dye, measuring dissolved O2, and measuring EH We believe that there is further need for a device that is scientifically sound and easy to use. Therefore, our goals were to develop and test a device that mimics natural soil processes, visually indicates soil reduction, and is robust. Our concept was to coat a rod or tube with a colored soil mineral that dissolves on reduction, insert the device into a soil, remove it after a few weeks or longer, and observe if some of the coating had been lost. If the coating was not dissolved, no reduction occurred, but if it was dissolved, reducing conditions must have prevailed. After trying several kinds of coatings and tubes, we chose ferrihydrite (FH) coating on polyvinyl chloride (PVC) pipe. We call the device an Indicator of Reduction in Soils (IRIS). As the study progressed we added semi‐quantitative interpretations by measuring depleted areas using a digital camera and image analysis. We tested IRIS tubes in the lab and in soils in Indiana, Minnesota, and North Dakota, and concluded they performed as expected. Reduction rates increased between February and April and were related to increasing soil temperature, turnover (flux) of soil OC, and content (inventory) of OC. Reduction rates decreased after April, presumably because the nutrient supply for microbes decreased.

Visual Assessment of Sulfate Reduction to Identify Hydric Soils

“…As anaerobic conditions develop in saturated soils, oxygen (O 2 ) is rapidly depleted, forcing microbes to couple organic matter oxidation with less thermodynamically favorable TEAs (Lambais et al, 2008). Alternate TEAs, commonly found in soils, in decreasing thermodynamic favor are nitrate (NO 3 -), manganese (Mn 4+ ), Fe 3+ , sulfate (SO 4 2-), and carbon dioxide (CO 2 ) (Vepraskas and Craft, 2016 (Castenson and Rabenhorst, 2006;Jenkinson and Franzmeier, 2006;Rabenhorst and Burch, 2006;Rabenhorst and Castenson, 2005 FeO(OH)(s) + e -+ 3H + (aq) « Fe 2+ (aq) + 2H 2 O(l) [2] Oxidized Fe 3+ is insoluble and colored, whereas reduced Fe 2+ is soluble and colorless. In reducing environments when Fe 3+ is reduced to Fe 2+ , the orange-colored Fe 3+ paint used in the construction of the IRIS tubes is reduced to soluble Fe 2+ and stripped from the tube.…”

“…In reducing environments when Fe 3+ is reduced to Fe 2+ , the orange-colored Fe 3+ paint used in the construction of the IRIS tubes is reduced to soluble Fe 2+ and stripped from the tube. Photographic analysis of the tubes can quantify the degree of reduction (Castenson and Rabenhorst, 2006;Jenkinson and Franzmeier, 2006). The assessment of IRIS tubes by Castenson and Rabenhorst (2006) found that when 30% of the ferrihydrite paint was removed from the tube, essentially all of the soils were under reducing conditions.…”

“…The major benefit that distinguishes IRIS tubes from other in situ methods that measure soil reduction is that they integrate a much larger area of the soil (i.e., versus measuring redox potential in a microsite associated with the Pt electrode tip). Because Fe is nearly ubiquitous in soil, IRIS tubes are a relevant measurement of reduction in soils nearly everywhere and play an important role in classifying hydric soils ( Jenkinson and Franzmeier, 2006 The black-colored FeS is unstable in aerobic conditions and thus will disappear when oxidized by O 2 on exposure to air (Rabenhorst et al, 2010;Stolt, 2005). The deposition of FeS requires a specific set of conditions, which include organic matter, a SO 4 2-and Fe 3+ source, anaerobic microorganisms, and favorable reducing conditions (Rabenhorst et al, 2010 …”

Relevance of the Lin's and Host hydropedological models to predict grape yield and wine quality

“…An alternative method for assessing water saturation and reducing condition is the use of IRIS (Indicator of Reduction In Soil) PVC tubes, coated with ferrihydrite (Fe(OH) 3 ) on the surface (Jenkinson and Franzmeier, 2006). During periods of reducing conditions, ferrihydrite painted on IRIS tubes is removed, through reduction and dissolution caused by heterotrophic microbes using Fe(III) as an electron acceptor while oxidizing soil organic matter.…”

“…In fact, at 0.35-0.50 m depth, the whole tube surface was uniformly discolored; on the contrary, in the other plots IRIS tubes only exhibited white spots of Fe removal, due to reduction processes occurring in microsites. Actually, in the pedoclimatic conditions under study, where short-term and locally restricted saturation occurs, the analysis of reduction pattern played a fundamental role to understand the removal mechanism of ferrihydrite (Jenkinson and Franzmeier, 2006). The spots of Fe removal present on most IRIS tubes were not only due to soil saturation, but also to proximity to an organic matter source like roots.…”