Martin C. Rabenhorst1, Rebecca R. Blank2, and Bruce R. James2. (1) Univ of Maryland, Dept. NRSL, 1112 HJ Patterson Hall, College Park, MD 20742, (2) Univ of Maryland, Dept. NRSL, 1112 HJ Patterson Hall, College Park, MD 20742
<>Wetland soils are distinguished by their being saturated, flooded or ponded such that they develop reducing conditions. Reducing soil conditions typically lead to distinctive biological communities adapted to life in an anaerobic environment, and also foster the development of particular biogeochemical processes that may provide environmental benefits. In order for reducing soil conditions to develop, the soil must, in addition to being saturated, contain oxidizable organic carbon which functions as an energy source for heterotrophic microorganisms. When such soils are warm enough for the microorganisms to be active, organic matter is oxidized and in the process electrons are transferred, initially to oxygen until it is depleted, and then to other electron acceptors such as NO3-, Mn+4, and Fe+3 causing reduction of these species to NO2-, Mn+2, and Fe+2. Reducing conditions in soils have traditionally been documented by measuring the voltage generated from a Pt electrode in conjunction with a reference electrode and comparing this to the standard hydrogen electrode (Eh). These data can be used together with measurements of pH and (assumptions regarding) ion activity to predict whether reduced or oxidized phases should be stable, thus documenting the oxidizing or reducing potential of the soil The redox status of the soil can also be estimated by using dyes specifically suited for identifying certain reduced species. For example, alpha,alpha-dipyridyl reacts with Fe+2 to produce a distinctive pink color Recently, IRIS (Indicator of Reduction In Soils) tubes have been introduced as a simple alternative approach for documenting reducing conditions in soils (Jenkinson, 2002). The basic concept is that a synthetic iron oxide is applied as a paint to PVC tubes (approximately 21 mm dia.) which are then inserted into the soil. Under wetland conditions, actively respiring microorganisms transfer electrons to the thin coating of iron oxides on the tube, causing the iron to become reduced and soluble, leaving portions of the white tube uncoated. The degree to which the tubes become stripped of the iron oxide paint is an indication of the degree to which microorganisms were using the iron oxides as an alternate electron acceptor>
During initial development of IRIS tubes, Jenkinson (2002) postulated that ferrihydrite should be utilized, because the poorly crystalline nature of the mineral more closely approximated newly formed iron oxides in the wetland soils that periodically went through reducing and oxidizing conditions. While this may be true, experimentation demonstrated that pure ferrihydrite would not adhere well to the PVC tubing, but rather the ferrihydrite coating would rub off easily. If however, the iron oxide is synthesized at a higher pH the ferrihydrite will transform progressively to goethite over time. By testing iron oxide paints comprised of varying proportions of ferrihydrite and goethite, it was demonstrated that when the paint contains at least 30-40% goethite, it adheres well to the PVC tubing. If there is less than 30% goethite (more than 70% ferrihydrite) the iron oxide paint will not adhere adequately to the PVC tubing and the IRIS tubes are not functional Based upon thermodynamics, ferrihydrite is predicted to be a less stable mineral than goethite meaning that it would be expected to be more easily reduced at higher redox potentials than would goethite. Therefore, one might hypothesize that IRIS tubes made using paint comprised of different proportions of ferrihydrite and goethite might perform differently when placed within certain soils. Therefore, an additional experiment was run to evaluate the performance of IRIS tubes that had been created using iron oxide paints with different proportions of ferrihydrite and goethite. Replicate tubes created using various proportions of ferrihydrite and goethite (ranging from 70:30 to 20:80) were placed in two different wetland soils for a two week period. The degree to which the iron oxide paint was removed was carefully quantified. The results of these field trials will be presented in this paper.
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