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Redox processes in a seasonally reduced riparian zone soil ecosystem Public Deposited

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  • Seasonal reducing conditions play an important role in the cycling of nutrients and soil pore water chemical constituents in the Willamette Valley. Long periods of soil saturation lead microorganisms utilizing organic matter as their carbon source to find alternative electron acceptors other than oxygen. Once oxygen has been depleted microorganisms sequentially use nitrate, manganese (IV), iron (III), and sulfate. Soil mineral solubility often controls the release of these pore water chemical constituents, and thus is an important component of the biogeochemistry of soils. Soil pore waters were sampled using pore water equilibration devices, or peepers. Peepers give a high resolution of the concentration of pore water chemical constituents with depth. They also sample pore waters without oxidizing the constituents and thus give a realistic view of the species in situ. Peepers have been widely used in sampling the pore waters of lake and ocean bottom sediment and peat bogs. However, no literature was found on the use of peepers in dense saturated mineral soils. The results from this experiment support the idea that carbon is an important determinant in the concentration of reduced species. All reduced species were most heavily concentrated at the depth that dissolved and solid soil organic carbon were most concentrated. Organic phosphorus was present in concentrations of an order of magnitude greater than inorganic phophorus. Organic matter apperars to be an important regulator of phosphorus. The soluble inorganic phosphorus concentration trend appeared to be correlated with the iron (II) concentration trend; thus, suggesting the importance of iron reduction in phosphate release. Inorganic phosphorus was found at concentrations of an order of magnitude greater under anaerobic conditions than under aerobic conditions. Minteqa2, a geochemical model, was used to evaluate which soil minerals are likely controlling the solubility of pore water chemical constituents. Ferrihydrite, goethite, green rust, and lepidocrocite all showed trends of increasing saturation index with increasing redox potential. The soil pore waters are over-saturated with respect to goethite and lepidocrocite. Both of these minerals have been observed in Willamette Valley soils under oxic conditions. Under-saturation was observed with respect to ferrihydrite and mackinawite suggesting dissolution of these mineral phases. Siderite was only slightly oversaturated. The thermodynamic modeling approach gives useful insight into the possible mineral solids that may form. However, identification of the important Fe solid phases awaits additional investigation using microscopic techniques such as, x-ray diffraction, Mossbauer spectroscopy, or other surface techniques.
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