Wednesday, November 7, 2007
324-13
Soil Reduction Rates In Relation to Iron Chemistry.
Amanda Zelasko, North Carolina State University, 1945 Shadow Glen Dr, Raleigh, NC 27604 and Dean Hesterberg, PO Box 7619, North Carolina State University, North Carolina State University, Department of Soil Science, Raleigh, NC 27695-7619.
Research has indicated that the rate of soil reduction following water saturation depends on the nature of the electron acceptors, particularly Fe(III) species, as well as the amount of organic matter as an electron donor. Observed changes in reduction rates during sequential oxidation-reduction cycles can impact wetland restoration and delineation and might be related to the chemical form and stabilities of Fe(III) solids. The objective of this research was to determine the role of Fe speciation on rates of soil reduction under water-saturated conditions. Soil samples from two wetland sites in North Carolina were incubated in specially designed redox incubators and subjected to three cycles of oxidation and reduction over 36 d. Pore water samples taken during the incubation were analyzed for iron and other components. Oxalate, citrate-bicarbonate-dithionate (CBD), and sodium pyrophosphate extractions were used to indicate poorly crystalline, crystalline, and organically bound forms of Fe in the soil samples, and Fe K-edge XAS analysis is being used to determine specific chemical or mineralogical forms of Fe in the soils samples before and after reduction cycles. Extractable Fe varied between soil samples, and the oxalate-to-CBD extractable Fe ratio was significantly higher (p < 0.05) in soils with organic C > 27 g kg-1. Dissolution of Fe(II) during reduction varied between soil samples and was correlated to soil reduction rate (r = 0.45). Dissolution of Fe(II) during reduction of soil samples was either high (up to 24 mg Fe/L), medium (up to 6 mg/L), or low (up to 1.5 mg/L), however, maximum concentrations of dissolved Fe(II) were not correlated to extractable forms of Fe. These results indicate that there is no simple relationship between extractable forms of Fe and soil reduction rate or Fe(II) dissolution.