Seasonal reductive dissolution of Fe- and Mn-oxides is a significant biogeochemical process in poorly drained Willamette Valley (OR) agricultural soils; in fact, as much as, 20% of the annual flow of electron reducing capacity may be coupled to microbial and abiotic reactions. The reduction/oxidation of mineral Fe and Mn pools may be linked to a number of important terrestrial processes, including: 1) the solubility and transport of inorganic P, 2) reductive dechlorination of agricultural chemicals, and 3) a possible flux of soluble Fe from wetlands to river and ocean environments. Analysis of pore waters suggests reduced conditions are stratified within the first 40 cm of the soil column. The most reduced zone (-200 mV) occurs at 10-20 cm below the soil surface and is consistent with the annual turnover of readily decomposable organic carbon. Soluble Fe(II) and Mn(II) concentrations reach values of 1,500 and 150 uM, respectively, in the late spring when saturated conditions persist as the soil warms. Soil solution chemical speciation modeling suggests that siderite (FeCO3), a "green rust mineral" (Fe(II)Fe(III)2(OH)8), and rhodocrocite (MnCO3) may control the solubility of Fe(II) and Mn(II). The dynamic nature of the soil solution pool is mirrored in the transformations of mineral phases; acid extractable (0.1 M) Fe(II) concentrations range from near 0 when the soil is fully oxidized in summer to over 50,000 umol kg-1 during periods of maximal reduction. The majority of the Fe and Mn in the oxidized and reduced mineral phases is x-ray amorphous. Lepidocrocite and ferrihydrite are observed under oxidizing conditions as Fe coatings in live root channels. A source of the ferrous iron may be a Fe-rich expandable clay mineral, which is abundant in the underlying clay layer and decreases in abundance towards the surface. Fe- and Mn-oxides exist in at least two physically definable pools: 1) a sand/fine pebble size pool consisting of oxide cemented authigenic nodules, and 2) a silt and finer sized matrix fraction. The nodules are significantly enriched in Fe and Mn (and other trace constituents) relative to the surrounding fine-grained matrix. The most poorly drained soils on the Willamette Valley landscape appear to be sources for the redistribution of Fe and Mn. They are depleted in both nodules and total concentrations of Fe and Mn relative to better drained adjoining soil bodies, yet continue to support high levels (e.g. 1,500 uM Fe(II)) of soluble redox active species.
1 Crop and Soil Science, 3017 ALS Bldg. Oregon State University, Corvallis, OR 97331-7306 United States.
2 USDA-ARS, 3450 SW Campus Way, Corvallis, OR 97331-7102 United States.
3 Geology, Portland State University P.O. Box 751, Portland, OR 97207-0751 United States.