Graduate Thesis Or Dissertation
 

Geochemical characteristics of iron-manganese nodules in seasonally-saturated soils of the Willamette Valley, Oregon

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  • This study was initiated to characterize the distribution of Fe-Mn nodules and to elucidate trends of Fe, Mn, and P relative to the intensity of the annual cycles of reduction and oxidation. Nodules were enriched in Fe, Mn, and P relative to the soil matrix making them an integral part of Fe, Mn, and P cycles. Nodules were sampled to 100 cm from three soils that differ by internal drainage: a well-drained Willamette silt loam (fine-silty, mixed, superactive, mesic Pachic Ultic Argixeroll); a somewhat-poorly drained Amity silt loam (fine-silty, mixed, mesic Agriaquic Xeric Argialboll); and a poorly-drained Dayton silt loam (fine, smectitic, mesic Typic Albaqualf) in Benton County, Oregon. Nodules comprised 1% to 4% (wt%) of the Willamette soil, 3 to 16% of the Amity soil, and 2% to 6% of the Dayton. Peak accumulations of nodules occurred in horizons above Bt horizons. Nodules accumulated up to 60% of the Fe-oxide pool and 90% of the Mn-oxide pool (dithionite-citrate-bicarbonate). The weakly crystalline Fe oxides (acid-ammonium- oxalate) ranged from 60% to 100% of the Fe-oxides in the Willamette nodules, 40% to Redacted for Privacy 90% of Amity nodule-Fe, and 30% to 70% of Dayton nodules. Where nodules were especially abundant, 40 to 60% of the soil-P pool was distributed among the nodules. Variations of Fe-Mn nodule morphology may be related to the duration and hydraulic properties of the high water table. Light-colored nodules that were predominantly irregular- and blocky-shaped were observed in zones that regularly contain seasonal high water table. Irregular-shaped nodules occurred in horizons where the water table fluctuated during the rainy season. Light-colored blocky nodules occurred mainly in surface horizons that were usnally saturated for the entire rainy season. Dark, spherical nodules were found in horizons that were well-aerated or seasonally reduced. When overlain by horizons containing light-colored irregular or blocky nodules, dark, blocky to spherical shaped nodules were in a zone of extended saturation with infrequent or rare fluctuations of the water table during the wet season.
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