Biogeochemistry of inorganic P, Mn, and Fe in seasonally reduced soils of the Willamette Valley, Oregon

Abstract

Saturated soils undergo reduced conditions as microorganisms consume oxygen and require other terminal electron accepting processes for the mineralization of soil organic carbon. Currently, the soil redox potential has not been considered as important when sampling these soils for Fe, Mn, and P04. Since Fe- and Mn-oxides are electron sinks for these reactions, the redox potential should affect their solubility in the soil. Phosphate solubility, should also be closely related to redox potential. Current methods for sampling Fe(II), Mn(II) and P04 under reduced conditions are inadequate since they do not protect the samples from contamination by 02. The presence of 02 results in oxidation and subsequent precipitation of Fe-phosphate from solution. The co-precipitation of Fe-phosphate lowers the amount of P04 and Fe(II) in solution leading to the conclusions that the concentration of these elements is less than exists in the natural state. These reduction and oxidation reactions are occurring in close to 200,000 acres of wetland soils in the southern Willamette Valley. On this scale, the wetlands play an important role in the non-point sources of phosphorus. The cyclic nature of reduced conditions on riparian soils of the Willamette Valley, and its effects on the solubility of Fe, Mn, and P04 is investigated in the second part of this thesis. Fe and P04 concentrations under dry conditions, representative of the dry summers, were almost nonexistent. Mn was more soluble under reduced conditions, but still a fair proportion of the total Mn was soluble even under dry conditions. Mn and P04 each had a relative redox potential in which their solubility decreased when the soil redox potential passed below their respective arbitrary value. This suggesting that at lower redox potentials, these ions formed new insoluble minerals.