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.
