These authors measured the thickness of the oxidized layer in flooded soils. For Eh measurements, a platinum electrode was pushed down through the soil at a rate of 2mm/hour, sufficiently slowly for the electrode tip to reach near-equilibrium conditions as it descended. Concentrations of reduced and oxidized forms of Manganese, Iron, Sulfur, and Nitrogen were also measured.
Within a few days of submergence, soils showed a clear redox-potential profile as measured by the platinum electrode, with the oxidized layer above the reduced layer, and a transition from above 200 mV to below 200 mV across a relatively narrow intermediate layer. The profiles as measured by the chemical species distribution were similar, though Mn showed a narrower and S a deeper oxidized layer, probably relating to the redox conditions needed to reduce the oxidized compounds present in the soil; sulfate, for example, requires lower Eh values to be reduced than do ferric oxides.
Manganese, Iron, and Sulfur did not diffuse appreciably in these experiments, but Nitrogen compounds did. These authors propose the following process is occurring in these flooded soils:
“…ammonium diffusion from the reduced layer to the oxidized layer -> ammonium oxidation to nitrate (nitrification) -> nitrate diffusion from the oxidized layer to the reduced layer -> denitrification…”To explain the observation that nitrate was absent from the reduced layer, and never very abundant in the oxidized layer, and that ammonium was rapidly depleted in the oxidized layer. In this system, nitrification and denitrification are occurring simultaneously at different positions and redox potentials.
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