Sequential Reduction and Oxidation of Inorganic Nitrogen, Manganese, and Iron in Flooded Soil

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Abstract
When a soil is flooded, the curtailment of O2 diffusion into the soil causes the microorganisms decomposing organic matter to switch from O2 to alternate electron acceptors. Three alternates utilized by facultative microorganisms when O2 becomes depleted are NO‐3, Mn4+ compounds, and Fe3+ compounds. We studied the sequence of reduction of these three redox systems under controlled redox potential conditions. The redox potential of soil suspensions was changed stepwise in 50‐mV increments from oxidized to reduced conditions and from reduced to oxidized conditions and maintained at each new potential for 15 d, at which time NO‐3, NH+4, Mn2+, and Fe2+ concentrations in the soil solution were analyzed. The results of this study showed that the oxidation and reduction of the three electron acceptors were sequential, with no overlap in the oxidation or reduction of the NO‐3 and Mn systems and little overlap in oxidation and reduction of the Mn and Fe systems. In the oxidized‐to‐reduced experiment, the critical redox potential at which all of the NO‐3was reduced and Mn2+ first appeared in the soil solution was approximately 200 mV. The critical redox potential at which Fe2+ appeared was 100 mV. For all three redox systems, the critical redox potentials for the oxidized‐to‐reduced transition were approximately 50 mV lower than for the reduced‐to‐oxidized transition.

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