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The floodwater NO3− removal rate of intermittently‐flooded fresh water swamp soils and continuously‐flooded saline marsh soils of southern Louisiana was quantitatively characterized in a laboratory study. Of the two areas studied, the marsh area was the more effective sink for NO3− contaminated waters with an average initial removal rate of 9.15 ppm N/day. After correcting for the rate of NO3− diffusion, the microbial NO3− removal rate was calculated to be 7.64 ppm N/day. The swamp soil had a removal rate of 4.38 ppm N/day. The microbial NO3− removal rate for this area, after correcting for diffusion, was 2.50 ppm N/day. Studies on samples of floodwater separated from the soil showed the active site of microbial NO3− reduction to be the soil‐water interface or within the soil, but not in the floodwater. Additions of organic matter to a mineral soil flooded for rice (Oryza sativa L.) culture decreased the thickness of the aerobic‐anaerobic zone at the soil‐water interface and increased the rate of NO3− reduction.

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The effect of various chemical oxidants on inhibition of sulfide (S2‐) in a previously reduced soil was studied. Oxygen, KNO3, MnO2, ferricitrophosphate, and FePO4 · 2H2O at rates equivalent to 1,000 ppm O2 (O2 added at rates of either 300 or 500 ppm) were both added prior to flooding and after soil reduction had reached a maximum. The results indicate that the more soluble oxidants had the greatest effects in maintaining more positive redox potentials and in delaying sulfate (SO42‐) reduction to S2‐. The soluble oxidants (KNO3 and ferricitrophosphate) delayed S2‐ production by 17 and 9 days, respectively. The less soluble compounds (MnO2 and FePO4 · 2H2O) were less effective in delaying S2‐ production but persisted longer in preventing maximum S2‐ buildup.Adding the oxidants after maximum S2‐ accumulation showed the most pronounced effect on S2‐ oxidation from KNO3 and the least effect from MnO2. Oxygen at the rate of 500 ppm was also a very effective oxidant. Sulfate reduction to S2‐ and S2‐ oxidation were apparently controlled by, or at best related to the redox potential since SO42‐ reduction and S2‐ oxidation both appeared to commence at potentials in the vicinity of −100 mV.

The O2 depletion rates, NO3− loss, and the effects of added O2 on NO3− disappearance and redox potential in four flooded or intermittently flooded soils from the swamp and coastal marshes of Louisiana were quantitatively characterized in a laboratory study. The NO3− added either to the shallow floodwater or mixed with the soil in a suspension rapidly disappeared. Eighty to ninety parts per million NO3− was lost from the soil suspensions in 1 to 4 days and from the floodwater over a soil in 10 to 20 days. No NO3− was lost from floodwater separated from the soils. Oxygen depletion in the soil suspensions occurred in 15 minutes to 4 hours. Redox potential curves exhibited a characteristic inflection after O2 disappearance in all soils studied. Nitrate disappearance did not appear to be inhibited by as much as 16 ppm O2 dissolved in the soil suspensions because the O2 was rapidly consumed.