LAUSR

ABSTRACT Unsaturated incubation (UI) of air-dried soil before flooding significantly reduced the reduction of redox potential and decreased the easily reducible iron (Fe) (ERFe). Antibacterial treatment of the soil decreased the ERFe level regardless of the moisture regime while fungicide treatment reduced the effects of UI. Treating soil under imposed UI with sucrose helped recover the ERFe to the non-UI level. The effects of UI disappeared after air-drying the soil; therefore, the decrease in ERFe—by imposing upland conditions before flooding—could be due to the decrease in the activity of iron-reducing bacteria owing to the depletion of respiratory substrates by bacterial activity during the upland conditions. These results indicate that a drastic change in soil moisture from dry to wet will increase the risk of iron toxicity by increasing the ERFe, but that maintaining an upland condition for more than one week before total submergence can reduce the risk of iron toxicity.