During the transition, soil-ORP steadily decreased reaching eventually values of pe = -3 – -6. Soil-pH did not drop below 6.5 due to buffering by the soil carbonates. NO₃ soil-solution concentrations decreased during flooding below 0.1 mM. Mn and Fe concentrations increased due to reductive-dissolution of their oxides. Their concentrations, however, were limited to ~ 0.1 mM by carbonate precipitation. Concentrations of O₂ decreased and of NH₄ increased under prolong flooding. SO₄ reduction, however, was observed only in several replicates of plant treatments. Significant difference in the redox status between the treatments was observed. Soils in the plant treatments flooded with effluents (PE treatments) were the most reduced treatments in terms of ORP decrease and redox processes rates, followed by plant treatments flooded with freshwater (PF), bare-soil treatments flooded with effluents (CE) and finally by bare-soil treatments flooded with freshwater (CF). The redox reaction that thermodynamically governs the electrode potential at the low ORP range (pe+pH < 6) as not been identified clearly yet.

This study clearly shows that plant growth and effluent irrigation can affect the rates of ORP decrease and of redox processes in the soil. The correlation between the soil-ORP and the reduction processes under flooding conditions is important for the understanding of the processes that occur during reduction cycles under irrigation/aerated conditions.