The Effect of Alternate Flooding and Drying on the pH and Redox of Submerged Rice Soil

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Abstract
The pH and redox relationships of a rice soil subjected to alternate flooding and drying were measured during the growth of three successive rice crops in the greenhouse. The objective was to explain why greater nitrogen losses occur under alternating flooding and drying than under continual submergence.At 2 cm below the soil‐water interface the pH dropped from 8.3 to approximately 7.0 in 40 days and remained there through 120 days. The redox (pe + pH) dropped from 16 to 4 under the same conditions. When alternate flooding and drying were imposed, the pH fluctuated from 8.0 during drying to near 7.6 during submergence while pe + pH shifted from 16 during drying to approximately 8 during flooding. Both pH and pe + pH dropped slightly with depth into the sediment reflecting greater accumulation of CO2 and depletion of O2.The findings in this study support the hypothesis that alternate flooding and drying causes the redox of rice soils to pass through the pe + pH range of 13 to 15 where theoretical considerations show that NO(g) is most stable and can function as a viable intermediate reaction product of denitrification in soils.