Dynamics of dissolved silicon in rice paddies under conditions of water conservation irrigation

PURPOSE: For conserving irrigation water for rice (Oryza sativa L.) cultivation, the repeated draining/flooding with minimal amounts of water (water conservative irrigation (WCI)) is recommended. WCI changes the reductive/oxidative state of iron (Fe), which may, in turn, change the dissolved silicon (Si) concentration in a soil solution. The purpose of the present study was to confirm the influence of WCI on the availability of Si by rice. MATERIALS AND METHODS: Seasonal variations in the soil solution Si concentration and the amount of Si absorbed by rice were compared between WCI and control plots in two fields. A microcosm experiment was also conducted to examine the effect of reduction/oxidation of Fe on the soil solution Si concentration. RESULTS AND DISCUSSION: In the field, the seasonal variation pattern of Fe in a soil solution differed between WCI and control plots, but the Si level did not, which was maintained at low levels during the WCI period probably due to active uptake by rice. The amount of Si absorbed by rice was similar between the two treatments. In the microcosm experiment, the soil solution concentration of Si decreased by 47 % along with the oxidation of Fe²⁺ in the first drainage. However, only the Fe²⁺ concentration was changed with further repetition of drainage and flooding, and total loss of Si from the soil solution due to intermittent flooding corresponded to 1 % of the Si absorbed by the rice in the field. CONCLUSIONS: Although WCI can change the seasonal variation in the soil solution Si concentration, its effect on the Si uptake by rice was insignificant.