Countermeasure by Means of Flooding in Latter Growth Stage to Restrain Cadmium Uptake by Lowland Rice

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  • 生育後期の湛水管理による水稲のカドミウム吸収抑制
  • セイイク コウキ ノ タンスイ カンリ ニ ヨル スイトウ ノ カドミウム キュウシュウ ヨクセイ

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This study was conducted to evaluate the effect of flooding in the latter growth stage (from the 15th day before heading to the 25th day after heading) on cadmium (Cd) uptake by lowland rice (Oryza sativa L. cv. Koshihikari) in a slightly polluted paddy field with Cd in the Jinzu River basin where the Cd content in brown rice was around 0.4mg kg^<-1>. With flooding in the latter growth stage Cd content in brown rice was 0.08mg kg^<-1> and as compared with a control plot managed by intermittent irrigation after midsummer drainage, percent of reduced Cd content was 84%. In addition, the probability that Cd content in brown rice would exceed 0.2mg kg^<-> was under 2%, taking annual change into consideration. With flooding in the latter growth stage Cd uptake by rice plants from the 15th day before heading to harvest time was reduced to 10% equivalency of the control plot. Considering changes of soil oxidation-reduction potential and pH, it was suggested that Cd in soil existed in insoluble form as sulfide in the flooding period. As to rice yield a clear influence from flooding was not found. Roots of the rice plants on the 20th day after heading were not injured under reductive soil conditions. Though trafficability of harvester on topsoil was degraded by flooding with a decrease of bulk density and hardness of surface soil, physical properties of the topsoil were improved by restricting the flooding period within the latter growth stage as compared with continued flooding from transplanting to the 25th day after heading. In conclusion, water management based on flooding for 40 days in the latter growth stage from the 15th day before heading to the 25th day after heading is efficient to decrease Cd content in brown rice as low as is reasonably achievable.

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