Effect of the concentration of radiocesium in irrigation water and soil on the concentration of radiocesium in crop

In 2011, radiocesium concentrations in brown rice grown in mountainous areas in Fukushima Prefecture, Japan, were found to exceed 500 Bq /kg. Contaminated irrigation water flowing from a nearby mountain forest was suspected as the source of the radiocesium. In this study, we investigated the migration of dissolved radiocesium (sup(137)Cs) from irrigation water into brown rice (Oryza sativa L. Hitomebore) and sunflower (Helianthus annuus L.) by determining the concentrations of sup(137)Cs in brown rice from plants grown in pots irrigated with water containing dissolved sup(137)Cs at a concentration of 0.1, 1.0, or 10 Bq /L. We found that when the sup(137)Cs concentration in the irrigation water was approximately 0.1 Bq /L, the amount of sup(137)Cs that migrated into the rice was below the provisional regulation value for sup(137)Cs in brown rice. For sunflower plants, the sup(137)Cs concentrations depended significantly on soil type when plants were irrigated with water containing sup(137)Cs at > 10 Bq /L, but not when plants were irrigated with water containing sup(137)Cs at < 1 Bq /L. For sunflower plants grown in soil with a high content of exchangeable potassium, the migration of sup(137)Cs from the irrigation water and the soil to the plants was reduced. We also investigated how sup(137)Cs uptake into brown rice obtained from plants grown in pots irrigated with sup(137)Cs-containing water (1.0 or 10 Bq /L) depended on the distribution of sup(137)Cs in the dissolved fraction, the soil-particle-bound fraction, or the organic-matter-bound fraction. For each sup(137)Cs fraction, the sup(137)Cs concentration in the brown rice increased in proportion to the concentration in the water. The dissolved sup(137)Cs was absorbed at a higher rate than the organic-matter-bound and soil-particle-bound sup(137)Cs. Absorption rates were higher in soil with a high exchangeable-potassium content than in soil with a low exchangeable-potassium content. Furthermore, the rates of absorption of dissolved and organic-matter-bound sup(137)Cs were similar for soils with the same exchangeable-potassium contents. We also conducted experiments to determine the growth stage at which brown rice took up the most sup(137)Cs from the contaminated irrigation water. Specifically, rice plants were grown in continuously flooded pots and irrigated with demineralized water prior to treatment with water containing dissolved sup(137)Cs during one of four periods: from transplanting to 28 days after transplanting (DAT), from 29 to 43 DAT, from 44 to 68 DAT (heading), and from 69 (heading) to 101 DAT (ripening). Irrigation with contaminated water from 44 to 68 DAT resulted in the highest concentrations of sup(137)Cs in the grain, hull, upper straw, and lower straw, compared with irrigation during the other periods. These results indicate that reduction of the migration of sup(137)Cs from irrigation water to brown rice will require that the use of sup(137)Cs-contaminated water (such as influent from mountains that have receive radioactive fallout) should be avoided during the heading stage. In addition, the potassium content in soil should be increased before the heading stage because high potassium content can suppress sup(137)Cs uptake. Increasing the exchangeable-potassium content in the soil also reduced sup(137)Cs migration from irrigation water to brown rice. For rice currently planted in the areas of Fukushima Prefecture mentioned above, measures to maintain high exchangeable-potassium content in the soil can be expected to lessen the impact of sup(137)Cs contamination in agricultural water, which is thought to be the main source of the high concentration of sup(137)Cs in the brown rice. It has been shown that suppression of sup(137)Cs uptake by zeolites is primarily the result of their ability to increase the amount of exchangeable potassium in soil. Thus, compared to rice, field crops are less susceptible to the influence of dissolved radiocesium (at concentrations such as 10 Bq /L) in irrigation water, by exceed the reference value (100 Bq /kg) of brown rice for high exchangeable-potassium content in the soil. Field crops are likely to exceed the reference value for radiocesium derived from irrigation water in the cultivation of horticultural crops is low. The sup(137)Cs concentration in brown rice can be determined to exceed the reference value (100 Bq /kg) as a result of the use of contaminated irrigation water and the exchange of K2O content (250 mg /kg dry soil) in the soil due to the application of KCl as a basal fertilizer.