Relationship Between Radiocesium Absorbed by Paddy Rice and Trapped by Zinc-Substituted Prussian Blue Sheet Buried in Soil

Rice absorbs radiocesium dissolved in soil solution. However, quantitative determination of dissolved radiocesium is difficult because large volume of water should be collected from paddy soil. We investigated whether a sheet capable of effectively trapping cesium (zinc-substituted Prussian blue (ZnPB) sheet) can be used to evaluate the level of dissolved Cs-137 to be absorbed by rice. A new device equipped with ZnPB sheet was invented to trap only radiocesium dissolved in soil solution, which is passed through a membrane filter covering the ZnPB sheet. A rice pot-culture experiment using three paddy soils with different Cs-137 concentrations and physico-chemical properties collected from Fukushima Prefecture was conducted under different fertilization or rice-planting treatments. The results showed that Cs-137 concentration in brown rice was positively correlated with the Cs-137 concentration in soil solution with a contribution ratio of regression R-2 of 0.91 and a p value less than 0.01, and also with the Cs-137-trapping rate by ZnPB sheet (amount of Cs-137 trapped by ZnPB sheet per unit area of sheet and unit time) buried in soil with a R-2 value of 0.89 and a p value less than 0.01. Moreover, field-cropping experiments conducted at the three paddy fields under conventional agricultural practices showed positive correlation between the Cs-137 concentration in brown rice and Cs-137-trapping rate by ZnPB sheet. Both the pot and field experiments showed that the 2 weeks of the early stage of rice growth is appropriate as the ZnPB buried period because K additive fertilization is still in time to restrict Cs-137 uptake by rice plant, and because the highest Cs-137-trapping rates of the ZnPB plate as compared with the following periods and high R-2 values of those regression curves. . The proposed method might be used to roughly evaluate the vulnerability of soil to Cs-137 transfer to rice and to identify the area in which effective countermeasure should be applied intensively.