Radiocaesium bioavailability to flooded rice in comparison with upland ryegrass

Attention is focused on mechanisms of the soil-plant transfer of radiocaesimn (R.Cs) to paddy rice in the area affected by the accident at the Ful"UShima Daiichi nuclear poWer plant. Paddy rice is cultivated in flooded soils, in contrast to tyegrass grovm in upland soil conditions. RCs transfer to paddy rice has importance as a major food crop, while rye grass is a major forage crop. It is speculated that flooding a soil can enl1ance the RCs transfer to crop plants due to accumulated ammonimn (NH4) in soils mobilizing RCs by the ion exchange in soils. Pot trials were perfonned to compare and analyse RCs transfer between paddy rice (flooded) and rye grass (Wlsaturated). Paddy soils (n - 9) collected from the Fukushima affected area were either cultivated with rice in flooded conditions or with iyegrass in unsaturated conditions, allowing pai.twise comparisons of the RCs b'3.llsfer. Against this speculation, the soil-plant transfer of RCs was 1.2-14-fold smaller for flooded rice than that for 1yegrass (P< 0.05). The RCs transfer to rice and iyegrass was negatively correlated to the radiocaesium interception potential (RIP) of soils and to the soil exchangeable potassium (K) content. The RCs mobility in soils, expressed as a soil solution-soil RCs concentration ratio, was on average 1.6-fold smaller for flooded soils than m1sahlrated soils, despite more NH4 accumulation i11 soil solution in flooded soils. The plant RCs concentration was not related to that in the soil solution but shaq>ly increased ·with the RCs/potassium (K) ratio in the soil solution with a larger slope for rice than ryegrass. A larger :fraction of soil pores filled with water in saturated soils increases the diffusivity ofK in soil solution and, hence, avoiding K depletion at the root sutface of rice and suppressing RCs uptake by cationic competition between K and RCs. It was revealed that the mechanisms mling RCs bioavailability to flooded lice are similar to what was previously established in unsaturated soils. The findings will contribute to the improved prediction of soil plant RCs transfer for rice grown in the Fukushima affected area.