Explaining the variation in ¹³⁷Cs aggregated transfer factor for wild edible plants as a case study on Koshiabura (Eleutherococcus sciadophylloides) buds

The aggregated transfer factor (T-ag) is commonly used to represent the actual transfer of radiocesium from soil to wild edible plants, but the values have shown substantial variation since the Fukushima nuclear accident. To elucidate the factors causing this variation, we investigated the effects of spatial scale and vertical Cs-137 distribution in the soil on the variation of T-ag-Cs-137 values for one of the most severely contaminated wild edible plants, Eleutherococcus sciadophylloides Franch. et Sav. (Koshiabura). The variation in T-ag-Cs-137 values was not reduced by direct measurement of Cs-137 deposition in soil samples from the Koshiabura habitat, as a substitute for using spatially averaged airborne survey data at the administrative district scale. The Cs-137 activity concentration in Koshiabura buds showed a significant positive correlation with the Cs-137 inventories only in the organic horizon of soil from the Koshiabura habitat. The ratio of Cs-137 inventories in the organic horizon to the total Cs-137 deposition in soil exhibited substantial variation, especially in broad-leaved deciduous forests that Koshiabura primarily inhabits. This variation may be the cause of the wide range of T-ag-Cs-137 values observed in Koshiabura buds when calculated from the total Cs-137 deposition in soil.