Explaining the variation in (137)Cs 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 (137)Cs distribution in the soil on the variation of T(ag)-(137)Cs values for one of the most severely contaminated wild edible plants, Eleutherococcus sciadophylloides Franch. et Sav. (Koshiabura). The variation in T(ag)-(137)Cs values was not reduced by direct measurement of (137)Cs deposition in soil samples from the Koshiabura habitat, as a substitute for using spatially averaged airborne survey data at the administrative district scale. The (137)Cs activity concentration in Koshiabura buds showed a significant positive correlation with the (137)Cs inventories only in the organic horizon of soil from the Koshiabura habitat. The ratio of (137)Cs inventories in the organic horizon to the total (137)Cs 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)-(137)Cs values observed in Koshiabura buds when calculated from the total (137)Cs deposition in soil.