Estimation of the soil depth at which 137Cs is taken up by roots of Eleutherococcus sciadophylloides (koshiabura), an edible wild tree species

Eleutherococcus sciadophylloides (commonly known as koshiabura) is a deciduous tree whose young buds are a popular wild food in Japan. After the 2011 Fukushima nuclear accident, radiocaesium contamination occurred in edible wild plants in forests, with particularly high levels in E. sciadophylloides buds. Consequently, restrictions on their consumption have impacted local cuisine and cultural traditions. To investigate the mechanisms underlying these high 137Cs concentrations, we employed a multifaceted approach combining root excavation, environmental DNA (eDNA) analysis, and Sr and Cs isotope analyses to cross-validate root depth and uptake dynamics. Root excavation revealed predominantly shallow root systems. eDNA analysis confirmed that the lateral roots collected from soil cores were most abundant within the 0–5 cm layer of the mineral soil horizon. Sr isotope analysis indicated that small E. sciadophylloides trees primarily absorbed cations from this layer. In contrast, Cs isotope analysis was insufficient to determine the soil depth of 137Cs uptake. This discrepancy may reflect contributions from both the root uptake of 137Cs leached from the organic horizon and the supply of 137Cs to buds through internal translocation within the tree. Our findings indicate that small E. sciadophylloides trees rely primarily on shallow root systems for nutrient uptake, specifically from the 0–5 cm soil layer where 137Cs has accumulated. This characteristic likely contributes to the high 137Cs levels observed in E. sciadophylloides buds. Further research is needed to clarify the relative contributions of root uptake and internal translocation to 137Cs transfer into buds.