Cs isotopic two-source mixing model reveals significant internal ¹³⁷ Cs translocation to the buds of <i>Eleutherococcus sciadophylloides</i> (Koshiabura)

This study proposes a two-source mixing model based on a Cs isotopic approach for the source apportionment of 137Cs; it is applicable to the 137Cs/133Cs ratios of two distinct sources and enables delineation of their mixtures. This approach was applied to identify the main pathway contributing to 137Cs accumulation in the buds of Eleutherococcus sciadophylloides (Koshiabura), an edible wild plant known for high 137Cs concentrations. 137Cs in current-year buds was partitioned into internal translocation and root uptake fractions based on two assumptions: (1) all 137Cs and 133Cs in buds originate exclusively from internal translocation or root uptake, with each source maintaining a constant 137Cs/133Cs ratio, and (2) the 137Cs/133Cs ratios in previous-year leaves and soil exchangeable fractions serve as proxies of internal translocation and root uptake, respectively. The 137Cs/133Cs ratios were obtained from the authors’ previous study on three small E. sciadophylloides trees. The representative ratio in the soil exchangeable fraction was calculated considering the vertical root distribution. The results showed that internal translocation accounted for 89%–99% of 137Cs in buds, indicating that it was the dominant pathway. Moreover, the 137Cs concentration in current-year buds attributed to internal translocation was higher than that in previous-year leaves. This internal redistribution during the spring growth enhances localized 137Cs accumulation and contributes to the consistently high concentrations observed in E. sciadophylloides buds. This study highlights the importance of internal translocation in 137Cs cycling within forest ecosystems.