Estimation of rooting depth of ¹³⁷Cs uptake by plants

Understanding the soil-to-plant transfer process of ¹³⁷Cs is essential for predicting the contamination levels of plants in contaminated areas. The rooting depth is considered one of the key factors explaining the difference in the activity concentration of ¹³⁷Cs in different plant species. In this study, the distributions of ¹³⁷Cs and ¹³³Cs in soils and plants were investigated, and the plants' rooting depth of ¹³⁷Cs uptake was estimated using the ¹³⁷Cs/¹³³Cs ratios in exchangeable fractions of soils and biological samples. The results showed that different plant species accumulate different levels of ¹³⁷Cs and ¹³³Cs. The ¹³⁷Cs/¹³³Cs ratios were fairly constant in plants of the same species. The average ¹³⁷Cs/¹³³Cs ratios in bamboo grasses and ferns were 0.015 ± 0.009 (n = 5) and 0.13 ± 0.04 Bq ng^-1 (n = 10) in Yamakiya, respectively. The percentage of ¹³⁷Cs in the exchangeable fraction of the uppermost soil layer was lower than that in the deeper soil layers. The activity concentrations of ¹³⁷Cs in the soil profiles decreased sharply with depth, whereas the depth distributions of ¹³³Cs were uniform. Therefore, the ¹³⁷Cs/¹³³Cs ratios were driven mainly by the ¹³⁷Cs activity concentrations in soil. The plants' rooting depths of ¹³⁷Cs uptake were estimated on the basis of the relationships between the averaged ¹³⁷Cs/¹³³Cs ratio in the soil layer and the ¹³⁷Cs/¹³³Cs ratio in the plant. The results indicate that the deeper-rooted species such as bamboo grasses have a lower accumulation of ¹³⁷Cs than the superficial-rooting species such as ferns. The soil-to-plant transfer factors would be determined using rooting depth by calculating the averaged activity concentration of ¹³⁷Cs within the estimated rooting depth.