Fine-root depth influences differences in soil-to-plant transfer of 137Cs and 133Cs

Abstract

The distribution of naturally occurring cesium-133 (¹³³Cs) in soil can be used to predict soil-to-plant transfer of cesium-137 (¹³⁷Cs was released during the 2011 Fukushima accident). However, when the distributions of ¹³⁷Cs and ¹³³Cs differ with soil depth, the relative amounts transferred to plants may vary with fine-root depth, since these roots are involved in nutrient absorption. Therefore, we investigated the influence of fine-root depth on soil-to-young-plant-tissue ¹³⁷Cs aggregated transfer factors (T_ags) and exchangeable ¹³³Cs T_ags (Young-shoot ¹³³Cs concentration/Soil exchangeable ¹³³Cs inventory) in 10 edible wild-plant species collected in Fukushima Prefecture between 2013 and 2017. The ¹³⁷Cs T_ag values were strongly positively correlated with those of exchangeable ¹³³Cs T_ag, implying that the distribution of ¹³³Cs can be used to predict the distribution of ¹³⁷Cs in plant bodies. However, in species with deep fine roots, ¹³⁷Cs T_ag values tended to be smaller than those estimated from exchangeable ¹³³Cs T_ag. Since ¹³³Cs is derived from minerals and is abundant in deep soil, such species likely absorbed large amounts of ¹³³Cs from deep soil layers. Relative to a simple regression model where variation in ¹³⁷Cs T_ag was explained only by exchangeable ¹³³Cs T_ag (adjusted R² = 0.59), a multiple regression model that included fine-root depth increased the explanatory power (adjusted R² = 0.82). Thus, when predicting plant ¹³⁷Cs concentrations using exchangeable ¹³³Cs T_ag, it is important to consider the effect of fine-root depth.