Effect of litter removal five years after the Fukushima accident on ¹³⁷Cs uptake by Japanese cedar

Removal of litter-associated ¹³⁷Cs from the forest floor (litter removal) can reduce the ¹³⁷Cs uptake by plants; however, the proposed effective period for litter removal was 1-2 years after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. This is because the ¹³⁷Cs in forest soil migrates rapidly from the litter to surface mineral soil layers in Japanese forests, and thus the effectiveness of litter removal will quickly become limited. However, it is unknown whether this approach can be applied to forests whose vertical migration of ¹³⁷Cs in the forest soil is relatively slow. Herein, we compared the ¹³⁷Cs activity concentration in the inner bark of the Japanese cedar (Cryptomeria japonica) between litter removal (conducted in September and October 2016, 5 years after the accident) and in control areas in Kawauchi Village, Fukushima Prefecture, where the vertical migration of ¹³⁷Cs was relatively slow from the litter to surface mineral soil layers. Air dose rates (ambient dose equivalent) in the litter removal area were significantly lower than those in the control area in 2022, and the ¹³⁷Cs inventory in the forest soil in litter removal area also tended to be lower than that in the control area. In Japanese cedars with similar levels of outer bark contamination, the ¹³⁷Cs activity concentration in the inner bark in the litter removal area was significantly lower than that in the control area, and consistent trends were also found when comparing the ¹³⁷Cs activity concentration in the leaves of Stephanandra incisa and Wisteria floribunda obtained from the same forest. Thus, the litter removal 5 years after the FDNPP accident may have reduced the ¹³⁷Cs uptake in Japanese cedar in an evergreen coniferous forest where the vertical migration of ¹³⁷Cs is relatively slow in the forest soil.