Downward migration of Cs promotes self-cleaning of forest ecosystem by reducing root uptake of Japanese cedar in Fukushima.

Approximately 70 % of the area highly Cs-contaminated by the Fukushima Daiichi Nuclear Power Plant accident is forested. Decontamination works in most of these forests have not progressed, and the forestry industry remains stagnant. Although the long-term dynamics of Cs in the forest ecosystem will be controlled by the amount of Cs absorbed by roots in the future, temporal changes in Cs of tree roots have rarely been reported. In the present study, we monitored the depth distribution of Cs in the soil and absorptive very fine (VF) roots of 0.5 mm or less in a Japanese cedar forest from 2011 to 2023. As a result, the Cs inventory in the mineral soil increased over time due to the migration from the forest canopy and litter layers, whereas that in the VF roots tended to decrease since 2020, although there was a large variation. Temporal decrease in the exchangeable Cs fraction with fixation and temporal increase in VF root biomass with their growth were not clearly observed, the Cs concentration in the VF roots at 0-2 cm decreased with the decrease in Cs concentration in the litter layers. Although the Cs concentration in the VF roots below 2 cm tended to increase with increasing Cs concentration in the soil at the same depth, the downward migration of Cs within the soil can reduce the amount of Cs absorbed by roots because the VF root biomass decreases exponentially with depth. In other words, Cs can be removed from the long-term active cycles of forest ecosystems as they migrate deeper into the soil. This natural migration process can be regarded as a "self-cleaning" of the forest ecosystem, the green and sustainable remediation using such self-cleaning should be actively adopted for the future forest management.