Decadal downward migration of 137Cs in forest soils after the Fukushima nuclear accident revealed by migration center analysis

Forest soils became the primary reservoir of 137 Cs following the Fukushima nuclear accident in March 2011; however, site-specific observations of its vertical migration with annual resolution encompassing the first decade post-deposition remain limited. This study analyzed one of the few such long-term records of 137 Cs depth profiles in forest soils to quantify decadal downward migration dynamics. The migration center, calculated in this study as the inventory-weighted mean depth from the litter surface, was used as a site-comparable indicator. Soil-core measurements collected repeatedly between 2011 and 2021 at cedar plantation (Ce) and deciduous broadleaved forest (DB) plots on Mount Tsukuba were analyzed. The observed mean migration center increased from 3.5 cm to 6.8 cm in Ce and from 3.8 cm to 7.5 cm in DB over the ten years. The three-parameter exponential model, including an asymptotic value, captured the decadal trends better than linear or simple exponential models, indicating that downward migration persisted through the first decade but became increasingly limited over time. The model predicted asymptotic migration-center depths of approximately 9 cm, suggesting a tendency toward long-term stabilization. In addition, this dataset was combined with published datasets from forest sites in Fukushima Prefecture. Across seven cedar plantation plots, the migration center showed a relatively consistent downward shift of approximately 4 cm, reaching nearly 8 cm below the litter surface during the first decade. The migration-center approach provides a common depth-based framework for comparing long-term radiocesium migration among forest sites and may facilitate interpretation by researchers and forest managers.