[en] Highlights: • ¹³⁷Cs vertical distribution in soil was observed at five forest sites for 4.4 years. • ¹³⁷Cs retention ratio of organic layer decreased rapidly with time. • ¹³⁷Cs migration from organic layer to mineral soil has fast and slow components. • ¹³⁷Cs transfer in mineral soil obeyed a diffusion equation model. • Model prediction indicated long-term retention of ¹³⁷Cs in surface mineral soil. - Abstract: Because of the Fukushima Dai-ichi Nuclear Power Plant accident, forest ecosystems in wide areas were contaminated with ¹³⁷Cs. It is important to characterize the behavior of ¹³⁷Cs after its deposition onto forest surface environments for evaluating and preventing long-term radiation risks. In the present study, ¹³⁷Cs vertical distributions in the soil profile were observed repeatedly at five forest sites with different vegetation types for 4.4 years after the accident in 2011, and ¹³⁷Cs migration in the organic layer and mineral soil was analyzed based on a comparison of models and observations. Cesium-137 migration from the organic layer to the underlying mineral soil was represented by a two-component exponential model. Cesium-137 migration from the organic layer was faster than that observed in European forests, suggesting that the mobility and bioavailability of ¹³⁷Cs could be suppressed rapidly in Japanese forests. At all sites, ¹³⁷Cs transfer in mineral soil could be reproduced by a simple diffusion equation model with continuous ¹³⁷Cs supply from the organic layer. The diffusion coefficients of ¹³⁷Cs in the mineral soil were estimated to be 0.042–0.55 cm² y⁻¹, which were roughly comparable with those of European forest soils affected by the Chernobyl Nuclear Power Plant accident. Model predictions using the determined model parameters indicated that 10 years after the accident, more than 70% of the deposited ¹³⁷Cs will migrate to the mineral soil but only less than 10% of the total ¹³⁷Cs inventory will penetrate deeper than 10 cm in the mineral soil across all sites. The results of the present study suggest that the ¹³⁷Cs deposited onto Japanese forest ecosystems will be retained in the surface layers of mineral soil for a long time.