Japanese forests were exposed to multiple sources of radioactive contamination. To acquire scientific guidance on forest management planning, it is crucial to understand the long-term radiocesium (¹³⁷Cs) distribution (and redistribution) over time. To obtain robust evidence of the residual global fallout of ¹³⁷Cs (¹³⁷Cs-GFO) after a few decades, we determined ¹³⁷Cs-GFO inventory in forest soil at 1171 soil pits of 316 plots evenly spaced across Japan from 2006 to 2011, shortly before the Fukushima Dai-ichi Nuclear Power Plant accident. The activity concentration measurements were performed using a NaI well-type scintillation counter. The average (±SD) ¹³⁷Cs-GFO in forest soil (0–30 cm from the surface) of the National Forest Soil Carbon Inventory (NFSCI) sampling plots uniformly extracted from the entire country was estimated to be 2.27 ± 1.73 kBq m⁻² (n = 316) as of Oct. 1, 2008. A high nationwide spatial variation was found in ¹³⁷Cs-GFO, where relatively high ¹³⁷Cs-GFO was found along the Sea of Japan compared with the total annual precipitation. We also obtained a reconstructed decay-corrected cumulative ¹³⁷Cs-GFO dataset from the fallout observatories as the initial ¹³⁷Cs-GFO. The cumulative ¹³⁷Cs-GFO of fallout observatories averaged 2.47 ± 0.95 kBq m⁻² (n = 39) as of Oct. 1, 2008 and displayed spatial variation similar to that in forest soil. To identify whether ¹³⁷Cs-GFO remains in forest soil across Japan, we examined a general linear mixed-effect model comparing ¹³⁷Cs-GFO between forest soil and the observatory under normalized annual precipitation and region. The model did not indicate a significant difference, but relatively lesser ¹³⁷Cs-GFO was found in forest soil, where the least-squares mean of ¹³⁷Cs-GFO in forest soils was 79.1% of that of the observatory. The variation in ¹³⁷Cs-GFO in forest soils within NFSCI sampling plots was 1.4 times greater than that among plots. The high spatial variation in ¹³⁷Cs-GFO within a 0.1-ha plot strongly suggested the redistribution of ¹³⁷Cs-GFO within the forest catchment. The vertical distribution pattern of ¹³⁷Cs-GFO across three depth layers indicated that the ¹³⁷Cs-GFO redistributions were likely attributed to the movements of sediments and mass. Moreover, when extracting soil pits assumed to have the least soil disturbance from the vertical distribution pattern, no significant difference in ¹³⁷Cs-GFO was observed between forest soil and observatory data. These findings provide important insights into the stability of ¹³⁷Cs-GFO in the forest ecosystem. Considering the potential hotspot where ¹³⁷Cs-GFO can accumulate deeper in the soil (>30 cm in depth), most ¹³⁷Cs-GFO has remained in the forest for decades. Our study offers microscale heterogeneous ¹³⁷Cs-GFO distribution in forests for ensuring long-term forest management planning necessary for both the long-term migration and local accumulation of ¹³⁷Cs in forests.

日本森林暴露于多种放射污染源。要获得有关森林管理规划的科学指导，至关重要的是要了解随时间推移的长期放射性铯（¹³⁷ Cs）分布（和再分布）。为获得几十年后全球¹³⁷ Cs（¹³⁷ Cs-GFO）残留的有力证据，我们确定了2006年至2011年日本全国316个样地的1171个土坑中森林土壤中的¹³⁷ Cs-GFO库存在福岛第一核电站事故之前。使用NaI阱型闪烁计数器进行活性浓度测量。平均值（±SD）¹³⁷从全国均匀提取的国家森林土壤碳清单（NFSCI）采样区的森林土壤（距地面0–30 cm）中的Cs-GFO估计为2.27±1.73 kBq m ^-2（n  = 316），于2008年10月1日。全国¹³⁷ Cs-GFO的空间变化较大，与日本年降水量相比，日本海沿岸的¹³⁷ Cs-GFO相对较高。我们还从辐射观测站获得了经过重建的衰减校正累积¹³⁷ Cs-GFO数据集，作为初始¹³⁷ Cs-GFO。沉降观测站的¹³⁷ Cs-GFO累积平均值为2.47±0.95 kBq m ^-2（n = 39），截至2008年10月1日，其空间变化与森林土壤相似。为了确定日本全国森林土壤中是否还残留有¹³⁷ Cs-GFO，我们研究了一个常规的线性混合效应模型，该模型比较了在正常年降水量和区域下森林土壤与天文台之间的¹³⁷ Cs-GFO。该模型没有显示出显着差异，但是在森林土壤中发现的¹³⁷ Cs-GFO相对较少，其中森林土壤中¹³⁷ Cs-GFO的最小二乘均值是观测站的79.1％。NFSCI样地内森林土壤中¹³⁷ Cs-GFO的变化比样地之间大1.4倍。¹³⁷的高空间变化0.1公顷土地中的Cs-GFO强烈建议在森林流域内重新分配¹³⁷ Cs-GFO。¹³⁷ Cs-GFO在三个深度层上的垂直分布模式表明¹³⁷ Cs-GFO的重新分布很可能归因于沉积物和物质的运动。此外，从垂直分布格局中提取假定土壤扰动最小的土坑时，在森林土壤和天文台数据之间未观察到¹³⁷ Cs-GFO的显着差异。这些发现提供了对¹³⁷ Cs-GFO在森林生态系统中稳定性的重要见解。考虑到潜在的热点¹³⁷Cs-GFO可以在土壤中积累更深（> 30 cm的深度），大多数¹³⁷ Cs-GFO在森林中保留了数十年。我们的研究提供了森林中微观尺度的¹³⁷ Cs-GFO的异质分布，以确保对¹³⁷ Cs的长期迁移和在森林中的局部积累所必需的长期森林管理计划。