Distribution, migration, and source of anthropogenic ²³⁶U in different forest vegetation zones of Changbai Mountain

This study presents the first systematic analysis of the distribution, migration characteristics, and sources of the anthropogenic radionuclide ²³⁶U in the vertical but distinct vegetation zones of Changbai Mountain (CBM) in northeastern China. Previous research in the CBM region has detected elevated levels of radionuclides, yet the behavior and inventory of ²³⁶U remain unclear. To address this, a total of 19 surface soil samples and 6 core samples were collected from different vertical vegetation zones in the CBM region, along with 5 comparative samples from northeastern China. These samples were chemically processed using a borate fusion method and analyzed by accelerator mass spectrometry (AMS). The results show that the ²³⁶U concentration in the surface soils of CBM region ranges from 0.34 × 10⁸-9.56 × 10⁸ atoms/g, significantly higher than those in the comparative regions of northeastern China. Spatial heterogeneity analysis among different vegetation zones indicates that post-depositional redistribution of ²³⁶U is jointly controlled by soil pH, organic matter content, and freeze-thaw erosion. The atomic ratio of ²³⁶U to ²³⁹Pu in the soil cores is 0.175 ± 0.008, which falls within the characteristic range for global fallout. Vertical profile analysis indicates a strong retention effect of ²³⁶U in the upper soil layers. These findings reveal the influence of terrain, climate, soil properties, and vegetation types on the migration behavior of ²³⁶U in complex forest vegetation zones.