Vertical distribution and temporal changes of ¹³⁷Cs in soil profiles under various land uses after the Fukushima Dai-ichi Nuclear Power Plant accident

We monitored the vertical distribution of ¹³⁷Cs in soil profiles under eight different land uses for the 2 y after the Fukushima Dai-ichi Nuclear Power Plant accident, and discussed the temporal changes in the early-stage of the migration and the determinants of the initial distribution. The soil samples were collected for four surveys using a scraper plate at each study site, which consisted of three forests (mixed forest, mature cedar, and young cedar), two grasslands (pasture and meadow) and three abandoned agricultural fields (farm land, tobacco field, and paddy field). The land use patterns have a large influence on some soil properties and the migration processes of ¹³⁷Cs above ground, resulting in different distribution of ¹³⁷Cs in those soil profiles. Specifically, the secondary deposition of ¹³⁷Cs from the coniferous canopy, retention of ¹³⁷Cs by litter layer, and the homogenization of ¹³⁷Cs concentrations in surface soil by natural soil mixing such as the disturbance by cattle grazing, roots growing and the formation of needle ice were important to cause redistribution of the deposited ¹³⁷Cs. Only in the paddy field, the ¹³⁷Cs inventory in subsurface soils (5–10 cm) gradually increased and comprised 26% of the total ¹³⁷Cs in 2 y, showing the downward migration of ¹³⁷Cs to subsurface soil. In the other sites, it was considered that ¹³⁷Cs were strongly adsorbed by soil particles and rarely migrated downward as soluble form. Vertical distributions during the first survey were able to be used as the initial distributions and were well fitted to the exponential equation. The distribution parameters α (relaxation depth) and β (relaxation mass depth), calculated by the exponential equation were correlated with RIP (r = −0.806, p < 0.05), macro pore (r = 0.651, p = 0.11), and dispersible fine particle content (r = 0.856, p < 0.05). It indicated that the initial distribution would be influenced by the Cs fixation ability of soil, and the penetration process of water and particles in soils. - Highlights: • We monitored the vertical distribution of ¹³⁷Cs in soils using the scraper plate. • The ¹³⁷Cs migrated from litter layer to soil surface by the litter decomposition. • The ¹³⁷Cs concentrations in surface soils were homogenized by natural soil mixing. • The downward migration of ¹³⁷Cs to the subsurface soil occurred in the paddy field