2019 Volume 88 Issue 2 Pages 150-163
We investigated the vertical 137Cs distribution in soil among five sod culture orchards with different soil textures over six years after 2011, when 137Cs fallout was released by the Fukushima Daiichi Nuclear Power Plant accident, to confirm the long-term 137Cs downward-migration into soil. At each orchard, soil cores were collected to a depth of 30 cm and subdivided into intervals of 3 to 9 cm. The 137Cs within the 3 cm of topsoil decreased from 84–94% during the first 7 months after deposition in 2011 to 41–75% in 2017. From 2012 onward, the vertical 137Cs profiles in the soils were explained by a two-component negative exponential model composed of a rapid and a slow component with a change of slope at a depth of 6 to 9 cm. It took 4 years after deposition to show a significant difference in the value of the average 137Cs migration distance (Md) among the orchards. The speed of 137Cs migration in the orchards during the 6 years after the accident year was 0.44 to 0.97 cm year−1 based on the Md. There was a significant positive correlation between Md and fine sand content in the 3 cm of topsoil between Md and the ratio of the total carbon content (TC) at a depth of 3 to 6 cm to that in the top 3 cm of soil. Furthermore, the percentage of exchangeable 137Cs (ex-137Cs) to 137Cs at 3 to 6 cm depth increased significantly in proportion to the ratio of TC at 3 to 6 cm depth to TC at 0 to 3 cm depth in soil collected in 2013. These findings indicate that one of the mobile forms of 137Cs was ex-137Cs combined with TC and that the fine sand content and TC influenced the 137Cs downward-migration in the 3 cm of topsoil in the orchard in which organic matter accumulated by sod culture.