Continuous nitrogen fertilization retards the vertical migration of Fukushima nuclear accident-derived cesium-137 in apple orchard soil

We monitored the levels of cesium-137 (¹³⁷Cs) in the soils of five orchards for six years following the Fukushima Daiichi Nuclear Power Plant accident on 11 March 2011 and found that the vertical distribution of accident-derived ¹³⁷Cs varied significantly among the orchards with varying land-use and fertilizer management. Based on these results, this study evaluated how nitrogen (N) fertilizer management may have affected the vertical migration of ¹³⁷Cs in the orchard soils. We selected an experimental orchard producing 'Jonathan' apples, where a long-term N-fertilizer trial has continued since 1973, with an N-fertilized plot (N plot; N added at 20 g m^-2 y^-1) and a non-fertilized plot (0 N plot). Five years after the accident, the vertical migration of accident-derived ¹³⁷Cs was significantly lower in the N plot (2.3 cm) than in the 0 N plot (4.3 cm), suggesting greater ¹³⁷Cs retention in the surface of the N plot. Application of a cesium bromide (¹³³CsBr) tracer suggested that the retarded vertical migration of ¹³⁷Cs in the N plot may be related to significantly lower amounts of exchangeable ¹³³Cs and significantly higher proportions of non-exchangeable ¹³³Cs in the upper 2 cm. We did not find any evidence of the aboveground plants contributing to more ¹³³Cs retention in the N plot. However, greater ¹³⁷Cs retention in the surface (0-2 cm) of the N plot may be due to more dissolved organic carbon (DOC), and high DOC may have contributed to deeper ¹³⁷Cs migration in both the plots. Our results suggest that continuous N fertilizer application significantly retarded the migration of ¹³⁷Cs by approximately one-half and resulted in less ¹³⁷Cs reaching the mobile exchangeable form in the deeper root-zone layers.