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 (Cs) 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 Cs 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 Cs 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 y) and a non-fertilized plot (0 N plot). Five years after the accident, the vertical migration of accident-derived Cs was significantly lower in the N plot (2.3 cm) than in the 0 N plot (4.3 cm), suggesting greater Cs retention in the surface of the N plot. Application of a cesium bromide (CsBr) tracer suggested that the retarded vertical migration of Cs in the N plot may be related to significantly lower amounts of exchangeable Cs and significantly higher proportions of non-exchangeable Cs in the upper 2 cm. We did not find any evidence of the aboveground plants contributing to more Cs retention in the N plot. However, greater Cs 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 Cs migration in both the plots. Our results suggest that continuous N fertilizer application significantly retarded the migration of Cs by approximately one-half and resulted in less Cs reaching the mobile exchangeable form in the deeper root-zone layers.