Exchangeability of ¹³⁷ Cs and K in soils of agricultural fields after decontamination in the eastern coastal area of Fukushima

To examine the exchangeability of 137Cs and K in soils of agricultural fields after decontamination, 173 soil samples were collected in 2016 from the plowed layer (0–15 cm depth) of decontaminated agricultural fields in Tomioka town, Fukushima prefecture, Japan. We investigated total 137Cs, exchangeable 137Cs, exchangeable K, and nonexchangeable K content for these soils. The total 137Cs content in soils was, on average, 1,200 Bq kg−1 (value range: 20–4,400 Bq kg−1), which was an 80% decrease from the values determined before the decontamination. This result indicated that the decontamination process considerably reduced the total 137Cs content in the plowed layer. The exchangeable K content in soils was on average 172 mg kg−1 (value range: 27.9 – 743 mg kg−1). More than 80% of soils showed the exchangeable K content lower than the recommended threshold value for paddy soils needed in order to reduce the 137Cs transfer from soil to plant (i.e. 200 mg kg−1). The nonexchangeable K content showed a wide range of values (value range: 98.8–1,280 mg kg−1) and no significant correlation with the exchangeable K content. Because nonexchangeable K is released more slowly but is comparable to exchangeable K as a reservoir of phytoavailable K, more frequent K application is recommended for soils with lower contents of both exchangeable and nonexchangeable K. The total 137Cs content in soils showed a significant positive correlation with the exchangeable K content in soils (p < 0.01), which means that decontamination efforts intended to reduce the total 137Cs content in soils might also lead to a reduction in the exchangeable K content. In conclusion, the application of K fertilizer is strongly recommended in the decontaminated agricultural fields because the decontamination causes a considerable decrease in exchangeable K content along with a reduction in total 137Cs content.