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

To examine the exchangeability of sup(137)Cs 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 sup(137)Cs, exchangeable sup(137)Cs, exchangeable K, and nonexchangeable K content for these soils. The total sup(137)Cs content in soils was, on average, 1,200 Bq /kg (value range: 20-4,400 Bq /kg), which was an 80% decrease from the values determined before the decontamination. This result indicated that the decontamination process considerably reduced the total sup(137)Cs content in the plowed layer. The exchangeable K content in soils was on average 172 mg /kg (value range: 27.9-743 mg /kg). 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 sup(137)Cs transfer from soil to plant (i.e. 200 mg /kg). The nonexchangeable K content showed a wide range of values (value range: 98.8-1,280 mg /kg) 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 sup(137)Cs 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 sup(137)Cs 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 sup(137)Cs content.