Abstract
Countermeasures to reduce radiocesium (134Cs and 137Cs) uptake by crops have been implemented in farmlands affected by the Fukushima nuclear accident in 2011. A widely practiced countermeasure is the application of potassium (K). Long-term soil K maintenance is a key issue due to the long physical half-life of 137Cs (30 years). Information on input and output pathways determining plant-available K budgets can provide a base for the development of maintenance strategies. Therefore, in this study we evaluated these pathways in paddy fields subjected to K fertilization as a countermeasure. We selected two fields with different soil textures and drainage conditions and quantified input and output via fertilization, irrigation, precipitation, straw return to soil, plant harvesting, surface runoff, and percolation during the cropping period in 2018. The major input pathways were fertilization, straw return, and irrigation due to a large inflow volume with spill-over irrigation. The major output pathways consisted of plant harvesting, surface runoff, and percolation. However, 85% of K in harvested plants was brought back by straw return; in practice, harvesting was a minor pathway. The K budgets during the study period were negative (−20 and −289 kg ha−1) and especially severe in clay loam soil with high output via percolation. This could probably be attributed to the low cation exchange capacity and high permeability from the low total C and clay contents. Losses via surface runoff stemmed from excessive irrigation volumes in both fields. Around 70% of the total K output via surface runoff and percolation was discharged before mid-summer drainage. Accordingly, controlling the irrigation volume during this period in addition to increasing cation exchange capacity and decreasing permeability may improve the negative budgets.
Highlights
The Fukushima Daiichi Nuclear Power Plant accident following the Great East Japan Earthquake on March 11, 2011 resulted in the emission of a large amount of radionuclides (131I, 134Cs, and 137Cs) into the atmosphere, thereby contaminating a wide area of eastern Japan. 131I has a short physical half-life (8 days) and does not remain in the environment for years. 134Cs and 137Cs, have long half-lives (2 and 30 years, respectively)
Numerous studies have shown that the removal of radiocesium by water discharge from watersheds [e.g., 1–4] is limited due to its strong adsorption by clay minerals in soil [5], leading to the potential for prolonged contamination
Topsoil removal at a depth of 5 cm has been performed in highly contaminated farmlands (>5000 Bq kg−1 in soil) as a countermeasure [12], it is difficult to remove radiocesium that migrates to the lower layer [8,13]
Summary
The Fukushima Daiichi Nuclear Power Plant accident following the Great East Japan Earthquake on March 11, 2011 resulted in the emission of a large amount of radionuclides (131I, 134Cs, and 137Cs) into the atmosphere, thereby contaminating a wide area of eastern Japan. 131I has a short physical half-life (8 days) and does not remain in the environment for years. 134Cs and 137Cs, have long half-lives (2 and 30 years, respectively). Most radiocesium has not been discharged from agricultural areas at the field scale [6,7,8,9]; countermeasures against the contamination of agricultural products have been implemented. Current guidelines [10] recommend maintaining the soil exchangeable K content to reduce radiocesium uptake by crops. This is based on the competition between K and Cs for uptake by plants. Topsoil removal at a depth of 5 cm has been performed in highly contaminated farmlands (>5000 Bq kg−1 in soil) as a countermeasure [12], it is difficult to remove radiocesium that migrates to the lower layer [8,13]. Soil K maintenance is an important long-term issue for the sustainable production of safe crops
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