Temporal dynamics of 137Cs distribution in soil and soil-to-crop transfer factor under different tillage systems after the Fukushima Daiichi Nuclear Power Plant accident in Japan

Modeling long-term transfers of radiocesium in farmland under different tillage and cover crop treatments

The 2011 nuclear accident at Japan's Fukushima Daiichi Nuclear Power Plant (FDNPP) prompted inquiries about the long-term transfer of Cesium-137 (137Cs) from soil to agricultural plants. In this context, numerical modeling is particularly useful for the long-term evaluation of the consequences of agroecosystem contamination. Agricultural practices, such as tillage and cover cropping, play key roles in 137Cs recycling in agroecosystems. In this study, we used 10-year monitoring data to develop a dynamic model to predict 137Cs redistribution (via uptake, litterfall, translocation, and percolation) under different tillage (no-tillage, NT; rotary cultivation, RC; moldboard plow, MP) and cover crop (rye; hairy vetch; fallow weed) treatments. The verification exercise and assessment results indicated the model's reliability, as the temporal dynamics of predicted values agreed with observed values. Tillage significantly influenced the 137Cs distribution in soil, thereby decreasing plant uptake of 137Cs, whereas cover crop exerted a minimal effect on 137Cs cycling. Furthermore, while the 137Cs concentrations in soybean grain under RC and NT treatments were comparable 62 years after the FDNPP accident, the concentration under MP treatment remained consistently the lowest. Despite natural decay being the main cause of the decreased global 137Cs level in the agroecosystem, with minimal losses from percolation to deeper soil layers and soybean harvesting, adopting an appropriate tillage practice was shown to promote a long-term reduction of 137Cs concentration in crops. Finally, to improve the model's accuracy, further research should consider incorporating the effects of soil properties and extreme weather events on 137Cs flow into the model, as these factors are essential for realizing improved agroecosystem predictions.

Phytotoxicity of radionuclides: A review of sources, impacts and remediation strategies

Various anthropogenic activities and natural sources contribute to the presence of radioactive materials in the environment, posing a serious threat to phytotoxicity. Contamination of soil and water by radioactive isotopes degrades the environmental quality and biodiversity. They persist in soils for a considerable amount of time and disturb the fauna and flora of any affected area. Hence, their removal from the contaminated medium is inevitable to prevent their entry into the food chain and the organisms at higher levels of the food chain. Physicochemical methods for radioactive element remediation are effective; however, they are not eco-friendly, can be expensive and impractical for large-scale remediation. Contrastingly, different bioremediation approaches, such as phytoremediation using appropriate plant species for removing the radionuclides from the polluted sites, and microbe-based remediation, represent promising alternatives for cleanup. In this review, sources of radionuclides in soil as well as their hazardous impacts on plants are discussed. Moreover, various conventional physicochemical approaches used for remediation discussed in detail. Similarly, the effectiveness and superiority of various bioremediation approaches, such as phytoremediation and microbe-based remediation, over traditional approaches have been explained in detail. In the end, future perspectives related to enhancing the efficiency of the phytoremediation process have been elaborated.

Aggregated transfer factors of fresh Cs and Sr pollution to various vegetables from six common European soils - mesocosm experiment

Aggregated transfer factors (Tag) were identified for three common vegetables grown in six common European soils freshly contaminated by 134Cs and 85Sr. The experiment was carried out as a mesocosm experiment in pots with an average soil weight of 15.8 kg per pot. The vegetables were grown one after the other during one vegetation season, in the order lettuce, onion, and radish (the order usually applied in private gardens and small farms). Despite the fact that lettuce was grown in the most contaminated soil, it had the lowest Tag (in m2/kg) of both radionuclides (3.6E-4 for Cs, 2.0E-2 for Sr), while onion had 6.4E-3 for Cs and 3.2E-2 for Sr and radish had 1.9E-3 for Cs and 8.1E-2 for Sr. Potassium supply did not show any statistically significant effect on Cs Tag; there was a significant impact of K on the decrease in Sr Tag. The experiments indicated that Tag is more affected by plant species than by soil type; therefore, selection of plants with a lower capacity to uptake radionuclides may be an important measure to reduce food contamination and thus minimize the committed effective dose.

Radiocesium distribution caused by tillage inversion affects the soil-to-crop transfer factor and translocation in agroecosystems

This study reports the translocation of cesium-137 (¹³⁷Cs) into deep soil layers, and the ¹³⁷Cs transfer from soil to soybean in farmland under three tillage (no tillage, NT; rotary cultivation, RC; moldboard plow; MP) treatments and an undisturbed grassland (GL) at eight years after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on 11 March 2011 in Japan. Tillage influences the ¹³⁷Cs distribution in the 0-30 cm of soil; the distribution of ¹³⁷Cs in the soil was uniform under RC and MP treatments, while in the grassland, most ¹³⁷Cs was concentrated on the soil surface (0-2.5 cm). The center of vertical ¹³⁷Cs radioactivity concentration (the thickness of the soil from surface which containing half of the ¹³⁷Cs inventory) in GL was 5.5 cm, which was shallower than that in farmland (9.5 cm in NT, 13.6 cm in RC and 15.2 cm in MP). Hence, the total translocation distance of ¹³⁷Cs 8 years after FDNPP accident showed the following trend: GL (2.4 cm) < NT (7.0 cm) < RC (10.0 cm) < MP (12.3 cm). Meanwhile, a significant positive correlation was observed between ¹³⁷Cs radioactivity concentration and organic carbon and nitrogen content in the soil. However, the ¹³⁷Cs radioactivity concentration in soybean grains was negatively correlated with the center of vertical ¹³⁷Cs radioactivity concentration but positively correlated with the ratio of exchangeable ¹³⁷Cs (ExCs) and K content in the soil. The ExCs/K and ¹³⁷Cs distributions in the soil were combined into a statistical model to predict the ¹³⁷Cs radioactivity concentration in soybean grain. The results revealed the magnitude of the impact of ¹³⁷Cs distribution on the ¹³⁷Cs transfer from soil to crop. The addition of the ¹³⁷Cs distribution dramatically improved the accuracy of the prediction model of ¹³⁷Cs radioactivity concentration in soybean.

Pre- and post-accident environmental transfer of radionuclides in Japan: lessons learned in the IAEA MODARIA II programme

An international review of radioecological data derived after the accident at the Fukushima Daiichi nuclear power plant was an important component of activities in working group 4 of the IAEA Models and data for radiological impact assessment, phase II (MODARIA II) programme. Japanese and international scientists reviewed radioecological data in the terrestrial and aquatic environments in Japan reported both before and after the accident. The environmental transfer processes considered included: (a) interception and retention radionuclides by plants, (b) loss of radionuclides from plant and systemic transport of radionuclides in plants (translocation), (c) behaviour of radiocaesium in soil, (d) uptake of radionuclides from soil by agricultural crops and wild plants, (e) transfer of radionuclides from feedstuffs to domestic and wild animals, (f) behaviour of radiocaesium in forest trees and forest systems, (g) behaviour of radiocaesium in freshwater systems, coastal areas and in the ocean, (h) transport of radiocaesium from catchments through rivers, streams and lakes to the ocean, (i) uptake of radiocaesium by aquatic organisms, and (j) modification of radionuclide concentrations in food products during food processing and culinary preparation. These data were compared with relevant global data within IAEA TECDOC-1927 'Environmental transfer of radionuclides in Japan following the accident at the Fukushima Daiichi Nuclear Power Plant'. This paper summarises the outcomes of the data collation and analysis within MODARIA II work group 4 and compares the Japan-specific data with existing radioecological knowledge acquired from past and contemporary radioecological studies. The key radioecological lessons learned are outlined and discussed.

Phytotoxicity mechanism of the natural radionuclide thorium in Vicia faba

Elucidation of the phytotoxic mechanisms of thorium (Th) is important for controlling Th accumulation in crops and improving the efficiency of phytoremediation. Here, we analyzed the subcellular distribution of Th in Vicia faba seedlings and the toxic reaction of seedlings to Th (5-40 μmol·L^-1) at the subcellular and cellular levels. Increasing the phosphate level in the culture medium from 0.01 to 0.1 mmol·L^-1 decreased the Th accumulation by the roots by 47-57%. Th was mainly distributed in the root cell walls (94-96%) and existed mainly in the form of residue (92-94%). Th accumulation in the root was similar to the changes observed for P, Ni, Cu, and Fe. High concentrations of Th (40 μmol·L^-1) induced abnormal root growth and leaf photosynthetic metabolism. At the cellular level, Th (40 μmol·L^-1) induced root edge cell death and inhibited root respiration and cell mitosis. SOD, POD and CAT activities were involved in the regulation of reactive oxygen species accumulation in the roots. Untargeted metabolomics identified 580 and 262 differentially expressed metabolites in roots and leaves. At the metabolic level, its toxicological mechanism involved a severe inhibition of the expression of nucleotides in roots and leaves.

Stabilization of caesium ions by simple organic molecules: crystal structures of Cs(OXL) (OXL = oxalurate anion) and the CsOH/cyanuric acid co-crystal Cs3(CYH3)4(OH)3 (CYH3 = cyanuric acid)

The reported X-ray crystal structures show that both the organic moieties form robust homomeric ribbons based on strong and articulated N–H⋯O hydrogen bonds.