Effectiveness of non-exchangeable potassium quantified by mild tetraphenyl­boron extraction in estimating radiocesium transfer to soybean in Fukushima.

Non-exchangeable K released from soil minerals can reduce radiocesium transfer to plants, as well as exchangeable K. We investigated the effect of non-exchangeable K on radiocesium transfer to soybean, and the non-exchangeable K extraction method most suitable for estimating the transfer risk. In Fukushima Prefecture, Japan, 106 soils were collected from 89 soybean fields during 2014-2018 to analyze non-exchangeable K contents using three methods: boiling nitric acid extraction, tetraphenyl­boron extraction, and mild tetraphenyl­boron extraction. The non-exchangeable K contents quantified by the former two methods were dependent on the amount of micas, which are K-bearing minerals. The non-exchangeable K content by mild tetraphenyl­boron extraction depended on the amount of K fertilizer application and K-fixing minerals but not on micas, indicating that it reflects fertilizer K fixed by the minerals. The soil-to-plant transfer factor of radiocesium was most correlated with the non-exchangeable K content by the mild extraction (rs = -0.67). This correlation was also stronger than that between exchangeable K and the transfer factor (rs = -0.40). As non-exchangeable K content increased, the exchangeable radiocesium fraction decreased, indicating that radiocesium was fixed together with K. Additionally, multiple regression analysis indicated that non-exchangeable K by the mild extraction significantly decreased the transfer factor even if the exchangeable radiocesium fraction was kept constant. Thus, the fixed K was considered to repress radiocesium transfer to soybean through both radiocesium fixation and K supply. With the criterion of total extracted K, the sum of exchangeable and non-exchangeable K, as 65 mg K2O 100 g-1 by the mild extraction, fields with high and low transfer factors were able to be differentiated more effectively than with a current criterion of exchangeable K as 50 mg K2O 100 g-1. The results revealed that mild tetraphenyl­boron extraction is effective for estimating radiocesium transfer to soybean.

Application of Finnish phlogopite to reduce radiocesium uptake by paddy rice.

Field and pot experiments were conducted to evaluate the effectiveness of coarse Finnish phlogopite application to reduce radiocesium uptake by paddy rice (Oryza sativa L.). The application of phlogopite was expected to reduce radiocesium uptake by crops through K supply and radiocesium retention. Three fields were set in Fukushima Prefecture, and coarse (mean particle size of 450 µm) phlogopite from Siilinjärvi (Finland) was applied at a rate of 5 t ha-1. Paddy rice was cultivated for 2-4 successive years. In all fields, the average 137Cs transfer factor (TF) of brown rice harvested from plots with added phlogopite was significantly lower than that of brown rice from plots without added phlogopite over the 2-4-year experiments. TF was decreased by up to 80% following phlogopite application, without an adverse effect on yield. Exchangeable K and soil solution K were higher in the soils with added phlogopite, suggesting K released from phlogopite reduced 137Cs uptake by paddy rice. Moreover, in a pot cultivation experiment, even when 55% of the total K was removed from phlogopite prior to application, the TF in pots with phlogopite application was less than half of that in pots without added phlogopite. The results from the field study and the pot cultivation experiment suggested that the application of Finnish phlogopite is effective to reduce the TF of brown rice. Exchangeable K and tetraphenylborate-extractable-K (TPB-K) at rooting stage, and soil solution K at tillering and heading stages showed significant negative correlation with TF. TPB-K was significantly positively correlated with soil solution K at tillering stage and heading stage, whereas exchangeable K at rooting stage did not exhibit significant correlation with soil solution K at heading stage. The results suggest that TPB-K is more reliable than exchangeable K, which could facilitate as a basis of K fertilizer recommendation for radiocesium-contaminated fields.

Comparative dynamics of potassium and radiocesium in soybean with different potassium application levels.

We conducted a field experiment in soybean with different levels of K application to elucidate the comparative dynamics of 137Cs and K. The inventory of K in the shoots increased substantially from the fifth trifoliate stage to the full seed stage, and as the absorption of K increased, so too did the absorption of 137Cs. Overall, the effect of K application was much greater in terms of 137Cs dynamics than K dynamics or biomass production. K application reduced not only the accumulation of 137Cs in the shoots, but also the distribution of 137Cs to the grains. However, the decrease of 137Cs distribution to the grain had a much smaller effect on 137Cs accumulation in the grains than 137Cs absorption. A positive correlation was also observed between the exchangeable 137Cs/K ratio in the soil and the 137Cs/K ratio in the shoots for each growth stage, and the 137Cs/K ratios in the shoots at the full seed and full maturity stage were much higher than those at the fifth trifoliate and full bloom stage under the same exchangeable 137Cs/K ratio in the soil. These findings suggest a decrease in the discrimination of 137Cs from K during absorption after the full bloom stage. As a result of this and the increase in soil-exchangeable 137Cs/K with growth, radiocesium was more transferable to the shoots after the full bloom stage. Overall, these results suggest that lowering the soil-exchangeable radiocesium/potassium ratio after the full bloom stage by increasing K availability could efficiently reduce the transfer of radiocesium to the grains.

Advanced approach for screening soil with a low radiocesium transfer to brown rice in Fukushima based on exchangeable and nonexchangeable potassium.

Phytoavailable K in soil is a key to control the transfer factor of radiocesium from soil to brown rice. The transfer factors were determined for paddy fields cultivated in 2017 and 2018 under different K fertilization regimes in Fukushima Prefecture, Japan. Two phytoavailable forms of K, the exchangeable and nonexchangeable K contents were investigated for the surface soil sampled after the transplanting and fertilization as well as after harvest of rice in the same paddy fields. The exchangeable K content largely decreased from after transplanting and fertilization to after harvest, and the exchangeable K of the soil after harvest was negatively correlated with the transfer factor (rs = -0.70, p < .001). Most soil samples after harvest showed that the transfer factors exponentially increased as the exchangeable K decreased; however, some of the samples indicated considerably low transfer factors (<0.005) despite being exchangeable K deficient, i.e., exchangeable K < 25 mg K2O 100 g-1. Even though this value before usual fertilization has been effectively used as a threshold to determine whether supplemental K fertilization is required to reduce the radiocesium content in brown rice, additional screening was needed to estimate this radiocesium transfer more precisely. Thus, we found that not only the exchangeable K but also nonexchangeable K contents had a negative correlation with the transfer factor (rs = -0.60, p < .001) of the soil samples after harvest but were not correlated with each other (rp = -0.10). Furthermore, the results revealed that soil with nonexchangeable K > 50 mg K2O 100 g-1 indicated a considerably low transfer factor, even if exchangeable K deficient. Thus, via our field-scale experiments, we concluded that the criterion nonexchangeable K > 50 mg K2O 100 g-1 can be used as another threshold for use along with that of exchangeable K to differentiate soil with a low radiocesium transfer rate from exchangeable K deficient soil.

Evaluation of 137Cs ageing by dynamics of 137Cs/133Cs ratio in Andosol paddy fields with/without potassium fertilizer application.

The mobility of 137Cs in soil decreases with time owing to fixation by micaceous minerals. Such ageing is a critical parameter for estimating and predicting annual change in 137Cs contamination risk of agricultural products. The decrease in the exchangeable fraction of 137Cs has traditionally been used as an index of the 137Cs ageing. Under field conditions, however, exchangeable 137Cs is influenced by several environmental factors. In this study, we propose a new index to evaluate the 137Cs ageing with minimum influence of environmental factors. The ratio of the exchangeable 137Cs fraction to exchangeable fraction of 133Cs ((137Cs/133Cs)exch) eliminates the influence of environmental factors on exchangeable 137Cs. We assessed the applicability of the (137Cs/133Cs)exch index, using a four-year field study of a rice paddy in allophanic Andosol, starting 200 days after the Fukushima Dai-ichi Nuclear Power Plant accident. The influence of K fertilization was also investigated. The 137Cs and 133Cs exchangeable fractions varied together, indicating that both were similarly controlled by environmental factors. The values of (137Cs/133Cs)exch decreased with time, reflecting 137Cs fixation by the ageing. The half-time of the (137Cs/133Cs)exch decline was 6.6-17.7 years. Relative to K fertilization, the lack of K fertilization seemed to affect the 137Cs ageing in two ways: the early 137Cs fixation progressed more rapidly, probably because fewer competing K+ ions were present, and the long-term ageing process was occasionally hampered, probably by the release of reserve K from micaceous minerals. The (137Cs/133Cs)exch values were similar to the ratio of the 137Cs to 133Cs transfer factor of the rice straw. Thus, we conclude that the (137Cs/133Cs)exch index is reliable for evaluating the 137Cs ageing, decrease in 137Cs mobility caused by the diffusion into micaceous mineral interlayer, in the field.

Rapid Quantification of Radioactive Strontium-90 in Fresh Foods via Online Solid-Phase Extraction-Inductively Coupled Plasma-Dynamic Reaction Cell-Mass Spectrometry and Its Comparative Evaluation with Conventional Radiometry.

This paper describes a rapid quantification method for radioactive strontium (90Sr) in fresh foods (perishable foods) and has been comparatively evaluated with the common classical radiometric quantification method. Inductively coupled plasma-dynamic reaction cell-mass spectrometry with online solid-phase extraction (cascade-ICP-MS) rapidly determines 90Sr in a pure water-based sample. Despite its advantages, its application to fresh foods (perishable foods) has not yet been reported; however, the analytical potential of this method for fresh foods must be evaluated. In this study, 90Sr was determined in 12 fresh foods via improved cascade-ICP-MS (Icas-ICP-MS). Addition and recovery tests were demonstrated using real samples of grape, apple, peach, Japanese pear, rice, buckwheat, soybean, spinach, shiitake mushroom, grass, sea squirt, and flounder. With a decomposed solution of Japanese pear, the measurement value coincided with the amount of spiked 90Sr. The reproducibility of the measurements was represented by relative standard deviations of 14.2 and 5.0% for spiked amounts of 20 and 200 Bq/kg, respectively (n = 10), and the recovery rates were 93.7 ± 7.1%. In this case, the limit of detection (LOD) was 2.2 Bq/kg (=0.43 pg/kg). These results were compared with the data obtained using a common classical radiometric quantification method (nitrate precipitation-low background gas flow counter (LBC) method) in the same samples. Both the methods showed equivalent performances with regard to reproducibility, precision, and LODs but different analysis times. Icas-ICP-MS required ∼22 min for analysis, whereas the nitrate precipitation-LBC method required 20 days, confirming that Icas-ICP-MS is the suitable method for analyzing 90Sr in fresh foods.

Phytoavailability of 137Cs and stable Cs in soils from different parent materials in Fukushima, Japan.

Weathered micaceous minerals (micas) are able to release potassium ion (K+) and fix caesium-137 (137Cs), both of which reduce soil-to-plant transfer of 137Cs. Among micas, trioctahedral micas such as biotite is expected to have a stronger ability to supply nonexchangeable K+ and a higher amount of Cs fixation sites than dioctahedral micas such as illite. Although biotite is predominant in granitic soils (G soils), illite is mainly dominant in sedimentary rock soils (S soils). Therefore, we hypothesized that G soils have a lower 137Cs transfer risk than S soils because of this difference in mineralogy. The objective of the present study was to determine the transfer factor (TF) of 137Cs and stable Cs (SCs) and to elucidate the determinant factors of TFs for G and S soils in Fukushima, Japan. Pot experiments were carried out with rice (Oryza sativa L. cv. Hokuriku 193) in G and S soils to determine the TF of 137Cs (TF-137Cs) and stable Cs (TF-SCs) under K-deficient conditions. TF-137Cs and TF-SCs were highly correlated, and both were significantly lower for G soils than for S soils. Higher TF values were shown for soils with lower amounts of exchangeable and nonexchangeable K or with higher percentages of exchangeable 137Cs (ex137Cs). The percentage of ex137Cs was negatively correlated with the amount of Cs fixation sites, represented by the radiocaesium interception potential. Thus, we concluded that smaller TF values for G soils were caused by a stronger ability to supply nonexchangeable K+ and a higher amount of Cs fixation sites. These findings will contribute to the establishment of soil screening techniques based on 137Cs transfer risk in Fukushima prefecture.

Influence of water management and fertilizer application on (137)Cs and (133)Cs uptake in paddy rice fields.

Cesium-137 derived from the Tokyo Electric Power Company's Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident contaminated large areas of agricultural land in Eastern Japan. Previous studies before the accident have indicated that flooding enhances radiocesium uptake in rice fields. We investigated the influence of water management in combination with fertilizers on (137)Cs concentrations in rice plants at two fields in southern Ibaraki Prefecture. Stable Cs ((133)Cs) in the plants was also determined as an analogue for predicting (137)Cs behavior after long-term aging of soil (137)Cs. The experimental periods comprised 3 y starting from 2012 in one field, and 2 y from 2013 in another field. These fields were divided into three water management sections: a long-flooding section without midsummer drainage, and medial-flooding, and short-flooding sections with one- or two-week midsummer drainage and earlier end of flooding than the long-flooding section. Six or four types of fertilizer subsections (most differing only in potassium application) were nested in each water management section. Generally, the long-flooding treatment led to higher (137)Cs and (133)Cs concentrations in both straw and brown rice than medial- and short-flooding treatments, although there were some notable exceptions in the first experimental year at each site. Effects of differing potassium fertilizer treatments were cumulative; the effects on (137)Cs and (133)Cs concentrations in rice plants were not obvious in 2012 and 2013, but in 2014, these concentrations were highest where potassium fertilizer had been absent and lowest where basal dressings of K had been tripled. The relationship between (137)Cs and (133)Cs in rice plants was not correlative in the first experimental year at each site, but correlation became evident in the subsequent year(s). This study demonstrates a novel finding that omitting midsummer drainage and/or delaying drainage during the grain-filling period enhances uptake of both (137)Cs and (133)Cs.

Evaluation of the cause of unexplained radiocaesium contamination of brown rice in Fukushima in 2013 using autoradiography and gamma-ray spectrometry.

The Great East Japan Earthquake on 11 March 2011, caused the release of radioactive materials from the Fukushima Daiichi Nuclear Power Plant (FDNPP), contaminating eastern Japan, particularly in part of Fukushima Prefecture. In 2012 and 2014, the radiocaesium concentration in brown rice did not exceed regulatory levels in Minamisoma City, Fukushima. However, in 2013, some radiocaesium concentrations in brown rice exceeded regulatory levels. In this work, autoradiograms showed that high radioactivity was present as contaminated spots on the panicles of rice and in brown rice in 2013. We evaluate the contribution of direct contamination to the radiocaesium concentration in brown rice and discuss the origin of radiocaesium contamination in brown rice using the (134)Cs/(137)Cs radioactivity ratio. Here, we show that the main cause of the unexplained radiocaesium contamination of brown rice in Minamisoma City in 2013 is the adherence of radioactive materials to the rice panicles, and these radioactive materials are associated with reactor units 2 or 3 of FDNPP.

Influence of the nonexchangeable potassium of mica on radiocesium uptake by paddy rice.

A pot cultivation experiment was conducted to elucidate the influence of the nonexchangeable potassium (K) of mica on radiocesium ((137)Cs) uptake by paddy rice (Oryza sativa L. cv. Koshihikari), and to evaluate the potential of mica application as a countermeasure to reduce radiocesium transfer from soil to paddy rice. The increase in the exchangeable K concentrations of soils, measured before planting, due to mica (muscovite, biotite, and phlogopite) application was negligible. However, in trioctahedral mica (biotite and phlogopite)-treated soil, the release of nonexchangeable K from the mica interlayer maintained the soil-solution K at a higher level during the growing season in comparison to the control, and consequently decreased the (137)Cs transfer factor for brown rice (TF). The sodium tetraphenylboron (TPB)-extractable K concentration of the soils, measured before planting, was strongly negatively correlated with the TF, whereas the exchangeable K concentration of the soils, also measured before planting, was not correlated with the TF. Therefore, we conclude that TPB-extractable K is more reliable than exchangeable K as a basis of fertilizer recommendations for radiocesium-contaminated paddy fields. Phlogopite-treated soils exhibited higher TPB-extractable K concentrations and lower TF values than biotite-treated soils. We thus conclude that phlogopite application is an effective countermeasure to reduce radiocesium uptake in paddy rice.