Influence of Soil Characteristics and Land Use Type on Existing Fractions of Radioactive 137 Cs in Fukushima Soils

: Radioactive cesium ( 137 Cs) in distinct soil fractions provides key information to its bioavailability, and therefore determining the effect of soil characteristics and land use types on existing fractions of 137 Cs in soils is important for predicting future 137 Cs mobility in Fukushima. Thus, the objective of this study was to investigate the influence of soil characteristics and land use types on sequentially extracted fractions of 137 Cs in Fukushima. In this study, five coniferous forest soils, eight arable soils, and eight paddy rice soils were sampled in 2012 and 2013. The 137 Cs in the soils were separated into four fractions; water ‐ soluble, exchangeable, organic matter ‐ bound and residual fractions. More than 90% of the soil 137 Cs fraction for arable and paddy rice soils was found in the residual fraction, implying significantly reduced bioavailable 137 Cs with higher fixation. In contrast, forest soils measured higher exchangeable and organic matter ‐ bound fractions of 5%–33% and 9%–44%, respectively, implying future 137 Cs mobility in the forest ecosystem. Correlation analysis showed a significant negative correlation ( p < 0.05) between the organic matter fraction and residual fraction in both arable and paddy rice soils. There was a significant positive correlation ( p < 0.05) for both exchangeable and residual fractions with cation exchange capacity (CEC), total carbon (TC) and total nitrogen (TN) values in arable soils. Organic matter content influenced both exchangeable and residual fractions. It was not clear whether organic matter played a direct role in 137 Cs fixation or mobility in the agricultural soils. In paddy rice soils, the organic matter fraction showed a significant negative correlation with TC and TN values. Soil pH was significantly negatively correlated ( p < 0.05) with both water ‐ soluble and residual fractions in forest soils but positively ( p < 0.1) with the organically bound 137 Cs fraction. investigate the existing 137 Cs fractions in various soils in by a sequential extraction procedure and to determine the influence of soil characteristics on the existing 137 Cs fractions, comparing 137 Cs mobility in both forest and agricultural land uses.


Soil Sampling and Preparation
Soil samples were collected from eight arable fields on 5 and 6 August 2013-referred to as Arable 1 to Arable 8-eight paddy rice fields on 8 and 9 December 2013-referred to as Paddy 1 to Paddy 8-and five coniferous forest sites on 31 October, 27 and 28 November 2012-referred to as Forest 1 to Forest 5-in Nihonmatsu City, Fukushima Prefecture, Japan, which is located 50 km away from the nuclear power plant (Figure 1) (Table 1).  The general soil types found at the sampling sites were Andosol, Cambisol, Haplic Fluvisol and Gleyic Fluvisol [21] (Table 1). Composite soil samples were collected at a surface soil plow depth of 0-10 cm by five-point diagonal sampling to give representative samples of each field, with the most extreme points, 1 and 5, measuring 16.2 km apart. For forest soils, a litter layer or cover consisting of easily recognizable, originally shaped organic components made up of periodically falling raw litter of mainly dead fallen leaves, branches, and early fermented and fragmented litter components were removed from the soil surface before sampling. Homogenized soils were air-dried for 2 weeks, crushed and sieved through a 2 mm mesh, and plant roots were removed. All analyses were carried out in 2014.

Soil Physical and Chemical Analysis
Soil particle size distribution analysis was performed by sedimentation and the pipette method [22] to determine the soil texture. Soil pH was determined by the electrode method. The pH was measured in a 1:2.5 (w/v) ratio of 10 g soil to 25 mL water supernatant suspension, after shaking for 1 h, using the Beckman PKG-260 pH meter (Beckman Coulter, California, USA) [23]. Total carbon (TC) and total nitrogen (TN) contents were determined using an automatic highly sensitive CN analyzer (Sumigraph NC-220F; Sumika Chemical Analysis Service Co., Osaka, Japan). Soil organic matter (OM) content was determined using the hydrogen peroxide digestion method [24]. Cation exchange capacity (CEC) was determined by the indophenol blue colorimetric method with a UV-visible spectrometer (UV mini 1240, Shimadzu, Japan). Exchangeable cations were extracted with 1 M ammonium acetate (NH4OAc, pH 7) using the Schollenberger method [25] 2.2.2. Measurement of 137 Cs Activity To measure the activity concentration of 137 Cs in the soil, an automatic gamma counter (2480 Wizard, Perkin-Elmer, Tokyo, Japan) equipped with a well-type NaI detector, with lead shielding, coupled to a multi-channel analyzer calibrated for the energy range 15-2048 keV with a maximum dead time of 2.5 μs was used. The energy and efficiency calibrations for this detector were performed using a blank (background) sample and a reference 137 Cs standard (Spectrum Techniques, Knoxville USA) with an activity of 10.878 kBq. Samples were loaded into 20 mL vials, and the 137 Cs activity of each sample was determined by counting gamma emissions for 1200 s. Cesium-137 activity concentrations were measured in 2014. The activity concentration in samples were expressed in Bq kg −1 dry weight. The detector efficiency for 137 Cs was 47% and the energy resolution was <10%.

Sequential Extraction of 137 Cs
Cesium-137 was sequentially extracted from 20 g soil (dry weight), which was sieved through a 2 mm sieve as mentioned previously for soil physical and chemical analysis. Prior to extraction, weighed samples were placed in 20 mL vials, and the initial 137 Cs activity of soil samples was measured. A modification of the sequential extraction procedure [26,27] was carried out as shown in Table 2, with four different 137 Cs fractions in soil extracted: water-soluble (W), exchangeable fraction (E), organic matter-bound fraction (O) and residual fraction (R). For the extraction procedure, each soil sample was weighed out into a 300 mL plastic bottle with the respective amounts of extractants added, and the samples were treated as described in Table 2. The extractants were distilled water, ammonium acetate (NH4OAc), and hydrogen peroxide (H2O2). For organic matter-bound extracted 137 Cs, soil samples were weighed out into a 500 mL glass beaker with 10 mL of 30% H2O2 to allow for organic matter digestion. Samples were further heated to ensure complete organic matter digestion, with 1 mL H2O2 added in time intervals until the supernatant was clear. All supernatants were filtered through a 0.45 μm membrane filter with a suction pump after shaking at the required temperatures, as described in Table 2. Prior to the next extraction, the soil extracts were washed by adding 100 mL distilled H2O. The difference in measured 137 Cs concentration between the immediate extraction and the previously extracted fraction was defined as the amount for each fraction. The residual fraction, which is mainly clay mineral-bound and strongly fixed, was determined from the remaining soil after the extraction of the organic matter fraction. The activities in each extract were expressed as a percentage of the initial activity in the soil.

Statistical Analysis
Correlations among 137 Cs fractions, between 137 Cs fractions and soil characteristics, and among soil characteristics were analyzed by Pearson Product Moment Correlation (SigmaPlot 12.5, Systat Software, San Jose, CA, USA).

Soil Physiochemical Properties
The pH was acidic to moderately alkaline, with values ranging from 5 to 7.5, 5.8 to 6.8 and 4.6 to 5.4 for arable, paddy rice and forest soils, respectively. Among the three land uses, arable soil recorded higher pH values, while forest soil recorded lower pH values (Table 3). CEC, TC, and TN were highest in forest soils and least in paddy rice soils, with some exceptions. Forest 4 recorded the highest CEC and TN values of 20.7 cmol 100 g −1 , and 7.1 g kg −1 respectively, but the highest TC value of 124 g kg −1 was recorded in Forest 5. The lowest CEC and TC values of 5.7 cmol 100 g −1 and 8.7 g kg −1 , respectively, were recorded in Paddy 4. TN values in arable and paddy rice soils were similar, except in Arable 4 and Arable 8, with values of 6.0 and 4.3 kg −1 . The soil texture of paddy rice soils was sandy loam for all fields except Paddy 1, which was sandy clay loam, and Paddy 3 and Paddy 4, which were featured by a loamy sand texture. Arable and forest soils were sandy loam to clay loam, depending on the relative amount of sand and clay fractions.
The results of sequential extraction showed that, in arable soils, both water-soluble and exchangeable fractions ranged from 1%-5% of the total 137 Cs extracted, while the organic matterbound fraction ranged from 6%-20% for all sites (Figure 2a).  For paddy rice soil, the water-soluble fraction was not detected as exchangeable and organic matter-bound fractions ranged from 1%-7% and 5%-10% respectively (Figure 2b). More than 90% residual fraction-extracted 137 Cs in both arable and paddy rice soils showed a stronger fixation compared to the earlier 137 Cs sequential extraction study by Saito et al. [

Watersoluble
Exchangeable Organic matter Residue fraction, 58% in the strongly fixed state and 17.5% in the residual fractions. The existing fraction of radioactive Cs when added to soil changes in the following order: water-soluble fraction, ion exchangeable and finally fixed fraction [29,30], with most radioactive Cs remaining strongly fixed to soil [15]. Previous research works, similar to this study, also showed a high fixation of 137 Cs to soil within two years after fall out [31][32][33] indicating progressive 137 Cs fixation with time, especially in the paddy rice soils (89%-97% residual fraction) as compared to in arable soils (72%-97%) (Figure 2a,b). In forest soils, the water-soluble fraction represented 0%-0.02% of total 137 Cs extracted with higher percentages of 5%-33% and 9%-44% representing exchangeable and organic matter-bound fractions, respectively, in contrast to those of both arable and paddy rice soils (Figure 2c). This shows a greater amount of easily mobile and bioavailable 137 Cs in forest soil than in the farmland soils. The residue, however, recorded lower fractions of 27%-79% in comparison to those of arable and paddy rice soils. Results from two independent preliminary surveys conducted six months after the accident by the Forestry and Forest Products Research Institute and Forestry Agency of Japan (FFPRI and FAJ) and by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) indicated a significant transfer of radiocesium from trees to the forest floor via litterfall and rain and further transfer to mineral soils (0-5 cm) via organic matter decomposition, within a one-year interval (August-September 2011 and 2012) of the survey. In this study, forest soils were sampled in 2012, which implies that this was the stage of radioactive 137 Cs transfer, or immediate transfer, from the forest floor and the sedimentation of organic matter to the soil. Thus, a lower amount of 137 Cs was bound to soil minerals and a higher proportion was bound to the mobile fractions (exchangeable and organic matter) in the forest soils.
In further discussions, the influence of soil characteristics and land use on the chemical fraction of 137 Cs will be analyzed.

Correlation of 137 Cs Fractions and Soil Characteristics
Results of correlation analysis among 137 Cs fractions showed a significant negative correlation between the organic matter fraction and the residual fraction for both arable (r 2 = 0.97, p < 0.05) and paddy rice soils (r 2 = 0.96, p < 0.05) ( Table 4). The negative correlation is evident as the lower amounts of organic-bound 137 Cs were mainly due to greater fixation over time. This was implied to be an inhibitory effect of the residual fraction on the organic-bound 137 Cs fraction and vice-versa. Larger proportions of 137 Cs are known to be associated with clay fractions, and clay adsorption largely accounts for 137 Cs retention in soils [6]. The retention of 137 Cs can increase with increasing clay content as shown by [7] who reported 69%-93% of 137 Cs to be found in the clay fraction of soils with a clay content of 24%-27%. Another suggested implication was the adsorption of 137 Cs from the organic fraction into the residual fraction over time. Similarly, a significant negative correlation between organic matter and the residual fraction was observed in forest soil (r 2 = −0.88, p < 0.1). A possible explanation for this relationship can be attributed to the inhibitory effect of organic matter bound to clay surfaces by limiting access to adsorption sites [34,35] which indirectly declined 137 Cs adsorption in the organic-rich forest soils (77-124 g kg −1 TC) ( Table 3). As organic matter has weak selectivity for 137 Cs, it may lead to more easily mobile and bioavailable fractions compared to agricultural soils.

Influence of Soil Characteristics on 137 Cs Availability
A significant positive correlation was obtained between the exchangeable fraction and CEC (r 2 = 0.76, p < 0.05), TC (r 2 = 0.80, p < 0.05) and TN (r 2 = 0.78, p < 0.05) values in arable soils (r 2 = p < 0.05) ( Table 5). Cs is present as a monovalent cation in soil and is retained at a cation exchange site whose capacity is expressed as CEC. W represents water-soluble fraction. E represents exchangeable fraction. O represents organic matter fraction. R represents residual fraction. CEC is the cation exchange capacity. TC and TN represent the total carbon and total nitrogen respectively. *, ** stand for significant correlation at p < 0.05 and p < 0.1 respectively; NA means not applicable, as a water-soluble faction was not detected.
A similar positive relationship was obtained by Matsunaga et al. [36], extracting water-soluble and exchangeable fractions in the same manner as in this study. In contrast, studies by Puhakainen et al. [27] and Koarashi et al. [8] showed no significant correlation of CEC with the exchangeable fraction and Cs retention respectively. CEC was therefore not considered to be a good measure for 137 Cs retention in soil. In addition, a significant positive correlation was obtained between 137 Cs in residue and CEC (r 2 = 0.84, p < 0.05), TC (r 2 = 0.73, p < 0.05) and TN (r 2 = 0.72, p < 0.05) values. Organic matter, as with clay minerals, is negatively charged, has high CEC and is capable of 137 Cs adsorption [8]. In the arable soils, the organic matter amounts and CEC values showed a proportionally high relationship, and it could be implied that the organic matter abundance influenced CEC, which further influenced the available exchangeable fraction and the strongly fixed residual fraction (Table  5). This notwithstanding, it could not be established clearly whether organic matter played a direct role in 137 Cs fixation or retention and mobility in arable soils.
In paddy rice soil, a significant negative correlation was obtained between organic matter fraction and TC (r 2 = −0.83, p < 0.05) as well as TN (r 2 = −0.80, p < 0.05) values. Previous studies have shown the soil removal in rice paddy fields in Fukushima to be a decontamination practice [37,38], and it has been suggested that the negative relationship was due to slower organic matter decomposition, most of which resulted from uncontaminated older residues after the decontamination practice.
A significant negative correlation was obtained between both water-soluble (r 2 = −0.91, p < 0.05) and residual fractions (r 2 = −0.92, p < 0.05). The fixation capacity of radioactive Cs is stronger in alkaline soil than in acidic soil. For example, the water-soluble fraction was increased at low pH, and the clay-fixed fraction was increased at high pH value [39]. In contrast to this study, as little as 0.02% ( Figure 2c) water-soluble fraction of the extracted 137 Cs was found for the acidic forest soil (Table 3). Such a contrasting relationship was also observed in the residual fraction, where acidity was seen to promote 137 Cs fixation. A significant positive correlation between pH and organic matter fraction (r 2 = 0.84, p < 0.1) was observed. 137 Cs + is adsorbed to the negatively charged carboxyl group of humus and forms a stable complex [15]. The negative charges increase with high pH values, and 137 Cs + is more likely to be adsorbed to the carboxyl group. The relationship implies that with high pH, the organic matter fraction is increased. Nevertheless, the pH of forest soils was not high enough to promote 137 Cs adsorption onto organic matter in this study. A possible explanation could be the high amount of organic matter content. A positive correlation (however not significant) between CEC and TC and TN values may have reflected the higher CEC values at high amounts of organic matter content (Table 3) and promoted the adsorption of 137 Cs into organic matter.

Influence of Land Use Types on 137 Cs Availability in Soils
The fixation of 137 Cs was greater in paddy rice soil than in arable soil (Figure 1). There are some reports that a cycle of drying and wetting is required to enhance 137 Cs fixation [40,41]. This condition was considered to be enhanced in the paddy rice soils due to repeated irrigation and drainage. In addition, organic matter inclusion by fertilization was suggested to have influenced the relationship between organic matter content and organic matter fraction in both arable and paddy rice soils, as sampling sites are used as agricultural production fields by farmers and as experimental fields by researchers.
In the forest ecosystem, 137 Cs reaches the soil by a litterfall process (in addition to air deposition by throughfall) and serves as a temporal storage for large 137 Cs accumulation. Koarashi et al. [8] reported that 50%-90% of radioactive Cs accumulates in the litter layer of the forest floor. As a dynamic component of the forest ecosystem, forest litter generally undergoes microbial decomposition within a few years [42,43] and may become available for uptake by trees and microorganisms. The bioavailable 137 Cs can form complex compounds with soluble organic matter produced by litter decomposition and transferred from the litter layer to soil mineral layer by downward seepage [44]. Such phenomena of uptake by trees and microorganisms as well as 137 Csorganic matter complexation are possible reasons for a low water-soluble fraction and a much increased organic matter fraction in the forest soils. In Japan, similar to as shown this study, most of the fallout 137 Cs in the 1960s remained in the shallow surface area even several decades after the main deposition in undisturbed forest [45]. Due to the typically high amounts of organic matter in the surface layer of forest, 137 Cs fixation in the mineral soil layer may be reduced [46]. Nonetheless, gradual oxidation and biodegradation can liberate 137 Cs adsorption to organic matter with time and thus further increase the available and easily extractable fractions, such as water-soluble and ion exchangeable fractions [32], which may not only be taken up by microorganisms and vegetation but also adsorbed into the mineral soil layer in forest soils (Table 5).

Conclusions
Results from the sequential extraction showed strong 137 Cs fixation in agricultural soils-in particular, paddy rice soils-in comparison to forest soils. Thus, the adverse effect on 137 Cs mobility and agricultural production is the gradual decrease of bioavailable 137 Cs for plant uptake. In contrast, the higher proportions of exchangeable and organic matter fractions observed in forest soils suggest future mobility and bioavailability of 137 Cs in forest ecosystems. A significant negative relationship (p < 0.05) between 137 Cs in the organic matter fraction and the residue in both arable and paddy rice soils suggest that the organic matter fraction inhibited 137 Cs fixation on soil minerals. In addition, CEC, TC, and TN showed positive relationships (p < 0.05) with both exchangeable fractions and residue, which suggests that high organic matter content increases exchangeable 137 Cs fractions. Thus, although 137 Cs retention in the organic matter fraction could not be observed, it is inferred that a high organic matter content leads to higher 137 Cs mobility. In forest soils, pH showed a negative relationship (p < 0.05) with the water-soluble fraction and the residue but a positive relationship (p < 0.1) with the organic matter fraction. It is suggested that high pH promotes 137 Cs retention in soil organic matter, and therefore soil pH needs to be considered in all land use types.
Author Contributions: Akwasi Dwira Mensah contributed to the writing of the original draft and the validation of the research work. Akimi Terasaki contributed to the curation, software and formal analysis of the data as well as the investigation and methodology of the research work. Dr. Han Phyo Aung was involved in the visualization and validation of the research data as well as the review and edit of the research paper. Prof. Hiroto Toda, Prof. Siaw Onwona and Dr. Richard Ansong Omari contributed in data validation and in the review and editing of the research paper. Prof. Sohzoh Suzuki and Prof. Haruo Tanaka were involved in the conceptualization, Validation as well as review and editing of the research paper. Prof. Sonoko Dorothea Bellingrath-Kimura, as the corresponding author and project administrator, contributed in the supervision, resource and funding acquisition for the research work. In addition, she contributed to the conceptualization, methodology, validation, and the review and editing of the research paper Funding: The research has no external funding and the APC was funded by Prof. Sonoko Dorothea Bellingrath-Kimura.

Conflicts of Interest:
The authors declare no conflict of interest.