Assessment of vertical radiocesium transfer in soil via roots
Introduction
In the aftermath of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, radionuclides have fallen into Japanese forests, and the radiocesium (137Cs) dynamics have been studied intensively (e.g., Hashimoto et al., 2013; Imamura et al., 2017; Iwagami et al., 2019; Kato et al., 2019; Komatsu et al., 2016; Kuroda et al., 2013; Ohashi et al., 2017; Shinomiya et al., 2014). Several studies have shown that most of the 137Cs was stored in the surface mineral soil layer, and the 137Cs activity concentration exponentially decreased with depth (e.g., Koarashi et al., 2012; Fujii et al., 2014, 2019; Imamura et al., 2017; Takahashi et al., 2018). Similar vertical patterns of 137Cs were also observed in previous studies after the global fallout and Chernobyl accident (e.g., Almgren and Isaksson, 2006; Koarashi et al., 2017). The cesium ion is generally fixed in micaceous clay minerals (e.g., illite and vermiculite) in the surface soil layer (e.g., Yamaguchi et al., 2012), and the dissolved 137Cs vertical migration rate in soils decreases with time because of the fixation of 137Cs in clay minerals (Kirchner et al., 2009). Thus, the dissolved 137Cs does not infiltrate easily from surface mineral soil to the deeper portions even with the passage of time. Global fallout occurred several decades before the Chernobyl accident, and the 137Cs in the former event clearly migrated deeper than that from Chernobyl more than 15 years after the accident (Almgren and Isaksson, 2006). This means that 137Cs gradually penetrated deeper in soil on a time scale of decades, despite the difficulty in the migration of dissolved 137Cs over time. To understand this long-term vertical migration of 137Cs, it is necessary to consider the effect of the transfer process that differs from the dissolved 137Cs vertical migration in a forest.
While the colloid-facilitated transport has been suggested as an alternative physical explanation of vertical migration of radionuclides in soils (e.g., Kretzschmar et al., 1999), this study focuses on a biological transport process via roots. The inputs of carbon and nutrients from fine roots (<2 mm) into soil belowground are crucial for understanding soil organic carbon and nutrient dynamics (Aerts et al., 1989; Jackson et al., 1997). The annual carbon input to soil matrix from fine roots is estimated to equal or exceed that from leaves (Jackson et al., 1997; Nadelhoffer and Raich, 1992). Nutrients are also released during root decomposition (Taiz et al., 2015). Rapid release of potassium from roots has been observed in a hinoki cypress (Chamaecyparis obtusa) forest (Fujii and Takeda, 2010). 137Cs generally exhibits chemical behavior similar to that of potassium (Yamaguchi et al., 2012), and it is thought that it is released into the soil matrix during the decomposition of root litter. In addition, translocation from foliar surfaces to the other tree components was observed after the nuclear power plant accident (e.g., Calmon et al., 2009; Tagami et al., 2012). Thus, it is expected that 137Cs reaches the roots via translocation process within plants. These transfer processes of 137Cs in soil via roots and translocation in trees have been clearly observed under controlled conditions (Waller and Olson, 1967). The 137Cs inventory in deeper soil layers is clearly smaller than that in the surface mineral soil layer a few years after the FDNPP accident (Imamura et al., 2017); hence, we hypothesized that the continuous supply of 137Cs into soil matrix via roots possibly increased the amount of 137Cs in the deeper soil layers on a time scale of decades. However, no studies have evaluated whether roots in a deep soil layer can contribute to changes in the 137Cs distribution of the soil matrix in the same layer under actual field conditions.
In this study, we report new results on 137Cs activity concentrations in roots and soil matrix. The results were obtained from the observation of four (0–5, 5–10, 10–15, and 15–20 cm) or six (0–5, 5–10, 10–15, 15–20, 20–25, and 25–30 cm) soil layers at seven study sites with different vegetation types in eastern Japan in 2013. Comparisons of 137Cs activity concentrations between roots and soil matrix at the same soil layer allowed us to determine the depth at which the 137Cs input of roots was relatively high. Additionally, we estimated the 137Cs inventory in roots and soil matrix to provide the potential contribution rate of root decomposition to the total amount of 137Cs inventory in each soil layer.
Section snippets
Materials and methods
Sampling was conducted at seven study sites (KU1–S, KU1-Q, KU1–H, KU2–S, KM, OG, and SK; Table 1) that were with different vegetation types in Fukushima (Kawamata Town and Kawauchi Village), Ibaraki (Kitaibaraki City), and Miyagi (Shiroishi City) Prefectures in 2013 (Fig. 1). KU1–S, KU2–S, KU1-Q, and KU1–H are located in Kawauchi Village. KU1–S and KU2–S are dominated by Japanese cedar (Cryptomeria japonica). KU1-Q and KU1–H were located near KU1–S. The dominant species in KU1-Q is konara oak (
Results and discussion
At two Japanese cedar forest sites in Kawauchi Village (KU1–S and KU2–S), the 137Cs activity concentrations in the roots in the shallow soil layer (0–5 or 0–10 cm) tended to be higher than those of the roots in a deeper soil layer (Fig. 2a and b). Similar vertical distributions of 137Cs activity concentration in roots were found at the hinoki cypress (KU1–H) and konara oak (KU1-Q) sites (Fig. 2c and d). At KM, where the deciduous broad-leaved forest floor is covered by kumaizasa, only the 137Cs
Funding
This study was funded by FFPRI [research grant #201126] and the Forestry Agency, Japan.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful to the Kanto Regional Forest Office of the Forestry Agency and the Kawauchi Village Office for the permission for investigation and sampling. The authors are also grateful to two anonymous reviewers for their constructive comments and suggestions. Many researchers and staff of the FFPRI also provided support for the field sampling and measurements of 137Cs activity concentrations.
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