Variability in radiocesium activity concentration in growing hardwood shoots in Fukushima, Japan.

The radiocesium contamination caused by the Fukushima Daiichi Nuclear Power Plant accident has made it difficult to use coppice woods as bed logs for mushroom cultivation. Evaluating the variability in the radiocesium activity concentration of logs is necessary in order to predict how many coppice woodlands are available for producing mushroom bed logs. To clarify the variability in radiocesium activity concentrations and to estimate the sample size required to estimate these concentrations with sufficient accuracy, we modeled the log-transformed radiocesium activity concentrations in growing shoots of hardwoods. We designed two models: (1) a model with mean concentrations that varied among stands with a standard deviation that was the same among stands, and (2) a model with varying means and standard deviations. We fit the data pertaining to only Quercus serrata to both models and calculated the widely applicable information criterion values. Consequently, we adopted the simpler model (1). Applying the selected model to data for all species, we examined the relationship between the number of measurement individuals and the predictive distribution of the expected concentration. Based on previous recommendations and measurement costs, we proposed that five individuals would be appropriate for estimating radiocesium activity concentration in a stand.

Ten-year trends in vertical distribution of radiocesium in Fukushima forest soils, Japan.

To elucidate interannual changes in the vertical distribution of 137Cs in forest ecosystems contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident, we investigated 137Cs inventories in forest soils (both organic and mineral soil horizons) at 10 sampling plots with different 137Cs deposition levels and dominant species for up to 10 years after the accident. We examined the temporal variation of the 137Cs inventories by depth with exponential regression models (assuming that the transition and partitioning of 137Cs are still active) and exponential offset regression models (assuming a shift to a stable 137Cs distribution, defined as the "quasi-equilibrium steady-state" in the Chernobyl accident). In the organic horizon, the 137Cs inventories were exponentially decreasing, and it might take more time to converge in the quasi-equilibrium steady-state at most plots. In the mineral soil horizon, most of 137Cs was found in the surface layer of the mineral soil horizon (0-5 cm). In this layer, the inventories first increased and then become relatively constant, and the exponential offset model was selected at most plots, suggesting entry into the quasi-equilibrium steady-state over the observation period. Although we also observed exponentially increasing trends in a lower layer (5-10 cm) of the mineral soil horizon, there was no clear increasing or decreasing trend of 137Cs inventory in the deeper mineral soil layers (10-15 and 15-20 cm). Our calculation of the relaxation depth and migration center revealed that downward migration of 137Cs is not significant in terms of the overall 137Cs distribution in the mineral soil horizon over 10 years.

Dynamics of radiocaesium within forests in Fukushima-results and analysis of a model inter-comparison.

Forests cover approximately 70% of the area contaminated by the Fukushima Daiichi Nuclear Power Plant accident in 2011. Following this severe contamination event, radiocaesium (137Cs) is anticipated to circulate within these forest ecosystems for several decades. Since the accident, a number of models have been constructed to evaluate the past and future dynamics of 137Cs in these forests. To explore the performance and uncertainties of these models we conducted a model inter-comparison exercise using Fukushima data. The main scenario addressed an evergreen needleleaf forest (cedar/cypress), which is the most common and commercially important forest type in Japan. We also tested the models with two forest management scenarios (decontamination by removal of soil surface litter and forest regeneration) and, furthermore, a deciduous broadleaf forest (konara oak) scenario as a preliminary modelling study of this type of forest. After appropriate calibration, the models reproduced the observed data reliably and the ranges of calculated trajectories were narrow in the early phase after the fallout. Successful model performances in the early phase were probably attributable to the availability of comprehensive data characterizing radiocaesium partitioning in the early phase. However, the envelope of the calculated model end points enlarged in long-term simulations over 50 years after the fallout. It is essential to continue repetitive verification/validation processes using decadal data for various forest types to improve the models and to update the forecasting capacity of the models.

Relationship between the activity concentration of 137Cs in the growing shoots of Quercus serrata and soil 137Cs, exchangeable cations, and pH in Fukushima, Japan.

Incorporation of radiocesium by plants via root uptake appears to be affected by some of the exchangeable cations in the soil and/or pH of the soil. However, few studies have examined the relationship between 137Cs in trees and soil properties in the area surrounding the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) after the accident in March 2011. To elucidate the relationships between the root uptake of 137Cs by deciduous broadleaved trees and soil properties, we measured the activity concentration of 137Cs in the growing shoots of coppiced konara oak (Quercus serrata) grown after the FDNPP accident and the amounts of total 137Cs; exchangeable (ex-) 137Cs, ex-K, ex-Mg, and ex-Ca; and pH (H2O) in soils collected from 34 forest stands in Fukushima between December 2016 and May 2017. Ex-137Cs showed a positive linear relationship with the activity concentration of 137Cs in the growing konara oak shoots, whereas ln-transformed ex-K, ex-Mg, ex-Ca, and pH (H2O) showed negative linear relationships with ln-transformed 137Cs activity concentrations in the growing shoots. However, only ex-137Cs and ex-K were identified as significant factors determining the activity concentration of 137Cs in konara oak according to multiple regression methods and a model selection using Akaike information criterion; ex-K had a stronger influence on the activity concentration of 137Cs in konara oak than ex-137Cs. In the present study, we demonstrated that soil ex-K negatively and non-linearly alters 137Cs activity concentration in deciduous broadleaved trees. We also noted that the relationship between 137Cs in deciduous broadleaved trees and soil ex-K in forests without K fertilization was similar to the relationships between 137Cs in other plants and ex-K in K-fertilized lands reported in previous studies of the FDNPP accident.

Potassium fertilisation reduces radiocesium uptake by Japanese cypress seedlings grown in a stand contaminated by the Fukushima Daiichi nuclear accident.

We analysed suppressive effects of potassium (K) fertilisation on radiocesium (137Cs) uptake by hinoki cypress (Chamaecyparis obtusa) seedlings from soils contaminated after the Fukushima Daiichi Nuclear Power Plant accident. Three-year-old seedlings were planted in a clear-cut forest (ca. 4 ha) during June-July 2014, and potassium chloride fertiliser (83 kg K ha-1) was applied twice (August 2014 and April 2015). 137Cs concentrations in the needles in the fertilised plots were one-eighth of those in the control (unfertilised) plots at the end of the second growing season (October 2015). Our results clearly indicated that K fertilisation reduced radiocesium transfer from soil to planted cypress seedlings. A linear mixed model analysis revealed that 137Cs concentrations in the needles were significantly affected by 137Cs inventory in the soil (Bq m-2) adjacent to the sampled seedlings, exchangeable K concentrations in surface mineral soils (0-5 cm) and fertilisation. The exchangeable K concentrations in surface soils in October 2015 did not differ from those in August 2014 (before fertilisation) in the fertilised plots and in the control plots. These results suggested that the levels of exchangeable K would temporarily increase by fertilisation during the growing season, and radiocesium uptake by tree roots was suppressed.

Seasonal and snowmelt-driven changes in the water-extractable organic carbon dynamics in a cool-temperate Japanese forest soil, estimated using the bomb-(14)C tracer.

Water-extractable organic carbon (WEOC) in soil consists of a mobile and bioavailable portion of the dissolved organic carbon (DOC) pool. WEOC plays an important role in dynamics of soil organic carbon (SOC) and transport of radionuclides in forest soils. Although considerable research has been conducted on the importance of recent litter versus older soil organic matter as WEOC sources in forest soil, a more thorough evaluation of the temporal pattern of WEOC is necessary. We investigated the seasonal variation in WEOC in a Japanese cool-temperate beech forest soil by using the carbon isotopic composition ((14)C and (13)C) of WEOC as a tracer for the carbon sources. Our observations demonstrated that fresh leaf litter DOC significantly contributed to WEOC in May (35-52%) when the spring snowmelt occurred because of the high water flux and low temperature. In the rainy season, increases in the concentration of WEOC and the proportion of hydrophobic compounds were caused by high microbial activity under wetter conditions. From summer to autumn, the WEOC in the mineral soil horizons was also dominated by microbial release from SOC (>90%). These results indicate that the origin and dynamics of WEOC are strongly controlled by seasonal events such as the spring snowmelt and the rainy season's intense rainfall.