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.

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.

Influence of Ectomycorrhizal Colonization on Cesium Uptake by Pinus densiflora Seedlings.

Radionuclides were deposited at forest areas in eastern parts of Japan following the Fukushima Daiichi Nuclear Power Plant incident in March 2011. Ectomycorrhizal (EM) fungi have important effects on radiocaesium dynamics in forest ecosystems. We examined the effect of colonization by the EM fungus Astraeus hygrometricus on the uptake of cesium (Cs) and potassium (K) by Pinus densiflora seedlings. Pine seedlings exhibited enhanced growth after the EM formation due to the colonization by A. hygrometricus. Additionally, the shoot Cs concentration increased after the EM formation when Cs was not added to the medium. This suggests that A. hygrometricus might be able to solubilize Cs fixed to soil particles. Moreover, the shoot K concentration increased significantly after the EM formation when Cs was added. However, there were no significant differences in the root K concentration between EM and non-EM seedlings. These results suggest that different mechanisms control the transfer of Cs and K from the root to the shoot of pine seedlings.

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.

Biomass and morphology of fine roots of sugi (Cryptomeria japonica) after 3 years of nitrogen fertilization.

Increasing nitrogen (N) deposition may affect carbon and nutrient dynamics in forest ecosystems. To better understand the effects of N deposition, we need to improve our knowledge of N effects on fine roots (roots <2 mm in diameter), as they are a key factor in carbon and nutrient dynamics. In this study, we fertilized 1 × 2 m plots in a sugi (Cryptomeria japonica) stand (336 kg ha(-) (1) y(-) (1)) for 3 years and evaluated the responses of the fine roots to high N load. After fertilization, the concentration of NO3-N in the soil of N-fertilized (NF) plots was five-times as large as that in the control plots and the effect was more remarkable in the subsurface soil than in the surface soil. The biomass of fine roots <2 mm in diameter appeared to be greater in the NF plots (88 ± 19 g m(-) (2)) than in the control plots (56 ± 14 g m(-) (2)), but this difference was not statistically significant. In both plots, 76% of the biomass was accounted for by fine roots that were <1 mm in diameter. In the surface soil, the specific root length of fine roots <1 mm in diameter was significantly greater, and the diameter of those fine roots was marginally smaller, in the NF plots than in the control plots. In addition, the concentration of N in fine roots <1 mm in diameter was marginally greater in the NF plots than in the control plots. There may have been increased production of thinner fine roots or increased root branching in the NF plots. This study suggests that, in general, high N load is likely to have positive effects on sugi in terms of fine root characteristics and the effects on fine-root morphology are more evident than the effects on fine-root biomass.

Growth and transpiration of Japanese cedar (Cryptomeria japonica) and Hinoki cypress (Chamaecyparis obtusa) seedlings in response to soil water content.

To investigate the effects of soil water content on growth and transpiration of Japanese cedar (Cryptomeria japonica D. Don) and Hinoki cypress (Chamaecyparis obtusa (Siebold et Zucc.) Endl.), potted seedlings were grown in well-watered soil (wet treatment) or in drying soil (dry treatment) for 12 weeks. Seedlings in the wet treatment were watered once every 2 or 3 days, whereas seedlings in the dry treatment were watered when soil water content (Theta; m3 m(-3)) reached 0.30, equivalent to a soil matric potential of -0.06 MPa. From Weeks 7 to 12 after the onset of the treatments, seedling transpiration was measured by weighing the potted seedlings. After the last watering, changes in transpiration rate during soil drying were monitored intensely. The dry treatment restricted aboveground growth but increased biomass allocation to the roots in both species, resulting in no significant treatment difference in whole-plant biomass production. The species showed similar responses in relative growth rate (RGR), net assimilation rate (NAR) and shoot mass ratio (SMR) to the dry treatment. Although NAR did not change significantly in either C. japonica or C. obtusa as the soil dried, the two species responded differently to the dry treatment in terms of mean transpiration rate (E) and water-use efficiency (WUE), which are parameters that relate to NAR. In the dry treatment, both E and WUE of C. japonica were stable, whereas in C. obtusa, E decreased and WUE increased (E and WUE counterbalanced to maintain a constant NAR). Transpiration rates were lower in C. obtusa seedlings than in C. japonica seedlings, even in well-watered conditions. During soil drying, the transpiration rate decreased after Theta reached about 0.38 (-0.003 MPa) in C. obtusa and 0.32 (-0.028 MPa) in C. japonica. We conclude that C. obtusa has more water-saving characteristics than C. japonica, particularly when water supply is limited.