Mirai Watanabe

Episode analysis of deposition of radiocesium from the Fukushima Daiichi nuclear power plant accident.

Chemical transport models played key roles in understanding the atmospheric behaviors and deposition patterns of radioactive materials emitted from the Fukushima Daiichi nuclear power plant after the nuclear accident that accompanied the great Tohoku earthquake and tsunami on 11 March 2011. However, model results could not be sufficiently evaluated because of limited observational data. We assess the model performance to simulate the deposition patterns of radiocesium ((137)Cs) by making use of airborne monitoring survey data for the first time. We conducted ten sensitivity simulations to evaluate the atmospheric model uncertainties associated with key model settings including emission data and wet deposition modules. We found that simulation using emissions estimated with a regional-scale (∼ 500 km) model better reproduced the observed (137)Cs deposition pattern in eastern Japan than simulation using emissions estimated with local-scale (∼ 50 km) or global-scale models. In addition, simulation using a process-based wet deposition module reproduced the observations well, whereas simulation using scavenging coefficients showed large uncertainties associated with empirical parameters. The best-available simulation reproduced the observed (137)Cs deposition rates in high-deposition areas (≥ 10 kBq m(-2)) within 1 order of magnitude and showed that deposition of radiocesium over land occurred predominantly during 15-16, 20-23, and 30-31 March 2011.

Uptake and translocation of radiocesium in cedar leaves following the Fukushima nuclear accident.

Cryptomeria japonica trees in the area surrounding Fukushima, Japan, intercepted (137)Cs present in atmospheric deposits soon after the Fukushima nuclear accident in March 2011. To study the uptake and translocation of (137)Cs in C. japonica leaves, we analyzed activity concentrations of (137)Cs and the concentration ratios of (137)Cs to (133)Cs ((137)Cs/(133)Cs) in old and new leaves of C. japonica collected from a forest on Mount Tsukuba between 9 and 15 months after the accident. Both isotopes were also analyzed in throughfall, bulk precipitation and soil extracts. Water of atmospheric and soil origin were used as proxies for deciphering the absorption from leaf surfaces and root systems, respectively. Results indicate that 20-40% of foliar (137)Cs existed inside the leaf, while 60-80% adhered to the leaf surface. The (137)Cs/(133)Cs ratios inside leaves that had sprouted before the accident were considerably higher than that of the soil extract and lower than that of throughfall and bulk precipitation. Additionally, more than 80% of (137)Cs in throughfall and bulk precipitation was present in the dissolved form, which is available for foliar uptake, indicating that a portion of the (137)Cs inside old leaves was presumably absorbed from the leaf surface. New leaves that sprouted after the accident had similar (137)Cs/(133)Cs ratios to that of the old leaves, suggesting that internal (137)Cs was translocated from old to new leaves. For 17 species of woody plants other than C. japonica, new leaves that sprouted after the accident also contained (137)Cs, and their (137)Cs/(133)Cs ratios were equal to or higher than that of the soil extract. These results suggested that foliar uptake and further translocation of (137)Cs is an important vector of contamination in various tree species during or just after radioactive fallout.

Pollution of montane soil with Cu, Zn, As, Sb, Pb, and nitrate in Kanto, Japan.

Soil cores and rainwater were sampled under canopies of Cryptomeria japonica in four montane areas along an atmospheric depositional gradient in Kanto, Japan. Soil cores (30cm in depth) were divided into 2-cm or 4-cm segments for analysis. Vertical distributions of elemental enrichment ratios in soils were calculated as follows: (X/Al)(i)/(X/Al)(BG) (where the numerator and denominator are concentration ratios of element-X and Al in the i- and bottom segments of soil cores, respectively). The upper 14-cm soil layer showed higher levels of Cu, Zn, As, Sb, and Pb than the lower (14-30cm) soil layer. In the four areas, the average enrichment ratios in the upper 6-cm soil layer were as follows: Pb (4.93)>or=Sb (4.06)>or=As (3.04)>Zn (1.71)>or=Cu (1.56). Exogenous elements (kg/ha) accumulated in the upper 14-cm soil layer were as follows: Zn (26.0)>Pb (12.4)>Cu (4.48)>or=As (3.43)>or=Sb (0.49). These rank orders were consistent with those of elements in anthropogenic aerosols and polluted (roadside) air, respectively, indicating that air pollutants probably caused enrichment of these elements in the soil surface layer. Approximately half of the total concentrations of As, Sb, and Pb in the upper 14-cm soil layer were derived from exogenous (anthropogenic) sources. Sb showed the highest enrichment factor in anthropogenic aerosols, and shows similar deposition behavior to NO(3)(-), which is a typical acidic air pollutant. There was a strong correlation between Sb and NO(3)(-) concentrations in rainfall (e.g., in the throughfall under C. japonica: [NO(3)(-)]=21.1 [dissolved Sb], r=0.938, p<0.0001, n=182). Using this correlation, total (cumulative) inputs of NO(3)(-) were estimated from the accumulated amounts of exogenous Sb in soils, i.e., 16.7t/ha at Mt. Kinsyo (most polluted), 8.6t/ha at Mt. Tsukuba (moderately polluted), and 5.8t/ha at the Taga mountain system (least polluted). There are no visible ecological effects of these accumulated elements in the Kanto region at present. However, the concentrations of some elements are within a harmful range, according to the Ecological Soil Screening Levels determined by the U.S. Environmental Protection Agency.

Effect of Antibiotics on Redox Transformations of Arsenic and Diversity of Arsenite-Oxidizing Bacteria in Sediment Microbial Communities

In the present study, we investigated the effect of antibiotics on microbial arsenate (As(V)) reduction and arsenite (As(III)) oxidation in sediments collected from a small pond and eutrophic lake. The As(V)-reducing activities were less susceptible to chloramphenicol in aerobic conditions than in anaerobic conditions. Aerobic As(V) reduction proceeded in the presence of diverse types of antibiotics, suggesting that As-resistant bacteria are widely antibiotic resistant. In contrast, some antibiotics, e.g., chloramphenicol, strongly inhibited aerobic As(III) oxidation. In addition, bacterial As(III) oxidase genes were scarcely amplified and Proteobacteria -related 16S rRNA genes drastically decreased in chloramphenicol-amended cultures. Erythromycin and lincomycin, which successfully target many Gram-positive bacteria, scarcely affected As(III) oxidation, although they decreased the diversity of As(III) oxidase genes. These results indicate that the aerobic As(III) oxidizers in the sediment cultures are mainly composed of Proteobacteria and are more sensitive to certain types of antibiotics than the aerobic As(V) reducers. Our results suggest that antibiotic disturbance of environmental microbial communities may affect the biogeochemical cycle of As.

Ecological niche separation in the Polynucleobacter subclusters linked to quality of dissolved organic matter: a demonstration using a high sensitivity cultivation-based approach

The free-living, cosmopolitan, freshwater betaproteobacterial bacterioplankton genus Polynucleobacter was detected in different years in 11 lakes of varying types and a river using the size-exclusion assay method (SEAM). Of the 350 strains isolated, 228 (65.1%) were affiliated with the Polynucleobacter subclusters PnecC (30.0%) and PnecD (35.1%). Significant positive correlations between fluorescence in situ hybridization and SEAM data were observed in the relative abundance of PnecC and PnecD bacteria to Polynucleobacter communities (PnecC + PnecD). Isolates were mainly PnecC bacteria in the samples with a high specific UV absorbance at 254 nm (SUVA(254) ), and a low total hydrolysable neutral carbohydrate and amino acid (THneutralCH + THAA) content of the dissolved organic matter (DOM) fraction, which is known to be correlated with a high humic content. In contrast, the PnecD bacteria were abundant in samples with high chlorophyll a and/or THneutralCH + THAA concentrations, indicative of primary productivity. With few exceptions, differences in the relative abundance of PnecC and PnecD in each sample, determined using a high-sensitivity cultivation-based approach, were due to DOM quality. These results suggest that the major DOM component in the field, which is allochthonously or autochthonously derived, is a key factor for ecological niche separation between PnecC and PnecD subclusters.

Potential for microbially mediated redox transformations and mobilization of arsenic in uncontaminated soils.

Surface soil samples, which had no significant As contamination, were examined for As(V) reduction, As(III) oxidation and As mobilization capability. All five soil samples tested exhibited microbial As(V)-reducing activities both in aerobic and anaerobic conditions. Under aerobic conditions when As(V) reduction had almost ceased, oxidation of As(III) to As(V) occurred, whereas only As(V) reduction was observed under anaerobic conditions. In cultures incubated with As(III), As(III) was oxidized by indigenous soil microbes only under aerobic conditions. These results indicate that microbial redox transformations of As are ubiquitous in the natural environment regardless of background As levels. Mobilization through microbially mediated As(V) and Fe(III) reduction occurred both in the presence and absence of oxygen. Significant variation in dissolved As occurred depending on the Fe contents of soils, and re-immobilization of As arose in the presence of oxygen, presumably as a consequence of dissolved As(III) and Fe(II) oxidation. There was no apparent correlation between dissolved Fe(II) and As, suggesting that reductive dissolution of Fe(III) minerals does not necessarily determine the extent of As release from soils.

Microbial biomass and nitrogen transformations in surface soils strongly acidified by volcanic hydrogen sulfide deposition in Osorezan, Japan

Abstract Volcanic acidification has created unique ecosystems that have had to adapt to the acidic environments in volcanic regions. To characterize the primary microbial properties of strongly acidified soils in such environments, we investigated microbial biomass, nitrogen transformations and other relevant chemical properties in the surface soils of solfatara and forests from Osorezan, a typical volcanic region in Japan, and compared the results to common Japanese forest soils. Soil microbial biomass C (MBC) and N (MBN) were determined using the chloroform fumigation–extraction method. Potential net N mineralization and net nitrification were measured in aerobic laboratory incubations. Long-term acidification in the Osorezan soils by volcanic hydrogen sulfide deposition caused low soil pH (3.0–3.8), base cation deficiency and increased concentrations of toxic ions such as Al3+. The proportions of MBC to total carbon (MBC/TC ratio) and MBN to total nitrogen (MBN/TN ratio) were lower than those in common Japanese forest soils. The extreme acidic conditions may have inhibited microbial survival in the Osorezan acid soils. Net N mineralization occurred at rates comparable to those in common Cryptomeria japonica forest soils, probably because of the presence of acid-tolerant soil microorganisms. Net nitrification was completely inhibited and autotrophic ammonia oxidizers were not detected by the MPN method. The inhibition of nitrification prevents nitrogen leaching from the soils, thus maintaining a nitrogen cycle in the volcanic acid region in which NH+ 4 (and NH3) is recycled among microorganisms and plants.

Behavior of dissolved radiocesium in river water in a forested watershed in Fukushima Prefecture

Dissolved radiocesium concentrations in river water in a high-dose-rate forest watershed in Fukushima Prefecture were investigated under base flow and storm flow conditions. Under base flow conditions, dissolved 137Cs concentrations in water (Bq/L) were relatively high in summer, and these levels were higher than particulate 137Cs concentrations (Bq/L). Under storm flow, particulate 137Cs concentration became dominant as the suspended solid concentration increased. Throughout the monitoring period, dissolved 137Cs concentrations in water (Bq/L) were higher under storm flow than base flow conditions and were positively correlated with runoff intensity. Factors influencing changes in dissolved 137Cs concentrations were investigated by measuring the 137Cs concentration of suspended solid (Bq/kg) and dissolved 137Cs of unsaturated soil water, throughfall, and rainfall, together with other main solute concentrations. The 137Cs concentration per unit weight of suspended solids in river water was not strongly correlated with runoff intensity. Additionally, dissolved 137Cs concentrations of soil water, groundwater, and rainfall were not detected, while higher dissolved 137Cs concentrations were detected in throughfall than river water. K+ concentrations were higher under storm flow than base flow, and dissolved organic carbon increased toward the peak flow rate. These findings suggested that one main factor influencing generation of dissolved 137Cs in the river water was leaching from organic material in flooded areas. However, further investigation is needed to clarify the dominant source of dissolved 137Cs in river water.

Effects of land-use type and nitrogen addition on nitrous oxide and carbon dioxide production potentials in Japanese Andosols

Abstract Land-use type and nitrogen (N) addition strongly affect nitrous oxide (N2O) and carbon dioxide (CO2) production, but the impacts of their interaction and the controlling factors remain unclear. The aim of this study was to evaluate the effect of both factors simultaneously on N2O and CO2 production and associated soil chemical and biological properties. Surface soils (0–10 cm) from three adjacent lands (apple orchard, grassland and deciduous forest) in central Japan were selected and incubated aerobically for 12 weeks with addition of 0, 30 or 150 kg N ha–1 yr–1. Land-use type had a significant (p < 0.001) impact on the cumulative N2O and CO2 production. Soils from the apple orchard had higher N2O and CO2 production potentials than those from the grassland and forest soils. Soil net N mineralization rate had a positive correlation with both soil N2O and CO2 production rates. Furthermore, the N2O production rate was positively correlated with the CO2 production rate. In the soils with no N addition, the dominant soil properties influencing N2O production were found to be the ammonium-N content and the ratio of soil microbial biomass carbon to nitrogen (MBC/MBN), while those for CO2 production were the content of nitrate-N and soluble organic carbon. N2O production increased with the increase in added N doses for the three land-use types and depended on the status of the initial soil available N. The effect of N addition on CO2 production varied with land use type; with the increase of N addition doses, it decreased for the apple orchard and forest soils but increased for the grassland soils. This difference might be due to the differences in microbial flora as indicated by the MBC/MBN ratio. Soil N mineralization was the major process controlling N2O and CO2 production in the examined soils under aerobic incubation conditions.

Coniferous coverage as well as catchment steepness influences local stream nitrate concentrations within a nitrogen-saturated forest in central Japan

High concentrations of nitrate have been detected in streams flowing from nitrogen-saturated forests; however, the spatial variations of nitrate leaching within those forests and its causes remain poorly explored. The aim of this study is to evaluate the influences of catchment topography and coniferous coverage on stream nitrate concentrations in a nitrogen-saturated forest. We measured nitrate concentrations in the baseflow of headwater streams at 40 montane forest catchments on Mount Tsukuba in central Japan, at three-month intervals for 1 year, and investigated their relationship with catchment topography and with coniferous coverage. Although stream nitrate concentrations varied from 0.5 to 3.0 mgN L-1, those in 31 catchments consistently exceeded 1 mgN L-1, indicating that this forest had experienced nitrogen saturation. A classification and regression tree analysis with multiple environmental factors showed that the mean slope gradient and coniferous coverage were the best and second best, respectively, at explaining inter-catchment variance of stream nitrate concentrations. This analysis suggested that the catchments with steep topography and high coniferous coverage tend to have high nitrate concentrations. Moreover, in the three-year observation period for five adjacent catchments, the two catchments with relatively higher coniferous coverage consistently had higher stream nitrate concentrations. Thus, the spatial variations in stream nitrate concentrations were primarily regulated by catchment steepness and, to a lesser extent, coniferous coverage in this nitrogen-saturated forest. Our results suggest that a decrease in coniferous coverage could potentially contribute to a reduction in nitrate leaching from this nitrogen-saturated forest, and consequently reduce the risk of nitrogen overload for the downstream ecosystems. This information will allow land managers and researchers to develop improved management plans for this and similar forests in Japan and elsewhere.