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Twelve years after the nuclear accident that occurred at the Fukushima Dai-ichi Nuclear Power Plant in March 2011, radiocesium contamination (with a large dominance of 137Cs, with a 30-years half-life) remains a major concern in various municipalities of Northeastern Japan. The Japanese authorities completed an unprecedented decontamination programme in residential and cultivated areas affected by the main radioactive plume (8953 km²). They implemented a complex remediation programme scheme relying on different decision rules depending on the waste type, its contamination level and its region of origin, after delineating different zones exposed to contrasted radiation rates. The central objective was not to expose local inhabitants to radioactive doses exceeding 1 mSv yr-1 in addition to the natural levels. At the onset of the full reopening of the Difficult-to-Return Zone in Spring 2023, the current review provides an update of a previous synthesis published in 2019 (Evrard et al., 2019). Although this ambitious remediation and reconstruction programme is almost completed, in the 12 municipalities of Fukushima Prefecture in which an evacuation order was imposed in at least one neighbourhood in 2011, from the 147,443 inhabitants who lived there before the accident, only 29.9 % of them had returned by 2020. Waste generated by decontamination and tsunami cleaning/demolition work is planned to have been fully transported to (interim) storage facilities by the end of 2023. The cost of the operations conducted between 2011–2020 for the so-called ‘nuclear recovery’ operations (including decontamination) was estimated by the Audit Board of Japan in 2023 to 6122.3 billion yen (~44 billion euro). Decontamination of cropland was shown to have impacted soil fertility, and potassium fertilization is recommended to limit the transfer of residual radiocesium to new crops. In forests that cover 71 % of the surface area of the Fukushima Prefecture and that were not targeted by remediation, radiocesium is now found in the upper mineral layer of the soil in a quasi-equilibrium state. Nevertheless, 137Cs concentrations in forest products (including wood for heating and construction, wild plants, wildlife game, mushrooms) often keep exceeding the threshold values authorized in Japan, which prohibits their exploitation in the area affected by the main plume. Radionuclides from forest were shown to be exported in dissolved and particle-bound forms to downstream river systems and floodplains, although multiple monitoring records showed the continuous decrease in radiocesium concentrations in both river water and sediment across the main plume between 2011–2021. Fish contamination is now generally found below the threshold limits although reputational damage remains a major concern for local fishing communities. The remobilisation of radiocesium from sediment accumulated in reservoirs of the region is also of potential concern as it may lead to secondary contamination of fish or irrigation waters supplied to decontaminated fields. Overall, this synthesis demonstrates the need to continue monitoring post-accidental radiocesium transfers in these environments and to keep sharing data in order to refine our predictive understanding of radiocesium mobility and consolidate the tools available to model contaminant transfers in ecosystems. In forests in particular, novel countermeasures and wood uses remain to be developed and tested. Furthermore, the hydrologic connectivity between ecosystems is of great influence on long term radiocesium transport. The consequences of extreme phenomena (e.g., typhoons, forest fires) that may become more frequent in the future as a result of global change in these contaminated environments should be further anticipated.

The majority of the area contaminated by the Fukushima Daiichi Nuclear Power Plant accident is covered with forests. We developed a dataset for radiocaesium (137Cs) in trees, soil, and mushrooms measured at numerous forest sites. The 137Cs activity concentration and inventory data reported in scientific journal papers written in English and Japanese, governmental reports, and governmental monitoring data on the web were collated. The ancillary information describing the forest stands were also collated, and further environmental information (e.g. climate) was derived from the other databases using longitude and latitude coordinates of the sampling locations. The database contains 8593, 4105, and 3189 entries of activity concentration data for trees, soil, and mushrooms, and 471 and 3521 entries of inventory data for trees and soil, respectively, which were collected from 2011 to 2017, and covers the entire Fukushima prefecture. The data can be used to document and understand the spatio-temporal dynamics of radiocaesium in the affected region and to aid the development and validation of models of radiocaesium dynamics in contaminated forests., Measurement(s) activity (of a radionuclide) • Cesium Cs 137 Technology Type(s) digital curation Factor Type(s) geographic location Sample Characteristic - Organism Cryptomeria japonica • Chamaecyparis obtusa • Pinus densiflora • Quercus serrata Sample Characteristic - Environment temperate evergreen needleleaf forest • temperate deciduous broadleaf forest • leaf • wood • litter layer • forest soil • mushroom (food source) • bark • pollen Sample Characteristic - Location Japan • Fukushima Prefecture Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.13166462

© 2019 Elsevier Ltd The role of organic matter (OM) in antimonate (further denoted as Sb(OH) 6 ) mobility in soil is unclear. The objective of this study was to evaluate Sb(OH) 6 –OM interaction. Antimonate solid:liquid distribution coefficients (K D ) were measured at low Sb concentrations in soil samples with a natural gradient in soil organic carbon (OC) that were collected from different depths of up to 3 m in two excavated soil profiles and in a subset of four soil samples with experimentally increased OM concentration from addition of Suwannee River OM. The K D values were related to soil properties by multiple linear regression and described with the CD–MUSIC model of ferrihydrite. The K D values ranged from 12 to 2800 L kg −1 and decreased strongly with increasing OC concentrations, when normalized to the amount of iron (Fe) and aluminium (Al) in acid oxalate extracts (r = −0.69; p < 0.0001). Experimentally increasing OC by ∼1.5 g kg −1 increased soluble Sb and decreased Sb(OH) 6 K D values by up to a factor of 8. The multiple regression model reveals that sorption of Sb(OH) 6 to Fe and Al hydroxides decreases with increasing pH and increasing dissolved organic carbon concentration. This effect could be explained with geochemical modelling by the competitive and electrostatic effects of adsorbed humic substances on Sb(OH) 6 surface complexation to the reactive surface sites of the Fe and Al hydroxides. Finally, both models could predict the in situ pore water Sb concentrations of unspiked samples, with a RMSE of 0.35 for the regression model and 0.43 for the geochemical model on the log 10 Sb concentrations. For these predictions, the 0.1 M Na 2 HPO 4 –extractable Sb concentration was measured and used to estimate the reversibly sorbed Sb pool. This study shows that increasing soil OM increases Sb(OH) 6 mobility at low soil Sb concentration, likely due to competitive sorption on Fe and Al hydroxides and a process based, geochemical model was calibrated to describe Sb(OH) 6 mobility in soil. ispartof: APPLIED GEOCHEMISTRY vol:104 pages:1-45 status: Published online

International audience; Storage of selenium and iodine can greatly vary between forest ecosystems, but the influence of tree species on partitioning and recycling of those elements remains elusive. In this study, contents of Se and I were measured in tree compartments, litterfall, humus, and soil horizons in monospecific stands of Douglas fir, pine, spruce, beech, and oak under identical climatic and edaphic conditions. The cycle of each element was characterized in terms of stocks and fluxes. Lowest concentrations were in wood (Se: 8–13 μg kg−1; I

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.

The spatial distribution and seasonal variations of atmospheric iodine (I), selenium (Se) and caesium (Cs) depositions remain unclear and this precludes adequate inputs for biogeochemical models. We quantified total concentrations and fluxes of these elements in rainfalls from 27 monitoring sites in France with contrasted climatic conditions; monthly measurements were taken over one year (starting in 2016/09). Since speciation of I and Se can impact their behaviour in the environment, analysis of their inorganic compounds was also conducted. Our results showed that annual I concentrations in rainfall were much higher than those of Se and Cs (annual means = 1.56, 0.044 and 0.005 μg L-1, respectively). The annual iodine concentrations were highly positively correlated with those of marine elements (i.e. Na, Cl and Mg), involving higher I concentrations under oceanic climate than for transition, continental and mountainous ones. Furthermore, common patterns were found between Se concentrations and both marine and terrestrial components consistent with the various sources of Se in atmosphere. The association of Cs with two anthropogenic components (i.e. NH4+ and NO3-) used in agriculture supports the hypothesis of its terrestrial origin (i.e. from atmospheric dusts) in rainfall. We found higher rainfall concentrations of I during the warmest months for all climates. However, no specific seasonal trend occurred for Se and Cs. On annual average, rainfall contained mostly unidentified selenium compounds (inorganic Se proportions = 25-54%) and equal proportions of inorganic and unidentified I compounds. Concentrations of iodate were higher under oceanic climate consistent with an iodine marine-origin. We thank the technical staff of RENECOFOR-ONF for providing samples for this study as well as datasets other than those for iodine, selenium and caesium. This work was financed by the Region Nouvelle Aquitaine and the Agence Nationale de la Recherche with funds allocated in the Investissements d'Avenir' framework program under the reference ANR11-RSNR-0002. The authors thank Vicki Moore for English language assistance. The authors would like to thank Guy Pernot from the computer graphics Unit of Andra for his help in maps edition. info:eu-repo/semantics/publishedVersion

Differences in the source and behaviour of

The levels of natural organic chlorine (Clorg) typically exceed levels of chloride in most soils and is therefore clearly of high importance for continental chlorine cycling. The high spatial variability raises questions on soil organic matter (SOM) chlorination rates among topsoils with different types of organic matter. We measured Clorg formation rates along depth profiles in six French temperate soils with similar Cl deposition using 36Cl tracer experiments. Three forest sites with different humus types and soils from grassland and arable land were studied. The highest specific chlorination rates (fraction of chlorine pool transformed to Clorg per time unit) among the forest soils were found in the humus layers. Comparing the forest sites, specific chlorination was highest in mull-type humus, characterized by high microbial activity and fast degradation of the organic matter. Considering non-humus soil layers, grassland and forest soils had similar specific chlorination rates in the uppermost layer (0–10 cm below humus layer). Below this depth the specific chlorination rate decreased slightly in forests, and drastically in the grassland soil. The agricultural soil exhibited the lowest specific chlorination rates, similar along the depth profile. Across all sites, specific chlorination rates were correlated with soil moisture and in combination with the patterns on organic matter types, the results suggest an extensive Cl cycling where humus types and soil moisture provided best conditions for microbial activity. Clorg accumulation and theoretical residence times were not clearly linked to chlorination rates. This indicates intensive Cl cycling between organic and inorganic forms in forest humus layers, regulated by humic matter reactivity and soil moisture, while long-term Clorg accumulation seems more linked with overall deep soil organic carbon stabilization. Thus, humus types and factors affecting soil carbon storage, including vegetation land use, could be used as indicators of potential Clorg formation and accumulation in soils.

Summary The arsenate (AsO4) and phosphate (PO4) mobility in aerobic soil is affected by soil organic matter (OM). This study was set up to quantify the interaction between OM and AsO4 with an observational, experimental and computational approach. The adsorption of AsO4 was measured with the radiotracer 73AsO4 in samples taken from different horizons of two soil profiles. In four samples, the OM concentration was increased experimentally. The AsO4 adsorption data were analysed with the CD-MUSIC model using ferrihydrite, with OM as competitor and isotopically exchangeable phosphorus of the soil as the total PO4 bound on the reactive surface. The solid–liquid distribution coefficient (KD) of 73AsO4 increased by more than two orders of magnitude with a decrease in total organic carbon (OC) concentration. The addition of Suwannee River OM (∼ 1 g OC kg-1) to samples with small OC (∼ 2 g kg-1) decreased the KD values 15-fold, whereas the effect was less (two-fold) in samples with a large OC (∼ 30 g kg-1). Soluble AsO4 and PO4 could be described well and simultaneously by introducing surface reactive OM (RO−) as an adjustable parameter in the geochemical model. The fitted RO− increased with increasing OC concentration, with a slope of 1.3 ± 0.15 mmol RO− g-1 OC. The amount of RO− expressed per mol iron (Fe) and aluminum (Al) hydroxides was maximum at a molar ratio of ∼ 0.34 at > 10 g OC kg-1 soil, which corresponds to earlier published capacities of an organo-mineral association that might affect potential soil C sequestration. Our research showed that OM enhanced the mobility of AsO4 and PO4; however, the surface reactive OM fraction needs experimental quantification. Highlights Quantification of organic matter–arsenate competition in soil. Include organic matter in geochemical models to predict arsenate and phosphate mobility in soil. Surface reactive organic matter is determined by total organic carbon and Fe and Al hydroxides. Arsenate adsorption is controlled naturally by organic matter concentration.

Because internal transfers can play a key role in radiocaesium persistence in trees, a reliable representation of radiocaesium recycling between tree organs in forest models is important for long-term simulations after radioactive fallout in Chernobyl and Fukushima. We developed an upgraded 2.0 version of the initial TRIPS (“Transfer of Radionuclides In Perennial vegetation System”) model involving explicit differentiation between tree organs (i.e., foliage, branches, stemwood and bark). The quality of TRIPS 2.0 was evaluated by testing model outputs against independent datasets for pine stands in Belarus and Ukraine. Scenarios involving “hot particle” deposits in forest remained challenging, but in all other scenarios generally positive verification results for soil and tree compartments indicated that the TRIPS 2.0 model adequately combines the major relevant processes. Interestingly, the response of stemwood contamination to changes in radiocaesium availability in soil, as determined by soil conditions, was shown to be more sensitive than for other tree compartments. We recommend the conceptual tree discretization of TRIPS 2.0 for generic forest modeling for two reasons: 1) regardless of different soil conditions, there was concurrent good agreement between simulations and data for individual tree compartments (foliage, branches, stemwood and bark), and 2) the measurements necessary to estimate internal tree transfers are easily accessible to usual field monitoring in forest biogeochemistry (for details, see Goor, F. & Thiry, Y., 2004. Science of the total environment, 325(1–3), 163–180).

It is unclear to what extent chlorine (Cl) and its long-lived isotope

International audience; Due to its longevity, radioisotope 129I is a health concern following potential releases in the environment which raises questions about residence and exposure times relevant for risk assessments. We deter¬mined 127I concentrations (as a surrogate for 129I) in a series of French forest soils (i.e. litters, humus and mineral soils) under different vegetation and climate conditions in order to identify the major processes affecting its accumulation and persistence in the soil column. The input fluxes linked to rainfall, throughfall and litterfall were also characterized. Main results obtained showed that: (i) rainfall iodine concentrations probably influenced those of litterfall through absorption by leaves/needles returning to the ground; (ii) throughfall was the major iodine input to soils (mean = 83%), compared to litterfall (mean = 17%); (iii) humus represented a temporary storage of iodine from atmospheric and biomass deposits; (iv) iodine concentrations in soils depended on both the iodine inputs and the soil's ability to retain iodine due to its organic matter, total iron and aluminium concentrations; (v) these soil properties were the main factors influencing the accumulation of iodine in the soil column, resulting in residence times of 419-1756 years; and (vi) the leaching of iodine-containing organic matter dissolved in soil solution may be an important source of labile organic iodine for groundwater and streams.

It is unclear to what extent sorption of antimony (Sb) in soils occurs over long time scales. This study was set up to monitor slow reactions (7 days to 6 months) of Sb in Sb(V)‐spiked soil samples during aerobic incubation. The solid–liquid distribution coefficients (KD) increased by factors of 3 to 6 between 7 and 190 days after spiking, depending on the soil. This increase in the KD value, hereafter named the Sb ageing factor, increased with increasing amorphous iron and aluminium hydroxide concentration and decreasing pH of the soil samples. Finally, ageing factors were highest in topsoils and could be important in preventing Sb from migrating downwards into groundwater bodies. HIGHLIGHTS: Quantification of antimonate ageing in soils. Topsoils could prevent downward leaching of Sb in the long‐term. Ageing of Sb increased in soils with increasing amorphous iron and aluminium hydroxide concentration and decreasing pH. Slow sorption reactions reduce Sb mobility in soils.

International audience; Selenium is a trace element and an essential nutrient. Its long-lived radioisotope, selenium 79 is of potential radio-ecological concern in surface environment of deep geological repository for high-level radioactive waste. In this study, the influence of environmental, climatic and geochemical conditions on stable Se (as a surrogate of 79Se) accumulation was statistically assessed (PCA analysis, Kruskall-Wallis and Spearman tests) based on the analysis of its concentration in litterfall, humus, and soil samples collected at 51 forest sites located in France. Selenium concentrations were in the ranges: 22–369, 57–1608 and 25–1222 μg kg−1 respectively in litterfall, humus, and soil. The proximity of the ocean and oceanic climate promoted Se enrichment of litterfall, likely due to a significant reaction of wet deposits with forest canopy. Se content was enhanced by humification (up to 6 times) suggesting that Se concentrations in humus were affected by atmospheric inputs. Selenium stock in humus decreased in the order of decreasing humus biomass and increasing turnover of organic matter: mor > moder > mull. Positive correlations between Se content and geochemical parameters such as organic carbon content, total Al and total Fe confirmed the important role of organic matter (OM) and mineral Fe/Al oxides in Se retention in soils.

A Correction to this paper has been published: https://doi.org/10.1038/s41597-021-00855-5.

International audience

This study investigated the temporal change in vertical distributions of radiocesium inventories in Japanese forest soils during the early phase (from 2011 to 2017) following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, using three simple parameters. We calculated the fraction in the organic layer (Fl/t), the migration center (Xc) and the relaxation depth (α) using 99 soil inventory data sets. Fl/t decreased significantly from 2011 to 2017 (logistic analysis, p

Aims The ubiquitous and extensive natural chlorination of organic matter in soils, leading to levels of chlorinated soil organic matter that often exceed the levels of chloride, remains mysterious in terms of its causes and regulation. While the composition of plant species and the availability of labile organic matter was recently shown to be important, the physical localization of chlorination in soils remains unclear but is a key for understanding regulation and patterns observed. Here we assess the relative importance of organic matter chlorination in (a) bulk soil, (b) the plant roots plus the rhizosphere zone surrounding the roots, and (c) above-ground plant biomass, in an experimental plant-soil system. Methods A radiotracer, 36Cl, was added to study translocation and transformations of Cl− and Clorg in agricultural soil with and without wheat (Triticum vulgare) over 50 days. Results The specific chlorination rates (the fraction of the added 36Cl− converted to 36Clorg per day) in soil with plants was much higher (0.02 d−1) than without plants (0.0007 d−1) at peak growth (day 25). The plant root and rhizosphere showed much higher formation of 36Clorg than the bulk soil, suggesting that the rhizosphere is a hotspot for chlorination in the soil. In addition, the treatment with plants displayed a rapid and high plant uptake of Cl−. Conclusions Our results indicate that the rhizosphere harbour the most extensive in-situ chlorination process in soil and that root-soil interaction may be key for terrestrial chlorine cycling. Funding agencies: Linkoping University; EDF, France; National Radioactive Waste Management Agency (Andra), France; Linkoping University, Sweden

The management of vast forested zones contaminated by radiocesium (rCs) following the Chernobyl and Fukushima fallout is of great social and economic concern in affected areas and requires appropriate dynamic models as predictive or questioning tools. Generally, the existing radio-ecological models need less fragmented data and more ecological realism in their quantitative description of the rCs cycling processes. The model TRIPS (“Transfer of Radionuclide In Perennial vegetation Systems”) developed in this study privileged an integrated approach which makes the best use of mass balance studies and available explicit experimental data for Scots pine stands. A main challenge was the differentiation and calibration of foliar absorption as well as root uptake in order to well represent the rCs biocycling. The general dynamics of rCs partitioning was simulated with a relatively good precision against an independent series of observed values. In our scenario the rCs biological cycling enters a steady-state about 15 years after the atmospheric deposits. At that time, the simulations showed an equivalent contribution of foliage and root uptake to the tree contamination. But the root uptake seems not sufficient to compensate the activity decline in the tree. The initial foliar uptake and subsequent internal transfers were confirmed to have a great possible impact on the phasing of tree contamination. An extra finding concerns the roots system acting as a buffer in the early period. The TRIPS model is particularly useful in cases where site-specific integrated datasets are available, but it could also be used with adequate caution to generic sites. This development paves the way for simplification or integration of new modules, as well as for a larger number of other applications for the Chernobyl or Fukushima forests once the appropriate data become available. According to the sensitivity analysis that involves in particular reliable estimates of net foliar uptake as well as root uptake not disconnected from rCs exchange reactions in soil.

Soil organic matter (SOM) affects arsenic (As) and antimony (Sb) mobility in soils under waterlogged conditions by acting as an electron donor, by catalyzing redox-cycling through electron shuttling and by acting as a competing ligand. This study was set up to disentangle these different effects of SOM towards As and Sb sorption in anaerobic soils. Nine samples were taken at different depths in an agricultural soil profile to collect samples with a natural SOM gradient (

It is unclear to what extent chlorine (Cl) and its long-lived isotope 36Cl are recycled in different terrestrial environments in response to time-variable inputs. A new version of a dynamic compartment model was developed to examine the transformation and transfer processes influencing the partitioning and persistence of both Cl and 36Cl in forest ecosystems. The model’s performance was evaluated by comparing simulations and field observations of scenarios of stable Cl atmospheric deposition and of global 36Cl fallout. The model reproduced Cl storage in soil reasonably well, despite wide heterogeneity in environmental conditions and atmospheric deposits. Sensitivity analysis confirmed that the natural production of organochlorine in soil plays a major role in Cl build-up and affects long-term Cl dynamics. The timeframe required for the soil organochlorine pool to reach equilibrium in a steady-state system was several thousands of years. Interestingly, root uptake flux, a predominant pathway of the inorganic cycle, was found to affect both inorganic and organic pools in soil, highlighting the importance of plant–soil interactions in Cl dynamics. Model outputs agreed well with local 36Cl measurements, and demonstrated that 90% of the 36Cl found in soil may have come from bomb-test fallout. The pattern of estimated 36Cl/Cl ratios showed that soil 36Cl was not in equilibrium with 36Cl levels in rain input in the post-bomb period. Complete recovery of a natural isotopic ratio in drainage water will need a time close to the residence time of organic 36Cl in soil: i.e., 800 years. A simple dynamic model concept was found to be suitable to illustrate the plant-soil interactions combining both the inorganic and organic Cl cycles acting over different time scales.

Our study focused on radiocaesium (137Cs) partitioning in forests, three vegetation periods after the Fukushima Daiichi nuclear power plant accident. 137Cs distribution in forest components (organic and mineral soil layers as well as tree compartments: stem, bark, needles, branches and roots) was measured for two Japanese cedar stand ages (17 and 33 years old). The results showed that around 85% of the initial deposit was found in the forest floor and topsoil. For the youngest stand almost 70% of the deposit is present in the forest floor, whereas for the oldest stand 50% is present in the 0–3 cm mineral soil layer. For trees, old and perennial organs (including dead and living needles and branches, litter fall and outer bark) directly exposed to the fallout remained the most contaminated. The crown concentrated 61–69% of the total tree contamination. Surprisingly the dead organs concentrated 25 ± 9% (young cedars) to 36 ± 20% (mature cedar) of the trees’ residual activity, highlighting the importance of that specific compartment in the early post-accident phase for Japanese cedar forests. Although the stem (including bark) represents the highest biomass pool, it only concentrates 3.3% and 4.6% of the initial 137Cs deposit for mature and young cedars, respectively.

International audience; This study aims at determining the chlorine and chlorine-36 fallout rates in an experimental beech forest site located in NE France (48 degrees 31'55 `' N, 5 degrees 16'8 `' E). A monthly record of Cl and Cl-36 concentrations in rainfall samples collected above the canopy was performed during two years, from March 2012 to February 2014. The results show that the Cl concentrations mainly originate from sea-spray while the Cl-36 concentrations originate from the stratosphere and therefore present a seasonal dependency. Abrupt and important inputs of Cl-36 from the stratosphere indeed yield sharp increases of the recorded concentrations during the spring-summer. We also show that a too short sampling period might bias the determined Cl-36 fallout rate. To smooth the seasonal and sporadic bursts of Cl-36, a minimum of 6 months sampling period is required. A mean Cl-36 fallout rate of (77 +/- 21) atoms m(-2) S-1 can be deduced from our study, which is 45% higher than the modelled value. This discrepancy suggests more studies aiming at measuring the Cl-36 fallout rate worldwide are necessary. (C) 2016 Elsevier B.V. All rights reserved.

Understanding hydrogen dynamics in soil organic matter is important to predict the fate of 3H in terrestrial environments. One way to determine hydrogen fate and to point out processes is to examine the isotopic signature of the element in soil. However, the non-exchangeable hydrogen isotopic signal in soil is complex and depends on the fate of organic compounds and microbial biosyntheses that incorporate water-derived hydrogen. To decipher this complex system and to understand the close link between hydrogen and carbon cycles, we followed labeled hydrogen and labeled carbon throughout near-natural soil incubations. We performed incubation experiments with three labeling conditions: 1 – 13C2H double-labeled molecules in the presence of 1H2O; 2 – 13C-labeled molecules in the presence of 2H2O; 3 – no molecule addition in the presence of 2H2O. The preservation of substrate-derived hydrogen after 1 year of incubation (ca. 5 % in most cases) was lower than the preservation of substrate-derived carbon (30 % in average). We highlighted that 70 % of the C–H bonds are broken during the degradation of the molecule, which permits the exchange with water hydrogen. Added molecules are used more for trophic resources. The isotopic composition of the non-exchangeable hydrogen was mainly driven by the incorporation of water hydrogen during microbial biosynthesis. It is linearly correlated with the amount of carbon that is degraded in the soil. The quantitative incorporation of water hydrogen in bulk material and lipids demonstrates that non-exchangeable hydrogen exists in both organic and mineral-bound forms. The proportion of the latter depends on soil type and minerals. This experiment quantified the processes affecting the isotopic composition of non-exchangeable hydrogen, and the results can be used to predict the fate of tritium in the ecosystem or the water deuterium signature in organic matter.

Consideration of natural formation of organochlorine compounds in soils is necessary in radioecology in order to understand chlorine radioisotope (36Cl) cycling in various environments for safety assessment purposes, but also in ecotoxicology because certain chlorinated organics in soils are toxic compounds. On the other hand, occurrence of organic chlorine in soils is poorly documented, especially in non-forest ecosystems. We measured total and organic chlorine concentrations in 51 French surface soils sampled from grassland, arable and forest sites on a national scale (French soil quality monitoring network) in order to characterize the variability of organic chlorine concentrations for these different land uses. While previous studies reported that the chlorination of soil organic matter is responsible for chlorine retention in temperate forest ecosystems, this study shows that the non-extractable organohalogen pool accounts for the majority (>80 % on an average) of the total measurable chlorine in grassland and agricultural soils. This suggests that natural chlorination is a widespread phenomenon in all kinds of soils. A multiple linear regression analysis performed on the dataset indicated that retention of organochlorine in soils is related to the organic carbon content, Cl input and soil pH.

Considered as one of the most available radionuclide in soil-plant system, ³⁶Cl is of potential concern for long-term management of radioactive wastes, due to its high mobility and its long half-life. To evaluate the risk of dispersion and accumulation of ³⁶Cl in the biosphere as a consequence of a potential contamination, there is a need for an appropriate understanding of the chlorine cycling dynamics in the ecosystems. To date, a small number of studies have investigated the chlorine transfer in the ecosystem including the transformation of chloride to organic chlorine but, to our knowledge, none have modelled this cycle. In this study, a model involving inorganic as well as organic pools in soils has been developed and parameterised to describe the biogeochemical fate of chlorine in a pine forest. The model has been evaluated for stable chlorine by performing a range of sensitivity analyses and by comparing the simulated to the observed values. Finally a range of contamination scenarios, which differ in terms of external supply, exposure time and source, has been simulated to estimate the possible accumulation of ³⁶Cl within the different compartments of the coniferous stand. The sensitivity study supports the relevancy of the model and its compartments, and has highlighted the chlorine transfers affecting the most the residence time of chlorine in the stand. Compared to observations, the model simulates realistic values for the chlorine content within the different forest compartments. For both atmospheric and underground contamination scenarios most of the chlorine can be found in its organic form in the soil. However, in case of an underground source, about two times less chlorine accumulates in the system and proportionally more chlorine leaves the system through drainage than through volatilisation.

International audience; This study was conducted to identify the principle selenate carrier phases for two selected soils, by comparing their reactivity with selenate to that of pure phases of the solids. Silica, calcium carbonate, aluminium hydroxide, goethite, bentonite and humic acid were selected as the main soil carrier phases. Comparisons were made first on the parameter values obtained with the best fit of a kinetic sorption model which can discriminate instantaneous sorption from kinetically limited sorption. Then comparisons were made of the ability for each solid to stabilise selenate by measuring the ratio of the partition coefficient for sorption (Kdsorption) over that of the desorption (Kddesorption). Kinetics and stabilisation were used to help elucidate the nature of interactions with the test solid phases for a large range of selenate concentrations. The experiments were conducted over 165 h in batch reactors, the solid being isolated from the solution by dialysis tubing, at two pH (5.4 and 8) and three selenate concentrations (1 ?? 10-3, 1 ?? 10-6 and 1 ?? 10-8 mol L-1). The results obtained showed that only aluminium hydroxide can sorb selenate throughout the studied pH range (pH 5.4 to 8.0). The sorption capacity on this mineral was high (Kdsorption \textgreater 100 to 1 ?? 104 L kg-1) and the selenate was mainly stabilized by the formation of inner sphere complexes. The sorption on goethite occurred at pH 5.4 (Kdsorption 52 L kg-1), mainly as outer sphere complexes, and was null at pH 8. On silica, a weak sorption was observed only at pH 5.4 and at 165 h (Kdsorption 4 L kg-1). On bentonite, calcium carbonate and humic acid no significant sorption was observed. Concerning the two soils studied, different behaviours were observed for selenate. For soil Ro (pH 5.4), Kdsorption was low (8 L kg-1) compared to soil Bu (pH 8) (70 L kg-1). The sorption behaviour of selenate on soil Ro was mainly due to outer sphere complexes, as for goethite, whereas for soil Bu the sorption was mainly attributed to inner sphere complexes followed by reduction mechanisms, probably initiated by microorganisms, in which no steady state was reached at the end of the 165 h experiments. The sorption of selenate decreased when concentrations reached 1 ?? 10-3 mol L-1, due to solid saturation, except for aluminium hydroxide. Reduction of selenate seemed also to occur on goethite and soil Ro, for the same concentration, but without preventing a decrease in sorption. Thus, this work shows that the comparison of selenate behaviour between soil and pure phases helps to elucidate the main carrier phases and sorption mechanisms in soil. ?? 2011 Elsevier Ltd.

Recent studies have shown that extensive chlorination of natural organic matter significantly affects chlorine (Cl) residence time in soils. This natural biogeochemical process must be considered when developing the conceptual models used as the basis for safety assessments regarding the potential health impacts of 36-chlorine released from present and planned radioactive waste disposal facilities. In this study, we surveyed 51 French forested areas to determine the variability in chlorine speciation and storage in soils. Concentrations of total chlorine (Cl(tot)) and organic chlorine (Cl(org)) were determined in litterfall, forest floor and mineral soil samples. Cl(org) constituted 11-100% of Cl(tot), with the highest concentrations being found in the humus layer (34-689 mg Cl(org) kg(-1)). In terms of areal storage (53 - 400 kg Cl(org) ha(-1)) the mineral soil dominated due to its greater thickness (40 cm). Cl(org) concentrations and estimated retention of organochlorine in the humus layer were correlated with Cl input, total Cl concentration, organic carbon content, soil pH and the dominant tree species. Cl(org) concentration in mineral soil was not significantly influenced by the studied environmental factors, however increasing Cl:C ratios with depth could indicate selective preservation of chlorinated organic molecules. Litterfall contributions of Cl were significant but generally minor compared to other fluxes and stocks. Assuming steady-state conditions, known annual wet deposition and measured inventories in soil, the theoretical average residence time calculated for total chlorine (inorganic (Cl(in)) and organic) was 5-fold higher than that estimated for Cl(in) alone. Consideration of the Cl(org) pool is therefore clearly important in studies of overall Cl cycling in terrestrial ecosystems.

Plantings of Scots pine (Pinus sylvestris L.) on a waste burial site in the Chernobyl Red Forest was shown to greatly influence the long term redistribution of radioactivity contained in sub-surfaces trenches. After 15 years of growth, aboveground biomass of the average tree growing on waste trench no.22 had accumulated 1.7 times more 137Cs than that of trees growing off the trench, and 5.4 times more 90Sr. At the scale of the trench and according to an average tree density of 3300 trees/ha for the study zone, tree contamination would correspond to 0.024% of the 137Cs and 2.52% of the 90Sr contained in the buried waste material. A quantitative description of the radionuclide cycling showed a potential for trees to annually extract up to 0.82% of the 90Sr pool in the trench and 0.0038% of the 137Cs. A preferential 90Sr uptake from the deep soil is envisioned while pine roots would take up 137Cs mostly from less contaminated shallow soil layers. The current upward flux of 90Sr through vegetation appeared at least equal to downward loss in waste material leaching as reported by Dewiere et al. (2004, Journal of Environmental Radioactivity 74, 139-150). Using a prospective calculation model, we estimated that maximum 90Sr cycling can be expected to occur at 40 years post-planting, resulting in 12% of the current 90Sr content in the trench transferred to surface soils through biomass turnover and 7% stored in tree biomass. These results are preliminary, although based on accurate methodology. A more integrated ecosystem study leading to the coupling between biological and geochemical models of radionuclide cycling within the Red Forest seems opportune. Such a study would help in the adequate management of that new forest and the waste trenches upon which they reside. © 2009 Elsevier Ltd. All rights reserved.

Compared to agricultural lands, forests are complex ecosystems as they can involve diverse plant species associations, several vegetative strata (overstorey, shrubs, herbaceous and other annual plant layer) and multi-layered soil profiles (forest floor, hemi-organic and mineral layers). A high degree of variability is thus generally observed in radionuclide transfers and redistribution patterns in contaminated forests. In the long term, the soil compartment represents the major reservoir of radionuclides which can give rise to long-term plant and hence food contamination. For practical reasons, the contamination of various specific forest products has commonly been quantified using the aggregated transfer factor (T(ag) in m(2)kg(-1)) which integrates various environmental parameters including soil and plant type, root distribution as well as nature and vertical distribution of the deposits. Long lasting availability of some radionuclides was shown to be the source of much higher transfer in forest ecosystems than in agricultural lands. This study aimed at reviewing the most relevant quantitative information on radionuclide transfers to forest biota including trees, understorey vegetation, mushrooms, berries and game animals. For both radiocaesium and radiostrontium in trees, the order of magnitude of mean T(ag) values was 10(-3)m(2)kg(-1) (dry weight). Tree foliage was usually 2-12 times more contaminated than trunk wood. Maximum contamination of tree components with radiocaesium was associated with (semi-)hydromorphic areas with thick humus layers. The transfer of radionuclides to mushrooms and berries is high, in comparison with foodstuffs grown in agricultural systems. Concerning caesium uptake by mushrooms, the transfer is characterized by a very large variability of T(ag), from 10(-3) to 10(1)m(2)kg(-1) (dry weight). For berries, typical values are around 0.01-0.1 m(2)kg(-1) (dry weight). Transfer of radioactive caesium to game animals and reindeer and the rate of activity reduction, quantified as an ecological half-life, reflect the soil and pasture conditions at individual locations. Forests in temperate and boreal regions differ with respect to soil type and vegetation, and a faster decline of muscle activity concentrations in deer occurs in the temperate zone. However, in wild boar the caesium activity concentration shows no decline because of its special feeding habits. In the late phase, i.e. at least a few months since the external radionuclide contamination on feed plants has been removed, a T(ag) value of 0.01 m(2)kg(-1) (fresh weight) is common for (137)Cs in the muscles of adult moose and terrestrial birds living in boreal forests, and 0.03 m(2)kg(-1) (fresh weight) for arctic hare. Radiocaesium concentrations in reindeer muscle in winter may exceed the summer content by a factor of more than two, the mean T(ag) values for winter ranging from 0.02 to 0.8 m(2)kg(-1) (fresh weight), and in summer from 0.04 to 0.4m(2)kg(-1). The highest values are found in the year of initial contamination, followed by a gradual reduction. In waterfowl a relatively fast decline in uptake of (137)Cs has been found, with T(ag) values changing from 0.01 to 0.002 m(2)kg(-1) (fresh weight) in the three years after the contaminating event, the rate being determined by the dynamics of (137)Cs in aquatic ecosystems.

Uptake of 137 Cs was measured in different agricultural plant species (beans, lettuce, barley and ryegrass) grown in 5 undisturbed soil monoliths covering major European soil types. The first cultivation was made three years after soil contamination and plants were grown during 3 successive years. The plant–soil 137 Cs transfer factors varied maximally 12-fold among soils and 35-fold among species when grown on the same soil. Single correlations between transfer factors and soil properties were found, but they varied widely with plant type and can hardly be used as a predictive tool because of the few soils used. The variation of 137 Cs concentrations in plants among soils was related to differences in soil solution 137 Cs and K concentrations, consistent with previous observations in hydroponics and pot trials. Absolute values of transfer factors could not be predicted based on a model validated for pot trials. The 137 Cs activity concentration in soil solution decreased significantly (11- to 250-fold) for most soils in the 1997–1999 period and is partly explained by decreasing K in soil solution. Transfer factors of lettuce showed both increasing and decreasing trends between 2 consecutive years depending on soil type. The trends could be explained by the variation in 137 Cs and K concentrations in soil solution. It is concluded that differences in 137 Cs transfer factors among soils and trends in transfer factors as a function of time can be explained from soil solution composition, as shown previously for pot trials, although absolute values of transfer factors could not be predicted.

The extent of radiocaesium in soil is very important to appreciate the risk of its recycling by the vegetation and thus the risk of food chain contamination. An intrinsic soil parameter, the radiocaesium interception potential (RIP), could be used to measure this retention. In this study, we tested the possibility to predict the RIP starting from a simple acid extraction in widely different soils coming from the whole world and pertaining to various soil reference groups of the WRB/FAO world soil classification. Our results show that a simple acid extraction could be used as an operational test to estimate the RIP whatever the soil type.

In soil, high variability of U bioavailability results in large range of apparent U toxic levels for plants. U toxicity on hairy roots of carrot was studied in nutrient gel with a standardized in vitro device. After exposure to 2.5 and 20 mg U L−1 for 34 days, U concentration ranged between 4 and 563 mg U kg−1 fresh weight which was in good accordance with U accumulation by roots of plant from contaminated soils. Threshold of U toxicity for root length decreased with time and a transient hormesis occurred for exposure to 2.5 and 5 mg U L−1. After 34 days and with root length as endpoint, significant toxicity appeared at a gel contamination level above 7.5 mg U L−1 corresponding to a maximum U concentration in the liquid phase of 0.8 mg L−1. The calculated EC50 for root length as a function of gel contamination was 9.4 mg U L−1. Lower threshold and EC50 were observed for biomass as endpoint (resp. 5 and 7.3 mg U L−1). The low values observed in this study could result from high sensitivity of carrot to U, high bioavailability of U in gel or absence of interferences with microorganisms. This in vitro device appeared adapted to study toxicity of U to plant roots in optimal conditions of both exposure and observations and is recommended to examine further physiological processes and the influence of microorganism interactions.

The objective of this study was to quantify the main terms of the water cycle in a Scots pine stand (Pinus sylvestris L.) growing on a sandy soil and to estimate the contribution of the shallow water table (0.80 m deep in spring) to the forest water use. Continuous monitoring was organized in 2005 to measure climate, throughfall, soil moisture, tree transpiration and water table variations at a half-hourly basis. Leaf area index seasonal dynamic was measured and roots were counted down to the bottom of the soil profile. Forest floor evapotranspiration was modelled with Granier et al. [Granier, A., Breda, N., Biron, P., Villette, S., 1999. A lumped water balance model to evaluate duration and intensity of drought constraints in forest stands. Ecol. Model. 116, 269-283]. From May to November, pine transpiration never exceeded 1.8S mm d(-1) and reached a total of 176.4 mm, which corresponded to 25% of potential evapotranspiration, whereas the understorey evapotranspiration was 130 mm (i.e. 18-20% of the stand water use). The maximum soil water reserve measured over the soil rooted zone was 250 mm, in which 145 mm was extractable water. A 3.5-week period with no rain was observed in June, which induced a regulation of pine transpiration when the soil extractable water reached 0.25 of its maximum value. We applied the water table fluctuation (WTF) method [White, W., 1932. A method for estimating groundwater supplies based on discharge by plants and evaporation from soil. US Geol. Survey Water Supply Paper 659-A. United States Government Printing Office, Washington, DC] to estimate the water table daily loss of water. A relationship was established with potential evapotranspiration and the actual transpiration fluxes of the stand. Yet, it was not possible to extract from the WTF results the part that was effectively contributing to actual transpiration. We applied then the WTF methodology on longer time intervals, with a focus on periods with no rains. From May to November, the contribution of the water table to forest transpiration reached 61%. During the drought period in June, the water table contributed to 98.5% of the water uptake by vegetation, through its contribution to the capillary rise above the water table. The presence of a groundwater table with a floor down to 180-200 cm allowed this stand to rely upon water that other-wise would have drained deeper. (C) 2008 Elsevier B.V. All rights reserved.

Radiocesium interception potential (RIP), a quantitative index of a frayed edge site of micaceous minerals, was investigated on test clay minerals (illite, kaolinite, montmorillonite and vermiculit...

cited By 0; International audience; Chloride is often considered as the main chlorine form in soils. However, recent studies show that chlorine is mostly present in soils as naturally produced organically bound molecules. The relative contribution of biotic, including microbial, and abiotic processes to formation of organically bound chlorine remains poorly understood. We performed a37Cl spiking batch experiment with a forest soil incubated under abiotic and biotic conditions over two time periods to simultaneously monitor the formation of organically bound chlorine from natural and tracer chlorine. To compare biotic and abiotic conditions without biased effect of sterilization technique for abiotic control, the soil was irradiated and reinoculated or not with soil microflora. Fifteen days after microbial inoculation, the natural non-extractable organic chlorine content in the inoculated soil was significantly higher than in the sterile soil, showing that microbial activity contributed to formation of organically bound chlorine. However, no significant difference was noted between the two incubation periods. The same trend was noted for tracer chlorine, yet without a significant difference. The present study shows that chlorination is mediated by microbial activity, but there is also some indication of abiotic formation of organically bound chlorine, with a non-extractable organic tracer chlorine formation of about 6% just after spiking in abiotic conditions. © 2016 Elsevier Ltd

International audience; A better understanding of selenium fate in soils at both short and long time scales is mandatory to consolidate risk assessment models relevant for managing both contamination and soil fertilization issues. The purpose of this study was thus to investigate Se retention processes and their kinetics by monitoring time-dependent distribution/speciation changes of both ambient and freshly added Se, in the form of stable enriched selenite-77, over a 2-years field experiment. This study clearly illustrates the complex reactivity of selenium in soil considering three methodologically defined fractions (i.e. soluble, exchangeable, organic). Time-dependent redistribution of Se-77 within solid-phases having different reactivity could be described as a combination of chemical and diffusion controlled processes leading to its stronger retention. Experimental data and their kinetic modeling evidenced that transfer towards less labile bearing phases are controlled by slow processes limiting the overall sorption of Se in soils. These results were used to estimate time needed for 77Se to reach the distribution of naturally present selenium which may extend up to several decades. Ambient Se speciation accounted for 60% to 100% of unidentified species as function of soil type whereas 77Se(IV) remained the more abundant species after 2-years field experiment. Modeling Se in the long-term without taking account these slow sorption kinetics would thus result in underestimation of Se retention. When using models based on Kd distribution coefficient, they should be at least reliant on ambient Se which is supposed to be at equilibrium. © 2016 Elsevier B.V.

International audience; The success of biofortification and phytoremediation practices, addressing Se deficiency and Se pollution issues, hinges crucially on the fate of selenium in the plant media in response to uptake, translocation and assimilation processes. We investigate the fate of selenium in root and shoot compartments after 3 and 6 weeks of experiment using a total of 128 plants grown in hydroponic solution supplied with 0.2, 2, 5, 20 and 100 mg L−1 of selenium in the form of selenite, selenate and a mixture of both species. Selenate-treated plants exhibited higher root-to-shoot Se translocation and total Se uptake than selenite-treated plants. Plants took advantage of the selenate mobility and presumably of the storage capacity of leaf vacuoles to circumvent selenium toxicity within the plant. Surprisingly, 28% of selenate was found in shoots of selenite-treated plants, questioning the ability of plants to oxidize selenite into selenate. Selenomethionine and methylated organo-selenium amounted to 30% and 8% respectively in shoots and 35% and 9% in roots of the identified Se, suggesting that selenium metabolization occurred concomitantly in root and shoot plant compartments and demonstrating that non-accumulator plants can synthesize notable quantities of precursor compound for volatilization. The present study demonstrated that non-accumulator plants can develop the same strategies as hyper-accumulator plants to limit selenium toxicity. When both selenate and selenite were supplied together, plants used selenate in a storage pathway and selenite in an assimilation pathway. Plants might thereby benefit from mixed supplies of selenite and selenate by saving enzymes and energy required for selenate reduction. © 2016 Elsevier Masson SAS

Much of the total pool of chlorine (Cl) in soil consists of naturally produced organic chlorine (Clorg). The chlorination of bulk organic matter at substantial rates has been experimentally confirmed in various soil types. The subsequent fates of Clorg are important for ecosystem Cl cycling and residence times. As most previous research into dechlorination in soils has examined either single substances or specific groups of compounds, we lack information about overall bulk dechlorination rates. Here we assessed bulk organic matter chlorination and dechlorination rates in coniferous forest soil based on a radiotracer experiment conducted under various environmental conditions (additional water, labile organic matter, and ammonium nitrate). Experiment results were used to develop a model to estimate specific chlorination (i.e., fraction of Cl(-) transformed to Clorg per time unit) and specific dechlorination (i.e., fraction of Clorg transformed to Cl(-) per time unit) rates. The results indicate that chlorination and dechlorination occurred simultaneously under all tested environmental conditions. Specific chlorination rates ranged from 0.0005 to 0.01 d(-1) and were hampered by nitrogen fertilization but were otherwise similar among the treatments. Specific dechlorination rates were 0.01-0.03d(-1) and were similar among all treatments. This study finds that soil Clorg levels result from a dynamic equilibrium between the chlorination and rapid dechlorination of some Clorg compounds, while another Clorg pool is dechlorinated more slowly. Altogether, this study demonstrates a highly active Cl cycling in soils.

International audience; After foliar interception of radioactive atmospheric fallout by forest trees, the short-term recycling dynamics of radiocesium from the tree to the soil as well as within the tree is a primary area of uncertainty in the modeling of the overall cycle. The partitioning of radiocesium transfers in a spruce tree exposed to aerial deposits was investigated during one growth season to reveal the dynamics and significance of underlying processes. The rate of radiocesium loss resulting from foliage leaching (wash-off) was shown to have a functional dependence on the frequency of rainy episodes in a first early stage (weathering 60% of initial contamination during 70 days) and on the amount of precipitation in a second stage (weathering 10% of initial deposits during the following 80 days). A classical single exponential decay model with offset and continuous time as predictor lead to a removal half-life t 1/2 of intercepted radiocesium of 25 days. During the growth season, the similar pattern of the internal 134 Cs content in new shoots and initially contaminated foliage confirmed that radiocesium was readily absorbed from needle surfaces and efficiently translocated to growing organs. In the crown, a pool of non-leachable 134 Cs (15–30%) was associated with the abiotic layer covering the twigs and needle surfaces. At the end of the growth season, 30% of the initial deposits were relocated to different tree parts, including organs like stemwood (5%) and roots (6%) not directly exposed to deposition. At the scale of the tree, 84% of the residual activity was assimilated by living tissues which corresponds to a foliar absorption rate coefficient of 0.25 year −1 for modeling purposes. According to the significant amount of radiocesium which can be incorporated in tree through foliar uptake, our results support the hypothesis that further internal transfers could supply the tree internal cycle of radiocesium extensively, and possibly mask the contribution of root uptake for a long time. © 2016 Elsevier Ltd

Acute K depletion in the rhizosphere can lead to increased root uptake of radiocesium. Two processes can govern this increase: the very low uptake of potassium and the weathering of Cs-fixing clay minerals. Their respective importance is, however, unknown. We investigated the effects of these processes on radiocesium mobilization by roots of willow (Salix viminalis L.) from three micas: muscovite, biotite, and phlogopite. Willows were grown in a mixed quartz-mica substrate with the three respective (134)Cs-contaminated micas as sole sources of potassium and radiocesium. After 7 wk of plant growth, the micas were partially weathered. The degree of mica weathering and the prevalent potassium concentration in the solution increased in the order muscovite (5-11 microM K) < biotite (25-32 microM K) < phlogopite (25-35 microM K). The mobilization and root uptake of radiocesium were negligible with muscovite but increased in the same order. These results show that mica weathering directly and chiefly governs the mobility of radiocesium in K-depleted rhizosphere soil. The low mobility of trace Cs in the muscovite rhizosphere is linked with the dioctahedral character of this mica, and hence to its very low alterability.

Potassium depletion in the soil solution around plant roots promotes the root uptake of radiocesium. However, it can also induce the transformation of mica through the release of interlayer K. In bulk soil, the formation of frayed edge sites (FES) with a high selectivity for Cs adsorption is usually related with mica weathering. We studied the effect of K level in the nutrient solution on the root-induced weathering of phlogopite as well as on the root uptake of radiocesium by willow (Salix viminalis L. var. Orm). The willows were grown for 7 wk in column lysimeters filled with a quartz-phlogopite mixed substrate continuously irrigated with nutrient solutions differing in K concentration (0-2 mM). From a potassium supply of 0.4 mM downward, we observed a decrease in root uptake of potassium as well as an increase in (i) potassium release from phlogopite, (ii) degree of transformation of phlogopite into vermiculite, and (iii) root uptake of radiocesium. Increasing K depletion had thus two effects: a decrease of the root uptake of potassium and an increase of phlogopite weathering in the rhizosphere, both of which promoted the root uptake of radiocesium.