Experimental evidence of large changes in terrestrial chlorine cycling following altered tree species composition

Organochlorine molecules (Clorg) are surprisingly abundant in soils and frequently exceed chloride (Cl(-)) levels. Despite the widespread abundance of Clorg and the common ability of microorganisms to produce Clorg, we lack fundamental knowledge about how overall chlorine cycling is regulated in forested ecosystems. Here we present data from a long-term reforestation experiment where native forest was cleared and replaced with five different tree species. Our results show that the abundance and residence times of Cl(-) and Clorg after 30 years were highly dependent on which tree species were planted on the nearby plots. Average Cl(-) and Clorg content in soil humus were higher, at experimental plots with coniferous trees than in those with deciduous trees. Plots with Norway spruce had the highest net accumulation of Cl(-) and Clorg over the experiment period, and showed a 10 and 4 times higher Cl(-) and Clorg storage (kg ha(-1)) in the biomass, respectively, and 7 and 9 times higher storage of Cl(-) and Clorg in the soil humus layer, compared to plots with oak. The results can explain why local soil chlorine levels are frequently independent of atmospheric deposition, and provide opportunities for improved modeling of chlorine distribution and cycling in terrestrial ecosystems.

Soil-to-plant transfer factors of radioactive Ca, Sm and Pd isotopes: critical assessment of the use of analogies to derive best-estimates from existing non-specific data

(45)Ca, (151)Sm and (107)Pd are three radionuclides present in low to intermediate in activity radioactive wastes for which no soil-to-plant Transfer Factors (TF) values are available to be used in biosphere models for Ecological Risk Assessment. In the absence of specific radioecological studies, this work reviews and analyzes the existing literature for stable isotopes of Pd, Sm and Ca in order to derive best estimates for TF values that could be used as Transfer Factors. Alternative methods of extrapolation are also critically assessed. The values have been classified according to climatic zone, plant class and soil type for each element. The overall geometric mean TF values (for all plants and conditions) was calculated as 8.4E-02 for Pd, for which the value of radioRu in TRS-472 is also available. The mean TF for Sm was 4.2E-04. This value was lower than the TF values for radioactive Ce that are proposed as alternative values for Sm in TRS-472. The former may be relevant for long term assessments and the latter could possibly used to describe the short term (151)Sm post-release behaviour. The mean value for Ca is 2.3E-01 but varies considerably among plants of a given class due to the variety of plant Ca uptake behaviors. Alternatively, to limit this variability, Ca data content for dry plant matter, as analyzed using the phylogenetic method, could be used to derive TF values if the conservation of isotopic ratio of (45)Ca to stable Ca in soils and in plants hypothesis is taken into account. The TF for Ca in sub-tropical zones is 10-fold lower than in temperate zones. There is a lot of data available about exchangeable Ca in soil, which mean that we could calculate an available TF. The analysis shows that Ca bioavailability is also a key factor within transfer.

Distribution and speciation of ambient selenium in contrasted soils, from mineral to organic rich

Selenium adsorption onto oxy-hydroxides mainly controls its mobility in volcanic soils, red earths and soils poor in organic matter (OM) while the influence of OM was emphasized in podzol and peat soils. This work aims at deciphering how those solid phases influence ambient Se mobility and speciation under less contrasted conditions in 26 soils spanning extensive ranges of OM (1-32%), Fe/Al oxy-hydroxides (0.3-6.1%) contents and pH (4.0-8.3). The soil collection included agriculture, meadow and forest soils to assess the influence of OM quality as well. Trace concentrations of six ambient Se species (Se(IV), Se(VI) and 4 organo-Se compounds) were analyzed by HPLC-ICP-MS in three extractants (ultrapure water, phosphate and sodium hydroxide) targeting Se associated to different soil phases. The Kd values determined from ultrapure water extraction were higher than those reported in commonly used short-term experiments after Se-spiking. Correlations of ambient Se content and distribution with soil parameters explained this difference by an involvement of slow processes in Se retention in soils. The 26 Kd values determined here for a wide variety of soils thus represent a relevant database for long-term prediction of Se mobility. For soils containing less than 20% OM, ambient Se solubility is primarily controlled by its adsorption onto crystalline oxy-hydroxides. However, OM plays an important role in Se mobility by forming organo-mineral associations that may protect adsorbed Se from leaching and/or create anoxic zones (aggregates) where Se is immobilized after its reduction. Although for the first time, inorganic Se(IV), Se(VI) and organo-Se compounds were simultaneously investigated in a large soil collection, high Se proportions remain unidentified in each soil extract, most probably due to Se incorporation and/or binding to colloidal-sized OM. Variations of environmental factors regulating the extent of OM-mineral associations/aggregation may thus lead to changes in Se mobility and bio-availability.

Recent developments in assessment of long-term radionuclide behavior in the geosphere-biosphere subsystem

Decisions on permitting, controlling and monitoring releases of radioactivity into the environment rely on a great variety of factors. Important among these is the prospective assessment of radionuclide behavior in the environment, including migration and accumulation among and within specific environmental media, and the resulting environmental and human health impacts. Models and techniques to undertake such assessments have been developed over several decades based on knowledge of the ecosystems involved, as well as monitoring of previous radionuclide releases to the environment, laboratory experiments and other related research. This paper presents developments in the assessment of radiation doses and related research for some of the key radionuclides identified as of potential significance in the context of releases to the biosphere from disposal facilities for solid radioactive waste. Since releases to the biosphere from disposal facilities involve transfers from the geosphere to the biosphere, an important aspect is the combined effects of surface hydrology, near-surface hydrogeology and chemical gradients on speciation and radionuclide mobility in the zone in which the geosphere and biosphere overlap (herein described as the geosphere-biosphere subsystem). In turn, these aspects of the environment can be modified as a result of environmental change over the thousands of years that have to be considered in radioactive waste disposal safety assessments. Building on the experience from improved understanding of the behavior of the key radionuclides, this paper proceeds to describe development of a generic methodology for representing the processes and environmental changes that are characteristic of the interface between the geosphere and the biosphere. The information that is provided and the methodology that is described are based on international collaborative work implemented through the BIOPROTA forum, www.bioprota.org.

Translocation of (125)I, (75)Se and (36)Cl to edible parts of radish, potato and green bean following wet foliar contamination under field conditions

Specific translocation factor values (ftr) for (129)I, (79)Se and (36)Cl following foliar transfer are still missing from the IAEA reference databases. The translocation of the short-lived isotopes, (125)I, (75)Se, and (36)Cl, to radish, potato and green bean edible parts was measured under field conditions following acute and chronic wet foliar contamination at various plant growth stages in the absence of leaching caused by rain. The translocation factors obtained for (125)I ranged from 0.8 to 2.6% for radish, from 0.1 to 2.3% for potato and from 0.1 to 2.6% for bean. The translocation factors obtained for (75)Se ranged from 6.3 to 21% for radish, from 1.6 to 32.6% for potato and from 7.7 to 22.8% for bean (values similar to Cs or even higher). The translocation factors obtained for (36)Cl were close to those for (75)Se and ranged from 4.3 to 28.8% for radish, from 0.5 to 31.5% for potato and from 4.3 to 16.3% for bean. Iodide showed the lowest apparent mobility because of its preferential fixation in or on the leaves and a significant amount was probably volatilized. Selenite internal transfer was significant and possibly followed the sulfur metabolic pathway. Chloride was very mobile and quickly diffused throughout the plant. The translocation factors varied with the growth stage and depended on the development state of the edible tissue and its associated sink strength for nutrients and assimilates. For radish, translocation was high during the early vegetative stages. For potato, wheat and bean, a major peak in translocation was seen during the flowering growth stage and the concomitant growth of potato tubers. An additive effect of successive contamination events on translocated elements was shown in radish but not in bean and potato. The highest translocation value obtained for an acute contamination event was shown to be an adequate, conservative indicator of chronic contamination in absence of specific values. Due to the absence of rain leaching during the experiment this study probably provides translocation values among the highest that could be recorded.

Translocation of 125I, 75Se and 36Cl to wheat edible parts following wet foliar contamination under field conditions

Apart from radiocaesium and radiostrontium, there have been few studies on the foliar transfer of radionuclides in plants. Consequently, specific translocation factor (ftr) values for (129)I, (79)Se and (36)Cl are still missing from the IAEA reference databases. The translocation of short - lived isotopes, (125)I and (75)Se, and of (36)Cl to wheat grain were measured under field conditions following acute and chronic wet foliar contamination at various plant growth stages in the absence of leaching caused by rain. The translocation factors ranged from 0.02% to 1.1% for (125)I (a value similar to Sr), from 0.1% to 16.5% for (75)Se, and from 1% to 14.9% for (36)Cl. Both (36)Cl and (75)Se were as mobile as Cs. The phenomenological analysis showed that each element displayed a specific behavior. Iodide showed the lowest apparent mobility because of its preferential fixation in or on the leaves and a significant amount probably volatilized. Selenite internal transfer was significant and possibly utilized the sulphur metabolic pathway. However bio - methylation of selenite may have led to increased volatilization. Chloride was very mobile and quickly diffused throughout the plant. In addition, the analysis underlined the importance of plant growth responses to annual variations in weather conditions that can affect open field experiments because plant growth stage played a major role in ftr values dispersion. The chronic contamination results suggested that a series of acute contamination events had an additive effect on translocated elements. The highest translocation value obtained for an acute contamination event was shown to be a good conservative assessment of chronic contamination if data on chronic contamination translocation are lacking. The absence of rain leaching during the experiment meant that this investigation avoided potential radionuclide transfer by the roots, which also meant that radionuclide retention on or in the leaves was maximized. This study was therefore able to obtain accurate translocation factors, which are probably among the highest that could be recorded.

Modelling of the natural chlorine cycling in a coniferous stand: implications for chlorine-36 behaviour in a contaminated forest environment

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.

Organic matter chlorination rates in different boreal soils: the role of soil organic matter content

Transformation of chloride (Cl(-)) to organic chlorine (Cl(org)) occurs naturally in soil but it is poorly understood how and why transformation rates vary among environments. There are still few measurements of chlorination rates in soils, even though formation of Cl(org) has been known for two decades. In the present study, we compare organic matter (OM) chlorination rates, measured by (36)Cl tracer experiments, in soils from eleven different locations (coniferous forest soils, pasture soils and agricultural soils) and discuss how various environmental factors effect chlorination. Chlorination rates were highest in the forest soils and strong correlations were seen with environmental variables such as soil OM content and Cl(-) concentration. Data presented support the hypothesis that OM levels give the framework for the soil chlorine cycling and that chlorination in more organic soils over time leads to a larger Cl(org) pool and in turn to a high internal supply of Cl(-) upon dechlorination. This provides unexpected indications that pore water Cl(-) levels may be controlled by supply from dechlorination processes and can explain why soil Cl(-) locally can be more closely related to soil OM content and the amount organically bound chlorine than to Cl(-) deposition.

Relevance of Radiocaesium Interception Potential (RIP) on a worldwide scale to assess soil vulnerability to 137Cs contamination

The extent of radiocaesium retention in soil is important to quantify the risk of further foodchain contamination. The Radiocaesium Interception Potential (RIP -Cremers et al., 1988, Nature 335, 247-249) is an intrinsic soil parameter which can be used to categorize soils or minerals in terms of their capacity to selectively adsorb radiocaesium. In this study, we measured RIP for a large soil collection (88 soil samples) representative of major FAO soil reference groups on a worldwide scale and tested the possibility to predict the RIP on the basis of other easily accessible or measurable soil data. We also compared RIP values with those obtained from separate chemical extraction experiments. The range of measured RIP values (1.8-13300 mmol kg(-1)) was shown to include nearly all possible cases of agricultural soil contamination. Only Podzols, Andosols and Ferralsols were clearly characterized by a very low RIP (<2000 mmol kg(-1)). On a worldwide scale, RIP was in fact slightly related to soil reference type or other simple major physicochemical parameters such as clay percentage or organic matter. Conversely our results indicated a link between the RIP and radiocaesium extractability across very different soils. We showed that, with the proposed scale of RIP values, a simple acid extraction method can provide an operational result highly predictive of potential RIP despite very contrasting soil properties. The RIP could be estimated from the empirical equation: RIP = (-31.701 ∗ log(AER) + 58.886)(2) where AER is the fraction of acid-extractable radiocaesium.

Sorption of selenate on soils and pure phases: kinetic parameters and stabilisation

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 (Kd(sorption)) over that of the desorption (Kd(desorption)). 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 (Kd(sorption) > 100 to 1 × 10(4) 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 (Kd(sorption) 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 (Kd(sorption) 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), Kd(sorption) 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.

Chloride and organic chlorine in forest soils: storage, residence times, and influence of ecological conditions

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.

Radiocesium transfer between Medicago truncatula plants via a common mycorrhizal network

Common mycorrhizal networks of arbuscular mycorrhizal fungi have been reported to transfer cesium between plants. However, a direct hyphae-mediated transfer (via cytoplasm/protoplasm) cannot be distinguished from an indirect transfer. Indeed, cesium released by the roots of the donor plant can be taken up by the receiver plant or fungal hyphae. In the present study, Medicago truncatula plants were connected by a common mycorrhizal network and Prussian Blue (ammonium-ferric-hexacyano ferrate) was added in the growth medium to adsorb the released radiocesium. A direct transfer of radiocesium to roots and shoots of the receiver plant was clearly demonstrated for the first time. Even though this transfer was quantitatively low, it suggested that shared mycorrhizal networks could contribute to the redistribution of this radionuclide in the environment, which otherwise would be restricted both in time and space. This finding may also help to understand the behaviour of its chemical analogue, potassium.

Impact of Scots pine (Pinus sylvestris L.) plantings on long term (137)Cs and (90)Sr recycling from a waste burial site in the Chernobyl Red Forest

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 (137)Cs than that of trees growing off the trench, and 5.4 times more (90)Sr. 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 (137)Cs and 2.52% of the (90)Sr 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 (90)Sr pool in the trench and 0.0038% of the (137)Cs. A preferential (90)Sr uptake from the deep soil is envisioned while pine roots would take up (137)Cs mostly from less contaminated shallow soil layers. The current upward flux of (90)Sr 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 (90)Sr cycling can be expected to occur at 40 years post-planting, resulting in 12% of the current (90)Sr 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.

Transfer parameter values in temperate forest ecosystems: a review

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.

Plant uptake of radiocaesium from artificially contaminated soil monoliths covering major European soil types

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.

Role and influence of mycorrhizal fungi on radiocesium accumulation by plants

This review summarizes current knowledge on the contribution of mycorrhizal fungi to radiocesium immobilization and plant accumulation. These root symbionts develop extended hyphae in soils and readily contribute to the soil-to-plant transfer of some nutrients. Available data show that ecto-mycorrhizal (ECM) fungi can accumulate high concentration of radiocesium in their extraradical phase while radiocesium uptake and accumulation by arbuscular mycorrhizal (AM) fungi is limited. Yet, both ECM and AM fungi can transport radiocesium to their host plants, but this transport is low. In addition, mycorrhizal fungi could thus either store radiocesium in their intraradical phase or limit its root-to-shoot translocation. The review discusses the impact of soil characteristics, and fungal and plant transporters on radiocesium uptake and accumulation in plants, as well as the potential role of mycorrhizal fungi in phytoremediation strategies.

Impact of arbuscular mycorrhizal fungi on uranium accumulation by plants

Contamination by uranium (U) occurs principally at U mining and processing sites. Uranium can have tremendous environmental consequences, as it is highly toxic to a broad range of organisms and can be dispersed in both terrestrial and aquatic environments. Remediation strategies of U-contaminated soils have included physical and chemical procedures, which may be beneficial, but are costly and can lead to further environmental damage. Phytoremediation has been proposed as a promising alternative, which relies on the capacity of plants and their associated microorganisms to stabilize or extract contaminants from soils. In this paper, we review the role of a group of plant symbiotic fungi, i.e. arbuscular mycorrhizal fungi, which constitute an essential link between the soil and the roots. These fungi participate in U immobilization in soils and within plant roots and they can reduce root-to-shoot translocation of U. However, there is a need to evaluate these observations in terms of their importance for phytostabilization strategies.

Evolution of pH, organic matter and (226)radium/calcium partitioning in U-mining debris following revegetation with pine trees

Natural attenuation processes resulting from the afforestation of some U-waste rock piles have the potential to limit the linkage of radioelements and other trace pollutants, thereby minimizing exposure risks. We determined the evolution of pH and organic matter and compared the (226)Ra and Ca extractability in pyrite-containing mining debris which was revegetated 35 years ago with Scots pine. Oxidation of sulphidic minerals remaining in the substrate appeared to dominate over acidification processes due to vegetation inputs and litter decomposition. The accumulation of organic matter in forest floor had a negligible effect on the (226)Ra upward recycling compared to the migration losses observed mainly from decarbonatation of the surface mining debris. (226)Ra was overall less soluble than Ca in the soil profile but NH(4)Ac-pH 5 had the capacity to extract a (226)Ra fraction of 31.1-41.5%, i.e. at least twice as much as for Ca. In deeper layers, a majority of both Ca and (226)Ra were extractable from the same non-specific adsorption pool, which mainly involved carbonate. In the upper acidified layer, the incorporation of organic matter had no effect on (226)Ra extractability. A further specific adsorption pool for (226)Ra was attributed to the formation of sparingly soluble Fe-Al oxyhydroxides. However, that specific (226)Ra-bearing phase was readily dissolved in NH(4)Ac-pH 5, indicating a relatively reversibility of the precipitation reaction of (226)Ra with amorphous oxide. Trees are effective at reducing hydrological release of many pollutants but in the mining debris studied, four decades of pine growth did not significantly promote (226)Ra remediation in the soil.

Radiocaesium accumulation in stemwood: integrated approach at the scale of forest stands for contaminated Scots pine in Belarus

Twenty years after the Chernobyl accident, root uptake from the surface layers of contaminated forest soils plays a major role in radiocaesium ((137)Cs) transfer to the trees and accumulation in perennial compartments, including stemwood. Trustworthy long-term predictions (modelling) of stemwood contamination with (137)Cs should accordingly be based on a reliable picture of this source-sink relationship. Considering the complexity of the processes involved in (137)Cs cycling in forest stands, elementary ratios like transfer factors (TF) were shown to be not very relevant for that purpose. At the tree level, alternatives like the wood immobilisation potential (WIP) have therefore been proposed in order to quantify the current net (137)Cs accumulation in stemwood. Our objective was here to compare WIP values determined for a series of contaminated forest stands in Belarus with the corresponding pools of (137)Cs available in the soil for root uptake. The comparison reveals that both indices are quite proportional, whatever the forest ecosystem features. This corroborates the relevancy of WIP as an indicator of the current (137)Cs root uptake by the trees, which could accordingly help to improve the existing models of (137)Cs cycling and the long-term management of contaminated forest ecosystems.

Effects of phosphorus fertilization on the availability and uptake of uranium and nutrients by plants grown on soil derived from uranium mining debris

Subterranean clover and barley were grown on a soil derived from uranium mining debris and fertilized with phosphate as a U immobilizing additive for in situ remediation. We investigated the beneficial effect of P fertilization in the range 0-500 mg P kg(-1) soil in terms of U extractability, plant biomass production and U uptake. Increasing P in the mining debris caused a significant decrease of the water-soluble U and NH(4)-Ac extractable U at pH 7 and 5. For both plant species, P fertilization considerably increased root and shoot dry matter up to a maximum observed for soil receiving 100 mg P kg(-1) while the soil-to-plant transfer of U was regularly decreased by increasing P content in soil. These observations show that P fertilization represents an in situ practical option to facilitate the revegetation of U-mining heaps and to reduce the risks of biota exposure to U contamination.

Rhizospheric mobilization and plant uptake of radiocesium from weathered micas: II. Influence of mineral alterability

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.

Rhizospheric mobilization and plant uptake of radiocesium from weathered micas: I. Influence of potassium depletion

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.

Radionuclide migration in forest ecosystems--results of a model validation study

The primary objective of the IAEA's BIOMASS Forest Working Group (FWG) was to bring together experimental radioecologists and modellers to facilitate the exchange of information which could be used to improve our ability to understand and forecast radionuclide transfers within forests. This paper describes a blind model validation exercise which was conducted by the FWG to test nine models which members of the group had developed in response to the need to predict the fate of radiocaesium in forests in Europe after the Chernobyl accident. The outcomes and conclusions of this exercise are summarised. It was concluded that, as a group, the models are capable of providing an envelope of predictions which can be expected to enclose experimental data for radiocaesium contamination in forests over the time scale tested. However, the models are subject to varying degrees of conceptual uncertainty which gives rise to a very high degree of divergence between individual model predictions, particularly when forecasting edible mushroom contamination. Furthermore, the forecasting capability of the models over future decades currently remains untested.

Uranium distribution and cycling in Scots pine (Pinus sylvestris L.) growing on a revegetated U-mining heap

We determined the uranium distribution in soil and its allocation in compartments of 35-year-old Scots pine developed on a revegetated U-mining heap. The processes controlling the dynamics of U recycling were identified and further quantified in terms of annual fluxes. As pine developed, an acid humus layer emerged leading to weathering of the alkaline mining debris but this had little effect on U mobility in the soil profile. Increased U mobility mainly involved a translocation of U to metal-humus chelates in surface layers. The root compartment accounted for 99.3% of the U budget in tree, thus serving as an effective barrier which restricts U uptake. The current root uptake and transfer of U to upper parts of the tree amounted to about 3g ha(-1) y(-1), i.e. less than 0.03% of the current NH4-exchangeable U pool in the soil (0-30 cm). Allocation and translocation pattern made it clear that a dominant fraction of the translocated U moves passively with the ascent xylem sap, most likely as a soluble complex, and steadily accumulates in the needles. Consequently, 97% of the U annual uptake is returned to the soil through litterfall. At the studied site, the risk of U dissemination due to biomass turnover or trunk harvest was low when considered in relation to the current "exemption level" for U.