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Published online 7 November 2005
Published in J Environ Qual 34:2174-2180 (2005)
DOI: 10.2134/jeq2004.0407
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Rhizosphere Conference

Rhizospheric Mobilization and Plant Uptake of Radiocesium from Weathered Micas

II. Influence of Mineral Alterability

Yves Thirya,*, Annick Gommersa, Anne Iserentantb and Bruno Delvauxb

a Radiation Protection Research Unit, Radioecology Section, SCK•CEN, Foundation of Public Utility, Boeretang 200, 2400 Mol, Belgium
b Unité Sciences du Sol, Université catholique de Louvain, Place Croix du Sud 2/10, 1348 Louvain-la-Neuve, Belgium

* Corresponding author (ythiry{at}sckcen.be)

Received for publication October 28, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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 134Cs-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 µM K) < biotite (25–32 µM K) < phlogopite (25–35 µM 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.

Abbreviations: FES, frayed edge sites • TF, transfer factor • XRD, X-ray diffraction


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
P OTASSIUM DEPLETION in the solution around root surfaces (Jungk et al., 1985; Jungk and Claassen, 1986) leads to a shift in the adsorption–desorption equilibrium toward an enhanced release of potassium from the solid phase (Hinsinger, 1998). This release can involve both exchangeable and "non-exchangeable" potassium (Hoagland and Martin, 1933; Markewitz and Richter, 2000; Niebes et al., 1993; Tributh et al., 1987) and results in the rapid formation of vermiculitic layers from pure micas (Hinsinger and Jaillard, 1993; Hinsinger et al., 1992; Mortland et al., 1956). The intensity of K release, however, largely depends on the alterability of mica. The dioctahedral minerals are generally more resistant to K release than the trioctahedral ones, because of (i) distinct orientation of the OH group in the octahedral layer (Bassett, 1960; Sawhney and Voigt, 1969; Schroeder, 1974) and (ii) length of the K–O bond (Leonard and Weed, 1970; Sawhney, 1972).

Potassium depletion also rules the acquisition of radiocesium by plants through processes affecting root uptake sensu stricto and mica weathering. First, decreasing K concentration below 1 mM significantly increases the root uptake of trace Cs (Smolders et al., 1996, 1997) because K and Cs ions use identical transport systems in plant cells (White and Broadley, 2000). Second, mica weathering forms vermiculitic frayed edge sites (FES) with very strong specificity for trace Cs fixation (Cremers et al., 1988; Maes et al., 1999). These FES control the mobility of radiocesium in soil and affect its phytoavailability (Kruytz et al., 2000; Thiry et al., 2000). As suggested earlier (Delvaux et al., 2000; Thiry et al., 2001), the rhizospheric mobilization of radiocesium involves both an acute K depletion and the weathering of FES bearing minerals. The respective importance of these processes is, however, unknown (Gommers et al., 2005). In particular, the possibility of extended mobilization of radiocesium in the rhizospheric environment of plants due to enhanced mica weathering has not been demonstrated.

In this paper, we will quantify the rhizospheric mobilization of radiocesium from three micas in a nutrient scenario characterized by acute K depletion, as the micas serve as sole sources of potassium. Micas with distinct susceptibility to weathering were tested: muscovite, biotite, and phlogopite. Muscovite is a dioctahedral mica, known to be more resistant to weathering than the trioctahedral minerals biotite and phlogopite (Fanning et al., 1989). We used high-yielding willows as their roots readily take up radiocesium (Gommers et al., 2000a), especially in acute K-depleted conditions (Gommers et al., 2000b).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Mica Characterization and Substrate Preparation
Three layer silicates were used, which were characterized by a 1.0-nm basal 001 spacing: a biotite (Ward's Natural Science Establishment, Rochester, NY), a phlogopite, and a muscovite (Cogebi, division of Compagnie Royale Asturienne des Mines S.A., Brussels, Belgium). Each mineral was milled and sieved to a particle size range of 100 to 200 µm. Total element contents were determined by inductive coupled plasma–atomic emission spectrometry (ICP–AES; Jarrell Ash, Franklin, MA), after borate fusion at 1000°C (Voïnovitch, 1988) and dissolution of fusion beads in 2 M HNO3. The Fe(II) content was determined separately by a modified Brinhmann method, and the Fe(III) content was computed as the difference between total Fe and Fe(II) contents.

Each mineral stock was equilibrated with the same nutrient solution devoid of potassium, until a constant K concentration in the solution was obtained below a level of 20 µM. The composition of the nutrient solution was as follows: 4.55 mM Ca (NO3)2, 1.45 mM MgSO4, 0.2 mM CaHPO4, 25 µM FeEDTA, 10 µM NaCl, 12 µM MnSO4, 43 µM H3BO3, 1.8 µM ZnSO4, 0.3 µM CuSO4, and 0.07 µM (NH4)6MO7O24. The pH was 6.2. After equilibration, a suspension (three replicates per mineral) was spiked with an identical amount of trace 134Cs+ with a specific activity of 113 MBq mg–1 of Cs, which was allowed to sorb for 7 d. The radiocesium content in the mineral particles as well as in solution was measured by {gamma}-counting (NaI detector; Minaxi {gamma} 5000 series; Packard, Meriden, CT) and the solid–liquid distribution coefficient Kd (Bq g–1 mica/Bq mL–1 solution) was calculated. The cation exchange capacity (CEC) was determined by AgTU (Chhabra et al., 1975). For each treatment, the 134Cs-contaminated mineral particles (100–200 µm) were mixed with quartz (500–1000 µm) at a ratio of 1 g mica to 99 g quartz. The quartz had been previously washed three times with 12 M HCl (50 mL per kg of quartz) and then abundantly rinsed with deionized water.

Experimental Setup
Nine lysimeter columns were filled with the quartz–mica mixture (three micas, three replicates). The experimental set up has been illustrated previously (see Fig. 1 in Gommers et al., 2005). The columns (height: 30 cm; diameter: 9 cm) were closed at the bottom with a 20-µm mesh net, allowing the excess solution to be collected below. A porous plate, previously saturated with the nutrient solution, was put on top of it to distribute the added nutrient solution evenly over the surface. One willow cutting was planted in each column. All cuttings were collected from 1-yr old branches on the same tree. The cropping systems were installed in a greenhouse with a cycle of 16 h of light and 8 h of dark and a minimum light intensity of 425 µmol photons m–2 s–1 being supplied by Philips (Eindhoven, the Netherlands) SON-T 400 daylight lamps (380–779 nm). The plants were grown for 7 wk at a mean temperature of 15°C during the night and 25°C during the day, and a relative humidity varying between 18 and 72%. For each treatment, the columns were continuously supplied with the same nutrient solution as described before. The flow was modified according to the growth rate of the plants and relative transpiration to obtain a percolate volume of >200 mL wk–1.

Percolate and Mineral Sampling and Analysis
During the experiment, the percolates were collected below each column, and the volume of each solution was measured. The different solutions were analyzed for K concentration by atomic absorption spectrometry (AAS) and for radiocesium content by {gamma}-counting.

At harvest, the plant shoots were cut just above the quartz–mica substrate and stored apart. The columns were then opened and the whole substrate was taken out from the column. The root system was carefully removed along with the adhering quartz–mica mixture, and was separated from the non-adhering substrate. The roots were gently shaken to remove part of the quartz–mica mixture that did not stick to the roots (Courchesne and Gobran, 1997): this part together with the non-adhering substrate was considered as bulk soil. Rinsing the roots with ethanol 99.8% further isolated the substrate firmly adhering to the root: this part was considered as rhizosphere soil.

For each treatment and repetition, a sample was taken from the bulk and rhizosphere soil. Mineral particles were separated from quartz by wet sieving with ethanol through a 0.5-mm sieve, and were dried at 30°C. Total 134Cs content of the particles from both rhizosphere and bulk soil as well as from the initial stock (i.e., just after equilibration) was determined by {gamma}-counting. Total K content was measured by ICP–AES, after fusion and wet digestion of the sample as described above. Exchangeable K+ and 134Cs+ contents were measured by AAS and {gamma}-counting in 0.006 M and 0.5 M CaCl2 extracts (0.1 g phlogopite to 10 mL solution), respectively. X-ray diffraction (Cu-K{alpha}, Bruker D8; Bruker, Rheinstetten, Germany) was performed on samples previously air-dried on glass plates.

Plant Sampling and Analysis
The shoots of each plant were separated into leaves and stems. After extraction of the root ball from the substrate and washing as described above, the roots were separated from the cuttings. Before analysis, the roots were washed free of phlogopite particles with ethanol. All plant materials were dried at 105°C and weighed to determine the net biomass produced during the experiment. For each plant compartment, the 134Cs content was determined by {gamma}-counting. The K content was determined by AAS after calcination at 550°C, and ash dissolution with 1.5 mL of 12 M HCl.

The mica-to-willow 134Cs transfer was first computed as a transfer factor (TF) for each plant part (leaf, stem, root, cutting):

where [134Cs]plant part was the radiocesium content in plant compartment (Bq g–1 dry wt.) and [134Cs]initial mica particles the radiocesium content of mica particles in the initial substrate (Bq g–1 dry wt.). A TF for the whole plant (TFtotal plant) was then calculated by using a weighed radiocesium plant content (i.e., by considering the respective biomass of the different plant compartments).

In our experimental conditions, the amount of both radiocesium and potassium exported with the percolates or through root uptake could only be derived from micaceous minerals which acted as the sole source of both elements. The initial stock of K and 134Cs in lysimetric column (total and exchangeable) was computed and further compared to the total amount of the respective element collected in the percolates or measured in plant.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Mineral Characteristics
The chemical composition of the three mineral samples, as calibrated at 100 to 200 µm, confirmed their identification as mica (Table 1). Each mineral was characterized by a typical large K2O content around 8 to 9%. The phlogopite presented as expected the largest MgO content compared to the other minerals. The X-ray diffraction (XRD) patterns of each mineral showed a typical 001 reflection at 1.0 nm (see bulk sample in Fig. 3). The value of the 060 reflection confirmed the dioctahedral character of muscovite (0.149 nm) and the trioctahedral character of phlogopite and biotite (0.153 nm) (not shown). For biotite, the slight asymmetry of the 060 reflection toward small angles, however, suggested the existence of some dioctahedral domains.


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  		Table 1. Elemental composition of the muscovite, biotite, and phlogopite mineral.

 


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  		Fig. 3. X-ray diffraction (XRD) patterns of the rhizosphere micas as compared to the XRD pattern of the bulk micas for each treatment. The intensity ratios of the 1.4- to 1.0-nm reflection (I1.4/I1.0) for rhizosphere micas are 0.025 in muscovite, 0.068 in biotite, and 0.300 in phlogopite.

 
General characteristics of each mineral after equilibration with the K-free nutrient solution and 134Cs contamination are given in Table 2. The values of the AgTU-CEC ranged between 41.4 (phlogopite) and 54.1 (biotite) cmolc kg–1. Both the total and exchangeable K contents differed very little between the three micas. The 134Cs solid–liquid distribution coefficient KdCs, as measured following initial contamination, was the largest in biotite, followed by phlogopite and then muscovite. Muscovite thus sorbed significantly less radiocesium than the trioctahedral micas. The 134Cs retention on specific adsorption sites is, however, expected to be higher for muscovite and biotite since the exchangeable 134Cs content represented a lower fraction of total 134Cs content in muscovite (4.7%) and biotite (4.0%) than in phlogopite (8.8%).


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  		Table 2. Specific characteristics of muscovite, biotite, and phlogopite particles (100–200 µm) after equilibration with the respective nutrient solution and contamination with radiocesium (average value ± standard deviation, n = 3).

 
Changes of Potassium and Cesium-134 Content in Percolates and Minerals
Variations in average potassium concentration in the percolates collected beneath the lysimeters are presented in Fig. 1 . For each mineral, the K level in solution slightly decreased in the first 10 d of plant growth. It progressively increased afterward for biotite and phlogopite until a maximum of 32 and 35 µM, respectively, before stabilizing at about 25 µM in both treatments. For muscovite, the K level in percolates increased to a maximum of 11 µM and then decreased to a final K concentration of 5 µM. During the period of maximum plant development (from the third week to the harvest), the K content in muscovite percolates was always inferior at least by a factor 2 to that of biotite and phlogopite. For radiocesium, its concentration in percolates differed very little between minerals and remained constant during the first 3 wk at average levels of 14 ± 2, 12 ± 2, and 15 ± 2 Bq L–1 for muscovite, biotite, and phlogopite, respectively, before decreasing to the respective final concentrations of 2 ± 1 (muscovite), 3 ± 1 (biotite), and 7 ± 1 (phlogopite) Bq L–1.



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  		Fig. 1. Potassium concentrations in the percolates of the different treatments (M, muscovite; B, biotite; P, phlogopite) during the experimental period.

 
Changes in some physicochemical characteristics measured in the bulk and rhizosphere substrate of each treatment are illustrated in Table 3. The bulk and rhizosphere muscovite particles did not differ in their total K content. In the case of phlogopite, there was a large and significant difference in the K content of the rhizosphere mica (5.2 mg g–1) as compared to the mica from the bulk soil fraction (70.6 mg g–1). For each mineral, there was a significant decrease in the exchangeable K in the rhizosphere mica as compared to the bulk soil fraction. Both the total and exchangeable 134Cs contents of micas were also lower in the rhizosphere mica than in the bulk soil fraction. The extent of the difference in total 134Cs content was similar in each treatment.


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  		Table 3. Total and exchangeable radiocesium and potassium concentrations in the various fractions of the micas particles (initial, bulk, and rhizosphere) for the different treatments (average value ± standard deviation, n = 3).

 
Total K and 134Cs content also differed between bulk and initial minerals (Table 3). Compared to the initial minerals, the reduction in total K content of the bulk minerals increased in the order muscovite (1.4%) < biotite (4.6%) < phlogopite (10.9%). The decrease was even higher for total 134Cs activity and ranged between 33 and 52%. This was probably due to a partial loss of the finest particles during the separation of quartz and micas by wet sieving at 100 µM since the differences could not be explained by exportation with percolates or plant uptake (see below).

Potassium and Cesium-134 Acquisition by Plant and Total Mobilization
A comparison between biomass production, K content in plant at harvesting, and 134Cs uptake as expressed by TF values is shown in Fig. 2 . The net biomass production per plant differed between the mineral treatments in the order: phlogopite (7.8 ± 1.3 mg) > biotite (5.7 ± 0.6 mg) > muscovite (2.3 ± 0.7 mg). The biotite plants produced only slightly less than the phlogopite plants, but significantly more than muscovite plants. The foliage of the muscovite plants presented K deficiency symptoms at the end of the experiment. Figure 2 shows that the average K concentration for the total plants was in fact minimum for muscovite and significantly increased in the order muscovite (0.30 ± 0.04 g 100 g–1) < biotite (0.51 ± 0.08 g 100 g–1) < phlogopite (0.87 ± 0.10 g 100 g–1). Similarly to the K acquisition, the 134Cs uptake by willow plants was the lowest for muscovite and maximum for phlogopite, with the biotite plants presenting an intermediate transfer factor value. Both the net biomass production and plant K content as well as the 134Cs uptake thus decreased in the same order: phlogopite > biotite > muscovite.



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  		Fig. 2. Net biomass (dry weight) produced by willow (A), total potassium content of plants (B), and radiocesium transfer factor (TF) (C) for each treatment (M, muscovite; B, biotite; P, phlogopite).

 
The initial stock of K and 134Cs in the lysimetric columns (total and exchangeable) and the amount of elements exported with the percolates or through root uptake are presented in Table 4. For each mineral, the total element mobilization (uptake + percolates) increased in the order muscovite < biotite < phlogopite and root uptake was a prevailing process of mobilization of both 134Cs and K. Compared to the amount of each element collected in the percolates, the amount exported by the plant was 3 to 11 times higher for 134Cs and 7 to 64 times higher for K. From Table 4, we calculated that the total amount of plant K accounted for 0.4% in muscovite, 0.9% in biotite, and 1.8% in phlogopite, of the initial mica K content per column. As compared to the initial quantity of exchangeable K per column, the plant K represented much larger quantities: 2.2-fold in muscovite, 4.9-fold in biotite, and 9.2-fold in phlogopite. Compared to K, the total plant 134Cs accounted for a larger proportion of the initial 134Cs content per column: 1.3% in muscovite, 1.9% in biotite, and 3.7% in phlogopite. The quantity of radiocesium in plants accounted for a limited fraction of the amount of exchangeable radiocesium per column (26% in muscovite, 47% in biotite, and 42% in phlogopite) whereas the quantity of potassium in plants exceeded by far the amount of exchangeable potassium per column.


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  		Table 4. Total amount of radiocesium and potassium exported with the percolates or by plants through root uptake in comparison with the initial stock of K and 134Cs in lysimetric column (total and exchangeable) for the different treatments (average value ± standard deviation, n = 3).

 
At harvesting, the rhizosphere soil was weighed and represented a minor fraction of the total soil for each mineral treatment: 5.6 ± 1.9% in muscovite, 6.9 ± 2.3% in biotite, and 9.8 ± 1.5% in phlogopite. Certain important differences in total potassium and radiocesium content between the bulk and rhizospheric soils (see Table 3) indicated, however, that a major portion of both elements was preferentially mobilized from the rhizospheric soil, possibly from the non-exchangeable pool of initial micas.

Mineral Transformation
X-ray diffraction patterns were performed on bulk and rhizosphere mica particles as illustrated in Fig. 3 . Irrespective of the nature of the micaceous mineral, the bulk particles exhibited the typical 001 reflection at 1.0 nm as a sole XRD feature in this diffraction domain. The rhizosphere micas behaved differently. All were partially weathered as a significant 001 reflection at 1.4 nm was clearly identified, showing the partial transformation of mica into vermiculite. However, the intensity ratio of the 1.4- to 1.0-nm reflection (I1.4/I1.0), as measured on XRD patterns, increased from 0.025 in muscovite to 0.068 in biotite and 0.300 in phlogopite. Biotite and muscovite were thus less weathered than phlogopite. These observations confirmed the varying weathering of micas, as influenced by mica type in conditions of root-induced K depletion (Hinsinger, 1990).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Weathering of Micas in the Rhizosphere of Willow
The XRD features did not show any transformation of mica into vermiculite before planting, just after equilibration with the K-free nutrient solution and 134Cs sorption, as well as in the bulk micas at harvesting. After the growing period, the partial transformation of mica into vermiculite was readily observed in the rhizosphere minerals, irrespective of the nature of the mica. However, the degree of transformation was the largest in phlogopite and decreased in the order phlogopite > biotite > muscovite. This conclusion is supported by several experimental features:

These features converge to show that mica weathering was induced by root uptake of potassium in K-depleted conditions, as demonstrated earlier (Hinsinger and Jaillard, 1993; Mortland et al., 1956). This uptake maintained an acute K depletion in the rhizosphere as the concentration of potassium were as low as 5 µM (muscovite) to 25 µM (biotite, phlogopite) in the lysimeter leachates.

Mica weathering rate depends on the structural characteristics of the layer silicates. The trioctahedral micas (biotite and phlogopite) are less stable than the dioctahedral one (muscovite) due to the orientation of the hydroxyl group in the octahedral layer. When perpendicular to the c basal plane, as in trioctahedral minerals, the repulsion between H and interlayer K ion is exacerbated, and thus interlayer K is more easily extracted (Bassett, 1960). The larger size of the octahedral layer in trioctahedral minerals produces furthermore longer K–O bonds, hence K ion is held less tightly than in dioctahedral minerals (Radoslovich, 1962; Sawhney, 1972). As such, other researchers have measured a weaker binding of K in biotite or phlogopite than in muscovite when extracted with different chemical reagents (Chute and Quirk, 1967; Leonard and Weed, 1970) or absorbed by plants (Boyle and Voigt, 1973; Hinsinger, 1990; Wentworth and Rossi, 1972). The resistance of muscovite to weathering is also reflected by the very low K level (2.5 µM) required to provoke K release from dioctahedral micas, as compared to the range in K level (58–430 µM) needed to induce similar process in trioctahedral micas (Sparks, 1987). Here, the biotite is less weatherable than the phlogopite in K-depleted conditions. This might be in contrast with previous results showing that within the trioctahedral mica group, interlayer K is released easier from biotite than from phlogopite. However, large differences exist between different biotites or different phlogopites in their ability to release potassium (Leonard and Weed, 1970; Newman, 1969). In the present study, the biotite exhibits some dioctahedral domain. Partial oxidation in biotite can increase the dioctahedral character of the mineral and decrease its ability to release interlayer K (Gilkes et al., 1972, 1973).

Mobilization of Radiocesium in the Rhizosphere of Willow
The pool of radiocesium and potassium absorbed by the willow plants or leached in the percolates evidently derived from the 134Cs-contaminated micas because the nutrient solution was devoid of both K and 134Cs. Just as for the release of potassium from mica, the total mobilization of radiocesium increased in the order muscovite < biotite < phlogopite and the export through root uptake was prevailing for both elements in each treatment. The mobilization of radiocesium was thus the smallest in the presence of muscovite despite the fact that K depletion was the most acute with this mineral. The root-induced mobilization of radiocesium thus increased with increasing transformation of mica into vermiculite: the larger the K release from mica, the larger the root uptake of both K and 134Cs.

Yet, the weathering of mica is the key process to generate frayed edge sites (FES) with very strong affinity for Cs ions and thus to fix trace Cs (Cremers et al., 1988; Maes et al., 1999). The selectivity of FES for trace Cs can be, however, heterogeneous, as some FES can exhibit exceedingly high selectivity of trace Cs against K (Wauters et al., 1996). These sites are most likely associated with FES-bearing minerals with a dioctahedral domain, as radiocesium release largely decreases after layer oxidation, even in strongly acid conditions (Maes et al., 1999). When fixed in the FES-bearing minerals, the release of radiocesium will thus strongly depend on mineral resistance to weathering, just as occurs for potassium (Fanning et al., 1989). As trace Cs is strongly and selectively sorbed against K in the FES, the factors governing K release will most likely determine the mobility of trace Cs in K-depleted rhizospheric environment. This hypothesis is strongly supported by our experimental data, as the 134Cs release and further root absorption increased in the order muscovite < biotite < phlogopite. In similar conditions of severe K depletion, radiocesium was indeed less released in muscovite than in phlogopite because muscovite was much less weathered than phlogopite. As such, just as for K, the release of trace Cs will depend on the susceptibility of the minerals to weathering and thus on their structural properties (Fanning et al., 1989).

Our results thus converge to show that weathering of FES-bearing mineral is a key process governing the mobility of trace Cs in the rhizosphere. In other words, K depletion is not the sole major driving force in root uptake of radiocesium: the stability of FES-bearing mineral or its resistance to rhizosphere weathering is a key parameter. This resistance depends on mineral properties, which are known to affect significantly the mobility of trace Cs at the solid–liquid interface (Maes et al., 1999). Similar rhizosphere processes are likely to occur in soils (Hinsinger, 1998). Such effects have been recently measured by simulating [K+, 137Cs+] root sink effects with sodium tetraphenylboron (NaTPB), a specific reagent which has long been used to assess K release from mica and to study mica weathering (Scott et al., 1960). In a wide variety of soils, the 137Cs rhizospheric mobilization was indeed strongly and positively correlated with 137Cs extraction yield in NaTPB (Delvaux et al., 2000), supporting that weathering of FES-bearing mineral is required to mobilize both K and trace Cs in soils. These observations further suggest that rhizosphere weathering processes may likely control the slow release of radiocesium from the solid phase in soil and its long-lasting transfer to living organisms in contaminated ecosystems (Smith et al., 1999). This would mean that characterizing mineral weathering rates would be as important in the assessment of radiocesium biorecycling in contaminated lands as it is in acid critical load studies in soils.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
We quantified the mica-to-plant transfer of radiocesium in acute K-depleted conditions. We used three micas as the sole source of potassium for plant nutrition: muscovite, biotite, and phlogopite. In the rhizosphere of willow plants, the micas partially weathered into vermiculite, thus producing Cs-specific frayed edge sites (FES).

The degree of mica weathering, the K release and uptake, and the 134Cs mica-to-plant transfer all increased in the order muscovite < biotite < phlogopite. Potassium depletion was most acute in the presence of muscovite and thus did not appear as the sole driving force in root uptake of trace Cs.

We conclude that rhizosphere weathering of FES-bearing mineral is a key process in plant acquisition of radiocesium. We believe that such rhizosphere processes should be refined into the estimation of radiocesium bioavailability in soil (i.e., through adequate chemical extraction) as well as into the modeling of radiocesium soil-to-plant transfer.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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