**2.3 Distribution of Cs (137Cs and 133Cs), K and Rb within** *Sphagnum* **plants**

Concentration values of Cs (137Cs and 133Cs) and neighboring alkali counterparts K and Rb in different segments of plant provide information on differences in their uptake, distribution and relationships. The averaged 137Cs activity concentrations in *Sphagnum* segments are presented in Figure 5a. Within the upper 10 cm from the capitulum, 137Cs activity concentration in *Sphagnum* plants was about 3350 Bq kg−1, with relatively small variations. Below 10-12 cm, the activity gradually declined with depth and in the lowest segments of *Sphagnum,* 137Cs activity concentrations was about 1370 Bq kg−1.

For individual samples, K concentrations ranged between 508 and 4970 mg kg−1 (mean 3096); Rb ranged between 2.4 and 31.4 mg kg−1 (mean 18.9) and 133Cs ranged between 0.046 and 0.363 mg kg−1 (mean 0.204): averaged concentrations of K, Rb and 133Cs in *Sphagnum* segments are presented in Figure 5b. Concentrations of Rb and 133Cs were constant in the upper 0-10 cm segments of *Sphagnum* moss and gradually declined in the lower parts of the plant length; whereas, the concentration of K decreased with increasing depth below 5 cm. Generally, the distribution of all three alkali metals was similar to 137Cs, but with a weaker increase of Rb towards the surface. The 137Cs activity concentrations had the highest coefficient of variation (standard deviation divided by the

Cesium (137Cs and 133Cs), Potassium

**segments of** *Sphagnum* **plants** 

Depth of segment, cm

part of the plant.

Rb and 133Cs).

parts (Figure 7).

and Rubidium in Macromycete Fungi and *Sphagnum* Plants 301

much the same down to about 16 cm, and displayed a slightly different pattern in the lower

**2.4 Mass concentration and isotopic (atom) ratios between 133Cs, K, Rb and 133Cs, in** 

Fig. 6. Ratios between K:137Cs, Rb:137Cs (scale values should be multiplied by 10−2), K:Rb (x102) and 133Cs:137Cs (x10−4) in *Sphagnum* segments. Calculations based on concentrations in mg kg−1 for stable isotopes and Bq kg−1 for 137Cs (+/− SE, n=13 for 137Cs; n=4 for each of K,

However, the isotopic (atom) ratios between 137Cs activity concentrations and mass concentrations of alkali metals, i.e. 137Cs/K, 137Cs/Rb and 137Cs/133Cs, had distinctively different pattern of distribution through the upper part (0-20 cm) of *Sphagnum* plants (Figure 7). The 137Cs/K ratio was relatively narrow through the upper part (0-16 cm) of *Sphagnum* plants and wider with increasing depth, whereas, the 137Cs/133Cs ratio was fairly constant through the upper part (0-12 cm) of *Sphagnum* plants and becomes narrower in the lower (14-20 cm) parts. The 137Cs/Rb ratio was constant through the middle part (4-16 cm) of *Sphagnum* plants and somewhat narrower in the uppermost (0-4 cm) and lowest (16-20 cm)

The distribution of the isotopic (atom) ratios between 137Cs activity concentrations and mass concentrations of alkali metals K and Rb through the upper part (0-20 cm) of *Sphagnum* plants are probably conditioned by at least three processes: physical decay of 137Cs atoms

uppermost (0-2 cm) and the lowest (18-20 cm) parts of the plant (Figure 6).

Ratios between mass concentrations of all three alkali metals and 137Cs activity concentrations, i.e. 133Cs:137Cs; K:137Cs, Rb:137Cs and 133Cs:137Cs, were constant through the upper part (0-16 cm) of *Sphagnum* plants (Figure 6). The ratio K/Rb was higher in the

Ratios K:137Cs, Rb:137Cs, K:Rb and Cs:137Cs

0 0.5 1 1.5 2 2.5 3

K:137Cs Rb:137Cs K:Rb

133Cs:137Cs

mean) in *Sphagnum* (43%). The coefficients of variation were 35% for K, 35% for Rb and 37% for 133Cs concentrations.

Two important features should be mentioned when discussing distributions of K, Rb, 133Cs and 137Cs in a *Sphagnum*-dominated peatland. Firstly, this type of peatland is extremely nutrient-poor, where only a few plant and fungal species producing small fruit bodies can grow and no mycorrhiza, except ericoid mycorrhiza, exists. Secondly, the upper part of the stratigraphy is composed of living *Sphagnum* cells that selectively absorb mineral ions from the surrounding water, and the binding of K, Rb and 133Cs can be at exchange sites both outside and inside the cell.

Fig. 5. 137Cs and alkali metals in Sphagnum: (a) average 137Cs activity concentration (kBq kg−1) in *Sphagnum* segments (+/− SE, n = 13); (b) average concentrations of K (scale values should be multiplied by 103), Rb (x101) and 133Cs (x10−1) (mg kg−1) in *Sphagnum* segments (+/− SE, n=4).

The distribution of 137Cs within *Sphagnum* plants was similar to stable K, Rb and 133Cs. The 137Cs activity concentrations and K, Rb and 133Cs concentrations were always highest in the uppermost 0-10 cm segments of *Sphagnum* (in the capitula and the subapical segments) and gradually decreased in older parts of plant. Such distribution could be interpreted as dependent on the living cells of capitula and living green segments in the upper part of *Sphagnum*. Similar patterns of K distribution within *Sphagnum* plants are reported (Hájek, 2008). 137Cs is taken up and relocated by *Sphagnum* plants in similar ways to the stable alkali metals, as the ratios between K, Rb, Cs and 137Cs in *Sphagnum* segments (Figure 6) were

mean) in *Sphagnum* (43%). The coefficients of variation were 35% for K, 35% for Rb and

Two important features should be mentioned when discussing distributions of K, Rb, 133Cs and 137Cs in a *Sphagnum*-dominated peatland. Firstly, this type of peatland is extremely nutrient-poor, where only a few plant and fungal species producing small fruit bodies can grow and no mycorrhiza, except ericoid mycorrhiza, exists. Secondly, the upper part of the stratigraphy is composed of living *Sphagnum* cells that selectively absorb mineral ions from the surrounding water, and the binding of K, Rb and 133Cs can be at exchange sites both

0

a

1234

137Cs, kBq kg−<sup>1</sup>

2

4

6

8

10

Depth of segment, cm

12

14

16

18

20

 Fig. 5. 137Cs and alkali metals in Sphagnum: (a) average 137Cs activity concentration (kBq kg−1) in *Sphagnum* segments (+/− SE, n = 13); (b) average concentrations of K (scale values should be multiplied by 103), Rb (x101) and 133Cs (x10−1) (mg kg−1) in *Sphagnum* segments

K

Rb

Cs

The distribution of 137Cs within *Sphagnum* plants was similar to stable K, Rb and 133Cs. The 137Cs activity concentrations and K, Rb and 133Cs concentrations were always highest in the uppermost 0-10 cm segments of *Sphagnum* (in the capitula and the subapical segments) and gradually decreased in older parts of plant. Such distribution could be interpreted as dependent on the living cells of capitula and living green segments in the upper part of *Sphagnum*. Similar patterns of K distribution within *Sphagnum* plants are reported (Hájek, 2008). 137Cs is taken up and relocated by *Sphagnum* plants in similar ways to the stable alkali metals, as the ratios between K, Rb, Cs and 137Cs in *Sphagnum* segments (Figure 6) were

37% for 133Cs concentrations.

outside and inside the cell.

012345

K, Rb, 133Cs, mg kg−<sup>1</sup>

0

b

2

4

6

8

10

Depth of segment, cm

12

14

16

18

20

(+/− SE, n=4).

much the same down to about 16 cm, and displayed a slightly different pattern in the lower part of the plant.

#### **2.4 Mass concentration and isotopic (atom) ratios between 133Cs, K, Rb and 133Cs, in segments of** *Sphagnum* **plants**

Ratios between mass concentrations of all three alkali metals and 137Cs activity concentrations, i.e. 133Cs:137Cs; K:137Cs, Rb:137Cs and 133Cs:137Cs, were constant through the upper part (0-16 cm) of *Sphagnum* plants (Figure 6). The ratio K/Rb was higher in the uppermost (0-2 cm) and the lowest (18-20 cm) parts of the plant (Figure 6).

Fig. 6. Ratios between K:137Cs, Rb:137Cs (scale values should be multiplied by 10−2), K:Rb (x102) and 133Cs:137Cs (x10−4) in *Sphagnum* segments. Calculations based on concentrations in mg kg−1 for stable isotopes and Bq kg−1 for 137Cs (+/− SE, n=13 for 137Cs; n=4 for each of K, Rb and 133Cs).

However, the isotopic (atom) ratios between 137Cs activity concentrations and mass concentrations of alkali metals, i.e. 137Cs/K, 137Cs/Rb and 137Cs/133Cs, had distinctively different pattern of distribution through the upper part (0-20 cm) of *Sphagnum* plants (Figure 7). The 137Cs/K ratio was relatively narrow through the upper part (0-16 cm) of *Sphagnum* plants and wider with increasing depth, whereas, the 137Cs/133Cs ratio was fairly constant through the upper part (0-12 cm) of *Sphagnum* plants and becomes narrower in the lower (14-20 cm) parts. The 137Cs/Rb ratio was constant through the middle part (4-16 cm) of *Sphagnum* plants and somewhat narrower in the uppermost (0-4 cm) and lowest (16-20 cm) parts (Figure 7).

The distribution of the isotopic (atom) ratios between 137Cs activity concentrations and mass concentrations of alkali metals K and Rb through the upper part (0-20 cm) of *Sphagnum* plants are probably conditioned by at least three processes: physical decay of 137Cs atoms

Cesium (137Cs and 133Cs), Potassium

0-10 cm length

10-20 cm length

1996; Bates, 1997).

K 0.562\*\*\*

K 0.856\*\*\*

high activity concentrations in the shoot apices.

Rb 0.893\*\*\* 0.632\*\*\*

Rb 0.950\*\*\* 0.952\*\*\*

(133Cs and 137Cs) in *Sphagnum* segments (\*\*\* p=0.001).

133Cs 0.840\*\*\* 0.792\*\*\* 0.802\*\*\*

133Cs 0.645\*\*\* 0.651\*\*\* 0.664\*\*\*

**2.6 Mechanisms of 137Cs and alkali metal uptake by** *Sphagnum* **plants** 

137Cs/133Cs − −0.262 0.270 −0.157

137Cs/133Cs − 0.122 0.219 −0.401

Table 8. Correlation coefficients between concentrations of potassium, rubidium and cesium

Presumably, 137Cs is bound within capitula, living green segments and dead brown segments of *Sphagnum* plants. According to Gstoettner and Fisher (1997), the uptake of some metals (Cd, Cr, and Zn) in *Sphagnum papillosum* is a passive process as they living and dead moss accumulate metal equally. For a wide range of bryophytes, Dragović et al. (2004) found 137Cs was primarily bound by cation exchange, with only a few percent occurring in biomolecules. *Sphagnum* mosses have remarkably high cation exchange capacity (Clymo, 1963), and according to Russell (1988), a high surface activity of *Sphagnum* is related to its high cation exchange capacity, which ranges between 90-140 meq/100 g. In a water saturated peat moss layer, water washes (1 L de-ionised water added to a column of about 1.4 L volume) removed about 60% of K from *Sphagnum* (Porter B. Orr, 1975), indicating this element was held on cation exchange sites. In turn, the desiccation of living moss usually causes cation leakage from cell cytoplasm, during which most of the effused K+ is retained on the exchange sites and reutilized during recovery after rewetting (Brown & Brümelis,

However, this is not necessarily the case for 137Cs, as 137Cs has a weaker correlation with K, especially in the uppermost parts of the plant, which means 137Cs uptake might be somewhat different from that of K. Even within the same segments of the plant, 137Cs activity concentrations has higher variation than K concentration. An even stronger decoupling between 137Cs and K is observed in the forest moss *Pleurozium schreberi,* in which 137Cs is retained to a higher degree in senescent parts (Mattsson & Lidén, 1975). However, close correlations, were found between Rb and 137Cs, which suggests similarities in their

and Rubidium in Macromycete Fungi and *Sphagnum* Plants 303

The relatively unchanged 137Cs/K, 137Cs/Rb and 137Cs/133Cs isotopic (atom) ratios in the upper 0-14 cm part of Sphagnum plant and the noticeable widening below 14-16 cm supported this assumption. An upward migration of 137Cs has been observed in earlier studies (Rosén et al., 2009); similarly, most 137Cs from the nuclear bomb tests from 1963 was retained in the top few cm of *Sphagnum* peat 20 years after, but there was also a lower peak at the level where the 1963 peat was laid down (Clymo, 1983): *Cladonia* lichens also retain

137Cs K Rb 133Cs

with time; attainment of equilibrium between stable 133Cs and 137Cs in the bioavailable fraction of peat soil; and, relation between cesium (133Cs and 137Cs), K and Rb when taken up by the *Sphagnum* plant.

Fig. 7. Isotopic (atom) ratios 137Cs/K (scale values should be multiplied by 10−12), 137Cs/Rb (x 10−09), and 137Cs/133Cs (x10−07) in *Sphagnum* segments. Calculations based on 137Cs activity concentrations and mass concentrations of K, Rb 133Cs (Eq. 2) (mean values, n=4 for each of 137Cs, K, Rb and 133Cs).

#### **2.5 Relationships between 133Cs, K, Rb and 133Cs in segments of** *Sphagnum* **plants**

Relationships between 133Cs, K, Rb and 133Cs in separate segments of *Sphagnum* plants is a tool allowing future investigate its uptake mechanism. There were close positive correlations between K, Rb and 133Cs mass concentrations and 137Cs activity concentrations in *Sphagnum* segments (Table 8). Correlation between 137Cs activity concentrations and Rb mass concentrations (r=0.950; p=0.001) and correlation between K and Rb mass concentrations (r=0.952; p=0.001) in 10-20 cm length of *Sphagnum* plants were highest, but 137Cs and K had a weaker correlation only when the upper 0-10 cm part of *Sphagnum* plants were analyzed (r=0.562; p=0.001). 137Cs/133Cs isotope (atom) ratios and mass concentrations of alkali metals (K, Rb and 133Cs) were not or negatively correlated (Table 8).

The marked decrease in 137Cs activity concentration below 14 cm (Figure 5a) raises the question as to at what depth the 1986 Chernobyl horizon was when the sampling was done. A peat core was sampled in May 2003 at Åkerlänna Römosse, an open bog about 14 km SW of Pålsjömossen, by van der Linden et al. (2008). Detailed dating by 14C wiggle-matching indicated the Chernobyl horizon was then at a depth of 17 cm. Depth-age data estimated a linear annual peat increment of 1.3 cm yr−1 over the last decade (R2=0.998), indicating the Chernobyl horizon would be at about 23 cm deep when the 137Cs sampling was done in 2007-08. Even if there are uncertainties in applying data from different peatlands, the Chernobyl horizon should be at, or below, the lowest segments sampled. Thus, an upward migration of 137Cs was obvious, but no downward migration could be tested in the study.

with time; attainment of equilibrium between stable 133Cs and 137Cs in the bioavailable fraction of peat soil; and, relation between cesium (133Cs and 137Cs), K and Rb when taken up

2.0 4.0 6.0 8.0 10.0 12.0

Isotopic (atom) ratios

137Cs/133Cs 137Cs/K 137Cs/Rb

Fig. 7. Isotopic (atom) ratios 137Cs/K (scale values should be multiplied by 10−12), 137Cs/Rb (x 10−09), and 137Cs/133Cs (x10−07) in *Sphagnum* segments. Calculations based on 137Cs activity concentrations and mass concentrations of K, Rb 133Cs (Eq. 2) (mean values, n=4 for each of

**2.5 Relationships between 133Cs, K, Rb and 133Cs in segments of** *Sphagnum* **plants**  Relationships between 133Cs, K, Rb and 133Cs in separate segments of *Sphagnum* plants is a tool allowing future investigate its uptake mechanism. There were close positive correlations between K, Rb and 133Cs mass concentrations and 137Cs activity concentrations in *Sphagnum* segments (Table 8). Correlation between 137Cs activity concentrations and Rb mass concentrations (r=0.950; p=0.001) and correlation between K and Rb mass concentrations (r=0.952; p=0.001) in 10-20 cm length of *Sphagnum* plants were highest, but 137Cs and K had a weaker correlation only when the upper 0-10 cm part of *Sphagnum* plants were analyzed (r=0.562; p=0.001). 137Cs/133Cs isotope (atom) ratios and mass concentrations

The marked decrease in 137Cs activity concentration below 14 cm (Figure 5a) raises the question as to at what depth the 1986 Chernobyl horizon was when the sampling was done. A peat core was sampled in May 2003 at Åkerlänna Römosse, an open bog about 14 km SW of Pålsjömossen, by van der Linden et al. (2008). Detailed dating by 14C wiggle-matching indicated the Chernobyl horizon was then at a depth of 17 cm. Depth-age data estimated a linear annual peat increment of 1.3 cm yr−1 over the last decade (R2=0.998), indicating the Chernobyl horizon would be at about 23 cm deep when the 137Cs sampling was done in 2007-08. Even if there are uncertainties in applying data from different peatlands, the Chernobyl horizon should be at, or below, the lowest segments sampled. Thus, an upward migration of 137Cs was obvious, but no downward migration could be tested in the study.

of alkali metals (K, Rb and 133Cs) were not or negatively correlated (Table 8).

by the *Sphagnum* plant.

Depth of segments, cm

137Cs, K, Rb and 133Cs).

The relatively unchanged 137Cs/K, 137Cs/Rb and 137Cs/133Cs isotopic (atom) ratios in the upper 0-14 cm part of Sphagnum plant and the noticeable widening below 14-16 cm supported this assumption. An upward migration of 137Cs has been observed in earlier studies (Rosén et al., 2009); similarly, most 137Cs from the nuclear bomb tests from 1963 was retained in the top few cm of *Sphagnum* peat 20 years after, but there was also a lower peak at the level where the 1963 peat was laid down (Clymo, 1983): *Cladonia* lichens also retain high activity concentrations in the shoot apices.


Table 8. Correlation coefficients between concentrations of potassium, rubidium and cesium (133Cs and 137Cs) in *Sphagnum* segments (\*\*\* p=0.001).

#### **2.6 Mechanisms of 137Cs and alkali metal uptake by** *Sphagnum* **plants**

Presumably, 137Cs is bound within capitula, living green segments and dead brown segments of *Sphagnum* plants. According to Gstoettner and Fisher (1997), the uptake of some metals (Cd, Cr, and Zn) in *Sphagnum papillosum* is a passive process as they living and dead moss accumulate metal equally. For a wide range of bryophytes, Dragović et al. (2004) found 137Cs was primarily bound by cation exchange, with only a few percent occurring in biomolecules. *Sphagnum* mosses have remarkably high cation exchange capacity (Clymo, 1963), and according to Russell (1988), a high surface activity of *Sphagnum* is related to its high cation exchange capacity, which ranges between 90-140 meq/100 g. In a water saturated peat moss layer, water washes (1 L de-ionised water added to a column of about 1.4 L volume) removed about 60% of K from *Sphagnum* (Porter B. Orr, 1975), indicating this element was held on cation exchange sites. In turn, the desiccation of living moss usually causes cation leakage from cell cytoplasm, during which most of the effused K+ is retained on the exchange sites and reutilized during recovery after rewetting (Brown & Brümelis, 1996; Bates, 1997).

However, this is not necessarily the case for 137Cs, as 137Cs has a weaker correlation with K, especially in the uppermost parts of the plant, which means 137Cs uptake might be somewhat different from that of K. Even within the same segments of the plant, 137Cs activity concentrations has higher variation than K concentration. An even stronger decoupling between 137Cs and K is observed in the forest moss *Pleurozium schreberi,* in which 137Cs is retained to a higher degree in senescent parts (Mattsson & Lidén, 1975). However, close correlations, were found between Rb and 137Cs, which suggests similarities in their

Cesium (137Cs and 133Cs), Potassium

**4. Acknowledgements** 

**5. References** 

ISSN 0033-8230

7364

9697

explain the accumulation in the top layer of the mosses.

(Swedish Nuclear Fuel and Waste Management Co).

and Rubidium in Macromycete Fungi and *Sphagnum* Plants 305

For *Sphagnum* the distribution of 137Cs can be driven by several processes: cation exchange is important and gives similar patterns for monovalent cations; uptake/retention in living cells; and downwash and upwash by water outside the plants. However, the most important mechanism is internal translocation to active tissue and the apex, which can

The authors gratefully acknowledge the Swedish University of Agricultural Sciences (SLU), Sweden, for supporting the project. We would like to express our thanks to Dr. I. Nikolova for her assistance with the experiments and to the staff of the Analytica Laboratory, Luleå, Sweden, for ICP-AES *and* ICP-SFMS analyses. The project was financially supported by SKB

Adema, E.; Baaijens, G.; van Belle, J.; Rappoldt, A.; Grootjans, A. & Smolders, A. (2006).

Baeza, A.; Hernández, S.; Guillén, F.; Moreno, J.; Manjón, J.L. & Pascual, R. (2004).

*Science of the Total Environment,* Vol.318, No.1-3, pp. 59-71, ISSN 0048-9697 Baeza, A.; Guillén, J.; Hernández, S.; Salas, A.; Bernedo, M.; Manjón, J. & Moreno, G. (2005).

Ban-nai T.; Yoshida S.; Muramatsu Y. & Suzuki A. (2005). Uptake of Radiocesium by Hypha

Bates, J. (1997). Effects of intermittent desiccation on nutrient economy and growth of two

Brown, D. & Brümelis, G. (1996). A biomonitoring method using the cellular distribution of

Brown, G. & Cummings, S. (2001). Potassium uptake and retention by *Oceanomonas* 

Brunner, I.; Frey, B. & Riesen, T. (1996). Influence of ectomycorrhization and

Bunzl, K. & Kracke, W. (1989). Seasonal variation of soil-to-plant transfer of K and fallout 134,137Cs in peatland vegetation. *Health Physics,* Vol.57, pp. 593-600, ISSN 0017-9078 Bystrzejewska-Piotrowska, G. & Bazala, M. (2008). A study of mechanisms responsible for

seedlings. *Tree Physiology,* Vol.16, pp. 705-711, ISSN 0829-318X

*Journal of Hydrology*, Vol.327, pp. 226– 234, ISSN 0022-1694

*Sciences,* Vol. 6, No.1, pp. 111-113, ISSN 1345-4749

*Letters,* Vol.205, No.1, pp. 37–41, ISSN 0378-1097

1185-1191, ISSN 0265-931X

Field evidence for buoyancy-driven water flow in a *Sphagnum* dominated peat bog.

Radiocaesium and natural gamma emitters in mushrooms collected in Spain.

Influence of the nutritional mechanism of fungi (mycorrhize/saprophyte) on the uptake of radionuclides by mycelium. *Radiochimica Acta,* Vol.93, No.4, pp. 233-238,

of Basidiomycetes – Radiotracer Experiments. *Journal of Nuclear and Radiochemical* 

ecologically contrasted mosses. *Annals of Botany,* Vol.79, pp.299–309, ISSN 0305-

metals in moss. *Science of the Total Environment,* Vol.187, pp. 153–161, ISSN 0048-

*baumannii* at low water activity in the presence of phenol. *FEMS Microbiology* 

cesium/potassium ratio on uptake and localization of cesium in Norway spruce

incorporation of cesium and radiocaesium into fruitbodies of king oyster mushroom (*Pleurotus eryngii*). *Journal of Environmental Radioactivity,* Vol.99, pp.

uptake and relocation: these observations complied with results reported for fungi (Vinichuk et al., 2010b; 2011).

Some lower parts of *Sphagnum* plants are still alive and able to create new shoots (Högström, 1997), however, although still connected to the capitulum, much of lower stem is dead. Thus, the decrease of 137Cs activity concentration in plant segments below 10 cm indicates a release of the radionuclide from the dying lower part of *Sphagnum* and internal translocation to the capitulum.

The mechanism of radiocesium and alkali metal relocation within *Sphagnum* is probably the same active translocation as described for metabolites by Rydin & Clymo (1989). Although external buoyancy-driven transport (Rappoldt et al., 2003) could redistribute 137Cs, field evidence suggests buoyancy creates a downward migration of K (Adema et al., 2006); thus, this mechanism appears unlikely. Likewise, a passive downwash and upwash (Clymo & Mackay, 1987) cannot explain accumulation towards the surface.

#### **3. Conclusions from the Swedish studies**

The concentrations of the three stable alkali elements K, Rb and 133Cs and the activity concentration of 137Cs were determined in various components of Swedish forests − bulk soil, rhizosphere, soil–root interface fraction, fungal mycelium and fungal sporocarps. The soil–root interface fraction was distinctly enriched with K and Rb, compared with bulk soil. Potassium concentration increased in the order bulk soil < rhizosphere < fungal mycelium < soil–root interface < fungal sporocarps, whereas, Rb concentration increased in the order bulk soil < rhizosphere < soil–root interface < fungal mycelium < fungal sporocarps.

Cesium was generally evenly distributed within bulk soil, rhizosphere and soil–root interface fractions, indicating no 133Cs enrichment in these forest compartments.

The uptake of K, Rb and 133Cs during the entire transfer process between soil and sporocarps occurred against a concentration gradient. For all three alkali metals, the levels of K, Rb and 133Cs were at least one order of magnitude higher in sporocarps than in fungal mycelium.

Potassium uptake appeared to be regulated by fungal nutritional demands for this element and fungi had a higher preference for uptake of Rb and K than for Cs. According to their efficiency of uptake by fungi, the three elements may be ranked in the order Rb+ > K+ > Cs+, with a relative ratio 100:57:32. Although the mechanism of Cs uptake by fungi could be similar to that of Rb, uptake mechanism for K appeared to be different. The variability in isotopic (atom) ratios of 137Cs/K, 137Cs/Rb and 137Cs/133Cs in the fungal sporocarps suggested they were independent on specific species of fungi. The relationships observed between concentration ratios 137Cs/133Cs and K, Rb and 133Cs in fungal sporocarps also varied widely and were inconsistent. The concentration of K, Rb and 133Cs in sporocarps appeared independent of the 137Cs/133Cs isotopic ratio.

The study of *S. variegatus* sporocarps sampled within 1 km2 forest area with high 137Cs fallout from the Chernobyl accident confirmed 133Cs and 137Cs uptake is not correlated with uptake of K; whereas, the uptake of Rb is closely related to the uptake of 133Cs. Furthermore, the variability in 137Cs and alkali metals (K, Rb and 133Cs) among genotypes in local populations of *S. variegatus* is high and the variation appears to be in the same range as found in species collected at different localities. The variations in concentrations of K, Rb and 133Cs and 137Cs activity concentration in sporocarps of *S. variegatus* appear to be influenced more by local environmental factors than by genetic differences among fungal genotypes.

For *Sphagnum* the distribution of 137Cs can be driven by several processes: cation exchange is important and gives similar patterns for monovalent cations; uptake/retention in living cells; and downwash and upwash by water outside the plants. However, the most important mechanism is internal translocation to active tissue and the apex, which can explain the accumulation in the top layer of the mosses.

#### **4. Acknowledgements**

304 Radioisotopes – Applications in Physical Sciences

uptake and relocation: these observations complied with results reported for fungi

Some lower parts of *Sphagnum* plants are still alive and able to create new shoots (Högström, 1997), however, although still connected to the capitulum, much of lower stem is dead. Thus, the decrease of 137Cs activity concentration in plant segments below 10 cm indicates a release of the radionuclide from the dying lower part of *Sphagnum* and internal translocation

The mechanism of radiocesium and alkali metal relocation within *Sphagnum* is probably the same active translocation as described for metabolites by Rydin & Clymo (1989). Although external buoyancy-driven transport (Rappoldt et al., 2003) could redistribute 137Cs, field evidence suggests buoyancy creates a downward migration of K (Adema et al., 2006); thus, this mechanism appears unlikely. Likewise, a passive downwash and upwash (Clymo &

The concentrations of the three stable alkali elements K, Rb and 133Cs and the activity concentration of 137Cs were determined in various components of Swedish forests − bulk soil, rhizosphere, soil–root interface fraction, fungal mycelium and fungal sporocarps. The soil–root interface fraction was distinctly enriched with K and Rb, compared with bulk soil. Potassium concentration increased in the order bulk soil < rhizosphere < fungal mycelium < soil–root interface < fungal sporocarps, whereas, Rb concentration increased in the order

Cesium was generally evenly distributed within bulk soil, rhizosphere and soil–root

The uptake of K, Rb and 133Cs during the entire transfer process between soil and sporocarps occurred against a concentration gradient. For all three alkali metals, the levels of K, Rb and 133Cs were at least one order of magnitude higher in sporocarps than in fungal mycelium. Potassium uptake appeared to be regulated by fungal nutritional demands for this element and fungi had a higher preference for uptake of Rb and K than for Cs. According to their efficiency of uptake by fungi, the three elements may be ranked in the order Rb+ > K+ > Cs+, with a relative ratio 100:57:32. Although the mechanism of Cs uptake by fungi could be similar to that of Rb, uptake mechanism for K appeared to be different. The variability in isotopic (atom) ratios of 137Cs/K, 137Cs/Rb and 137Cs/133Cs in the fungal sporocarps suggested they were independent on specific species of fungi. The relationships observed between concentration ratios 137Cs/133Cs and K, Rb and 133Cs in fungal sporocarps also varied widely and were inconsistent. The concentration of K, Rb and 133Cs in sporocarps

The study of *S. variegatus* sporocarps sampled within 1 km2 forest area with high 137Cs fallout from the Chernobyl accident confirmed 133Cs and 137Cs uptake is not correlated with uptake of K; whereas, the uptake of Rb is closely related to the uptake of 133Cs. Furthermore, the variability in 137Cs and alkali metals (K, Rb and 133Cs) among genotypes in local populations of *S. variegatus* is high and the variation appears to be in the same range as found in species collected at different localities. The variations in concentrations of K, Rb and 133Cs and 137Cs activity concentration in sporocarps of *S. variegatus* appear to be influenced more by local environmental factors than by genetic differences among fungal

bulk soil < rhizosphere < soil–root interface < fungal mycelium < fungal sporocarps.

interface fractions, indicating no 133Cs enrichment in these forest compartments.

Mackay, 1987) cannot explain accumulation towards the surface.

**3. Conclusions from the Swedish studies** 

appeared independent of the 137Cs/133Cs isotopic ratio.

(Vinichuk et al., 2010b; 2011).

to the capitulum.

genotypes.

The authors gratefully acknowledge the Swedish University of Agricultural Sciences (SLU), Sweden, for supporting the project. We would like to express our thanks to Dr. I. Nikolova for her assistance with the experiments and to the staff of the Analytica Laboratory, Luleå, Sweden, for ICP-AES *and* ICP-SFMS analyses. The project was financially supported by SKB (Swedish Nuclear Fuel and Waste Management Co).
