14 Cesium ( 137 Cs and 133 Cs ) , Potassium and Rubidium in Macromycete Fungi and Sphagnum Plants

1.1 Cesium ( 137 Cs and 133 Cs), potassium and rubidium in macromycete fungi Radiocesium (137Cs) released in the environment as result of nuclear weapons tests in the 1950s and 1960s, and later due to the Chernobyl accident in 1986, is still a critical fission product because of its long half-life of 30 years and its high fission yield. The study of the cesium radioisotope 137Cs is important, as production and emission rates are much higher than other radioisotopes. This chapter comprises results obtained in several experiments in Swedish forest ecosystems and aims to discuss the behavior of cesium isotopes (137Cs and 133Cs) and their counterparts potassium (K) and rubidium (Rb) in the ”soil-fungi-plants transfer“ system. The chapter consists of two parts: one mainly dealing with 137Cs, 133Cs, K and Rb in forest soil and macromycete fungi, and the other with the same isotopes in separate segments of Sphagnum plants. The bioavailability of radionuclides controls the ultimate exposure of living organisms and the ambient environment to these contaminants. Consequently, conceptually and methodologically, the understanding of bioavailability of radionuclides is a key issue in the field of radioecology. Soil-fungi-plants transfer is the first step by which 137Cs enters food chains.

and in the uptake of nutrients from soil into plants via the formation of symbiotic mycorrhizal associations (Read & Perez-Moreno, 2003).The fungi facilitate nutrient uptake into the host plant, both as a consequence of the physical geometry of the mycelium and by the ability of the fungi to mobilize nutrients from organic substrates through the action of extracellular catabolic enzymes (Leake & Read, 1997).In addition to acquiring essential macronutrients, mycorrhizal fungi are efficient at taking-up and accumulating microelements (Smith & Read, 1997), this ability results in the accumulation of non-essential elements and radionuclides, particularly 137 Cs and can have important consequences for the retention, mobility and availability of these elements in forest ecosystems (Steiner et al., 2002).Although fungal biomass, in comparison to plant biomass, is relatively low in forest soil (Dighton et al., 1991;Tanesaka et al., 1993), many fungal species accumulate more 137 Cs than vascular plants do and 137 Cs activity concentrations in many fungi are 10 to 100 times higher than in plants (Rosén et al., 2011).Fungi (particularly sporocarps) accumulate 137 Cs against a background of low 137 Cs activity concentrations, thus, the contribution of fungi to 137 Cs cycling in forest systems is substantial.Fungi are important in radiocesium migration in nutrient poor and organic rich soils of forest systems (Rafferty et al., 1997).In organic matter, the presence of single strains of saprotrophic fungi considerably enhances the retention of Cs in organic systems (Parekh et al., 2008): ≈ 70% of the Cs spike is strongly (irreversibly) bound (remains non-extractable) compared to only ≈ 10% in abiotic (sterilized) systems.Fungal mycelium may act as a sink for radiocesium (Dighton et al., 1991;Olsen et al., 1990), as it contains 20-30% 137 Cs in soil inventories, and as much as 40% of radiocesium can leached from irradiated samples compared to control samples (Guillitte et al., 1994).Mycelium in upper organic soil layers may contain up to 50% of the total 137 Cs located within the upper 0-10 cm layers of Swedish and Ukrainian forest soils (Vinichuk & Johanson 2003).In terms of the total radiocesium within a forest ecosystem, fungal sporocarps contain a small part of activity and may only account for about 0.5 % (McGee et al., 2000) or even less − 0.01 to 0.1% (Nikolova et al., 1997) of the total radiocesium deposited within a forest ecosystem.However, these estimates are based on the assumption radionuclide concentration in fungal sporocarps is similar to that of the fungal parts of mycorrhizae (Nikolova et al., 1997).The activity concentration in sporocarps is probably higher than in the mycelium (Vinichuk & Johanson, 2003, 2004) and sporocarps constitute only about 1% of the total mycelia biomass in a forest ecosystem.Due to the high levels of 137 Cs in sporocarps, their contribution to the internal dose in man may be high through consumption of edible mushrooms (Kalač, 2001).Consequently, the consumption of sporocarps of edible fungi (Skuterud et al., 1997) or of game animals that consumed large quantities of fungi with high 137 Cs contents (Johanson & Bergström, 1994) represents an important pathway by which 137 Cs enters the human food system.The 137 Cs activity concentration in edible fungi species has not decreased over the last 20 years (Suillus variegatus) or significantly increased (Cantharellus spp.) (Mascanzoni, 2009;Rosén et al., 2011).

1.1.2
137 Cs, 133 Cs and alkali metals in fungi Although fungi are important for 137 Cs uptake and migration in forest systems and since the Chernobyl accident, fungal species may contain high concentrations of radiocesium, the reasons and mechanisms for the magnitude higher concentration of radiocesium in fungi bioavailable fraction of soils within forest ecosystems is reported Karadeniz & Yaprak (2007) but in cultivated soils, equilibrium between fallout 137 Cs and stable 133 Cs among exchangeable, organic bound and strongly bound fractions has not reached, even though most 137 Cs was deposited on the soils more than 20 years before (Tsukada, 2006).The important roles fungi play in nutrient uptake in forest soils, in particular its role in 137 Cs transfer between soil and fungi, requires better understanding of the mechanisms involved.Although transfer of radioactive cesium from soils to plants through fungi is well researched, there is still limited knowledge on natural stable 133 Cs and other alkali metals (K and Rb) and the potential role as a predictor for radiocesium behavior, and less is known about the relationships between 133 Cs and other alkali metals (K and Rb) during uptake by fungi.To explore mechanisms governing the uptake of radionuclides ( 137 Cs) data on uptake of stable isotopes of alkali metals (K, Rb, 133 Cs) by fungal species, and the behavior of the three alkali metals K, Rb and 133 Cs in bulk soil, fungal mycelium and sporocarps are required.Therefore, an attempt was made to quantify the uptake and distribution of the alkali metals in the soil-mycelium-sporocarp compartments and to study the relationships between K, Rb and 133 Cs in the various transfer steps.Additionally, the sporocarps of ectomycorrhizal fungi Suillus variegatus were analyzed to determine whether i) Cs ( 133 Cs and 137 Cs) uptake was correlated with K uptake; ii) intraspecific correlation of these alkali metals and 137 Cs activity concentrations in sporocarps was higher within, rather than among different fungal species; and, iii) the genotypic origin of sporocarps affected uptake and correlation.Substantial research in this area has been conducted in Sweden after the fallout from nuclear weapons tests and the Chernobyl accident.Some results are published in a series of several articles in collaboration with Profs K.J. Johanson, H. Rydin and Dr. A. Taylor (Vinichuk et al., 2004;2010a;2010b;2011).This chapter aims to summarize the acquired knowledge from studies in Sweden and to place them in a larger context.The results are summarized and discussed and address the issues of K, Rb and 133 Cs concentrations in soil fractions and fungal compartments (Section 1.3.);concentration ratios of K, Rb and 133 Cs in soil fractions and fungi (Section 1.4); relationships between K, Rb and 133 Cs in soil and fungi (Section 1.5); the isotopic (atom) ratios 137 Cs/K, 137 Cs/Rb and 137 Cs/ 133 Cs in fungal species (Section 1.6); K, Rb and Cs ( 137 Cs and 133 Cs) in sporocarps of a single species (Section 1.7); mechanisms of 137 Cs and alkali metal uptake by fungi (Section 1.8); Cs ( 137 Cs and 133 Cs), K and Rb in Sphagnum plants (Section 2); distribution of Cs ( 137 Cs and 133 Cs), K and Rb within Sphagnum plants (Section 2.3); mass concentration and isotopic (atom) ratios between 137 Cs, K, Rb and 133 Cs in segments of Sphagnum plants (Section 2.4); relationships between 137 Cs, K, Rb and 133 Cs, in segments of Sphagnum plants (Section 2.5); mechanisms of 137 Cs and alkali metal uptake by Sphagnum plants (Section 2.6); and conclusions from the Swedish studies (Section 3).Before presenting and discussing results a short description of study area, study design and methods used is presented (section 1.2).

Study area, study design and methods for results presented 1.2.1 Study area
The K, Rb and 133 Cs concentrations in soil fractions and fungal compartments were studied in an area located in a forest ecosystem on the east coast of central Sweden (60°22′N, 18°13′E).The soil was a sandy or clayey till and the humus mainly occurred in the form of mull.A more detailed description of the study area is presented by Vinichuk et al. (2010b).

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Sporocarps of ectomycorrhizal fungi Suillus variegatus was studied in an area located about 40 km north-west of Uppsala in central Sweden (N 60°08'; E 17°10').The forest is located on moraine and is dominated by Scots pine (Pinus sylvestris) and Norway spruce (Picea abies), with inserts of deciduous trees, primarily birch (Betula pendula and Betula pubescens).The field layer consisted mainly of the dwarf shrubs bilberry (Vaccinium myrtillus L.), lingonberry (Vaccinium vitis-idaea L.) and heather Calluna vulgaris L.): for details about the area and sampling see Dahlberg et al. (1997).

Study design
For studies of K, Rb and 133 Cs concentrations in soil fractions and fungal compartments, samples of soil and fungal sporocarps were collected from 10 sampling plots during September to November 2003.Four replicate soil samples were taken, with a cylindrical steel tube with a diameter of 5.7 cm, from around and directly underneath the fungal sporocarps (an area of about 0.5 m 2 ) and within each 10 m 2 area to a depth of 10 cm.Soil cores were divided horizontally into two 5-cm thick layers.Sporocarps of 12 different fungal species were collected and identified to species level, and the 137 Cs activity concentration in fresh material was determined.The sporocarps were dried at 35°C to constant weight and concentrations of 133 Cs, K and Rb were determined.A selection of dried sporocarps of S. variegatus (n=51), retained from a study by Dahlberg et al. (1997) on the relationship between 137 Cs activity concentrations and genotype identification, was used.The sporocarps were collected once a week during sporocarp season (end of August through September) in 1994 and were taken from five sampling sites (100 to 1600 m 2 in size) within an area of about 1 km 2 .Eight genotypes with 2 to 8 sporocarps each were tested (in total 32 sporocarps) and are referred to here as individual genotypes.Sporocarps within genotypes were spatially separated by up to 10-12m.All genotypes were used for the estimation of correlation coefficients, but only genotypes with at least four sporocarps were included in the alkali metal analysis.In addition, 19 individual sporocarps with unknown genotype (i.e.not tested for genotype identity) were included: these sporocarps consisted of both the same and different genotypes.The combined set of sporocarps refers to all sporocarps: for further details about the sampling and identification of genotypes see Dahlberg et al. (1997).The 137 Cs activity concentration values corrected to sampling date and expressed as kBq kg −1 dry weight (DW) for each sporocarp, as reported by Dahlberg et al. (1997), were used. 133Cs concentrations in soil fractions and fungal compartments, fungal mycelia were separated from the soil samples (30-50 g, 0-5 cm layer depth) under a dissection microscope (magnification X64) with forceps and by adding small amounts of distilled water to disperse the soil.The prepared fraction of mycelium (30−60 mg DW g -1 soil) was not identified to determine of the mycelia extracted from the soil samples and the sporocarps belonged to the same species, as it assumed a majority of the prepared mycelia belonged to the same species as the nearby sporocarps.The method for mycelium preparation is described in Vinichuk & Johanson (2003).Mycelium samples were dried at 35°C to constant weight for determination of K, Rb and 133 Cs.

Methods
The soil samples (0-5 cm layer) were partitioned by the method described in Gorban & Clegg (1996).First, soil was gently sieved through a 2 mm mesh giving a bulk soil fraction.The remaining soil aggregates containing roots were further crumbled and gently squeezed between the fingers: this was called the rhizosphere fraction.The residue (finest roots with adhering soil particles) was called the soil-root interface fraction.Nine samples of bulk soil fraction and mycelium, 12 samples of fungal sporocarps, and six samples of rhizosphere and soil-root interface fraction were analyzed for K, Rb and 133 Cs.The 137 Cs activity concentrations in the bulk soil samples and sporocarps were determined with calibrated HP-Ge detectors, corrected to sampling date and expressed as Bq kg −1 DW.The measuring time employed provided a statistical error ranging between 5 and 10%.For element analyses, a 2.5 g portion of each sample was analyzed by inductively coupled plasma in the laboratories of ALS Scandinavia (Luleå, Sweden) with recoveries 97-101% for K; 97.5-99.4% for Rb, and 93.7-102.5% for, 133 Cs.For soil, CRM SO-2 (heavy metals in soil) was used which had no certified values for K, Rb or 133 Cs.Element concentrations in the analyzed fractions are reported as mg kg −1 DW.For element analyses (K, Rb and 133 Cs) of S. variegatus sporocarps, aliquots of about 0.3 g of each sample were analyzed by the same technique.Element concentrations are reported as mg kg − 1 DW and the isotopic ratio of 137 Cs/ 133 Cs was calculated with Equations 1 and 2 (Chao et al., 2008): where: A is the 137 Cs radioactivity (Bq kg −1 ); λ is the disintegration rate of 137 Cs 7.25 x10 −10 s −1 ; a is the atomic weight of cesium (132.9);N is the Avogadro number, which is 6.02 x10 23 ; and, 133 C and C are the 133 Cs concentration (mg g −1 ).Eq. ( 1) can be simplified to Eq. 2: where: A is the 137 Cs activity concentration in Bq kg −1 and 133 C is the 133 Cs concentration in mg kg −1 .Thus, the units of the isotope ratio are dimensionless.Relationships between K, Rb, 133 Cs and 137 Cs concentrations in different fractions and sporocarps of S. variegatus were identified by Pearson correlation coefficients.Correlation coefficients were analyzed in five separate sets of samples: in four sets, all samples had known genotype identity, and in the last set, there was a combined set of samples containing both genotypes that had been tested by somatic incompatibility sporocarps and genotypes that had not been tested.Correlation analyses for genotypes with three or less sporocarps were omitted.All statistical analyses were run with Minitab® 15.1.1.0.(© 2007 Minitab Inc.) software, with level of significance of 5% (0.05), 1% (0.01) and 0.1% (0.001).

K, Rb and 133
Cs concentrations in soil fractions and fungal compartments K, Rb and 133 C s c o n c e n t r a t i o n s v a l u e s i n s o i l f r actions and fungal compartments are necessary for calculating the concentration ratio at each step of its transfer in the soil-fungi system, differences in the uptake between elements and the relationships.This in turn will be the main reason for the different K, Rb and 133 Cs concentrations observed in sporocarps of various fungal species.Concentrations of K, Rb and 133 Cs in bulk soil were not significantly different from those in the rhizosphere, although the values for all three elements were slightly higher in the rhizosphere fraction (Table 1 Table 1.Mean concentrations of K, Rb and 133 Cs (mg kg −1 DW (standard deviation)) in soil fractions and fungi 1 .
Potassium concentrations were higher in both the soil-root interface and fungal mycelium fractions than in the bulk soil and rhizosphere fraction.A comparison of K, Rb and 133 Cs concentrations revealed fungal sporocarps accumulated much greater amounts of these elements than mycelium.For example, K concentrations in fungal sporocarps collected from the same plots where soil samples and mycelium were extracted were about 15 times higher than K concentrations found in mycelium.The concentrations of Rb in fungal sporocarps were about 18-fold higher than in corresponding fungal mycelium, and those of 133 Cs were about 7-fold higher (Table 1).Thus, potassium concentration increased in the order bulk soil<rhizosphere<fungal mycelium<soil-root interface<fungal sporocarps and was higher in the soil-root interface fraction and fungi than in bulk soil.The high concentrations of K in fungal sporocarps may reflect a demand for this element as a major cation in osmoregulation and that K is an important element in regulating the productivity of sporophore formation in fungi (Tyler, 1982).Rb in mycelium was 3.5-fold higher than in bulk soil and 2.5-fold higher than in rhizosphere, and concentrations increased in the order bulk soil<rhizosphere<soil-root interface<fungal mycelium<fungal sporocarps.The concentrations of Rb were slightly higher in the soil-root interface fraction than in bulk soil; thus, fungi appeared to have high preference for this element, as the accumulation of Rb by fungi, and especially fungal sporocarps, was pronounced.Rubidium concentrations in sporocarps were more than one order of magnitude higher than those in mycelium extracted from soil of the same plots where fungal sporocarps were sampled.The ability of fungi to accumulate Rb is documented: mushrooms accumulate at least one order of magnitude higher concentrations of Rb than plants growing in the same forest (Yoshida & Muramatsu, 1998).Concentrations of stable cesium varied considerably among samples but no significant differences were found among the different fractions analyzed.Cesium concentrations increased in the order soil-root interface<bulk soil<rhizosphere<fungal mycelium<fungal sporocarps, and were only significantly higher in fungal sporocarps, compared with bulk soil.Stable 133 Cs was generally evenly distributed within bulk soil, rhizosphere and soil-root interface fractions, indicating no 133 Cs enrichment in those forest compartments.However, 133 Cs concentrations in sporocarps were nearly one order of magnitude higher than those found in soil mycelium.
The differences between fungal species in their preferences for uptake of 137 Cs or stable 133 Cs appear to reflect the location of the fungal mycelium relative to that of cesium within the soil profile (Rühm et al., 1997).Unlike 137 Cs, stable 133 Cs originates from soil; therefore, the amount of unavailable 133 Cs, compared to the total amount of 133 Cs, in soil presumably higher than that of 137 Cs.As a result, stable 133 Cs is considered less available for uptake as it is contained in mineral compounds and is difficult for fungi or plants to access: the concentration ratio of stable 133 Cs in mushrooms is lower than for 137 Cs (Yoshida & Muramatsu, 1998).The differing behavior of the natural and radioactive forms of 133 Cs may derive from their disequilibrium in the ecosystem (Horyna & Řanad, 1988).

Concentration ratios of K, Rb and 133 Cs in soil fractions and fungi
The concept of concentration ratios (CR, defined as concentration of the element (mg kg −1 DW) in a specific fraction or fungi divided by concentration of the element (mg kg −1 DW) in bulk soil) is widely used to quantify the transfer of radionuclides from soil to plants/fungi.This approach allows the estimation of differences in uptake of elements.
The elements concentration ratio data followed a similar pattern, but the enrichment of all three elements in fungal material was more evident, particularly in the sporocarps (Table 2).
Thus, for all three alkali metals studied, the levels of K, Rb, 133 Cs and 137 Cs in sporocarps were at least one order of magnitude higher than those in fungal mycelium (Table 2).The concentration ratios for each element varied considerably between the species sampled.The saprotrophic fungus Hypholoma capnoides had the lowest values and the mycorrhizal fungus Sarcodon imbricatus had the highest.Sporocarp:bulk soil concentration ratios are presented in Table 3. Sarcodon imbricatus accumulates nearly 100 000 Bq kg -1 of 137 Cs, giving TF values (defined as 137 Cs activity concentration (Bq kg −1 DW) in fungi divided by 137 Cs deposition (kBq m −2 )) about 22 (Vinichuk & Johanson, 2003).The sporocarps of Sarcodon imbricatus had distinctively high concentration ratios of Rb and 133 Cs than other species analyzed.The mycorrhizal fungus Cantharellus tubaeformis, is another species showing relatively high concentration ratios, particularly for K and Rb.Cantharellus tubaeformis accumulates several tens of thousands Bq kg -1 of 137 Cs (Kammerer et al., 1994).Among those with moderate concentration ratios for each element are Boletus edulis, Tricholoma equestre, Lactarius scrobiculatus and Cortinarius spp.Thus, the levels of K, Rb, 133 Cs and 137 Cs in sporocarps were at least one order of magnitude higher than those in fungal mycelium indicating biomagnification through the food web in forest ecosystems. 1 Saprophyte, all other analyzed fungal species are ectomycorrhizal Table 3. Element concentration ratios (mg kg −1 DW in fungi)/(mg kg −1 DW in bulk soil) in fungi for fungal sporocarps.

Relationships between K, Rb and 133 Cs in soil and fungi
Although correlation analysis may be not definitive, it is a useful approach for elucidating similarities or differences in uptake mechanisms of cesium ( 137 Cs and 133 Cs), K and Rb: close correlation between elements indicates similarities in their uptake mechanisms.No significant correlations between K in soil and in either mycelium (r=0.452,ns) or in sporocarps (r=0.338,ns) has been identified and sporocarp Rb and 133 Cs concentrations were unrelated to soil concentrations, however, in mycelium both elements were correlated with soil concentrations (Rb: r=0.856, p=0.003;Cs: r=0.804, p=0.009).There was a close positive correlation (r=0.946,p=0.001) between the K:Rb ratio in soil and in fungal mycelium (Figure 1b) and this relationship was also apparent between soil and sporocarps, but was weak and not significant (r=0.602,ns: Figure1b).The K: 133 Cs ratio in soil and fungal components had a different pattern: the K:Cs ratio in mycelium was closely positively correlated (r=0.883,p=0.01) to the K: 133 Cs ratio in soil (Figure 1a), but was relatively weakly and non-significantly correlated to soil in fungal sporocarps.No significant correlations were found between the concentrations of the three elements in fungi, soil pH or soil organic matter content (data not shown).The competition between K, Rb and 133 Cs in the various transfer steps was investigated in an attempt to estimate the relationships between the concentrations of these three elements in soil, mycelia and fungal sporocarps.The lack of a significant correlation between K in soil and in either mycelium or sporocarps indicated a demand for essential K in fungi, regardless of the concentration of this element in soil.Regardless of fungal species, K concentration in fungi appears to be controlled within a narrow range, (Yoshida & Muramatsu, 1998), and supports the claim K uptake by fungi is self-regulated by the internal nutritional requirements of the fungus (Baeza et al., 2004).The relationships observed between K:Rb and K: 133 Cs ratios in fungal sporocarps and soil mycelia, with respect to the soil in which they were growing (Figure 1), also indicated differences in uptake of these alkali metals by fungi.Although correlation analyses is not the best tool for analyzing the uptake mechanism, the closest positive correlations between K:Rb ratios in fungal mycelium and in soil indicated similarities in the uptake mechanism of these two elements by fungi, although the relationships between K: 133 Cs ratios in soil mycelium and in soil were less pronounced.These findings were in good agreement with the suggestion by Yoshida & Muramatsu (1998) that there might be an alternative pathway for 133 Cs uptake into cells and the mechanism of 133 Cs uptake by fungi could be similar to that for Rb, as 133 Cs does not show a good correlation with K.The high efficiency of Rb uptake by fungi indicates Rb, but not 133 Cs, eventually replaces essential K due to K limitation (Brown & Cummings, 2001) and Rb has the capacity to partially replace K, but 133 Cs does not (Wallace, 1970 and references therein).Forest plants apparently discriminate between K+ and Rb+ in soils and a shortage of K+ favors the uptake of the closely related Rb+ ion (Nyholm & Tyler, 2000), whereas, increasing K+ availability in the system decreases Rb+ uptake (Drobner & Tyler, 1998).These results provided new insights into the use of transfer factors or concentration ratios.

The isotopic (atom) ratios
137 Cs/K, 137 Cs/Rb and 137 Cs/ 133 Cs in fungal species The isotopic ratios of 137 Cs/K, 137 Cs/Rb and 137 Cs/ 133 Cs in the fungal sporocarps belonging to different species were used to interpret the distribution of 137 Cs and the alkali metals in fungi and to provide better understanding of its uptake mechanisms.Measurements of trace levels of stable 133 Cs could be another way of obtaining information about the biological behavior of 137 Cs.To obtain better estimates, the isotopic ratios for fungal sporocarps in this  (Vinichuk et al., 2010b) were calculated and compared with estimates calculated in similar studies by Yoshida & Muramatsu (1998).Mean values of isotopic ratios of 137  The activity concentrations of 137 Cs in fungal sporocarps were about 13 to 16 orders of magnitude lower than mass concentrations of K, 10 to 13 orders of magnitude lower than mass concentrations for Rb, and 8 to 9 orders of magnitude lower than mass concentrations for 133 Cs.Isotopic (atom) ratios in the fungal sporocarps collected in Sweden were two-three orders of magnitude narrower than those collected in Japan, which reflected the level of 137 Cs concentrations in mushrooms: the median value for all fungi species was 4151 Bq kg −1 DW in Swedish forests and 135 Bq kg −1 DW in Japanese forests.Isotopic (atom) ratios of 137 Cs/K, 137 Cs/Rb, 137 Cs/ 133 Cs were variable in both datasets and appeared independent of specific species of fungi.These ratios might reflect the isotopic ratios of soil horizons from which radiocesium is predominantly taken up and be a possible source of the variability in isotopic ratios in fungal fruit bodies.Rühm et al. (1997) used the isotopic ratio 134 Cs/ 137 Cs to localize mycelia of fungal species in situ; alternatively, the isotopic (atom) ratio 137 Cs/ 133 Cs can be used to localize fungal mycelia in situ.However, this approach is only appropriate for organic soil layers, which contain virtually no or very little clay mineral to which cesium can bind.The isotopic ratios 137 Cs/ 133 Cs in fruit bodies of fungi were similar to those found in organic soil layers of forest soil (Rühm et al., 1997;Karadeniz & Yaprak, 2007).
The relationships observed between the concentration ratios 137 Cs/ 133 Cs and K, Rb and 133 Cs in fungal sporocarps also varied widely and were inconsistent (Table 4).The concentration of K, Rb and 133 Cs in sporocarps appeared independent of the 137 Cs/ 133 Cs isotopic ratio, suggesting differences in uptake of these alkali metals by fungi and complex interactions between fungi, their host and the environment.

K, Rb and Cs ( 137 Cs and 133 Cs) in sporocarps of a single species
Most results presented in this Chapter are already published (Vinichuk et al., 2011), and are based on sporocarp analysis of different ectomycorrhizal and saprotrophic fungal species.Fungal accumulation of 137 Cs is suggested to be species-dependent, thus, 137 Cs activity concentration and mass concentration of K, Rb and 133 Cs in fungal sporocarps belonging to the mycorrhizal fungus Suillus variegatus were analyzed.S. variegatus form mycorrhiza with Scots pine and predominantly occur in sandy, acidic soils and have a marked ability to accumulate radiocesium (Dahlberg et al., 1997): as this is an edible mushroom, high radiocesium contents present some concern with regard to human consumption.The concentrations of K (range 22.2-52.1 g kg −1 ) and Rb (range 0.22-0.65 g kg −1 ) in sporocarps of S. variegatus varied in relatively narrow ranges, whereas, the mass concentration of 133 Cs had a range of 2.16 to 21.5 mg kg −1 and the activity concentration of 137 Cs ranged from 15.8 to 150.9 kBq kg −1 .Both 133 Cs and 137 Cs had wider ranges than K or Rb within sporocarps from the same genotype or across the combined set of sporocarps (Table 5).The mean of the 137 Cs/ 133 Cs isotopic ratio in the combined set of sporocarps was 2.5 x 10 −7 (range 8.3 x 10 −8 and 4.4 x 10 −7 ).The 137 Cs/Cs isotopic ratios from identified genotypes were site-genotype dependent: the ratio values of genotypes at site 4 were about two-times higher than the ratios of genotypes at site 2 (Table 6).-1 2-2 4-3 4-4 4-5 4-6 7-7 6-  Similarly, in results obtained from a previous study (Vinichuk et al. 2004)  Thus, the study of S. variegatus revealed no significant correlations between 133 Cs mass concentration or 137 Cs activity concentration and the concentration of K in sporocarps, either within the whole population or among the genotypes.Potassium, 133 Cs and 137 Cs within the four genotypes were also not correlated, with one genotype exception (Table 7).However, the exception was conditional due to a one single value.Three of four analyzed sporocarp genotypes had high correlation between K and Rb: the forth was only moderately correlated (Table 7).However, the correlations between 137 Cs and K and Rb and 133 Cs in the four genotypes were inconsistent (Table 3).Potassium, Rb, 133 Cs and 137 Cs were correlated in genotype 2-1 (due to one single value), whereas, no or negative correlations were found between the same elements/isotopes for the other three genotypes.In two of four genotypes, the 137 Cs/ 133 Cs isotopic ratio was not correlated with 133 Cs, K or Rb; however, there was a negative correlation with Rb in one genotype (2-2) and positive correlation with 133 Cs in another (4-3) (Table 7).Data obtained for S. variegatus supported results from earlier studies (Ismail, 1994;Yoshida & Muramatsu, 1998) on different species of fungi, suggesting cesium ( 137 Cs and 133 Cs) and K are not correlated in mushrooms.Thus, correlation analysis may be a useful, although not definitive, approach for elucidating similarities or differences in uptake mechanisms of cesium ( 137 Cs and 133 Cs) and K.The concentration of K in sporocarps appeared independent of the 137 Cs/ 133 Cs isotopic ratio in both the whole population (Figure 3) and among the genotypes, with one exception (Table 7).The absence of correlation between 137 C (or 133 Cs) and K in fungi may be due to the incorporation of K being self-regulated by the nutritional requirements of the fungus, whereas, incorporation of 137 Cs is not self-regulated by the fungus (Baeza et al., 2004).Although K and cesium ( 133 Cs and 137 Cs) concentrations did not correlate within S. variegatus, both K + and Cs + ions may compete for uptake by fungi.In experiments under controlled conditions and with sterile medium (Bystrzejewska-Piotrowska & Bazala, 2008), the competition between Cs + and K + depends on Cs + concentration in the growth medium and on the path of Cs + uptake.In studies of Cs uptake by hyphae of basidiomycete Hebeloma vinosophyllum when grown on a simulated medium (Ban-Nai et al., 2005), the addition of monovalent cations of K + , Rb + , and NH 4 + reduced uptake of Cs.In addition, radiocesium transport by arbuscular mycorrhizal (AM) fungi decreases if K concentration increases in a compartment accessible only to AM (Gyuricza et al., 2010), and a higher Cs:K ratio in the nutrient solution increases uptake of Cs by ectomycorrhizal seedlings (Brunner et al., 1996).

Site
A noticeable (20-60%) and long-lasting (at least 17 years) reduction in 133 Cs activity concentration in fungal sporocarps in situ due to a single K fertilization of 100 kg ha −1 in a Scots pine forest is reported by Rosén et al., (2011).The relation between 137 Cs and K, and Rb and 133 Cs within S. variegatus (Figure 2) was similar to an earlier report on different species of fungi (Yoshida & Muramatsu, 1998).Rubidium concentration in sporocarps was positively correlated with 133 Cs and 137 Cs, but generally negatively correlated with 137 Cs/ 133 Cs isotopic ratio, i.e. a narrower 137 Cs/ 133 Cs ratio in sporocarps resulted in higher Rb uptake by fungi.This ratio may reflect the soil layers explored by the mycelia (Rühm et al., 1997), as fungi have a higher affinity for Rb than for K and cesium (Ban-Nai et al., 2005;Yoshida & Muramatsu, 1998), and Rb concentrations in sporocarps can be more than one order of magnitude greater than in mycelium extracted as fungal sporocarps from soil of the same plots (Vinichuk et al., 2011).Soil mycelia always consist of numerous fungal species and the intraspecific relationships between soil mycelia and sporocarps has not yet been estimated; however, the development of molecular methods with the ability to mass sequence environmental samples in combination with quantitative PCR may now enable such analysis to be conducted.Mass concentration of 133 Cs and activity concentration of 137 Cs have different relations in fungal sporocarps: in three of four genotypes, there was a high correlation, two of which were significant (r=0.908** and r=979*), and there was no correlation in the fourth genotype (r=−0.263,Table 7), whereas, correlation between 137 Cs and 133 Cs within the whole population was only moderate (r=0.605*** Figure 2).In terms of 133 Cs and 137 Cs behavior, there would be no biochemical differentiation, but there could be differences in atom abundance and isotopic disequilibrium within the system.Fungi have large spatiotemporal variation in 133 Cs and 137 Cs content in sporocarps of the same species and different species (de Meijer et al., 1988), and the variation in K, Rb, 133 Cs and 137 Cs concentrations within a single genotype appeared similar, or lower, than the variation within all genotypes.The results for 137 Cs and alkali elements in a set of samples of S. variegatus, collected during the same season and consisting of sporocarps from both different and the same genotype, indicated the variability in concentrations was similar to different fungal species collected in Japan over three years (Yoshida & Muramatsu, 1998).The relatively narrow range in K and Rb variation and the higher 133 Cs and 137 Cs variations might be due to different mechanisms being involved.The differences in correlation coefficients between 137 Cs and the alkali metals varied among and within the genotypes of S. variegatus, suggesting both interspecific and intrapopulation variation in the uptake of K, Rb, stable 133 Cs and, 137 Cs and, their relationships could be explained by factors other than genotype identity.The variability in 137 Cs transfer depends on the sampling location of fungal sporocarps (Gillett & Crout, 2000), for S. variegatus, these interaction factors might include the spatial pattern of soil chemical parameters, heterogeneity of 137 Cs fallout, mycelia location, and heterogeneity due to abiotic and biotic interactions increasing over time (Dahlberg et al., 1997).Within the combined set of sporocarps the concentration of Rb and 137 Cs activity concentration in S. variegatus sporocarps were normally distributed but the frequency distribution of 133 Cs and K was not: asymmetry of 137 Cs frequency distributions is reported in other fungal species (Baeza et al., 2004;Gaso et al., 1998;Ismail, 1994).According to Gillett & Crout (2000), the frequency distribution of 137 Cs appears species dependent: high accumulating species tend to be normally distributed and low accumulating species tend to be log-normally distributed.However, lognormal distribution is almost the default for concentration of radionuclides and is unlikely to be a species-specific phenomenon, as it also occurs in soil concentrations, which implies normal distribution would not be expected, even if large set of samples were analyzed.

Mechanisms of 137 Cs and alkali metal uptake by fungi
Generally, little is known about the mechanisms involved in the uptake and retention of radionuclides by fungi.Studies of uptake mechanisms and affinity for alkali metals in fungi are scarce, but some results are reviewed by Rodríguez-Navarro (2000).Compared to plants, fungal fruit bodies can be characterized by high 137 Cs, 133 Cs and Rb concentrations and low calcium (Ca) and strontium (Sr) concentrations.In a laboratory experiment with the woodinhabiting mushroom Pleurotus ostreatus (Fr.)Kummer Y-l (Terada et al., 1998), 137 Cs uptake by mycelia decreased with increasing of 133 Cs, K or Rb concentration in the media, and K uptake by mycelia decreased with increasing of 133 Cs concentration.In an experiment with pure cultures of mycorrhizal fungi (Olsen et al., 1990) some species had preference for Cs over K and in the experiments with yeast (Conway & Duggan, 1958), K had preference over Cs and the affinity for alkali metal uptake decreased in the order K + < Rb + < Cs + followed by Na + and Li + , with a relative ratio of 100:42:7:4:0.5.Fungi (mycelium and sporocarps) have a higher affinity for uptake of Rb and K to Cs, and based on the CR values for fungal sporocarps (Table 3), alkali metal can be ranked in the order Rb + > K + > Cs + , with a relative ratio of 100:57:32, which is within the range of 100:88:50 derived by Yoshida & Muramatsu (1998).The affinity for an alkali metal depends on the nutritional status of the organism, which at least partly explains differences reported between field experiments and laboratory experiments with a good nutrient supply.The mycorrhizal species Sarcodon imbricatus was found to be the most efficient in accumulating K, Rb and Cs, which was in agreement with results obtained by Tyler (1982), where a mean CR for Rb in litter decomposing fungus Collybia peronata was reported to be 41, and the mean CR for Rb in Amanita rubescens, which is mycorrhizal with several tree species, was above 100.However, lower 40 K content for mycorrhizal species is reported by Römmelt et al. (1990), which means mycorrhizal species do not necessarily accumulate alkali metals more efficiently than saprotrophic ones.Accumulation of stable and radioactive cesium by fungi is apparently species-dependent but is affected by local environmental conditions.According to de Meijer et al. (1988), the variation in concentrations of stable and radioactive cesium in fungi of the same species is generally larger than the variation between different species and the variation in 137 Cs levels within the same genet of S. varegatus is as large as within non-genet populations of the species (Dahlberg et al., 1997), suggesting both interspecific and intrapopulation variation in the uptake of K, Rb, stable 133 Cs and 137 Cs, and that their relationships can be explained by factors other than genotype identity (Vinichuk et al., 2011).There is about two orders of magnitude variation in Cs uptake, with the highest CR value in e.g. S. imbricatus (256) and the lowest in Lactarius deterrimus (2.6), although other studies (Seeger & Schweinshaut, 1981) report the highest accumulation of stable Cs is in Cortinarius sp.

Introduction
Peatlands are areas where remains of plant litter have accumulated under water-logging as a result of anoxic conditions and low decomposability of the plant material.They are generally nutrient-poor habitats, particularly temperate and boreal bogs in the northern hemisphere, in which peat formation builds a dome isolating the vegetation from the surrounding groundwater.Hence, bogs are ombrotrophic, i.e. all water and nutrient supply to the vegetation is from aerial dust and precipitation, resulting in an extremely nutrientwww.intechopen.com

Cesium (
137 Cs and 133 Cs), Potassium and Rubidium in Macromycete Fungi and Sphagnum Plants 297 poor ecosystem often formed and dominated by peat mosses (Sphagnum).Sphagnumdominated peatlands with some groundwater inflow (i.e.weakly minerotrophic 'poor fens') are almost as nutrient poor and acid as true bogs.Sphagnum plants absorb and retain substantial amounts of fallout-derived radiocesium, and some attention has been given to the transfer of the radioactive cesium isotope 137 Cs within raised bogs (Bunzl & Kracke, 1989;Rosén et al., 2009), and relatively high 137 Cs bioavailability to bog vegetation and mosses in particular are found (Bunzl & Kracke, 1989).The transfer of 137 Cs within a peatland ecosystem is different from that in forest or on agricultural land.In soils with high clay content, there is low bioavailability and low vertical migration rate of radiocesium due to binding to some clay minerals (Cornell, 1993).In nutrientpoor but organic-matter-rich forest soils, the vertical migration rate of 137 Cs is also low, but bioavailability is often high, particularly for mycorrhizal fungi (Olsen et al., 1990;Vinichuk & Johansson, 2003;Vinichuk et al., 2004;2005).In forests and pastures, extensive fungal mycelium counteracts the downward transport of 137 Cs by an upward translocation flux (Rafferty et al., 2000); this results in a slow net downward transport of 137 Cs in the soil profile.In peatlands, 137 Cs appears to move through advection in peat water (review by Turetsky et al., 2004).Small amounts of clay mineral in the peat reduce Cs mobility (MacKenzie et al., 1997), but most Sphagnum peat is virtually clay mineral free organic matter.In wet parts of open peatlands that lack fungal mycelium, the downward migration of 137 Cs in the Sphagnum layers is expected to be faster than in forest soil and Cs is continuously translocated towards the growing apex of the Sphagnum shoots, where it is accumulated.Some attempts have been made to investigate whether 137 C is associated with essential biomacromolecules in mosses and to determine the 137 Cs distribution among intracellular moss compartments (Dragović et al., 2004).The chemical behavior of radiocesium is expected to be similar to that of stable 133 Cs and other alkali metals, i.e.K, Rb, which have similar physicochemical properties.Moreover, stable 133 Cs usually provides a useful analogy for observing long-term variation and transfer parameters of 137 Cs in a specific environment, particularly in peatlands that are cut off from an input of stable Cs from the mineral soil.As the relationship between K and Rb in fungi is not clearly understood, whether Cs follows the same pathways as K in Sphagnum is also unclear.Thus, the 137 Cs activity concentration and mass concentration of K, Rb and 133 Cs was analyzed within individual Sphagnum plants (down to 20 cm depth) growing on a peatland in eastern central Sweden and its distribution in the uppermost capitulum and subapical segments of Sphagnum mosses were compared to determine the possible mechanisms involved in radiocesium uptake and retention within Sphagnum plants.Additionally, the isotopic (atom) ratios of 137 Cs/K, 137 Cs/Rb and 137 Cs/ 133 Cs within individual Sphagnum plants were recorded for determining the distribution of 137 Cs and alkali metal, and to obtain a better understanding of the uptake mechanisms and the biological behavior of 137 Cs in nutrient-poor Sphagnum dominated ecosystem.There are few studies on the influence of alkali metals (K, Rb, 133 Cs) on 137 Cs distribution and cycling processes in peatlands.Plant species growing on peat have varying degree capacities for influencing uptake and binding of radionuclides, but no systematic study has covered all the dominant species of Sphagnum peatlands their competition for radionuclides and nutrients.The important role of Sphagnum mosses in mineral nutrient turnover in nutrient-poor ecosystems, in particular their role in 137 Cs uptake and binding, necessitates a clear understanding of the mechanisms involved.The general aim was to gain better insight into mechanisms governing the uptake of both radionuclides ( 137 Cs) and stable isotopes of alkali metals (K, Rb, 133 Cs) by Sphagnum mosses.The specific aim was to compare the distribution of 137 Cs, K, Rb and 133 Cs in the uppermost capitulum and subapical segments of Sphagnum mosses to be able to discuss the possible mechanisms involved in radiocesium uptake and retention within Sphagnum plants.Most results obtained in this study are published in collaboration with Prof. H. Rydin (Vinichuk et al., 2010a).

Study area
The study area was a small peatland (Palsjömossen) within a coniferous forest in eastern central Sweden, about 35 km NW of Uppsala (60°03′40′′N, 17°07′47′′E): the peatland area sampled was open and Sphagnum-dominated (Figure 4).A weak minerotrophic influence was indicated by the dominance of Sphagnum papillosum, and the presence of Carex rostrata, Carex pauciflora and Menyanthes trifoliata (fen indicators in the region).The area had scattered hummocks, mostly built by Sphagnum fuscum, and was dominated by dwarf-shrubs such as Andromeda polifolia, Calluna vulgaris, Empetrum nigrum and Vaccinium oxycoccos.Sampling was within a 25 m −2 low, flat 'lawn community' (Rydin & Jeglum, 2006) totally covered by S. papillosum, S. angustifolium and S. magellanicum with an abundant cover of Eriophorum vaginatum.The water table was generally less than 15 cm below the surface: surface water was pH 3.9-4.4(June 2009).papillosum, in a few cases S. angustifolium or S. magellanicum).In the laboratory, the fresh, individual, erect and tightly interwoven Sphagnum plants were sectioned into 1 cm (0-10) or 2 cm (10-20 cm) long segments down to 20 cm from the growing apex.The 137 Cs activity concentrations were measured in fresh Sphagnum segments.Thereafter, the samples were dried at 40°C to constant weight and analyzed for K, Rb and 133 Cs.The activity concentration (Bq kg −1 ) of 137 Cs in plant samples was determined by calibrated HP Ge detectors.Statistical error due to the random process of decay ranged between 5 and 10%.Plant material was measured in different geometries filled up, except a few samples that contained about 1 g of dry material.All 137 Cs activity concentrations were recalculated to the sampling date and expressed on a dry mass basis.The analysis of Sphagnum segments for K, Rb and Cs was by a combination of ICP-AES and ICP-SFMS techniques at ALS Scandinavia AB.For K concentration determination, ICP-AES was used and for 133 Cs and Rb, ICP-SFMS was used.The detection limits were 200 mg kg −1 for K, 0.04 mg kg −1 for 133 Cs and 0.008 mg kg −1 for Rb.The isotopic (atom) ratio of 137 Cs/ 133 Cs was calculated with Equations 1 and 2 (Chao et al., 2008)

Distribution of Cs (
137 Cs and 133 Cs), K and Rb within Sphagnum plants Concentration values of Cs ( 137 Cs and 133 Cs) and neighboring alkali counterparts K and Rb in different segments of plant provide information on differences in their uptake, distribution and relationships.The averaged 137 Cs activity concentrations in Sphagnum segments are presented in Figure 5a.Within the upper 10 cm from the capitulum, 137 Cs 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, 137 Cs 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 133 Cs ranged between 0.046 and 0.363 mg kg −1 (mean 0.204): averaged concentrations of K, Rb and 133 Cs in Sphagnum segments are presented in Figure 5b.Concentrations of Rb and 133 Cs 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 137 Cs, but with a weaker increase of Rb towards the surface.The 137 Cs activity concentrations had the highest coefficient of variation (standard deviation divided by the mean) in Sphagnum (43%).The coefficients of variation were 35% for K, 35% for Rb and 37% for 133 Cs concentrations.Two important features should be mentioned when discussing distributions of K, Rb, 133 Cs and 137 Cs 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 133 Cs can be at exchange sites both outside and inside the cell.The distribution of 137 Cs within Sphagnum plants was similar to stable K, Rb and 133 Cs.The 137 Cs activity concentrations and K, Rb and 133 Cs 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 137 Cs in Sphagnum segments (Figure 6) were  133 Cs, K, Rb and 133 Cs, in segments of Sphagnum plants Ratios between mass concentrations of all three alkali metals and 137 Cs activity concentrations, i.e. 133 Cs: 137 Cs; K: 137 Cs, Rb: 137 Cs and 133 Cs: 137 Cs, 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: 137 Cs, Rb: 137 Cs (scale values should be multiplied by 10 −2 ), K:Rb (x10 2 ) and 133 Cs: 137 Cs (x10 −4 ) in Sphagnum segments.Calculations based on concentrations in mg kg −1 for stable isotopes and Bq kg −1 for 137 Cs (+/− SE, n=13 for 137 Cs; n=4 for each of K, Rb and 133 Cs).

Mass concentration and isotopic (atom) ratios between
However, the isotopic (atom) ratios between 137 Cs activity concentrations and mass concentrations of alkali metals, i.e. 137 Cs/K, 137 Cs/Rb and 137 Cs/ 133 Cs, had distinctively different pattern of distribution through the upper part (0-20 cm) of Sphagnum plants (Figure 7).The 137 Cs/K ratio was relatively narrow through the upper part (0-16 cm) of Sphagnum plants and wider with increasing depth, whereas, the 137 Cs/ 133 Cs 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 137 Cs/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 137 Cs 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 137 Cs atoms

Relationships between 133 Cs, K, Rb and 133 Cs in segments of Sphagnum plants
Relationships between 133 Cs, K, Rb and 133 Cs in separate segments of Sphagnum plants is a tool allowing future investigate its uptake mechanism.There were close positive correlations between K, Rb and 133 Cs mass concentrations and 137 Cs activity concentrations in Sphagnum segments (Table 8).Correlation between 137 Cs 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 137 Cs 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/ 133 Cs isotope (atom) ratios and mass concentrations of alkali metals (K, Rb and 133 Cs) were not or negatively correlated (Table 8).The marked decrease in 137 Cs 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 14 C 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 (R 2 =0.998), indicating the Chernobyl horizon would be at about 23 cm deep when the 137 Cs 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 137 Cs was obvious, but no downward migration could be tested in the study.The relatively unchanged 137 Cs/K, 137 Cs/Rb and 137 Cs/ 133 Cs 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 137 Cs has been observed in earlier studies (Rosén et al., 2009); similarly, most 137 Cs 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.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 137 Cs 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 137 Cs, as 137 Cs has a weaker correlation with K, especially in the uppermost parts of the plant, which means 137 Cs uptake might be somewhat different from that of K.Even within the same segments of the plant, 137 Cs activity concentrations has higher variation than K concentration.An even stronger decoupling between 137 Cs and K is observed in the forest moss Pleurozium schreberi, in which 137 Cs is retained to a higher degree in senescent parts (Mattsson & Lidén, 1975).However, close correlations, were found between Rb and 137 Cs, which suggests similarities in their 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 137 Cs 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 137 Cs, 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.

Conclusions from the Swedish studies
The concentrations of the three stable alkali elements K, Rb and 133 Cs and the activity concentration of 137 Cs 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 133 Cs enrichment in these forest compartments.The uptake of K, Rb and 133 Cs 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 133 Cs 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 137 Cs/K, 137 Cs/Rb and 137 Cs/ 133 Cs in the fungal sporocarps suggested they were independent on specific species of fungi.The relationships observed between concentration ratios 137 Cs/ 133 Cs and K, Rb and 133 Cs in fungal sporocarps also varied widely and were inconsistent.The concentration of K, Rb and 133 Cs in sporocarps appeared independent of the 137 Cs/ 133 Cs isotopic ratio.
The study of S. variegatus sporocarps sampled within 1 km 2 forest area with high 137 Cs fallout from the Chernobyl accident confirmed 133 Cs and 137 Cs uptake is not correlated with uptake of K; whereas, the uptake of Rb is closely related to the uptake of 133 Cs.Furthermore, the variability in 137 Cs and alkali metals (K, Rb and 133 Cs) 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.For Sphagnum the distribution of 137 Cs 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.

Fig. 1 .
Fig. 1.Ratio of (a) K: 133 Cs and (b) K:Rb in fungal sporocarps (♦, solid line) and soil mycelium (○, dotted line) in relation to the soil in which they were growing.** p=0.01, *** p=0.001 and Rubidium in Macromycete Fungi and Sphagnum Plants 289 study the concentrations of K in sporocarps of S. variegatus were not related to the concentrations of 137 Cs (r=0.103) or 133 Cs (r=−0.066) in the combined data set (Figure 2: c, b).In contrast, the concentrations of K and Rb were significantly correlated in the combined dataset (r=0.505, Figure 2: a).Rubidium was strongly correlated with stable 133 Cs (r=0.746) and moderately correlated with 137 Cs (r=0.440) and K (r=0.505: Figure 2: d, e, a).Both 133 Cs and 137 Cs were significantly correlated in the combined dataset (Figure 2: f).The 137 Cs/ 133 Cs isotopic ratio in the combined dataset was not correlated to K concentration, but correlated moderately and negatively with both 133 Cs (r=−0.636)and Rb (r=−0.500)concentrations (Figure 3: a, c, b).
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 part of the plant.
attainment of equilibrium between stable 133 Cs and 137 Cs in the bioavailable fraction of peat soil; and, relation between cesium ( 133 Cs and 137 Cs), K and Rb when taken up by the Sphagnum plant.
and Rubidium in Macromycete Fungi and Sphagnum Plants 303 ).

Table 4 .
Cs/K,137Cs/Rb and 137 Cs/133Cs in the fungal sporocarps, and range and correlation coefficients between concentration ratios 137 Cs/ 133 Cs and K, Rb and 133 Cs are presented in Table4.Isotopic (atom) ratios of 137 Cs/K, 137 Cs/Rb, 137 Cs/ 133 Cs, correlation coefficients between isotopic ratios 137 Cs/ 133 Cs and mass concentrations of K, Rb and 133 Cs in fungal sporocarps (n = number of sporocarps analyzed).

Table 5 .
Potassium, rubidium and cesium ( 133 Cs) mass concentrations and 137 Cs activity concentrations in sporocarps of S. variegatus (DW) from identified and unknown genotypes, where n = number of sporocarps of each genotype analyzed, M = mean, SD = standard deviation, CV = coefficient of variation.www.intechopen.com

Table 7 .
Correlation coefficients between concentrations of potassium, rubidium and cesium ( 133 Cs and 137 Cs) in genotypes of S. variegatus with more than four sporocarps analyzed 1 .
. Relationships between K, Rb and 133 Cs concentrations in different Sphagnum segments were determined by Pearson Potassium and Rubidium in Macromycete Fungi and Sphagnum Plants

Mechanisms of 137 Cs 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 The variations in concentrations of K, Rb and 133 Cs and 137 Cs activity concentration in sporocarps of S. variegatus appear to be influenced more by local environmental factors than by genetic differences among fungal genotypes. www.intechopen.com