**2. Cs (137Cs and 133Cs), K and Rb in** *Sphagnum* **plants**

#### **2.1 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 nutrient-

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 137Cs, 133Cs 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), 137Cs uptake by mycelia decreased with increasing of 133Cs, K or Rb concentration in the media, and K uptake by mycelia decreased with increasing of 133Cs 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

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 40K 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.

report the highest accumulation of stable Cs is in *Cortinarius* sp.

**2. Cs (137Cs and 133Cs), K and Rb in** *Sphagnum* **plants** 

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 137Cs 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 133Cs and 137Cs, 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)

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 nutrient-

**1.8 Mechanisms of 137Cs and alkali metal uptake by fungi** 

(1998).

**2.1 Introduction** 

poor ecosystem often formed and dominated by peat mosses (*Sphagnum*). *Sphagnum*dominated 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 137Cs within raised bogs (Bunzl & Kracke, 1989; Rosén et al., 2009), and relatively high 137Cs bioavailability to bog vegetation and mosses in particular are found (Bunzl & Kracke, 1989).

The transfer of 137Cs 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 137Cs 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 137Cs by an upward translocation flux (Rafferty et al., 2000); this results in a slow net downward transport of 137Cs in the soil profile.

In peatlands, 137Cs 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 137Cs 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 137C is associated with essential biomacromolecules in mosses and to determine the 137Cs distribution among intracellular moss compartments (Dragović et al., 2004).

The chemical behavior of radiocesium is expected to be similar to that of stable 133Cs and other alkali metals, i.e. K, Rb, which have similar physicochemical properties. Moreover, stable 133Cs usually provides a useful analogy for observing long-term variation and transfer parameters of 137Cs 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 137Cs activity concentration and mass concentration of K, Rb and 133Cs 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 137Cs/K, 137Cs/Rb and 137Cs/133Cs within individual *Sphagnum* plants were recorded for determining the distribution of 137Cs and alkali metal, and to obtain a better understanding of the uptake mechanisms and the biological behavior of 137Cs in nutrient-poor *Sphagnum* dominated ecosystem. There are few studies on the influence of alkali metals (K, Rb, 133Cs) on 137Cs 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

Cesium (137Cs and 133Cs), Potassium

software.

and Rubidium in Macromycete Fungi and *Sphagnum* Plants 299

correlation coefficients. All statistical analyses were with Minitab (© 2007 Minitab Inc.)

Fig. 4. The study area of peatland, Palsjömossen: *Sphagnum*-dominated bog.

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

segments of *Sphagnum,* 137Cs activity concentrations was about 1370 Bq kg−1.

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

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

their role in 137Cs 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 (137Cs) and stable isotopes of alkali metals (K, Rb, 133Cs) by *Sphagnum* mosses. The specific aim was to compare the distribution of 137Cs, K, Rb and 133Cs 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).

#### **2.2 Study area and methods 2.2.1 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).

#### **2.2.2 Methods**

Samples of individual *Sphagnum* shoots that held together down to 20 cm were randomly collected in 2007 (May and September) and 2008 (July, August and September). Thirteen samples of *Sphagnum* plants were collected and analyzed; three in 2007 and 10 sets in 2008. Each sample consisted of approximately 20–60 individual *Sphagnum* plants (mostly *S. 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 137Cs 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 133Cs.

The activity concentration (Bq kg−1) of 137Cs 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 137Cs 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 133Cs and Rb, ICP-SFMS was used. The detection limits were 200 mg kg−1 for K, 0.04 mg kg−1 for 133Cs and 0.008 mg kg−1 for Rb. The isotopic (atom) ratio of 137Cs/133Cs was calculated with Equations 1 and 2 (Chao et al., 2008). Relationships between K, Rb and 133Cs concentrations in different *Sphagnum* segments were determined by Pearson

their role in 137Cs uptake and binding, necessitates a clear understanding of the mechanisms

The general aim was to gain better insight into mechanisms governing the uptake of both radionuclides (137Cs) and stable isotopes of alkali metals (K, Rb, 133Cs) by *Sphagnum* mosses. The specific aim was to compare the distribution of 137Cs, K, Rb and 133Cs 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

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

Samples of individual *Sphagnum* shoots that held together down to 20 cm were randomly collected in 2007 (May and September) and 2008 (July, August and September). Thirteen samples of *Sphagnum* plants were collected and analyzed; three in 2007 and 10 sets in 2008. Each sample consisted of approximately 20–60 individual *Sphagnum* plants (mostly *S. 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 137Cs activity concentrations were measured in fresh *Sphagnum* segments. Thereafter, the samples were

The activity concentration (Bq kg−1) of 137Cs 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 137Cs 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 133Cs and Rb, ICP-SFMS was used. The detection limits were 200 mg kg−1 for K, 0.04 mg kg−1 for 133Cs and 0.008 mg kg−1 for Rb. The isotopic (atom) ratio of 137Cs/133Cs was calculated with Equations 1 and 2 (Chao et al., 2008). Relationships between K, Rb and 133Cs concentrations in different *Sphagnum* segments were determined by Pearson

dried at 40°C to constant weight and analyzed for K, Rb and 133Cs.

involved.

et al., 2010a).

**2.2.1 Study area** 

**2.2 Study area and methods** 

was pH 3.9–4.4 (June 2009).

**2.2.2 Methods** 

correlation coefficients. All statistical analyses were with Minitab (© 2007 Minitab Inc.) software.

Fig. 4. The study area of peatland, Palsjömossen: *Sphagnum*-dominated bog.
