Migration and bioavailability of 137Cs in forest soil of southern Germany

Journal of Environmental Radioactivity 100 (2009) 315–321

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Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad

Migration and bioavailability of

137

Cs in forest soil of southern Germany

I. Konopleva a, E. Klemt a, A. Konoplev b, G. Zibold a, * a b

Hochschule Ravensburg-Weingarten, University of Applied Sciences, 88250 Weingarten, Germany Scientific Production Association ‘‘TYPHOON’’, Obninsk, Russia

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 23 January 2009

To give a quantitative description of the radiocaesium soil–plant transfer for fern (Dryopteris carthusiana) and blackberry (Rubus fruticosus), physical and chemical properties of soils in spruce and mixed forest stands were investigated. Of special interest was the selective sorption of radiocaesium, which was determined by measuring the Radiocaesium Interception Potential (RIP). Forest soil and plants were taken at 10 locations of the Altdorfer Wald (5 sites in spruce forest and 5 sites in mixed forest). It was found that the bioavailability of radiocaesium in spruce forest was on average seven times higher than in mixed forest. It was shown that important factors determining the bioavailability of radiocaesium in forest soil were its exchangeability and the radiocaesium interception potential (RIP) of the soil. Low potassium concentration in soil solution of forest soils favors radiocaesium soil–plant transfer. Ammonium in forest soils plays an even more important role than potassium as a mobilizer of radiocaesium. The availability factor – a function of RIP, exchangeability and cationic composition of soil solution – characterized reliably the soil–plant transfer in both spruce and mixed forest. For highly organic soils in coniferous forest, radiocaesium sorption at regular exchange sites should be taken into account when its bioavailability is considered. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: 137 Cs Bioavailability Forest soil Ammonium Potassium RIP Exchangeability

1. Introduction The majority of soils in semi-natural ecosystems (forests and bogs) of the northern hemisphere are acidic, rich in organic material and poor in nutrients such as potassium (Delvaux et al., 2000). Soils in southern Germany contain present radiocaesium inventories in the range 10–60 kBq m2 (a legacy from the Chernobyl accident, 1986), which appears in solution only at trace contamination level. For surface soils rich in clays (as is the case in soils used for agriculture), most of this radiocaesium is adsorbed at the selective sites of clay minerals and is, therefore, not available for plant uptake. In forest soils, however, competition for the radiocaesium between plant roots and the relatively small number of selective sorption sites available leads to bio-recycling of the radiocaesium by root uptake. To date, it has not been possible to considerably minimize the 137Cs soil–plant transfer in forest soil and thus reduce the radiation dose to humans due to ingestion of forest products such as mushrooms and game meat. One reason is that the process of 137Cs transfer from forest soil to plants is rather complex and still a matter of considerable research.

* Corresponding author. University of Applied Science, P.O. Box 1261, D-88241 Weingarten, Doggenriedstr., Germany. Tel.: þ49 751 94011. E-mail address: [email protected] (G. Zibold). 0265-931X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2008.12.010

As an example, enhanced activity concentrations of 137Cs in wild boar from Upper Swabia (south-west Germany) have been reported since 2003, with measured values reaching above 8000 Bq/kg of fresh mass (Klemt et al., 2005). The increase with respect to previous years has been attributed to the consumption by the boars of highly contaminated deer truffles (Elaphomyces granulatus fr.) (Fielitz, 2005). Deer truffles grow in spruce forest soil at depths of about 7 cm under the surface, in the humus horizon. This organic soil horizon contains the maximum fraction of the radiocaesium in the soil profile which is taken up and accumulated by deer truffles. In forest soils, humus is formed by litter decomposition. Litter is of key importance in radionuclide redistribution in forest soil. The rate of litter decomposition is one of the most important factors in the 137 Cs downward migration. This rate is governed by the type of vegetation, climatic conditions, and the composition of the underlying mineral layer. Litter decomposes and transforms into humus with different rates depending on its composition and, in particular, on its nitrogen content (Dushofur, 1998). Deciduous litter decomposes faster than coniferous litter and forms a thin humus layer near the soil surface. Also, in deciduous forest the 137Cs downward migration to the mineral layer has been shown to be faster (Scheglov, 1997). In contrast, coniferous litter, which is generally depleted in nitrogen content and releases antibacterial substances, changes very slowly and forms a thick humus horizon. This horizon, situated

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near the soil surface, accumulates radionuclides for a long time and is the main source for the 137Cs uptake by grazing plants and mushrooms. The retention of the 137Cs in the humus layer of forest soils has been attributed mainly to the presence of clay minerals (Maes et al., 1998; Kruyts et al., 2004) and microbiological immobilization (Guillitte et al., 1994; Brukmann and Wolters, 1994). The 137Cs has been shown to be selectively sorbed and fixed by clay minerals (2:1 structure) if present in organic rich soils even in small quantities (Valcke, 1993; Rigol et al., 1998). The highly selective sites are located at the expanded edges of the clay particle interlayers and are called ‘‘frayed edge sites’’ (FES) (Cremers et al., 1988). The bioavailability of the 137Cs in soils depends on the number of these selective sorption sites (FES), the cationic composition of soil solution and the portion of exchangeable radiocaesium in soil (Konoplev et al., 1999). The 137Cs concentration in the soil solution is the key characteristic determining its uptake by plants (Konoplev et al., 1993). The common measure of the radionuclide exchange between soil and soil solution is the solid–liquid distribution coefficient (Kd). Since part of the caesium in the soil solid phase is not available for an exchange with the solution because of its irreversible sorption by minerals, it is preferable to use the exchangeable solid–liquid 137 Cs distribution coefficient (Kex d ) to describe the partition of between soil and soil solution and to assess the 137Cs concentration in the soil solution. The Kex d is defined as the ratio of the concentration of radionuclide reversibly sorbed by the solid phase to its concentration in the liquid phase (Konoplev et al., 1992). The Kex d may be quantitatively estimated in terms of the radiocaesium interception potential, RIP, and the concentration of competing ions in the soil solution (Sweeck et al., 1990). Note that RIP is a product of the capacity of the highly selective frayed edge sites (FES) and KFES c (Cs/M), the selectivity coefficient of Cs in relation to competing cations (M):

RIPðMÞ ¼ KcFES ðCs=MÞ  ½FES

(1)

NHþ 4

þ

where M ¼ K or depending on cationic scenario. Values of RIP(M) characterize the potential ability of soils to selectively and reversibly adsorb Csþ and the values allow the calculation of Kex d for other ion scenarios on the basis of the following equation:

Kdex ðCsÞ ¼ h

K

þ

i SS

RIPðKÞ þKcFES ðNH4 =KÞ

i h  NHþ 4

(2) SS

Therefore, the solid–liquid distribution of 137Cs is essentially governed by the FES capacity and the soil solution concentrations of 137 Cs and the Kþ and NHþ 4 . The FES are highly selective for 137 Cs are generally very small in concentrations of stable Cs and comparison to the number of competing ions in the soil solution (e.g. 1 kBq 137Cs corresponds to about 1012 M). Therefore, 137Cs in most soils and sediments is a trace contaminant and is more or less completely sorbed on FES. In this case the strong influence of the capacity of FES on 137Cs soil–plant transfer becomes plausible. In soils with a large content of organic matter the FES are not the only controlling factor and the regular exchange complex plays an important role. In the high organic matter soils, the Kex d value is calculated on the basis of the equation:

Kdex ðCsÞ ¼ KdFES ðCsÞ þ KdRES ðCsÞ

Kdex ðCsÞ ¼ h

Kþ

i SS

RIPðKÞ h i þKcFES ðNH4 =KÞ NHþ 4

þh SS

Kex þ NH4ex h i i þ NHþ 4

Kþ

SS

SS

(4) Forest soils have important characteristic features: (1) the presence of litter and organic horizons with different thicknesses, (2) a depth distribution of roots characterized by plant species, and (3) a heterogeneous distribution of radiocaesium in the soil profile depending on sorption characteristics of the soil horizons. Nevertheless it is still common to use the aggregated transfer factor Tag to quantify the transfer of 137Cs from soil to plants, mushrooms or game meat. The aggregated transfer factor Tag (in m2/kg) is defined by the caesium activity concentration (in Bq/kg) of the dry mass of the plants, divided by the total inventory (in Bq/m2) of the soil. Thus, the value of Tag is related to the total inventory, which means that the geometry of plant roots, the depth distribution of 137 Cs, and the availability of the 137Cs to plants in the root zone layer are not taken into consideration. This causes high variability of Tag. Measured values of Tag for a plant can vary by a factor 100–1000, depending on conditions in soil and sampling (IAEA, 1994). An improved description of the soil–plant transfer can be achieved if the activity concentration of the plant is related to the activity concentration in the soil horizon where the plant roots are located (Konoplev et al., 1996, 1998). According to Fig. 1 this would be the Of-horizon for wood sorrel and the Oh horizon for fern. This transfer factor is named TF which is also known as the concentration ratio, CR. Several efforts have been undertaken to predict TF values using soil chemical properties, e.g. the exchangeability of radionuclides in the soil (Oughton et al., 1992; Konoplev et al., 1993), the concentrations of radionuclides and of competing cations in the soil solution (Shaw et al., 1992; Smolders et al., 1997; Roca et al., 1997). Concerning 137Cs, a fruitful hypothesis has been to assume that the TF is proportional to the fraction of 137Cs in the root exchange complex, which depends on the composition of the soil solution (Smolders et al., 1997). Using this hypothesis, Konoplev et al. (1996, 1998) and Konoplev and Konopleva (1999) developed a model which describes the bioavailability of 137Cs in soil as expressed in the form:

TF ¼ A  B ¼

aex RIP



½KSS þKcFES ðNH4 =KÞ  ½NH4 SS pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi B ½CaSS þ½MgSS

(5)

In this formula, A is the ‘‘availability’’ factor, B characterizes the ability of a specific plant to sorb radiocaesium on the root exchange complex and transfer it through the cell wall, aex is the exchangeability of radiocaesium in soil, RIP is a measure for the selective

(3)

where RES are the Regular Exchange Sites on the external surfaces of clay particles and humus substances. Therefore, Kex d (Cs) is calculated as

Fig. 1. The root distribution of fern (Dryopteris carthusiana) and wood sorrel (Oxalis acetosella) in the different soil horizons as compared to the depth distribution of 137Cs in the soil of a spruce forest (Klemt et al., 1996).

I. Konopleva et al. / Journal of Environmental Radioactivity 100 (2009) 315–321 þ sorption of 137Csþ, KFES c (NH4/K) is the selectivity coefficient of NH4 ions with respect to the competing ion Kþ, in rectangular brackets [ ] are the cation concentrations in the soil solution. This model was successfully tested for forest soils from Upper Swabia, Russia, Sweden and Switzerland (Konoplev et al., 1999, 2000). Delvaux et al. (2000) discovered low concentrations of soluble K in the rhizosphere to be the cause of increased uptake of 137Cs and they verified the above relation between TF and RIP for ryegrass (Lolium). Kruyts et al. (2004) showed that the accumulation of organic material in the top soil can cause a decrease of RIP in the thick humus horizons of forest soil and thus lead to an increase of the transfer of 137Cs. The objective of this paper was to study the migration and bioavailability of 137Cs in soils of ‘‘spruce forest’’ and ‘‘mixed spruce and beech forest’’ in the pre-alpine region in south-western Germany in connection with soil characteristics and soil solution composition. Special attention was devoted to the effect of potentially elevated ammonium concentrations in humified layers of forest soils.

2. Materials and methods 2.1. Sampling sites, altitude, precipitation, soil type, geology The area is located 30 km north of Lake Constance in the south of Germany (Center of the forest: Gauss Krueger coordinates (PD) 3552980; 5300190, see Fig. 2). It is characterized by a mixture of forest areas (about 25%), agricultural land, bogs and small lakes at an altitude of about 650 m. Mean annual temperature is about 8  C. At Bad Schussenried the following average values for the years 1980–2005 were recorded: maximum temperature: 17.8  C in July, minimum temperature: 1.1  C in January, maximum precipitation: 116 mm in July, minimum precipitation: 49 mm in February, and a total precipitation of 916 mm on average per year. The annual precipitation varies locally between 700 and 1400 mm. Altdorfer Wald comprises about 60 km2 of forest mainly spruce, Picea abies and mixed forest (beech, Fagus, and spruce, P. abies). The main type of soil is Luvisol with a tendency to podsolic Luvisol belonging to the soil family mottled loam. The geology of the bedrock is mainly moraine. A schematic map of the area under study and the sampling sites are presented in Fig. 2.

317

Soil and plant sampling: Soil material was taken as a monolith. A volume of dimension of about 30 cm  20 cm area, and depth of about 25 cm was dug and transported to the laboratory. In the laboratory this sample was divided according to the different horizons. Area, thicknesses, and weight of the horizons were measured. After removal of stones and tree roots, the soil material was dried in air and sieved using a mesh size of 2 mm. At each sampling site all plants growing on the monolith were collected, separated according to their species (fern, blackberry), dried at 105  C and crushed in a mixer. Plant samples located within a circle of about 20 m around the monolith were also harvested and prepared in the same way. The 137Cs activity concentration was determined by gamma spectrometry using HPGe detectors. Measuring times were chosen in order to achieve a statistical uncertainty smaller than 5%. On September 15th 2005, the 137Cs inventory in the forest soil varied at the sites studied between 8000 and 26,000 Bq/m2. Soil pH was measured in 0.01 M CaCl2 using a solid–liquid (S/L) ratio of 1:2.5 for mineral layers and in S/L ratio of 1:5 for organic layers after 2 h of equilibration. Soil texture was determined using a hydrometer method after destruction of organic matter with 30% hydrogen peroxide (Gee and Bauder, 1986). Classification of the soil by grain size was done according to USDA (US Department of Agriculture) standard. The organic matter content in samples was determined by loss on ignition (10 h at 450  C). RIP determination (Wauters et al., 1996): The soil sample (about 1 g) was equilibrated with a mixed potassium–calcium solution (0.5 mM KCl þ 100 mM CaCl2) to mask the RES by Ca2þ and to saturate FES by Kþ. After pre-saturation (3  20 h), a phase separation was conducted by centrifugation and the soil sample was equilibrated with the same K–Ca solution spiked with 137Cs. After 24 h the distribution coefficient Kd (137Cs) was obtained by measuring the Cs activity remaining in the solution. The product of Kd (137Cs) and the potassium concentration represents the value of RIP. The RIP in a NH4 scenario, RIP(NH4), was measured in the same way, only NH4Cl was used instead of KCl. 137 Cs exchangeability and exchangeable cations: After determining its 137Cs activity, the soil sample (50 g) was equilibrated with 1 M NH4OAc during 24 h using a solid–liquid ratio of 1:10 for mineral layers and of 1:20 for organic layers. The soil suspension was centrifuged and the solution was filtered through a 0.45-mm membrane filter. The ratio of 137Cs in the solution to that in the soil is its exchangeability. Contents of exchangeable Ca, Mg, and K were measured in a 1 M NH4OAc extract. Exchangeable ammonium was determined in the same way using a 2 M KCl extract. Soil solution isolation: To collect soil solution, a syringe without plunger was supplied with a paper filter and with glass fiber as a plug. Then it was filled with a soil sample. The syringe with the soil was centrifuged for 1 h at 1700 g. The soil solution was filtered through a 0.45-mm membrane filter. Cation content (K, Ca, and Mg) in soil solution was determined using atomic absorption spectrophotometry (AAS). The uncertainties are 0.1 mg/L for K and Mg and 0.2 mg/L for Ca. The NH4 in soil solution was measured with a colorimetric method by indophenol reaction (Krom, 1980). The uncertainty was about 5% as tested by replicates.

3. Results and discussion 3.1. Depth distribution of

137

Cs in forest soil

Typical depth distributions of 137Cs in soil of mixed forest (a) and spruce forest (b) are shown in Fig. 3. It can be noted that 19 years after deposition still more than 50% of the 137Cs activity is located in the upper 10-cm soil layer with a peak of the activity concentration in the Ah horizon. The organic horizons in spruce forest soils are thicker and richer in 137Cs than those in mixed forest soils. In spruce forest soils, plant roots are located mainly in the Oh horizon, whereas in mixed forest soils roots are mainly in the Ah horizon. Our data show that in spruce forest about 50% of the total 137Cs inventory is located in the root zone; in mixed forest it is about 30%. Fig. 3 shows that the organic Oh horizon in spruce forest soil accumulates 137Cs and thus prevents its downward migration. As a result, the penetration of 137Cs to deeper soil layers of spruce forest is more limited in comparison to that in a mixed forest. 3.2. Main soil characteristics

Fig. 2. Sites under study in forest Altdorfer Wald. The location of the cities Weingarten and Ravensburg are indicated.

Main soil characteristics of the root zone layers are given in Table 1. All soils were acidic with pH values between 3.6 and 5.8 in mixed forest, and pH values between 2.8 and 3.4 in spruce forest. Soil textures ranged from sandy clay loam to sandy loam. Sand is

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a

137Cs,

0

500

Bq/(m2 cm) 1000

1500

2000

L Of

4.5 6.5

Ah B

Depth, cm

13

22

Mixed forest

30

b

137Cs,

0

Bq/(m2 cm)

500

1000

1500

2000

L Of

3

Oh 7

Ah

Depth, cm

B 11 13

18

Spruce forest

23 Fig. 3. Depth distribution of the mineral horizons are below.

137

Cs activity concentration in soil of a mixed forest (a) and of a spruce forest (b). Organic horizons are above the maximum of the distribution,

the dominant grain size fraction with 46–65% in the mineral horizon B. Organic humified layers of forest soils could be characterized by elevated levels of ammonium concentrations. Ammonium

effectively mobilizes 137Cs from soil sorption sites causing its higher mobility and plant availability (Sanchez et al., 1999). At all 10 sampling sites the vertical distribution of ammonium concentration in the soil solution was measured. It is known that

Table 1 Texture, grain size and physico-chemical soil properties of root zone layers. Site

Type of forest

Depth, cm

Horizon

pH, CaCl2

OMa, %

1 2 3 4 5 6 7 8 9 10

Mixed Mixed Mixed Mixed Mixed Spruce Spruce Spruce Spruce Spruce

4.5–5.5 4.2–6.2 2.5–4.3 2–3.5 5.5–6.5 3–4 2–3.5 7–9 5.5–7.5 3–7

Ah Ah Ah Ah Ah Oh Oh Oh Oh Oh

3.9 4.1 5.6 3.6 5.8 3.3 3.4 2.8 3.2 2.9

22 14 17 24 17 59 68 57 76 77

a b c

Grain sizeb

Texture

Clay, %

Silt, %

Sand , %

32 28 20 28 22 27 18 25 27 23

21 26 16 23 25 22 17 23 23 17

46 46 63 49 53 51 65 52 51 60

Organic matter content determined as losses on ignition at 450  C. Grain size distribution was determined for B soil layers. TRB is total reserve in bases.

Sandy clay loam Sandy clay loam Sandy clay loam Sandy clay loam Sandy clay loam Sandy clay loam Sandy loam Sandy clay loam Sandy clay loam Sandy clay loam

Exchangeable cations K, cmol kg1

NH4, cmol kg1

Ca, cmol kg1

Mg, cmol kg1

0.3 0.3 0.3 0.3 0.4 0.6 0.6 0.9 0.7 0.7

0.2 0.2 0.2 0.2 0.2 0.6 1.1 0.6 1.3 1.1

20.2 16.2 29.4 9.3 57.9 24.1 21.3 8.4 35.5 12.1

1.7 1.2 9.5 2.6 2.8 3.2 4.5 2.2 3.4 4.4

TRBc, cmol kg1 22.4 17.8 39.4 12.4 61.2 28.5 27.6 12.1 40.8 18.4

I. Konopleva et al. / Journal of Environmental Radioactivity 100 (2009) 315–321

800

Soil solution NH4 concentration, mM 0.0

0.2

0.3

0.4

0.5

coniferous forest

Of

600

mixed forest

RIP, meq/kg

Soil horizons

0.1

319

Oh Ah

400

200 Ah

0 B

5

0

10

15

20

25

30

Clay content, % Fig. 5. Dependence of RIP on the fraction of clays in the soil.

Fig. 4. Mean NH4 concentration in the soil solution of the root zone of forest soil.

ammonification is a bacterial mineralization of organic substances and it is accelerated with increasing concentrations of dissolved organic matter and oxygen. In agreement with this idea, highest ammonium concentrations were measured in well aerated Of horizons. Root zone layers in spruce and mixed forest differ essentially in organic matter content (Table 1); however, differences in soil solution ammonium concentrations in those layers are not pronounced in both types of forest (Fig. 4). On average, the soil solution potassium concentration in the root zone exceeded the NHþ 4 concentrations by about a factor of 1.7 as shown in Table 2. Potassium concentrations ranged from 0.07 to 0.53 mM and ammonium concentrations from 0.05 to 0.25 mM. Low soil solution Kþ concentrations in the root zone have an essential influence on the plant availability of 137Cs (Zhu and Smolders, 2000). The dependence of the 137Cs uptake by plants on potassium in the Kþ concentration range of less than 1 mM has been well studied (Smolders et al., 1996; Waegeneers et al., 2001). The CF (the plant to solution concentration ratio) for 137Cs is reduced with increasing K in solution and the largest effect of K on Cs uptake was found in the Kþ concentration range less than 0.25 mM (Zhu and Smolders, 2000). So, observed low concentrations of potassium in soil solution may cause higher 137Cs soil–plant transfer. 3.3. Radiocaesium interception potential of the soils under study The RIP value characterizes the ability of soils to selectively adsorb Csþ. The RIP values were determined for Oh and Ah horizons of the soils under study. Fig. 5 illustrates the dependence of RIP on

the fraction of clay in soil. Although the clay mineralogy can differ for different layers and sites, a linear dependence between RIP and the clay fraction was found. This allows the assumption that the clay mineralogy is more or less similar at all investigated sites. The exchangeability of 137Cs obtained by extraction with NH4OAc varies in a range from 1.8 to 5.7% in spruce forests and is generally higher in comparison with mixed forests with 0.4 to 3.4%. Our data show that the higher values of the 137Cs exchangeability correspond to lower values of RIP; however, this correlation is rather weak. In the series of mixed forest soils, we have found a negative linear correlation (r ¼ 0.9) between exchangeability and RIP(K) values. This fact indicates that the same kind of clay minerals could be responsible for 137Cs selective sorption and interlayer fixation, and that 137Cs is adsorbed in the mineral part of mixed forest soil. For coniferous forests, RIP(K) values appeared to be inversely proportional to the exchangeability (not shown in the figures). A set of RIP(NH4) was measured in order to estimate the selectivity coefficient Kc(NH4/K) on FES (Table 2). The values of KFES c (NH4/ K) are in a range from 1.4 to 2.8. Taking into account the measured potassium and ammonium concentrations this means that in most sites ammonium plays a more important role than potassium as a competing cation of radiocaesium for the selective sorption sites FES (Table 2). The RIP(K) values ranged from 23 to 149 mequiv./kg in the Oh horizon in coniferous forest (sites 6–10) and decreased with increasing organic matter content. The RIP values are proportional to the portion of clay minerals in Oh and Ah layers (see Fig. 5). The small portion of clay minerals in the humus layers of spruce forest

Table 2 2þ Selected characteristics of soils, 137Cs exchangeability aex, soil solution Kþ, NHþ and Mg2þ concentrations, percentage of 137Cs on RES, calculated availability factors (A), 4 , Ca 137 Cs soil–plant transfer factors attributed to root zone TF. Site

aex, %

RIP(K), mequiv./kg

RIP(NH4), mequiv./kg

KFES c (NH4/K)

K, mM

NH4, mM

Ca, mM

Mg, mM

% KRES d

A, 104 mM1/2 kg/mequiv.

TF, Bq kg1/Bq kg1 Fern

Blackberry

1 2 3 4 5 6 7 8 9 10

1.3 0.9 2.0 0.4 3.4 1.8 1.8 2.8 3.1 5.7

632 548 396 231 224 149 135 91 43 23

278 251 154 113 129 54 77 65 29 16

2.3 2.2 2.6 2.0 1.7 2.8 1.8 1.4 1.5 1.5

0.14 0.10 0.07 0.24 0.19 0.15 0.13 0.31 0.53 0.17

0.08 0.11 0.07 0.13 0.05 0.08 0.23 0.25 0.16 0.15

0.63 0.52 0.70 1.16 1.43 0.87 0.75 0.18 2.57 0.30

0.13 0.16 0.67 0.71 0.27 0.40 0.38 0.10 0.56 0.09

1 1 2 3 3 11 16 16 34 49

0.08 0.07 0.11 0.05 0.31 0.35 0.56 3.21 2.07 7.80

0.03 0.04 0.12 0.13 0.13 0.20 0.60 0.34 0.58 1.05

0.005 0.003 0.02 0.02 0.007 0.05 0.06 0.06 0.13 0.52

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Availability factor A 10-4mM1/2kg/meq

320

100

1.2 FES

TF, Bq kg-1/Bq kg-1

10 FES + RES 1

0.1

Fern 0.8

0.4

R2 = 0.56

R2 = 0.90 Black berry

0.01 10

100

1000

0.0

RIP (K) meq/kg

0

2

4

6 -4

Fig. 6. Availability factor A versus RIP(K) in logarithmic scale. Mixed forest stands are found to the lower right.

8

10

1/2

Availability factor A, 10 mM kg/meq Fig. 8. Transfer factor TF for the root zone versus availability factor A for fern and blackberry.

is caused by the low rate of litter decomposition. Coniferous litter decomposes very slowly and forms a thick humus horizon like mor or moder. In the soil samples taken in the Oh layers with the mor humus, the RIP values were almost negligible, and notable values were observed in the samples from Oh horizons with moder humus. For soils in mixed forests essentially higher RIP(K) values as compared with spruce forests have been observed in the range from 224 to 632 mequiv./kg. Just on the basis of this comparison we may expect that radiocaesium soil–plant transfer for the mixed forests should be lower than that for spruce forests.

3.4. Predicted availability factor A and measured values of RIP In Fig. 6 the predicted ‘‘availability’’ factor A is plotted versus the measured values of RIP, in logarithmic scale. A dependence (squares) is shown taking into consideration the selective sorption of Csþ on FES only. It can be fitted by a straight line with a negative slope of (1.7  0.2). We also considered sorption both on FES and RES and this dependence is shown as triangles. In this case factor A was calculated according to the following equation:

10

A ¼ 

aex KdFES

þ

KdRES

1   pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ½CaSS þ½MgSS

(6)

RES where (KFES d þ Kd ) was calculated according to Eq. (4). For spruce forest with small FES values, the A values are now smaller and the slope of the straight line decreased slightly (1.5  0.2). In soils of mixed forests the selective sorption of Csþ on FES dominates as indicated by the negligible decrease of the A values for higher RIP values. Fig. 7 presents the dependence of TF attributed to the root zone for fern and blackberry on the RIP of the correspondent soil layer for both types of forest. The graph clearly shows that RIP plays a predominant role in characterizing forest soils in terms of radiocaesium bioavailability. In Fig. 8 predicted values of the availability factor A versus measured values of the transfer factor TF are plotted in linear scales. The linear dependency between A and TF is better fulfilled for blackberry than for fern. It can be seen from Figs. 7 and 8 that as expected (Drissner et al., 1998) fern has substantially higher TF values in comparison to blackberry. It is important to note that the dependencies presented in Figs. 7 and 8 are comprising both spruce and mixed forests. These dependencies can be described by a single function for both forest types.

TF, Bq kg-1/Bq kg-1

4. Conclusions 1

0.1

0.01

0.001 10

100

1000

RIP, meq/kg Fig. 7. Dependence of the 137Cs transfer factor TF attributed to the root zone for fern (upper) and blackberry (lower) on radiocaesium interception potential RIP of the corresponding soil layer.

1. Soil–plant transfer factors of 137Cs attributed to the root zone in soils of spruce forest are on average an order of magnitude higher than that in soils of mixed forest. 2. Relatively low potassium concentrations in the soil solution of the root zone (less than 0.25 mM in most cases) for all investigated sites are in favor of an elevated soil–plant transfer. 3. Concentrations of ammonium and potassium in root zones have been found to be close to each other. However, taking into account that ammonium is a more efficient competitor to radiocaesium than potassium (Kc(NH4/K) varies from 1.4 to 2.8) we conclude that in most investigated sites ammonium is a more important mobilizer of radiocaesium than potassium. 4. Besides the ratio of the ammonium to the potassium concentration in soil solution, the most important factors determining the radiocaesium bioavailability in forest soils and finally the aggregated transfer factor Tag are the 137Cs exchangeability and the RIP value in the root zone.

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