499
The concentrations of K, Rb and Cs in spruce needles (Picea abies
Karst.) and in the associated soils
Armin Wyttenbachl, Sixto Bajol, Jiirg Bucherz, Verena Furrerl, Patrick Schleppil and Leonhard Toblerl
1 Paul
Scherrer Institut, CH-5232 Villigen PSI, Switzerland
Landschaft, CH-8903 Birmensdorf, Switzerland
2 Forschungsanstalt fur Wald, Schnee und
Angenommen: 3 1. Marz 1995
-
Summary Zusammenfassung
nK elemend comnrfationsof K. Rb and Cc were delemined in needles from individual spruce tms as a function of the needle age class.
The concentrations am highest in cumnt year's needles and decrease
smoothly in older needles, approaching a constant value. Rb and Cs
show similar behaviour. whmas that of K differs in so far as its rclative
decnasc with lime is less pronounced. Intm- and intersite variation of the
needk concentrations 811: largest for Cs and smallest for K. Individual
trees show a highly significant conelation between the log (Cs) and log
(Rb) values.
Total soil concentrations wen determined at 9 sites with different parent material. InIra- and intersite variations of K are compnrahle for soils
and for neodlcs. whcrcas those for Rb and Cs arc much smaller for soils
than for needles. All the elements arc conelated in soil samples.
That is no close connection between soil and needle concentrationsat
the various sites. However. the results suggest that needle concentrations
of Rb and Cs. but noi of K. are strongly dependent on the sorp(ion propcnies of the associated soils.
1 Introduction
The elements K, Rb and Cs are chemically closely related. They arc alkali metals and their ions have many properties in common. For plants, K is an essential major nutritional element, whiie Rb and Cs occur in much smaller
amounts and are not essential.
The occurrence of K in soils and plants, its uptake from
soil into plants and its role in plants are well documented
(Scheffer, 1984, Murschncr, 1986). In contrast, such information on Rb and Cs is much more scarce, This is partially
due to their nonessential and - when present at natural concentrations - nontoxic nature, but is also an analytical problem. Cs is usually present in plants in very small amounts,
and this poses difficulties to most analytical techniques.
Z. Pflanzenernahr. Bodenk,, 158,499-504 (1995)
Die K-, Rb- und Cs-Konzentrationen in
Fichtennadeln (Piceu d i e s Karst) und den
dazugehorigen Biiden
Die Gchalte der Elemenre K. Rh and Cs wurdcn in Nadeln untcrschiedlichen Alters von einzelnen Fichten gemessen. Die KOn7EIItrah
nen sind jewcils am gr(iSten in jungcn Nadeln und nehmen mit zunehmender Nadelalterskliwc sukzcssive bis auf c k n konstantcn Weit ab.
Dabei i n das Verhalten von Rh und Cs sehr tlhnlich. w h m d K cine
deutlich kleinerc Abnahmc zigt. Die Streuung der Nodclgehalte ist am
gr(l6en fUr Cs und tun kinsten fUr K,und &warsowohl zwischcn BUumen auf einem Scpndoit als auch zwkchcn Standurtmittcln.Die Ochalte
von Cs und Rb zeigen eine hochsignifikante Komlation ihrer lognrilhmisfhen Weitc.
In Bodenprufilen auf 9 Standonen (iiber unterschiedlichem Muttergestein) wurden die Toulgehalte von K, Rb und CYgemessen. D i m Elemcntc sind in Bodenpmben mwohl innerhalb eims Standom ah auch
zwischen Standonen kornlien. lhre Sauung in Bodenprobm ist fUr K
vergleichbar mil dcrjenigen in Nadeln, wtihnnd sic f i r Rb und Cs in
B e n vie1 kleiner als in Nadeln isi.
Zwischen den N&l- und Bodengehalten bsteht keine engc Bezichung, Die Ergebnisse deuten jedoch dnrauf hin. da8 die Nadclgehaltc
von Rb und Cs,nicht aber diejenigen von K. stark von den A d m o n s c i p e n s c b n der jeweiligcn StandortsWen abhtingen.
Information on the radioactive 1 3 T s is much more abundant than on stable Cs. However, a meaningful interpretation of the results of 137Cs in many cases necessitates the
knowledge of the data for the stable element.
It was the aim of the present work to determine the concentrations of K, Rb and Cs in needles of spruce trees and
in the associated soils, and to investigate the relationships
existing between the three elements. Since K, Rb and Cs
were determined in the same samples, it is certain that they
were subject to the same environmental influences. While
the variation between plant samples was kept as small as
possible by the restriction to Norway spruce and by the separate treatment of the needle age classes, variation between
soil data was enhanced by the selection of sites over different parent materials. Due to the rnultielement nature of the
0 VCH Verlagsgesellschaft mbH, D-6945 1 Weinhelm, 1995
0044-3263195fo510-0499 $5.00
+ .2S/O
500
analytical technique used, information on many other elements were obtained together with the data for K, Rb and
cs.
2 Methods
2.1 Sampling
Samples were collected at 9 sites, which will be referred to by their
acronyms. Sites were about 100 m x 100 m with an uniform bedrock.
From 5 to 9 trees and one or two soil profiles were sampled per site. One
site (WIN) differed from the others by its size (12 km x 12 km) and by the
greater number of profiles (39) and trees (35) sampled; this site is mostly
covered by moraines, but bedrock is not quite as uniform as at the other
sites.
Trees were sampled by cutting branches from the upper third of the
crown in November, taking the 7th whirl if possible. All trees of one site
were sampled in the same year. The trees had an age from 50 to 250 years.
Twigs were cut from the branches according to the age of their needles,
usually collecting age classes 1 to 5. Adhering dust and aerosols were
removed by washing with tetrahydrofuran/toluene (Wyttenbach et al.,
1992).
The twigs were dried, the needles separated from the twig axis and finally ground. Resulting samples contained needles of a given age class from
one tree only.
Soil samples were taken by horizons down to the subsoil (about 100
cm), dried and sieved (2 mm). An aliquot was then ground and sieved
again (0.5 mm). At WIN, sampling was restricted to the topmost 50 cm.
2.2 Elemental analysis
K, Rb and Cs along with other elements were determined both in plant
and soil samples by instrumental neutron activation analysis (Wyttenbach
et al., 1992). Detection limits were 100 pg/g, 100 ng/g and 3 ng/g, respectively. This was sufficient for all samples except for 2 % of the plant samples with Cs. The results for the reference materials NIST-1571 (orchard
leaves) and IAEA-Soil-5, which were inserted as control samples at frequent intervals, were in agreement with accepted values. All analytical
results are reported with respect to needles dried at 85°C and to soils dried
at 65°C. Cleaning the needles prior to analysis is important for samples
with low Cs concentrations (< 10 n&), since dust and aerosols on the needle surface would increase these results by about 30 %. Cleaning would
not have been necessary for Rb and K. Leaching of endogenous K, Rb and
Cs from the needles by the washing procedure is < 0.1 %.
2.3 Abbreviations
AC is the needle age class; AC1 denotes needles of the current year,
AC2 those sprouted one year before, and so on. The term biodynamic
behaviour is used for the change of the concentration in successive ACs,
with all ACs sampled at the same date. Variability is expressed as the coefficient of variation (CV) with normal distributions and as the geometric
standard deviation (GSD) with other distributions.
Wyttenbach, Bajo, Bucher, Furrer, Schleppi and Tobler
typical example is given in Fig. 1. This decrease is always
found and does not depend on the concentration level,
which for Rb and Cs differs by orders of magnitude
between sites. The elements Rb and Cs (both for individual
trees and for site means) always show a very similar biodynamic behaviour; the ratio Rb/Cs is therefore practically
constant for the different ACs. Potassium differs from Rb
and Cs at all sites in the manner shown in Fig. 1; owing to
these differences, the ratio K/Rb (or K/Cs) is not constant in
different ACs, but increases by a factor of 1.7 on average at
all sites when going from AC1 to AC3.
25 j
Ab
20-
9
\
?,\
11
K
1.0-
0.5-
1
2
3
4
nredle age char
5
Figure 1: The biodynamic curves of K, Rb and Cs at site REG. Points are
experimental site means (n = 8 trees), curves are given by eq. 1 fitted to
the experimental data. Normalization of the concentrations is done such as
to give 1 for the mean of all 5 age classes. The normalization value is 473 1
Fg/g for K, 2.8 pg/g for Rb and 13 ng/g for Cs. The ratio K/Rb rises from
1097 in ACI to 2266 in AC 5 at this site.
Abbildung 1: Die biodynamischen Kurven von K, Rb und Cs in Fichtennadeln vom Standort REG. Die Punkte sind die Mittelwerte von 8 Baumen, die Kurven wurden durch Anpassung von Gleichung 1 an die experimentellen Daten erhalten. Die Werte jedes Elementes sind so nominiert,
dal3 der Mittelwert aller 5 Altersklassen gleich 1 wird. Der Nominierungswert betragt 4731 pg/g fur K, 2.8 pg/g fur Rb und 13 ng/g fur Cs. An diesem Standort nimmt das K/Rb-Verhaltnis von 1097 in Altersklasse 1 bis
2266 in Altersklasse 5 zu.
3.2 Needle concentrations: intra- and intersite variation
3 Results
3.1 Influence of the needle age class
The concentrations of K, Rb and Cs depend strongly on
the AC, becoming smaller with increasing needle age. A
In view of the great influence of the needle age on the
concentrations, all subsequent data are given for ACl
exclusively. The variation among individual trees within
sites is always found to increase as K < Rb < Cs. The mean
CV for K is 18 % with little difference between the various
K, Rb and Cs in spruce needles
Table 1: Site means (a) of K, Rb, Cs in spruce needles (age class 1).
Tabelle I: K, Rb und Cs in Fichtennadeln der Altersklasse 1. Mittelwerte
der Standorte (a).
WIN
AUE
CHA
REG
ALP
VIL
GRO
SAR
SCH
35
8
8
8
5
6
5
5
9
5298
6387
3974
6595
5216
6733
4944
6061
7655
11.1
2.17
4.60
11.1
5.57
24.0
6.01
24.3
27.8
5.55
384
42.0
513
31.0
647
32.6
19940
146
all sites (n = 9):
geometricmean 5780
13.1
GSD
1.22 3.96
477
575
166
27 1
188
17.5
9.6
9.4
.38
2441
1388
713
1097
940
160
159
186
52
194
414
232
247
200
109
60
50
7
440
3.62
109
3.44
sites. Contrary to this the variations of Rb and Cs differ
considerably between the various sites (Tobler et al., 1994).
At WIN, where more trees were sampled than at other sites,
it was found that the distribution of Rb and Cs is lognormal
and that of K is normal (Wyttenbach et al., 1991).
Mean concentrations for the individud sites are given in
Tab. 1. The variation among sites increases again as K < Rb
< Cs. The ratio Maxmin for site means is only 1.9 for K,
but 67 for Rb and 1800 for Cs. The range for all individual
trees is 2750 - 8700 pg/g K, 0.2-194 pgfg Rb and 3 - 28000
ng/g Cs. As a consequence of the different intersite variations for K, Rb and Cs, the ratios K/Rb, Rb/Cs and K/Cs are
also very variable.
3.3 Correlations between needle concentrations
119
12.5
48
11.4
(a) Site means are arithmetic for K and geometric for Rb and Cs.
(b) Sites are ordered according to the Cs-value.
Sites are identified by the first 3 letters of their location:
Winterthur ZH, Auenstein AG, Chaneaz VD, Regelsberg SG, Alpthal
SZ, Villingen, Grossalp GR, Sardona SG, Schluchsee.
All sites are in Switzerland, except VIL and SCH (Black Forest. Germany).
Correlations were calculated between the logarithms of
the concentrations (AC1) of individual trees from all sites
except WIN. This site was excluded in order to avoid an
unduly large influence by the great number of trees from
this site. Although the correlations between Rb and K (n =
54, r = 0.450) and between Cs and K (r = 0.422) are significant at the 1 % level, the relationships are rather weak, as
shown by the low r values. On the other hand, the correla-
Table 2: K, Rb and Cs (pg/g) at various soil depths (site WIN)
Tabelle 2: K, Rb und Cs (pgjg) in verschiedenen Bodentiefen (Standort WIN)
Depth
cm
horizon
0- 5
5 - 20
20 - 50
Ah
Ah, 1
Bv
all
n
cs
LO1 (a)
K
mean
cv %
Rb
mean
cv %
mean
cv %
%
37
39
39
13033
13972
14821
18
20
21
82.0
92.6
95.6
18
20
21
5.12
5.20
5.63
23
25
27
16.0
9.5
4.6
115
13958
20
90.2
21
5.32
25
K/Rb
Rb/Cs
K/Cs
155
195
190
142
I36
150
138
155
94
17.0
23.3
27.0
21.9
13.8
6.0
8.0
13.4
4.2
2642
4545
5135
3 108
1879
893
1101
2086
395
148
1.24
12.7
1.91
1879
2.28
154
167
258
8.8
14.8
1358
2486
(a) Loss on ignition (500"C, 2h)
Table 3: K, Rb and Cs (p&) in soils. Site means and comparison with literature data.
Tabelle 3: K, Rb and Cs (Iim
&
Boden.
)
Mittelwerte der Standorte und Vergleicb mit Literaturangaben
Site
bedrock
WIN
AUE
CHA
REG
ALP
VIL
GRO
moraine
moraine
moraine
molasse
flysch
red beds (a)
moraine (b)
quarzite
granite
SAR
SCH
soil
gleyic Luvisol
gleyic Cambisol
gleyic Luvisol
gleyic Cambisol
humic Gleysol
dystric Cambisol
eutric Cambisol
orthic Podzol
orthic Podzol
this work
geometric mean
geometric standard deviation
Literature
data
Tibet
China
USA
Illinois
(C)
(C)
(d)
(e)
a) Buntsandstein
b) Derived mostly from limestone
n
K
Rb
cs
115
4
5
2
6
3
4
4
2
14'000
15'000
19'000
18'600
18'300
15'300
24'200
33'200
90
77
100
81
137
122
111
156
352
5.3
3.3
3.7
3.7
9.9
20.5
13.9
11.6
84.0
9
17'900
1.37
121
1.59
> 200
> 200
> 200
> 200
20'500
17'900
15'000
133
107
58
89
11'500
c) Tim et al., 1993
d) Shacklette and Boerngen, 1984
9.5
2.85
15.1
7.2
e) Jones, 1989
502
tion between Rb and Cs is very strong (r = 0.976, p <
0.0001): the regression is given by log(Cs) =
(- 3.17 k .08) + (1.97 f .06) log(Rb), when the concentrations are in pg/g. It should be noted that the slope of this
log-log regression is not unity, meaning that the ratio
between Rb and Cs is not constant, but depends strongly on
the magnitude of the variables. This latter effect is also seen
for the Rb/Cs ratios of the site means (Tab. 1).
3.4 Soil concentrations: influence of soil depth, intraand intersite variation
The change of the concentrations with soil depth was evaluated in 39 profiles at WIN (Tab. 2). There is a slight
increase for all 3 elements with depth. These changes are
completely explained by the dilution of the mineral soil
with organic matter near the surface; the same observation
was made in profiles at the other sites. Intrasite variation for
each of the 3 elements is about 20 %. Mean soil data for all
sites (Tab. 3) show considerably larger variations, with
intersite variation being smallest for K, intermediate for Rb,
and largest for Cs.
3.5 Correlations between soil concentrations
At individual sites K, Rb and Cs are always highly correlated. The intrasite correlation at WIN is weakest for Cs-K
(n = 115, r = 0.824) and strongest for Rb-K (r = 0.906). The
linear regressions have an insignificant intercept, indicating
a constant ratio between any two elements in all samples.
Site means (Tab. 3, n = 9) also are correlated, with Cs-K
(r = 0.859), Rb-K (r = 0.937) and Cs-Rb (r = 0.971). However the significance of these intersite correlations is less,
and the scatter of the data larger than for the intrasite correlations.
4 Discussion
4.1 Biodynamics
The decrease of the concentrations with needle AC (Fig.
1) is the result of the net translocation of the element from
older to younger needles. There is a smooth decrease
towards a low, constant value. This behaviour can be
described by the function
where c(t) is the concentration in AC t (t=l,..S), and A,B,
and h are constants. With Rb and Cs it has been shown that
neither h nor the ratio B/A vary much among the different
sites, whereas A and B show large variations (Tohler et al.,
1994).
The decrease of the concentrations of K with the age of
spruce needles is well documented (e.g. Rademacher et al.,
1992), and may also be successfully approximated by eq. 1.
Wyttenbuch,Bujo, Bucher, Furrer, Schleppi and Tohler
The content of a needle can thus be described as the result
of import and export, where the export exceeds the import
in each successive AC by an ever decreasing amount, as is
reflected in the exponential decrease of the biodynamic
curves. In old needles (AC4 and AC5) the export is almost
equal to the import since the concentrations no longer
change significantly. The export partially satisfies the
requirements of the newly formed needles. It is suggested
that the compartmentation of K within needle cells
(Schmidt et al., 1989) plays a role in these dynamics, different pools having necessarily different mobilities. Dilution
effects by increasing needle weights on the biodynamic
curves are almost negligible, as may be seen from the fact
that needle weight (on average over all sites) increases only
by a factor of 1.21 when going from AC 1 to AC4, whereas
Rb concentration decreases by a factor of 2.51. The
observed decrease also cannot be attributed to foliar leaching since experiments with artificial acid mist and rain
failed to show any effect on the concentration of K in
needles (Kelly and Strickland, 1986, Pfirrmann et al.,
1990).
Investigations on the biodynamics of stable Rb and Cs are
rare. Robarge et al. (1989) showed that both elements
decrease in needles of Picea rubens and Abies fraseri
between AC1 and AC3, in agreement with the present findings. Ample evidence, however, is available for the high
mobility of 137Cs within the plant. After deposition as
radioactive fallout and foliar uptake, 137Cs is efficiently distributed to other plant organs. A similar behaviour is known
for R6Rb applied to leaves (Percy and Baker, 1988).
The nonessential elements Rb and Cs can be described as
approximately following the same biophysical and biochemical processes as the essential K. Because of the similar properties of these ions, they compete in absorption and
transport mechanisms, even in the very selective active
transport across cell membranes (Rohson and Pitman,
1983). That this competition is not absolute can, however,
be seen from our results where Rb and Cs appear more
mobile than K, pointing to some differences in their compartmentation or transport. 86Rb is often used as a radioactive tracer for K in plant-physiological studies, but the
assumption that Rb behaves exactly like K is brought into
question by the present findings as well as by those of Stone
and Kszystyniak (1977).
For the sake of completeness, it should be added that Na
shows a biodynamic behaviour entirely different from its
homologues K, Rb and Cs, with concentrations increasing
linearly with age. This contrast obviously reflects the physiological discrimination between Na and K, allowing distinct transport and compartmentation (Flowers and Liiuchli,
1983). As a smaller but more hydrated ion, Na can indeed
be well separated from K by carriers transporting ions
across membranes. On the other hand, biodynamic curves
similar to K, Rb and Cs are also observed for other mobile
elements such as C1, Cu, Mg, P, and, in some instances, Co,
Mn and Zn (Wyttenbach et al., 1995a).
503
K, Rb and Cs in spruce needles
4.2 Concentrations in needles of AC1
4.4 Comparison of needle and soil data
Potassium: The values for individual trees at a given site
are normally distributed and have a rather narrow range.
The different site means to do not vary greatly. The deficiency limit for spruce needles is given as 3500 pg/g
(Anonymous, 1986). Since no site mean and only 6 individual trees out of 89 are below this value, poor K availability
does not seem a problem in the present investigation.
Rubidium and Cesium: One characteristic of Rb and Cs
is their lognormal distribution on most sites and their large
range, both with individual trees and with site means. Cs
has indeed been shown to have the largest variation among
all 25 elements investigated in parallel work. Another characteristic of Rb and Cs is their marginal correlation with K,
as opposed to the extremely strong, though nonlinear, correlation between Rb and Cs. The pair Rb-Cs has the strongest correlation among the 300 possible combinations from
all 25 investigated elements. Paired determinations of Rb
and Cs in the same samples are rare. However, it is remarkable that the few values published for various conifers all
follow the correlation found for Norway spruce (Wyttenbach et al., 1995b). This correlation therefore seems to be
generally valid for conifer needles, irrespective of the species or provenance.
The comparison between needle and soil data is made
with the assumption that the soil data, normally coming
from only one or two profiles per site, are sufficiently
representative for the needle data, which come from several
trees per site. Since soil data were found to vary considerably less both on a site and between sites than do the needle
data, this assumption seems to present no problem.
The needle and soil data have some common characteristics, in particular 1) the variation among sites (GSD in Tab.
1 and 3) is largest for Cs and smallest for K. With Cs and
Rb, however, this effect is much more pronounced in needles than in soils. 2) The needle data can be grouped roughly into low (WIN, AUE, CHA, REG, ALP), medium (VIL,
GRO, SAR) and high (SCH) values, with the grouping
being stronger for Cs and Rb than for K. The soil data show
approximately the same grouping, but it is much less pronounced. Spearman rank correlation between needles and
soil data (n = 9 sites means) is not significant for K (r, =
0.033), but significant for Rb (r, =.700, p < 0.05) and for Cs
(rs =.857, p < 0.01). This means that needle concentrations
of K are hardly influenced by the total soil concentrations,
as it is expected for a selective uptake; Rb and Cs show
some connection between the needles and the soil, but since
the correlations are nor linear, this connection is not a simple proportionality.
On the other hand, needle and soil data differ in various
aspects, in particular 1) the close correlation between Rb
and K present in the soil is not reflected in the needles, as is
evidenced by the much larger variation of K/Rb for needles
(GSD 3.44) than for soils (GSD 1.24), and 2) the large
range of Rb and Cs and their strong correlation in needles
are not shown by the soil. From these differences it is evident that the variation in the total soil concentrations does
not sufficiently explain the variation in the needle concentrations.
Nevertheless, the extremely strong correlation between
the Rb and Cs needle concentrations clearly suggests that a
common factor is responsible for their covariation. Since
the Cs concentration changes by two orders of magnitude
when the Rb concentration changes by one order (as is indicated by the slope of 2 of the log-log correlation), it is evident that this common factor acts much more strongly on
Cs than on Rb. The comparatively small variation of the K
concentrations further indicates that this factor has only a
small influence on the uptake of this element. We consider
it as unlikely that this factor represents a selective uptake or
discrimination of Rb and Cs by the plant with respect to K,
but favour the hypothesis of a soil related factor. Various
soil properties have been discussed to influence the avaitability of 137Cs from soils. These include pH, organic matter, exchange capacity, clay content and clay composition
(Kiihn et al., 1984, and StefSens et al., 1988). Neither the pH
nor the organic matter content of the present sites show any
correlation with the needle data. Clay content has not been
4.3 Concentrations in soils
As might be expected from the average composition of
the upper continental crust (K 28200 pg/g, Rb 112 pglg, Cs
3.7 pg/g), all sites show K >> Rb > Cs. Variation among
sites is moderate, being smallest for K and largest for Cs.
The soil data for K, Rb and Cs seem to be mostly inherited
from the parent material as there is no significant variation
with depth within the profiles (Tab. 2).
The highest values for all 3 elements, but especially for
Rb and Cs, are found at SCH. This is in accordance with
the known enrichment of Rb and Cs in granites (Mason and
Moore, 1982). The lowest values for Rb and Cs are found at
sites over fluvioglacial deposits (AUE, CHA, WIN). All
sites are well within the range given in literature. Mean values from this work correspond closely to mean soil values
from China and Tibet, whereas Rb in soils from the United
States is somewhat lower (Tab. 3).
At a given site, the ratio between any two elements is
more constant than the elemental concentrations themselves
as evidenced by the strong intrasite correlations. This is due
to the fact that in individual soil samples elemental concentrations are derived mainly from minerals (feldspar and
micas) with a constant ratio between the three elements.
Comparing the various sites (Tab. 3), it is found that only
the ratio K/Rb is reasonably uniform, whereas K/Cs and
Rb/Cs show larger variations. The larger intersite variation
of K/Cs and Rb/Cs in soils must be related to the different
enrichment of Cs during metamorphic processes involved
in the formation of the various bedrocks.
Wyttenbach, Bajo, Bucher, Furrer, Schleppi and Tobler
determined but its influence is probable. Brouwer et al.,
(1983), Scheffer and Schachtschahel (1984) and Cornell
(1993) have shown that certain clay minerals selectively
adsorb large monovalent cations, thereby decreasing their
concentration in the soil solution, and that this adsorption
increases strongly as K < Rb < Cs, the order given by the
intersite variability of the needle concentrations in the
present investigation. Sites with small Cs and Rb needle
concentrations and a large Rb/Cs ratio (WIN, AUE, CHA,
REG, ALP) are thus expected to have a high content of clay
and strongly adsorbing minerals, sites with intermediate
concentrations and ratios (VIL, GRO, SAR) to have a medium content, and the site SCH, where c s and Rb needle concentrations are largest and the Rb/Cs ratio is smallest, to
have a small clay content. These expectations are not unreasonable in view of the involved soils. Fluvioglacial deposits, such as those present at WIN, AUE and CHA are
known to contain large amounts of the strongly adsorbing
minerals illite, biotite and chlorite (Griitter et al., 1990). On
the other hand, the sandy soil at SCH is likely to contain
collapsed micas that do not easily adsorb Rb and Cs.
If the variations of Rb and Cs between sites are due to different clay contents, then the variation between individual
trees on a given site, which is much smaller than the intersite variation, must mainly be due to small-scale local variations of the clay content. Some additional minor variation
may also arise from differences in the absorption, transportation and storage behaviour of the individual trees. The
dominant influence of the clay on the availability of Rb and
Cs is probably the reason why the needle values of all conifers (although representing different species of most diverse
provenance and therefore presumably growing on different
soils) follow the same correlation between Cs and Rb as
found for Norway spruce in the present work.
Acknowledgements
We thank Mrs. Briitsch (PSI) and Mr. Zimmermann (WSL) for their help
with the collection and analysis of the samples, and the staff of the
SAPHIR Reactor (PSI) for providing irradiation facilities. Partial support
for this work was provided by the Swiss National Foundation (Grant 3130755.91).
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