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Plant Soil (2008) 302:213–220
DOI 10.1007/s11104-007-9471-6
Clay minerals as a soil potassium reservoir: observation
and quantification through X-ray diffraction
Pierre Barré & Christophe Montagnier &
Claire Chenu & Luc Abbadie & Bruce Velde
Received: 2 August 2007 / Accepted: 30 October 2007 / Published online: 22 November 2007
# Springer Science + Business Media B.V. 2007
C. Montagnier
Centre de Versailles-Grignon, INRA,
Unité expérimentale grandes cultures,
route de Saint-Cyr,
78026 Versailles Cedex, France
long term fertilization experiment which allowed one
to address the following questions: (1) Do fertilization
treatments induce some modifications (as seen from
X-ray diffraction measurements) on soil 2:1 clay
mineralogy? (2) Are soil 2:1 clay mineral modifications related to soil K budget in the different plots?
(3) Do fertilizer treatments modify clay Al, Si, Mg, Fe
or K elemental content? (4) Are clay mineral
modifications related to clay K content modifications?
(5) Are clay mineral changes related to clay Al, Si,
Mg or Fe content as well as those of K content? Our
results showed that K fertilization treatments considered in the context of soil K budget are very
significantly related to 2:1 soil clay mineralogy and
clay K content. The 2:1 clay mineral modifications
observed through X-ray measurements were quantitatively correlated with chemically analyzed clay K
content. Clay K content modifications are independent from clay Al, Si, Mg or Fe contents. These
results show that the soil chemical environment can
modify interlayer site occupations (illite content)
which suggests that high level accumulation of
potassium can occur without any modification of the
clay sheet structure. This study therefore validates the
view of 2:1 clay minerals as a K reservoir easily
quantifiable through X-ray observations.
C. Chenu
Laboratoire BioEMCO, UMR 7618,
Bâtiment EGER,
78850 Thiverval Grignon, France
Keywords Abiotic/biotic interactions . Clay minerals .
Ecosystem functioning . Long term experiment .
Potassium cycle . Soil
Abstract Potassium (K) is a major element for plant
growth. The K+ ions fixed in soil 2:1 clay mineral
interlayers contribute to plant K nutrition. Such clay
minerals are most often the majority in temperate
soils. Field and laboratory observations based on Xray diffraction techniques suggest that 2:1 clay
minerals behave as a K reservoir. The present work
investigated this idea through data from a replicated
Responsible Editor: Thomas B. Kinraide
P. Barré (*) : L. Abbadie
Laboratoire BioEMCO,UMR 7618,
Ecology department, Ecole Normale Supérieure,
46 rue d’Ulm,
75230 Paris cedex 05, France
e-mail: [email protected]
P. Barré : B. Velde
UMR 8538, Laboratoire de Géologie, ENS-CNRS,
Ecole Normale Supérieure,
24 rue Lhomond,
75231 Paris Cedex 05, France
Potassium (K) is a major element for plant nutrition.
Available soil K is a limiting factor in many
agricultural systems (e.g. Evans and Sorger 1966;
Kilmer et al. 1968; Oosterhuis and Berkowitz 1996)
and K fertilizers are broadly used in agriculture all
over the world. Potassium availability can also limit
primary production in natural ecosystems. Indeed, a
review by Tripler et al. (2006) showed that K
fertilization increases primary production in forest
ecosystems. Soil potassium availability is also involved in many other ecological processes such as the
outcome of inter-specific competition (Tilman et al.
1999). Improving the knowledge about mineral K
resources is therefore needed for a better understanding of both natural and cultivated ecosystems.
Soil 2:1 clay minerals play an important role for
soil K availability. Indeed, several studies showed that
the presence of such clay minerals, even in subsidiary
quantities, increases effective soil K availability (e.g.
Arkcoll et al. 1985; Delvaux et al. 1990; Poss et al.
1991). The particular importance of such clay layers
relies on their ability to adsorb and release K+ ions
from 2:1 clay mineral interlayer sites (e.g. Hinsinger
2002); these adsorption and release phenomenon can
occur over a short period (e.g. Jungk and Claassen
1986). Although, the contribution of clay mineral
interlayer K to plant nutrition was often considered to
be of minor importance, it is now suspected that it
might contribute to a significant part if not the
majority of the K supply in many soils (Hinsinger
2002; Øgaard and Krogstad 2005; Andrist-Rangel et
al. 2006). Observation and quantification of interlayer
K dynamics is therefore most likely a key-parameter
for understanding the soil K cycle.
Potassium ions are usually anhydrous in soil clay
mineral interlayer sites. Soil clay mineral layers which
contain anhydrous K+ ions have a 1 nm basal spacing
and are traditionally called illite or “illite-like” in the
present work. One finds discrete illite in soils, i.e.
structures which are exclusively composed of anhydrous potassic interlayers, but generally the greatest part
of 1 nm layers in soil clays is associated with smectite
layers in mixed layer crystallites. These clay mineral
populations are called interstratified illite−smectite and
are widespread especially in temperate soils (Wilson
1999; Velde 2001). Barré et al. (2007a) have hypothesized that 2:1 soil clay minerals can behave as a huge
Plant Soil (2008) 302:213–220
K reservoir whose dynamic could be observed through
X-ray diffraction measurements. Based on a compilation of published data, it was suggested that in a K-rich
context some 2:1 layers are able to fix K+ ions forming
a 1 nm layer and in a K-poor context some of the
collapsed layers could release K+ ions and in doing so,
reopen as smectite layers. This hypothesis was successfully tested in laboratory experiments (Barré et al.
2007b). It was demonstrated that K adsorption or
release on a soil clay fraction dominated by 2:1 clay
minerals could be observed and even quantified
through X-ray measurements. The aim of the present
study is to provide field evidence for this qualitative
and quantitative relationship between 2:1 clay mineralogy (illite layer content) and soil K cycle.
In the present study we investigated fertilizer
effects, based on X-ray observations and chemical
analysis of soil clay fractions from a long term P/K
fertilization experiment performed in the INRA
Grignon station (Yvelines, France). The following
questions have been raised: (1) Did fertilization
treatments induce some modifications of soil 2:1
clay mineralogy ? (2) Were soil 2:1 clay mineral
modifications related to soil K budget? (3) Did
fertilization treatments modify clay Al, Si, Mg, Fe
or K content? (4) Are the observed, X-ray diffraction clay mineral modifications related to clay K
content modifications? (5) Are clay K content
modifications related to changes in clay Al, Si, Mg
or Fe content?
Fertilization treatments
Different P and K fertilization treatments were applied
on different plots in cropped fields at the Grignon
experimental station (Yvelines, France) of the French
Institute for Agronomical Research (INRA). No other
fertilizers were added during this experiment. The soil
is a Luvisol, its ploughed layer having the following
characteristics: sand/silt/clay=9/68/23; pH (H20)=
7.8; pHKCl=7.1; organic C=1.4%; N total=0.14%;
CaCO3 =3%; CEC=14 cmolc/kg.
In the beginning of the experiment in 1959, the
field was divided in 20 plots (7 m×15 m). Fifteen of
these plots received about 70 kg of P and 70 kg of K
per year during 7 years. The five other plots did not
Plant Soil (2008) 302:213–220
Sampling procedure
receive any fertilizer input from 1959 to the present
day. These latter plots will be thereafter referred as
0init. In 1966, a new fertilizer treatment was applied.
This treatment was stopped in 10 of the 15 plots
previously treated until 1978. In five other plots P and
K fertilizer treatments were continued. These plots
will be thereafter referred as PKinit. The amounts of P
and K fertilizer treatments were respectively 100 and
180 kg ha−1 year−1 during this period. In 1979, the
plots treated from 1959 to 1965 and not treated from
1966 to 1978 were divided in two parts. A new
fertilizer treatment was applied to the 20 smaller plots
(3.5×15 m) from 1979 up to the present day. Among
the 20 smaller plots, five received 100 kg/ha of P and
200 kg/ha of K each year from 1979, five received
only K, five only P and five neither K nor P. These
four groups will be thereafter referred respectively as
PK, K, P and 0. The treatments were not modified for
the 10 largest plots: the five 0init plots received no
input and the five PKinit received 100 kg/ha of P and
200 kg/ha of K per year. Figure 1 presents the
distribution of the plots in the field and summarizes
the quantities of P and K received from the beginning
of the experiment. Each treatment was therefore
replicated fivefold.
Three soil cores (diameter=5 cm; length=20 cm)
were sampled near the center of each plot in October
2006. Twenty centimeters corresponds approximately
to the plough depth. The three cores were pooled into
one sample which was dried at 60°C, sieved at 2 mm
and thoroughly homogenised.
Clay fractions recovering procedure
We used a physical fractionation procedure to recover
the soil clay fraction (<2 μm). For each sample, 20 g
of soil were shaken with 100 ml of deionized water at
175 rpm on a shaking plate during 16 h. The
suspension was then ultrasonified at 100 J/ml, sieved
at 200 μm and centrifuged to separate the 0–2 μm
fraction. For that purpose, the suspension was
centrifuged at 70×g during 9 min allowing sand and
silt fractions to sediment. The supernatant containing
the clay fraction was poured in a beaker. This
procedure was repeated until the supernatant was
quite clear. All the supernatant collected for a given
soil sample were pooled. A few milliliters of a 0.5 M
strontium chloride solution were added to each pooled
supernatant to reach a 0.01 M. About three drops of
the flocculated clays were poured on a glass slide for
oriented X-ray diffraction. The remaining clay fractions were dried at 60°C.
Potassium budget in the plots
All plots were cropped with the same plants during
the experiment. The crop rotation and the mean yields
per fertilizer treatments were determined. We estimated the K content of the matter exported by the crops
according to the values determined by the French
committee for optimized fertilization (COMIFER
2007). For each fertilizer treatment, the mean K
budget was calculated by subtracting the K added as
fertilizers and the K exported in the crops (mean
yields×K content) since 1959. These budgets are
reported on Table 1.
Fig. 1 Experimental design
of the long term fertilization
experiment in the INRA
Grignon station (Yvelines,
15 m
Chemical analysis of the clay fractions
Sub-samples of the 30 clay fractions were compacted
into 6 mm diameter and 2 mm thick disks using a
hand press and mounted on glass slides and were
subjected to X-ray fluorescence analysis of major
elements under SEM (scanning electron microscope,
EDS-X-ray fluorescence analysis) treatment. For
details, see Velde (2006). This technique allowed us
added quantities since 1959
K (kg/ha)
P (kg/ha)
Plant Soil (2008) 302:213–220
Table 1 Mean potassium budget for each treatment
Fertilization treatments
Total K exported (kg/ha)
Total K imported (kg/ha)
K budget (kg/ha)
The K imported corresponds to the K added as fertilizer from 1959 and the K exported corresponds to the K exported in the crops
from 1959
to determine Si, Al, Mg, Fe and K contents in the clay
XRD pattern acquisition and decomposition in the 2:1
clay minerals range
The oriented preparations were analysed using an Xray diffractometer using Cu Kα radiation. The XRD
diffractograms were obtained at 0.05° step intervals
for 3 s counting time in the range of 4 to 13° 2θ.
The XRD diffractograms, accumulated as intensityposition counts, could be treated numerically using
curve decomposition methods given in Lanson (1997)
using a four-point smoothing routine of a background
subtracted spectrum. The 30 diffractograms were
decomposed into five component curves. Peak positions, intensities and width at half height (WHH) were
determined for each X-ray pattern. The area of each
curve is approximated by multiplying the intensity by
The following clay mineral identification procedure based on the observations of Moore and
Reynolds (1997), Righi et al. (1995) and Velde
(2001) was used to attribute a clay mineral population
to the peaks:
1. Illite was considered as two components of the
peaks at near 1 nm, poorly crystallized illite (PCI)
and well crystallized illite (WCI). PCI have a
greater peak width than WCI (about 0.9° vs 0.4°
2θ using Cu radiation). PCI have 1.03–1.05 nm
spacing whereas WCI have a spacing near 1 nm
(for details, see Meunier and Velde 2004).
2. Smectite–illite mixed layer minerals have positions ranging from 1.15 to 1.5 nm depending on
the proportion of illite and smectite layers. These
phases show peak widths of 1.5 to 2° 2θ using Cu
radiation. There are generally two illite−smectite
mixed layer components in soils which was the
case for our samples. The mixed layered phase
with the highest spacing was referred as SI
(smectite-rich) and the other as IS (illite-rich).
Both are disordered mixed layer minerals according to the criteria of Moore and Reynolds (1997).
3. Chlorite or HI (hydroxyl interlayer phases) or soil
vermiculite shows 1.42–1.44 nm spacing and has
a narrow peak (near 0.3–0.5° 2θ at peak half
height using Cu radiation). These minerals were
found in subsidiary quantities in our samples.
Calculation of X-ray pattern centre of gravity
The peak position of an interstratified illite−smectite
clay mineral depends on the relative abundance of
each type of layer. The higher the illite content is, the
smaller the position in nanometers. One can also
consider that the relative peak area of a clay mineral
population gives an indication on its relative abundance. Therefore, anhydrous K layers in the clay
assemblage decrease the centre of gravity position of
the X-ray pattern by increasing the relative area of
illite peaks and by shifting the position of interstratified peaks towards smaller positions in nanometers.
Based on this, the measurements of the centre of
gravity position in the 2:1 clay mineral range should
be related to the anhydrous K layer content of the
whole clay assemblage and may therefore be used as
an approximation of relative K content. Further
details can be found in Barré et al. (2007b).
The centre of gravity (cg) of the X-ray patterns was
easily calculated as follows:
cg ¼
.X X
ai posi
area of the peak i (WHH×intensity)
position of the peak i
Plant Soil (2008) 302:213–220
Peak WHH, intensity and position were all determined by the Decomp program (Lanson 1997). We
used the positions measured in nanometers rather than
in 2θ angle and the cg position will therefore be
thereafter given also in nanometers. Figure 2 shows a
decomposed X-ray pattern and the measurement of
the position of its centre of gravity.
of the clay minerals sampled in the 30 plots (F3,26 =
17.61, P=0.0001). We found a significant “fertK”
effect (F1,26 =14.25, P<0.001) which indicated that the
K fertilization treatment modified soil clay minerals.
There is no effect of the P fertilization (F1,26 =0.32, P=
0.57) nor an effect of the interaction between P and K
fertilizations (F1,26 =0.02, P=0.90). Figure 3 presents
cg positions plotted against the added K quantities.
Statistical analysis
All analyses were performed with SAS software
(version 8.2). We tested with the GLM procedure if
the fertilization treatments had an effect on the
position of the centre of gravity. The effects of the
model were “fertK”, “fertP” and the interaction
“fertP×fertK”. We also tested if the K budgets and
the position of the centre of gravity are related. Then,
we tested if the fertilization treatments had an effect
on clay Al, Si, Mg, Fe and K content. The effects of
this model were also “fertK”, “fertP” and the
interaction “fertP × fertK”. We performed linear
regressions between cg and clay K content and
between clay K content and clay Al, Si, Mg and Fe
contents with the GLM procedure.
Are soil 2:1 clay mineral modifications related to soil
K budget?
We calculated the mean positions of the centre of
gravity of the X-ray patterns of the clay minerals for
each fertilization treatment as well as the mean K
budget. Figure 4 presents the mean position of the
centre of gravity plotted against soil K budget. There
is a strongly significant linear relationship between
the centre of gravity positions and the soil K budget
(F1,4 =122.17, P<0.001, R2 =0.97). The higher the K
input the lower the position of the centre of gravity
and conversely, the higher the K output the higher the
position of the centre of gravity indicating a greater
presence of non-potassic smectites minerals.
Do fertilization treatments modify Al, Si, Mg, Fe or K
content of soil clay fraction?
Do fertilization treatments modify soil clay minerals?
The fertilizer treatments very significantly influenced
the position of the centre of gravity of the X-ray patterns
Position: 1.46 nm
Relative area: 0.57
Position: 1.42 nm
Relative area: 0.02
Position: 1.02 nm
Relative area: 0.14
Position: 1.16 nm
Relative area: 0.22
Position: 1 nm
Relative area: 0.05
Fig. 2 Decomposed X-ray
pattern of a clay fraction
from a K parcel. The bold
line represents the measured
diffractogram. Each curve
corresponds to an identified
clay mineral population.
The cg position was calculated as follow: cg=0.57×
1.31 nm
There is no significant effect of the fertilization
treatments on clay Al, Si, Mg of Fe contents. The
calculated F and P-values were respectively for Al,
SI, Mg and Fe: F3,26 =0.10, P=0.96; F3,26 =0.10, P=
0.96; F3,26 =1.03, P=0.39; F3,26 =0.65, P=0.59). At
of gravity position
* = centre
cg = 1.31 nm
º2θ Cu Kα
Plant Soil (2008) 302:213–220
cg position (nm)
clay K content (%)
Fig. 3 Relationship between cg position of X-ray patterns and
K fertilization. The equation of the linear regression is: Y=
the opposite, there is a significant effect of the
fertilizer treatment on the clay K content (F3,26 =
6.37, P=0.002). In particular, there is a significant
effect of the K fertilizer on clay K content (F1,26 =
8.54, P=0.007) and neither an effect of the P fertilizer
treatment (F1,26 =0.16, P=0.69) nor effect of an
interaction (F1,26 =0.1, P=0.74).
Are soil clay mineral modifications related to K
content of the soil clay fraction?
Figure 5 presents the cg position of the X-ray patterns
plotted against the measured K content of the clay
fraction. There is a strongly significant negative linear
relationship between cg positions and K content
(F1,28 =59.43, P<0.0001, R2 =0.68). Clay mineral
modifications are therefore strongly correlated with
the total K content of the clay fraction. It is also
interesting to consider the equation of the regression:
Y=−0.072 (±0.009) X+1.52 (±0.03).
This means that if the clay assemblage is K free the
cg position would be 1.52 nm which is exactly the
cg position (nm)
K budget (kg/ha)
Fig. 4 Relationship between cg position and soil K budget. The
equation of the linear regression is: Y=−4.20.10−6(±5.5.10−7)×X+
Fig. 5 Relationship between cg position of X-ray patterns and
clay K content
position of a bi-hydrated (K free) smectite. Moreover,
if we consider the cg position of a mica or a pure illite
peak (1 nm), the K content of this clay mineral would
be 7.21% according to the equation. A 7.21% K
content is in the range of the K content for illite
minerals (e.g. Newman 1987).
Are clay mineral changes related to clay Al, Si, Mg or
Fe content as well as those of K content?
There is no relationship between Al and K contents
(F1,28 =2.11, P=0.16). There is also no relationship
between Si, Mg, Fe and K content (F1,28 =1.81, P=
0.19; F1,28 =0.35, P=0.56; F1,28 =0.01, P=0.90). Clay
K content modifications are independent from Al, Si,
Mg or Fe contents.
The abscissa position of the cg of X-ray patterns
decreased significantly with K fertilizer addition.
There is also a very significant relationship between
the soil K budget and the abscissa position of the cg
of X-ray patterns. Our K budgets have been simplified because we did not take into account the aerial K
deposition and K leaching by water soil interaction
from the plots. However, the order of magnitude of K
leaching and K deposition generally considered in
temperate agrosystems (about 5–20 kg ha−1 year−1)
are significantly smaller than the quantities of K
brought as fertilizers or exported in the crops in our
study. Moreover, as deposition and leaching have
antagonistic effects on the K budget, ignoring both
phenomenon reduces the error on K budgets. Though
less informative than a complete K budget, we
Plant Soil (2008) 302:213–220
therefore consider that our simplified budget is
relevant to support our conclusions: the K budgets
in the plots are quantitatively related to clay mineral
modifications. This observation supports those general conclusions by Tributh et al. (1987) and Velde and
Peck (2002) for similar studies.
The modifications observed through X-ray measurements are linked to the modifications of total clay
K content. This confirms the quantitative relationship
under field condition, for a given clay assemblage,
between the position of the centre of gravity of X-ray
patterns and the measured clay K content observed in
a laboratory experiment performed on “simplified”
soils (Barré et al. 2007b). Taken together, all these
results show that if the K budget was positive the soil
clay minerals of the plot became more potassic with
anhydrous (1 nm) layers whereas the quantity of
anhydrous K layers decreased if the K budget was
negative. Indeed, a clay mineral assemblage with a
higher anhydrous K interlayer quantity has a higher
total K content and at the same time its X-ray pattern
has a lower cg position (more layers collapsed to
1 nm). Consequently, the present study provides the
first direct demonstration at the field scale, supported
by appropriate statistical analysis, that soil 2:1 clay
minerals behave as a K reservoir with a capacity to
respond to potassium availability.
Contrary to the clay K content, the fertilization
treatments did not modify Al, Si, Mg or Fe content of
the clay fraction. Moreover, clay K content variations
are not correlated with the variation of any of these
elements. As Al, Si, Mg and Fe are the most abundant
cations in the clay mineral sheets, these results suggest
that interlayer K fixation or release does not imply that
modifications of the major ions inside clay sheets
occurred. However, we did not test if the oxydation
degree of the Fe cations was modified by the fertilization treatments. If suggested by our study, we therefore
cannot claim that interlayer site occupation modifications are independent from layer charge modifications
and such a demonstration needs further research.
Our data allow an estimation of the order of
magnitude of the size of the K reservoir. The mean
difference between total K content of the clay fractions
of the plots which received the higher quantity of K
fertilizer (“PKinit”) and those which did not receive any
input (“0init”) is 2,4 g of K per kilogram of clays. If we
consider a soil with a density of 1,000 kg/m3 and a 20%
clay content, this means that the fertilizer treatments led
to a difference of about 1 t of K per hectare in the clay
fraction of the 0–20 cm zone. This is much more than
the K quantity needed yearly by plants (50 to 80 kg
ha−1 year−1 on average (Schlesinger 1991, p159;
Hinsinger 2002)). The reservoir could therefore largely
supply the whole K plant needs during the growing
season and take back the K yearly released as throughfall or during the litter decomposition. It means that the
net K flux could be nearly zero for the reservoir at the
yearly scale. Moreover, each year, the values of positive
or negative K fluxes are small compared to the K
quantity in the reservoir. These arguments may explain
why such an important mechanism for the K cycle in
terrestrial ecosystems has not been documented until
now. Indeed, the differences between soil clay mineralogy before and after the growing seasons should be
subtle and localized in the vicinity of roots. It means
that even if clay mineral modification occurs after the
growing season, the difference is not too diluted to be
observed in the bulk soil. Moreover, if the annual net
flux of K between soil 2:1 clay minerals and the
vegetation is nearly zero, even long term experiments
would not allow one to observe clay modifications.
That is why, drastic K fertilization treatments coupled
with the use of numerical X-ray pattern decomposition
program were needed to demonstrate that soil 2:1 clay
minerals behave as a K reservoir. The kinetics of the
filling and emptying of the reservoir in field condition
remains nonetheless an open question which needs
further investigations.
Clay minerals are also involved in the major soil
processes such as aggregate formation, soil organic
matter adsorption and soil cation exchange capacity
all considered to be properties vital to healthy plant
growth. Our study showed that clay minerals could be
modified within 46 years, i.e. rapidly compared to
assumed pedogenetic time scales. In particular, K
uptake in the vicinity of roots should strongly modify
clay minerals. We must therefore wonder if these
modifications have an influence on soil properties
influenced by clay minerals. If so, clay mineral
modifications should have some consequences for
the functioning of the rhizosphere. Such an idea is
supported by the study by Li et al. (2004) which
showed that the presence of K ions in interlayer sites
modifies pesticide sorption on smectite clays. Given
that clay minerals can react at the plant life time scale
and that they are involved in major soil processes, we
suggest that clay mineral analysis should be an
important perspective for a better understanding of
ecosystem functioning.
Our data show that soil 2:1 clay minerals behave as a
K reservoir. The filling or emptying of this reservoir
could be followed efficiently through X-ray measurements of materials formed under field conditions.
This reservoir is likely to have a key role for K cycle
in soils. The most evident implications are that it
could obviously supply short term K plant needs and
preserve long term ecosystem productivity by reducing K leaching. Our work also validates the use of the
centre of gravity position to qualitatively and quantitatively study clay mineral modifications in soil
dominated by illite and mixed layer illite−smectite
clay minerals which open many perspectives for soil
K cycle understanding from roots to ecosystem scale.
Acknowledgments We deeply thank the technical team of the
Grignon experimental station which has performed precisely
the fertilization protocol since 1959. We also acknowledge
Pascal Denoroy from the INRA Bordeaux who contributed to
develop the standard mineral contents of cultivated crops and
indicated us these values. We also thank François Rassineux of
the ERM society for his help for the clay elemental analyses
and the Hydrasa laboratory (Université de Poitiers, France) for
the use of their X-ray diffractometers. The help of Jacques
Mériguet for the field work was also much appreciated. We
acknowledge Gérard Lacroix, Elisa Thébault and Céline Hauzy
for their helpful comments on the manuscript.
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