3 Results and Discussion
3.1 SWRC
Figure 2 shows the modelled SWRCs for the measured means of the silty
sand and clayey loam used in our study. The agreement between the fitted
curves and the measured data was very good (R2=1.0 for
the clayey loam and 0.98 for the silty sand, respectively). The silty
sand’s SWRC was steeper and declined faster than the SWRC of the clayey
loam. A sudden steepening of the slope indicates a distinct air-entry
tension value, common for coarse soils (Wassar et al., 2016). The SWRC
for the clayey loam showed a very high water retention at a suction head
of 15,000 cm H2O. This water, held in the smallest pore
spaces, is considered immobile or residual water. Soils with a high clay
content appear to have a larger bound/residual water pool (Adams et al.,
2019). The residual water content for the clayey loam was 26.4±1.1 Vol
%, whereas the silty sand’s residual water content was 3.7±0.4 Vol %.
Differences in SWRCs are dependent on various soil properties such as
bulk density, organic carbon, soil texture and aggregate size (Lipiec et
al., 2007). Sandy soils involve mainly capillary binding, and therefore
release most of the water at higher potentials, while clayey soils, with
adhesive and osmotic binding, release water at lower (more negative)
potentials (Binkley and Fisher, 2012). Cryogenic water extraction
following pF 3 resulted in a mean water content for the silty sand of
4.8±2.6 Vol % and for the clayey loam of 28.7±0.9 Vol % indicating
that water extraction was not fully complete when compared to the values
at the end of the pressure extraction. Cryogenic water extraction
following pF 4.2 led to mean volumetric water contents of 3.8±1. Vol %
and 26.1±1.3 %, respectively, which is comparable to the residual water
content obtained via pressure plate extraction.
3.2 pF effects on isotopic
composition
When now comparing the stable isotope values in dual isotope space from
the sampling along the pF curve, most of the soil water isotope values
plotted below and slightly to the right of the Global Meteoric Water
Line (GMWL) (Fig. 3), whereas the introduced isotopic label (DIW)
plotted on the GMWL. In general, the pF curve extracts from the silty
sand and the clayey loam showed a similar isotopic composition and
extraction behavior. With increasing pF values, the
δ2H and δ18O composition tended to
get heavier and moved up and slightly to the right of the GMWL (apart
from pF 4.2). Only extracts from pF 4.2 showed a depletion in heavy
isotopes for both soil types in comparison to the introduced label
(DIW). This depletion was more pronounced for the silty sand extracts.
When looking at the cryogenic extracts, the clayey loam cryo extracts
taken after pF 4.2 did not show significant differences to the silty
sand’s extracts at pF 4.2, the silty sand’s cryo extracts taken after pF
4.2 and the clayey loam cryo extracts taken after pF 3. However, they
differed statistically significantly from all other pressure extracts.
The silty sand’s cryo extracts taken after pF 3 showed the largest mean
difference to the DIW in 18O-direction (3.3‰) and
+12.4‰ in 2H-direction, whereas the cryo extracts
taken after pF 4.2 and the pressure extracts from pF 4.2 differed from
the DIW mainly in negative 2H-direction by-9.35‰ and
-9.84‰, respectively. For the clayey loam, the pF 1.8 pressure extract
showed the largest mean difference to the DIW (+1.19‰ for
δ18O and +3.62‰ for δ2H). For both
soil types, the extracts from pF 1.4 showed the smallest mean difference
to the DIW in 18O-direction (-0.03‰ for the silty sand
and +0.06‰ for the clayey loam). For δ2H, the mean
difference to the DIW was within the range of the measurement accuracy
of the isotope analysis (-0.12‰ for the silty sand and +0.40‰ for the
clayey loam, respectively).
In general, the isotope values for the silty sand showed a much larger
SD than the values of the clayey loam extracts (Fig. 3). For the silty
sand, the cryo extracts from pF 3 were enriched in comparison to the
DIW, whereas, the cryo extracts from pF 4.2 were depleted in heavy
isotopes. Both differed significantly between each other (p=0.006). Such
depletion could be an artifact of the cryogenic extraction and has been
observed during water recovery tests of the same soil types in a dataset
by Orlowski et al. (2013) and by others (e.g., Adams et al., 2019).
However, in the study by Orlowski et al. (2013), the clayey loam cryo
extracts showed a much larger depletion than the silty sand extracts,
which was further dependent on water extraction times. The SDs of the
δ18O values of the silty sand ranged between 0.02 and
0.73‰ and for the clayey loam between 0.07and 0.22‰ (pF 1.4–4.2).
Whereas for δ2H, the SDs of the silty sand varied
between 0.10 and 9.03 and for the clayey loam between 0.30 and 2.21.
Except for the SD of 9.03 (pF 4.2, silty sand), the SDs of the
δ2H and δ18O values were either
within the range of the measurement accuracy for our isotope analyses
(±0.2/±0.8‰ for δ18O/δ2H) or only
slightly higher.
Lin et al. (2018) and Lin and Horita (2016) showed that the equilibrium
fractionation factor changes if the vapor pressure controls the quantity
of water adsorbed on a surface. These surface isotope effects are much
stronger for 2H than for 18O (Chen
et al., 2016; Lin et al., 2018). Gaj and McDonnell (2019) found out that
soil tension affects the equilibrium fractionation factor for soil
tensions above pF 3.1 (1,260 hPa). Their study included the exact same
soil types as used in our study. In their study, the tension effect on
the equilibrium fractionation factor increased linearly with increasing
soil tension, which was independent on soil texture. The higher the soil
tension, the farther away the isotope values plot from the introduced
isotope label in their water recovery study using the water-vapor
equilibrium method by Wassenaar et al. (2008). The authors hypothesized
that adhesion is the cause of the additional fractionation on the water
vapor isotopic composition. In our study, liquid water extracts were
compared and not vapor samples. Nevertheless, our work showed that under
dry conditions, soil tension is the main driver for isotope
fractionation leading to isotopically enriched water extracts (see Fig.
3). According to Gaj and McDonnell (2019) immobile water at high soil
tension would be depleted in the heavy isotopic species, which was only
the case for the pF 4.2 extracts in our study but we did not take
samples between pF 3 and 4.2, which could have further underlined this
hypothesis. Nevertheless, this does not explain the isotopically
enriched values of the silty sand’s cryo extracts taken after pF 3 (Fig.
3). We therefore suggest that future studies should consider testing
soil tension effects for a variety of different soil types and at much
higher resolution than in our study. However, if our findings are
supported by others in future experiments, this would have important
consequences for recent research on plant water uptake studies and
interpreting the water isotopic composition of mobile and bulk water in
soils (with respect to the tension water is held in the soil). We know
that plants can apply high tensions to withdraw soil water, especially
under dry conditions and that responses to water stress are
species-specifically different (Fotelli et al., 2000). Given our
findings, this would imply that isotopically more enriched water would
be taken up by plants during dryer conditions (at higher tensions).
However, soil properties and the soil’s water retention characteristics
affect the isotopic composition of this water pool. Further, water
uptake strategies of plants are highly species-specific and are not only
influenced by soil water availability (Larcher, 2003). In contrast,
Vargas et al. (2017) showed that avocado plants might preferentially
take up 1H and 16O, leaving the
remaining pool of water in the soil enriched. This discrimination was a
function of the soil water loss and soil type. Barbeta et al. (2020)
recently conducted a drought experiment with Fagus sylvaticawhere they compared the soil and stem water isotopic compositions. Under
drier conditions, the authors observed soil-stem isotopic offsets. They
hypothesized that adsorbed water dominates the fraction of bulk soil
water under dry conditions. Depending on the balance between the
isotopic enrichment caused by evaporation and the depletion caused by
the higher fraction of adsorbed water, the isotopic composition of bulk
soil water may therefore exhibit different trends with regard to its2H and 18O composition. This further
affects the observed soil-stem isotopic offset (Barbeta et al., 2020).
Lu (2016) demonstrated that the fraction of adsorbed water varies highly
depending on the soil type and may range from 1.7% VWC in sandy soils
to 12.8% VWC in silty clay soils. In the experiment by Barbeta et al.
(2020), soil types showed a significant effect on the drying rate. Thus,
exploring soil water retention characteristics with regard to plant
water availabilities is important when comparing soil and plant water
isotopic compositions and drawing assumptions with respect to plant
water uptake depths, times and water stress responses.
3.3 Isotopic variation over
time
Figure 4 depicts the δ2H and δ18O
variation over 7 days for the two soil types when permanently exposed to
a pressure of 15 bar in the pressure extractor (experiment 2). In
general, the δ2H and δ18O values
showed a similar trend. The silty sand extracts’ isotope values started
off within the range of the DIW values, approached LW values over time
and then decreased again. More specifically, the δ-values started to
decrease after 30 min of extraction. From 150 min to 7,200 min the
δ-values increased until the final sample reached
approximately the starting value of -58.7‰ for δ2H.
For silty sand’s δ18O values, the last sample’s
δ18O value was by 0.5‰ higher than the starting value
of -8.5‰. For silty sand’s δ2H values, the point of
greatest difference to the starting value was reached after 60 min and
it seemed like a plateau was reached which remained constant (within the
range of measurement inaccuracy) for the next five values. The greatest
SDs for the silty sand isotope values were observed at 7,200 min
extraction time for both isotopes.
For the clayey loam, the isotopic variation over time was generally
smaller. Interestingly, the δ18O values remained close
to the DIW value (-8.6±0.2‰) almost over the entire 7 day extraction
period, which was not the case for the silty sand extracts. This is
surprising since LW was used for spiking the soil samples and DIW for
wetting the ceramic plates of the extractors. The δ2H
values remained within the range of the starting value (-58.7‰) and the
DIW (-58.4‰), respectively, until they slightly decreased from 60 min
onwards. The difference to the LW was smallest after 150 min extraction
time. Surprisingly, the δ-values never reached the LW isotope values
used for spiking, neither for δ2H nor for
δ18O. The last sample’s δ-values (at
10,080 min) were even more positive than the DIW signature and showed
the greatest difference to both the DIW and LW.
Figure 5 shows the mean differences over time between the clayey loam
and silty sand extracts compared to the DIW’s and LW’s
δ-values. Statistically, mean δ2H
and δ18O differences to the LW were significantly
different from zero for both soil types (p=0.00). However, the mean
δ2H and δ18O differences to the DIW
were only significantly different for the silty sand (p=0.00 for
δ18O and δ2H) but nor for the clayey
loam (p>0.05 for both isotopes).
Mean differences for the clayey loam extracts to the DIW ranged from
-2.7‰ to 0.1‰ for δ2H and from -0.4‰ to 0.1‰ for
δ18O, respectively. Mean differences to the LW showed
a larger variation: from 3.6‰ to 6.3‰ for δ2H and from
0.8‰ to 1.3‰ for δ18O. For the δ2H
values of the silty sand extracts, mean differences to the DIW ranged
from -5.6‰ to 0.0‰ and to the LW from 0.7‰ to 6.3‰. For
δ18O, mean differences to the DIW ranged from -1.1‰ to
0.6‰ and to the LW from 0.1‰ to 1.8‰. For the δ18O
values of the clayey loam extracts, largest differences occurred at
extraction times of 80 min (to DIW) and 10,080 min (to LW). Generally,
δ-value differences were smaller to DIW than to LW for the clayey loam.
This was not the case for the silty sand extracts. Interestingly, silty
sand extracts from 60 to 105 min showed the smallest isotopic difference
to the introduced LW, which was used for spiking the soil samples.
Thus, our null hypothesis that soil water collected sequentially over a
period of 7 days under a 15 bar pressure would not differ isotopically
from the introduced isotopic label did not hold true. The time at which
the water draining from the pressure extractor did play a crucial role
for the recovery of the introduced isotopic label. We further observed
that the clayey loam extracts seemed to have liberated water from the
ceramic plates, which was imprinted in the extracted isotopic signature
(Fig. 4 and 5). However, the spiked label for the ceramic plates could
still be recovered via cryogenic vacuum extraction and did not differ
statistically significantly from the DIW (see Fig. 5). Thus, not as
expected, we observed a temporal change (over 7 days) of the isotopic
composition of the water extracted at 15 bar. Our findings suggest that
isotopically different fractions of water in the two soil types were
released over time. Since the silty sand releases water much faster than
the clayey loam (Fig. 2), the introduced isotopic label was visible in
the extracts after 60 min. However, for the clayey loam, it is obscure
that even under 15 bar pressure a different soil water fraction can
remain almost constant over time (Fig. 4) and the introduced isotopic
label could not be recovered even when considering the given SD. This
somehow contradicts the findings of others (Sprenger et al., 2016;
Vargas et al., 2017) that tightly bound soil water quickly exchanges
with mobile water in soils. Barbeta et al. (2020) demonstrated in a
drought experiment with Fagus sylvatica an opposite isotope trend
for δ2H and δ18O in soil water when
the permanent wilting point had been reached and water potentials fell
below -1MPa. They argue that soil evaporative enrichment creates a
stronger enrichment in 18O than in2H and surface isotope effects are much stronger for2H than for 18O (Chen et al., 2016;
Lin et al., 2018). Given this, it is possible that soil water
δ18O enriches while soil water δ2H
becomes depleted, at least when the soil water balance is dominated by
root water uptake.
Gaj and McDonnell (2019) hypothesized that the soil water and soil vapor
fractionation at high soil tension are driven by the surface properties
and the ionic strength of the remaining soil solution. This implies that
e.g. the interlayer space of clay minerals and mineral surfaces impact
the amount and strength at which water is held in the soil (Gaj et al.,
2017; Oerter et al., 2014). Further, water retention and O and H
interactions with the soil matrix are higher for clay soils than for
sandy soils (Thielemann et al., 2019). Adams et al. (2019) showed that
the retention increased with increasing clay and silt contents. Our
silty sand consisted of 92.7% sand and only 4.8% silt but our clayey
loam had a clay fraction of 41.9%, which is rich in Vermiculite (43.4
%) (see Orlowski et al., 2018a). This is a 2:1 clay with a medium
shrink-swell capacity but high cation exchange capacity. This has been
shown to affect mineral-water interactions and thus cause isotope
fractionation effects (Gaj et al., 2017; Meißner et al., 2014; Oerter et
al., 2014). This might explain why in our experiment 2, the clayey loam
water extracts showed a different extraction behavior to the silty sand
when exposed to the highest pressure level (Fig. 5 and 6) and the clayey
loam soil water most likely interacted with the ceramic plate water.
Nevertheless, our results showed an extraction time-dependent effect on
soil water held at 15bar. This might have implications on how we sample
and interpret plant available soil water. If at a certain point in time
plants would apply a constant tension to take up soil water, the timing
of sampling for studying plant water uptake patterns would be highly
relevant; since given our findings at 15bar pressure, the soil water
isotopic composition would change over time. We admit that this is
highly speculative but underlines the need for more research on
time-variant changes in soil water pools relevant for plant water supply
by e.g. simultaneously applying high-resolution in-situ isotope
measurements at the soil and plant level.
3.4 Isotopic variation over time in dual isotope
space
While testing whether soil water collected sequentially over a period of
7 days at 15 bar, we observed different effects on 2H
and 18O when compared to the cryogenically extracted
samples and the introduced labels (to the soils and ceramic plates). For
δ18O, the DIW showed no statistically significant
differences to the clayey loam extracts and the cryogenically extracted
silty sand. However, for δ2H, DIW was significantly
different to all tested subgroups (Fig. 6). Interestingly, the LW was
statistically similar to the cryogenically extracted water from the
ceramic plates for both isotopes. Additionally for δ2H
there were no significant differences to the cryogenically extracted
silty sand samples (p=0.41). The clayey loam extracts were statistically
similar to the cryogenically extracted silty sand samples for
δ18O, which was not true for δ2H
(p=0.001). For δ18O, the cryogenically extracted
clayey loam samples showed no significant differences to the silty sand
samples from the pressure extractor but were significantly different to
the clayey loam samples (p=0.006). The cryogenically extracted silty
sand samples on the other hand did not differ significantly from the
silty sand samples from the pressure extractor. This did not hold true
for δ2H. But the cryogenically extracted silty sand
samples were statistically similar to the cryogenically extracted
ceramic plates and the cryogenically extracted clayey loam samples for
δ2H. Given the statistical differences between the LW
(used for spiking the soil samples) and the pressure plate extracts, we
had to reject our null hypothesis. Soil water collected sequentially
over 7 days at 15 bar did not show the same isotopic composition as the
water used for spiking the samples. The water draining from the clayey
loam surprisingly showed statistical similarities to the
δ18O of the DIW (p=0.67) used for rewetting the
ceramic plates of the extractor. This was not the case for
δ2H. However, over time the LW must have exchanged the
DIW of the ceramic plates, since we did not find statistical differences
between the ceramic plate water and the LW for both isotopes. This leads
to the conclusion that there was an isotopic exchange between the
ceramic plate water and the water to be extracted from the saturated
soil samples. Surprisingly, the exchange did not occur in experiment 1,
which leads us to the conclusion that it is pressure level dependent. If
it would have occurred, the extracted water isotopic composition would
have plotted closer to the spiked isotopic label.
Through the cryogenic extraction of the silty sand, the isotopic
composition of the water changed in a way that the δ2H
values became more negative and the δ18O values became
more positive in comparison to the water extracted from the pressure
plates. Interestingly, the cryogenically extracted water from the clayey
loam was more depleted in both heavy isotopes (2H and18O) than the clayey loam water extracted via the
pressure plate but also the cryogenically extracted water from the silty
sand was more depleted than the silty sand water from the pressure plate
extraction. For a spiking experiment with the same soil types, Orlowski
et al. (2013) observed that the cryogenically extracted silty sand water
was more enriched in heavy isotopes and showed a smaller deviation from
the spike water than the clayey loam, which plotted furthest away from
the spike water. Here we saw a similar behavior with the cryogenically
extracted clayey loam samples being more depleted but only the hydrogen
isotopic composition of the silty sand extracts did not differ
significantly from the introduced LW. Thus, we observed a deviation from
the water used for spiking for both soil types.
When plotting the data in dual isotope space, we found statistically
significant linear regressions for the samples of the different
subgroups. The cryogenically extracted silty sand samples showed a much
higher correlation among each other (R2=0.97) than the
cryogenically extracted clayey loam samples (R2=0.62).
The same was true for the clayey loam (R2=0.65) and
silty sand samples (R2=0.96) from the pressure
extractor (Fig. 6). The slopes of the different regression lines were
very similar: The silty sand’s line had a slope of 4.01, the clayey
loam’s of 4.50 and the cryo silty sand’s of 4.14; only the cryo clayey
loam’s slope of 5.81 was slightly higher than the others. The intercepts
of the regression lines from the cryogenically extracted samples and the
pressure plate extraction for both soil types showed a difference of
approximately 5. In the study by Gaj and McDonnell (2019), which
included the same soil types, the slope of the regression lines
decreased with decreasing grain size. Their sandy soils plotted on a
regression line with a slope of 3 and the clayey soils’ regression line
had a slope of 2. Such strong grain size dependency was not reflected in
the slopes of the regression lines in our study. Much rather was there a
cryogenic extraction induced effect on the clayey loam as previously
observed by Orlowski et al. (2013).
In summary, the soil water sampled sequentially over 7 days at 15 bar
deviated from the spiking water. The observed deviations changed over
the time of the experiment and were larger for the clayey loam than for
the silty sand and also different for the two isotopes
(2H and 18O).