The number of ecohydrological studies involving water stable isotope measurements has been increasing steadily due to technological (e.g., field-deployable laser spectroscopy and cheaper instruments) and methodological (i.e., tracer approaches or improvements in root water uptake models) advances in recent years. This enables researchers from a broad scientific background to incorporate water-isotope-based methods into their studies.
Several isotope effects are currently not fully understood but might be essential when investigating root water uptake depths of vegetation and separating isotope processes in the soil–vegetation–atmosphere continuum. Different viewpoints exist on (i) extraction methods for soil and plant water and methodological artifacts potentially introduced by them, (ii) the pools of water (mobile vs. immobile) measured with those methods, and (iii) spatial variability and temporal dynamics of the water isotope composition of different compartments in terrestrial ecosystems.
In situ methods have been proposed as an innovative and necessary way to address these issues and are required in order to disentangle isotope effects and take them into account when studying root water uptake depths of plants and for studying soil–plant–atmosphere interaction based on water stable isotopes. Herein, we review the current status of in situ measurements of water stable isotopes in soils and plants, point out current issues and highlight the potential for future research. Moreover, we put a strong focus and incorporate practical aspects into this review in order to provide a guideline for researchers with limited previous experience with in situ methods. We also include a section on opportunities for incorporating data obtained with described in situ methods into existing isotope-enabled ecohydrological models and provide examples illustrating potential benefits of doing so. Finally, we propose an integrated methodology for measuring both soil and plant water isotopes in situ when carrying out studies at the soil–vegetation–atmosphere continuum. Several authors have shown that reliable data can be generated in the field using in situ methods for measuring the soil water isotope composition. For transpiration, reliable methods also exist but are not common in ecohydrological field studies due to the required effort. Little attention has been paid to in situ xylem water isotope measurements. Research needs to focus on improving and further developing those methods.
There is a need for a consistent and combined (soils and plants) methodology for ecohydrological studies. Such systems should be designed and adapted to the environment to be studied. We further conclude that many studies currently might not rely on in situ methods extensively because of the technical difficulty and existing methodological uncertainties. Future research needs to aim on developing a simplified approach that provides a reasonable trade-off between practicability and precision and accuracy.
Since the presentation of the heavily debated “two water worlds hypothesis” (McDonnell, 2014) the attention of many ecohydrologists – especially those working with water isotopes – has been focusing on what was termed as “ecohydrological separation”. In the original hypothesis, the authors claim that based on the studies of Brooks et al. (2010) and Goldsmith et al. (2012) plants in some watersheds prefer water which is “more difficult” for them to access (i.e., soil water with relatively higher matric potential) over “easier” accessible water sources (i.e., soil water with low matric potential that eventually becomes stream water).
The discussion remains controversial, with a number of critical responses. Sprenger et al. (2016), for instance, offer a simple and logic explanation for “ecohydrological separation”: “subsequent mixing of the evaporated soil water with nonfractionated precipitation water could explain the differences in the isotopic signal of water in the top soil and in the xylem of plants on the one hand and groundwater and streamwater on the other hand” (refer to Fig. 8 in Sprenger et al., 2016). Hence, the authors question “if ecohydrological separation is actually taking part or if instead the soil water undergoes isotopic changes over space (e.g., depth) and time (e.g., seasonality) leading to distinct isotopic signals between the top soil and subsoil, which will directly affect the isotopic signal of the root water.” (Sprenger et al., 2016). Furthermore, plant physiological (rooting depth and water potential of plants) and aspects such as nutrient availability or the interplay between water demand vs. water availability were completely neglected in the theory (which the authors themselves admit; McDonnell, 2014). Especially the latter aspects have been omitted in many studies (partially this might be because many of those were conducted by hydrologists, not plant experts). Plants might not want the “easily accessible” water, for instance, if this water is poor in dissolved oxygen or nutrients and therefore use the “less available” water preferentially. An example for this might be tropical catchments, where soils are often nutrient-poor, but stream or fresh rainwater contains the majority of nutrients. Recently, Dubbert et al. (2019) stated that isotopic differences between soil, plant and groundwater can be fully explained by spatiotemporal dynamics and that based on a pool-weighted approach, the effect of different water pools should be negligible. Lastly, Barbeta et al. (2020) carried out a systematic experiment to study the isotopic offset between soil and stem water and found that differences are “likely to be caused by water isotope heterogeneities within the soil pore and stem tissues… than by fractionation under root water uptake” (Barbeta et al., 2020).
Nevertheless – whether one agrees with the theory or not – the hypothesis had a significant impact in terms of (i) questioning the comparability of ecohydrological studies because of methodological artifacts (e.g., mobile vs. bound soil water, soil and plant water extraction methods, and organic contamination), (ii) testing existing and developing novel methods to investigate fundamental processes at the soil–vegetation–atmosphere continuum in an integrated manner, and finally (iii) questioning a number of concepts that have been applied for many years but now appear in a new light (e.g., root water uptake studies and the incorporation of isotope effects).
Consequently, many researchers have been focusing on these issues since and a number of publications have been pointing out current limitations and
ways forward (Berry et al., 2018; Bowling et al., 2017; Brantley et al., 2017; Dubbert et al., 2019; Penna et al., 2018; Sprenger et al., 2016). One
of the most pressing issues identified is the establishment of a consistent, homogenized method for the analysis of water stable isotopes allowing for
a solid analysis and interpretation of water isotopes in soils and plants and comparison with each other. Berry et al. (2018) postmarked current
methods applied in ecohydrology as “shotgun” methods, which is a suitable metaphor to describe how many studies are carried out. What they call for
is establishing consistent and continuous methods of monitoring. Due to partially striking differences in
Certainly, not all source water studies are biased. Due to the systematic evaluations carried out in recent years and despite all the controversy on
methodological aspects (Gaj et al., 2017; Millar et al., 2018; Orlowski et al., 2013, 2016a, b, 2018a, b; Thoma et al., 2018) it can be stated that in (i) soils that
contain a high portion of sand (low portion of clay), (ii) studies using isotopically labeled tracers (
However, there are a number of isotope effects that clearly complicate the idealized situation, where one takes a xylem sample from a tree (unfractionated mixture of all water sources) in addition to sampling a soil profile (and perhaps groundwater) and subsequently determines root water uptake depths. An updated view of the isotope effects potentially affecting water sources and consumers, depicted in Fig. 1, emphasizes the sheer complexity that now is questioning many water uptake studies.
A compilation of isotope effects potentially affecting the soil and plant water isotope composition.
In addition to the isotope effects summarized in Fig. 1, there might be methodological alteration of the isotope composition caused by different
extraction methods extracting different water pools and organic contamination causing an offset of
The community seems to agree on three key challenges (Brantley et al., 2017; Dubbert et al., 2019; Sprenger et al., 2016; Stumpp et al., 2018; Werner and Dubbert, 2016): (i) to develop consistent and comparable methods for a holistic monitoring of soil–plant–atmosphere interaction; (ii) to further investigate, disentangle and quantify the abovementioned isotope effects by increasing the spatiotemporal resolution of water isotope measurements at the soil–vegetation–atmosphere interface; and (iii) to decrease the uncertainty when studying root water uptake by integrated measurements of sources and consumers into one framework. In other words, we need combined in situ systems for measuring both soil and xylem water isotopes in a higher spatiotemporal resolution in order to achieve an integrated analysis of soils and plants using the same methodology and ultimately, measure the same water pools (Sprenger et al., 2016). While it might be possible to achieve a high temporal resolution by destructive sampling, a number of disadvantages are associated with that: for instance, the experimental plot is disturbed multiple times; small-scale heterogeneity might bias the outcomes; and longer-term studies in a high temporal resolution are basically impossible. For plants, a high frequency of destructive sampling might harm the plant irreversibly. Lastly, when carrying out longer-term studies the time and costs associated with destructive sampling and analysis might outbalance effort and benefits. Hence, in situ methods are essential for the detection, analysis and interpretation of related isotope effects (see Fig. 1).
This review aims at summarizing recent advances in in situ water isotope measurement techniques for soils (depth-dependent and bulk soil) and plants (xylem and transpiration via physical leaf chambers). We begin with an overview of in situ studies in the compartments of soils and vegetation. From thereon, we focus in separate sections on main issues emerging from the existing studies, namely (i) materials and measurement systems; (ii) calibration, standardization and validation; and (iii) comparability with water extraction studies and measurement of natural abundances of water isotopes. We then conclude and propose ways forward in terms of a combined approach for a consistent, integrated method in order to study the temporal dynamics of processes at the soil–vegetation–atmosphere continuum.
A number of early partially in situ studies (pre-IRIS; isotope-ratio infrared spectrometry) exist, where researchers collected soil water vapor. For the sake of completeness and
acknowledging these pioneering efforts, those will be summarized briefly. Thoma et al. (1979) directed water from up to 25
With the introduction of IRIS, rapid progress in terms of continuous measurements and field-deployable systems began. Koehler and Wassenaar (2011)
were the first to show that unattended, continuous measurements of the water isotope composition of natural water samples (lakes, rivers and groundwater)
based on isotopic equilibration between liquid and vapor phases are possible by using a gas-permeable membrane contactor connected to a laser
spectroscope. A similar gas-permeable membrane system was tested by Munksgaard et al. (2011). The first reported in situ measurement
of soils was reported by Herbstritt et al. (2012). A microporous hydrophobic membrane contactor was combined with an isotope laser spectrometer and
tested for both pure liquid water and water that was directed through a soil column. The authors determined isotopic-equilibrium fractionation factors
for a range of temperatures by fitting the empirical factors
Though insightful for testing the liquid–vapor isotopic equilibrium for continuous measurements and the effect of contactor membranes on isotopic equilibrium and fractionation factors, the approaches of the abovementioned studies were not applied further for soil water isotope measurements. Instead, two different types (or “families”) of gas-permeable membrane probes evolved, which both are based on similar principles but differ in design and level of complexity.
The first of these types of membrane systems was introduced by Volkmann and Weiler (2014) and thereafter used mainly by this research group for
measuring soil and later also xylem water isotopes (compare Sect. 2.3). The authors developed specific probes for the purpose of sampling soil water
vapor. The main elements of these probes are a microporous membrane (Porex, Aachen, Germany); a mixing chamber; and a sample, dilution and –
optionally – a throughflow line. The principle of operating the probes is based on drawing soil water vapor into the water isotope analyzer via the
sampling line (30–35
The second type of gas-permeable probes originates from the study of Rothfuss et al. (2013) and has been applied in different forms and by different
groups since then (see below). A major advantage of the gas-permeable membrane used (Accurel®PP V8/2HF, Membrana GmbH, Germany;
0.155
Another group from the United States developed a system for in situ measurements of soils and has applied the same type of gas-permeable membrane
probes in several studies (Oerter et al., 2017, 2019; Oerter and Bowen, 2017, 2019). In principle, the authors use the same methodology as presented
by the group around Rothfuss but provide a more flexible design of probes and a stand-alone solution for true field measurements (Fig. 4). Their
system – to date – probably constitutes the most complete in terms of field deployability and calibration, and the results reflect that (in
particular see Oerter et al., 2017 and Fig. 4 and Sect. 3). The authors further present a novel approach for correcting their samples by including
water and clay content (see Sect. 3 calibration). In a primer, Oerter et al. (2017) used a vapor-permeable membrane technique and measured soil water
isotopes in situ at four sites in North America and validated the water vapor probe method with the bag equilibration method and vacuum extraction
with subsequent liquid water analysis. The authors found that the accuracy of the three compared methods in their study is equivalent, with increased
ease of use in its application, and sample throughput rates of seven samples per hour by using the vapor probes. In fact, RMSE of the vapor probe method
for
The isotopic composition of soil evaporation, transpiration and evapotranspiration can be measured in situ using laser spectrometers coupled to
different chamber systems. These chamber-based in situ techniques were among the earliest development steps of in situ water isotope monitoring,
well before the development of membrane-based approaches. There are two types of chamber systems to measure soil evaporation and plant transpiration
fluxes and their isotope composition: flow-through steady-state (Dubbert et al., 2013) and closed-chamber systems (Wang et al., 2013). In a closed
chamber the amount of water vapor will, upon closure of the chamber, increase over time, while the
The isotope composition of soil evaporation has been predominantly used to achieve a better understanding of the dynamics of hydrological processes
(Braud et al., 2005b, a, 2009a, b; Haverd et al., 2011) and to partition evapotranspiration into its components: soil evaporation and plant
transpiration (e.g., Dubbert et al., 2013, 2014b; Haverd et al., 2011; Rothfuss et al., 2012, 2010; Williams et al., 2004; Yepez et al., 2007,
2003). Prior to the development of IRIS there were few studies measuring
Therefore, first approaches to combine soil gas exchange chambers and laser spectrometers concentrated on conducting sensitivity analysis of the CG
model towards its input parameter (relative humidity, soil temperature, soil water content, and the isotope composition of soil water and atmospheric
vapor; Braud et al., 2005a, b, 2009a, b; Dubbert et al., 2013; Haverd et al., 2011; Rothfuss et al., 2010, 2012). Conclusively, the correct
estimation of the evaporation front is particularly important. Usually the soil layer with the highest
In any case, direct in situ measurements of soil evaporation are mostly limited to laboratory studies conducting sensitivity analysis of the Craig–Gordon model and its input parameters as well as calculating kinetic fractionation (for a recent paper, see Quade et al., 2018). It is often not
technically possible to observe
Direct estimation of
In situ observations of
Similar to in situ soil evaporation isotope observations, in situ observations of
Direct in situ estimates of the isotope composition of transpiration have also been used to derive root water uptake proportions (Kühnhammer
et al., 2020; Volkmann et al., 2016b) by assuming isotopic steady state and substituting
For the direct measurement of plant xylem water isotopes, only two studies are reported to date. Volkmann et al. (2016a) present field observations of the xylem water
isotope composition of two adult field maple trees (
In situ measured xylem water isotopes (
With the obtained data, Volkmann et al. (2016a) demonstrated that temporal changes as well as spatial patterns of integration in xylem water isotope
composition can be resolved through in situ measurement. In both studied trees, diurnal cycles of xylem water isotopes were found. However, the
authors could not prove whether this is a true diurnal cycle or introduced through imperfect accounting for temperature-dependent liquid–vapor
fractionation at the probe interface. The authors achieved a median precision of 1.1 ‰ for
Marshall et al. (2020) tested an alternative method for measuring the isotope composition of tree xylem and showed that both natural abundances and
highly enriched
Xylem water
With the additional measures taken and the developed model, Marshall et al. (2020) suggest that this deviation was due to nonequilibrium conditions
in the upper borehole due to its small diameter (relative humidity of sample air was 98
Concluding this section, Table S1 in the Supplement provides information on all reviewed studies, details on the setup and main findings as well as advantages and disadvantages of the applied methodologies.
Apparent from the review of studies is that in situ measurements are still in the development stage; hence, applied methods and approaches vary greatly. In this section, we pick out key aspects that need to be considered and propose a way towards more comparable and homogeneous setups. The biggest and most critical issues emerging from the existing studies are (i) the materials and approaches used for sampling the water vapor, (ii) the calibration of the system, (iii) the avoidance of condensation, and (iv) the method of validating the in situ data compared to other methods and interpreting it best. We focus in this section on methods for obtaining in situ depth profiles of soil water isotopes and the measurement of xylem water isotopes due to the fact that methods for monitoring bulk soil evaporation and transpiration at the leaf level have been discussed previously in detail (Soderberg et al., 2012; Song et al., 2015a, b).
Most of the reviewed studies used gas-permeable membranes (e.g., Accurel PP V8/2HF, Membrana GmbH; 0.2
The number of in- and outlets of the probes depends on the measurement approach. In general, two of these exist: (i) a pull-only system (e.g., Volkman and Weiler, 2014), where water is drawn simply through the gas-permeable membrane by the force of the vacuum pump of the laser spectrometer. Such a system in fact requires only one capillary and thus is the simplest of the setups. However, it should be considered that a notable amount of air is drawn from the media to be measured (soil or plant). This could be especially relevant for applications in tree xylem, as it might increase the risk of cavitation and hence, damage the plant. A pull-only method might not even be possible in trees due to the different structure of xylem compared to soil. The extracted volume of soil water vapor can easily be calculated by multiplying the flow rate with the measurement time. Most studies use (ii) probes with two capillaries: one in- and one outlet (e.g., Oerter et al., 2017; Rothfuss et al., 2013). This changes the approach drastically, because now dry air is pushed through the inlet (via a dry-gas supply), entering the membrane from one side and leaving it at the outlet. During the passage of the dry air, water from the soil air diffuses into the membrane and exchanges isotopically through the gas-permeable membrane. Unless soils are extremely dry, saturated sample air can be assumed to be in isotopic equilibrium with liquid soil water. However, the isotopic-equilibrium fractionation factor could be affected by soil water tension (Gaj and McDonnell, 2019) as well as wettability, texture and chemical composition of soil surfaces (Gaj et al., 2019). Directing air into the system in the push-through method has two consequences. First, one needs to get rid of this excess air before it enters the laser spectrometer to avoid damage. This is commonly achieved by an open split just before the analyzer inlet. Second, the chance of external water vapor entering the stream can be excluded, as long as air is coming out of the open split, which is a clear advantage over the push-only method, where it needs to be assured that all connections are airtight.
The pull-only system can also be operated with an additional inlet capillary or tube connected to a reservoir with drying agent. Doing so, atmospheric or dry air (via passage through a drying agent) is drawn into the gas-permeable probe and equilibrated therein during the passage. Flow rates, however, are not adjustable using this approach.
It needs to be carefully decided which approach is to be used, and, ultimately, this depends on the application (e.g., tracer test, measuring natural abundances and long- vs. short-term measurements). A pull-only system is technically much easier to build, install and maintain and is also cheaper, but it it is critical to avoid external air entering the system at any of the connections. The push-through approach is more flexible, and flow rates can be adjusted, but it requires more maintenance, connections (for provision and control of dry air at the inlet) and valves.
Figure 4 depicts a schematic of an in situ soil water isotope system (reprinted with permission from Oerter and Bowen, 2017).
Schematic drawings showing
Condensation (or better: avoidance of it) is the most critical practical issue for all in situ approaches, regardless if soils or plants are
measured. If condensation occurs inside of the tubing or inside the chamber, the Dry air is diluted directly in the membrane system (Volkmann et al., 2016a; Volkmann and Weiler, 2014) or shortly after (Oerter et al., 2017;
Oerter and Bowen, 2017, 2019; Rothfuss et al., 2013, 2015, Kühnhammer et al., 2020). This way, the water vapor concentration of the system is
lowered, and condensation is less likely. The tube is heated (suggested by Gaj et al., 2016). Assuring that the temperature of the transport line is always warmer than the temperature at
the sampling location will avoid condensation to occur. Even in warm climates this might be necessary as solely the temperature difference between the
location where water vapor is equilibrated (i.e., inside of the gas-permeable probe) and the sampling line is decisive if condensation occurs or not
(refer to section recommendations for further elaboration on this issue).
Flushing the system with dry air prior to the measurement removes water that condensated before the current measurement (Kühnhammer et al., 2020; Volkmann and Weiler, 2014). An ideal system would include different measures to automatically ensure the prevention of condensation both during and in between measurements. During measurements condensation is prevented by dilution with dry air and heating of tubing prior to that point. A three-way valve directly after the measurement point could be included to remove liquid water from the gas-permeable tubing or borehole without having to pass it through the whole system. In between measurements it could be used to cut off the measurement point from the rest of the system while decreasing the relative humidity from the sampling point to the analyzer via the dilution line.
Condensation occurs whenever the temperature inside the sampling line (e.g., inside of a soil gas probe) is cooler than on the outside (e.g., atmospheric air). This is often likely and will affect the water isotope data tremendously. Hence, it is of utmost importance to include measures to avoid it in the sampling design while checking measurements for it regularly, and it is best to avoid it altogether. However, it is not always easy to identify. For this reason, we present three examples of (raw) isotope measurements in Fig. 5 which depict (i) a “good” measurement cycle, (ii) a measurement cycle initially influenced by condensation but then turning into a clean measurement once the condensation disappears, and (iii–iv) a bad measurement cycle with condensation affecting the complete data. Figure 5 shows extracts from data collected by the authors during a field campaign in Costa Rica in the beginning of 2019.
Measurement cycle of an in situ system switching through different probes. Shown are water vapor concentration (wvmr – water vapor mixing ratio – in ppmV) and the raw vapor values for
The calibration of water
Ideally, isotope standards are prepared in the same medium that is measured. That means one should use soil standards when measuring soil water
isotopes and use water standards when measuring liquid water samples, as well as use the same probes (e.g., membrane material) and sample flow
rates. Gaj and McDonnell (2019) provided empirical evidence that soil matrix effects can affect the fractionation factors in soils and need to be
accounted for. The clear advantage of this is that such mineral-mediated isotope effects can be incorporated into the calibration procedure using soil
standards in a way that the standard will be affected in the same way as the measured sample. However, one might also argue against this, as pre-drying
the soil (e.g., at 105
In regard to chamber-based measurements, correction has mostly been done with liquid standards injected into the instrument in the past. However, when integrating chambers in a larger in situ framework, we recommend using water equilibration standards instead. Obviously, the background dry gas is of major importance here, as the air matrix of the standard should be the same as that of the sample.
Because of the influence of different water vapor concentrations on measured
Schmidt et al. (2010) investigated concentration effects on IRIS
The water vapor concentration when carrying out in situ measurements of the isotope composition of soil and xylem water is affected by the temperature of the media of interest but also soil moisture or stem water content. Indirectly, the flow rate chosen by the user also affects the water vapor concentration (if flow rates are too high, saturation will not be reached). The interplay of those factors is complex and not trivial to account for (refer to Sects. 3.4 and 5 for elaborations on this issue). In soil and leaf chambers, relative humidity and vapor pressure deficit affect the water vapor concentration of the measurand.
Recent research has shown that especially in clay-rich soils, an offset in comparison to water used for spiking can be observed due to tightly bound water (Gaj et al., 2017; Newberry et al., 2017a, b; Oerter et al., 2014). This creates a real challenge for any soil water isotope measurement and was discussed heavily (Orlowski et al., 2013, 2016a, b; Sprenger et al., 2016). It has to be noted, that those studies investigated destructively sampled and therefore unstructured soils. Under natural conditions soil structure might, however, play a significant role in soil-intern isotopic differences. To date, it is not clear how to best handle these additional factors. As stated above, a preparation of isotope standards in the same soil that is to be measured seems to be the most promising approach, and Oerter et al. (2017) provide an innovative procedure to calibrate their data (see Sect. 3.4).
In addition, spectral contamination of IRIS measurements caused by organic compounds has been discussed frequently and was recognized as a major
source of error when extracting water from plant tissues (Barbeta et al., 2019; Brand et al., 2009; Brantley et al., 2017; Hendry et al., 2011;
Martín-Gómez et al., 2015; Millar et al., 2018; Newberry et al., 2017a; Penna et al., 2018; West et al., 2010, 2011). It is not known to
date if this plays a role for in situ approaches (refer Sect. 5). Volkmann et al. (2016a) speculated in their study that organic contamination
might be one of the reasons for the observed discrepancies in their dataset. For liquid water samples, a method for correcting for the influence of
organic substances exists (Barbeta et al., 2020; Lin et al., 2019; Schultz et al., 2011; Wu et al., 2013). Thereby, deionized water is spiked with
varying amounts of methanol and ethanol to create correction curves for
Finally, the issues of carrier gases and biogenic matrix effects have been raised recently. Gralher et al. (2016) tested how different mixtures of
As for the measurement of liquid water samples, it is recommended to always use a drift standard that can be measured either after each run (e.g., after measuring one soil profile or a set of tree replicates) or after a certain time. A linear correction similar to the regression for water concentration can then be performed.
All of the presented studies are based on isotopic exchange between the air outside and inside of the gas-permeable probe. Ideally, equilibrium
fractionation is achieved during the passage of the air through the membrane. The isotope composition of water (soil or xylem) can then be calculated
applying the well-established equations for equilibrium fractionation (see Horita and Wesolowski, 1994; Majoube, 1971):
The final step is – similar to liquid water isotope measurements – the normalization to the VSMOW scale (we spare the procedure here, as this is widely known and sufficiently documented).
As shown in the previous section, calibration protocols for addressing the abovementioned steps vary greatly. Not always all the steps are
addressed – either because it was not relevant for the particular investigation or because it was simply neglected. Thus, it is necessary to
introduce a way of assessing the measurements. Across studies, trueness, precision and reproducibility of in situ methods are generally good. For an
evaluation of accuracy, the reviewed publications compared the obtained isotope composition either with cryogenically extracted samples (Gaj et al.,
2016; Soderberg et al., 2012; Volkmann et al., 2016a; Volkmann and Weiler, 2014), results from direct bag equilibration methods (Pratt et al., 2016)
or both (Oerter et al., 2017). Further, theoretical approaches (mass balance calculations and modeling) have been applied to reproduce the in situ
measurements (Rothfuss et al., 2013, 2015; Soderberg et al., 2012). The agreement of soil profiles extracted with vacuum extraction at deeper soil
layers is generally better. In the upper soil layers, partially large differences (
The validation of the xylem water isotope in situ measurements of Volkmann et al. (2016a) yielded good results in terms of precision (median of
1.1 ‰ for
The closest agreement of the reviewed studies when comparing in situ derived data with other methods was achieved in the study of Oerter et al. (2017). Both in terms of measurement and data handling, their methodology appears to be the most complete at present. In addition, the authors propose a novel, innovative way of calibrating in situ data of soil water isotopes. Oerter and Bowen (2019) proposed an updated approach including the correction for carrier gas effects and also introducing the installation of soil water isotope probes in direct contact with roots or the rhizosphere. A reprint of their isotope depth profiles determined with gas-permeable soil gas probes, direct equilibration and vacuum-extracted profiles is shown in Fig. 6.
Comparison of soil water
We propose here an adaptation (more general) of the procedure used by the authors:
Collect samples from each soil depth interval from the site of interest, and dry soil in an oven; place samples in gastight bags or containers
(e.g., 0–10, 10–50 and Add different amounts of isotope standard with known Add soil temperature sensors to standard bags or containers. Measure standard preparations under a range of temperatures (e.g., 0–35 Perform a multilinear regression analysis (e.g., “nlme” package in R) in order to estimate theoretical liquid water standard values using the
parameters Select a best-fit equation for estimation of Perform a statistical analysis regarding the goodness of estimation and which parameters explain variation in estimated liquid Apply final equations to the dataset, with a consequent check of isotope standards throughout the measurement campaign using derived equations.
A procedure like this has several advantages: first, it uses additional information that might have an influence on the measurements, such as clay and
water content. Second, it incorporates these information into one procedure, namely a multilinear regression. Third, an extra calculation step for
the vapor–liquid conversion that exists in several forms can be avoided. Finally, the derived relationships can be objectively assessed using goodness-of-fit measures, tested throughout the measurement period and, if required, adapted later. Thus, we recommend this way of calibration and derivation
of liquid water
The movement of water in an ecosystem is often measured at specific points, e.g., transpiration of one or a few leaves, sap flow in one or a few trees, and soil moisture at certain depths in a soil profile. This is also true for new approaches measuring water stable isotopes in situ; i.e., the limitations of destructive sampling in regard to spatial resolution remain (though portable probes exist that might remedy this situation). In order to obtain reliable estimates of the measured variables for a catchment or even an ecosystem, those point measurements have to be upscaled to a wider area. This can be done by transferring the observations made and the knowledge gained into mechanistic, physically based models (e.g., Crow et al., 2005). Models can also help to identify the dominating processes that govern water fluxes, and residence times across the soil–plant–atmosphere continuum and are used to investigate subsurface processes that cannot be measured easily like root water uptake and preferential flow as well as percolation and mixing of soil water and groundwater recharge (Sprenger et al., 2016). A better mechanistic understanding and parametrization of these hydrological processes will in turn benefit models across scales – from field sites (e.g., Sprenger et al., 2015) to catchments (e.g., Birkel et al., 2014) up to global-scale earth system models (Clark et al., 2015).
At the catchment scale, tracer-aided modeling has become a significant research topic due to the higher availability of datasets on water stable isotopes measured in precipitation and streamflow (Birkel and Soulsby, 2015). By adding a travel time component, these approaches enable a combined representation of water velocity and celerity and ultimately allow for better representing ecosystem solute transport and getting the right model output for the right reasons (McDonnell and Beven, 2014). It was shown that incorporating soil water isotope data into rainfall–runoff modeling improved the identifiability of parameters when simulating the stream water isotope composition (Birkel et al., 2014). However, Knighton et al. (2017) point out that in some catchments, isotope variation of streamflow might not react strongly to vadose-zone ecohydrological processes, and depending on the research question, model performance should be evaluated also including a comparison of modeled and measured soil water isotopes of the unsaturated soil. Furthermore, it is not clear how (isotopic) heterogeneity of soil and plant water values affect catchment-scale flux estimations, as such high-resolution measurements are just becoming available now. This illustrates the need for a better mechanistic understanding of subcatchment processes and a concurrent comparison of model estimations and field measurements.
To address this, an increasing number of ecohydrological models were adapted in the last years to incorporate the movement of water stable isotopes between ecosystem water pools. The low temporal resolution that is usually associated with destructive sampling of water stable isotopes as compared to other soil physical and plant physiological measurements (e.g., soil moisture, matric potential and sap flow) limited their application in the past (Meunier et al., 2017). The continued and more in-depth observation of water stable isotopes in vadose-zone water pools and plant water uptake will hence likely provoke the addition and revision of ecohydrological processes in isotope-enabled land surface models (Stumpp et al., 2018).
Table S2 summarizes physically based models that are able to simulate water movement and the water stable isotope composition in different ecosystem water pools and specifically, different depths of the vadose zone and/or plant water. As presented in situ approaches measure the water stable isotope composition in field studies with a certain level of limited spatial resolution; we focus on process models on the plot to catchment scales and spare listing isotope-enabled land surface models. We also include applications of the respective models that focus on investigating water fluxes and their isotope dynamics. A detailed description and comparison of listed models is beyond the scope of this review. Rather we want to illustrate the broad variety of options and benefits from incorporating water stable isotope data collected in situ in plant water and across soil profiles into isotope-enabled ecohydrological models. We further aim to encourage collaborations between field scientists and modelers. Both field measurements as well as modeling approaches are becoming increasingly complex and require substantial training and experience. Conclusively, it might be unrealistic to have both carried out by the same person. In addition, modelers and field scientists often speak “a different language”, i.e., look at processes from different angles. We therefore would like to stress here that increased collaboration is inevitable. This might also include publication of “cleaned” datasets and offering them to the community, as is common in other disciplines.
Observed differences of isotope composition of bulk soil and mobile water and current discussions on the two-water world hypothesis, motivated
Sprenger et al. (2018) to incorporate two soil pore domains, i.e., mobile and bulk soil water, into vadose-zone modeling. They showed that accounting
for both slow and fast water flow components with differing isotope composition and isotopic exchange via water vapor improved the simulation of soil
water isotope dynamics. Also focusing on isotopic effects on soil water, Rothfuss et al. (2012) used data from a controlled monolith experiment to
calibrate SiSPAT-Isotope with measured soil volumetric water content and
To advance the understanding of root water uptake and specifically assess the age of water used by two tree species (
On the catchment scale, Knighton et al. (2020) used xylem isotopes (seasonal resolution) and soil water isotopes (weekly resolution) in the fully
distributed model EcH2O-iso to investigate the importance of tree water storage and mixing. When including this storage component, they found a better
agreement between simulated and observed
While the models and applications described above investigate water movement at the plot and catchment scales, water stable isotopes are also included in multiple land surface models, e.g., iCLM4 (isotopically enabled Community Land Model; Wong et al., 2017), ECHAM5-JSBACH-wiso (Haese et al., 2013), Iso-MATSIRO (Yoshimura et al., 2006), NASA GISS (Goddard Institute for Space Studies) ModelE (Aleinov and Schmidt, 2006), ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems; Risi et al., 2016), that can be coupled to atmospheric general circulation models (e.g., Risi et al., 2016). If model parts function as stand-alone applications to test particular ecohydrological processes (e.g., soil evaporation or root water uptake) but can also be integrated into larger-scale models that combine modules that describe different water fluxes between system components, the effect of one particular process on the whole system can be observed. By coupling the 1-D model Soil–Litter–Iso to a land surface model, Haverd and Cuntz (2010) demonstrated the importance of including a litter component into the model to better reproduce the evapotranspiration flux and its isotope composition at a forested site in Australia. Risi et al. (2016) performed sensitivity tests to the ORCHIDEE land surface models parameters to identify the potential of using water stable isotope measurements to better represent ecohydrological processes. They conclude that to best inform their type of model, water stable isotopes should concurrently be sampled in all ecosystem water pools. The authors point out that soil water isotope vertical variations are important to investigate and improve the realistic representation of infiltration pathways.
In contrast to physically based models that aim at realistically describing physical processes of water and energy fluxes over time with mathematical
equations and usually need substantial computing power, conceptual models are less complex and faster due to their spatial integration but rely on
calibration parameters reducing their physical realism (Asadollahi et al., 2020). Storage selection functions are a recent approach combining water
flow and transport processes to represent the effect of storage and biogeochemical processes on the water age distribution of catchment outflow
(Rinaldo et al., 2015). Asadollahi et al. (2020) used water stable isotope data of lysimeter experiments to compare this approach with
physically based HYDRUS-1D simulations. They explain similarities and differences between modeled lysimeter drainage and evapotranspiration and
discuss age dynamics of different water fluxes. Taking advantage of the high temporal resolution of in situ data of the isotope composition of xylem
water isotope, storage selection functions could also be used to investigate the importance and the effect of tree water storage and internal mixing
on the
Concurrent measurements of the water stable isotope composition in plant xylem and potential plant water sources, like different soil depths and groundwater, represent an indispensable approach to determine root water uptake patterns and the relative contribution of present water sources. Rothfuss and Javaux (2017) reviewed different methods to determine root water uptake depths. Most commonly, purely statistical approaches (i.e., mixing models) are used. While these can also benefit from a better representation of the temporal variability enabled by in situ measurements (Kühnhammer et al., 2020), efforts should be directed at using physically based models. Those models, only accounting for 4 % of reviewed studies (Rothfuss and Javaux, 2017), enable a better mechanistic understanding of root water uptake and help to improve its representation in land surface models.
These examples show numerous ways in which water stable isotopes as tracers of ecosystem water fluxes can be used to evaluate and improve physically based soil–vegetation–atmosphere models. On the other hand, modeling approaches provide a more integrated (spatially and temporally) view on water fluxes and can inform field scientists by optimizing sampling in respect to its timing and temporal and spatial resolution, as well as identifying compartments and fluxes that play a critical role in the specific investigated ecosystem. Key challenges will be how to deal with natural heterogeneity across different scales and ecosystem water pools in order to correctly upscale in situ point measurements (Penna et al., 2018). Furthermore, accounting for temporal dynamics of water stable isotopes measured in different ecosystem compartments, i.e., soils, plants and atmosphere, into only one model might require incorporating many more processes and parameters, which therefore potentially decreases parameter identifiability. It is, however, important to address these issues and explore the use of new in situ data to improve the physically based representation and parametrization of key ecohydrological processes on the local scale in order to improve predictions of large-scale models.
The goal of this review was to summarize the current state of in situ approaches for measuring and modeling the water stable isotope composition of soil water, evaporation and plant water (in both xylem and leaf transpiration) and point out current issues and challenges. We further aimed to provide a hands-on guide to basic principles and difficulties associated with applying in situ methods. Based on this, we propose to combine applications of in situ investigations in different compartments of the soil–plant–atmosphere continuum in the future.
In situ measurements are an inevitable step for any holistic study within the critical zone. The current design of many ecohydrological studies is
still based on destructive sampling at discrete points in time and space. The number of artifacts (potential isotope effects) and methodological
constraints (limited spatiotemporal resolution and issues of measuring different water pools with different extraction methods) associated with that
(refer to introduction) are increasingly questioning established methodologies. While certainly – apart from advancements in in situ methods – new
protocols for destructive sampling and analysis are needed in order to account for the findings of the last decade, in situ methods provide an
elegant way of overcoming a number of current limitations. For instance, the water pools measured in soils and plants using in situ methods are
ultimately the same, i.e., the mobile fraction that actively takes part in water fluxes and exchange. Using any extraction method, the risk of
extracting and comparing different water pools is high (an extraction temperature of 105
Another example is the high temporal resolution that can be achieved with such measurements which resolves the issue of lag time and enables the
investigation of non-steady-state conditions. Hence, in situ methods will be highly useful for any study involving rapid changes of environmental
conditions, e.g., root water uptake studies, water partitioning and nighttime transpiration. They will also benefit long-term studies, such as
monitoring combined reaction of soils and plants to droughts or extreme events. Moreover, high-frequency in situ monitoring can elevate tracing of
the water cycle via isotopic labeling (
Having that said, it should always be carefully evaluated if an in situ approach is required for the purpose of the study – or if destructive
sampling is sufficient. When carrying out in situ studies, the aim of the study determines the design of the system to be used, and a good starting
point would be to clarify the following aspects:
Is the particular study a long-term study (weeks to months) or a rather short-term study (days)? Is the goal to obtain data in a high temporal or spatial resolution (or both)? (This aspects aims to define if the system needs to be portable or rather stationary.) Is it a tracer experiment, or is the goal to obtain natural abundances of soil and plant water isotopes?
The setup of any in situ system is neither simple nor easy; stand-alone or even plug-and-play approaches are still not available. In order to obtain
reliable isotope data, daily maintenance and troubleshooting is inevitable at present. Developing an automated, portable system including
isotope standard measurements, probes, valve systems, mass flow controllers, temperature controls, etc. that requires less maintenance is highly
desirable. The complicated technical setup and calibration process as well as the vast amount of data created which needs to be processed carefully
might be a reason why only a few research groups have conducted in situ studies so far. We hope to shed light on some of the technical aspects
involved and clarify those through this review.
Despite the abovementioned issues, in situ approaches for monitoring depth-dependent soil water isotopes employing gas-permeable probes
have advanced tremendously in recent years. It now seems feasible to obtain measurements of natural abundances of soil water isotopes in a high
temporal frequency. For monitoring the isotope composition of xylem water in situ under field conditions, on the other hand, there is only
one existing study applying isotopic labeling (Volkmann et al., 2016a, b). Future efforts should be directed towards testing and improving the methods
suggested and developing novel approaches with the ultimate goal of measuring natural abundances of plant water isotopes in situ (Beyer et al., 2019;
Kühnhammer et al., 2020; Marshall et al., 2020). Subsequently, continuous soil and plant water isotope measurements should be combined (for a
recent example, see Orlowski et al., 2020). Chamber-based measurements of transpired and evaporated water vapor are well established and have mainly
been employed in frameworks focusing on partitioning of ecosystem evapotranspiration (Dubbert et al., 2013, 2014a, b; Rothfuss et al., 2012) or
studying isotopic fractionation during soil evaporation (Or et al., 2013) and the leaf water isotope composition (Cernusak et al., 2016; Song et al.,
2013, 2015a; Wu et al., 2013). They have also been used in ecohydrological studies tackling issues, such as root water uptake depths (e.g., Volkmann
et al., 2016a). However, given the critical and often violated assumptions of isotopic steady state of transpiration (i.e.,
Despite the great advances in monitoring depth-dependent soil water isotopes in situ, there is no generally accepted calibration protocol existing yet (such as van Geldern and Barth, 2012, for water samples). Hence, homogenization of calibration and validation protocols is required. We propose here to make such a development based on the ideas of Oerter et al. (2017), which is – in the authors' opinion – the most complete of all currently existing approaches. It also provides an objective way of handling the data (via statistical measures) and is very flexible in including or excluding additional factors that might be relevant (e.g., mineral-mediated fractionation). In terms of calibration, we further suggest that laboratory standards are provided using the same media that is to be measured (e.g., use standards prepared and measured in soils when measuring soils in situ) in order to fulfill the assumption of the identical-treatment principle, which has been violated in a number of studies. We contacted the authors of the original bag equilibration method (Wassenaar et al., 2008) with this question and obtained the following response: “We and others have wrestled with this and you are correct the original publication is technically not an identical treatment. I suppose the real question is how much does either approach matter in practice vs its convenience – are we talking only 10th's of a permil bias (maybe not an issue) or a lot more (worrisome)?” They also noted that “it is also not identical treatment if you dry and wet soil or sand with lab standard waters, as some soils may have more potential for bound residual water or isotope exchange with clay particles, for example, or if the soil standard properties differ a lot from field samples.” (Leonard Irwin Wassenaar, personal communication, 2019). For this reason, an ideal preparation of soil standards does not exist at present. However, running laboratory tests before in situ measurements using soil from different depths (e.g., A and B horizon) from the site to be measured, oven-drying it, spiking it with different water contents, and measuring it over a range of temperatures and water vapor concentrations will give a sound baseline for calibrating the on-site data. For the field calibration, soil standards (e.g., two to three) for each soil horizon should then be prepared and measured for each sequence in the field. We further propose installing TDR (time domain reflectometry) probes in each of the standard bags to keep track of the water content and temperature which is needed for the calibration.
For validation, it has been shown that a comparison of cryogenically extracted samples, although this has been the standard method for decades, with equilibration methods is not feasible for soil samples because different water pools are measured with the two approaches. The same might be true for plant samples. There is an urgent need to develop alternative ideas. For soils, a comparison of in situ data with destructive sampling and using the bag equilibration method might be a way. However, the issue of spatial heterogeneity between the two measures remains. For plants, the bag equilibration method might also be feasible but has not been tested thoroughly.
For both soils (e.g., the upper soil layers) and plants, the effect of organic contaminants (such as volatile organic compounds) on in situ measurements needs to be evaluated, and measures need to be developed to correct for it during postprocessing. Such might be included into the multistep procedure suggested by Oerter et al. (2017). As stated, a method for correcting liquid water samples for the influence of organic substances already exists (Lin et al., 2019; Schultz et al., 2011; Wu et al., 2013) and could be easily adopted to vapor phase measurements. However, it needs to be determined before if contamination even plays a role for data obtained in situ.
Another recommendation related to data treatment is the establishment of a way of evaluation if equilibrium conditions prevailed at the site of isotope exchange during the in situ measurement (e.g., inside of the gas-permeable soil or tree probe). All reviewed studies presented herein use some sort of equilibrium vapor–liquid conversion (e.g., Horita et al., 2008; Majoube, 1971). Only one of them (Marshall et al., 2020) evaluated if this assumption actually was true for their particular setup (flow rate, exchange length, etc.).
An idealized, yet complicated, in situ system depicting all relevant components for a complete measurement of water isotopes of soils (depth-dependent and bulk soil) and plants (in tree xylem and at the leaf level).
To estimate relative humidity (per definition the ratio of actual to saturated water vapor pressures) in boreholes, the ratio of “water vapor
concentration” (in
One might argue that via the equal-treatment principal, saturation is theoretically not necessary because it can be accounted for during
calibration. However, this would require, for instance, for soil samples, a preparation of soil standards with the
In the concluding section, we propose a combined soil–plant in situ monitoring system which – in the authors' opinion – represents a holistic way of investigating dynamic ecohydrological processes at the interfaces of soil, vegetation and atmosphere.
The authors of this study have been involved in the development of in situ methods for nearly a decade. Based on this literature review and their own experiences, an “ideal” system is presented in Fig. 7.
The – admittedly highly complex – system depicted in Fig. 7 combines measurements of all compartments covered in this review. A setup like this
would enable one to monitor the complete cycling of water through soils and plants: (i) gas-permeable probes for measuring depth-dependent soil water
isotope ratios (supported by soil moisture and temperature sensors for the equilibrium calculations), (ii) soil chambers for monitoring the isotope composition
of evaporation, (iii) stem probes or stem boreholes (supported by thermocouples for the equilibrium calculations), and (iv) leaf chambers for monitoring
the isotope composition of transpiration and finally the monitoring of atmospheric water vapor. Ideally, these fluxes are all controlled by one
valve or manifold system. Through the inlet of each measurement stream, dry air with the required flow rate (MFC 1) can be directed through the
probes or chambers. At the same time, it can be used to flush the systems prior to the measurement sequence with dry air (diving air, synthetic air or
When reading through this explanation, the reader probably gets the impression that this is very complicated. Admittedly, it is; and despite its complexity critical minds might still request it if the suggested procedure is a true identical treatment. However, a holistic approach for all relevant isofluxes would have an enormous potential for improving process understanding (e.g., travel times, water sourcing, fractionation and storage times) and isotope-enabled modeling.
It is, thus, the task of the community to further improve, but also simplify, in situ measurements. We encourage the community to carry out and test in situ systems. The increased technical effort for the setup is often compensated by far with the higher spatial (if using probes as a mobile version) and temporal resolution.
Lastly, it needs to be clear to anybody applying in situ methods that a higher uncertainty has to be expected when working with such methods. While future efforts should certainly be directed to decrease those uncertainties as much as possible, it is equally important to communicate those uncertainties. Many of the “old” studies are employing a very low number of samples, for instance, for plant source water studies. They often end up with strong statements but completely neglect the dynamic character of natural systems. Thus, only a (perhaps very small and biased) part of the story is reported. In order to improve the understanding of ecohydrological processes, it is inevitable to develop ready-to-use in situ monitoring systems; it is crucial for the community to further develop such methods and make them accessible to a larger group of researchers and practitioners in the near future.
No data sets were used in this article.
The supplement related to this article is available online at:
MB had the initial idea of the paper and created the storyline. MB and MD wrote the initial text and compiled the reviewed studies. KK wrote Sect. 4 and improved the text upon the initial version.
The authors declare that they have no conflict of interest.
We thank Youri Rothfuss, Erik Oerter and one anonymous reviewer for their constructive and helpful comments during the review process; Christine Stumpp for editing; and Matthias Cuntz for his feedback on modeling. We kindly appreciate additional input and ideas on the paper from Leonard Irwin Wassenaar, Matthias Sprenger and Lukas Kleine. Their advice and response to specific requests helped to improve this paper and make this a useful contribution.
This research has been supported by the Volkswagen Foundation (contract no. A122505; reference no. 92889).
This paper was edited by Christine Stumpp and reviewed by Youri Rothfuss and one anonymous referee.