Cryogenic Vacuum Distillation Research Articles

Analyses of water dependency of Haloxylon ammodendron in arid regions of Iran using stable isotope technique.

The complex relationships within desert ecosystems and their environmental conditions are reflected in patterns of plant water use. Thus, understanding the sources of water used by plants in these areas is crucial for effective resource management. In this study, we investigated the water use pattern of Haloxylon ammodendron in Semnan province, in the central plateau of Iran, using the stable isotope analysis. We employed a simple, homemade cryogenic vacuum distillation (CVD) system to directly extract water from soil samples and different plant components for subsequent analysis by mass spectrometer. The contribution of each possible water source to the plant xylem water was estimated using the IsoSource mixing model. The pattern of 18O values in the xylem water of H. ammodendron indicated its reliance on groundwater as a primary water resource during the wet season. Additionally, the correlation of sand particles with both δ2H and δ18O was found to be 0.32. Moreover, the 18O values of H. ammodendron xylem water were mainly similar to those of groundwater, suggesting the species' dominant use of groundwater. The findings of this study provide valuable insights for strategically planting H. ammodendron to mitigate impacts on groundwater resources and ensure long-term sustainability in arid and semi-arid regions.

Cryogenic vacuum distillation vs Cavitron methods in ecohydrology: Extraction protocol effects on plant water isotopic values

Stable isotope ratios of hydrogen and oxygen in plant water are widely used for water tracing in ecohydrology studies. In this approach, plant water is extracted for isotopic analysis of δ2H and δ18O. Among the extraction methods, cryogenic vacuum distillation (CVD) has been the most popular, but its impact on the isotopic composition of plant water is currently under debate. Newer Cavitron method have been proposed to replace CVD method for xylem water extraction. These recommendations have been largely based on comparisons between Cavitron-xylem water with low temperatures CVD-xylem water. However, the CVD protocol (extraction temperature and time) varies widely across laboratories, and no direct, systematic comparison has yet been made between extraction temperature and time protocols vs. the Cavitron approach. Here we compared the isotopic values of xylem water from the same tree obtained through CVD extraction at 60 °C (60, 120, 240, 360 min), 100 °C (30, 60, 120, 240 min), 140 °C (15, 30, 60, 120, 240 min), and 200 °C (15, 30 min). Subsequently, we compared the results of CVD-xylem water with Cavitron-xylem water. Our data show that isotopic values for CVD-xylem water became more positive with increasing extraction time under the same extraction temperature. Such extractions also became more isotopically positive with increasing extraction temperature for the same extraction time. When total extraction efficiency exceeded 98 %, there was no δ18O difference in CVD-xylem water among any of the different protocols (p > 0.05). However, lower extraction temperatures resulted in more negative δ2H when compared to higher temperature extraction (p < 0.05). Cavitron-xylem water was close to CVD-xylem water (average difference of 3 ‰ for δ2H and 0.5 ‰ for δ18O, n = 79) when total extraction efficiency for CVD was below 98 %. But for extraction efficiencies beyond 98 %, the Cavitron-xylem water was more negative in δ2H (17.3 ‰, n = 70) and δ18O (1.7 ‰, n = 70) than CVD-xylem water. Compared to Cavitron-xylem water, CVD-xylem water at 200 °C with extraction efficiency > 98 % was closer to the soil water. Further study is necessary to conduct a complete cross-comparison between Cavitron and CVD at different temperatures with different species at various water and salt stress conditions. But our results suggest that abandoning CVD for plant water maybe premature until such complete comparison work is done.

δ2H isotopic offsets in xylem water measurements under cryogenic vacuum distillation: Quantifying and correcting wood‐water hydrogen exchange influences

AbstractStable isotopes δ18O and δ2H are used to infer vegetation water sources. In some studies, significant xylem water δ2H offsets from potential source waters have been observed. The offsets appear to be more prevalent with cryogenic vacuum distillation (CVD) of plant water. Hypothesized mechanisms for these offsets include changes during plant water uptake and transport, and methodological problems. We propose that a large portion of the offsets are due to hydrogen isotope exchange between xylem water and non‐crystalline hydroxyl groups of wood cellulose and hemicellulose during CVD. We present a method for estimating the hypothesized isotopic exchange between wood tissues and water, which is the result of Rayleigh and equilibrium fractionation. To estimate the exchange, we use published wood properties for North American tree species and isotope chemical relationships as a function of moisture content, CVD temperature and water extraction efficiency. A simple model of exchange between xylem water and hydroxyl groups captures the range of observations in studies in which CVD and non‐CVD methods were compared. To evaluate the model, we compared observed δ2H offsets (sw‐excess) values from two field datasets (90°C, n = 364, and 170°C, n = 43) to δ2H offsets estimated with our chemical model. We found good agreement between observed and estimated δ2H offsets for samples extracted at 90°C (r2 = 0.69) but not for samples extracted at 170°C (r2 = 0.20). The offset may be eliminated by increasing the extraction temperature to 229°C or by adding a standard sufficient to raise the moisture content to &gt;150%. A correction can also be approximated by applying a theoretical calculation based on the extraction temperature, moisture content and water extraction efficiency.

How do non‐halophyte locust trees thrive in temperate coastal regions: A study of salinity and multiple environmental factors on water uptake patterns

AbstractUnderstanding plant water use patterns is crucial for comprehending the dynamics of the soil–plant‐atmosphere continuum and evaluating the adaptability of plants across diverse ecosystems. However, there remains a gap in our comprehension of non‐halophyte plants' water uptake patterns and driving factors in temperate coastal regions. For this reason, we used locust trees (a widely planted non‐halophyte tree species in northern China) as a study subject. We collected water isotope data (δ2H and δ18O) for locust trees xylem and soil over two consecutive growing seasons. The MixSIAR model was used along with five distinct sets of input data (single isotopes, uncorrected dual isotopes, and corrected dual isotopes incorporating δ2H data obtained by soil water line or cryogenic vacuum distillation methods) to infer water utilization patterns. The results indicated that locust trees primarily absorb shallow soil water (0–20 cm, 29.4% ± 16.9%) and deep soil water (120–180 cm, 24.7% ± 5.8%). Pearson's correlation analysis revealed the key driving factors behind water uptake patterns were vegetation transpiration and soil salinity. Remarkably, the build up of salts in the lower soil layer (60–120 cm) hinders the absorption of water by plants. To prevent high salt concentrations from affecting water uptake in non‐halophyte plants, we recommend implementing sufficient irrigation from March to April each year to meet the water needs of plant growth and regulate the accumulation of salts in various soil layers. This study reveals the dynamic water utilization strategy of non‐halophyte plants in temperate coastal regions, offering valuable information for water resources management.

Comparative Analysis of Water Isotopic Compositions: Evaluating Isotope Analyzer for Soil and Extraction Method for Stem Water

Stable isotopes of water (δ2H and δ18O) are reliable tracers for the investigation of plant–soil–water interactions in forest ecosystems. However, variations in isotopic compositions may arise due to differences in analytical instruments and water extraction methods. In this study, we conducted three different experiments to identify isotopic differences caused by analytical and methodological variations. First, we analyzed soil water by using the two most commonly applied methods: isotope ratio mass spectrometry (IRMS) and cavity ring-down spectroscopy (CRDS). Second, we compared the isotopes in xylem water extracted from the stems of nine tree species using cryogenic vacuum distillation (CVD) with different heating times. Third, we compared the compositions in xylem water extracted with three different methods: mechanical squeezing using a pressure chamber (PC), an induction module (IM), and CVD. The differences in isotopic composition between IRMS and CRDS were significant but minimal. Soil properties were not significant factors contributing to differences between the two instruments. For the xylem water extraction with CVD, each of the nine tree species required heating for more than three hours. Significant differences were observed in δ2H among the three extraction methods for xylem water. Xylem water extracted by CVD showed more depleted values compared to those obtained by PC and IM. Our results highlight the importance of considering analytical and methodological variations in stable isotope analysis.

Subsurface mixing of different soil water pools in a permafrost-underlain headwater catchment, Qilian Mountains, Northeastern Tibetan Plateau

Recently studies postulate that there are two distinct soil water pools, i.e., mobile and immobile soil water (MSW and IMSW), and different soil waters have different mobility to generate runoff. Evaluating the influences of soil thawing on the subsurface mixing between different soil waters is important for understanding subsurface hydrological processes. In this study, the isotopic differences between different soil waters were evaluated in a permafrost-underlain headwater catchment by comparing the variations of stable isotopes and seasonal origin of MSW and bulk soil water (BSW), including both IMSW and MSW. BSW was sampled using soil augering and cryogenic vacuum distillation in thawed and frozen soil layers, and MSW was collected using a zero-tension lysimeter in thawed soil layers. The results show the following: (1) The average δ2H values of MSW, thawed BSW, and frozen BSW were similar during the spring snowmelt period (SSP), this can be attributed to weak evaporation and their similar origin, which is supported by the high line-conditioned excess values and their similar seasonal origin index values. (2) The enriched MSW collected during the SRP compared to the spring MSW and was associated with enriched summer precipitation, and the IMSW experienced strong evaporation during the period from the SSP to the SRP, both leading to enriched summer BSW compared to the spring BSW. (3) The frozen and thawed BSW had very similar isotopic compositions and seasonal origin, indicating the close hydrological connection between them and the important recharge of the melting frozen soil to soil water or runoff. The different freeze–thaw stages and soil moisture conditions controlled the stable isotope variations and mixing of different soil waters, this results in the different mobility of the mobile/immobile soil waters involved in the runoff generation processes during the SRP and the SSP, and therefore these features are useful to verify the isotopes-aided precipitation–runoff models applied in such catchments.

Insights into tension‐mediated and antecedent water effects on soil water isotopic composition

AbstractUsing isotopic spike experiments, we investigated the existence and magnitude of soil‐mediated isotopic effects and of the interaction between isotopically distinct soil water pools, both associated in isotopic mismatches between water extracted from soil and soil water taken up by the roots. For this, we applied and compared four established techniques commonly used for the extraction of water (vapor) from soil, three of them relying on destructive soil sampling (cryogenic vacuum distillation, centrifugation, and direct water vapor equilibration), and one being a nondestructive in situ online technique. We observed an almost complete mixing of sequentially added, isotopically distinct water samples to a pure quartz sand (memory effect). The isotopic composition of water held at high soil tension in the pure quartz sand (pF = 2) as well as in a sandy soil (pF = 1.8 and 3) deviated considerably from that of the added water (tension effect). However, we could attribute this deviation not exclusively to a soil‐mediated effect but also to methodological shortcomings during our experiments. Finally, we found the following decreasing trend in precision as well as in accuracy of the used water extraction methods: in situ online &gt; centrifugation &gt; direct water vapor equilibration &gt; cryogenic vacuum distillation. The investigation of isotopic fractionation of soil water due to physicochemical processes in soil can be facilitated if the experimental techniques used do not involve isotopic fractionation. In addition, methodological uncertainties and inaccuracies can be minimized by method standardization, increasing the potential of water stable isotopic monitoring in ecohydrological studies.

Uncorrected soil water isotopes through cryogenic vacuum distillation may lead to a false estimation on plant water sources

Abstract Successful use of stable isotopes (δ2H and δ18O) in ecohydrological studies relies on the accurate extraction of unfractionated water from different types of soil samples. Cryogenic vacuum distillation (CVD) is a common laboratory‐based technique used for soil water extraction; however, the reliability of this technique in reflecting soil water δ2H and δ18O is still of concern. This study examines the reliability of a newly developed automatic cryogenic vacuum distillation (ACVD) system. We further assessed the impacts of extraction parameters (i.e. extraction time, temperature and vacuum) and soil properties on the recovery of soil water δ2H and δ18O for the ACVD and traditional cryogenic vacuum distillation (TCVD) systems. Finally, we investigated the potential influence of CVD (ACVD and TCVD) technique on the prediction of plant water uptake through a sensitivity analysis. Both ACVD and TCVD similarly extracted water from the rewetted soils, but none of the CVD systems successfully recovered the isotopic signatures of doped water from soil materials. Mean δ2H offsets of extracted soil water were −2.6 ± 1.3‰ and −2.4 ± 1.7‰ for ACVD and TCVD, respectively, while mean δ18O offsets were −0.16 ± 0.14‰ and −0.39 ± 0.37‰. The isotopic offsets of CVD systems were positively correlated with soil clay content, and negatively correlated with soil water content. Using corrected soil data (with CVD offsets) could improve the prediction of plant water uptake based on its high correlation with the environmental factors. This study identifies the isotopic offsets of CVD systems (i.e. ACVD and TCVD) and provides possible solutions for better predicting plant water sources. Even though, the wide use of CVD techniques probably induce noticeable uncertainties in the prediction of plants water uptake depths. The dataset of soil water extraction in this study will have implications for the technological development of CVD techniques.

Ecohydrological niche segregation among desert shrubs in a gypsum-calcareous formation, north-western Iran

ABSTRACT Background Xerophilic subshrubs exhibit multiple functional types and frequently show hydrological niche segregation. In the poorly studied Irano-Turanian gypsum deserts, knowledge of the ecohydrological strategies of different plant species is essential to understand community complexity in these vulnerable ecosystems. Aim We studied the ecohydrological strategies of five co-existing subshrub members of Caryophyllales, ascertaining if their rooting architecture, gypsum affinity or photosynthetic pathway determined their water uptake, and if gypsum crystallisation water could be a relevant water source for plants in different seasons. Methods We conducted soil and xylem sampling for isotope analyses in spring and summer and extracted water by cryogenic vacuum distillation. Oxygen and hydrogen isotope compositions were determined and compared with visual representation and Bayesian Mixing Models to determine species ecohydrological strategies. Results Species – season interactions were related to differences in xylem sap isotopic composition. Three basic strategies relying on contrasting the use of free topsoil moisture and deep soil water could be detected and were in part explained by rooting architecture. Plant gypsum affinity and photosynthetic pathways did not have a significant effect on the water sources used by the plants. Conclusions Ecohydrological niche segregation was explained partly by rooting architecture and species-specific traits. Gypsum crystallisation water was not used in summer by the studied species.

Technical note: On uncertainties in plant water isotopic composition following extraction by cryogenic vacuum distillation

Abstract. Recent studies have challenged the interpretation of plant water isotopes obtained through cryogenic vacuum distillation (CVD) based on observations of a large 2H fractionation. These studies have hypothesized the existence of an H-atom exchange between water and organic tissue during CVD extraction with the magnitude of H exchange related to relative water content of the sample; however, clear evidence is lacking. Here, we systematically tested the uncertainties in the isotopic composition of CVD-extracted water by conducting a series of incubation and rehydration experiments using isotopically depleted water, water at natural isotope abundance, woody materials with exchangeable H, and organic materials without exchangeable H (cellulose triacetate and caffeine). We show that the offsets between hydrogen and oxygen isotope ratios and expected reference values (Δ2H and Δ18O) have inversely proportional relationships with the absolute amount of water being extracted, i.e. the lower the water amount, the higher the Δ2H and Δ18O. However, neither Δ2H nor Δ18O values, were related to sample relative water content. The Δ2H pattern was more pronounced for materials with exchangeable H atoms than with non-exchangeable H atoms. This is caused by the combined effect of H exchange during the incubation of materials in water and isotopic enrichments during evaporation and sublimation that depend on absolute water amount. The H exchange during CVD extraction itself was negligible. Despite these technical issues, we observed that the water amount-dependent patterns were much less pronounced for samples at natural isotope abundance and particularly low when sufficiently high amounts of water were extracted (&gt;600 µL). Our study provides new insights into the mechanisms causing isotope fractionation during CVD extraction of water. The methodological uncertainties can be controlled if large samples of natural isotope abundance are used in ecohydrological studies.

Comparative cryogenic extraction rehydration experiments reveal isotope fractionation during root water uptake in Gramineae.

Determining whether isotope fractionation occurs during root water uptake is a prerequisite for using stem or xylem water isotopes to trace water sources. However, it is unclear whether isotope fractionation occurs during root water uptake in gramineous crops. We conducted prevalidation experiments to estimate the isotope measurement bias associated with cryogenic vacuum distillation (CVD). Next, we assessed isotope fractionation during root water uptake in two common agronomic crops, wheat (Triticum aestivum L.) and maize (Zea mays L.), under flooding after postdrought stress conditions. Cryogenic vacuum distillation caused significant depletion of 2 H but negligible effects on 18 O for both soil and stem water. Surprisingly CVD caused depletion of 2 H and enrichment of 18 O in root water. Stem and root water δ18 O were more than soil water δ18 O, even considering the uncertainty of CVD. Soil water 18 O was depleted compared with irrigation water 18 O in the pots with plants but enriched relative to irrigation water 18 O in the pots without plants. These results indicate that isotope fractionation occurred during wheat and maize root water uptake after full irrigation and led to a heavy isotope enrichment in stem water. Therefore, the xylem/stem water isotope approach widely used to trace water sources should be carefully evaluated.

A comparative study of plant water extraction methods for isotopic analyses: Scholander-type pressure chamber vs. cryogenic vacuum distillation

Abstract. Recent tracer-based studies using stable isotopes of hydrogen and oxygen showed that different methods for extracting water from plant tissues can return different isotopic compositions due to the presence of organic compounds and because they extract different plant water domains. One of the most used methods to extract plant water is the cryogenic vacuum distillation (CVD), which tends to extract total plant water. Conversely, the Scholander-type pressure chamber (SPC), which is commonly used by tree physiologists to measure water potential in plant tissues and determine plant water stress, is expected to extract only the more mobile plant water (i.e., xylem and inter-cellular water). However, only few studies reported the application of SPC to extract plant water for isotopic analyses, and therefore, inter-method comparisons between SPC and CVD are of great value. In this work, we analyzed the variability in the isotopic composition of plant water extracted by SPC and CVD, also considering the potential variability in the isotopic signature of the plant water extracted by CVD from various tissues (i.e., leaves, twig without bark, twig with bark, twig close to the trunk of the tree, and wood core) and from different plant species (i.e., alder, apple, chestnut, and beech). The extraction of plant water by SPC is simple, can be carried out in the field, and it does not require specific laboratory work as in the case of CVD. However, the main limitation of SPC is the very small water volume that can be extracted from the lignified twigs under water stress conditions compared to CVD. Our results indicated that plant water extracted by SPC and CVD were significantly different. The difference in the isotopic composition obtained by the two extraction methods was smaller in the beech samples compared to alder, apple, and chestnut samples. The isotopic signature of alder, apple, and chestnut plant water extracted by SPC was more enriched in δ2H and δ18O, respectively, than the samples obtained by CVD. We conclude that plant water extraction by SPC is not an alternative for CVD as SPC mostly extracts the mobile plant water, whereas CVD tends to retrieve all water stored in the sampled tissue from both living and dead cells. However, studies aiming to quantify the relative contribution of the soil water sources to transpiration should rely more on the isotopic composition of xylem water (which is theoretically sampled by SPC) than the isotopic composition of total plant water (sampled by CVD), which also contains a fraction of water that could be stored in plant tissues for a longer time.

Withdrawn: Similar Isotopic Biases of Plant Stem Bulk Water From Different Water Sources by Cryogenic Vacuum Distillation Demonstrated Through Rehydration Experiments

AbstractZhao, P., Sprenger, M., Barzegar, R., Tang, X., &amp; Adamowski, J. (2022). Similar isotopic biases of plant stem bulk water from different water sources by cryogenic vacuum distillation demonstrated through rehydration experiments. Geophysical Research Letters, 49, e2021GL096474. The above article from Geophysical Research Letters, published online on 30 March 2022 in Wiley Online Library (http://wileyonlinelibrary.com), has been withdrawn by agreement among the authors, the Editor‐in‐Chief Harihar Rajaram, the American Geophysical Union, and Wiley Periodicals, LLC. The withdrawal has been agreed because the authors discovered, after acceptance of the article but before final publication, some accidental transcription/typo errors in the data (for some δ2H and δ18O values) which would affect some of the results and cannot be sufficiently corrected to the authors' satisfaction.

Isotopic Heterogeneity of Stem Water in Conifers Is Correlated to Xylem Hydraulic Traits and Supports Multiple Residence Times

The isotopic composition of xylem water is frequently measured to identify sources of plant water uptake and evaluate the ecosystem water budget. The most common approach to sample xylem water is cryogenic vacuum distillation (CVD). However, the water recovered by CVD is total xylem water from the complex xylem tissue, including living xylem parenchyma cells, embolized tracheary conduits, and small or disconnected conduits that may have a different isotopic composition from water conducted through conduits of the dominant flow from roots to leaves. The isotopic composition of water in the dominant flow network is likely more representative of the isotopic composition of daily transpiration whereas the total xylem water likely integrates water with a longer residence time that may undergo exchange with organic compounds. An alternative extraction method using a pressure chamber (PC) can capture predominantly the transpiration-stream water through the dominant flow network. We compared the offsets in the isotopic composition of water recovered using CVD and PC from eight conifer species that vary in xylem anatomical and functional traits. The PC method accessed a significantly distinct isotopic domain of stem xylem water compared to the total xylem water accessed by CVD (δ2H, p = 0.012; δ18O, p = 0.028). The difference between δ2H of stem water extracted by PC and CVD methods (Δ2Hstem) was significantly correlated with stem water content (p = 0.048) and the mean Δ2Hstem for each species had a significant relationship with species-specific xylem vulnerability to cavitation (i.e., ψ50) from literature values (p = 0.030). We found a significant positive relationship between Δ2Hstem and Δ18Ostem across all trees sampled (p = &amp;lt;0.001). These results support the existence of isotopically heterogeneous water pools, but we cannot exclude potential CVD artifacts contributing to a portion of the Δ2Hstem offsets. Our data suggest additional mechanisms of incomplete mixing and variable residence time in xylem conduits may contribute to isotopic heterogeneity proposed by previous work. Future work should consider using the PC method for assessing the isotopic composition of daily scale transpiration and determining species-specific xylem anatomical properties that could explain isotopic differences between various xylem water pools.

Organic contamination in online laser-based plant stem and leaf water isotope measurements for pre-extracted samples

ABSTRACT Water stable isotopes have been widely used as natural tracers to investigate soil–plant–atmosphere interactions. Recent developments in induction module cavity ring-down spectroscopy (IM-CRDS) have made it possible to rapidly complete isotope analyses, and to combust co-extracted organic compounds at the same time. However, the agreement between IM-CRDS and isotope ratio mass spectrometry (IRMS) analyses has generally been poor and was primarily attributable to spectral interference of IM-CRDS. Here we evaluated the impacts of organic contamination on the isotope ratios using IM-CRDS with two different methods. No spectral interference was observed for solid samples measured directly by IM-CRDS, whereas clear organic contamination occurred in isotope analyses for pre-extracted plant stem and leaf samples. Our results demonstrate that IM-CRDS can fully combust co-extracted organic compounds by in-line oxidation in the direct measurement of solid samples, although this may not guarantee that the IM-CRDS can obtain better isotopic data than IRMS. It may be risky to evaluate the performance of IM-CRDS by measuring pre-extracted water samples because cryogenic vacuum distillation is likely to introduce extra organic compounds, which may not be fully removed during subsequent IM-CRDS measurement. In addition, spectral variables are useful for post-processing corrections.

Determining the stable isotope composition of porewater using low temperature multi-step extraction for low water content soils

The δ2H and δ18O values (isotope values) of porewater in soils have been used as natural tracers in many research disciplines. Conventional methods to determine the isotope values of porewater require extraction of water from the soil. To extract soil porewater cryogenic vacuum distillation is a commonly used technique. Based on extraction techniques porewater can be considered to be composed of ‘extractable’ and ‘non-extractable’ two components. Based on the goal of most studies, porewater consists of ‘active’ and ‘inactive’ two components, of which only the active part is of interest. Because different porewater components are interrelated through isotope exchange, in principle the conventional distillation technique cannot extract the active porewater without changing its original isotopic composition, regardless of whether recovery of the active porewater is complete or not. Thus, using the isotope results determined on the extracted water to represent the active porewater in soil may lead to variable results. Here, we describe a new distillation strategy for extraction of porewater for determining active porewater isotope values. The extraction uses a lower extraction temperature that is comparable to natural conditions. The isotope values are determined on the extracted water corresponding to different recoveries resulted from multi-step extraction. The final results of the active porewater can be derived indirectly from the multi-step extraction. This study demonstrates that in some cases the conventional one-step extraction technique may not be capable for accurate determination of isotope values of the active porewater. The multi-step extraction technique described in this paper can be used to give more reliable results. Multi-step extraction is similar to direct equilibration methods in that it determines the porewater component which plays an active part in the water cycle, regardless of whether extraction of the porewater is complete or not. Multi-step extraction method could provide an alternative approach to direct equilibration methods in some special cases, or provide a way to test the reliability of other methods for the determination of δ2H and δ18O values of soil porewater.

A combination of soil water extraction methods quantifies the isotopic mixing of waters held at separate tensions in soil

Abstract. Measurements of the isotopic composition of separate and potentially interacting pools of soil water provide a powerful means to precisely resolve plant water sources and quantify water residence time and connectivity among soil water regions during recharge events. Here we present an approach for quantifying the time-dependent isotopic mixing of water recovered at separate suction pressures or tensions in soil over an entire moisture release curve. We wetted oven-dried, homogenized sandy loam soil first with isotopically “light” water (δ2H =-130 ‰; δ18O =-17.6 ‰) to represent antecedent moisture held at high matric tension. We then brought the soil to near saturation with “heavy” water (δ2H =-44 ‰; δ18O =-7.8 ‰) that represented new input water. Soil water samples were subsequently sequentially extracted at three tensions (“low-tension” centrifugation ≈0.016 MPa; “mid-tension” centrifugation ≈1.14 MPa; and “high-tension” cryogenic vacuum distillation at an estimated tension greater than 100 MPa) after variable equilibration periods of 0 h, 8 h, 1 d, 3 d, and 7 d. We assessed the differences in the isotopic composition of extracted water over the 7 d equilibration period with a MANOVA and a model quantifying the time-dependent isotopic mixing of water towards equilibrium via self-diffusion. The simplified and homogenous soil structure and nearly saturated moisture conditions used in our experiment likely facilitated rapid isotope mixing and equilibration among antecedent and new input water. Despite this, the isotope composition of waters extracted at mid compared with high tension remained significantly different for up to 1 d, and waters extracted at low compared with high tension remained significantly different for longer than 3 d. Complete mixing (assuming no fractionation) for the pool of water extracted at high tension occurred after approximately 4.33 d. Our combination approach involving the extraction of water over different domains of the moisture release curve will be useful for assessing how soil texture and other physical and chemical properties influence isotope exchange and mixing times for studies aiming to properly characterize and interpret the isotopic composition of extracted soil and plant waters, especially under variably unsaturated conditions.

Bound and mobile soil water isotope ratios are affected by soil texture and mineralogy, whereas extraction method influences their measurement

AbstractQuestions persist about interpreting isotope ratios of bound and mobile soil water pools, particularly relative to clay content and extraction conditions. Interactions between pools and resulting extracted water isotope composition are presumably related to soil texture, yet few studies have manipulated the bound pool to understand its influence on soil water processes. Using a series of drying and spiking experiments, we effectively labelled bound and mobile water pools in soils with varying clay content. Soils were first vacuum dried to remove residual water, which was then replaced with heavy isotope‐enriched water prior to oven drying and spiking with heavy isotope‐depleted water. Water was extracted via centrifugation or cryogenic vacuum distillation (at four temperatures) and analysed for oxygen and hydrogen isotope ratios via isotope ratio mass spectrometry. Water from centrifuged samples fell along a mixing line between the two added waters but was more enriched in heavy isotopes than the depleted label, demonstrating that despite oven drying, a residual pool remains and mixes with the mobile water. Soils with higher clay + silt content appeared to have a larger bound pool. Water from vacuum distillation samples have a significant temperature effect, with high temperature extractions yielding progressively more heavy isotope‐enriched values, suggesting that Rayleigh fractionation occurred at low temperatures in the vacuum line. By distinctly labelling bound and mobile soil water pools, we detected interactions between the two that were dependent on soil texture. Although neither extraction method appeared to completely extract the combined bound and mobile (total water) pool, centrifugation and high temperature cryogenic vacuum distillations were comparable for both δ2H and δ18O of soil water isotope ratios.

Calibration method affects the measured δ2H and δ18O in soil water by direct H2Oliquid–H2Ovapour equilibration with laser spectroscopy

AbstractThe direct H2Oliquid–H2Ovapour equilibration method utilizing laser spectroscopy (DVE‐LS) is a way to measure soil pore water stable isotopes. Various equilibration times and calibration methods have been used in DVE‐LS. Yet little is known about their effects on the accuracy of the obtained isotope values. The objective of this study was to evaluate how equilibration time and calibration methods affect the accuracy of DVE‐LS. We did both spiking and field soil experiments. For the spiking experiment, we applied DVE‐LS to four soils of different textures, each of which was subjected to five water contents and six equilibration times. For the field soil experiment, we applied three calibration methods for DVE‐LS to two field soil profiles, and the results were compared with cryogenic vacuum distillation (CVD)‐LS. Results showed that DVE‐LS demonstrated higher δ2H and δ18O as equilibration time increased, but 12 to 24 hr could be used as optimal equilibration time. For field soil samples, DVE‐LS with liquid waters as standards led to significantly higher δ2H and δ18O than CVD‐LS, with root mean square error (RMSE) of 8.06‰ for δ2H and 0.98‰ for δ18O. Calibration with soil texture reduced RMSE to 3.53‰ and 0.72‰ for δ2H and δ18O, respectively. Further, calibration with both soil texture and water content decreased RMSE to 3.10‰ for δ2H and 0.73‰ for δ18O. Our findings conclude that the calibration method applied may affect the measured soil water isotope values from DVE‐LS.

Rainwater use by cotton under subsurface drip and center pivot irrigation

To increase the efficiency by which agronomic crops use water from both irrigation and rain during the growing season requires quantifying the proportion of rainfall used by the crop for a rain event. The rainfall pattern in the Texas High Plains (THP) is characterized by isolated thunderstorms of high rates and of short duration, where <1% of the storms produce rain events>50 mm and 80% of total rain events are < than 13 mm. The primary source of irrigation-water in the THP is pumped from the Ogallala Aquifer (OA), which has a different isotopic (δ18O) signature compared to rainfall-captured water. Given this difference, it is feasible to quantify changes in the δ18O signature of the plant water as the plant uptakes the rain- and irrigation-water stored in the soil. To this end, cotton (Gossypium hirsutum L.) was grown and irrigated with subsurface drip and center pivot and under dryland conditions. The irrigation-water was pumped from the OA and rainfall was collected in a rain gauge with mineral oil to prevent evaporation. Additionally, plant and soil samples were collected before and after each rain event. Thereafter, water was extracted from the collected soil and plant samples using cryogenic vacuum distillation and analyzed for δ18O (‰) using a Liquid-Water Isotope Analyzer. The difference in isotope concentrations in the extracts showed a change δ18O (‰) of the cotton petiole water toward that of the rainwater signature of 17% for sub-surface drip, 32% for dryland, and 63% for center pivot irrigation. These results imply that the application of irrigation-water with a sprinkler center pivot results in increased rainwater use in cotton compared to that of sub-surface drip.