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Unravelling the complexities of transpiration can be assisted by understanding the oxygen isotope composition of transpired water vapour (δE). It is often assumed that δE is at steady state, thereby mirroring the oxygen isotope composition of source water (δsource), but this assumption has never been tested at the whole-tree scale. This study utilized the unique infrastructure of 12 whole-tree chambers enclosing Eucalyptus parramattensis E.C.Hall trees to measure δE along with concurrent temperature and gas exchange data. Six chambers tracked ambient air temperature and six were exposed to an ambient +3 °C warming treatment. Day time means for δE were within 1.2‰ of δsource (-3.3‰) but varied considerably throughout the day. Our observations show that E. parramattensis trees are seldom transpiring at isotopic steady state over a diel period, but transpiration approaches source water isotopic composition over longer time periods., (© The Author(s) 2024. Published by Oxford University Press.)

Tamarix ramosissima has an important role in stabilizing sand dunes in desert ecosystems. Understanding the water use strategies of T. ramosissima is essential to understand its adaptations on coppice dunes. We utilized the stable isotopes δ 2 H and δ 18 O in soil water, groundwater, and xylem water to identify monthly differences in water sources. Additionally. we explored rooting depth using 2 H 2 O as an artificial tracer. In May, T. ramosissima derived 75% of its water from shallow and middle soil layers. In July, it absorbed 90% water from middle and deep soil layers. In August and September, it acquired approximately 80% of its water from deep soil layers. The labelling using 2 H as an artificial tracer indicated that the root system of T. ramosissima could reach depths >500 cm in the coppice dunes. 2 H absorption was observed at depths of 100, 200, 300 and 400 cm. Soil water is the dominant water source for T. ramosissima in the coppice dunes because groundwater is at depths >30 m. The flexible water-use strategies of T. ramosissima enable it to effectively utilize different available water sources to adapt to the arid environment. These findings improve our understanding of water uptake patterns and drought adaptation strategies of T. ramosissima in the coppice dunes of desert ecosystems., (© 2024 Wiley‐VCH GmbH. Published by John Wiley & Sons Ltd.)

Increases in temperatures and atmospheric CO2 concentration influence the growth performance of trees worldwide. The direction and intensity of tree growth and physiological responses to changing climate do, however, vary according to environmental conditions. Here we present complex, long-term, tree-physiological responses to unprecedented temperature increase in East Asia. For this purpose, we studied radial growth and isotopic (δ13C and δ18O) variations using tree-ring data for the past 100 yr of dominant Quercus mongolica trees from the cool-temperate forests from Hallasan, South Korea. Overall, we found that tree stem basal area increment, intercellular CO2 concentration and intrinsic water-use efficiency significantly increased over the last century. We observed, however, short-term variability in the trends of these variables among four periods identified by change point analysis. In comparison, δ18O did not show significant changes over time, suggesting no major hydrological changes in this precipitation-rich area. The strength and direction of growth-climate relationships also varied during the past 100 yr. Basal area increment (BAI) did not show significant relationships with the climate over the 1924-1949 and 1975-1999 periods. However, over 1950-1974, BAI was negatively affected by both temperature and precipitation, while after 2000, a temperature stimulus was observed. Finally, over the past two decades, the increase in Q. mongolica tree growth accelerated and was associated with high spring-summer temperatures and atmospheric CO2 concentrations and decreasing intrinsic water-use efficiency, δ18O and vapour pressure deficit, suggesting that the photosynthetic rate continued increasing under no water limitations. Our results indicate that the performance of dominant trees of one of the most widely distributed species in East Asia has benefited from recent global changes, mainly over the past two decades. Such findings are essential for projections of forest dynamics and carbon sequestration under climate change., (© The Author(s) 2024. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permission@oup.com.)

Tracer injection has long been recognized as a valuable tool for delineating tree hydraulics and assessing water transport pathways. Recently, isotope tracers have emerged as innovative instruments for investigating tree hydraulics, providing new insights into tree water dynamics. Nevertheless, there is a critical need for further research to comprehensively grasp water movement and distribution within trees. A previously introduced technique for analyzing the isotopic ratio of water in wet tissues, offering millimeter-scale resolution for visualizing tracer movement, faces challenges due to its underdeveloped sample preparation techniques. In this study, we introduced an H 2 O tracer into S. gracilistyla samples, exclusively comprising indeterminate roots, stems, and leaves, cultivated through hydroponics and grown within the current year. Our objective was to assess the axial distribution of the tracer in the xylem. Additionally, we devised a novel method for preparing frozen wet tissue samples, enhancing the repeatability and success rate of experiments. The results demonstrated that all frozen wet tissue samples exhibited an average water loss rate of less than 0.6%. Isotopic analysis of these samples unveiled a consistent decline in tracer concentration with increasing height in all Salix specimens, with three out of five samples revealing a significant isotope gradient. Our findings affirm the efficacy and practicality of combining isotopic labeling with freezing, stabilization, and preparation techniques. Looking ahead, our isotopic labeling and analysis methods are poised to transcend woody plants, finding extensive applications in plant physiology and ecohydrology.18 O tracer into S. gracilistyla samples, exclusively comprising indeterminate roots, stems, and leaves, cultivated through hydroponics and grown within the current year. Our objective was to assess the axial distribution of the tracer in the xylem. Additionally, we devised a novel method for preparing frozen wet tissue samples, enhancing the repeatability and success rate of experiments. The results demonstrated that all frozen wet tissue samples exhibited an average water loss rate of less than 0.6%. Isotopic analysis of these samples unveiled a consistent decline in tracer concentration with increasing height in all Salix specimens, with three out of five samples revealing a significant isotope gradient. Our findings affirm the efficacy and practicality of combining isotopic labeling with freezing, stabilization, and preparation techniques. Looking ahead, our isotopic labeling and analysis methods are poised to transcend woody plants, finding extensive applications in plant physiology and ecohydrology., (© 2024 Scandinavian Plant Physiology Society.)

We investigated the stable isotope hydrology of Sable Island, Nova Scotia, Canada over a five year period from September, 2017 to August, 2022. The δ 2 O values of integrated monthly precipitation were weakly seasonal and ranged from -66 to -15 ‰ and from -9.7 to -1.9 ‰, respectively. Fitting these monthly precipitation data resulted in a local meteoric water line (LMWL) defined by: δ 18 O values of integrated monthly precipitation were weakly seasonal and ranged from -66 to -15 ‰ and from -9.7 to -1.9 ‰, respectively. Fitting these monthly precipitation data resulted in a local meteoric water line (LMWL) defined by: δ 2 H = 7.22 ± 0.21 · δ 18 O + 7.50 ± 1.22 ‰. Amount-weighted annual precipitation had δ 2 O values more consistent with mean annual precipitation or mixing of freshwater with local seawater. Surface waters had more positive values and showed evidence of isolation from the groundwater system. The stable isotopic compositions of plant (leaf) water, on the other hand, indicate plants use groundwater as their source. Fog had δ 18 O values of -36 ± 11 ‰ and -6.1 ± 1.4 ‰, respectively. Deep groundwater had more negative δ 2 H and δ H values of horsehair were statistically significant and suggest variations in water use may exist between spatially disparate horse communities. Our results establish an important initial framework for ongoing isotope studies of feral horses and other wildlife on Sable Island.18 O values than mean annual precipitation, suggesting recharge occurs mainly in the winter, while shallow groundwater had δ 2 H and δ 18 O values more consistent with mean annual precipitation or mixing of freshwater with local seawater. Surface waters had more positive values and showed evidence of isolation from the groundwater system. The stable isotopic compositions of plant (leaf) water, on the other hand, indicate plants use groundwater as their source. Fog had δ 2 H and δ 18 O values that were significantly more positive than those of local precipitation, yet had similar 17 O-excess values. δ 2 H values of horsehair from 4 individuals lacked seasonality, but had variations typical to those of precipitation on the island. Differences in mean δ 2 H values of horsehair were statistically significant and suggest variations in water use may exist between spatially disparate horse communities. Our results establish an important initial framework for ongoing isotope studies of feral horses and other wildlife on Sable Island.

In Taiwanese volcanic watersheds, we investigated stable water isotopes in meteoric water, plants, and thermal water. Meteoric water exhibited a seasonal cycle, with heavier isotopes in winter and lighter ones in summer, especially in the southern region. The northern monsoon signal lagged the south by two weeks. In the Tatun mountains, young water fractions indicated prevalent old water sources. In the northern watershed, streamwater mainly came from the winter monsoon, while the southern one was influenced by alternating monsoons. Both indices indicated that winter plants depended on summer rainfall. Streamwater and plants had distinct sources in winter, supporting ecohydrological separation. Thermal spring water's d -excess helped identify water-rock interactions, with low d value signaling such interactions. The topographic wetness index showed a higher summer monsoon contribution to southern streamwater but a lower one to plants. The mean linear channel direction significantly affected the monsoon contribution fraction, with northeast-oriented channels vulnerable to northeastward winter monsoons. Finally, we developed a model illustrating hydrological processes on short and long timescales. Our findings enhance our understanding of hydrological disturbances' impact on water resources and ecosystems.

Measurements of oxygen isotope enrichment of leaf water above source water (Δ 18 O LW ) can improve our understanding of the interaction between leaf anatomy and physiology on leaf water transport. Models have been developed to predict Δ 18 O LW such as the string-of-lakes model, which describes the mixing of leaf water pools, and the Péclet effect model, which incorporates transpiration rate and the mixing length between unenriched xylem and enriched mesophyll water in the mesophyll (L m ) or veins (L v ). Here we compare measurements and models of Δ 18 O LW on two cell wall composition mutants grown under two light intensities and relative humidities to evaluate cell wall properties on leaf water transport. In maize (Zea mays), the compromised ultrastructure of the suberin lamellae in the bundle sheath of the ALIPHATIC SUBERIN FERULOYL TRANSFERASE mutant (Zmasft) reduced barriers to apoplastic water movement, resulting in higher E and, potentially, L v and, consequently, lower Δ 18 O LW . The difference in Δ 18 O LW in cellulose synthase-like F6 (CslF6) mutants and wild-type of rice (Oryza sativa) grown under two light intensities co-varied with stomatal density. These results show that cell wall composition and stomatal density influence Δ 18 O LW and that stable isotopes can facilitate the development of a physiologically and anatomically explicit water transport model., (© 2023 John Wiley & Sons Ltd.)

It is known that the oxygen isotope composition of CO 2 -in-air, when stored over longer time periods in glass sample flasks, tends to drift to more negative values while the carbon isotope composition remains stable. The exact mechanisms behind this drift were still unclear. New experimental results reveal that water already inside the flasks during sampling plays a major role in the drift of the oxygen isotopes. A drying method to remove any water sticking to the inner walls by evacuating the flasks for more than 72 h while heating to 60 °C significantly decreases drift of the oxygen isotopes. Moreover, flasks not dried with this method showed higher differences among drift rates of individual flasks. This is explained through the buildup of H 2 O molecules sticking to the inner walls. Humidity of the air samples in the flasks as well as surface characteristics will lead to differences among flasks. Results also show that permeability of water is higher through Viton O-ring flask seals than through polychlorotrifluoroethylene (PCTFE) shaft seals, and that the stability of flasks sealed with the latter is significantly better over time.

Rationale: Water is the medium of life, is involved in biochemical reactions, and is exchanged among internal pools and with the water in the external environment of organisms. Understanding these processes can be improved by isotopically labeling the metabolic water that is produced inside the cells of organisms during aerobic respiration., Methods: Here we describe a new method for isotopically labeling cellular water by incubating microbes and plant tissues in air enriched in 18 O 2 . As oxygen gas is reduced during respiration, H 2 18 O is produced. The rate of H 2 18 O production and the synthesis of biomolecules that incorporate 18 O from H 2 18 O can be quantified using cavity ringdown spectrometry and isotope ratio mass spectrometry., Results: For Escherichia coli in solution culture, soil microbial communities, and respiring tissues of plants, the amount of H 2 18 O produced was strongly correlated with that of 18 O 2 consumed during incubations. Measurements of 18 showed that metabolic water was an important substrate in biosynthesis reactions.2 showed that metabolic water was an important substrate in biosynthesis reactions., Conclusions: Any organism with aerobic respiration is amenable to labeling with 18 O 2 , and the method described here enables a new approach to investigate questions regarding plant and microbial physiology. In plants, 18 O introduced as metabolic water could be tracked as it moves between living cells and exchanges with external water. For probing soil microbial physiology, the method described here has the advantage over the application of exogenous H 2 18 O of not increasing the soil moisture, a disturbance that can affect microbial metabolism., (© 2022 John Wiley & Sons Ltd.)

The local meteoric water line (LMWL) is an important basis for tracing the regional hydrological processes with stable isotopes. The establishment of LMWL, which can represent the overall characteristics of stable isotopes of local precipitation, is crucial for accurately revealing the hydrological processes. The influences of different temporal scales and regression methods on the established LMWL were analyzed and compared based on nine years of stable isotopic data of precipitation in Changwu Tableland, a typical area of the inland monsoon region of Northwest China. The results showed that, for different regression methods, the LMWL established by stable isotopes of annual precipitation was basically stable, whereas the LWML established by each precipitation event and the monthly precipitation data showed significant differences with different regression methods. The LMWL from the ordinary least squares regression (OLSR), major axis regression (MA) and reduced major axis regression (RMA) methods were significantly different based on the data of precipitation event, monthly precipitation data, and annual precipitation data, respectively. Only when OLSR, MA and RMA considering precipitation weighting were used, the LMWL established by these scale data was relatively close. This suggested that special attention should be paid to the selection of precipitation stable isotope data scale and regression method when LMWL was established in areas with temporal heterogeneity of precipitation and stable isotopic characteristics. For regions where stable isotope observations of precipitation were difficult and data were limited, the precipitation weighted major axis regression or reduced major axis regression methods are recommended during the establishment of representative LMWL.

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 δ 18 O were more than soil water δ 18 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., (© 2022 The Authors. New Phytologist © 2022 New Phytologist Foundation.)

Contact electrification between water and a solid surface is crucial for physicochemical processes at water-solid interfaces. However, the nature of the involved processes remains poorly understood, especially in the initial stage of the interface formation. Here we report that H 2 O 2 is spontaneously produced from the hydroxyl groups on the solid surface when contact occurred. The density of hydroxyl groups affects the H 2 O 2 yield. The participation of hydroxyl groups in H 2 O 2 generation is confirmed by mass spectrometric detection of 18 O in the product of the reaction between 4-carboxyphenylboronic acid and 18 O-labeled H 2 O 2 resulting from 18 O 2 plasma treatment of the surface. We propose a model for H 2 O 2 generation based on recombination of the hydroxyl radicals produced from the surface hydroxyl groups in the water-solid contact process. Our observations show that the spontaneous generation of H 2 O 2 is universal on the surfaces of soil and atmospheric fine particles in a humid environment.

Monitoring changes in stable oxygen isotope ratios in molecular oxygen allows for studying many fundamental processes in bio(geo)chemistry and environmental sciences. While the measurement of [Formula: see text]O/[Formula: see text]O ratios of [Formula: see text] in gaseous samples can be carried out conveniently and from extracting moderately small aqueous samples for analyses by continuous-flow isotope ratio mass spectrometry (CF-IRMS), oxygen isotope signatures, [Formula: see text]O, could be overestimated by more than 6[Formula: see text] because of interferences from argon in air. Here, we systematically evaluated the extent of such Ar interferences on [Formula: see text]O/[Formula: see text]O ratios of [Formula: see text] for measurements by gas chromatography/IRMS and GasBench/IRMS and propose simple instrumental modifications for improved Ar and [Formula: see text] separation as well as post-measurement correction procedures for obtaining accurate [Formula: see text]O. We subsequently evaluated the consequences of Ar interferences for the quantification of O isotope fractionation in terms of isotope enrichment factors, [Formula: see text], and [Formula: see text]O kinetic isotope effects ([Formula: see text]O KIEs) in samples where [Formula: see text] is consumed and Ar:[Formula: see text] ratios increase steadily and substantially over the course of a reaction. We show that the extent of O isotope fractionation is overestimated only slightly and that this effect is typically smaller than uncertainties originating from the precision of [Formula: see text]O measurements and experimental variability. Ar interferences can become more relevant and bias [Formula: see text] values by more than [Formula: see text] in aqueous samples where fractional [Formula: see text] conversion exceeds 90%. Practically, however, such samples would typically contain less than 25 [Formula: see text]M of [Formula: see text] at ambient temperature, an amount that is close to the method detection limit of [Formula: see text]O/[Formula: see text]O ratio measurement by CF-IRMS., (© 2022. The Author(s).)

Spatiotemporal variations in soil water content (SWC) and soil water stable isotopic compositions (SWSIC; 2 H/ 1 H (δD) and 18 O/ 16 O (δ 18 O)) provide critical information on elucidating land surface processes across scales. Meanwhile, little is known about the spatiotemporal characteristics of SWSIC and its driving factors. Therefore, it's necessary to improve tracer techniques of SWSIC by interpreting their spatiotemporal variability patterns as well as the correlations with other factors such as texture, soil depth and vegetation. To this end, the spatiotemporal variations in SWC and SWSIC along with their controlling factors were jointly investigated based on seven field campaigns over roughly a two-year period at an agricultural field in North China Plain. Two transects, vegetated and bared, were considered. The results of vegetated transect showed that both SWC and SWSIC exhibited considerable spatiotemporal variabilities at the field scale of ~100 m, with SWSIC displaying more complex patterns. Overall, the spatial variations in SWSIC were larger in wet seasons than in dry seasons, which decreased with increasing soil depth, largely due to less impacts of precipitation inputs and soil evaporation on SWSIC dynamics at deeper depths. The temporal stability analysis (TSA) showed that there existed temporal persistence of the spatial structure of SWSIC, particularly at deeper soil depths. Moreover, the SWSIC data in our study showed that the effect of vegetation on the SWSIC dynamics was noticeable with shading effects, root distribution and water uptake, which caused much lesser degrees of soil evaporation at the vegetated transect. What's more, the representative sites for monitoring spatial average δD values were identified, demonstrating the viability of using the TSA method to estimate the spatial average SWSIC values at field scales. These findings can improve the interpretation of SWSIC data for practical applications., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

We compiled hydrogen and oxygen stable isotope compositions (δ 2 H and δ 18 O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ 2 H was more closely correlated with δ 2 H of xylem water or atmospheric vapour, whereas leaf water δ 18 O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ 2 H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ 18 O. We next expressed leaf water as isotopic enrichment above xylem water (Δ 2 H and Δ 18 O) to remove the impact of xylem water isotopic variation. For Δ 2 H, leaf water still correlated with atmospheric vapour, whereas Δ 18 O showed no such correlation. This was explained by covariance between air relative humidity and the Δ 18 O of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that 2 H and 18 O in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water., (© 2022 The Authors New Phytologist © 2022 New Phytologist Foundation.)

Serving as a medium between source water and cellulose, leaf water contributes to the isotope ratios ( δ 18 O, δ 2 H of leaf water on the southern Tibetan Plateau. The δ 18 H of leaf water were relatively low when precipitation events occurred. In particular, δ 2 H of leaf water became extremely depleted 5 h after the precipitation event. Our findings demonstrate that precipitation can modify the isotope ratios of leaf water from external and internal causes. First, precipitation events affect meteorological elements, lead to decreases in leaf transpiration, and immediately weaken the isotope enrichment of leaf water ('rapid effect' of precipitation). Second, precipitation events affect the internal plant-soil water cycle process, causing the plant to preferentially use deeper soil water, and the corresponding isotope ratios of leaf water exhibit extremely low values 5 h after precipitation events ('delay effect' of precipitation). This study suggests that researchers need to be cautious in separating the signals of precipitation and hydrological processes when interpreting isotope records preserved in tree-ring cellulose archives from the Tibetan Plateau.δ 18 O and δ 2 H of leaf water were relatively low when precipitation events occurred. In particular, 18 O and 2 H of leaf water became extremely depleted 5 h after the precipitation event. Our findings demonstrate that precipitation can modify the isotope ratios of leaf water from external and internal causes. First, precipitation events affect meteorological elements, lead to decreases in leaf transpiration, and immediately weaken the isotope enrichment of leaf water ('rapid effect' of precipitation). Second, precipitation events affect the internal plant-soil water cycle process, causing the plant to preferentially use deeper soil water, and the corresponding isotope ratios of leaf water exhibit extremely low values 5 h after precipitation events ('delay effect' of precipitation). This study suggests that researchers need to be cautious in separating the signals of precipitation and hydrological processes when interpreting isotope records preserved in tree-ring cellulose archives from the Tibetan Plateau.

The doubly labelled water (DLW) method is widely used to determine energy expenditure. In this work, we demonstrate the addition of the third stable isotope, 17 O, to turn it into triply labelled water (TLW), using the three isotopes measurement of optical spectrometry. We performed TLW ( 2 H, 18 O and 17 O) measurements for the analysis of the CO 2 production (r CO2 ) of mice on different diets for the first time. Triply highly enriched water was injected into mice, and the isotope enrichments of the distilled blood samples of one initial and two finals were measured by an off-axis integrated cavity output spectroscopy instrument. We evaluated the impact of different calculation protocols and the values of evaporative water loss fraction. We found that the dilution space and turnover rates of 17 O and 18 O were equal for the same mice group, and that values of r CO2 calculated based on 18 O- 2 H, or on 17 O- 2 H agreed very well. This increases the reliability and redundancy of the measurements and it lowers the uncertainty in the calculated r CO2 to 3% when taking the average of two DLW methods. However, the TLW method overestimated the r CO2 compared to the indirect calorimetry measurements that we also performed, much more for the mice on a high-fat diet than for low-fat. We hypothesize an extra loss or exchange mechanism with a high fractionation for 2 H to explain this difference., (© 2022. The Author(s).)

Rationale: Blood water oxygen isotope compositions can provide valuable insights into physiological processes and ecological patterns. While blood samples are commonly drawn for medical or scientific purposes, blood fractions are infrequently measured for oxygen isotopic compositions (δ 18 O) because such measurements are time consuming and expensive., Methods: We sampled blood from sheep, goats, and iguanas raised in field and animal laboratories into serum, EDTA, heparin, and uncoated plastic vials commonly used in medical and scientific research, then separated red blood cell (RBC) and plasma or serum blood fractions. These were injected into helium-flushed Exetainer tubes where they naturally outgassed endogenous CO 2 (goat blood), or into He- and CO 2 -flushed tubes (iguana blood). The CO 2 gas was sampled on a GasBench II system, and δ 18 O was measured by an isotope ratio mass spectrometer (IRMS)., Results: Repeated δ 18 O measurements were stable over multiple days. The addition of desiccated blood solids to water standards had little impact on their δ 18 O measurements, suggesting that organic molecular constituents within blood serum and plasma do not interfere with blood water δ 18 O values. We observed slight but statistically significant δ 18 O offsets between plasma, serum and RBC fractions. Mass-dependent body water turnover times for iguanas were derived from the data., Conclusions: We demonstrate that a simple blood-CO 2 equilibration method using the GasBench can quickly, reliably and accurately characterize water δ 18 O in the plasma, RBC, and whole blood fractions of mammalian and reptilian blood samples (precision ≤ 0.1‰). This method will expand the application of blood stable isotope analysis in physiological and medical research., (© 2022 John Wiley & Sons Ltd.)

With the Quanfuzhuang River basin located at Hani Rice Terrace core region as study area, we analyzed the isotopic composition and the effects of 12 surface water sampling sites for the forest landscape type and terrace landscape type from May 2015 to April 2016. The results showed that: 1) For the variation of isotope composition, both the average value and the variation range of δ 18 O in surface water under forest patches were smaller than that under terrace patches. 2) The overall elevation effect of the hydrogen and oxygen stable isotopes in surface water was obvious, except that in August and March, which could be expressed as the linear regression equation δ 18 O=-0.012 H +13.84 ( r =-0.83, n =12). 3) The altitude gradient of δ 18 O in surface water was -1.2‰·(100 m) -1 , which was not the true altitude gradient affected by precipitation but by landscape gradient of δ 18 O in surface water between forest patches and terrace patches. 4) Under the "Forest-Terrace" landscape pattern, the δ 18 O differences in surface water between forest patches and terrace patches enhanced the elevation effect. Therefore, when landscape heterogeneity was strong, isotopic effect was strengthened, even with opposite isotope effect.

Background and Aims: Gypsum drylands are widespread worldwide. In these arid ecosystems, the ability of different species to access different water sources during drought is a key determining factor of the composition of plant communities. Gypsum crystallization water could be a relevant source of water for shallow-rooted plants, but the segregation in the use of this source of water among plants remains unexplored. We analysed the principal water sources used by 20 species living in a gypsum hilltop, the effect of rooting depth and gypsum affinity, and the interaction of the plants with the soil beneath them., Methods: We characterized the water stable isotope composition, δ 2H and δ 18O, of plant xylem water and related it to the free and gypsum crystallization water extracted from different depths throughout the soil profile and the groundwater, in both spring and summer. Bayesian isotope mixing models were used to estimate the contribution of water sources to plant xylem sap., Key Results: In spring, all species used free water from the top soil as the main source. In summer, there was segregation in water sources used by different species depending on their rooting depth, but not on their gypsum affinity. Gypsum crystallization water was the main source for most shallow-rooted species, whereas free water from 50 to 100 cm depth was the main source for deep-rooted species. We detected plant-soil interactions in spring, and indirect evidence of possible hydraulic lift by deep-rooted species in summer., Conclusions: Plants coexisting in gypsum communities segregate their hydrological niches according to their rooting depth. Crystallization water of gypsum represents an unaccounted for, vital source for most of the shallow-rooted species growing on gypsum drylands. Thus, crystallization water helps shallow-rooted species to endure arid conditions, which eventually accounts for the maintenance of high biodiversity in these specialized ecosystems., (© The Author(s) 2021. Published by Oxford University Press on behalf of the Annals of Botany Company.)

Recent evidence has revealed the emergence of a megadrought in southwestern North America since 2000. Megadroughts extend for at least 2 decades, making it challenging to identify such events until they are well established. Here, we examined tree-ring growth and stable isotope ratios in Pinus ponderosa at its driest niche edge to investigate whether trees growing near their aridity limit were sensitive to the megadrought climatic pre-conditions, and were capable of informing predictive efforts. During the decade before the megadrought, trees in four populations revealed increases in the cellulose δ 13 C content of earlywood, latewood, and false latewood, which, based on past studies are correlated with increased intrinsic water-use efficiency. However, radial growth and cellulose δ 18 O were not sensitive to pre-megadrought conditions. During the 2 decades preceding the megadrought, at all four sites, the changes in δ 13 C were caused by the high sensitivity of needle carbon and water exchange to drought trends in key winter months, and for three of the four sites during crucial summer months. Such pre-megadrought physiological sensitivity appears to be unique for trees near their arid range limit, as similar patterns were not observed in trees in ten reference sites located along a latitudinal gradient in the same megadrought domain, despite similar drying trends. Our results reveal the utility of tree-ring δ 13 C to reconstruct spatiotemporal patterns during the organizational phase of a megadrought, demonstrating that trees near the arid boundaries of a species' distribution might be useful in the early detection of long-lasting droughts., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Rationale: This study determines if saliva collection procedures for the doubly labeled water (DLW) method, used for measuring total energy expenditure (TEE), are comparable to urine and plasma collection. Both the cavity ring-down spectroscopy (CRDS) and isotope ratio mass spectrometry (IRMS) analyses techniques are compared., Methods: Saliva specimens were collected from participants for the DLW method. The specimens were collected under different conditions: after consumption of tap water, after chewing gum, and during exposure to conditions of high and low relative humidity. The isotopes in saliva were compared with simultaneous plasma and urine collection. TEE calculated from saliva and analyzed using CRDS was compared to that of plasma analyzed using IRMS., Results: The within-individual variances were not significantly different between the saliva specimens (0.4‰) and plasma (0.3‰). After the oral dose of DLW, the saliva specimens displayed a shorter equilibration time to urine. When participants consumed 500 mL of tap water, the enrichment of saliva specimens reached a new plateau value faster than urine. Saliva collection exposed to high ambient humidity conditions was slightly less enriched as compared to low-humidity conditions while urine enrichment was unaffected. In contrast, whereas the within-individual effects of gum chewing during saliva collection on 18 O were unaffected, the abundance of 2 H in saliva was slightly lower after chewing the gum. The within-individual difference between TEE calculated from saliva and that calculated from plasma analyzed using IRMS did not differ from zero, and the standard deviation was not different from that predicted by a propagation of error analysis based on analytical error alone., Conclusions: Our findings support using saliva specimens for the DLW method. The analysis of plasma and urine, however, requires reducing the memory effect resulting from contaminants. Also, it should be performed in a manner that minimizes exposure to air where specimens may be exposed to evaporation or contamination from water vapor during sampling., (© 2021 John Wiley & Sons Ltd.)

The Canary Islands, an archipelago east of Morocco's Atlantic coast, present steep altitudinal gradients covering various climatic zones from hot deserts to subalpine Mediterranean, passing through fog-influenced cloud forests. Unlike the majority of the Canarian flora, Pinus canariensis C. Sm. ex DC. in Buch grow along most of these gradients, allowing the study of plant functioning in contrasting ecosystems. Here we assess the water sources (precipitation, fog) of P. canariensis and its physiological behavior in its different natural environments. We analyzed carbon and oxygen isotope ratios of water and organics from atmosphere, soil and different plant organs and tissues (including 10-year annual time series of tree-ring cellulose) of six sites from 480 to 1990 m above sea level on the Canary Island La Palma. We found a decreasing δ18O trend in source water that was overridden by an increasing δ18O trend in needle water, leaf assimilates and tree-ring cellulose with increasing altitude, suggesting site-specific tree physiological responses to relative humidity. Fog-influenced and fog-free sites showed similar δ13C values, suggesting photosynthetic activity to be limited by stomatal closure and irradiance at certain periods. In addition, we observed an 18O-depletion (fog-free and timberline sites) and 13C-depletion (fog-influenced and fog-free sites) in latewood compared with earlywood caused by seasonal differences in: (i) water uptake (i.e., deeper ground water during summer drought, fog water frequency and interception) and (ii) meteorological conditions (stem radial growth and latewood δ18O correlated with winter precipitation). In addition, we found evidence for foliar water uptake and strong isotopic gradients along the pine needle axis in water and assimilates. These gradients are likely the reason for an unexpected underestimation of pine needle water δ18O when applying standard leaf water δ18O models. Our results indicate that soil water availability and air humidity conditions are the main drivers of the physiological behavior of pine along the Canary Island's altitudinal gradients., (© The Author(s) 2021. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

The procedure of calibrating in-house water standards suitable for routine analyses of triple-isotope composition of water samples using Picarro L2140- i CRDS analyser is presented and discussed. Such standards are indispensable for achieving and maintaining high quality of isotope analyses of water in terms of their precision and accuracy. A set of seven different water standards consisting of three in-house standards and four secondary standards commercially available was calibrated against VSMOW2/SLAP2 primary reference materials. The calibrated standards cover a wide range of isotopic composition, with δ values ranging from close to zero to the values comparable with SLAP2. The apparent consistency of the calibrated values of δ 2 H, δ 18 O and d -excess with corresponding certified values for commercially available USGS47-50 standards and the consistency of the calibrated values of δ 17 O and Δ 17 O with its literature values for USGS47-48 standards confirm the high quality of the performed calibration. Moreover, the calibration exercise allowed to obtain δ 17 O and Δ 17 O values for USGS49 and USGS50 standards, not reported so far.

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.

Dew is an important water resource for plants in most deserts. The mechanism that allows desert plants to use dew water was studied using an isotopic water tracer approach. Most plants use water directly from the soil; the roots transfer the water to the rest of the plant, where it is required for all metabolic functions. However, many plants can also take up water into their leaves and stems. Examining the dew water uptake pathways in desert plants can lend insight on another all water-use pathways examination. We determined where and how dew water enters plants in the water limited Negev desert. Highly depleted isotopic water was sprayed on three different dominant plant species of the Negev desert-Artemesia sieberi, Salsola inermis and Haloxylon scoparium-and its entry into the plant was followed. Water was sprayed onto the soil only, or on the leaves/stems only (with soil covered to prevent water entry via root uptake). Thereafter, the isotopic composition of water in the roots and stems were measured at various time points. The results show that each plant species used the dew water to a different extent, and we obtained evidence of foliar uptake capacity of dew water that varied depending on the microenvironmental conditions. A. sieberi took up the greatest amount of dew water through both stems and roots, S. inermis took up dew water mainly from the roots, and H. scoparium showed the least dew capture overall.

The impacts of climate change on precipitation and the growing demand for water have increased the water risks worldwide. Water scarcity is one of the main challenges of the 21st century, and the assessment of water risks is only possible from spatially distributed records of historical climate and levels of water reservoirs. One potential method to assess water supply is the reconstruction of oxygen isotopes in rainfall. We here investigated the use of tree-ring stable isotopes in urban trees to assess spatial/temporal variation in precipitation and level of water reservoirs. We analyzed the intra-annual variation of δ 13 C and δ 18 O in the tree rings of Tipuana tipu trees from northern and southern Metropolitan Area of São Paulo (MASP), Brazil. While variation in δ 13 C indicates low leaf-level enrichments from evapotranspiration, δ 18 O variation clearly reflects precipitation extremes. Tree-ring δ 18 O was highest during the 2014 drought, associated with the lowest historical reservoir levels in the city. The δ 18 O values from the middle of the tree rings have a strong association with the mid-summer precipitation (r = -0.71), similar to the association between the volume of precipitation and its δ 18 O signature (r = -0.76). These consistent results allowed us to test the association between tree-ring δ 18 O and water-level of the main reservoirs that supply the MASP. We observed a strong association between intra-annual tree-ring δ 18 O and the water-level of reservoirs in the northern and southern MASP (r = -0.94, r = -0.90, respectively). These results point to the potential use of high-resolution tree-ring stable isotopes to put precipitation extremes, and water supply, in a historical perspective assisting public policies related to water risks and climate change. The ability to record precipitation extremes, and previously reported capacity to record air pollution, place Tipuana tipu in a prominent position as a reliable environmental monitor for urban locations., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

The use of stable isotopes ( δ 2 O) is widespread in water resources studies. In the Latin America and the Caribbean (LAC) region, the application of isotope techniques has increased in the past decade, but there remains room to gain self-reliance in environmental isotope studies, necessitating easy and fast access to good-quality isotope data. To that end, in 2018 the IAEA carried out the first regional interlaboratory comparison exercise, testing the analytical performance of 25 laboratories using isotope-ratio mass spectrometry and laser absorption spectroscopy. The three test samples covered a commonly observed range of 0 to -16 ‰ δ 18 O) is widespread in water resources studies. In the Latin America and the Caribbean (LAC) region, the application of isotope techniques has increased in the past decade, but there remains room to gain self-reliance in environmental isotope studies, necessitating easy and fast access to good-quality isotope data. To that end, in 2018 the IAEA carried out the first regional interlaboratory comparison exercise, testing the analytical performance of 25 laboratories using isotope-ratio mass spectrometry and laser absorption spectroscopy. The three test samples covered a commonly observed range of 0 to -16 ‰ δ 18 O and 0 to -115 ‰ δ 2 H. z- and ζ-scores were used to benchmark laboratories' performance against a strict criterion. We found that 81% of the laboratories had satisfactory performance ( | z | ¯ ≤ 2) for δ 2 H but only 54% achieved similar scores for δ 18 O. Only a minor fraction of results (12% for δ 2 H and 15% for δ 18 O) were unsatisfactory. The larger number of questionable results for δ 18 O confirmed the challenges in laser absorption spectroscopy for this isotope. Besides instrumental performance, the sample throughput, laboratory reference materials, and data post-processing were contributing factors to inaccurate or imprecise performance.

We have used stable water isotopes (δ 18 O, δ 2 H) in combination with lumped-parameter modeling for characterizing unsaturated flow in two lysimeters vegetated with maize. The lysimeters contained undisturbed soil cores dominated by sandy gravel (Ly1) and clayey sandy silt (Ly2). Stable water isotopes were analyzed in precipitation and lysimeter outflow water over about 3 years. The mean transit time of water T and dispersion parameter P D , obtained from modeling, were higher for the silt soil in Ly2 than for the gravel soil in Ly1 (T of 362 vs. 129 d, P D of 0.7 vs. 0.12). The consideration of preferential flow (PF) paths could substantially improve the model curve fits, with 13 and 11% contribution of PF for Ly1 and Ly2 as best estimates. Different assumptions were compared to estimate the input function, that is, stable water isotope content in the recharging water. Using the isotopic composition of precipitation as input (no modification) resulted in reasonable model estimations. Best model fits for the entire observation were obtained by weighting the recharging isotopes according to average precipitation within periods of 3 and 6 months, in correspondence to changing vegetation phases and seasonal influences. Input functions that consider actual evapotranspiration could significantly improve modeling at some periods, however, this led to deviations between modeled and observed δ 18 O at other periods. This may indicate the influence of variable flow, so that dividing the whole observation period into hydraulically characteristic sub-periods for lumped-parameter modeling (which implements steady-state flow) is recommended for possible further improvement., (© 2019 The Authors. Groundwater published by Wiley Periodicals, Inc. on behalf of National Ground Water Association.)

One of the many difficulties associated with identifying undocumented border crossers stems from an inability to narrow down the search area for the region of origin and family members to obtain family reference samples for DNA comparison. While the geography of regions of origins is wide, the biological profiles of the undocumented border crossers often show strong similarities, young and male. The isotopic composition of human bones, teeth, and hair has been demonstrated to be useful biomarkers for tracing locations and movements of individuals and for aiding in the identification of human remains. Data obtained from human remains can be compared to and aligned with various reference sources, such as soils and bedrock, archaeological remains, or cultural data. Here, the spatial deficiency in isotopic reference data for Mexico, specifically for oxygen (δ 18 O) isotopes in tap water, is being addressed through the collection and analysis of over 150 water samples and explored with tooth enamel isotopic values from recently identified Mexican nationals. The isoscape was developed using a Spine with Barriers interpolation method and shows sufficient detail to narrow down the regions where specific isotopic values are represented. The individuals were plotted on the developed isoscape using conditional correlations. The methodology was successful in assigning the correct regions of origin for the two individuals, which shows the tremendous potential of the developed isoscape. Nonetheless, there is more research needed to further improve upon this geolocation method, including analysis of multiple isotopes in different tissues, and the development of new isotopic methods., (© 2020 American Academy of Forensic Sciences.)

Clarifying 18 O isotope composition of leaf water (δ L,b ) would provide theoretical refe-rence for the study of leaf physiology and forest hydrology. We continuously monitored the concentration of atmospheric water vapor (W a ) and 18 O isotope composition of atmospheric water vapor (δ v ) at the canopy of Platycladus lateralis plantation in the mountain area of Beijing. We analyzed the effects of kinetic fractionation coefficients Δ k1 (32%) and Δ k2 (28%) on the prediction of δ L,b by combining the measured leaf water 18 O isotope (δ x ) and δ L,b of P. lateralis. The results showed that the diurnal variation of W a was irregular. Atmospheric relative humidity (RH) showed a "V" shape of diurnal variation, and stomatal conductance (g s ) increased first and then decreased at the diurnal scale. W a , RH, and g s showed a significant negative correlation with δ L,b when isotopes approached a steady-state equilibrium around noon. The kinetic fractionation coefficient Δ k1 and Δ k2 were applied to the Craig-Gordon model to predict δ L,b under the isotopic quasi-steady-state condition. The results showed that the predicted values of Δ k2 approached the observed values of δ L,b . This result indicated that the application of Δ k2 to the model was more consistent with the change of water isotope concentration in the leaves of P. lateralis in the mountain area of Beijing. These results would improve our understanding of water isotope enrichment model and evapotranspiration resolution model in leaves.

Strategies for deep soil water acquisition (WA deep ) are critical to a species' adaptation to drought. However, it is unknown how WA deep determines the abundance and resource economy strategies of understorey shrub species. With data from 13 understorey shrub species in subtropical coniferous plantations, we investigated associations between the magnitude of WA deep , the seasonal plasticity of WA deep , midday leaf water potential (Ψ md ), species abundance and resource economic traits across organs. Higher capacity for WA deep was associated with higher intrinsic water use efficiency, but was not necessary for maintaining higher Ψ md in the dry season nor was it an ubiquitous trait possessed by the most common shrub species. Species with higher seasonal plasticity of WA deep had lower wood density, indicating that fast species had higher plasticity in deep soil resource acquisition. However, the magnitude and plasticity of WA deep were not related to shallow fine root economy traits, suggesting independent dimensions of soil resource acquisition between deep and shallow soil. Our results provide new insights into the mechanisms through which the magnitude and plasticity of WA deep interact with shallow soil and aboveground resource acquisition traits to integrate the whole-plant economic spectrum and, thus, community assembly processes., (© 2019 The Authors. New Phytologist © 2019 New Phytologist Trust.)

Mesophyll conductance (g m ) is the diffusion of CO 2 from intercellular air spaces (IAS) to the first site of carboxylation in the mesophyll cells. In C 3 species, g m is influenced by diverse leaf structural and anatomical traits; however, little is known about traits affecting g m in C 4 species. To address this knowledge gap, we used online oxygen isotope discrimination measurements to estimate g m and microscopy techniques to measure leaf structural and anatomical traits potentially related to g m in 18 C 4 grasses. In this study, g m scaled positively with photosynthesis and intrinsic water-use efficiency (TE i ), but not with stomatal conductance. Also, g m was not determined by a single trait but was positively correlated with adaxial stomatal densities (SD ada ), stomatal ratio (SR), mesophyll surface area exposed to IAS (S mes ) and leaf thickness. However, g m was not related to abaxial stomatal densities (SD aba ) and mesophyll cell wall thickness (T CW ). Our study suggests that greater SD ada and SR increased g m by increasing S mes and creating additional parallel pathways for CO 2 diffusion inside mesophyll cells. Thus, SD ada , SR and S mes are important determinants of C 4 -g m and could be the target traits selected or modified for achieving greater g m and TE i in C 4 species., (© 2019 The Authors. New Phytologist © 2019 New Phytologist Trust.)

Quantum mechanical calculations can be useful in predicting equilibrium isotopic fractionations of geochemical reactions. However, these computational chemistry methods vary widely in their effectiveness in the prediction of various physical observables. Most studies employing the approach known as density functional theory (DFT) to model these observable quantities focus on predictive accuracy for energetics and geometries. In this study, several density functionals are evaluated against experimental bond lengths, harmonic vibrational frequencies, frequency shifts upon isotopic substitution, and 18 O/ 16 O isotopic fractionation between CO 2 (g) and H 2 O(g). Successful prediction of harmonic vibrational frequencies strongly correlates with successful prediction of isotopic fractionation, despite the possible introduction of errors by the harmonic approximation. Harmonic experimental frequencies, not anharmonic ones, must be used when comparing spectra and when predicting isotope fractionation. The B3LYP and X3LYP functionals perform more accurately in the evaluation of both harmonic vibrational frequencies and isotopic fractionation factors using the 6-311+G(d,p) and 6-311++G(2d,p) basis sets, achieving fractionation factor errors of 0.2-0.6‰ at 25 °C out of a total fractionation of 51‰. Error cancellation between vibrational frequencies and the harmonic approximation is crucial to their success. The above combination of exchange-correlation functionals and basis sets also well predicts the vibrational properties of interacting CO 2 and H 2 O molecules, suggesting that they may be applicable to more complex geochemical reactions involving C and O isotopic fractionations.

Isotopic composition of gross rainfall has been extensively used as a conservative tracer to track water movement and other hydrological processes in vegetated ecosystems. Recent studies from forest ecosystems, however, demonstrated that vegetation canopies can alter the isotopic composition of rainwater during rainfall partitioning into throughfall and stemflow, likely leading to errors and biases in aforementioned studies. No known studies, to date, had investigated this topic in shrub-dominated arid and semi-arid ecosystems where water is typically the driving factor in ecological, hydrological and biogeochemical processes. In this study, event-based gross rainfall, the throughfall and stemflow induced by shrubs of Caragana korshinskii were measured and samples were collected within a water-limited arid desert ecosystem of northern China, and their water stable isotopes ( 18 O and 2 H) were also analyzed in the laboratory. We mainly aimed to investigate whether there is an isotopic enrichment or depletion in stemflow and throughfall in comparison to gross rainfall, and to evaluate the possible underlying mechanisms. Our results indicated an enrichment of both isotopes in stemflow, while a general more depletion in throughfall than in gross rainfall, which is presumably affected by a combinative effects of canopy evaporation, isotopic exchange, and selective canopy storage. Deuterium excess of stemflow were found to be significantly higher (P < 0.05) than that of gross rainfall and throughfall. Moreover, we detected the pronounced "amount effect", with a significant (P < 0.05) negative relationship between isotopic composition and the amount of gross rainfall, throughfall, and stemflow, respectively. Our study is expected to contribute to an improved understanding of physical processes and water routing in shrub canopies within vast arid desert ecosystems., (Copyright © 2019. Published by Elsevier B.V.)

Understanding the water isotopes in feed products derived from grass is fundamental for tracing domestic animal products. Grass silage water was reported to have fewer heavy isotopes than fresh grass, but it is still unknown whether dew formation (either dewfall or dewrise), exchange with soil water, or other processes override the expected enrichment of heavy isotopes due to wilting. The isotopic variations of water (δ 2 H, δ 18 O) in fresh grass and cut grass during wilting on soil and on plastic were compared in this study. Drying enriched heavier isotopes, but this was overridden by three processes that finally caused low δ 2 H and δ 18 O values: (i) the adsorption of humidity from the surroundings, (ii) the exchange with humidity, and (iii) the depletion of heavy water isotopes close to organic surfaces, called the surface effect, which was the most dominant effect at the end of drying when the water content became low.

The mean residence time of soil water (MRT sw ) for forestland and shrubland in a water source area of Yuanyang Terrace, southwestern China, was estimated using stable isotope tracer tests and the sine-wave regression model. Stable isotope analyses from precipitation and soil water were performed in 2015. The δ 2 H/δ 18 O relationship of precipitation resulted in δ 2 H = 7.31δ 18 O + 1.49, which is nearly identical to the local meteoric water line in Kunming, southwestern, China. The MRT sw was simulated at five depth ranges (0-20, 20-40, 40-60, 60-80, 80-100 cm) of the two vegetation types by precipitation δ 18 O input data and soil water δ 18 O output data. The results showed that the MRT sw values of the forestland and shrubland both increased with soil depth. However, differences in the MRT sw of the forestland (between day 53 and 94) and of the shrubland (between day 76 and 142) were discussed. Regarding the physical properties of the soil profiles from the sample plots, non-capillary porosity decreased with soil depth in the forestland (from 48.5 to 20.5 %), and was clearly higher than that in the shrubland (from 38.8 to 18.7 %). Therefore, non-capillary porosity (macropores) could be a factor that shortens the mean residence time of soil water.

A thorough evaluation of measurement uncertainty together with control of short-term and long-term precision of measurements should be a basis of any successful quality assurance/quality control (QA/QC) strategy aimed at maintaining a high quality of the analytical process. Here we present the results of a comprehensive assessment of the analytical performance of a Picarro L2140-i CRDS laser spectrometer analysing δ 2 H, δ 18 O and δ 17 O in water. The assessment is based on results obtained during 15 months of continuous operation of this instrument (February 2017 to May 2018). The short-term precision of measured and derived quantities was 0.11, 0.036, 0.028, 0.23 ‰ and 11 per meg, for δ 2 H, δ 18 O, δ 17 O, d-excess and Δ 17 O, respectively, and is comparable to the precision reported by the manufacturer. The long-term precision of the L2140-i, defined as standard uncertainty of the time series of 153 analyses of a laboratory standard conducted throughout 15 months, was roughly two times lower (0.24, 0.053, 0.038, 0.37 ‰ and 21 per meg, for δ 2 H, δ 18 O, δ 17 O, d-excess and Δ 17 O). In-depth assessment of the measurement uncertainty of a single analysis revealed that assigned uncertainty of the calibration standards is an important component of the uncertainty budget, especially in case of δ 2 H analysis.

We used two types of soil with different physicochemical properties (loam and sand), oven-dried them, and then added the known isotopic composition mineral water that was reference water to compose the soil-water mixture with different soil water contents (loam: 0.15, 0.20, 0.30 g·g -1 ; sand: 0.10 g·g -1 ). After that, we set up different equilibrium time (loam: 3, 6, 12, 24, 48, 72, 96 h; sand: 96 h) to ensure that the dry soil particles were well mixed with the added water. The soil water was extracted by mechanical centrifugation and cryogenic vacuum extraction after equilibrium, and their isotope composition was analyzed. Results showed that the isotopic values of soil water extracted by mechanical centrifugation method had no significant difference in same water content with different equilibration times, but were more enriched compared with the reference water isotopic value. The maximum enrichment for hydrogen and oxygen isotope was 7.38‰ and 1.24‰, respectively. In contrast, cryogenic vacuum extraction method resulted in more depleted soil water isotopes than reference water, with the maximum depletion for hydrogen and oxygen isotope being 6.27‰ and 1.03‰, respectively. Moreover, the degree of depletion increased with the increases of equilibrium time (less than 24 h) at low water content, and became stable after 24 h. With the increases of soil water content, the isotopic composition of the extracted soil water was less affected by the two extraction methods. The water isotope value of loam that had high clay content, was more sensitive to the extraction method than the sandy soil that had low clay content. The difference of isotopic composition caused by extraction methods did not affect the plant water source segmentation.

18 O/ 16 O isotope effects were observed at the cathode of a polymer electrolyte membrane fuel cell at 25 and 35°C. Results of experiments in which the 18 O/ 16 O isotope ratios of the oxygen gases supplied to and exhausted from the cell were measured revealed that the lighter isotope 16 O reacted more preferentially to form water molecules at the cathode than the heavier one, 18 O. The value of the oxygen isotope separation factor, S 1 , defined as the ratio of the 18 O/ 16 O isotope ratios of the oxygen gases supplied to and exhausted from the cell, ranged from 1.0030 to 1.0139, and tended to decrease with decreasing rate of oxygen utilisation (θ) and with increasing flow rate of the feed oxygen gas (D F ). The value of another separation factor, S 2 , defined as the ratio of the 18 O/ 16 O isotope ratios of the exhausted oxygen gas and oxygen having reacted to form water molecules at the cathode, ranged from 1.0049 to 1.0304. The S 2 value was much less affected by the change in θ and D F than the S 1 value with the majority of the S 2 value being in the range of 1.0240-1.0304.

Rationale: Oxygen isotope analysis of water molecules of liquid foodstuffs is commonly performed under isotopic equilibrium between water in the solution and the vapour water, assuming that the liquid water activity is equal to unity and that liquid water is an ideal mixture of H 2 O isotopologues. A priori this behaviour is not realistic for all foodstuffs, which frequently are very concentrated solutions. In this paper we mainly consider "balsamic vinegar" with the aim of defining an appropriate procedure of oxygen isotope ratio analysis of water molecules in these concentrated solutions., Methods: Isotope ratio mass spectrometry (IRMS) measurements of the oxygen isotope ratios (δ 18 O values) were carried out on CO 2 equilibrated with water molecules at 22 ± 0.1°C. Three independently calibrated, very low salinity waters were used as standards., Results: For grape must and wine vinegar (density < 1.15 g/cm 3 ) the δ 18 O values for water determined directly on these solutions are "true" values. On the contrary, for balsamic vinegar with density higher than 1.15-1.20 g/cm 3 , the δ 18 O values obtained directly on the solutions are systematically different from those obtained on water produced by distillation of the same samples at 70°C under vacuum., Conclusions: In the case of balsamic vinegar with density higher than 1.15-1.20 g/cm 3 , to avoid severe systematic errors, the isotopic analyses must be carried out on water obtained by distillation under stirring., (© 2018 John Wiley & Sons, Ltd.)

Deficit irrigation has usually improved crop water use efficiency (WUE), but there are still gaps in our understanding of the mechanisms. Four irrigation treatments were a conventional furrow irrigation (CFI), border irrigation (BI), alternate furrow irrigation (AFI), and an AFI (M/2) (the amount of irrigation was 50% of the AFI). The volume of irrigation water applied were nearly the same for CFI, BI, and AFI. The isotope (δ 18 O and δD) method was used to quantify corn root water uptake (RWU) during 2013-2014. Compared to CFI and BI, corn yield and WUE were 17.0-30.2% and 13.3-33.8% higher in AFI, respectively. No significant yield reduction were observed between AFI and AFI (M/2) . Corn RWU was more from deeper soil with increasing growth stage for AFI (M/2) , AFI, and CFI, but from shallower depth for BI. The depth for RWU varied in the order of AFI (M/2)  > AFI > CFI > BI. The maximum root density was in the depth of 40-80 cm at the growing stage in AFI, and 4-26% more water was extracted from the wetter and deeper root zones. The WUE increased under deficit irrigation, and stimulated the root growth with attendant decreases in water loss to deep percolation.

The study aims to assess variability in leaf water isotopic enrichment occurring in the field under natural conditions. We focused on seasonal variation and difference between sun-exposed and shaded leaves. Isotopic composition (δ 18 O, δ 2 H) of leaf water was monitored in a beech tree (Fagus sylvatica L.) growing in the forest-meadow ecotone together with δ 18 O ( 2 H) of water compartments which are in close relation to this signal, namely twig and soil water. The sampling was carried out in approximately two-week intervals during five consecutive vegetation seasons. The δ 18 O ( 2 H) data showed a distinct seasonal pattern and a consistency in relative differences between the seasons and sample categories. Leaf water was the most isotopically enriched water compartment. The leaf water enrichment decreased toward the autumn reflecting the change in δ 18 O ( 2 H) of source water and evaporative demands. The soil and twig water isotopic signal was depleted against current precipitation as it partly retained the isotopic signature from winter precipitation however the seasonal pattern of soil and twig water followed that of precipitation. No significant differences between sun-exposed and shaded samples were detected. Nevertheless, the observed strong seasonal pattern of isotope composition of leaf, twig and soil water should be taken into account when using leaf water enrichment for further calculations or modeling., (Copyright © 2018 Elsevier GmbH. All rights reserved.)

Rationale: The stable isotope ratios of water (δ 2 H and δ 18 O values) have been widely used to trace water in plants in a variety of physiological, ecohydrological, biogeochemical and hydrological studies. In such work, the analyte must first be extracted from samples, prior to isotopic analysis. While cryogenic vacuum distillation is currently the most widely used method reported in the literature, a variety of extraction-collection-analysis methods exist. A formal inter-method comparison on plant tissues has yet to be carried out., Methods: We performed an inter-method comparison of six plant water extraction techniques: direct vapour equilibration, microwave extraction, two unique versions of cryogenic vacuum distillation, centrifugation, and high-pressure mechanical squeezing. These methods were applied to four isotopically unique plant portions (head, stem, leaf, and root crown) of spring wheat (Triticum aestivum L.). Extracted plant water was analyzed via spectrometric (OA-ICOS) and mass-based (IRMS) analysis systems when possible. Spring wheat was grown under controlled conditions with irrigation inputs of a known isotopic composition., Results: The tested methods of extraction yielded markedly different isotopic signatures. Centrifugation, microwave extraction, direct vapour equilibration, and high-pressure mechanical squeezing produced water more enriched in 2 H and 18 O content. Both cryogenic vacuum distillation systems and the high-pressure mechanical squeezing method produced water more depleted in 2 H and 18 O content, depending upon the plant portion extracted. The various methods also produced differing concentrations of co-extracted organic compounds, depending on the mode of extraction. Overall, the direct vapor equilibration method outperformed all other methods., Conclusions: Despite its popularity, cryogenic vacuum distillation was outperformed by the direct vapor equilibration method in terms of limited co-extraction of volatile organic compounds, rapid sample throughput, and near instantaneous returned stable isotope results. More research is now needed with other plant species, especially woody plants, to see how far the findings from this study could be extended., (Copyright © 2018 John Wiley & Sons, Ltd.)

The stable isotopes of water are extensively used as tracers in many fields of research. For this use, it is essential to know the isotope fractionation factors connected to various processes, the most important of which being phase changes. Many experimental studies have been performed on phase change fractionation over the last decades. Whereas liquid-vapour fractionation measurements are relatively straightforward, vapour-solid and liquid-solid fractionation measurements are more complicated, as maintaining equilibrium conditions when a solid is involved is difficult. In this work, we determine the ice-liquid isotope fractionation factors in an indirect way, by applying the Van't Hoff equation. This equation describes the relationship of the fractionation factors with isotope-dependent temperature changes. We apply it to the recently experimentally determined isotope dependences of the triple point temperature of water [Faghihi V, Peruzzi A, Aerts-Bijma AT, et al. Accurate experimental determination of the isotope effects on the triple point temperature of water. I. Dependence on the 2 H abundance. Metrologia. 2015;52:819-826; Faghihi V, Kozicki M, Aerts-Bijma AT, et al. Accurate experimental determination of the isotope effects on the triple point temperature of water. II. Combined dependence on the 18 O and 17 O abundances. Metrologia. 2015;52:827-834]. This results in new values for the 2 H (deuterium) and 18 O fractionation factors for the liquid-solid phase change of water, which agree well with existing, direct experimental data [Lehmann M, Siegenthaler U. Equilibrium oxygen- and hydrogen-isotope fractionation between ice and water. J Glaciol. 1991;37:23-26]. For 2 H, the uncertainty is improved by a factor of 3, whereas for 18 O the uncertainty is similar. Our final results are α S-L ( 2 H/ 1 H) = 1.02093(13), and α S-L ( 18 O/ 16 O) = 1.002909(25), where the latter is the weighted average of the previous experimental study and this work.

In South-western Morocco, water scarcity and high temperature are the main factors determining species survival. Argania spinosa (L.) Skeels is a tree species, endemic to Morocco, which is suffering from ongoing habitat shrinkage. Argan trees play essential local ecological and economic roles: protecting soils from erosion, shading different types of crops, helping maintain soil fertility and, even more importantly, its seeds are used by the local population for oil production, with valuable nutritional, medicinal and cosmetic purposes. The main objective of this study was to identify the sources of water used by this species and to assess the effect of water availability on the photosynthetic rate and stem water potential in two populations: one growing on the coast and a second one 10 km inland. Stem water potential, photosynthetic rate and xylem water isotopic composition (δ18O) were seasonally monitored during 2 years. Trees from both populations showed a similar strategy in the use of the available water sources, which was strongly dependent on deep soil water throughout the year. Nevertheless, during the wet season or under low precipitation a more complex water uptake pattern was found with a mixture of water sources, including precipitation and soil at different depths. No evidence was found of the use of either groundwater or atmospheric water in this species. Despite the similar water-use strategy, the results indicate that Argania trees from the inland population explored deeper layers than coastal ones as suggested by more depleted δ18O values recorded in the inland trees and better photosynthetic performance, hence suggesting that the coastal population of A. spinosa could be subjected to higher stress.

The history of the growth of continental crust is uncertain, and several different models that involve a gradual, decelerating, or stepwise process have been proposed 1-4 . Even more uncertain is the timing and the secular trend of the emergence of most landmasses above the sea (subaerial landmasses), with estimates ranging from about one billion to three billion years ago 5-7 . The area of emerged crust influences global climate feedbacks and the supply of nutrients to the oceans 8 , and therefore connects Earth's crustal evolution to surface environmental conditions 9-11 . Here we use the triple-oxygen-isotope composition of shales from all continents, spanning 3.7 billion years, to provide constraints on the emergence of continents over time. Our measurements show a stepwise total decrease of 0.08 per mille in the average triple-oxygen-isotope value of shales across the Archaean-Proterozoic boundary. We suggest that our data are best explained by a shift in the nature of water-rock interactions, from near-coastal in the Archaean era to predominantly continental in the Proterozoic, accompanied by a decrease in average surface temperatures. We propose that this shift may have coincided with the onset of a modern hydrological cycle owing to the rapid emergence of continental crust with near-modern average elevation and aerial extent roughly 2.5 billion years ago.

Understanding species-specific changes in water-use patterns under recent climate scenarios is necessary to predict accurately the responses of seasonally dry ecosystems to future climate. In this study, we conducted a precipitation manipulation experiment to investigate the changes in water-use patterns of two coexisting species (Achnatherum splendens and Allium tanguticum) to alterations in soil water content (SWC) resulting from increased and decreased rainfall treatments. The results showed that the leaf water potential (Ψ) of A. splendens and A. tanguticum responded to changes in shallow and middle SWC at both the control and treatment plots. However, A. splendens proportionally extracted water from the shallow soil layer (0-10cm) when it was available but shifted to absorbing deep soil water (30-60 cm) during drought. By contrast, the A. tanguticum did not differ significantly in uptake depth between treatment and control plots but entirely depended on water from shallow soil layers. The flexible water-use patterns of A.splendens may be a key factor facilitating its dominance and it better acclimates the recent climate change in the alpine grassland community around Qinghai Lake.

The oxygen ( 18 O/ 16 O) isotope analysis of hair is commonly applied to reconstruct an individual's residence history. However, region-of-origin as determined from oxygen isotope values (δ 18 O) alone is often spatially indistinct. Adding additional geochemical recorders can refine region-of-origin estimates. In this capacity, strontium ( 87 Sr/ 86 Sr) isotope analysis has attracted increased interest. While 87 Sr/ 86 Sr reflects the influences of local geology, 87 Sr/ 86 Sr of hair includes both external environmental signals as well as the internal dietary indicators. To better understand the impact of these contributions to the spatial signal encoded within 87 Sr/ 86 Sr of hair, human hair was collected from three locations within Salt Lake City, Utah along with the donor's sex. The 87 Sr/ 86 Sr and δ 18 O of hair and local tap water were measured. There were no significant relationships between sex and either δ 18 O or 87 Sr/ 86 Sr of hair, nor between collection location and the δ 18 O of hair. However, we found significant associations between collection location and 87 Sr/ 86 Sr of hair. These findings suggest that interactions with local water may be an important source of Sr to human hair and that the 87 Sr/ 86 Sr of hair may have the capacity to record differences in 87 Sr/ 86 Sr of tap waters on small spatial scales.

Rationale: The doubly labelled water (DLW) method is the reference method for the estimation of free-living total energy expenditure (TEE). In this method, where both 2 H and 18 O are employed, different approaches have been adopted to deal with the non-conformity observed regarding the distribution space for the labels being non-coincident with total body water. However, the method adopted can have a significant effect on the estimated TEE., Methods: We proposed a Bayesian reasoning approach to modify an assumed prior distribution for the space ratio using experimental data to derive the TEE. A Bayesian hierarchical approach was also investigated. The dataset was obtained from 59 adults (37 women) who underwent a DLW experiment during which the 2 H and 18 O enrichments were measured using isotope ratio mass spectrometry (IRMS)., Results: TEE was estimated at 9925 (9106-11236) [median and interquartile range], 9646 (9167-10540), and 9,638 (9220-10340) kJ·day -1 for women and at 13961 (12851-15347), 13353 (12651-15088) and 13211 (12653-14238) kJ·day -1 for men, using normalized non-Bayesian, independent Bayesian and hierarchical Bayesian approaches, respectively. A comparison of hierarchical Bayesian with normalized non-Bayesian methods indicated a marked difference in behaviour between genders. The median difference was -287 kJ·day -1 for women, and -750 kJ·day -1 for men. In men there is an appreciable compression of the TEE distribution obtained from the hierarchical model compared with the normalized non-Bayesian methods (range of TEE 11234-15431 kJ·day -1 vs 10786-18221 kJ·day -1 ). An analogous, yet smaller, compression is seen in women (7081-12287 kJ·day -1 vs 6989-13775 kJ·day -1 )., Conclusions: The Bayesian analysis is an appealing method to estimate TEE during DLW experiments. The principal advantages over those obtained using the classical least-squares method is the generation of potentially more useful estimates of TEE, and improved handling of outliers and missing data scenarios, particularly if a hierarchical model is used., (© 2017 The Authors. Rapid Communications in Mass Spectrometry Published by John Wiley & Sons Ltd.)