A method for in situ monitoring of the isotope composition of tree xylem water using laser spectroscopy

Coping with spectral interferences when measuring water stable isotopes of vegetables

RATIONALE

Laser-based analyzers are widely used in ecohydrology to analyze plant water isotopic compositions (δ18O and δ2H). The suitability of three different water extraction and isotope equilibration techniques was compared. We examined whether co-extracted volatile organic compounds (VOCs) affect laser-based isotope measurements and used the instrument's spectral parameters to post-correct for interfering VOCs.

METHODS

Cryogenic vacuum extraction, vapor headspace equilibration in bags, and vapor equilibration in situ probes were used to extract liquid water or water vapor for laser-based isotope analysis (cavity ring-down spectrometry, CRDS). Isotope data were calibrated by standards for each method separately. Spectral parameters of the instrument, appropriate to identify spectral interferences with MeOH and CH4, were identified and used for post-correction. Differences between the three methods and between the origins of the vegetables were identified by statistical tests.

RESULTS

VOCs were found in various amounts for the three different methods. They were co-extracted or co-equilibrated during the different extraction or equilibration methods. Correlation coefficients of isotope data and "CH4" (spectral parameter) were 0.99 or better; however, slopes for δ18O were similar on different instrument types but different for δ2H. Our correction approach improved results and inter-comparability of the methods considerably without knowing the chemical composition of the plant sap.

CONCLUSIONS

All three methods were sensitive enough to distinguish and resolve differences in natural abundance. Data quality was improved by the "CH4 correction" approach but could probably be optimized by a plant species-specific correction. Standardized tools for contaminant removal or post-correction applications from manufacturers, in particular for vapor-mode analysis, are still needed.

Two common pitfalls in the analysis of water-stable isotopologues with cryogenic vacuum extraction and cavity ring-down spectroscopy

Water stable isotopologue analysis is widely used to disentangle ecohydrological processes. Yet, there are increasing reports of measurement uncertainties for established and emerging methods, such as cryogenic vacuum extraction (CVE) or cavity ring-down spectroscopy (CRDS). With this study, we investigate two pitfalls, that potentially contribute to uncertainties in water-stable isotopologue research. To investigate fractionation sources in CVE, we extracted pure water of known isotopic composition with cotton, glass wool or without cover and compared the isotopologue results with non-extracted reference samples. To characterise the dependency of δ2H and δ18O on the water mixing ratio in CRDS, which is of high importance for in-situ applications with large natural variations in mixing ratios, we chose samples with a large range of isotopic compositions and determined δ2H and δ18O for different water mixing ratios with two CRDS analysers (Picarro, Inc.). Cotton wool had a strong fractionation effect on δ2H values, which increased with more 2H-enriched samples. δ2H and δ18O values showed a strong dependency on the water mixing ratio analysed with CRDS with differences of up to 34.5‰ (δ2H) and 3.9‰ (δ18O) for the same sample at different mixing ratios. CVE and CRDS, now routinely applied in water stable isotopologue research, come with pitfalls, namely fractionation effects of cover materials and water mixing ratio dependencies of δ2H and δ18O, which can lead to erroneous isotopologue results and thus, invalid conclusions about (ecohydrological) processes. These practical issues identified here should be reported and addressed adequately in water-stable isotopologue research.

Interaction between beech and spruce trees in temperate forests affects water use, root water uptake pattern and canopy structure

Beneficial and negative effects of species interactions can strongly influence water fluxes in forest ecosystems. However, little is known about how trees dynamically adjust their water use when growing with interspecific neighbours. Therefore, we investigated the interaction effects between Fagus sylvatica (European beech) and Picea abies (Norway spruce) on water-use strategies and aboveground structural characteristics. We used continuous in situ isotope spectroscopy of xylem and soil water to investigate source water dynamics and root water uptake depths. Picea abies exhibited a reduced sun-exposed crown area in equally mixed compared with spruce-dominated sites, which was further correlated to a reduction in sap flow of -14.5 ± 8.2%. Contrarily, F. sylvatica trees showed +13.3 ± 33.3% higher water fluxes in equally mixed compared with beech-dominated forest sites. Although a significantly higher crown interference by neighbouring trees was observed, no correlation of water fluxes and crown structure was found. High time-resolved xylem δ2H values showed a large plasticity of tree water use (-74.1 to -28.5‰), reflecting the δ2H dynamics of soil and especially precipitation water sources. Fagus sylvatica in equally mixed sites shifted water uptake to deeper soil layers, while uptake of fresh precipitation was faster in beech-dominated sites. Our continuous in situ water stable isotope measurements traced root water uptake dynamics at unprecedented temporal resolution, indicating highly dynamic use of water sources in response to precipitation and to neighbouring species competition. Understanding this plasticity may be highly relevant in the context of increasing water scarcity and precipitation variability under climate change.

Deep roots mitigate drought impacts on tropical trees despite limited quantitative contribution to transpiration

Deep rooting is considered a central drought-mitigation trait with vast impact on ecosystem water cycling. Despite its importance, little is known about the overall quantitative water use via deep roots and dynamic shifts of water uptake depths with changing ambient conditions. Knowledge is especially sparse for tropical trees. Therefore, we conducted a drought, deep soil water labeling and re-wetting experiment at Biosphere 2 Tropical Rainforest. We used in situ methods to determine water stable isotope values in soil and tree water in high temporal resolution. Complemented by soil and stem water content and sap flow measurements we determined percentages and quantities of deep-water in total root water uptake dynamics of different tree species. All canopy trees had access to deep-water (max. uptake depth 3.3 m), with contributions to transpiration ranging between 21 % and 90 % during drought, when surface soil water availability was limited. Our results suggest that deep soil is an essential water source for tropical trees that delays potentially detrimental drops in plant water potentials and stem water content when surface soil water is limited and could hence mitigate the impacts of increasing drought occurrence and intensity as a consequence of climate change. Quantitatively, however, the amount of deep-water uptake was low due to the trees' reduction of sap flow during drought. Total water uptake largely followed surface soil water availability and trees switched back their uptake depth dynamically, from deep to shallow soils, following rainfall. Total transpiration fluxes were hence largely driven by precipitation input.

Challenges in studying water fluxes within the soil-plant-atmosphere continuum: A tracer-based perspective on pathways to progress

Tracing and quantifying water fluxes in the hydrological cycle is crucial for understanding the current state of ecohydrological systems and their vulnerability to environmental change. Especially the interface between ecosystems and the atmosphere that is strongly mediated by plants is important to meaningfully describe ecohydrological system functioning. Many of the dynamic interactions generated by water fluxes between soil, plant and the atmosphere are not well understood, which is partly due to a lack of interdisciplinary research. This opinion paper reflects the outcome of a discussion among hydrologists, plant ecophysiologists and soil scientists on open questions and new opportunities for collaborative research on the topic "water fluxes in the soil-plant-atmosphere continuum" especially focusing on environmental and artificial tracers. We emphasize the need for a multi-scale experimental approach, where a hypothesis is tested at multiple spatial scales and under diverse environmental conditions to better describe the small-scale processes (i.e., causes) that lead to large-scale patterns of ecosystem functioning (i.e., consequences). Novel in-situ, high-frequency measurement techniques offer the opportunity to sample data at a high spatial and temporal resolution needed to understand the underlying processes. We advocate for a combination of long-term natural abundance measurements and event-based approaches. Multiple environmental and artificial tracers, such as stable isotopes, and a suite of experimental and analytical approaches should be combined to complement information gained by different methods. Virtual experiments using process-based models should be used to inform sampling campaigns and field experiments, e.g., to improve experimental designs and to simulate experimental outcomes. On the other hand, experimental data are a pre-requisite to improve our currently incomplete models. Interdisciplinary collaboration will help to overcome research gaps that overlap across different earth system science fields and help to generate a more holistic view of water fluxes between soil, plant and atmosphere in diverse ecosystems.

Plant water uptake modelling: added value of cross-disciplinary approaches

In recent years, research interest in plant water uptake strategies has rapidly increased in many disciplines, such as hydrology, plant ecology and ecophysiology. Quantitative modelling approaches to estimate plant water uptake and spatiotemporal dynamics have significantly advanced through different disciplines across scales. Despite this progress, major limitations, for example, predicting plant water uptake under drought or drought impact at large scales, remain. These are less attributed to limitations in process understanding, but rather to a lack of implementation of cross-disciplinary insights into plant water uptake model structure. The main goal of this review is to highlight how the four dominant model approaches, that is, Feddes approach, hydrodynamic approach, optimality and statistical approaches, can be and have been used to create interdisciplinary hybrid models enabling a holistic system understanding that, among other things, embeds plant water uptake plasticity into a broader conceptual view of soil-plant feedbacks of water, nutrient and carbon cycling, or reflects observed drought responses of plant-soil feedbacks and their dynamics under, that is, drought. Specifically, we provide examples of how integration of Bayesian and hydrodynamic approaches might overcome challenges in interpreting plant water uptake related to different travel and residence times of different plant water sources or trade-offs between root system optimization to forage for water and nutrients during different seasons and phenological stages.

Tracing plant source water dynamics during drought by continuous transpiration measurements: An in-situ stable isotope approach

The isotopic composition of xylem water (δ_X ) is of considerable interest for plant source water studies. In-situ monitored isotopic composition of transpired water (δ_T ) could provide a nondestructive proxy for δ_X -values. Using flow-through leaf chambers, we monitored 2-hourly δ_T -dynamics in two tropical plant species, one canopy-forming tree and one understory herbaceous species. In an enclosed rainforest (Biosphere 2), we observed δ_T -dynamics in response to an experimental severe drought, followed by a ² H deep-water pulse applied belowground before starting regular rain. We also sampled branches to obtain δ_X -values from cryogenic vacuum extraction (CVE). Daily flux-weighted δ¹⁸ O_T -values were a good proxy for δ¹⁸ O_X -values under well-watered and drought conditions that matched the rainforest's water source. Transpiration-derived δ¹⁸ O_X -values were mostly lower than CVE-derived values. Transpiration-derived δ² H_X -values were relatively high compared to source water and consistently higher than CVE-derived values during drought. Tracing the ² H deep-water pulse in real-time showed distinct water uptake and transport responses: a fast and strong contribution of deep water to canopy tree transpiration contrasting with a slow and limited contribution to understory species transpiration. Thus, the in-situ transpiration method is a promising tool to capture rapid dynamics in plant water uptake and use by both woody and nonwoody species.

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.

Continuous in situ measurements of water stable isotopes in soils, tree trunk and root xylem: Field approval

RATIONALE

New methods to measure stable isotopes of soil and tree water directly in the field enable us to increase the temporal resolution of obtained data and advance our knowledge on the dynamics of soil and plant water fluxes. Only few field applications exist. However, these are needed to further improve novel methods and hence exploit their full potential.

METHODS

We tested the borehole equilibration method in the field and collected in situ and destructive samples of stable isotopes of soil, trunk and root xylem water over a 2.5-month experiment in a tropical dry forest under natural abundance conditions and following labelled irrigation. Water from destructive samples was extracted using cryogenic vacuum extraction. Isotope ratios were determined with IRIS instruments using cavity ring-down spectroscopy both in the field and in the laboratory.

RESULTS

In general, timelines of both methods agreed well for both soil and xylem samples. Irrigation labelled with heavy hydrogen isotopes clearly impacted the isotope composition of soil water and one of the two studied tree species. Inter-method deviations increased in consequence of labelling, which revealed their different capabilities to cover spatial and temporal heterogeneities.

CONCLUSIONS

We applied the novel borehole equilibration method in a remote field location. Our experiment reinforced the potential of this in situ method for measuring xylem water isotopes in both tree trunks and roots and confirmed the reliability of gas permeable soil probes. However, in situ xylem measurements should be further developed to reduce the uncertainty within the range of natural abundance and hence enable their full potential.

Drought reduces water uptake in beech from the drying topsoil, but no compensatory uptake occurs from deeper soil layers

The intensity and frequency of droughts events are projected to increase in future with expected adverse effects for forests. Thus, information on the dynamics of tree water uptake from different soil layers during and after drought is crucial. We applied an in situ water isotopologue monitoring system to determine the oxygen isotope composition in soil and xylem water of European beech with a 2-h resolution together with measurements of soil water content, transpiration and tree water deficit. Using a Bayesian isotope mixing model, we inferred the relative and absolute contribution of water from four different soil layers to tree water use. Beech took up more than 50% of its water from the uppermost 5 cm soil layer at the beginning of the 2018 drought, but then reduced absolute water uptake from the drying topsoil by 84%. The trees were not able to quantitatively compensate for restricted topsoil water availability by additional uptake from deeper soil layers, which is related to the fine root depth distribution. Absolute water uptake from the topsoil was restored to pre-drought levels within 3 wk after rewetting. These uptake patterns help to explain both the drought sensitivity of beech and its high recovery potential after drought release.

Radial and axial water movement in adult trees recorded by stable isotope tracing

The capacity of trees to release water from storage compartments into the transpiration stream can mitigate damage to hydraulic functioning. However, the location of these 'transient' water sources and also the pathways of water movement other than vertical through tree stems still remain poorly understood. We conducted an experiment on two tree species in a common garden in eastern Australia that naturally grow in regions of high (Eucalyptus tereticornis, 'Red Gum') and low (Eucalyptus sideroxylon, 'Ironbark') annual precipitation rates. Deuterium-enriched water (1350% label strength) was directly introduced into the transpiration stream of three trees per species for four consecutive days. Subsequently, the trees were felled, woody tissue samples were collected from different heights and azimuthal positions of the stems, and stable isotope ratios were determined on the water extracted from all samples. The presence/absence of the tracer along the radial and vertical stem axes in combination with xylem hydraulic properties inferred from sapflow, leaf and stem water potentials, wood moisture contents and anatomical sapwood characteristics elucidated species-specific patterns of short-term stem water storage and movement. The distribution of water isotopes at natural abundance among woody tissues indicated systematic differences with highest values of sapwood water and lower values in inner bark and heartwood. Presence of tracer in water of the inner bark highlighted the importance of this tissue as capacitor. Although injected at the northern side of stems, tracer was also discovered at the southern side, providing empirical evidence for circumferential flow in sapwood, particularly of Ironbark. Greater vertical water transport in Red Gum compared with more radial and circumferential water transport in Ironbark were associated with species-specific sapwood anatomy. Our study highlights the value of combining information from stable isotope tracers and wood anatomy to investigate patterns of water transport and storage of tall trees in situ.

A starting guide to root ecology: strengthening ecological concepts and standardising root classification, sampling, processing and trait measurements

In the context of a recent massive increase in research on plant root functions and their impact on the environment, root ecologists currently face many important challenges to keep on generating cutting-edge, meaningful and integrated knowledge. Consideration of the below-ground components in plant and ecosystem studies has been consistently called for in recent decades, but methodology is disparate and sometimes inappropriate. This handbook, based on the collective effort of a large team of experts, will improve trait comparisons across studies and integration of information across databases by providing standardised methods and controlled vocabularies. It is meant to be used not only as starting point by students and scientists who desire working on below-ground ecosystems, but also by experts for consolidating and broadening their views on multiple aspects of root ecology. Beyond the classical compilation of measurement protocols, we have synthesised recommendations from the literature to provide key background knowledge useful for: (1) defining below-ground plant entities and giving keys for their meaningful dissection, classification and naming beyond the classical fine-root vs coarse-root approach; (2) considering the specificity of root research to produce sound laboratory and field data; (3) describing typical, but overlooked steps for studying roots (e.g. root handling, cleaning and storage); and (4) gathering metadata necessary for the interpretation of results and their reuse. Most importantly, all root traits have been introduced with some degree of ecological context that will be a foundation for understanding their ecological meaning, their typical use and uncertainties, and some methodological and conceptual perspectives for future research. Considering all of this, we urge readers not to solely extract protocol recommendations for trait measurements from this work, but to take a moment to read and reflect on the extensive information contained in this broader guide to root ecology, including sections I-VII and the many introductions to each section and root trait description. Finally, it is critical to understand that a major aim of this guide is to help break down barriers between the many subdisciplines of root ecology and ecophysiology, broaden researchers' views on the multiple aspects of root study and create favourable conditions for the inception of comprehensive experiments on the role of roots in plant and ecosystem functioning.

Organic contamination detection for isotopic analysis of water by laser spectroscopy

RATIONALE

Hydrogen and oxygen stable isotope ratios (δ² H, δ¹⁷ O, and δ¹⁸ O values) are commonly used tracers of water. These ratios can be measured by isotope ratio infrared spectroscopy (IRIS). However, IRIS approaches are prone to errors induced by organic compounds present in plant, soil, and natural water samples. A novel approach using ¹⁷ O-excess values has shown promise for flagging spectrally contaminated plant samples during IRIS analysis. A systematic assessment of this flagging system is needed to prove it useful.

METHODS

Errors induced by methanol and ethanol water mixtures on measured IRIS and isotope ratio mass spectrometry (IRMS) results were evaluated. For IRIS analyses both liquid- and vapour-mode (via direct vapour equilibration) methods are used. The δ² H, δ¹⁷ O, and δ¹⁸ O values were measured and compared with known reference values to determine the errors induced by methanol and ethanol contamination. In addition, the ¹⁷ O-excess contamination detection approach was tested. This is a post-processing detection tool for both liquid and vapour IRIS triple-isotope analyses, utilizing calculated ¹⁷ O-excess values to flag contaminated samples.

RESULTS

Organic contamination induced significant errors in IRIS results, not seen in IRMS results. Methanol caused larger errors than ethanol. Results from vapour-IRIS analyses had larger errors than those from liquid-IRIS analyses. The ¹⁷ O-excess approach identified methanol driven error in liquid- and vapour-mode IRIS samples at levels where isotope results became unacceptably erroneous. For ethanol contaminated samples, a mix of erroneous and correct flagging occurred with the ¹⁷ O-excess method. Our results indicate that methanol is the more problematic contaminant for data corruption. The ¹⁷ O-excess method was therefore useful for data quality control.

CONCLUSIONS

Organic contamination caused significant errors in IRIS stable isotope results. These errors were larger during vapour analyses than during liquid IRIS analyses, and larger for methanol than ethanol contamination. The ¹⁷ O-excess method is highly sensitive for detecting narrowband (methanol) contamination error in vapour and liquid analysis modes in IRIS.

Soil gas probes for monitoring trace gas messengers of microbial activity

Soil microbes vigorously produce and consume gases that reflect active soil biogeochemical processes. Soil gas measurements are therefore a powerful tool to monitor microbial activity. Yet, the majority of soil gases lack non-disruptive subsurface measurement methods at spatiotemporal scales relevant to microbial processes and soil structure. To address this need, we developed a soil gas sampling system that uses novel diffusive soil probes and sample transfer approaches for high-resolution sampling from discrete subsurface regions. Probe sampling requires transferring soil gas samples to above-ground gas analyzers where concentrations and isotopologues are measured. Obtaining representative soil gas samples has historically required balancing disruption to soil gas composition with measurement frequency and analyzer volume demand. These considerations have limited attempts to quantify trace gas spatial concentration gradients and heterogeneity at scales relevant to the soil microbiome. Here, we describe our new flexible diffusive probe sampling system integrated with a modified, reduced volume trace gas analyzer and demonstrate its application for subsurface monitoring of biogeochemical cycling of nitrous oxide (N₂O) and its site-specific isotopologues, methane, carbon dioxide, and nitric oxide in controlled soil columns. The sampling system observed reproducible responses of soil gas concentrations to manipulations of soil nutrients and redox state, providing a new window into the microbial response to these key environmental forcings. Using site-specific N₂O isotopologues as indicators of microbial processes, we constrain the dynamics of in situ microbial activity. Unlocking trace gas messengers of microbial activity will complement -omics approaches, challenge subsurface models, and improve understanding of soil heterogeneity to disentangle interactive processes in the subsurface biome.

Tracing plant–environment interactions from organismal to planetary scales using stable isotopes: a mini review

Natural isotope variation forms a mosaic of isotopically distinct pools across the biosphere and flows between pools integrate plant ecology with global biogeochemical cycling. Carbon, nitrogen, and water isotopic ratios (among others) can be measured in plant tissues, at root and foliar interfaces, and in adjacent atmospheric, water, and soil environments. Natural abundance isotopes provide ecological insight to complement and enhance biogeochemical research, such as understanding the physiological conditions during photosynthetic assimilation (e.g. water stress) or the contribution of unusual plant water or nutrient sources (e.g. fog, foliar deposition). While foundational concepts and methods have endured through four decades of research, technological improvements that enable measurement at fine spatiotemporal scales, of multiple isotopes, and of isotopomers, are advancing the field of stable isotope ecology. For example, isotope studies now benefit from the maturation of field-portable infrared spectroscopy, which allows the exploration of plant–environment sensitivity at physiological timescales. Isotope ecology is also benefiting from, and contributing to, new understanding of the plant–soil–atmosphere system, such as improving the representation of soil carbon pools and turnover in land surface models. At larger Earth-system scales, a maturing global coverage of isotope data and new data from site networks offer exciting synthesis opportunities to merge the insights of single-or multi-isotope analysis with ecosystem and remote sensing data in a data-driven modeling framework, to create geospatial isotope products essential for studies of global environmental change.

Stem water cryogenic extraction biases estimation in deuterium isotope composition of plant source water

The hydrogen isotope ratio of water cryogenically extracted from plant stem samples (δ²H_{stem_CVD}) is routinely used to aid isotope applications that span hydrological, ecological, and paleoclimatological research. However, an increasing number of studies have shown that a key assumption of these applications-that δ²H_{stem_CVD} is equal to the δ²H of plant source water (δ²H_source)-is not necessarily met in plants from various habitats. To examine this assumption, we purposedly designed an experimental system to allow independent measurements of δ²H_{stem_CVD}, δ²H_source, and δ²H of water transported in xylem conduits (δ²H_xylem) under controlled conditions. Our measurements performed on nine woody plant species from diverse habitats revealed a consistent and significant depletion in δ²H_{stem_CVD} compared with both δ²H_source and δ²H_xylem Meanwhile, no significant discrepancy was observed between δ²H_source and δ²H_xylem in any of the plants investigated. These results cast significant doubt on the long-standing view that deuterium fractionation occurs during root water uptake and, alternatively, suggest that measurement bias inherent in the cryogenic extraction method is the root cause of δ²H_{stem_CVD} depletion. We used a rehydration experiment to show that the stem water cryogenic extraction error could originate from a dynamic exchange between organically bound deuterium and liquid water during water extraction. In light of our finding, we suggest caution when partitioning plant water sources and reconstructing past climates using hydrogen isotopes, and carefully propose that the paradigm-shifting phenomenon of ecohydrological separation ("two water worlds") is underpinned by an extraction artifact.

Xylem sap phosphorus sampling using microdialysis-a non-destructive high sampling frequency method tested under laboratory and field conditions

For a better understanding of plant nutrition processes, it is important to study the flux of nutrients within plants. However, existing xylem sap sampling methods are typically destructive and do not allow for repeated, highly frequent measurements of nutrient concentration. In this paper, we present a novel use of microdialysis (MD) for characterizing xylem sap phosphate (PO43-) concentration as a possible alternative to destructive sampling. First, MD probes were tested under laboratory conditions in vitro, in a stirred solution test, and in vivo, using beech tree stem segments. Exponential decline in the relative recovery (RR) with an increasing MD pumping rate allows for determining an optimal sampling interval (i.e., the maximum amount of sample volume with the minimum required concentration). The RR changed only minimally, with a change in the simulated sap flow velocity during the in vivo stem segment test. This suggests that MD can be applied over a range of naturally occurring sap flow velocities. Differences in the ionic strength between the xylem sap and the perfusate pumped through the MD did not influence the RR. Then, MD was successfully applied in a 24 h field campaign in two beech trees of different ages and allowed for in situ assessments of the diurnal variation of PO43- concentration and (together with xylem flow measurements) flux variability in living trees. Both beech trees exhibited the same diurnal pattern in PO43- concentrations with higher concentrations in the younger tree. The xylem PO43- concentration measured with MD was in the same order of magnitude as that received through destructive sampling in the younger tree. The MD probes did not show a decline in RR after the field application. We showed that MD can be applied to capture the PO43- concentration dynamics in the xylem sap with bihourly resolution under field conditions.

Borehole Equilibration: Testing a New Method to Monitor the Isotopic Composition of Tree Xylem Water in situ

Forest water use has been difficult to quantify. One promising approach is to measure the isotopic composition of plant water, e.g., the transpired water vapor or xylem water. Because different water sources, e.g., groundwater versus shallow soil water, often show different isotopic signatures, isotopes can be used to investigate the depths from which plants take up their water and how this changes over time. Traditionally such measurements have relied on the extraction of wood samples, which provide limited time resolution at great expense, and risk possible artifacts. Utilizing a borehole drilled through a tree's stem, we propose a new method based on the notion that water vapor in a slow-moving airstream approaches isotopic equilibration with the much greater mass of liquid water in the xylem. We present two empirical data sets showing that the method can work in practice. We then present a theoretical model estimating equilibration times and exploring the limits at which the approach will fail. The method provides a simple, cheap, and accurate means of continuously estimating the isotopic composition of the source water for transpiration.

Investigating the root plasticity response of Centaurea jacea to soil water availability changes from isotopic analysis

Root water uptake is a key ecohydrological process for which a physically based understanding has been developed in the past decades. However, due to methodological constraints, knowledge gaps remain about the plastic response of whole plant root systems to a rapidly changing environment. We designed a laboratory system for nondestructive monitoring of stable isotopic composition in plant transpiration of a herbaceous species (Centaurea jacea) and of soil water across depths, taking advantage of newly developed in situ methods. Daily root water uptake profiles were obtained using a statistical Bayesian multisource mixing model. Fast shifts in the isotopic composition of both soil and transpiration water could be observed with the setup and translated into dynamic and pronounced shifts of the root water uptake profile, even in well watered conditions. The incorporation of plant physiological and soil physical information into statistical modelling improved the model output. A simple exercise of water balance closure underlined the nonunique relationship between root water uptake profile on the one hand, and water content and root distribution profiles on the other, illustrating the continuous adaption of the plant water uptake as a function of its root hydraulic architecture and soil water availability during the experiment.

Water Stable Isotopes in Ecohydrological Field Research: Comparison Between In Situ and Destructive Monitoring Methods to Determine Soil Water Isotopic Signatures

Ecohydrological isotope based field research is often constrained by a lack of temporally explicit soil water data, usually related to the choice of destructive sampling in the field and subsequent analysis in the laboratory. New techniques based on gas permeable membranes allow to sample soil water vapor in situ and infer soil liquid water isotopic signatures. Here, a membrane-based in situ soil water vapor sampling method was tested at a grassland site in Freiburg, Germany. It was further compared with two commonly used destructive sampling approaches for determination of soil liquid water isotopic signatures: cryogenic vacuum extraction and centrifugation. All methods were tested under semi-controlled field conditions, conducting an experiment with dry-wet cycling and two isotopically different labeling irrigation waters. We found mean absolute differences between cryogenic vacuum extraction and in situ vapor measurements of 0.3-14.2‰ (δ¹⁸O) and 0.4-152.2‰ (δ²H) for soil liquid water. The smallest differences were found under natural abundance conditions of ²H and ¹⁸O, the strongest differences were observed after irrigation with labeled waters. Labeling strongly increased the isotopic variation in soil water: Mean soil water isotopic signatures derived by cryogenic vacuum extraction were -11.6 ± 10.9‰ (δ¹⁸O) and +61.9 ± 266.3‰ (δ²H). The in situ soil water vapor method showed isotopic signatures of -12.5 ± 9.4‰ (δ¹⁸O) and +169.3 ± 261.5‰ (δ²H). Centrifugation was unsuccessful for soil samples due to low water recovery rates. It is therefore not recommended. Our study highlights that the in situ soil water vapor method captures the temporal dynamics in the isotopic signature of soil water well while the destructive approach also includes the natural lateral isotopic heterogeneity. The different advantages and limitations of the three methods regarding setup, handling and costs are discussed. The choice of method should not only consider prevailing environmental conditions but the experimental design and goal. We see a very promising tool in the in situ soil water vapor method, capturing both temporal developments and spatial variability of soil water processes.

Water fluxes mediated by vegetation: emerging isotopic insights at the soil and atmosphere interfaces

Plants mediate water fluxes within the soil-vegetation-atmosphere continuum. This water transfer in soils, through plants, into the atmosphere can be effectively traced by stable isotopologues of water. However, rapid dynamic processes have only recently gained attention, such as adaptations in root water uptake depths (within hours to days) or the imprint of transpirational fluxes on atmospheric moisture, particularly promoted by the development of real-time in-situ water vapour stable isotope observation techniques. We focus on open questions and emerging insights at the soil-plant and plant-atmosphere interfaces, as we believe that these are the controlling factors for ecosystem water cycling. At both interfaces, complex pictures of interacting ecophysiological and hydrological processes emerge: root water uptake dynamics depend on both spatiotemporal variations in water availability and species-specific regulation of adaptive root conductivity within the rooting system by, for example, modulating soil-root conductivity in response to water and nutrient demands. Similarly, plant water transport and losses are a fine-tuned interplay between species-specific structural and functional strategies of water use and atmospheric processes. We propose that only by explicitly merging insights from distinct disciplines - for example, hydrology, plant physiology and atmospheric sciences - will we gain a holistic picture of the impact of vegetation on processes governing the soil-plant-atmosphere continuum.

A comparison of extraction systems for plant water stable isotope analysis

RATIONALE

The stable isotope ratios of water (δ² H and δ¹⁸ 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.

The effect of 18 O-labelled water vapour on the oxygen isotope ratio of water and assimilates in plants at high humidity

Our understanding of how temporal variations of atmospheric water vapour and its isotopic composition (δ¹⁸ O_V ) influence water and assimilates in plants remains limited, restricting our ability to use δ¹⁸ O as a tracer of ecophysiological processes. We exposed oak (Quercus robur) saplings under wet and dry soil moisture conditions to ¹⁸ O-depleted water vapour (c. - 200‰) at high relative humidity (c. 93%) for 5 h, simulating a fog event. We then traced the step change in δ¹⁸ O_V into water and assimilates (e.g. sucrose, hexoses, quercitol and starch) in the leaf lamina, main veins and twigs over 24 h. The immediate δ¹⁸ O_V effect was highest for δ¹⁸ O of leaf lamina water, but 40% lower on δ¹⁸ O of main vein water. To a smaller extent, we also observed changes in δ¹⁸ O of twig xylem water. Depending on the individual assimilation rate of each plant, the ¹⁸ O-label was partitioned among different assimilates, with highest changes in δ¹⁸ O of starch/sucrose and lowest in δ¹⁸ O of quercitol. Additionally, ¹⁸ O-label partitioning and allocation towards leaf starch and twig phloem sugars was influenced by the plant water status. Our results have important implications for water isotope heterogeneity in plants and for our understanding of how the δ¹⁸ O signal is incorporated into biomarkers.

Membrane inlet laser spectroscopy to measure H and O stable isotope compositions of soil and sediment pore water with high sample throughput

RATIONALE

The fast and accurate measurement of H and O stable isotope compositions (δ² H and δ¹⁸ O values) of soil and sediment pore water remains an impediment to scaling-up the application of these isotopes in soil and vadose hydrology. Here we describe a method and its calibration to measuring soil and sediment pore water δ² H and δ¹⁸ O values using a water vapor-permeable probe coupled to an isotope ratio infrared spectroscopy analyzer.

METHODS

We compare the water vapor probe method with a vapor direct equilibration method, and vacuum extraction with liquid water analysis. At a series of four study sites in a managed desert agroecosystem in the eastern Great Basin of North America, we use the water vapor probe to measure soil depth profiles of δ² H and δ¹⁸ O values.

RESULTS

We demonstrate the accuracy of the method to be equivalent to direct headspace equilibration and vacuum extraction techniques, with increased ease of use in its application, and with analysis throughput rates greater than 7 h^-1 . The soil depth H and O stable isotope profiles show that soil properties such as contrasting soil texture and pedogenic soil horizons control the shape of the isotope profiles, which are reflective of local evaporation conditions within the soils.

CONCLUSIONS

We conclude that this water vapor probe method has potential to yield large numbers of H and O stable isotope analyses of soil and sediment waters within shorter timeframes and with increased ease than with currently existing methods. Copyright © 2016 John Wiley & Sons, Ltd.

Correcting Laser-Based Water Stable Isotope Readings Biased by Carrier Gas Changes

Recently, laser-based water stable isotope spectrometers have become popular as they enable previously impossible approaches of environmental observations. Consequently, they have been subjected to increasingly heterogeneous atmospheric conditions. However, there is still a severe lack of data on the impact of nonstandardized gas matrices on analyzer performances. Against this background, we investigated the influence of changing proportions of N2, O2, and CO2 in the carrier gas on the isotope measurements of a typical laser-based water stable isotope analyzer (Picarro L2120-i). We combined environmentally relevant mixtures of N2, O2, and CO2 with referenced, flash-evaporated water and found that isotope readings of the same water were altered by up to +14.57‰ for δ(18)O and -35.9‰ for δ(2)H. All tested relationships between carrier gas changes and respective isotope readings were strongly linearly correlated (R(2) > 0.99). Furthermore, an analyzer-measured variable allowed for reliable postcorrection of the biased isotope readings, which we additionally tested on field data. Our findings are of importance for environmental data obtained by analyzers based on the same technology. They are relevant for assays where inconsistent gas matrices or a mismatch in this regard between unknown and reference analyses cannot be excluded, which is in particular common when investigating the soil-vegetation-atmosphere continuum.