HS10.5

Processes in the rooting zone and root water uptake decisively affect ecosystem resilience to stressors such as drought or deposition of air pollutants. However, the rooting zone is literally a black box. Water stable isotopes may shed light on some parts of this black box and tell us about water residence times, mixing of different water pools, plant water sources, preferential flow, soil evaporation, and many more. While this flexibility to use stable water isotopes is advantageous, it also entails many degrees of freedom. The additional collinearity of the hydrogen and oxygen isotopes leads to a largely underdetermined problem to solve. Bayesian mixing models help quantify the resulting uncertainty and add some constraints to the system by prior knowledge. Particularly in the case of disentangling plant water sources, additional data such as soil water content, matric potential or sap flow are needed since i) the isotope gradient smoothes with soil depth and ii) isotope-derived relative changes in root water uptake cannot be translated to absolute ones. Framing this ancillary data in a physically based water balance model may refine our predictions and help to trace water and related nutrient fluxes through the belowground. This contribution will summarize some of the experience gained during an in-situ monitoring study conducted in the drought summer of 2018 and a one-year sampling campaign at ten long-term ecosystem monitoring sites across Switzerland.

How to cite: Meusburger, K., Bernhard, F., and Gessler, A.: What stable water isotopes may tell us about belowground processes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1182, https://doi.org/10.5194/egusphere-egu22-1182, 2022.

Since the `60s, stable isotopes of hydrogen and oxygen have proven to be excellent tracers for water flows in different systems. Nowadays, such tracers still represent a formidable resource for eco-hydrologists who study water spatio-temporal dynamics and transit time distributions in the Critical Zone (CZ). Unfortunately, many issues remain unsolved, especially when looking at the regolith-trees continuum in which a complex mixture of isotopic fractionation processes may occur (evaporation, transpiration, adsorption on soil clays and oxides, microorganism's activity, mixing of water of different ages). Moreover, the lack of standard protocols for sampling and analysis represents a limitation when determining which water source(s) trees uptake. Indeed, by using sap cryogenically extracted from tree cores (recognized as the standard protocol), several studies have indicated discrepancies between the isotopic signatures of xylem sap extracted from plants and the potential water source(s).

In this context, we propose to look at the water which flowing in the xylem vessels, by directly sampling sap from tree roots and branches. It is our hypothesis that root water would represent a more reliable fraction to identify the source(s) of water that trees absorbed from the different regolith compartments. Additionally, we also aim to observe how O and H isotopic composition of sap is evolving from the roots to the leaves. Inside this pathway, the absorbed water would be impacted by various processes, such as evaporation from the bark and mixing with other water pools (e.g. storage water), which would lead to a change in the isotopic composition of the sap which will be observed in the one extracted from the branches.

To reach our objectives, we sampled water from diverse CZ compartments at three European beech (Fagus sylvatica L.) stands in the Weierbach Experimental Catchment (Hissler et al. 2021). These stands are located along a catena from the highest elevation (plateau) to the stream (riparian zone). Rainfall, throughfall, soil solutions at 20, 40 and 60 cm depth, groundwater and streamwater were collected. For beech sap sampling, we developed and applied an in-situ extraction using suction under vacuum. This method allowed us to collect sap from roots, stems and branches, separately. At the same time, tree cores from the trunks were collected and stored at -20 °C before cryogenically extracting their water content under vacuum.

When reported in a δ²H vs. δ¹⁸O diagram, our results clearly illustrate that beech sap samples extracted with this techniques plot in a different area than the water extracted using cryogenic extraction. The root sap samples fall on the Local Meteoric Water Line (LMWL) in the same field as specific water sources (soil solutions, groundwater, streamwater). Moreover, the sap samples collected from branches are also located on the LMWL but presented a significant enrichment in ¹⁸O and ²H in comparison to the root sap samples. These new results allow us to calculate more accurately the contribution of the regolith water pools to the tree uptake and to discuss the origin of the fractionation that happened for both O and H isotopes in the tree water pathway.

How to cite: Montemagno, A., Hissler, C., Bense, V., Teuling, A. J., and Pfister, L.: New approaches to sap extraction from trees and comparison with conventional methods: new avenues for H and O stable isotopes ecohydrology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4249, https://doi.org/10.5194/egusphere-egu22-4249, 2022.

Recent studies applying stable isotopes of hydrogen and oxygen showed that different methods for extracting water from plant tissues can return different isotopic composition. One of the most used methods to extract plant water is the cryogenic vacuum distillation (CVD), which tends to extract total plant water. Conversely, the Scholander pressure chamber (SPC), which is commonly used by tree physiologists to measure water potential in plant tissues, likely accesses only the mobile plant water (i.e., xylem and inter-cellular water). However, only few studies reported the application of SPC to extract plant water for isotopic analyses, and therefore, an inter-method comparison between SPC and CVD is needed.

In this study, we analyzed the variability in the isotopic signature of plant water extracted by SPC and CVD. Furthermore, we considered the potential variability in the isotopic composition of the plant water extracted from various tissues by CVD (i.e., leaves, twig without bark, twig with bark, twig close to the trunk of the tree, and wood core), and from different tree species (i.e., alder, apple, chestnut and beech) located in three different study areas in northern Italy.

Results indicate that plant waters extracted by SPC and CVD were significantly different, likely due to the extraction of different plant water domains. The difference in the isotopic composition obtained by the two extraction methods was smaller in the beech samples compared to alder, apple and chestnut samples. The signature of alder, apple and chestnut plant water extracted by SPC was more enriched in heavy isotopes than the samples obtained by CVD (except for the leaf water obtained by CVD, which also had a marked evaporative signature). We conclude that plant water extraction by SPC does not represent an alternative for CVD, as SPC likely extracts mostly the mobile plant water, whereas CVD tends to retrieve all water stored in the sampled tissues. However, studies aiming to quantify the relative contribution of the water sources to transpiration should rely more on the isotopic composition of xylem water transpiring during the sampling day (theoretically sampled by SPC), than the isotopic composition of total plant water (sampled by CVD), which also contains a fraction of water that could be stored in plant tissues for a long time.

Keywords: stable isotopes of hydrogen and oxygen; cryogenic vacuum distillation; Scholander pressure chamber; plant water; xylem water.

How to cite: Zuecco, G., Amin, A., Frentress, J., Engel, M., Marchina, C., Anfodillo, T., Borga, M., Carraro, V., Scandellari, F., Tagliavini, M., Zanotelli, D., Comiti, F., and Penna, D.: Comparing plant water extraction methods for isotopic analyses: the Scholander pressure chamber vs. the cryogenic vacuum distillation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10019, https://doi.org/10.5194/egusphere-egu22-10019, 2022.

The isotopic signature of xylem water (δ_X) is of great interest for plant source water studies. δ_X is usually derived by destructive sampling and subsequent cryogenic vacuum extraction (CVE). However, numerous studies have criticized this approach due its methodological constraints and analytical artifacts. New in situ methods to derive δ_X have been proposed in recent years. Yet, they are still in the development- and test phase, and their application highly intrusive. Gas exchange chamber techniques, on the other hand, have been well established for decades and allow the isotopic signature of transpired water (δ_T) to be monitored in high temporal resolution. As δ_T values approach δ_x values when transpiration is at isotopic steady state, measurements of δ_T may provide a relatively non-intrusive method to derive δ_X values.  

While conducting a large-scale long-term drought experiment of 90 days in a model rainforest ecosystem (Biosphere 2, WALD project), we monitored dynamics in δ_T values in two tropical plant species, one understory and one canopy species. Severe drought was ended with a deep water pulse strongly enriched in ²H. By connecting flow-through leaf chambers to a water isotope analyzer, we measured δ_T in a 2-h resolution and observed its response to increasing drought, deep labelling and subsequent recovery. Parallel to continuous measurements of δ_T, branch samples were collected at 5 time points throughout the experiment to determine δ_X values from CVE.

We found that δ_T values provide a good proxy for δ_X values when using the daily averages of δ_T values, weighted by the transpiration flux; derived δ_X values matched well with isotopic compositions of soil water. In situ-δ¹⁸O_X agreed well with values from CVE. CVE-δ²H_X values, however, were strongly enriched in ²H in comparison to in situ derived values, which is probably linked to the isotopic effects of CVE on δ²H as recently found in several studies. Moreover, by adding a ²H deep water pulse, δ_T allowed us to distinguish clearly between deep and shallow soil water use as well as show uptake velocities of newly added water. Monitoring  δ_T using gas exchange chambers provides a good proxy for δ_X values to address research questions concerning plant-available water sources and their usage, and at the same time give additional information on the plant water status. 

How to cite: Kübert, A., Dubbert, M., Kuehnhammer, K., Beyer, M., van Haren, J., Meredith, L. K., Ladd, S. N., and Werner, C.: Deriving xylem water isotopic compositions from in situ transpiration measurements: opportunity for plant source water identification?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11593, https://doi.org/10.5194/egusphere-egu22-11593, 2022.

Isotope ratios of soil water and atmospheric water vapor have been used to estimate soil evaporation fluxes and to partition evapotranspiration at local (plot, stand) scales, but the application of these methods has been limited by 1) challenges associated with data acquisition, and 2) the complexity of and lack of consensus about appropriate data interpretation methods. New initiatives that have expanded access to data, such as the U.S. National Ecological Observatory Network (NEON), are beginning to address the first of these limitations. In order to make progress toward the second, we link a model of soil water and water isotope balance, based on the widely used Noah land surface model, to a range of core NEON measurements and ancillary field-collected data using a Bayesian hierarchical framework. This model framework allows self-consistent treatment of the water and isotope cycles, including representation of uncertainties and differing assumptions, and simultaneous optimization of all model parameters conditioned on all data using Markov-Chain Monte Carlo sampling. We test the framework by applying it to estimate evapotranspiration partitioning at a dryland NEON site in central Utah and show that the posterior estimates give reasonable and useful constraints on flux rates and provide constraints on model parameters that could inform our understanding of soil properties and isotopic systematics in the system. This flexible framework for interpretation of water isotope data in evapotranspiration studies is amenable to application across ecosystems and at sites with different levels of data availability in support of cross-site syntheses and validation/testing of earth system models.

How to cite: Bowen, G., Austin, P., Allen, S., Anderegg, W., Good, S., Noone, D., and Still, C.: Quantifying and partitioning evapotranspiration using Bayesian inversion of an isotope-enabled soil water balance model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10699, https://doi.org/10.5194/egusphere-egu22-10699, 2022.

The WATSON COST Action (CA19120; https://watson-cost.eu/) started in September 2020. It aims to integrate and synthesize current interdisciplinary scientific knowledge on the use of the stable isotopes of water to understand the mixing and partitioning of water in the Earth’s Critical Zone. The network is organized into working groups that focus on a major scientific challenge: 1) groundwater recharge and soil water mixing processes; 2) vegetation water uptake and transpiration; and 3) catchment-scale residence time and travel times. A fourth working group organizes the network and dissemination activities.

WATSON aims at better connecting academia and stakeholders from industry, non-profit organizations, and government agencies. WATSON fosters the exchange of information and expertise among scientists and stakeholders, builds capacity in the use of the latest isotope approaches and translates scientific cutting-edge knowledge into tangible outputs and recommendations on how to use stable water isotopes to effectively address water management needs. 

Our poster describes the WATSON network, as well as its activities. These include the preparation of an open-access database of water isotope-based studies in the Critical Zone, the development of protocols for water sampling and stable isotope analysis, the organization of virtual and in-person meetings, seminars, training schools, and the exchange of students, researchers, and technicians via short term scientific missions.

How to cite: Barbeta, A., Coenders-Gerrits, M., Geris, J., Jakovljević, T., Llorens, P., Marttila, H., Nerantzis, K., Orlowski, N., Öztürk, E. Z., Penna, D., Popp, A. L., Rothfuss, Y., Scandellari, F., Stockinger, M., Stumpp, C., van Meerveld, I., von Freyberg, J., Vreča, P., and Žvab Rožič, P.: The WATSON COST Action: Water isotopes in the critical zone from groundwater recharge to plant transpiration, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6602, https://doi.org/10.5194/egusphere-egu22-6602, 2022.

Urban green spaces are valuable infrastructure in the urban environment because they facilitate natural stormwater management through rainwater retention, decelerated runoff and enhanced evapotranspiration which can also mitigate heat stress. Investigating the complex interactions of water flux partitioning of incoming precipitation into “green” (i.e. evaporation and transpiration) and “blue” (surface runoff and groundwater recharge) water fluxes through urban vegetation is crucial to understand what types of landcover might best balance water re-distribution for a particular geographical setting and provide a cooling effect whilst not compromising groundwater recharge.

Stable water isotopes are very useful tools to investigate these complex processes. So far, studies investigating high-resolution ecohydrological process dynamics at the urban soil-plant-atmosphere interface, e.g. canopy evaporation, with stable isotopes are rare. Here, we conducted novel field experiments using direct in-situ monitoring of the isotope composition of evaporated atmospheric moisture at different heights above the soil surface, plant xylem and soil water in different types of urban greenspaces in Berlin, Germany. Results show a more homogenous spatio-temporal distribution of water vapour signals within the elevation profile of urban trees compared to grasslands, reflecting continuous interplay of interception evaporation, transpiration and soil evaporation. Additionally, grasslands showed a lower impact on the isotopic composition of atmospheric water vapor, mainly reflecting higher evaporative losses close to the ground surface. Complex patterns of precipitation fractionation under contrasting urban vegetation canopies were also revealed. Topsoil moisture rates strongly depended on the soil type and less on the above vegetation type.

The collected data on the redistribution of urban water in different types of green spaces is very helpful for the development of isotope aided ecohydrological models. This knowledge can further support valuable decision-making for sustainable urban development across scales.

How to cite: Ring, A.-M., Tetzlaff, D., Kleinschmit, B., and Soulsby, C.: Urban Moisture (Re)cycling: quantifying canopy effects using in-situ water stable isotope monitoring, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-339, https://doi.org/10.5194/egusphere-egu22-339, 2022.

Understanding water availability and sources for crop transpiration is essential for sustainable management of water resources in agriculture, especially under changing climatic conditions. Identifying the origin of water accessed by crops is particularly critical in typical rain-fed agroecosystems, such as vineyards in Mediterranean regions where viticulture is one of the mainstays of the local economy.

As far as we know, no study so far has attempted to analyse root water uptake dynamics on hilly vineyards where the hillslope topography plays a role on water redistribution and availability for grapevine uptake. For this reason, we instrumented two plots within a vineyard in the famous Chianti wine region in Tuscany, Italy. The vineyard is cultivated with 11-year-old grapevines either on 1103 Paulsen or 420A rootstocks, which are typically characterized by a deeper and shallower rooting system, respectively. We aimed to test the hypothesis that i) grapevines located at the hillslope bottom took up water from shallower soil layers compared to grapevines located at the hillslope top due to lateral downslope redistribution of infiltrated rainwater; and that ii) grapevines with rootstock 1103 Paulsen took up water from deeper soil layers compared to the grapevines with rootstock 420A.

We monitored precipitation and temperature as well as soil moisture at 30 and 60 cm depth at two hillslope locations (top and bottom, 140 and 115 m asl, respectively) from April to October 2021. We collected samples for isotopic analysis from rainfall, soil at 30 and 60 cm, shoots and leaves from two adjacent grapevines for each rootstock at the two hillslope locations. Additional water samples were taken through the application of sealed plastic bags around some top branches and collection of water that had transpired and condensed on the bag walls. Water from soil samples, leaves and shoots was extracted though cryogenic vacuum distillation. The isotopic composition was determined through laser spectroscopy or, for organically-contaminated samples, mass spectrometry.

Preliminary results show that soil moisture was higher at the hillslope bottom than at the top, and higher at 60 than 30 cm depth. The isotopic composition of soil water was statistically different at the two depths with more enriched values in the shallower layer, as expected in Mediterranean climates; however, no statistical difference was observed between soil water at the two hillslope locations. The isotopic composition of plant water between the two locations was statistically different for bag (transpired) water and leaf water (although the latter was highly fractionated), but not for shoot water, allowing us only to partially accept hypothesis i). However, the isotopic composition of bag, leaf, and shoot water was always similar for 1103 Paulsen and 420A grafted plants, suggesting that grapevines with different rootstock took up soil water from the same depth in the study vineyard, and leading us to reject hypothesis ii).

These results contribute to better understand water uptake sources in economically-valuable agroecosystems such as vineyards.

How to cite: Penna, D., Benettin, P., Dani, A., Manca di Villahermosa, F. S., Verdone, M., Pastacaldi, G., Andreotti, C., and Tagliavini, M.: Do hillslope position and rootstock matter in root water uptake by grapevines? A case study in a Tuscany vineyard, Italy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3448, https://doi.org/10.5194/egusphere-egu22-3448, 2022.

Rapid alteration of Earth’s climate amplifies concerns over the future quantity and quality of freshwater resources. Earth’s warming atmosphere is known to store and transport more water vapor at higher velocities than historic climatic conditions, augmenting the magnitude and frequency of extreme weather events like intense rainfall and droughts. Warming can accelerate evapotranspiration by elevating vapor pressure gradients, whereas atmospheric CO₂ enrichmentcan suppress transpiration as plants preferentially close their stomata. Despite the potential hydrological implications, no study to date has comprehensively explored how these global change factors, when combined, impact the transit of moisture through the soil-plant-atmosphere continuum. In a montane grassland we tracked an extensive deuterium-labelled rainfall event following a severe experimental drought period under current versus simulated future (+300 ppm CO₂ and +3°C warming) conditions. We monitored stable isotopes of water in soil and evapotranspiration vapor, to partition signatures of evaporation, transpiration, drainage and soil pore water and quantify transit times through each ecohydrological compartment. Preliminary results suggest that under future conditions (+300 ppm CO₂ and 3°C) drought can drastically increase the retention time of water in the rootzone, which intermittently forces plants to return older water to the atmosphere. We intend to use these findings to directly quantify the water age preference of evaporative and drainage fluxes under a range of climate scenarios.

How to cite: Radolinski, J., Wachter, H., Birk, S., Brüggemann, N., Herndl, M., Kahmen, A., Kübert, A., Schaumberger, A., Stumpp, C., Vremec, M., Werner, C., and Bahn, M.: Impact of elevated CO2, temperature, and drought on summer ecohydrological moisture cycling and water transit times in montane grassland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9888, https://doi.org/10.5194/egusphere-egu22-9888, 2022.

Topography plays a major role in mediating subsurface water redistribution and ultimately water availability for tree transpiration. Trees located in valley bottoms commonly benefit from greater accessibility to groundwater and wetter soil from lateral redistribution of water compared to trees growing upslopes. However, water availability and movement in the subsurface may differ according to subsurface properties (permeability, soil texture, geology) and climatic regimes. The spatially and seasonally variable water accessibility along a hillslope affects species composition, stand structure, and biomass productivity resulting in areas which will be more likely impacted by potential water shortages. So far, the understanding of how hydrological processes occurring at the hillslope scale affect tree water use is still limited, rising the need of measurements at hillslope-level to allow deeper comprehension of forest dynamic and survival.

Here, we set up a comparative study on a gentle and very steep forested hillslope located in the Weierbach catchment (Luxembourg) and the Re della Pietra catchment (Italy), respectively. We aimed at testing if different climatic and hydrological conditions, i.e., meteorological forcing, groundwater depth, soil moisture, and water redistribution affect water use patterns of beech trees (Fagus sylvatica L.) along hillslopes.

We monitored soil moisture, groundwater level, sap velocity, and hydro-meteorological variables and determined the isotopic composition of precipitation, soil water, groundwater, and xylem water to estimate tree water sources. The combination of these measurements allows us to link the transpiration response of trees to water availability along the two different hillslopes.We carried out biweekly field campaigns during the growing season 2019 and 2020 in Weierbach catchment and throughout 2021 in Re della Pietra catchment to sample xylem water, soil water at different depths, groundwater, stream water, and precipitation.

Trees in the Weierbach catchment rely on water stored in the unsaturated zone regardless of the hillslope position and the hydrologic conditions of the season. On the contrary, preliminary results from Re della Pietra suggest position-specific water use strategy. Trees located at the footslope experienced longer vegetative period compared to plants located at the midslope and hilltop locations due do larger soil moisture content recorded at the footslope. Additionally, xylem water of footslope trees displayed lighter isotopic composition compared to other trees, suggesting the use of a less fractionated water sources.

We argue that contrary to the Weierbach catchment where subsurface hillslope structure promotes vertical water flux over lateral redistribution in the vadose zone, the steep hillslope on the Re della Pietra catchment experiences shallow lateral downslope water redistribution which results in substantial differences in vadose zone water supply between hillslope positions.

How to cite: Fabiani, G., Klaus, J., Schoppach, R., and Penna, D.: Contrasting tree water use strategies along hillslopes in forested catchments in Luxembourg and Italy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5313, https://doi.org/10.5194/egusphere-egu22-5313, 2022.

Stable isotopes (²H and ¹⁸O) are common natural tracers for the investigation of water transport in the soil-plant-atmosphere continuum. Isotopic data can be coupled with soil water content data to inversely estimate soil hydraulic and transport parameters. The calibration of a hydrological model by inverse modelling is a prerequisite to determine the temporal origin of xylem water taken by roots.

In this study, we used isotopic data to calibrate Hydrus-1D via inverse modelling to simulate one-dimensional water flow and isotope transport in a controlled soil-plant-atmosphere system. We propose the following protocol i) to estimate root water uptake transit time of irrigation water, and ii) to assess the sensitivity of the transit time distribution to the variation in the water available for root uptake.

The dataset was obtained from an isotope-tracing experiment carried out between May and July 2018 on an olive tree placed in a pot inside a glasshouse. Meteorological variables and sap flow were monitored at 5-minute intervals, whereas shallow soil moisture (0-6 cm depth) was measured manually with an impedance probe at the daily timescale. The olive tree was irrigated with water of known isotopic composition. The pot surface was covered by a plastic sheet to avoid evaporation. Soil at different depths, twigs, wood cores and root samples were collected weekly for isotopic analyses. Water from soil and the xylem tissues was extracted by cryogenic vacuum distillation. Based on the results of a previous study carried out on the same dataset, we considered that no isotopic fractionation occurred during the water uptake and the transport within the olive tree.

We used soil water content and δ¹⁸O data at different soil depths to optimize flow (soil hydraulic and root water uptake parameters) and transport (longitudinal dispersivity) parameters. Numerical simulations of isotope transport were validated with sap flow data (compared to actual transpiration) and δ¹⁸O in xylem water. Given that the timing of irrigation water for plant transpiration is fundamental for assessing the vulnerability of olive trees to drought, we will be proposing various scenarios based on different irrigation events to mimic drought periods. Based on these scenarios, we will be evaluating the sensitivity of the root water uptake transit time to the different water availability in the soil profile. Afterwards, the same protocol will be exploited to determine the root water uptake transit time for different tree species under various environmental conditions.

Keywords: stable isotopes, HYDRUS-1D, root water uptake, transit time, soil water.

How to cite: Todini, D. and the PRIN WATZON: Assessing root water uptake transit time by simulating isotope transport in Hydrus-1D, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9870, https://doi.org/10.5194/egusphere-egu22-9870, 2022.