Stable isotopes to study water and nutrient dynamics in the soil-plant-atmosphere continuum

Stable isotopes are powerful tools for tracing fluxes of water and associated nutrients in the soil-plant-atmosphere continuum. They are increasingly used by various disciplines to better understand the functioning of the soil-plant-atmosphere system. While new methods allow measurements at high spatial and temporal resolution, studies applying tracer methods are now tackling complex interactions between soil processes, plant physiology and ecology, and variable atmospheric drivers. As such, methodological developments and changes are happening quickly and have a strong bearing on process understanding and interpretation of findings. This session aims to address the current state of the art for methods, applications, and process interpretations using stable isotopes in the critical zone and to foster interdisciplinary exchange. We welcome experimental and modeling studies that present methodological developments and applications of isotope tracers to improve the actual knowledge of the water and nutrient exchanges at the soil-plant-atmosphere interfaces. Studies that seek to cross disciplinary boundaries and reveal new eco-hydrological process understanding are especially welcome.

Co-organized by BG2/SSS11
Convener: Natalie Orlowski | Co-conveners: Adrià BarbetaECSECS, Josie GerisECSECS, Jana von FreybergECSECS
vPICO presentations
| Mon, 26 Apr, 09:00–10:30 (CEST)

Session assets

Session materials

vPICO presentations: Mon, 26 Apr

Chairpersons: Adrià Barbeta, Josie Geris, Natalie Orlowski
Plant water isotopes, water uptake and/or evapotranspiration studies
Michael Singer, Christopher Sargeant, John Stella, Kelly Caylor, Dar Roberts, Li Kui, Kudzai Kaseke, Marc Mayes, Lissa Pelletier, Jared Williams, Maria Warter, Romy Sabathier, Conor McMahon, Christopher Kibler, Bryn Morgan, and Melissa Rohde

Isotopic tracing of water sources for plants is an increasingly common method that supports insight into climatic controls on water availability to plants and their use of this available water, especially in water-limited environments where isotopic endmembers are distinct. Recent advances in this field of research have enabled characterization of annual and seasonal water use by plants, whose water sources vary in contribution along a continuum between groundwater (isotopically light) to infiltrated precipitation (isotopically heavy). Xylem samples are commonly used to characterize real-time uptake of water from roots, and they can be contextualized with respect to endmember water sources via sampling of root zone water, providing these endmembers are isotopically distinct. The time integration of seasonally varying water source usage results in the annually recorded isotopic signal recorded in tree ring cellulose for temperate trees and shrubs, which reflects the dominant water source used in the season of growth. This has enabled dendro-isotopic methods that are commonly used to reconstruct past climates (isotopically light = colder/wetter; isotopically heavy = warmer/drier). However, questions have arisen about the utility of these annually integrated dendro-isotopic signatures, given the strong seasonal variations of water use that are particularly pronounced in dryland ecosystems, including notable water source switching by plants.      

In our recent work, we have been pushing isotopic methods in new directions to better understand what plants can tell us about how climate affected hydrology across dryland regions, and about the associated plant responses. Drylands pose interesting research challenges, since water is typically the key limiting factor on dryland plant growth, and it is fundamental to the health, functioning, composition, distribution, and evolution of vegetation communities. In drylands, water availability to plants may vary dramatically across space and time, creating challenges for simple analyses of annual water use signatures. To aid the understanding of climatically-controlled ecohydrology in drylands, we have developed a new tool (ISO-Tool) based on established biochemical fractionation theory, which allows for back-calculation of water sources used for growth from tree-ring isotopes. This tool generates critical knowledge for evaluating dendro-isotopic signatures within the same reference frame as sampled endmember water sources, and it can be used for both annual and seasonal analyses of plant water use. We have also been working on a set of interdisciplinary metrics we call water stress indicators (WSIs), which support corroboration of information on climatic forcing, water availability, plant water uptake, and ecological health of terrestrial vegetation.   

Using these new methods, we have been able to identify important hydroclimatic gradients in water usage for the same species that reflect the local expression of climate into plant-available water. We have also begun to understand the whole continuum from climate forcing to root-zone water availability to tree growth to canopy health. We believe this broader continuum perspective is critical for tackling key ecohydrological questions especially in drylands, where we expect large variability in water availability across space and time.         

How to cite: Singer, M., Sargeant, C., Stella, J., Caylor, K., Roberts, D., Kui, L., Kaseke, K., Mayes, M., Pelletier, L., Williams, J., Warter, M., Sabathier, R., McMahon, C., Kibler, C., Morgan, B., and Rohde, M.: Stable isotope insights into dryland ecohydrology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8405, https://doi.org/10.5194/egusphere-egu21-8405, 2021.

Jesse Radolinski, Maud Tissink, and Michael Bahn

Global change in the Anthropocene will impose various combinations of warming, atmospheric CO2 levels , and moisture availability on terrestrial ecosystems. A warming climate may increase vapor pressure gradients near plant and soil evaporation fronts driving higher rates of evapotranspiration (ET), whereas atmospheric CO2 enrichment (eCO2) can trigger stomatal closure, suppressing transpiration. Our best depiction of future water resources comes from controlled climate-manipulation experiments; however, climate change factors (e.g., eCO2, warming, and drought) are primarily studied in isolation, limiting the scope of inference. Here we use a series of chamber measurements taken throughout the 2020 growing season to quantify the individual and combined effects of elevated atmospheric CO2 (+300 ppm), warming (+ 3°C) and recurrent drought on evapotranspiration and plant water use in a mountain grassland. Though water use efficiency (WUE) was nearly identical between “future” (+300 ppm CO2 and 3°C) and “current” (ambient conditions) systems during drought simulations, the future plots maintained a 2-3 fold higher WUE with the twice the inter-measurement variance during the post-drought recovery period. The isotopic signatures of droughted plots were generally isotopically depleted compared to their non-drought counterparts at peak drought, and the future drought systems had 20 ‰ lighter bulk ET δ2H compared to plots receiving warming alone. Altogether these preliminary results suggest that 1) drought under a future warmer climate and eCO2 may drive grassland ecosystems to conserve water; 2) when warmed, mountain grasslands may preferentially return recently fallen precipitation to the atmosphere, whereas 3) drought can induce preferential withdrawal of older water storage. Future work will include the use of StorAge Selection (SAS) modeling to characterize the preference of water residence time to atmospheric fluxes under a changing climate.

How to cite: Radolinski, J., Tissink, M., and Bahn, M.: Evapotranspiration flux dynamics in a changing climate, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14393, https://doi.org/10.5194/egusphere-egu21-14393, 2021.

Javier de la Casa, Adrià Barbeta, Asun Rodriguez-Uña, Lisa Wingate, Jérôme Ogeé, and Teresa E. Gimeno


Long-standing ecological theory establishes that the isotopic composition of the plant water reflects that of the root-accessed sources, at least in non-saline or non-xeric environments. However, a growing number of studies challenge this assumption by reporting plant-source offsets in water isotopic composition, for a wide range of ecosystems. We conducted a global meta-analysis to systematically quantify the magnitude of this plant-source offset in water isotopic composition and its potential explanatory factors. We compiled 108 studies reporting dual water isotopic composition (δ2H and δ18O) of plant and source water. From these studies, we extracted the δ2H and δ18O of both plant and source waters for 223 plant species from artic to tropical biomes. For each species and sampling campaign, within each study, we calculated the mean line conditioned excess (LC-excess), with the slope and intercept of the local meteoric water line, and the mean soil water line conditioned excess (SWL-excess), from the slope and intercept of the soil water evaporation line. For each study site and sampling campaign, we obtained land surface temperature and volumetric soil water from the ERA5 database. For each study species, we recorded the functional type, leaf habit and for those available wood density. We found, on average, a significantly negative SWL-excess: plant water was systematically more depleted in δ2H than soil water. In > 90% of the cases with significantly negative SWL-excess, we also found negative LC-excess values, meaning that access to sources alternative to soil water was unlikely to explain negative SWL-excess values. 

Calculated SWL-excess was affected by temperature and humidity: there were larger mismatches between plant and source water in isotopic composition in colder and wetter sites. Angiosperms, broadleaved and deciduous species exhibited more negative SWL-excess values than gymnosperms, narrow-leaved and evergreen species. Our results suggest that when using the dual isotopic approach, potential biases in the adscription of plant water sources are more likely in broadleaved forests in humid, and cold regions. Potential underlying mechanism for these isotopic mismatches will be discussed.


How to cite: de la Casa, J., Barbeta, A., Rodriguez-Uña, A., Wingate, L., Ogeé, J., and Gimeno, T. E.: A global meta-analysis reveals a significant offset in δ2H between plant water and its sources, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12207, https://doi.org/10.5194/egusphere-egu21-12207, 2021.

Magali F. Nehemy, Benettin Paolo, Andrea Rinaldo, and Jeffrey J. McDonnell

Isotopic tracing is de rigueur in ecohydrology and for quantifying tracing water sources that contribute to xylem water. But, tree transpiration is not a one dimensional process from roots to leaves. Three dimensional storages actively participate in water transport within the stem complicating in unknown ways, the otherwise straightforward tracing from source to xylem. Phloem is the largest elastic storage and works as a hydraulic capacitor, and as such is of great importance to tree water transport and functioning. Water stored in phloem moves into xylem vessels buffering changes in xylem water potential and sustaining tree hydraulic integrity. Although phloem water is of great importance to transpiration, we lack understanding about the relationship between xylem and phloem water isotopic composition. Assessing the isotopic composition of phloem is a needed next step to fully comprehend patterns of tree water use and improve understanding about isotopic offset between xylem and source water. Here we show daily and sub-daily dual-isotope measurements of phloem water in relation to xylem and leaf water in Salix viminalis along with high-resolution measurements of plant water status and transpiration rates in a large lysimeter. We found that phloem was more depleted in heavier isotopes than xylem and leaves. On average δ2H phloem water was 2.05 ‰ and δ18O phloem water was 0.66 ‰ more negative than xylem water. The largest difference observed between phloem and xylem isotopic composition occurred at night during a period of tree water deficit. Although, there was variability in the observed difference between xylem and phloem throughout the experiment, xylem and phloem isotopic composition were highly correlated (δ2H r = 0.89; δ18O r = 0.75). Our sub daily measurements showed that xylem and phloem differences decreased during predawn and morning compared to previous evening and midday measurements. We observed that the δ2H difference between phloem and xylem increased with the increase in daily use of phloem water storages, while δ18O difference between phloem and xylem increased with transpiration rate. Our results show that xylem and phloem isotope composition are in sync and that observed differences can be related to changes in plant water status and possible fractionation associated with transport within phloem-xylem. Further studies are necessary to understand how phloem affects source water interpretations across different tree species and larger trees, where phloem contribution to daily transpiration may be larger.

How to cite: Nehemy, M. F., Paolo, B., Rinaldo, A., and McDonnell, J. J.: The isotopic composition of phloem water and its relations to xylem water at daily and sub-daily resolutions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11487, https://doi.org/10.5194/egusphere-egu21-11487, 2021.

Ginevra Fabiani, Daniele Penna, and Julian Klaus

Obtaining reliable estimates of isotopic composition of xylem water transported in tree stems is crucial for ecohydrological studies. In most tree species xylem consists of two physiologically different parts, sapwood and heartwood. The former functions as the flowpath for sap flow, whereas the latter does not conduct water and provides mechanical support to the stem. However, some studies highlighted that water stored in heartwood might sustain transpiration by providing water during dry periods. Therefore, assessing how the isotopic composition in sapwood and heartwood compartments changes over time is critical to explain tree hydraulic.

Typically, studies rely on wood cores from the tree trunk to isotopically characterize xylem water in order to assess water sources for tree use (e.g., soil water from different depths, groundwater, stream water or a mixture of those), and only few studies specified which functional portion of the wood was sampled. There is currently a lack of knowledge on the possible isotopic difference between heartwood and sapwood potentially leading to uncertainties on the origin of the extracted water.

In the present study, we investigate four forest species characterised by different xylem anatomy, wood density, and timing of physiological activity to evaluate the degree of differentiation in isotopic composition between sapwood and heartwood.

We carried out biweekly sampling campaigns over one growing season (March-October 2020) in a central European forest (Luxembourg) to assess sapwood and heartwood isotopic composition and water content of European beech (Fagus sylvatica), sessile oak (Quercus petraea), douglas fir (Pseudotsuga menziesii), and spruce (Picea abies).

Preliminary results showed a temporal variation in isotopic composition both in sapwood and heartwood for all investigated species. In conifers, we found a stronger difference in isotopic composition and water content between sapwood and heartwood compared to broadleaved species suggesting a larger degree of compartmentalization. Heartwood displays consistently heavier oxygen delta values compared to sapwood in all species whereas sapwood shows heavier hydrogen delta values compared to heartwood only in conifers. These preliminary results suggest that the occurrence of potential mixing between older water stored in the heartwood and newly uptake water flowing in the sapwood should be taken into account in tree water uptake studies.

How to cite: Fabiani, G., Penna, D., and Klaus, J.: Sapwood-Heartwood isotopic composition in four forest species: implications for isotopes studies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2945, https://doi.org/10.5194/egusphere-egu21-2945, 2021.

Stefano Brighenti, Giacomo Bertoldi, Agnese Aguzzoni, Damiano Zanotellii, Nikolaus Obojes, Massimo Tagliavini, Giulia Zuecco, Marco Borga, Daniele Penna, Ginevra Fabiani, and Francesco Comiti

Different water sources can contribute to plant transpiration in Alpine environments, such as rainfall, snowmelt, irrigation and/or stream waters that are temporarily stored in the vadose and saturated zones. Particularly, the proportion of water uptake from different soil depths can strikingly differ depending on the species and the local environmental conditions such as the availability of freshwater resources, and local climatic and pedological settings.

We aim at estimating the relative contributions of different water sources (i.e., soil water at various depths and groundwater) to tree transpiration with the use of stable water isotopes. Our work is part of a wider national project (WATZON: WATer mixing in the critical ZONe) studying the relationship between plants, soil and water in contrasting natural and semi-natural environments of Italy. Here we report the results of monitoring activities in two different ecosystems in South-Tyrol (Eastern Italian Alps): an apple orchard growing on a deep (>2.5 m) sandy soil of the Adige floodplain (Binnenland), and a sub-alpine conifer forest located on steep slopes with a shallow (10-60 cm) skeletal soil (Mazia, 2000 mt a.s.l.), where we selected European larch (Larix decidua) as a model-species. Water (precipitation, stream water, groundwater), soil at different depths and twigs samples were collected fortnightly from May to November 2020, and weather conditions (automatic stations), soil parameters (moisture and temperature) at different depths and sapflow were continuously recorded over the entire period.

At both locations, precipitation waters had a heavier isotopic composition than stream water and groundwater, that did not show any significant difference between each other in terms of isotopic signature. While all these potential water sources plotted on the local meteoric water line, shallow soil water samples (5-15 cm) deviated from it revealing a stronger and more variable evaporative fractionation when compared with those of deeper soil (25-65 cm). Xylem water samples from apple trees at Binnenland overlapped with soil water samples, more consistently at 10-30 cm depths. This water mostly derived from infiltrated rainwater but with a non-negligible contribution from groundwater during July and August. In contrast, xylem water from larch trees at Mazia plotted on the local meteoric water line, and had an isotopic composition more similar to that of precipitation than soil water even for samples collected after several days of drying out. As sapflow measurements of larches revealed a continuous transpiration, it is unlikely that trees took up water only soon after precipitation events. Instead, we hypothesize that larches at Mazia likely rely on a water pool which is different from the soil (e.g., rock moisture).

These contrasting ecohydrological systems reveal different strategies of water use by dwelling plants in natural and anthropic systems, showing a distinct sensitivity and resilience to changing climate.

How to cite: Brighenti, S., Bertoldi, G., Aguzzoni, A., Zanotellii, D., Obojes, N., Tagliavini, M., Zuecco, G., Borga, M., Penna, D., Fabiani, G., and Comiti, F.: Apple trees, larches and their water uptake: distinct ecohydrological systems in contrasting environmental settings?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8211, https://doi.org/10.5194/egusphere-egu21-8211, 2021.

Daniele Penna, Jason Frentress, Damiano Zanotelli, Francesca Scandellari, Agnese Aguzzoni, Michael Engel, Massimo Tagliavini, and Francesco Comiti

Understanding the dynamics and sources of root water uptake in agricultural systems is becoming increasingly important for implementing efficient and sustainable water resources management and, at the same time, for optimizing crop yield and quality under changing climatic conditions. In this work, we adopted the stable isotope approach to investigate the water sources accessed by apple trees in two orchards growing in the upper Etsch/Adige valley (South Tyrol, Eastern Italian Alps). We tested the general hypothesis that soil water, composed of a mixture of rain and irrigation water, was the main source for tree transpiration but that river water and groundwater mixed with soil water and contributed to root uptake for trees growing close to the river and with higher water table. Our results revealed that apple trees during the 2015 and 2016 growing seasons relied mostly on soil water present in the upper 20-40 cm of soils, with an apparently negligible contribution of groundwater and river water, irrespective of the field position across the valley bottom. The isotopic composition of xylem water did not reflect the one of irrigation water (and neither that of groundwater) but rather of rainfall and throughfall, as well as that of soil water. We related this “hidden” tracer signature of irrigation water to the effect of soil evaporation that strongly modified its original isotopic composition: irrigation and rain water infiltrated into the soil and mixed with isotopically fractionated soil water, and trees took up a mixture of water with different isotopic composition compared to the one of the original irrigation source. This work contributes to improve the understanding of water uptake strategies in Alpine apple orchards and paves the way for further analysis on the proportion of irrigation and rain water used by apple trees in mountain agroecosystems.

How to cite: Penna, D., Frentress, J., Zanotelli, D., Scandellari, F., Aguzzoni, A., Engel, M., Tagliavini, M., and Comiti, F.: Water sources for apple trees in Alpine orchards: where does irrigation water go?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9289, https://doi.org/10.5194/egusphere-egu21-9289, 2021.

Zhiqing Lan, Han Chen, Han Li, Jinhui Jeanne Huang, Edward McBean, Jiawei Zhang, and Junjie Gao

The scarcity of water resources is an important issue in urbanization. Urban forest land water consumption accounts for a large part of urban water resources, the study of water uptake patterns in urban forest area is crucial for urban water saving and precision irrigation, but no identified research have investigated water uptake patterns in urban forest area until now. In this study, we measured the deuterium isotope ratio (δD) and the oxygen isotope ratio (δ18O) of precipitation, irrigation water, xylem water and soil water sources in a locust tree forest in Jinnan District of Tianjin City, China across 2019-2020. Water sources proportion in the root zone area of different growing seasons were obtained by IsoSource model, MixSIR model and SIAR model. Results show that there is a significant difference in soil moisture content between different stand age locust trees in time and depth variation. The trend of soil moisture of different stand ages of locust in time sequence intend to increase first and then decrease, the most significantly change of soil water content happened in shallow layer (0~40 cm). The change in vertical depth is about the same. The soil profile of 0-200 cm was discretized into three layers. The shallow layer (0~40 cm) soil water δD and δ18O fluctuated widely and decreased with the depth increased. This study revealed the dynamic replenishment of the root zone water in urban forest land, and provides insights into reforestation and water management in urban area.

How to cite: Lan, Z., Chen, H., Li, H., Huang, J. J., McBean, E., Zhang, J., and Gao, J.: Applying stable water isotopes to determine root water uptake patterns in an urban forest land, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8136, https://doi.org/10.5194/egusphere-egu21-8136, 2021.

Gunther Liebhard, Andreas Klik, Christine Stumpp, Peter Strauß, and Reinhard Nolz

Freshwater is a scarce resource facing a growing demand. One aspect of this growing demand arises from the expansion and intensification of crop production on irrigated land. To preserve valuable water resources, agricultural water management must aim at an efficient use of water. This can be approached by facilitating sufficient water supply for optimal crop transpiration (T) and thus crop production, and at the same time reduce unproductive water losses due to soil evaporation (E). In this regard, knowing the ratios of E and T and how they are affected by environmental and management conditions is required to develop, adapt, and evaluate agricultural practices with respect to efficient water use.

The study aimed at applying a modified E-T-partitioning method to evaluate irrigation and how varying water availability affected E and T ratios. Field experiments were conducted 2019 in Groß-Enzersdorf in the agricultural region Marchfeld east of Vienna, Austria (48°12’ N, 16°34’ E; 157 m elevation a.s.l., average annual precipitation of approx. 540 mm). A conventionally managed field was planted with soybean (glycine max l. merr) and irrigated twice with a hose reel irrigation machine. Partitioning of evapotranspiration (ET) was analyzed using an adapted water balance and stable isotope mass balance method. Monitoring throughout the soybean vegetation period comprised weekly analyses of the isotopic composition of soil samples, the profile water content in 10 cm steps down to 80 cm, weather data, the isotopic signatures of precipitation and irrigation water, ET, and crop growing stages. ET was measured with eddy covariance technique, and isotopic fractionation for determining E and T ratios was calculated from measured boundary conditions.

Weekly T ratios from blossom to beginning of maturation of soybean ranged from 56 to 84 %, which is in agreement with studies based on comparable partitioning methods. The relation between E and T did not only progress according to the canopy development but also responded to water availability in the rooting zone. During the vegetative growth stage, for example, the proportion of T was larger at partial canopy cover and sufficient water availability (from spring precipitation) compared to full canopy cover and when facing soil water stress. When soil surface was dry, E dropped to almost zero. On the other hand, a wetted surface substantially raised the E, even under closed canopy. Multiple small rain events during full canopy cover mainly contributed to E. As the analyses sufficiently revealed the relations of E and T ratios to changing boundary conditions, the method proved useful to evaluate irrigation events and strategies and deduce further improvements. In case of using a hose reel irrigation machine, the results suggest an intensification of the individual watering. However, a quantitative relationship between irrigation and the amount of water used for T would require shorter evaluation intervals of 2-3 days.

How to cite: Liebhard, G., Klik, A., Stumpp, C., Strauß, P., and Nolz, R.: Assessment of evaporation and transpiration ratios under varying moisture conditions in a soybean field., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8855, https://doi.org/10.5194/egusphere-egu21-8855, 2021.

Rossella Guerrieri, Anna Barceló, Stefania Mattana, Joan Calíz, Emilio Casamayor, David Elustondo, Sofie Hellsten, Giorgio Matteucci, Päivi Merilä, Greg Michalski, Manuel Nicolas, Anne Thimonier, Elena Vanguelova, Arne Verstraeten, Peter Waldner, Mirai Watanabe, Josep Peñuelas, and Maurizio Mencuccini

There is mounting evidence demonstrating that fluxes and chemical composition of precipitation is substantially changed after passing through tree canopies, particularly in the case of atmospheric nitrogen (N) compounds, with important implications on forest N cycling. However, the processes underpinning those changes – beyond the leaf retention and/or leaching of N compounds - have been less investigated. In a previous study we provided isotopic evidence that biological nitrification in tree canopies was responsible for significant changes in the amount of NO3- from rainfall to throughfall across two UK forests at high nitrogen (N) deposition. This finding strongly suggested that forest canopies are not just passive filters for precipitation water and dissolved nutrients, and that the microbial life hidden within them can be responsible for transforming atmospheric N before it reaches the soil. We extended the isotopic approach at the European scale, and combined it to next-generation sequence analyses with the aim of elucidating canopy nitrification and identify phyllosphere microbes responsible for it. Specifically, in this study we: 1) estimated the relative contribution of NO3- derived from biological canopy nitrification vs. atmospheric deposition by using δ18O and δ17O of NO3- in rainfall and throughfall water; 2) quantified the functional genes related to nitrification, and finally 3) characterized the microbial communities harboured in tree canopies (i.e., phyllosphere) and in the underlying soils for two dominant tree species in Europe (Fagus sylvatica L. and Pinus sylvestris L.) using metabarcoding techniques. We considered twelve sites included in the European ICP Forests monitoring network, chosen along climate and N deposition gradients, spanning from Fennoscandia to the Mediterranean. We will show that presence of nitrifying microbes (as assessed through qPCR) and their activity (as derived from δ18O and δ17O) were detected in the tree canopies across most of the sites, and that canopy nitrification was significantly correlated with atmospheric N deposition. Finally, we will discuss differences in microbial community structure and composition across phyllosphere (and between the two tree species considered), water and soil samples in the investigated forests. Our study demonstrates the potential of integrating stable isotopes with microbial analyses to advance our understanding on canopy-atmosphere interactions and their contribution to N cycling.

How to cite: Guerrieri, R., Barceló, A., Mattana, S., Calíz, J., Casamayor, E., Elustondo, D., Hellsten, S., Matteucci, G., Merilä, P., Michalski, G., Nicolas, M., Thimonier, A., Vanguelova, E., Verstraeten, A., Waldner, P., Watanabe, M., Peñuelas, J., and Mencuccini, M.: Nitrification in tree canopies of European forests: evidence from oxygen isotopes in nitrate and microbial analyses in rainfall and throughfall water. , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12351, https://doi.org/10.5194/egusphere-egu21-12351, 2021.

Isotopic spatiotemporal dynamics of subsurface water pools
Filip Muhic, Pertti Ala-Aho, Hannu Marttila, and Björn Klöve

Research on ecohydrological separation and plant water use have been increasing in the last few years, and various studies indicate that trees can use winter precipitation as a dominant water source during the growing season. Such studies are of great importance to northern regions, where soil water recharge timing is predicted to be significantly altered due to climate change. In order to assess plant water use in sub-arctic environment, it is necessary to understand how soil water pools under different land covers evolve throughout the year and how cryogenic processes alter the isotope input signal. This field study was conducted from May 2019 to June 2020 in Pallas catchment, located in sub-arctic conditions in Finnish Lapland. Soil cores up to 1 meter depth with 5 cm increments and xylem water of dominant tree species were collected in 4 locations, ranging from forest to shrubland/forest transitional area, and to forested peatland. All locations are positioned on a snow survey, in the vicinity of previously installed groundwater wells and snow lysimeters, and within 2 kilometers of rain gauge. Additional spatial samples of topsoil and xylem water were collected throughout the catchment during 2019 growing season. Relative proportions of tree source water were calculated by Bayesian mixing model MixSIAR. We produce new data set that displays plot and catchment scale soil water heterogeneities in a snow dominated environment, and examine: i) How soil properties affect isotopic composition of soil water?; ii) What is the effect of rising groundwater level on soil water isotope composition?; and iii) How snowpack thickness and melt timing modify soil water isotope patterns? We analyze if these varying pools of water are reflected in tree xylem water. Soil water isotope dynamics under deep snowpack, during and after snowmelt reveal how snow accumulation and melt timing and magnitude influence plant available water for growing season.

How to cite: Muhic, F., Ala-Aho, P., Marttila, H., and Klöve, B.: Drivers of spatial and temporal soil water isotope variability in a sub-arctic catchment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14845, https://doi.org/10.5194/egusphere-egu21-14845, 2021.

Wenyi Qin, Guo Chen, Peng Wang, Xiang Wang, and Xiaozhen Li

Hydrogen and oxygen stable isotope compositions in soil waters have been widely used to investigate hydrologic cycles, particularly for understanding plant water usage. However, most studies of soil water isotopes have traditionally ignored the importance of O-horizon that may potentially influence the accurate evaluation of hydrologic processes, especially in alpine regions where O-horizon are thick due to low temperatures. Therefore, we investigated the isotopic differences (via mean effect size, lnRR) of waters from O-horizon and 0–10 cm soil layer in grasslands and woodlands of Western Sichuan alpine regions and evaluated the influences of climatic and biotic factors on observed differences. The results indicated that the δ2H and δ18O of O-horizon water were significantly higher than those of the 0–10 cm soil layer in grasslands, but these differences were not significant in woodlands. The influence of climatic factors on lnRR was limited relative to biotic factors, and the influence of climate contrasted with expectations based on evaporation principles. Rather, above ground biomass (AGB) was the most important factor associated with lnRR and it was significantly correlated with lnRR between and within soil waters from two vegetation types. Consequently, the observed differences were mainly related to vegetation conditions that influence microclimates within canopies. Therefore, investigations of hydrological processes may inaccurately estimate their influences when not separately considering the high stable isotopes values of O-horizon in grasslands of alpine regions with thin soil layers. In particular, the influence of O-horizon should especially be considered when AGB was lower than 100 t/hm2 not only in grassland but also in other vegetation types.

How to cite: Qin, W., Chen, G., Wang, P., Wang, X., and Li, X.: The influences of climatic and biotic parameters on the isotopic offset among topsoil waters in typical vegetation types in alpine, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5182, https://doi.org/10.5194/egusphere-egu21-5182, 2021.

Qin Liu, Tiejun Wang, Cong-qiang Liu, and Xi Chen

Soil water stable isotope compositions (SWSI; i.e., δD and δ18O) and soil moisture content (SMC) are widely used to illuminate water exchange processes across the atmosphere-land interface. Thus, the knowledge of spatiotemporal dynamics of these two variables is critical to help our understanding of relevant ecohydrological processes. However, in comparison to the efforts for elucidating the spatiotemporal variability in SMC, much less attention was paid to understand the spatiotemporal variability in SWSI, which also raises the question as to whether SWSI and SMC share similar spatiotemporal features. To this end, the spatiotemporal dynamics of SWSI and SMC were jointly investigated on a karst hillslope with eight sampling campaigns among two years. The method of temporal stability analysis (TSA) was adopted to evaluate the spatiotemporal patterns of SWSI and SMC in this study. Generally, both δD and δ18O exhibited considerable temporal and spatial variations; meanwhile, the variations in δD and δ18O values were relatively smaller than that of SMC. In addition, in comparison with the spatial pattern of SMC, there were no clear relationships between the standard deviation (SD) and the spatial mean of δD or δ18O. However, the SD of line-conditioned excess (lc-excess) and its mean values displayed a strong negative correlation, indicating that the spatial variations in lc-excess increased with soil evaporation. Moreover, SWSI displayed weaker temporal stability than SMC and no clear controlling factors were identified, suggesting that the spatiotemporal dynamics of SWSI might be more complex than that of SMC. This study provided comprehensive field evidence that there existed profound spatiotemporal variability in SWSI and its spatiotemporal features were different from SMC, highlighting that the spatiotemporal variability in SWSI needs to be considered in isotope-based estimations and it should be investigated separately from the spatiotemporal characteristics of SMC in future studies.

How to cite: Liu, Q., Wang, T., Liu, C., and Chen, X.: Characterizing the spatiotemporal dynamics of soil water stable isotopes on a karst hillslope in southwestern China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-640, https://doi.org/10.5194/egusphere-egu21-640, 2021.

Vesna Zupanc, Matjaž Glavan, Miha Curk, Urša Pečan, Michael Stockinger, Gerhard Kammerer, Devakunjari Vadibeler, and Christine Stumpp

Environmental tracers, present in the environment and provided by nature, provide integrative information about both water flow and transport. For studying water flow and solute transport, the hydrogen and oxygen isotopes are of special interest, as their ratios provide a tracer signal with every precipitation event and are seasonally distributed. In order to follow the seasonal distribution of stable isotopes in the soil water and use this information for identifying hydrological processes and hydraulic properties, soil was sampled three times in three profiles, two on Krško polje aquifer in SE Slovenia and one on Ljubljansko polje in central Slovenia. Isotope composition of soil water was measured with the water-vapor-equilibration method. Based on the isotope composition of soil water integrative information about water flow and transport processes with time and depth below ground were assessed. Porewater isotopes were in similar range as precipitation for all three profiles.  Variable isotope ratios in the upper 60 cm for the different sampling times indicated dynamic water fluxes in this upper part of the vadose zone. Results also showed more evaporation at one sampling location, Brege. The information from stable isotopes will be of importance for further analyzing the water fluxes in the vadose zone of the study sties. 
This research was financed by the ARRS BIAT 20-21-32 and IAEA CRP 1.50.18 Multiple isotope fingerprints to identify sources and transport of agro-contaminants.  

How to cite: Zupanc, V., Glavan, M., Curk, M., Pečan, U., Stockinger, M., Kammerer, G., Vadibeler, D., and Stumpp, C.: Characterization of hydraulic properties in the upper vadose zone for two aquifers in Slovenia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2407, https://doi.org/10.5194/egusphere-egu21-2407, 2021.

Methodological studies
Devakunjari Vadibeler, Michael Stockinger, and Christine Stumpp

Stable water isotopes of oxygen (d18O) are used as tracers to study soil pore water. One method to measure d18O of soil samples is the direct liquid-vapor equilibration (DLVE) method. In this method, test samples are stored in Ziploc bags and equilibrated for three days. After equilibration, the headspace gas is measured using laser spectrometry. The DLVE method requires minimum sample handling, enables direct isotopic measurements without the need of extracting the water, and is highly reliable and comparatively cheaper than other measurement methods. However, the influence of different soil textures and saturation levels on the δ18O isotope when using the DLVE method is not well understood yet. In this study, three different soil textures (sand, organic carbon rich silt and kaolinite) were oven-dried for three days at 105°C and saturated to different saturation levels (100%, 80%, 60% and 40%) in laboratory cylinders for a week. The samples were saturated using tap water of known isotopic value and stored in Ziploc bags for different amounts of time. The samples were analyzed after 1, 2, 3, 4 and 7 days using cavity ring down spectroscopy (CRDS), and the isotopic ratios recorded after storage were compared with the isotopic measurements obtained before the sample equilibration. The resulting isotopic deviations were less than the CRDS measurement precision after one day of sample storage for sandy soil regardless of their saturation levels. Likewise, one day was also adequate for 100%, 80% and 40% saturated kaolinite, with 100% saturation allowing for up to seven days of sample storage with only small isotopic deviations (±0.43‰). Contrary to this, for organic-silty soil the required equilibration time depended on the saturation level. The findings from this laboratory-based analysis enhance the understanding of the impact of soil texture and saturation level on the DLVE method.

How to cite: Vadibeler, D., Stockinger, M., and Stumpp, C.: Effect of different soil textures and saturation levels on the equilibration time of oxygen isotopes using the direct liquid-vapor equilibration method, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3012, https://doi.org/10.5194/egusphere-egu21-3012, 2021.

Ruth-Kristina Magh, Nils Henriksson, Hyungwoo Lim, Reimo Lutter, Tomas Lundmark, and John D. Marshall

Identifying tree water sources has long been an issue since obtaining samples was labor intensive and lacked high time resolution because of the destructive sampling procedure. It was previously shown that the “borehole equilibrium method” (Marshall et al. 2020) allows in situ measurements of xylem sap isotopic composition. While the advantage in using this method is its ability to monitor isotopic composition of xylem continuously and rapidly with immediate data availability, disadvantages are the limited number of trees that can be observed and that the laser has to be present in the field. Here, we propose cheaper and more field-deployable elaboration of the method based on the same principle as to use for tracer pulse-chasing experiments in forested ecosystems.

We installed boreholes in tree stems and sealed them on both sites using brass fittings with a pierceable chlorobutyl septum. The water vapor inside the sealed borehole was assumed to reach isotopic equilibrium with the liquid water in the xylem due to diffusion within seconds and was sampled using gas-tight syringes. The 20ml sample was then injected in a dry air stream connected to a Picarro L2130-I cavity ring-down absorption spectrometer (CRDAS). Standards of known isotopic composition were injected the same way. The peaks, rather than plateaus, of isotopic ratios measured from these injections were weighted by the water vapor amount, giving results accurate enough to distinguish between xylem water of natural abundance and water enriched in deuterium (average SD for 2H 5.2‰ and 18O 1.9‰ for natural abundance samples). To test this method in the field, we labeled 1m2 of soil at different soil depths with 15.5 L of water enriched in 2HHO (δ2H +220000 ‰) in a Scots pine forest in northern Sweden. Trees within a 10m radius from the labeled center were monitored continuously, allowing daily measurements of up to 120 trees for six weeks. Depending on soil depth the uptake dynamics varied over time, with the peaks from the shallowest soil injections  occurring within two weeks, while for the deeper soil layers the contribution to transpiration lagged behind approx. four weeks, likely due to a combination of lower root density and reduced hydraulic conductivity at greater depth.  The strength of the peaks was correlated with distance from the labeled soil patch.

We were able to show that this method works to chase an artificially enriched water pulse through a natural forested ecosystem. At the same time, this adaptation allows the method to become even cheaper than its precursor as it requires much less tubing and fewer fittings. Lastly, we consider it more field-deployable because it does not require the CRDAS to be in the field.

How to cite: Magh, R.-K., Henriksson, N., Lim, H., Lutter, R., Lundmark, T., and Marshall, J. D.: Borehole equilibration 2.0 or how to chase tracer pulses in a forested ecosystem, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8871, https://doi.org/10.5194/egusphere-egu21-8871, 2021.

Nunzio Romano, Carolina Allocca, Luisa Stellato, Fabio Marzaioli, and Paolo Nasta

The stable isotope composition of water (δ18O and δ2H) represents a useful tool to distinguish among different water pools along the soil-plant-atmosphere continuum. Using δ2H and δ18O as tracers helps gain a better understanding of plant root water uptake and dominant ecohydrological processes. To determine which pools of water are used for plant physiologic functions and returned to the atmosphere by transpiration, a common approach is to analyze the isotopic composition of water in both soil and plant. Cryogenic water extraction (CWE; Orlowski et al., 2016) is the most widely used laboratory-based technique to extract water from soil samples for isotopic analysis. However, recent studies have shown that the extraction conditions (time, temperature, and vacuum) and soil physical and chemical properties may affect the extracted soil-water isotope composition even significantly.

We have developed an efficient and cost-effective cryogenic vacuum equipment to extract water from soil or vegetation and this presentation aims at discussing some preliminary results. The equipment has been specifically designed to meet the following requirements: i) enable to quantify the accuracy of a CWE continuous flow extraction line, and ii) identify a specific extraction standard protocol for soil and vegetation samples. Two experiments have been carried out to evaluate the isotope fractionation induced by the system and how different operational parameters (i.e. times and temperature of extraction) can affect the results. Firstly, a known water isotopic ratio was processed by the vacuum system to determine the measurement accuracy and reproducibility by comparing pre- and post-processed water isotopic signatures. The likely causes of observed biases induced by sample processing are assessed and a relevant correction procedure is suggested. Subsequently, measurements were carried out on replicated samples taken from two differently-textured soils that, after being dried, were saturated in the laboratory up to different water content values with water of known isotopic composition. Also, plant samples were collected from plants grown in a greenhouse and irrigated with water of known isotopic composition.

Water from all samples was extracted by our CWE system and then analyzed using an isotope ratio mass spectrometer in Gas Bench mode for analyses and in temperature conversion elemental analysis (TC/EA) mode for. Preliminary results have quantified the isotope fractions on average of -1.6 ‰ for δ18O and 14.2 ‰ for δ2H. Normalization of stable isotopes from unknown samples according to observed fractionation has enabled the observed bias to become virtually zero, leading to a replicate reproducibility of δ18O and δ2H for soil water of 0.6 ‰ and 3 ‰, respectively. The analyses carried out up to now did not find statistical evidence that the soil types and soil-water contents may affect the extraction method and the accuracy of our protocol.

How to cite: Romano, N., Allocca, C., Stellato, L., Marzaioli, F., and Nasta, P.: Development of cryogenic extraction system for δ18O and δ2H measurement of water in soils and plants., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14274, https://doi.org/10.5194/egusphere-egu21-14274, 2021.

Anam Amin, Giulia Zuecco, Chiara Marchina, Michael Engel, Daniele Penna, Jeffrey J. McDonnell, and Marco Borga

Plant transpiration is a main component of the global water cycle and plays a key role in regulating ecohydrological process. Stable isotopes of oxygen and hydrogen are often used for the identification and quantification of plant water sources in ecohydrology. However, the isotopic tracing technique assumes that the isotopic signal in the water taken up by the plants remains unaltered during uptake at the soil-roots interface and transport to the distal twigs, i.e., isotopic fractionation does not occur during the water uptake and along the transport pathway. Nevertheless, recent studies showed that isotopic fractionation can occur under different environmental conditions. In this study, we performed a simple experiment with two olive (Olea europaea) trees utilizing labelled water to test isotopic fractionation of plant water during uptake and transport within the plants under controlled conditions. In addition, we performed the cryogenic vacuum distillation in two different laboratories to examine any possible effects of the extraction system on the isotopic composition of plant water extracts.

We set up the olive trees in pots inside a glasshouse and measured sap flow rates with Granier thermal dissipation probe, and shallow soil moisture by using a portable soil moisture probe. Air temperature, global solar radiation, and relative humidity were measured by a weather station installed inside the glasshouse nearby the olive trees. We irrigated the two plants with water of known isotopic composition and sampled the twigs, wood cores, roots, and soils at different depths (0-5, 5-15, and 15-25 cm). We extracted plant and soil waters by means of cryogenic vacuum distillation performed in two different laboratories.

Our results showed that the plant water samples reflected the isotopic signature of labelled water and mobile soil water, suggesting no isotopic fractionation during water transport. No significant differences were detected for twigs and wood cores extracted from distinct sections of the tree. However, only significant differences were obtained between plant tissue water (twigs, cores) and cryogenically-extracted deep soil water (i.e., >15 cm depths). Furthermore, we found no significant effects of the two cryogenic extraction systems on the isotopic composition of water extracts. Our results indicate that isotopic fractionation might not occur during root water uptake and transport processes in olive trees, at least under the specified experimental conditions, validating the conventional isotope-tracing approach. Further work both in the field and under controlled conditions, and on different plant species, is needed to check for this consistency, as well as testing other plant water extraction methods.


Keywords: olive tree; stable isotope analysis; plant water; cryogenic vacuum distillation; fractionation; labelled water.

How to cite: Amin, A., Zuecco, G., Marchina, C., Engel, M., Penna, D., McDonnell, J. J., and Borga, M.: A simple glasshouse experiment to test the isotopic fractionation in olive trees, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9300, https://doi.org/10.5194/egusphere-egu21-9300, 2021.