scholarly journals In situ measurements of soil and plant water isotopes: a review of approaches, practical considerations and a vision for the future

2020 ◽  
Vol 24 (9) ◽  
pp. 4413-4440
Author(s):  
Matthias Beyer ◽  
Kathrin Kühnhammer ◽  
Maren Dubbert
Keyword(s):  
Water Uptake ◽  
Isotope Composition ◽  
Isotope Effects ◽  
Root Water Uptake ◽  
Water Isotopes ◽  
Future Research ◽  
Plant Water ◽  
In Situ Methods ◽  
Water Isotope

Abstract. The number of ecohydrological studies involving water stable isotope measurements has been increasing steadily due to technological (e.g., field-deployable laser spectroscopy and cheaper instruments) and methodological (i.e., tracer approaches or improvements in root water uptake models) advances in recent years. This enables researchers from a broad scientific background to incorporate water-isotope-based methods into their studies. Several isotope effects are currently not fully understood but might be essential when investigating root water uptake depths of vegetation and separating isotope processes in the soil–vegetation–atmosphere continuum. Different viewpoints exist on (i) extraction methods for soil and plant water and methodological artifacts potentially introduced by them, (ii) the pools of water (mobile vs. immobile) measured with those methods, and (iii) spatial variability and temporal dynamics of the water isotope composition of different compartments in terrestrial ecosystems. In situ methods have been proposed as an innovative and necessary way to address these issues and are required in order to disentangle isotope effects and take them into account when studying root water uptake depths of plants and for studying soil–plant–atmosphere interaction based on water stable isotopes. Herein, we review the current status of in situ measurements of water stable isotopes in soils and plants, point out current issues and highlight the potential for future research. Moreover, we put a strong focus and incorporate practical aspects into this review in order to provide a guideline for researchers with limited previous experience with in situ methods. We also include a section on opportunities for incorporating data obtained with described in situ methods into existing isotope-enabled ecohydrological models and provide examples illustrating potential benefits of doing so. Finally, we propose an integrated methodology for measuring both soil and plant water isotopes in situ when carrying out studies at the soil–vegetation–atmosphere continuum. Several authors have shown that reliable data can be generated in the field using in situ methods for measuring the soil water isotope composition. For transpiration, reliable methods also exist but are not common in ecohydrological field studies due to the required effort. Little attention has been paid to in situ xylem water isotope measurements. Research needs to focus on improving and further developing those methods. There is a need for a consistent and combined (soils and plants) methodology for ecohydrological studies. Such systems should be designed and adapted to the environment to be studied. We further conclude that many studies currently might not rely on in situ methods extensively because of the technical difficulty and existing methodological uncertainties. Future research needs to aim on developing a simplified approach that provides a reasonable trade-off between practicability and precision and accuracy.

scholarly journals X Water Worlds and how to investigate them: A review and future perspective on <i>in situ</i> measurements of water stable isotopes in soils and plants

2019 ◽  
Author(s):  
Matthias Beyer ◽  
Maren Dubbert
Keyword(s):  
Stable Isotopes ◽  
Water Uptake ◽  
Isotope Effects ◽  
Root Water Uptake ◽  
Future Research ◽  
Plant Water ◽  
Research Needs ◽  
In Situ Methods ◽  
Water Isotope

Abstract. The number of ecohydrological studies involving water stable isotopes has been increasing steadily due to technological (i.e. field deployable laser spectroscopy and cheaper instruments) and methodological (i.e. tracer approaches or improvements in root water uptake models) advances in recent years. This enables researchers from a broad scientific background to incorporate water isotope-based methods into their studies. Several isotope effects are currently not fully understood, but essential when investigating root water uptake depths of vegetation and disentangle isotope processes at the soil-vegetation-atmosphere continuum. In particular different viewpoints exist on i) extraction methods for soil and plant water and methodological artefacts potentially introduced by them; ii) the pools of water measured with those methods and iii) spatiotemporal issues related with water stable isotope research. In situ methods have been proposed as an innovative and necessary way to address these issues and are required in order to disentangle isotope effects and take them into account when studying root water uptake depths of plants and for studying soil-plant-atmosphere interaction based on water stable isotopes. Herein, we review the current status of in situ measurements of water stable isotopes in soils and plants, point out current issues and highlight potential for future research. Moreover, we put a strong focus and incorporate practical aspects into this review. Finally, we propose an integrated methodology for measuring both soil and plant water isotopes when carrying out studies at the soil-vegetation-atmosphere interface. For all in situ methods, extreme care needs to be taken particularly during set-up in order to obtain reliable data. In situ methods for soils are well established. For transpiration, reliable methods also exist but are not common in ecohydrological field studies due to the required effort. Little attention has been payed to in situ xylem water isotope measurements. Research needs to focus on improving and further developing those methods. There is a need for a consistent and combined (soils and plants) methodology for ecohydrological studies. Such systems should be designed and adapted to the environment to be studied. We further conclude that many studies currently might not rely on in situ methods extensively because of the technical difficulty. Hence, future research needs to aim on developing a simplified approach that provides a reasonable trade-off between practicability and precision/accuracy.

Tracing plant water fluxes in ecosystems by stable isotopes along the soil-plant and plant-atmosphere interfaces

2020 ◽  
Author(s):  
Christiane Werner
Keyword(s):  
Water Uptake ◽  
Large Scale ◽  
Temporal Dynamics ◽  
Root Water Uptake ◽  
Plant Water ◽  
Water Fluxes ◽  
Terrestrial Vegetation ◽  
Species Specific ◽  
The Impact

&lt;p&gt;Terrestrial vegetation is a main driver of ecosystem water fluxes, as plants mediate the water fluxes within the soil-vegetation-atmosphere continuum. Stable isotopologues of water are efficient tracer to follow the water transfer in soils, uptake by plants, transport in stems and release into the atmosphere through stomata. The development of in-situ methods coupled to isotope spectroscopy does now enable real-time in-situ water vapour isotopologue measurements revealing high spatial and temporal dynamics, such as adaptations in root water uptake depths (within hours to days) or the impact of transpirational fluxes on atmospheric moisture.&lt;/p&gt;&lt;p&gt;Examples will be given how isotopes can be used to inform the complex interplay between plant ecophysiological adaptations and hydrological processes. For example, root water uptake is not solely driven by soil water availability but has to be understood in the context of species-specific regulation of active zones in their rooting system determining the conductivity between soil and roots regulating uptake depths. The latter has also to be evaluated in context of the nutrient demand and the spatial nutrient availability. Similarly, plant water transport and losses are a fined tuned interplay between species-specific structural and functional adaptations and atmospheric processes.&lt;/p&gt;&lt;p&gt;Finally, first data of a large-scale ecosystem labelling experiment at the Biosphere 2 tropical rainforest of the B2 Wald, Atmosphere, and Live Dynamics (B2WALD) will be presented.&lt;/p&gt;

Inferring plant physiologic parameters for root water uptake modelling from high frequency in-situ isotope measurements

2020 ◽  
Author(s):  
Stefan Seeger ◽  
Michael Rinderer ◽  
Markus Weiler
Keyword(s):  
Soil Moisture ◽  
Water Uptake ◽  
Root Distribution ◽  
Fine Root ◽  
Root Water Uptake ◽  
Plant Water ◽  
Plant Water Uptake ◽  
Fine Root Distribution ◽  
Isotope Measurements

&lt;p&gt;In the face of global climate change, a well-informed knowledge of plant physiologic key parameters is essential to predict the behavior of ecosystems in a changing environment. Many of these parameters may be determined with lab or pot experiments, but it could prove problematic to transfer results obtained in a such experiments with small trees to fully grown trees. Therefore, new approaches to determine relevant parameters for mature trees are still required. Regarding plant water uptake, parameters related to fine root distribution (maximum depth, depth distribution and rhizosphere radius) and parameters describing the physiological limits of root water uptake are important, but usually hard or costly to assess for fully grown trees.&amp;#160; In-situ isotope probes (Volkmann et al. 2016a&amp;#160; &amp; 2016b) are a promising recent development that offer new possibilities for the investigation of plant water uptake and associated physiological parameters.&lt;/p&gt;&lt;p&gt;In this study we used in-situ stable water isotope probes in soil (six depths from 10 to 100 cm) and in tree xylem of mature (140 years) European beech trees (three heights between 0 and 8 m). With those probes, we monitored soil and xylem isotope signatures after an isotopically labeled (Deutrium-Excess = 100 &amp;#8240;) irrigation pulse equivalent to 150 mm of precipitation and foursubsequent natural precipitation events over a period of twelve weeks with a high temporal resolution (six or more measurements per probe per day). Those measurements were complemented with measurements of soil moisture and sap flow dynamics. We interpolated our measured soil isotope and soil moisture data in order to obtain spatially and temporally continuous data for those soil parameters. Then we used this data as an input to the Feddes-Jarvis plant water uptake model, in order to predict the isotopic signature of plant water uptake at daily time steps. With the help of our observed isotopic signatures, we were able to directly constrain the critical water potential parameter of the Feddes model as well as the underlying fine root distribution. Furthermore, the observed dampening of the breakthrough curve of our Deuterium-labeling pulse allowed us to infer information on the rhizosphere&amp;#160; radius and water transport velocities in the fine roots and stem between the points of root water uptake and the eight meter stem height.&lt;/p&gt;&lt;p&gt;With our field experiment we showed that in-situ isotope measurements in soil profiles and in tree xylem sap can help to constrain plant water uptake modelling parameters. Future experiments might use this approach to scrutinize lab-scale derived hypothesizes regarding tree water uptake and to investigate the temporal and spatial dynamics of root water uptake in the field.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;em&gt;Volkmann, T. H., Haberer, K., Gessler, A., &amp; Weiler, M. (2016a). High&amp;#8208;resolution isotope measurements resolve rapid ecohydrological dynamics at the soil&amp;#8211;plant interface. New Phytologist, 210(3), 839-849.&amp;#160;&lt;/em&gt;&lt;/p&gt;&lt;p&gt;&lt;em&gt;Volkmann, T. H., K&amp;#252;hnhammer, K., Herbstritt, B., Gessler, A., &amp; Weiler, M. (2016b). A method for in situ monitoring of the isotope composition of tree xylem water using laser spectroscopy. Plant, cell &amp; environment, 39(9), 2055-2063.&amp;#160;&lt;/em&gt;&lt;/p&gt;&lt;p&gt;&lt;em&gt;Jarvis, N. J. (1989). A simple empirical model of root water uptake. Journal of Hydrology, 107(1-4), 57-72.&amp;#160;&lt;/em&gt;&lt;/p&gt;

Monitoring tree species-specific water uptake strategies via continuous in-situ stable water isotope measurements

2020 ◽  
Author(s):  
Natalie Orlowski ◽  
Stefan Seeger ◽  
David Mennekes ◽  
Hugo de Boer ◽  
Markus Weiler ◽  
...  
Keyword(s):  
Water Uptake ◽  
Temporal Resolution ◽  
Tree Species ◽  
Isotopic Signature ◽  
Plant Water ◽  
Destructive Sampling ◽  
Species Specific ◽  
Isotope Measurements ◽  
Water Isotope

&lt;p&gt;Water isotope tracing techniques in combination with laser-based isotopic analyses have advanced our understanding of plant water uptake patterns providing opportunities to carry out observational studies at high spatio-temporal resolution. Studying these highly dynamic processes at the interface between soils and trees can be challenging under natural field conditions, as available water resources are difficult to control. On the other hand, the results of small pot experiments in the greenhouse using tree seedlings are often difficult to transfer to mature trees. Here, we setup a controlled outdoor large pot experiment with three different, 4-6 meter high and 20 year old trees: &lt;em&gt;Pinus pinea, Alnus &lt;span&gt;spaethii&lt;/span&gt; and Quercus suber.&lt;/em&gt; We took advantage of stable water isotope techniques by tracing plant water uptake from the root zone through the xylem via isotopically labelled irrigation water. We combined ecohydrological observations of sapflow, photosynthesis, soil moisture and temperature and soil matrix potential with high resolution measurements of water stable isotopes in soils and trees to understand how soil water is used by different tree species. We monitored the isotopic composition of soil and xylem water in high temporal resolution with in-situ isotope probes installed at different depths in the soil and different heights in the tree stem. We further compared the water isotopic composition of our in-situ monitoring setup with destructive sampling methods for soil and plant water (vapour equilibration method and cryogenic extraction).&lt;/p&gt;&lt;p&gt;Our results from the continuous monitoring showed a distinct difference in the xylem sap isotopic signature between&lt;em&gt; Quercus&lt;/em&gt; on the one hand and &lt;em&gt;Alnus&lt;/em&gt; and &lt;em&gt;Pinus&lt;/em&gt; on the other hand. This is likely due to different water use strategies of these tree species. The tree xylem isotopic signature of &lt;em&gt;Alnus&lt;/em&gt; and &lt;em&gt;Pinus&lt;/em&gt; responded to the isotopic label within one day and six days at 15 cm and 150 cm stem height, respectively. The peak isotopic signature in the tree xylem due to the label application was similar to the isotopic signature of the soil in 30 cm (for &lt;em&gt;Alnus&lt;/em&gt;) and 15&amp;#160;cm (for &lt;em&gt;Pinus&lt;/em&gt;). &lt;em&gt;Quercus&lt;/em&gt; showed a delayed and much slower increase in the xylem isotopic signature in response to the label and the highest values were significantly lower than the corresponding soil isotopic signatures. Our methodological comparison showed that the isotopic signature of the destructive samples (from both methods) had a larger spread and this spread tended to become larger with subsequent labeling. Destructive soil samples showed a wider isotopic variation than destructive xylem samples. The in-situ isotope measurements in comparison showed a relative constant small to medium spread for soil and xylem isotopic measurements. Our in-situ isotope probes therefore seem to be a potential alternative or supplement to destructive sampling offering much higher temporal resolution. The continuation of the labeling experiments in 2020 will allow us to further study tree-species specific water uptake strategies, which will become important under future climatic conditions in terms of development of adaptation strategies for sustainable forest management.&lt;/p&gt;

Tracing dynamic water uptake and transport from root to canopy by online monitoring of water isotopes in an enclosed tropical forest in response to drought

2020 ◽  
Author(s):  
Kathrin Kuehnhammer ◽  
Joost van Haren ◽  
Angelika Kuebert ◽  
Maren Dubbert ◽  
Nemiah Ladd ◽  
...  
Keyword(s):  
Water Uptake ◽  
Deep Water ◽  
Sap Flow ◽  
Water Storage ◽  
Root Water Uptake ◽  
Water Isotopes ◽  
Hydraulic Redistribution ◽  
Stem Water ◽  
Stem Water Storage

&lt;p&gt;&lt;em&gt;Online&lt;/em&gt; (or: &lt;em&gt;in situ&lt;/em&gt;) methods for measuring soil and plant water isotopes have been identified as an innovative and crucial step to address recently identified issues in studying water uptake using stable isotope techniques.&lt;/p&gt;&lt;p&gt;During a controlled three month drought and rewetting experiment at the Biosphere 2 (B2) enclosed rainforest, a recently developed online method for measuring stem water isotopes (&lt;em&gt;Marshall et al., 2019&lt;/em&gt;), namely &amp;#8216;stem borehole equilibration&amp;#8217;, was combined with &lt;em&gt;online&lt;/em&gt; monitoring of soil water isotopes and transpired water isotopes as well as sap flow and stem water storage. This enabled us to study root water uptake depths of different tree species and dynamic changes during the dry down and rewetting. After two months of drought, the system was supplied with isotopically labelled water (deuterated water) from down below via a pipe system spanning across the complete B2 rainforest in order to identify deep water uptake of the rainforest trees and hydraulic redistribution.&lt;/p&gt;&lt;p&gt;Results show that &amp;#8211; as expected &amp;#8211; all monitored trees responded to the drought by changing their root water uptake towards deeper soil depths while sap flow rates of most trees decreased. When rewetting the system, deep water uptake from the base of B2 (between 2.5m and 4m soil depth) was identified in all large, mature trees (Clitoria faichildiana, Hibiscus tilliaceus, Hura crepitans, Pachira aquatica). No deep water uptake was found in the smaller trees (mainly Pachira aquatica). Furthermore, stem water storage was notably different between species and affected their adaptation to drought and response to rewetting. The labelled water was also identified in the transpired water more than one month after re-starting rainfall at B2. &amp;#160;However, no hydraulic redistribution was identified.&lt;/p&gt;&lt;p&gt;The holistic approach for monitoring the interactions of soils and plants provides inevitable insights into the adaptation of (enclosed) rainforests under drought and might have implications for natural rainforests. In particular, the capability of large trees to develop deep roots and the role of stem water storage are important elements for adaptation to climatic changes and need to be studied further under &amp;#8216;real&amp;#8217; conditions.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Marshall, J.D., Cuntz, M., Beyer, M., Dubbert, M., K&amp;#252;hnhammer, K., 2019. Borehole equilibration: testing a new method to monitor the isotopic composition of tree xylem water in situ. Front. Plant Sci.&lt;/p&gt;

scholarly journals Xylem water in riparian Willow trees (Salix alba) reveals shallow sources of root water uptake by in situ monitoring of stable water isotopes

2021 ◽  
Author(s):  
Jessica Landgraf ◽  
Dörthe Tetzlaff ◽  
Maren Dubbert ◽  
David Dubbert ◽  
Aaron Smith ◽  
...  
Keyword(s):  
Isotopic Composition ◽  
Soil Water ◽  
Water Uptake ◽  
Sap Flow ◽  
Root Water Uptake ◽  
In Situ Monitoring ◽  
Water Isotopes ◽  
Stable Water Isotopes ◽  
Salix Alba

Abstract. Root water uptake is an important critical zone process, as plants can tap various water sources and transpire these back into the atmosphere. However, knowledge about the spatial and temporal dynamics of root water uptake and associated water sources at both high temporal resolution (e.g. daily) and over longer time periods (e.g. seasonal) is still limited. We used cavity ring-down spectroscopy (CRDS) for continuous in situ monitoring of stable water isotopes in soil and xylem water for two riparian willow (Salix alba) trees over the growing season (May to October) of 2020. This was complemented by isotopic sampling of local precipitation, groundwater and stream water in order to help constrain the potential sources of root water uptake. A local flux tower, together with sap flow monitoring, soil moisture measurements and dendrometry were also used to provide the hydroclimatic and ecohydrological contexts for in situ isotope monitoring. In addition, bulk samples of soil water and xylem water were collected to corroborate the continuous in situ data. The monitoring period was characterised by frequent inputs of precipitation, interspersed by warm dry periods which resulted in variable moisture storage in the upper 20 cm of the soil profile and dynamic isotope signatures. This variability was greatly damped in 40 cm and the isotopic composition of the sub-soil and groundwater was relatively stable. The isotopic composition and dynamics of xylem water was very similar to that of the upper soil and analysis using a Bayesian mixing model inferred that overall ~90 % of root water uptake was derived from the upper soil profile. Sap flow and dendrometry data indicated that soil water availability did not seriously limit transpiration during the study period, though there was a suggestion that deeper (> 40 cm) soil water might provide a higher proportion of root water uptake (~30 %) in a drier period in the late summer. The study demonstrates the utility of prolonged real time monitoring of natural stable isotope abundance in soil-vegetation systems, which has great potential for further understanding of ecohydrological partitioning under changing hydroclimatic conditions.

C-dots as a novel silica-based fluorescent nanoparticle tracer to investigate plant hydraulics

2020 ◽  
Author(s):  
Kanishka Singh ◽  
Benjamin Hafner ◽  
James Knighton ◽  
M. Todd Walter ◽  
Taryn Bauerle
Keyword(s):  
Water Uptake ◽  
Plant Tissue ◽  
Imaging System ◽  
Root Water Uptake ◽  
Nano Particles ◽  
Water Isotopes ◽  
Hydraulic Redistribution ◽  
Severe Drought ◽  
Stable Water Isotopes ◽  
Plant Hydraulics

&lt;p&gt;Forest cover exerts a significant control on the partitioning of precipitation between evapotranspiration and surface runoff. Thus, understanding how plants take up and transpire water in forested catchments is essential to predict flooding potential and hydrologic cycling. A growing literature underscores the importance of integrating whole-plant hydraulics, including such processes as the spatial variability of root distribution and the temporally dynamic nature of root water uptake by depth in understanding the relationship between changes in vegetation and hydrology. The analysis of stable isotopes of water (&lt;sup&gt;18&lt;/sup&gt;O and &lt;sup&gt;2&lt;/sup&gt;H) sourced from soils and plant tissue has enabled the estimation of tree root water uptake depths and water use strategies. Despite the general acceptance of stable water isotopic data to estimate plant hydraulic dynamics, this methodology imposes assumptions that may produce spurious results. For example, end member mixing analysis neglects time-delays during tree-water storage. Also, it is likely that hydraulic redistribution processes of plants, which transport water across soil depths and both into and out of plant tissue, modify &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O and &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H; the isotopic signature of a collected sample may thus reflect a history of transport and exposure to fractionating processes not accounted for in analysis. We tested the feasibility of C-dots, core-shell silica polyethylene-glycol coated fluorescent nano-particles (5.1&amp;#160;nm diameter) in 20&amp;#160;&amp;#181;mol/l solution with H&lt;sub&gt;2&lt;/sub&gt;O labeled with a near-infrared fluorophore, cyanine 5.5 (excitation maximum of 646 nm, emission maximum of 662 nm), as an alternative to stable water isotopes in the investigation of plant hydraulics. We examined the absorption and transport of C-dots through soil, as well as roots and aerial structures of Eastern hemlock (Tsuga canadensis), Eastern white pine (Pinus strobus), and white spruce (Picea glauca) saplings (n = 12 each) via an IVIS-200 luminescence in-situ imaging system. We compared the fluid mechanics, residence times and mixing schemes of C-dots with &lt;sup&gt;2&lt;/sup&gt;H-labeled water during transport within these plant species to establish the nanoparticles as a viable alternative through a split-root hydraulic redistribution experiment under moderate and severe drought conditions. We present a residence-time distribution to elucidate the mixing scheme of C-dot solution and calibration curves to aid future studies. This research is the premier assessment of this nanoparticle as an alternative tracer to stable water isotopes, and as such may yield insights for broader applications.&lt;/p&gt;

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