EGU22-11593
https://doi.org/10.5194/egusphere-egu22-11593
EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Deriving xylem water isotopic compositions from in situ transpiration measurements: opportunity for plant source water identification? 

Angelika Kübert1,2, Maren Dubbert3, Kathrin Kuehnhammer4, Matthias Beyer4, Joost van Haren5, Laura K. Meredith6, S. Nemiah Ladd1,7, and Christiane Werner1
Angelika Kübert et al.
  • 1Ecosystem Physiology, University of Freiburg, Freiburg, Germany
  • 2Institute for Atmospheric and Earth System Research, University of Helsinki, Helsinki, Finland
  • 3Isotope Biogeochemistry & Gas Fluxes, Landscape Functioning, ZALF, Müncheberg, Germany
  • 4Institute for Geoecology, Technical University of Braunschweig, Braunschweig, Germany
  • 5Biosphere 2 and Honors College, University of Arizona, Tucson, United States of America
  • 6School of Natural Resources and the Environment, University of Arizona, Tucson, United States of America
  • 7Department of Environmental Sciences, University of Basel, Basel, Switzerland

The isotopic signature of xylem water (δX) is of great interest for plant source water studies. δX 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 δX 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 (δT) to be monitored in high temporal resolution. As δT values approach δx values when transpiration is at isotopic steady state, measurements of δT may provide a relatively non-intrusive method to derive δX 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 δT 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 δT in a 2-h resolution and observed its response to increasing drought, deep labelling and subsequent recovery. Parallel to continuous measurements of δT, branch samples were collected at 5 time points throughout the experiment to determine δX values from CVE.

We found that δT values provide a good proxy for δX values when using the daily averages of δT values, weighted by the transpiration flux; derived δX values matched well with isotopic compositions of soil water. In situ-δ18OX agreed well with values from CVE. CVE-δ2HX 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, δT allowed us to distinguish clearly between deep and shallow soil water use as well as show uptake velocities of newly added water. Monitoring  δT using gas exchange chambers provides a good proxy for δX 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.