Deriving xylem water isotopic compositions from in situ transpiration measurements: opportunity for plant source water identification?&#160;
Authors: Kübert Angelika , Dubbert Maren , Kuehnhammer Kathrin , Beyer Matthias , van Haren Joost , Meredith Laura K. , Ladd S. Nemiah , Werner Christiane 
Published: March 2022

&lt;p&gt;The isotopic signature of xylem water (&amp;#948;&lt;sub&gt;X&lt;/sub&gt;) is of great interest for plant source water studies. &amp;#948;&lt;sub&gt;X&lt;/sub&gt; 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 &amp;#948;&lt;sub&gt;X&lt;/sub&gt; 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 (&amp;#948;&lt;sub&gt;T&lt;/sub&gt;) to be monitored in high temporal resolution. As &amp;#948;&lt;sub&gt;T&lt;/sub&gt; values approach &amp;#948;&lt;sub&gt;x&lt;/sub&gt; values when transpiration is at isotopic steady state, measurements of &amp;#948;&lt;sub&gt;T&lt;/sub&gt; may provide a relatively non-intrusive method to derive &amp;#948;&lt;sub&gt;X&lt;/sub&gt; values. &amp;#160;&lt;/p&gt;&lt;p&gt;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 &amp;#948;&lt;sub&gt;T&lt;/sub&gt; values in two tropical plant species, one understory and one canopy species. Severe drought was ended with a deep water pulse strongly enriched in &lt;sup&gt;2&lt;/sup&gt;H. By connecting flow-through leaf chambers to a water isotope analyzer, we measured &amp;#948;&lt;sub&gt;T&lt;/sub&gt; in a 2-h resolution and observed its response to increasing drought, deep labelling and subsequent recovery. Parallel to continuous measurements of &amp;#948;&lt;sub&gt;T&lt;/sub&gt;, branch samples were collected at 5 time points throughout the experiment to determine &amp;#948;&lt;sub&gt;X&lt;/sub&gt; values from CVE.&lt;/p&gt;&lt;p&gt;We found that &amp;#948;&lt;sub&gt;T&lt;/sub&gt; values provide a good proxy for &amp;#948;&lt;sub&gt;X&lt;/sub&gt; values when using the daily averages of &amp;#948;&lt;sub&gt;T&lt;/sub&gt; values, weighted by the transpiration flux; derived &amp;#948;&lt;sub&gt;X&lt;/sub&gt; values matched well with isotopic compositions of soil water. In situ-&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;X&lt;/sub&gt; agreed well with values from CVE. CVE-&amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;X&lt;/sub&gt; values, however, were strongly enriched in &lt;sup&gt;2&lt;/sup&gt;H in comparison to in situ derived values, which is probably linked to the isotopic effects of CVE on &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H as recently found in several studies. Moreover, by adding a &lt;sup&gt;2&lt;/sup&gt;H deep water pulse, &amp;#948;&lt;sub&gt;T&lt;/sub&gt; allowed us to distinguish clearly between deep and shallow soil water use as well as show uptake velocities of newly added water. Monitoring&lt;sub&gt;&amp;#160; &lt;/sub&gt;&amp;#948;&lt;sub&gt;T&lt;/sub&gt; using gas exchange chambers provides a good proxy for &amp;#948;&lt;sub&gt;X&lt;/sub&gt; 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.&amp;#160;&lt;/p&gt;