Ecohydrological travel times derived from in situ stable water isotope measurements in trees during a semi-controlled pot experiment

Abstract

Abstract. Tree water uptake processes and ecohydrological travel times have gained more attention in recent ecohydrological studies. In situ measurement techniques for stable water isotopes offer great potential to investigate these processes but have not been applied much to tree xylem and soils so far. Here, we used in situ probes for stable water isotope measurements to monitor the isotopic signatures of soil and tree xylem water before and after two deuterium-labeled irrigation experiments. To show the potential of the method, we tested our measurement approach with 20-year-old trees of three different species (Pinus pinea, Alnus incana and Quercus suber). They were planted in large pots with homogeneous soil in order to have semi-controlled experimental conditions. Additional destructive sampling of soil and plant material allowed for a comparison between destructive (cryogenic vacuum extraction and direct water vapor equilibration) and in situ isotope measurements. Furthermore, isotope-tracer-based ecohydrological travel times were compared to travel times derived from sap flow measurements. The time to first arrival of the isotope tracer signals at 15 cm stem hight were ca. 17 h for all tree species and matched well with sap-flow-based travel times. However, at 150 cm stem height tracer-based travel times differed between tree species and ranged between 2.4 and 3.3 d. Sap-flow-based travel times at 150 cm stem hight were ca. 1.3 d longer than tracer-based travel times. The isotope signature of destructive and in situ isotope measurements differed notably, which suggests that the two types of techniques sampled water from different pools. In situ measurements of soil and xylem water were much more consistent between the three tree pots (on average standard deviations were smaller by 8.4 ‰ for δ2H and by 1.6 ‰ for δ18O for the in situ measurements) and also among the measurements from the same tree pot in comparison to the destructive methods (on average standard deviations were smaller by 7.8 ‰ and 1.6 ‰ for δ2H and δ18O, respectively). Our study demonstrates the potential of semi-controlled large-scale pot experiments and very frequent in situ isotope measurements for monitoring tree water uptake and ecohydrological travel times. It also shows that differences in sampling techniques or sensor types need to be considered when comparing results of different studies and within one study using different methods.

On the urgent need for standardization in isotope‐based ecohydrological investigations

“…A further issue is the lack of comparability between the various types of extraction approaches. Extraction method intercomparisons utilizing soils spiked with waters of known isotope values, natural bulk soils, and plants grown in controlled and natural conditions showed significant differences in the isotopic values of analytes extracted by the tested methods (Barbeta et al, 2020;Kelln et al, 2001;Kübert et al, 2020;Mennekes et al, 2021;Millar et al, 2018Millar et al, , 2019Orlowski et al, 2019;Orlowski, Pratt, & McDonnell, 2016;Zuecco et al, 2022). In the case of soils, the research found some extraction approaches were closer to their 'truth value' than others or that isotope values of extracted analytes were statistically different between methods.…”

“…Beyond potential extraction induced errors (Chen et al, 2020;Fischer et al, 2019), we suggest that the greatest concern is the wide variety of extraction approaches being utilized and the lack of unified standard operating procedures (SOP) to guide their use (see Gralher et al, 2021;Orlowski et al, 2018). Research showed that significant differences were found in the isotopic composition of analytes extracted by various tested extraction methods (Barbeta et al, 2022;Kelln et al, 2001;Mennekes et al, 2021;Millar et al, 2018Millar et al, , 2019Orlowski et al, 2018;Orlowski, Pratt, & McDonnell, 2016;Zuecco et al, 2022). See Table 2 for extraction method error ranges.…”

Modelling temporal variability of in situ soil water and vegetation isotopes reveals ecohydrological couplings in a riparian willow plot

“…Due to the relatively young uptake water ages of willows in this study, the inclusion of any cell water storage (∼ 16 L d −1 if 20 % of transpiration, ∼ 600 L total storage to maintain transpiration for 1 week) may significantly increase the mean age of xylem water and would increase the total vegetation mixing time. Given the limited studies describing the ages of water and tracers in transpiring trees (Sprenger et al, 2019;Mennekes et al, 2021;Benettin et al, 2021), more studies quantifying the transport through the xylem, as conducted here, and the inclusion of additional mixing within vegetation (e.g. Steppe et al, 2006) would be beneficial to further constrain plant water use estimations.…”

Causes and Factors of Cryogenic Extraction Biases on Isotopes of Xylem Water

“…For example, stem water encompasses sap water, living parenchyma, phloem cell water, and the intercellular space between xylem cells (Jupa et al, 2016;Tyree & Zimmermann, 2002). While the transit time of sap water varies from hours to days in plants (Mennekes et al, 2021), tissue water may have a much longer residence time (Berghuijs & Allen, 2019). Due to the differing mobility, physiological processes, and degree of influence of external environmental conditions, the sap water and tissue water in xylem possess differing isotopic compositions (De Deurwaerder et al, 2020;Ellsworth & Sternberg, 2015).…”