Ecohydrological Partitioning in the Critical Zone: Analysis of the Soil-Plant-Atmosphere Flows by Means of Stable Water Isotopes

Tree and small catchment scale studies are important in getting a detailed understanding of ecohydrological processes and the interactions between plants and water fluxes with respect to specific geology, climate, physiographic and topographic settings. However, such studies cannot explain the processes ongoing at large spatial and temporal scales and/or across the globe. Recent global-scale analyses based on water stable isotopes (2H and 18O) quantified the contribution of different water sources to plant transpiration. However, no previous studies have estimated the depth distribution of soil water uptake by plants, and its relation to climate, and plant group at the global scale. Recently, numerous ecohydrological studies explored the controls of climate and physiographic settings on isotopic composition of plant water. However, the assessment of the critical postulates (i.e., no isotopic fractionation in plant tissues) behind the use of stable water isotopes have not been widely tested. Furthermore, recent literature has raised a concern regarding the lack of standardized methodologies for plant and soil materials sampling and water extraction. The accuracy of different water extraction methodologies and the technical parameters (such as system setup, extraction time and temperature), which may affect the isotopic composition of the soil and plant extracted waters and thus, our interpretation of the results, still need to be thoroughly investigated. To deal with these issues, this thesis presents an integrated outlook by exploring plant water-relations through use of stable water isotopes. This approach moved from a global scale assessment of the soil water source exploited by plants and the main drivers (i.e., climate and plant functional type) controlling the isotopic compositions of xylem water. This analysis revealed soil water as main water source for plants. A new graphical inference method has been developed for the quantification of soil water depths contribution. The results indicated the largest overlap (up to 100%) for shallow soil water and xylem water in cold zone, while the overlap between deep soil water and xylem water was largest for arid and tropical climate zones (i.e. >75%). This method has a great prospective to be tested and applied at other study sites around the globe. Such global or catchment scale water source quantification by stable water isotopes is based on fundamental underlying postulation, henceforth, as a second objective in this thesis, it was attempted to test the isotopic fractionation of plant water along the pathway from roots to leaves under a greenhouse setup. Besides this, the measured isotopic compositions were also evaluated towards their sensitivity to the plant water extraction method via an interlaboratory comparison of commonly used cryogenic vacuum distillation system for plant materials. The resulting isotopic composition of different plant tissue water (i.e., stem and core) showed no fractionation with some exception to the root water and soil water samples at depths >15 cm, which were isotopically enriched as compared to the irrigation water. The samples extracted in two labs were comparable, nevertheless this controlled experiment further highlights the urgency of such comparisons to be performed for other existing plant water extraction methodologies and across other plant species. Thereupon a subsequent comparison of different plant water extraction techniques (Scholander-type pressure chamber vs. cryogenic vacuum distillation) was performed. Results indicated a significant difference in the isotopic values obtained by the two extraction methods and this difference was smaller for beech samples compared to the chestnut samples. In addition, different results were observed for δ2H and δ18O, i.e. larger differences between the plant water extraction methods and the samples were found for δ2H compared to δ18O.