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| Hauptseite > Publikationsdatenbank > Ecohydrological response of a grassland species to drought from an isotopic and hydraulic perspective |
| Hauptseite > Publikationsdatenbank > Ecohydrological response of a grassland species to drought from an isotopic and hydraulic perspective |
| Dissertation / PhD Thesis | FZJ-2025-01627 |
2025
Université de Liège
Gembloux, Belgien
Abstract: Water flow from the soil through the plants and into the atmosphere is not only passively determined by above- and below-ground environmental conditions. Plants actively control water loss through certain physiological properties (e.g., stomatal closure) following external and internal, chemical and physical triggers (e.g., internal signaling with abscisic acid, production of mucilage, extreme dry conditions around the roots or the leaves). The study of the degree and timing of the vegetation’s water loss prevention sheds light into not only the resilience and plasticity of a certain species, but also into the feedbacks between environmental conditions and plant physiology that ultimately determine water flow in the soil-plant-atmosphere continuum. Understanding the dynamic interactions between biotic and abiotic processes underlying water fluxes across scales has become even more relevant since the frequency of weather extremes (e.g., drought and heatwaves) is observed to increase due to climate change, affecting especially water-scarce ecosystems such as grasslands. These ecosystems provide forage for livestock, are biodiversity hotspots, can store significant amounts of carbon, and represent a considerable area of agricultural land (in Europe, over 30%). The threats posed by drought, reduced precipitation, and heatwaves (e.g., loss of biodiversity, carbon-storage capacity and productivity) are not only environmental, but also societal and financial.Water stable isotopic monitoring has been used for several decades as a tracer tool in ecohydrological studies aiming at elucidating interactions between biotic and abiotic processes. More specifically, quantifying root water uptake and investigating its dynamics and drivers with isotopic measurements of soil and plant water in linear mixing models is now a relatively common strategy. Insights into the mechanisms at the soil-root interface influencing water uptake, especially in dry soils, have been gained with physically-based transfer models, i.e., accounting for root hydraulic properties. This mechanistic understanding of the feedbacks in the soil-plant-atmosphere continuum ultimately enables accurate water flux estimations across scales and accurate predictions of the impacts of climate change. Thus, vulnerability of grasslands to hydrological extremes can be investigated in part through the assessment of the drought response of the species in this ecosystem at the single plant and community scale.Nevertheless, the potential of water stable isotopes for root water uptake quantification is limited by the uncertainties associated with the techniques used to extract soil and plant water. Firstly, the reliance of many established water recovery techniques (e.g., cryogenic vacuum distillation, centrifugation, direct water vapor equilibration) on destructive sampling and water extraction in the lab leads to non-negligible measurement uncertainty. In situ non-destructive techniques that rely on continuous soil water vapor isotopic measurements have contributed to reducing this uncertainty source while providing estimates of root water uptake patterns at higher temporal resolution, which is of key interest for modeling purposes. Secondly, questions remain regarding possible fractionating effects due to isotope interactions between soil water and soil texture. It has been hypothesized that these fractionating processes result in heterogeneous soil water isotopic compositions, challenging the assumption made in isotopic mixing models that roots take up water from a well-mixed soil water source.In chapter 1, I outline the theoretical framework about grasslands, their response to drought and vulnerability to hydrological extremes, water stable isotopes and processed-based transfer modelling. In this section, I also formulate the main and specific objectives, as well as accompanying hypothesis of this doctoral project. In chapters 2 through 4, I describe in detail the methodological framework, present and discuss the results of the three studies constituting this doctoral project:1. In the first study (chapter 2), I investigated the potential soil-texture- and soil-water-tension-related isotopic fractionating effects on soil water though a comparison of water extracted via pressure, with three destructive isotopic techniques and measured with a non-destructive one in two types of soil. Describing potential soil water isotopic fractionation caused by a particular soil via non-destructive determinations was a prerequisite to eliminate potential biases in the second study.2. In a second study, I assessed the ecohydrological response of the forb species Centaurea jacea, native to grasslands in Europe, to varying above- and below-ground conditions in a semi-automated laboratory experiment at the single-plant scale. For this, plant physiological and environmental conditions were continuously monitored. This species was the dominant forb species in the semi-natural temperate grassland, where the ecohydrological assessment of the response to drought and nitrogen loading of the plant community was performed in the framework of the same research program the present doctoral project was embedded in.3. In the third and final study, I modelled root water uptake and the hydraulic parameters of this laboratory experiment with a macroscopic mechanistic hydraulic model to describe links between processes at the soil-root and plant-atmosphere interfaces. I also compared isotope and physically-based root water uptake patterns.From the experiments in the first study, I concluded that i) two isotopically distinct water pools successively added to a “chemically inert” soil (quartz sand) mix almost completely; ii) the isotopic composition of soil water changed as a function of soil water potential in a loamy sand due most likely to methodological constraints and enhanced by low soil water potential values; and iii) the isotopic determinations of three destructive methods (cryogenic vacuum distillation, centrifugation, and direct water vapor equilibration) and the in situ non-destructive online technique were not comparable. Since the discrepancies between the isotopic determinations of the in situ method and the spike water were rooted in methodological issues, we determined that no additional corrections of the soil water δ-values from this method using the loamy sand were necessary.The root water uptake profiles obtained in the second study enabled by the semi-automated experimental setup evidenced a consistent reliance of Centaurea jacea on water in shallow depths: up to 79% of water uptake occurred in soil layer 0-15 cm, while up to 44% occurred in soil layer 45-60 cm. Moreover, this grassland drought-resistant species was able to maintain high transpiration rates – by withstanding very low leaf water potential values – and relatively constant water use efficiency in dry conditions, traits also observed in the field in other studies. In the final days of the drought part of our experiment, I observed a steady decrease in canopy conductance at relatively high soil water content.To test the hypothesis that this decrease in canopy conductance might be related to a decrease in root or soil conductivity, the ecohydrological assessment was broadened by adding a hydraulic perspective. In these final days of the drought part of the experiment, a steep decrease of the root system conductance and hydraulic conductance near the soil-root interface of C. jacea under dry conditions was modelled using a macroscopic mechanistic hydraulic model. Consequently, there were slight discrepancies between the isotope- and hydraulic-based root water uptake patterns, since not only root density or water availability but hydraulic states in the soil and the plant were part of the analysis. Assumptions regarding overestimations of soil water content and absorbing root surface were also part of this extended isotopic and hydraulic approach.A summary of the main findings and the perspective for each study is found in chapter 5. An important task in future studies is disentangling methodology-related and “naturally” occurring fractionating processes in ecohydrological systems and determining the biases and uncertainties they introduce in isotope-based root water uptake quantification. These biases and uncertainties could be minimized through standardized methodological frameworks overarching experimental design, sampling and isotopic determinations. Investigating isotope discrimination in the soil (due to texture, tension or root presence), during transport into the roots (due to e.g., mycorrhizal activity or the root membrane) or through the xylem (due to e.g., molecular transport mechanisms or mixing of storage and conduit water) is still needed. By addressing these knowledge gaps, the potential of water stable isotopes as tracers in the mechanistic understanding of still unknown biotic-abiotic dynamic processes and the identification of climate-related breaking or tipping points in the soil-plant-atmosphere continuum will increase. Moreover, fully coupling isotope- and physically-based transfer models accounting also for relevant and until now overlooked processes influencing water, carbon, and nutrient cycling (e.g., fine root biomass) without making them unnecessarily complicated can assist in achieving this mechanistic understanding and provide accurate predictions of the impacts of climate change on grasslands and the related ecological, social and financial consequences.
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