Nära till naturen : en diskussion om riktlinjer för grundtillgång på friluftsmarker nära tätorter /

This study tested a method to quantify and locate hydraulic lift (HL, defined as the passive upward water flow from wetter to dryer soil zones through the plant root system) by combining an experiment using the stable water isotope 1H218O as a tracer with a soil–plant water flow model. Our methodology consisted in (i) establishing the initial conditions for HL in a large rhizobox planted with Italian ryegrass (Lolium multiflorum Lam.), (ii) labeling water in the deepest soil layer with an 18O-enriched solution, (iii) monitoring the water O isotopic composition in soil layers to find out changes in the upper layers that would reflect redistribution of 18O-enriched water from the bottom layers by the roots, and (iv) comparing the observed soil water O isotopic composition to simulation results of a three-dimensional model of water flow and isotope transport in the soil–root system. Our main findings were that (i) the depth and strength of the observed changes in soil water O isotopic composition could be well reproduced with a modeling approach (RMSE = 0.2‰, i.e., equivalent to the precision of the isotopic measurements), (ii) the corresponding water volume involved in HL was estimated to account for 19% of the plant transpiration of the following day, i.e., 0.45 mm of water, and was in agreement with the observed soil water content changes, and (iii) the magnitude of the simulated HL was sensitive to both plant and soil hydraulic properties

Partition de l'évapotranspiration réelle en évaporation du sol et transpiration des couverts végétaux à partir du traçage isotopique (18O) en milieu contrôlé : expérimentation et modélisation SiSPAT_Isotope

Evapotranspiration (ET) is a major component of continental precipitation recycling. In order to improve our knowledge of the water cycle at different scales, it is therefore crucial to know the amount and origin of water lost to the atmosphere through ET (i.e. Soil Evaporation and/or Plant Transpiration). ET from a monolith of soil with a tall fescue cover (Festuca arundinacea L.) was measured in a series of controlled conditions (climat chamber) experiments over the course of the growing period. Heavy stable isotopes were used to partition ET into Soil Evaporation and Plant Transpiration components at different stages of growth from bare soil to full canopy. The contribution of Soil Evaporation to Evapotranspiration decreased during the experiment from 100% (bare soil) to 94% (at 16 days after seeding), 83% (at 28 days), 70% (at 36 days) and finally dropped down to 5% (at 43 days). Experiment under controlled conditions allowed each partition value to be associated with a detailed description of vegetation characteristics (Leaf Area Index, Root Distribution Densitiy) as well as soil and climat conditions. The nature and amount of data gathered here gave us the opportunity to evaluate the performance of a model of heat, water and isotopes transfer through the soil, vegetation and atmosphere, SiSPAT_Isotopes (Simple Soil Plant Atmosphere Transfer model, Braud et al., 1995; 2000; 2002). The model proved reliable under the monolith experimental conditions. Moreover, this study enabled the evaluation of the model's isotopes transfer module within the soil and the plant: isotopic profiles and fluxes within the soil as well as plant extraction depths could be modelled. It was possible to simulate continuously the experiment and analyse into more details the outputs of the model that cannot be normally measured.L'évapotranspiration est un terme majeur du recyclage des précipitations au niveau des surfaces continentales. En vue de compléter notre connaissance du cycle de l'eau à différentes échelles, il est donc essentiel de connaître à la fois les quantités d'eau libérées par évapotranspiration dans l'atmosphère ainsi que leurs origines possibles (évaporation du sol et/ou transpiration des couverts végétaux). L'utilisation des isotopes stables et lourds de l'eau lors d'une série d'études réalisées sur monolithes de sol en milieu contrôlé (réacteur biogéochimique) nous a permis de déterminer l'évolution de la partition de l'évapotranspiration en évaporation du sol et transpiration des plantes au cours du développement d'un couvert de fétuque élevée. Il a fallu pour cela maintenir le système sol-plante en régime hydrique permanent de manière à ce qu'il atteigne l'état isotopique stationnaire. La contribution de l'évaporation du sol à l'évapotranspiration réelle a diminué durant l'expérience de 100% (sol nu) à 94% (16 jours suivant le semis), 83% (28 jours), 70% (36 jours) pour finalement atteindre 5% (43 jours). Le recours au milieu contrôlé nous a permis d'atteindre l'état isotopique stationnaire recherché et associer à chaque valeur de partition une description détaillée du couvert végétal (indice foliaire, profil de densité racinaire) ainsi que des conditions climatiques dans l'atmosphère du réacteur et hydriques dans le sol. La nature et la quantité de données à disposition permettent l'évaluation du fonctionnement d'un modèle de flux de chaleur, d'eau et d'isotopes à travers le continuum sol-plante-atmosphère SiSPAT_Isotopes (Simple Soil Plant Atmosphere Transfer model, Braud et al., 1995 ; 2000 ; 2002). On a pu constater que la structure du modèle était suffisamment souple pour être adaptée aux conditions expérimentales. Ce travail a tout particulièrement permis l'évaluation du module de transport des isotopes dans le sol et la plante : on a pu modéliser les profils ainsi que les flux d'isotopes dans le sol et déterminer les profondeurs d'extraction racinaire. Il a été possible de simuler l'expérience en continu, y compris en régime transitoire et d'analyser plus en détail, en s'appuyant sur les sorties du modèle non accessibles à la mesure, les hypothèses faites lors de l'expérience, notamment l'atteinte de régime hydrique permanent et d'état isotopique stationnaire

Monitoring water stable isotope composition in soils using gas-permeable tubing and infrared laser absorption spectroscopy

Abstract: The water stable isotopologues 1H2H16O and 1H218O are powerful tracers of processes occurring in nature. Their slightly different masses as compared to the most abundant water isotopologue (1H216O) affect their thermodynamic (e.g. during chemical equilibrium reactions or physical phase transitions with equilibration) and kinetic (liquid and vapor phases transport processes and chemical reactions without equilibration) properties. This results in measurable differences of the isotopic composition of water within or between the different terrestrial ecosystem compartments (i.e. sub-soil, soil, surface waters, plant, and atmosphere). These differences can help addressing a number of issues, among them water balance closure and flux partitioning from the soil-plant-atmosphere continuum at the field to regional scales. In soils particularly, the isotopic composition of water (δ2H and δ18O) provides qualitative information about whether water has only infiltrated or already been re-evaporated since the last rainfall event or about the location of the evaporation front. From water stable isotope composition profiles measured in soils, it is also possible, under certain hypotheses, to derive quantitative information such as soil evaporation flux and the identification of root water uptake depths. In addition, water stable isotopologues have been well implemented into physically based Soil–Vegetation–Atmosphere Transfer models (e.g. SiSPAT-Isotope; Soil–Litter iso; TOUGHREACT) and have demonstrated their potential. However, the main disadvantage of the use of stable isotopes in soil water studies is that, contrary to other state variables (e.g. water content and tension) that can be monitored over long periods (e.g. by time-domain reflectometry, capacitive sensing, tensiometry or micro-psychrometry), stable isotope compositions are analyzed following destructive sampling, and thus are available only at a given time. As a consequence, there are important time discrepancies between soil water and stable isotope information which greatly limit the insight potential of the latter. Recently, a novel technique based on direct infrared laser absorption spectroscopy was developed that allows simultaneous and direct measurements of 1H216O, 1H2H16O and 1H218O composition in water vapour, which constitutes a major breakthrough in stable isotope analysis. Many applications can be found in the literature for varying temporal and spatial scales. In this study, we present a simple methodology for monitoring soil liquid water stable isotope composition (δ2H, δ18O) in a non-destructive manner by sampling and measuring water vapour equilibrated with soil water, using gas-permeable polypropylene tubing and a cavity ring-down laser absorption spectrometer. We will first give a detailed presentation of our laboratory controlled experimental setup and water vapour sampling protocol. We will then show that, following a preliminary calibration in a saturated fine sand, it is possible to follow the changes of isotopic composition of soil water over a wide range of water availability conditions. Limits of this technique as well as advices to potential field users will be given

Reviews and syntheses: Isotopic approaches to quantify root water uptake: a review and comparison of methods

Plant root water uptake (RWU) has been documented for the past five decades from water stable isotopic analysis. By comparing the (hydrogen or oxygen) stable isotopic compositions of plant xylem water to those of potential contributive water sources (e.g., water from different soil layers, groundwater, water from recent precipitation or from a nearby stream), studies were able to determine the relative contributions of these water sources to RWU. In this paper, the different methods used for locating/quantifying relative contributions of water sources to RWU (i.e., graphical inference, statistical (e.g., Bayesian)multi-source linear mixing models) are reviewed with emphasis on their respective advantages and drawbacks. The graphical and statistical methods are tested against a physically based analytical RWU model during a series of virtual experiments differing in the depth of the groundwater table, the soil surface water status, and the plant transpiration rate value. The benchmarking of these methods illustrates the limitations of the graphical and statistical methods while it underlines the performance of one Bayesian mixing model. The simplest two-end-member mixing model is also successfully tested when all possible sources in the soil can be identified to define the two end-members and compute their isotopic compositions. Finally, the authors call for a development of approaches coupling physically based RWU models with controlled condition experimental setups

Isotopic approaches to quantifying root water uptake and redistribution: a review and comparison of methods

Plant root water uptake (RWU) and release (i.e., hydraulic redistribution – HR, and its particular case hydraulic lift – HL) have been documented for the past five decades from water stable isotopic analysis. By comparing the (hydrogen or oxygen) stable isotopic composition of plant xylem water to those of potential contributive water sources (e.g., water from different soil layers, groundwater, water from recent precipitation or from a nearby stream) authors could determine the relative contributions of these water sources to RWU. Other authors have confirmed the existence of HR and HL from the isotopic analysis of the plant xylem water following a labelling pulse. In this paper, the different methods used for locating / quantifying relative contributions of water sources to RWU (i.e., graphical inference, statistical (e.g., Bayesian) multi-source linear mixing models) are reviewed with emphasis on their respective advantages and drawbacks. The graphical and statistical methods are tested against a physically based analytical RWU model during a series of virtual experiments differing in the depth of the groundwater table, the soil surface water status, and the plant transpiration rate value. The benchmarking of these methods illustrates the limitations of the graphical and statistical methods (e.g., their inability to locate or quantify HR) while it underlines the performance of one Bayesian mixing model, but only when the number of considered water sources in the soil is the highest to closely reflect the vertical distribution of the soil water isotopic composition. The simplest two end-member mixing model is also successfully tested when all possible sources in the soil can be identified to define the two end-members and compute their isotopic compositions. Finally, future challenges in studying RWU with stable isotopic analysis are evocated with focus on new isotopic monitoring methods and sampling strategies, and on the implementation of isotope transport in physically based RWU models

Monitoring water stable isotope composition in soils using gas-permeabletubing and infrared laser absorption spectroscopy

The water stable isotopologues 1H2H16O and 1H182 O are powerful tracers of processes occurring in nature. Theirslightly different masses as compared to the most abundant water isotopologue (1H162 O) affect their thermodynamic(e.g. during chemical equilibrium reactions or physical phase transitions with equilibration) and kinetic(liquid and vapor phases transport processes and chemical reactions without equilibration) properties. This resultsin measurable differences of the isotopic composition of water within or between the different terrestrial ecosystemcompartments (i.e. sub-soil, soil, surface waters, plant, and atmosphere). These differences can help addressing anumber of issues, among them water balance closure and flux partitioning from the soil-plant-atmosphere continuumat the field to regional scales. In soils particularly, the isotopic composition of water (2H and 18O) providesqualitative information about whether water has only infiltrated or already been re-evaporated since the last rainfallevent or about the location of the evaporation front. From water stable isotope composition profiles measured insoils, it is also possible, under certain hypotheses, to derive quantitative information such as soil evaporation fluxand the identification of root water uptake depths. In addition, water stable isotopologues have been well implementedinto physically based Soil–Vegetation–Atmosphere Transfer models (e.g. SiSPAT-Isotope; Soil–Litter iso;TOUGHREACT) and have demonstrated their potential. However, the main disadvantage of the isotope methodologyis that, contrary to other soil state variables that can be monitored over long time periods, 2H and 18O aretypically analyzed following destructive sampling.Here, we present a non-destructive method for monitoring soil liquid water 2H and 18O over a wide range ofwater availability conditions and temperatures by sampling and measuring water vapor equilibrated with soil waterusing gas-permeable polypropylene tubing and a cavity ring-down laser absorption spectrometer. By analyzingwater vapor 2H and 18O sampled with the tubing from a fine sand for temperatures ranging between 8–24°C,we demonstrate that (i) our new method is capable of monitoring 2H and 18O in soils online with high precisionand, after calibration, also with high accuracy, (ii) our sampling protocol enabled detecting changes of 2Hand 18O following non-fractionating addition and removal of liquid water and water vapor of different isotopiccompositions, and (iii) the time needed for the tubing to monitor these changes is compatible with the observedvariations of 2H and 18O in soils under natural conditions

Instrumental Approaches to Source Partitioning of CO2_{2} and H2_{2}O Fluxes

How does the biosphere react on global change and local land use management? The land surface currently acts as a sink for anthropogenic emissions from fossil fuels, but an additional CO2 release is caused by land use change. The sensitivities of photosynthetic CO2 uptake and respiratory CO2 release to environmental parameters remain uncertain. One possible way to disentangle the flux of greenhouse gases is source partitioning, e.g. into photosynthesis and respiration (CO2) or into evaporation and transpiration (H2O).The BMBF-funded project IDAS-GHG (Instrumental and Data-driven Approaches to Source-Partitioning of Greenhouse Gas Fluxes: Comparison, Combination, Advancement) aims at comparing and improving existing methods for partitioning of CO2 and H2O fluxes into their respective raw components. Data-driven approaches use existing (raw or processed) data of typical eddy-covariance stations. Instrumental approaches of source partitioning require additional measurements at different parts of ecosystems and different methods, e.g. soil-flux chamber measurements, profile measurements or tracer measurements (isotopes).We present preliminary results of a profile measurement system involving a small elevator continuously moving up and down. It measures changes in the concentration of CO2 and H2O at a high vertical and temporal resolution between the soil surface, the plant canopy and the atmosphere. Tests were carried out at the TERENO research site of Selhausen (Lower Rhine Embayment in the river Rur catchment (50°52'09’’N, 06°27'01’’E, 104.5 m MSL, Germany) on a winter wheat field for a growing season from seeding to harvest (April - August 2015).The half hourly mean profiles of CO2 and H2O show the effects of soil respiration and photosynthetic carbon assimilation very clearly, varying both during the daily cycle and during the growing season.An additional way to partition CO2 and H2O fluxes is through measurements of concentration profiles of their stable isotopologues (13CO2, 12C18O16O, 1H2H16O, and 1H218O). Following controlled-conditions experiments in the laboratory on soil columns in autumn and winter 2015, a quantum-cascade dual isotope laser will be deployed at the Selhausen test site in a low-flow (i.e., soil atmosphere and chamber measurements) and high flow (i.e., Eddy-Covariance measurements) configurations for comparison with the above-mentioned profile measurement system