Partitioning Water Vapor Fluxes by the Use of Their Water Stable Isotopologues: From the Lab to the Field

Water stable isotopes are powerful tracers for partitioning of the terrestrial ecosystem water vapor fluxes into process-based components, i.e. evapotranspiration (ET) into soil evaporation (E) and plant transpiration (T). The isotopic methodology for ET artitioning is based on the fact that E and T have distinct water stable isotopic compositions, which in turn are due to each flux being differently affected by isotopic kinetic effects. To use stable isotopologues of water in ET partitioning studies, a good knowledge of the isotopic (equilibrium and kinetic) fractionation effects is crucial. While the temperature-dependent equilibrium fractionation factor is well characterized (Majoube 1971), the kinetic fractionation factor (αK), relevant, e.g., during soil evaporation, needs further investigation. In order to address this knowledge gap, we conducted a series of three different long-term bare soil evaporation experiments (differing in soil-water availability and aerodynamic conditions) to obtain αK values from the collected isotopic data and the inversion of a well-known resistance-totransfer model (i.e., the Craig and Gordon (1965) model). The isotopic composition of the soil water (δs) vapor was monitored non-destructively by using gas-permeable tubing (Rothfuss et al. 2013).The Craig and Gordon (1965) model was used in two different approaches. The first approach uses the Keeling (1958) plot to obtain values for the isotopic composition of the evaporation (δE). The second approach uses the slope of the linear regression between δs 2H and δs 18O. Results showed that the largest source uncertainty in the computation of αK stemmed from the uncertainty associated with the δE values modeled with the Keeling (1958) plot method. In the second approach αK values werewithin the theoretical range proposed by Dongmann et al. (1974) and Mathieu and Bariac (1996), which pointed to the prevalence of the turbulent transport of water vapor under saturated and unsaturated soil 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

Determination of kinetic isotopic fractionation of water during bare soil evaporation

A process-based understanding of the water cycle in the atmosphere is important for improving meteorological and hydrological forecasting models. Usually only net fluxes of evapotranspiration – ET are measured, while land-surface models compute their raw components evaporation –E and transpiration –T. Isotopologues can be used as tracers to partition ET, but this requires knowledge of isotopic kinetic effects which impact the stable isotopic composition of water pools (e.g., soil, plant, surface waters) during phase change and vapor transport by soil evaporation and plant transpiration. Craig and Gordon (1965) introduced the kinetic fractionation in their model. It’s defined as the ratio of the transport resistances in air of the isotopologue to the most abundant isotopologue. Previous studies conducted laboratory experiments for free evaporating water (Merlivat, 1978. Cappa et al. 2003) or bare soil evaporation (Braud et al. 2009) with only a low temporal resolution. The goal of this study is to provide estimates of this factor at higher temporal resolution. We performed a soil evaporation laboratory experiment to determine the kinetic fractionation factor by applying the Craig and Gordon model. A 0.7 m high column (0.48 m i.d.) was filled with silt loam (20.1 % sand, 14.9 % loam, 65 % silt) and saturated with water of known isotopic composition. Soil volumetric water content, temperature and the isotopic composition of the soil water vapor were measured at six different depths. At each depth microporous polypropylene tubing allowed the sampling of soil water vapor and the measurement of its isotopic composition in a non-destructive manner with high precision and accuracy as detailed in Rothfuss et al. (2013). In addition, atmospheric water vapor was sampled at seven different heights up to one meter above the surface. Finally, air relative humidity and temperature were monitored at one meter height. Results showed that soil and atmospheric isotopic composition profiles could be monitored at high temporal and vertical resolutions during the course of the experiment. The kinetic fractionation factor could be calculated by using an inverse Graig and Gordon model and the Keeling plot method at high temporal resolution over a long period. We observed an increasing isotopic composition in the evaporating water vapor due to more enriched surface water. This leads to a higher transport resistances and an increasing kinetic fractionation factor

Partitioning evapotranspiration fluxes with water stable isotopic measurements: from the lab to the field

Water stable isotopes are powerful tools for partitioning net into raw water fluxes such as evapotranspiration (ET) into soil evaporation (E) and plant transpiration (T). The isotopic methodology for ET partitioning is based on the fact that E and T have distinct water stable isotopic compositions, which in turn relies on the fact that each flux is differently affected by isotopic kinetic effects.An important work to be performed in parallel to field measurements is to better characterize these kinetic effects in the laboratory under controlled conditions. A soil evaporation laboratory experiment was conducted to retrieve characteristic values of the kinetic fractionation factor (αK) under varying soil and atmospheric water conditions. For this we used a combined soil and atmosphere column to monitor the soil and atmospheric water isotopic composition profiles at a high temporal and vertical resolution in a nondestructive manner by combining micro-porous membranes and laser spectroscopy. αK was calculated by using a well-known isotopic evaporation model in an inverse mode with the isotopic composition of E as one input variable, which was determined using a micro-Keeling regression plot.Knowledge on αK was further used in the field (Selhausen, North Rhine-Westphalia, Germany) to partition ET of catch crops and sugar beet (Beta vulgaris) during one growing season. Soil and atmospheric water isotopic profiles were measured automatically across depths and heights following a similar modus operandi as in the laboratory experiment. Additionally, a newly developed continuously moving elevator was used to obtain water vapor isotopic composition profiles with a high vertical resolution between soil surface, plant canopy and atmosphere. Finally, soil and plant samples were collected destructively to provide a comparison with the traditional isotopic methods. Our results illustrate the changing proportions of T and E along the growing season and demonstrate the applicability of our new non-destructive approach to field conditions

A high-resolution measurement technique for vertical CO2_2 and H2_2O profiles within and above crop canopies and its use for flux partitioning

We present a portable elevator-based setup for measuring CO$_2$, water vapor, temperature and wind profiles from the soil surface to the surface layer above crop canopies. The end of a tube connected to a closed-path gas analyzer is continuously moved up and down over the profile height (currently 2 m), while concentrations are logged at a frequency of 20 Hz. Temperature and wind speed are measured at the same frequency by a ventilated finewire thermocouple and a hotwire, respectively, and all measurements are duplicated as a continuous fixed-height measurement at the top of the profile. Test measurements were carried out at the TERENO research site of Selhausen (50°52'09’’N, 06°27'01’’E, 104.5 m MSL, Germany, ICOS site DE-RuS) in winter wheat, winter barley and a catch crop mixture during different stages of crop development and different times of the day (spring 2015 to autumn 2016). We demonstrate a simple approach to correct for time lags, and the resulting half-hourly mean profiles of CO$_2$ and H$_2$O over height increments of 2.5 cm. These results clearly show the effects of soil respiration and photosynthetic carbon assimilation, varying both during the daily cycle and during the growing season.Post-harvest measurements over bare soil and short intercrop canopy (<20 cm) were analyzed in the framework of Monin-Obukhov similarity theory to check the validity of the measurement and raw data processing approach. Derived CO$_2$ and latent heat fluxes show a good agreement to eddy-covariance measurements.In a next step, we applied a dispersion matrix inversion (modified after Warland and Thurtell 2000, Santos et al. 2011) to the concentration profiles to estimate the vertical source and sink distribution of CO$_2$ and H$_2$O. First results showed reasonable values for evaporation, transpiration and aboveground net primary production, but a likely overestimation of soil respiration. We discuss possible causes associated with exchange processes near the soil surface below a dense canopy, and the potential use of the wind and temperature profiles in efforts to improve the dispersion parametrization in this region. Santos, E.A., Wagner-Riddle, C., Warland, J.S. and Brown, S. (2011): Applying a Lagrangian dispersion analysis to infer carbon dioxide and latent heat fluxes in a corn canopy. Agricultural and Forest Meteorology 151: 620-632.Warland, J.S., Thurtell, G.W. (2000): A Lagrangian solution to the relationship between a distributed source and concentration profile. Boundary-Layer Meteorology 96: 453-471

Vergleich datenbasierter und instrumenteller Ansätze zum Source-Partitioning von Kohlenstoffdioxidflüssen in einem Winterweizenbestand

Wie reagiert die Biosphäre auf den Globalen Wandel und die lokale Landbewirtschaftung, und wie wirkt sie sich wiederum auf den Klimawandel aus? Die Landoberfläche kann zum jetzigen Zeitpunkt ca. 33 % (±17 %) des Kohlenstoffdioxids (CO2) aus der Verbrennung fossiler Brennstoffe aufnehmen. Dem gegenüber steht allerdings eine zusätzliche CO2-Abgabe von 14 % (±10 %) aus Landnutzungsänderungen (IPCC 2013). Photosynthetische CO2-Aufnahme und respiratorische CO2-Abgabe werden unterschiedlich von Umweltfaktoren wie Temperatur, CO2-Konzentration, und Wasserverfügbarkeit beeinflusst. Diese Faktoren sind wiederum dem globalen Wandel unterworfen. Um diese Wechselwirkungen analysieren zu können, müssen Nettoflüsse von Treibhausgasen, wie sie beispielsweise mit der Eddy-Kovarianz-Methode gemessen werden können, in ihre Einzelbeiträge zerlegt werden. Derartige Versuche, den CO2-Fluss in Photosynthese und Respiration oder den latenten Wärmefluss in Evaporation und Transpiration aufzuschlüsseln, werden unter dem Begriff “Source Partitioning” zusammengefasst.Das BMBF-geförderte Forschungsprojekt IDAS-GHG (Instrumental and Data-driven Approaches to Source-Partitioning of Greenhouse Gas Fluxes: Comparison, Combination, Advancement) hat die Zielsetzung, existierende Ansätze zum Source-Partitioning von Treibhausgasflüssen systematisch miteinander zu vergleichen und zu verbessern. Diese lassen sich in zwei Gruppen gliedern: Datenbasierte Ansätze nutzen bestehende (Roh)daten aus der Eddy-Kovarianz-Messung. Instrumentelle Ansätze hingegen beinhalten die Durchführung zusätzlicher Messungen, wie z. B. Kammer- und Profilmessungen, oder die Verwendung von Tracern (z. B. stabile Isotope), die Auskunft über die Herkunft der Gasmoleküle geben können.In unserer Präsentation werden einige dieser Methoden am Beispiel des Messstandorts Selhausen beschrieben. Der Standort befindet sich im TERENO-Observatorium Eifel/Niederrheinische Bucht in der intensiv landwirtschaftlich genutzten niederrheinischen Bördelandschaft. Der Untersuchungszeitraum erstreckt sich über die Vegetationsperiode eines Winterweizenfeldes von der Aussaat im Oktober 2014 über die Ernte hinaus bis Ende September 2015. Im Messfeld installiert ist eine dauerhafte Eddy-Kovarianz-Station und ein automatisches Bodenrespirations-Kammersystem mit bis zu vier Langzeitkammern. Zusätzlich wurden stichprobenartig Profilmessungen der CO2- und H2O-Konzentrationen mit einem eigens gebauten Liftsystem durchgeführt.Mithilfe der gemessenen Eddy-Kovarianz-Daten zeigen wir einen Vergleich bestehender Ansätze zum Source-Partitioning des Netto-Ökosystem-Austauschs in Bruttoprimärproduktion (Photosynthese) und Ökosystematmung (Respiration). Unter Verwendung von Kammermessungen wird dieser um die Terme Nettoprimärproduktion und Bodenrespiration erweitert.Das Profilsystem misst Änderungen der Konzentration von CO2 und H2O mit einer hohen vertikalen und zeitlichen Auflösung zwischen Bodenoberfläche, Pflanzenbestand und Atmosphäre. Die Profile im Halbstundenmittel bilden den Effekt der photosynthetischen Kohlenstoff-Assimilation und Bodenatmung deutlich ab und geben somit qualitative Informationen über Quell- und Senkbereiche. Im nächsten Schritt wird versucht, diese zu quantifizieren und mit den bereits beschriebenen Ansätzen zu vergleichen

Investigation of Kinetic Isotopic Fractionation of Water During Bare Soil Evaporation

The kinetic fractionation factor (αK) controls to a large extent the isotopic enrichment of surface waters during evaporation (E). In contrast to the well-known vapor-to-liquid isotopic equilibrium fractionation factor, αK has still not yet been properly characterized for soil water evaporation. In this study, we investigated the αK daily dynamics during a series of three laboratory experiments differing in soil water availability and aerodynamic conditions. For this, we applied a commonly-used isotopic evaporation model and tested it in two different approaches. First, a two-end member mixing model (“Keeling plot”) was fitted to the measured isotopic composition of the laboratory air water vapor to obtain αK. In a second approach, αK was obtained from the slope of the “evaporation line” in a dual isotopic coordinate system. For both methods, the isotopic composition of the soil water was determined non-destructively and online by sampling the soil water vapor with gas-permeable microporous tubing. Results highlighted the limitation of the first approach, as the determination of the isotopic composition of E with the Keeling plot was challenging with the laboratory setup. The second approach provided αK values within the range (α_K^(2_H ) = 1.0132 ±0.0013; α_K^(〖18〗_O ) = 1.0149 ±0.0012) reported in the literature and pointed to the prevalence of turbulent water vapor transport under water-saturated soil conditions, but also at soil water content significantly lower than the saturated value. In a third experiment, temporal dynamics of the atmospheric water vapor intrusion in the topmost soil layer could be observed during an isotopic labeling pulse