Thierry Bariac

Concentration effects on laser‐based δ18O and δ2H measurements and implications for the calibration of vapour measurements with liquid standards

Recently available isotope ratio infrared spectroscopy can directly measure the isotopic composition of atmospheric water vapour (δ(18) O, δ(2) H), overcoming one of the main limitations of isotope ratio mass spectrometry (IRMS) methods. Calibrating these gas-phase instruments requires the vapourisation of liquid standards since primary standards in principle are liquids. Here we test the viability of calibrating a wavelength-scanned cavity ring-down spectroscopy (CRDS) instrument with vapourised liquid standards. We also quantify the dependency of the measured isotope values on the water concentration for a range of isotopic compositions. In both liquid and vapour samples, we found an increase in δ(18) O and δ(2) H with water vapour concentration. For δ(18) O, the slope of this increase was similar for liquid and vapour, with a slight positive relationship with sample δ-value. For δ(2) H, we found diverging patterns for liquid and vapour samples, with no dependence on δ-value for vapour, but a decreasing slope for liquid samples. We also quantified tubing memory effects to step changes in isotopic composition, avoiding concurrent changes in the water vapour concentration. Dekabon tubing exhibited much stronger, concentration-dependent, memory effects for δ(2) H than stainless steel or perfluoroalkoxy (PFA) tubing. Direct vapour measurements with CRDS in a controlled experimental chamber agreed well with results obtained from vapour simultaneously collected in cold traps analysed by CRDS and IRMS. We conclude that vapour measurements can be calibrated reliably with liquid standards. We demonstrate how to take the concentration dependencies of the δ-values into account. Copyright

Water transfer and mobile water content measurement in a cultivated crusted soil

In crusted soils, runoff and erosion at the surface are strongly controlled by soil infiltrability. An in situ hydrodynamic characterization of a cultivated crusted soil was conducted to define the factors that reduce infiltrability. Experiments were carried out on three layers of two different profiles in the topsoil: (i) on the surface crust, which was either sedimentary or structural, (ii) within the underlying soil, and (iii) at the plow pan surface. A structural crust is the result of gradual coalescing of aggregates by raindrop compaction, whereas a sedimentary crust is formed by deposition of the particles suspended in overland flow. The purpose here was to characterize water transfer as a function of vertical heterogeneity. A tension disc infiltrometer, along with an 18O solution, was used to create a near-saturated flow. Hydrodynamic properties and mobile water fraction of the soil surface were inferred from the cumulative infiltration and the soil solute concentration at the end of the experiments. Visual observations of X-ray images obtained from thin sections were used to emphasize some of the conclusions about the hydrodynamic characterization. Results of infiltration in soil covered with either more or less developed crusts were compared. Then, comparisons were made between the infiltrability of the underlying soil, which was covered by sedimentary or structural crusts. Finally, estimated values of hydraulic conductivity and the mobile water fraction for each layer of the two profiles provided information on water transfer. Results showed that the fraction of the soil surface covered by sedimentary crusts and structural crusts was an important factor for infiltrability (the cumulative infiltration at t = 5000 s varied between 11.5 mm and 14.8 mm in soil covered by a sedimentary crust, whereas the variation was between 18 mm and 22 mm in soil covered by a structural crust). On the other hand, infiltrability did not depend on the developmental stage of the surface crusts as the differences between the cumulative infiltration in more or less developed crusts were not significantly different at P = 0.05 (according to the Student’s t test). The sedimentary crust seemed to protect the underlying soil from aggregate coalescence. Thus, collapsing was less important in the underlying plowed material covered by a sedimentary crust. As a consequence, the mobile water fraction and effective mean pore size estimations showed that in the case of strongly collapsed material, coalescing increased the pore connection: water transferred through small but well connected pores (the effective mean pore size was λm = 0.105 mm, and the mobile water fraction was f = 0.93). When there was less collapsed underlying soil, the pores participating in transfer were bigger but less connected (λm = 1.2 mm and f = 0.5). The plow pan did not show strong impermeable behavior because the macropores made by roots were not sealed by plowing.

Reducing and correcting for contamination of ecosystem water stable isotopes measured by isotope ratio infrared spectroscopy.

Concern exists about the suitability of laser spectroscopic instruments for the measurement of the (18)O/(16)O and (2)H/(1)H values of liquid samples other than pure water. It is possible to derive erroneous isotope values due to optical interference by certain organic compounds, including some commonly present in ecosystem-derived samples such as leaf or soil waters. Here we investigated the reliability of wavelength-scanned cavity ring-down spectroscopy (CRDS) (18)O/(16)O and (2)H/(1)H measurements from a range of ecosystem-derived waters, through comparison with isotope ratio mass spectrometry (IRMS). We tested the residual of the spectral fit S(r) calculated by the CRDS instrument as a means to quantify the difference between the CRDS and IRMS δ-values. There was very good overall agreement between the CRDS and IRMS values for both isotopes, but differences of up to 2.3‰ (δ(18)O values) and 23‰ (δ(2)H values) were observed in leaf water extracts from Citrus limon and Alnus cordata. The S(r) statistic successfully detected contaminated samples. Treatment of Citrus leaf water with activated charcoal reduced, but did not eliminate, δ(2)H(CRDS) - δ(2)H(IRMS) linearly for the tested range of 0-20% charcoal. The effect of distillation temperature on the degree of contamination was large, particularly for δ(2)H values but variable, resulting in positive, negative or no correlation with distillation temperature. S(r) and δ(CRDS) - δ(IRMS) were highly correlated, in particular for δ(2)H values, across the range of samples that we tested, indicating the potential to use this relationship to correct the δ-values of contaminated plant water extracts. We also examined the sensitivity of the CRDS system to changes in the temperature of its operating environment. We found that temperature changes ≥4 °C for δ(18)O values and ≥10 °C for δ(2)H values resulted in errors larger than the CRDS precision for the respective isotopes and advise the use of such instruments only in sufficiently temperature-stabilised environments.

Spatial variation and temporal persistence of grapevine response to a soil texture gradient

Abstract Studying the water transport in the soil-plant system requires information on the spatio-temporal variability of both subsystems and the ability to assess the impact of the soil heterogeneity and of the biological responses on the coupling between vegetation and its substrate. This study was conducted for 2 years in a vineyard in the Aude Valley, France, by measuring the particle size distribution of the topsoil, the instantaneous isotopic ratios ( 18 O 16 O , 2 H 1 H ) of leaf water, annual shoot biomass production, and interannual persistence of this biomass along a 360 m transect. The resultant spatial series were analysed for their correlations and converted to spectra. Changes in the isotopic ratios along the transect reflect the soil texture gradient, suggesting that the vines root deeper on the gravel layers than elsewhere. This could provide a mechanism for the partial decoupling between soil and vegetation, and thus explain (1) the strong temporal persistence of the vegetation pattern, (2) the low overall correlation between biomass production and soil texture. The spectra show that this correlation concentrates at specific scales which correspond to a minimum variability in the shoot biomass. In this case, therefore, soil texture plays only a minor role in determining the spatial heterogeneity of shoot biomass in grapevine.

The Water Isotopic Version of the Land-Surface Model ORCHIDEE: Implementation, Evaluation, Sensitivity to Hydrological Parameters

Land-Surface Models (LSMs) exhibit large spread and uncertainties in the way they partition precipitation into surface runoff, drainage, transpiration and bare soil evaporation. To explore to what extent water isotope measurements could help evaluate the simulation of the soil water budget in LSMs, water stable isotopes have been implemented in the ORCHIDEE (ORganizing Carbon and Hydrology In Dynamic EcosystEms: the land-surface model) LSM. This article presents this implementation and the evaluation of simulations both in a stand-alone mode and coupled with an atmospheric general circulation model. ORCHIDEE simulates reasonably well the isotopic composition of soil, stem and leaf water compared to local observations at ten measurement sites. When coupled to LMDZ (Laboratoire de Meteorologie Dynamique-Zoom: the atmospheric model), it simulates well the isotopic composition of precipitation and river water compared to global observations. Sensitivity tests to LSM (Land-Surface Model) parameters are performed to identify processes whose representation by LSMs could be better evaluated using water isotopic measurements. We find that measured vertical variations in soil water isotopes could help evaluate the representation of infiltration pathways by multi-layer soil models. Measured water isotopes in rivers could help calibrate the partitioning of total runoff into surface runoff and drainage and the residence time scales in underground reservoirs. Finally, co-located isotope measurements in precipitation, vapor and soil water could help estimate the partitioning of infiltrating precipitation into bare soil evaporation.

Écoulements hypodermiques et transferts de solutés dans les placages éoliens du Sahel : étude par traçage isotopique et chimique sous pluies simulées

Aeolian microdunes are key elements of the ecology of the Sahel of Burkina Faso. The aim of the study is to observe the flow and magnitude of water and solutes through the microdunes by artificial isotope (oxygen 18) and chemical (chloride) tracing using demineralised water. It is shown that the quantity of sub-surface water flow is of secondary importance to that of surface runoff (about 5 %), although its importance with regard to solute transport is far from negligible (more than 20 %).