Publications (12)48.28 Total impact
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Article: Reducing and correcting for contamination of ecosystem water stable isotopes measured by isotope ratio infrared spectroscopy.
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ABSTRACT: 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.Rapid Communications in Mass Spectrometry 01/2012; 26(2):141-53. · 2.79 Impact Factor -
Article: Concentration effects on laser-based δ18 O and δ2 H measurements and implications for the calibration of vapour measurements with liquid standards.
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ABSTRACT: 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 © 2010 John Wiley & Sons, Ltd.Rapid Communications in Mass Spectrometry 12/2010; 24(24):3553-61. · 2.79 Impact Factor -
Article: Concentration effects on laser‐based δ18O and δ2H measurements and implications for the calibration of vapour measurements with liquid standards
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ABSTRACT: Recently available isotope ratio infrared spectroscopy can directly measure the isotopic composition of atmospheric water vapour (δ18O, δ2H), 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 δ18O and δ2H with water vapour concentration. For δ18O, the slope of this increase was similar for liquid and vapour, with a slight positive relationship with sample δ-value. For δ2H, 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 δ2H 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 © 2010 John Wiley & Sons, Ltd.Rapid Communications in Mass Spectrometry 11/2010; 24(24):3553 - 3561. · 2.79 Impact Factor -
Article: Biotic and abiotic factors affecting the δ 13 C of soil respired CO 2 in a Mediterranean oak woodland
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ABSTRACT: The flux (R s) and carbon isotopic composition (δ 13 C Rs) of soil respired CO 2 was measured every 2 h over the course of three diel cycles in a Mediterranean oak woodland, together with measurements of the δ 13 C composition of leaf, root and soil organic matter (δ 13 C SOM) and metabolites. Simulations of R s and δ 13 C Rs were also made using a numerical model parameterised with the SOM data and assuming short-term production rates were driven mainly by temperature. Average values of δ 13 C Rs over the study period were within the range of root metabolite and average δ 13 C SOM values, but enriched in 13 C relative to the bulk δ 13 C of leaf, litter, and roots and the upper soil organic layers. There was good agreement between model output and observed CO 2 fluxes and the underlying features of δ 13 C Rs . Observed diel variations of 0.5 ‰ in δ 13 C Rs were predicted by the model in response to temperature-related shifts in production rates along a ∼ 3 ‰ gradient observed in the profile of δ 13 C SOM . However, observed δ 13 C Rs varied by over 2 ‰, indicating that both dynamics in soil respiratory metabolism and physical processes can influence short-term variability of δ 13 C Rs .Isotopes in Environmental and Health Studies 01/2010; 45:343-359. · 0.90 Impact Factor -
Article: The impact of soil microorganisms on the global budget of delta18O in atmospheric CO2.
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ABSTRACT: Improved global estimates of terrestrial photosynthesis and respiration are critical for predicting the rate of change in atmospheric CO(2). The oxygen isotopic composition of atmospheric CO(2) can be used to estimate these fluxes because oxygen isotopic exchange between CO(2) and water creates distinct isotopic flux signatures. The enzyme carbonic anhydrase (CA) is known to accelerate this exchange in leaves, but the possibility of CA activity in soils is commonly neglected. Here, we report widespread accelerated soil CO(2) hydration. Exchange was 10-300 times faster than the uncatalyzed rate, consistent with typical population sizes for CA-containing soil microorganisms. Including accelerated soil hydration in global model simulations modifies contributions from soil and foliage to the global CO(18)O budget and eliminates persistent discrepancies existing between model and atmospheric observations. This enhanced soil hydration also increases the differences between the isotopic signatures of photosynthesis and respiration, particularly in the tropics, increasing the precision of CO(2) gross fluxes obtained by using the delta(18)O of atmospheric CO(2) by 50%.Proceedings of the National Academy of Sciences 12/2009; 106(52):22411-5. · 9.68 Impact Factor -
Article: Biotic and abiotic factors affecting the delta(13)C of soil respired CO(2) in a Mediterranean oak woodland.
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ABSTRACT: The flux (R(s)) and carbon isotopic composition (delta(13)C (Rs)) of soil respired CO (2) was measured every 2 h over the course of three diel cycles in a Mediterranean oak woodland, together with measurements of the delta(13)C composition of leaf, root and soil organic matter (delta(13)C (SOM)) and metabolites. Simulations of R(s) and delta(13)C (Rs) were also made using a numerical model parameterised with the SOM data and assuming short-term production rates were driven mainly by temperature. Average values of delta(13)C (Rs) over the study period were within the range of root metabolite and average delta(13)C (SOM) values, but enriched in (13)C relative to the bulk delta(13)C of leaf, litter, and roots and the upper soil organic layers. There was good agreement between model output and observed CO (2) fluxes and the underlying features of delta(13)C (Rs). Observed diel variations of 0.5 per thousand in delta(13)C (Rs) were predicted by the model in response to temperature-related shifts in production rates along a approximately 3 per thousand gradient observed in the profile of delta(13)C (SOM). However, observed delta(13)C (Rs) varied by over 2 per thousand, indicating that both dynamics in soil respiratory metabolism and physical processes can influence short-term variability of delta(13)C (Rs).Isotopes in Environmental and Health Studies 12/2009; 45(4):343-59. · 0.90 Impact Factor -
Article: Carbon isotope fractionation during photorespiration and carboxylation in Senecio.
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ABSTRACT: The magnitude of fractionation during photorespiration and the effect on net photosynthetic (13)C discrimination (Delta) were investigated for three Senecio species, S. squalidus, S. cineraria, and S. greyii. We determined the contributions of different processes during photosynthesis to Delta by comparing observations (Delta(obs)) with discrimination predicted from gas-exchange measurements (Delta(pred)). Photorespiration rates were manipulated by altering the O(2) partial pressure (pO(2)) in the air surrounding the leaves. Contributions from (13)C-depleted photorespiratory CO(2) were largest at high pO(2). The parameters for photorespiratory fractionation (f), net fractionation during carboxylation by Rubisco and phosphoenolpyruvate carboxylase (b), and mesophyll conductance (g(i)) were determined simultaneously for all measurements. Instead of using Delta(obs) data to obtain g(i) and f successively, which requires that b is known, we treated b, f, and g(i) as unknowns. We propose this as an alternative approach to analyze measurements under field conditions when b and g(i) are not known or cannot be determined in separate experiments. Good agreement between modeled and observed Delta was achieved with f = 11.6 per thousand +/- 1.5 per thousand, b = 26.0 per thousand +/- 0.3 per thousand, and g(i) of 0.27 +/- 0.01, 0.25 +/- 0.01, and 0.22 +/- 0.01 mol m(-2) s(-1) for S. squalidus, S. cineraria, and S. greyii, respectively. We estimate that photorespiratory fractionation decreases Delta by about 1.2 per thousand on average under field conditions. In addition, diurnal changes in Delta are likely to reflect variations in photorespiration even at the canopy level. Our results emphasize that the effects of photorespiration must be taken into account when partitioning net CO(2) exchange of ecosystems into gross fluxes of photosynthesis and respiration.Plant physiology 11/2008; 148(4):2013-20. · 6.53 Impact Factor -
Article: To concentrate or ventilate? Carbon acquisition, isotope discrimination and physiological ecology of early land plant life forms.
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ABSTRACT: A comparative study has been made of the photosynthetic physiological ecology and carbon isotope discrimination characteristics for modern-day bryophytes and closely related algal groups. Firstly, the extent of bryophyte distribution and diversification as compared with more advanced land plant groups is considered. Secondly, measurements of instantaneous carbon isotope discrimination (Delta), photosynthetic CO(2) assimilation and electron transport rates were compared during the drying cycles. The extent of surface diffusion limitation (when wetted), internal conductance and water use efficiency (WUE) at optimal tissue water content (TWC) were derived for liverworts and a hornwort from contrasting habitats and with differing degrees of thallus ventilation (as intra-thalline cavities and internal airspaces). We also explore how the operation of a biophysical carbon-concentrating mechanism (CCM) tempers isotope discrimination characteristics in two other hornworts, as well as the green algae Coleochaete orbicularis and Chlamydomonas reinhardtii. The magnitude of Delta was compared for each life form over a drying curve and used to derive the surface liquid-phase conductance (when wetted) and internal conductance (at optimal TWC). The magnitude of external and internal conductances, and WUE, was higher for ventilated, compared with non-ventilated, liverworts and hornworts, but the values were similar within each group, suggesting that both factors have been optimized for each life form. For the hornworts, leakiness of the CCM was highest for Megaceros vincentianus and C. orbicularis (approx. 30%) and, at 5%, lowest in C. reinhardtii grown under ambient CO2 concentrations. Finally, evidence for the operation of a CCM in algae and hornworts is considered in terms of the probable role of the chloroplast pyrenoid, as the origins, structure and function of this enigmatic organelle are explored during the evolution of land plants.Philosophical Transactions of The Royal Society B Biological Sciences 09/2008; 363(1504):2767-78. · 6.40 Impact Factor -
Article: Evaporation and carbonic anhydrase activity recorded in oxygen isotope signatures of net CO2 fluxes from a Mediterranean soil
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ABSTRACT: The oxygen stable isotope composition (δ18O) of CO2 is a valuable tool for studying the gas exchange between terrestrial ecosystems and the atmosphere. In the soil, it records the isotopic signal of water pools subjected to precipitation and evaporation events. The δ18O of the surface soil net CO2 flux is dominated by the physical processes of diffusion of CO2 into and out of the soil and the chemical reactions during CO2–H2O equilibration. Catalytic reactions by the enzyme carbonic anhydrase, reducing CO2 hydration times, have been proposed recently to explain field observations of the δ18O signatures of net soil CO2 fluxes. How important these catalytic reactions are for accurately predicting large-scale biosphere fluxes and partitioning net ecosystem fluxes is currently uncertain because of the lack of field data. In this study, we determined the δ18O signatures of net soil CO2 fluxes from soil chamber measurements in a Mediterranean forest. Over the 3 days of measurements, the observed δ18O signatures of net soil CO2 fluxes became progressively enriched with a well-characterized diurnal cycle. Model simulations indicated that the δ18O signatures recorded the interplay of two effects: (1) progressive enrichment of water in the upper soil by evaporation, and (2) catalytic acceleration of the isotopic exchange between CO2 and soil water, amplifying the contributions of ‘atmospheric invasion’ to net signatures. We conclude that there is a need for better understanding of the role of enzymatic reactions, and hence soil biology, in determining the contributions of soil fluxes to oxygen isotope signals in atmospheric CO2.Global Change Biology 08/2008; 14(9):2178 - 2193. · 6.86 Impact Factor -
Article: Carbon isotopes and water use efficiency: sense and sensitivity.
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ABSTRACT: We revisit the relationship between plant water use efficiency and carbon isotope signatures (delta(13)C) of plant material. Based on the definitions of intrinsic, instantaneous and integrated water use efficiency, we discuss the implications for interpreting delta(13)C data from leaf to landscape levels, and across diurnal to decadal timescales. Previous studies have often applied a simplified, linear relationship between delta(13)C, ratios of intercellular to ambient CO(2) mole fraction (C (i)/C (a)), and water use efficiency. In contrast, photosynthetic (13)C discrimination (Delta) is sensitive to the ratio of the chloroplast to ambient CO(2) mole fraction, C (c)/C (a) (rather than C (i)/C (a)) and, consequently, to mesophyll conductance. Because mesophyll conductance may differ between species and over time, it is not possible to determine C (c)/C (a) from the same gas exchange measurements as C (i)/C (a). On the other hand, water use efficiency at the leaf level depends on evaporative demand, which does not directly affect Delta. Water use efficiency and Delta can thus vary independently, making it difficult to obtain trends in water use efficiency from delta(13)C data. As an alternative approach, we offer a model available at http://carbonisotopes.googlepages.com to explore how water use efficiency and (13)C discrimination are related across leaf and canopy scales. The model provides a tool to investigate whether trends in Delta indicate changes in leaf functional traits and/or environmental conditions during leaf growth, and how they are associated with trends in plant water use efficiency. The model can be used, for example, to examine whether trends in delta(13)C signatures obtained from tree rings imply changes in tree water use efficiency in response to atmospheric CO(2) increase. This is crucial for predicting how plants may respond to future climate change.Oecologia 04/2008; 155(3):441-54. · 3.41 Impact Factor -
Article: Variations in 13C discrimination during CO2 exchange by Picea sitchensis branches in the field.
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ABSTRACT: We report diurnal variations in (13)C discrimination ((13)Delta) of Picea sitchensis (Bong.) Carr. branches measured in the field using a branch chamber technique. The observations were compared to predicted (13)Delta based on concurrent measurements of branch gas exchange. Observed (13)Delta values were described well by the classical model of (13)Delta including isotope effects during photorespiration, day respiration and CO(2) transfer through a series of resistances to the sites of carboxylation. A simplified linear of model (13)Delta did not capture the observed diurnal variability. At dawn and dusk, we measured very high (13)Delta values that were not predicted by either of the said models. Exploring the sensitivity of (13)Delta to possible respiratory isotope effects, we conclude that isotopic disequilibria between the gross fluxes of photosynthesis and day respiration can explain the high observed (13)Delta values during net photosynthetic gas exchange. Based on the classical model, a revised formulation incorporating an isotopically distinct substrate for day respiration was able to account well for the high observed dawn and dusk (13)Delta values.Plant Cell and Environment 06/2007; 30(5):600-16. · 5.22 Impact Factor -
Article: The impact of soil microorganisms on the global budget of δ18O in atmospheric CO2
[show abstract] [hide abstract]
ABSTRACT: Improved global estimates of terrestrial photosynthesis and respiration are critical for predicting the rate of change in atmospheric CO2. The oxygen isotopic composition of atmospheric CO2 can be used to estimate these fluxes because oxygen isotopic exchange between CO2 and water creates distinct isotopic flux signatures. The enzyme carbonic anhydrase (CA) is known to accelerate this exchange in leaves, but the possibility of CA activity in soils is commonly neglected. Here, we report widespread accelerated soil CO2 hydration. Exchange was 10–300 times faster than the uncatalyzed rate, consistent with typical population sizes for CA-containing soil microorganisms. Including accelerated soil hydration in global model simulations modifies contributions from soil and foliage to the global CO18O budget and eliminates persistent discrepancies existing between model and atmospheric observations. This enhanced soil hydration also increases the differences between the isotopic signatures of photosynthesis and respiration, particularly in the tropics, increasing the precision of CO2 gross fluxes obtained by using the δ18O of atmospheric CO2 by 50%.
Top co-authors
Top Journals
Institutions
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2009–2012
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French National Centre for Scientific Research
Lyon, Rhone-Alpes, France -
Université Pierre et Marie Curie Paris 6
Paris, Ile-de-France, France
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2010
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University of California, Los Angeles
- Department of Atmospheric and Oceanic Sciences (AOS)
Los Angeles, CA, USA
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