The spatial variations in the stable carbon isotope composition (d 13 C) of air and leaves (total... more The spatial variations in the stable carbon isotope composition (d 13 C) of air and leaves (total matter and soluble sugars) were quantified within the crown of a well-watered, 20-year-old walnut tree growing in a low-density orchard. The observed leaf carbon isotope discrimination (D) was compared with that computed by a three-dimensional model simulating the intracanopy distribution of irradiance, transpiration and photosynthesis (previously parameterized and tested for the same tree canopy) coupled to a biophysically based model of carbon isotope discrimination. The importance of discrimination associated with CO2 gradients encountered from the substomatal sites to the carboxylation sites was evaluated. We also assessed by simulation the effect of current irradiance on leaf gas exchange and the effect of long-term acclimation of photosynthetic capacity and stomatal and internal conductances to light regime on intracanopy gradients in D. The main conclusions of this study are: (i) leaf D can exhibit important variations (5 and 8‰ in total leaf material and soluble sugars, respectively) along light gradients within the foliage of an isolated tree; (ii) internal conductance must be taken into account to adequately predict leaf D , and (iii) the spatial variations in D and water-use efficiency resulted from the short-term response of leaf gas exchange to variations in local irradiance and, to a much lesser extent, from the long-term acclimation of leaf characteristics to the local light regime.
Concern exists about the suitability of laser spectroscopic instruments for the measurement of th... more 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.
Rainfall recycling by evapotranspiration from continental surfaces is certainly the most unknown ... more Rainfall recycling by evapotranspiration from continental surfaces is certainly the most unknown component of the global water cycle. This is due to the large variability of rainfall as well as the heterogeneity of these continental surfaces, both in time and space. Traditional measuring methods such as sap flow, micro lysimeter, water and energy balance estimation (Bowen ratio, eddy correlation) have been used since the 70s for a monitoring of real evapotranspiration fluxes over crops and others plant covers. A complementary method consists in using isotopic biogeochemistry. When making specific hypothesis, it is possible to identify and quantify the different sources of the atmospheric water vapour (vegetation and soil at different scales). Analysis of the heavy stable isotopic ratios of water in both liquid and vapour phases: 18O and 2H can allow determining the history of the water in the soil since the last rainfall event (infiltration, re-evaporation) or the root extraction de...
Recently available laser instruments can directly measure the isotopic composition of water vapou... more Recently available laser instruments can directly measure the isotopic composition of water vapour (delta18O, delta2H) in air. Here, we evaluate the calibration of a wavelength-scanned cavity ring-down spectrometer (CRDS) 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
The response of plants to increasing atmospheric CO2 concentrations will have an important influe... more The response of plants to increasing atmospheric CO2 concentrations will have an important influence on biogeochemical cycles in the future. These responses are physiologically driven, but have important feedbacks to water and nutrient cycles as the plants adjust photosynthetic activity within the constraints of these other cycles. Leaf scale manipulations of CO2 concentration provide us with a wealth of information on the short-term, biochemical response of leaf photosynthesis, but these responses do not translate to whole plant responses under sustained growth at higher CO2 concentrations, as has been evidenced by free air enrichment studies. Here we report on a study into whole plant responses to CO2 concentration using a new, 10m3 isotope-biogeochemistry growth chamber housing small (1.5m) potted Pinus nigra trees under controlled conditions. This chamber is capable of controlling the climatic and CO2 conditions and designed for sampling biogeochemical pools for isotopic analysi...
ABSTRACT The contribution of diffusion processes and leaf temperature variations to the spatial d... more ABSTRACT The contribution of diffusion processes and leaf temperature variations to the spatial distribution of leaf water isotopes.
Sustainable use of Genetic Diversity in Forage and Turf Breeding, 2010
ABSTRACT Depending on root depth and the vertical distribution of water, the soil contributes to ... more ABSTRACT Depending on root depth and the vertical distribution of water, the soil contributes to satisfy the transpiration demand of grass crops. Using soil humidity measurements repeated in time at different depth, the depth of water extraction (DWE) of a species can be measured in pure swards. However, that may be irrelevant in mixed swards because (i) roots of all species can be found in all horizons and (ii) the relationship between root density, soil humidity and water extraction is complex. To date, the use of natural abundance of 18O is the only way to precisely compare the DWE of plants sharing the same ground. The work reported here describes the results obtained in a mixture of Dactylis glomerata, Lolium perenne, Festuca arundinacea grown in dense sward and believed to exhibit various abilities to exploit soil resources. Pure stands were also analysed and compared to the mixed crop. The results showed that when the soil profile exhibited a monotonous gradient of natural 18O abundance in water, the ranking of the DWE of three species was possible and explained the differences in plant water status observed in summer. KeywordsPerennial ryegrass-Tall fescue-Cocksfoot-Water use-Isotopic signature- 18O
The response of plants to increasing atmospheric CO2 concentrations is a key element shaping futu... more The response of plants to increasing atmospheric CO2 concentrations is a key element shaping future biogeochemical cycles. While leaf scale manipulations of CO2 concentration provide us with a wealth of information on the biochemical response of leaf photosynthesis, these responses do not necessarily translate to whole plant responses at higher CO2 concentrations. Evidence from free air enrichment studies show different responses of plant gas exchange parameters in their degree of acclimation to long-term increases in atmospheric CO2, relative to those predicted from the instantaneous responses of leaf-level measurements. However, free air enrichment studies are also limited by the fact that they represent a single step change in CO2 and it is therefore of interest to understand how physiological responses derived from leaf-scale increases in CO2 compare to those from exposing the whole plant to increases CO2 across a range of elevated CO2 concentrations. Here we report on a study i...
The spatial variations in the stable carbon isotope composition ( d d d d 13 C) of air and leaves... more The spatial variations in the stable carbon isotope composition ( d d d d 13 C) of air and leaves (total matter and soluble sugars) were quantified within the crown of a well-watered, 20year-old walnut tree growing in a low-density orchard. The observed leaf carbon isotope discrimination ( D D D D ) was compared with that computed by a three-dimensional model simulating the intracanopy distribution of irradiance, transpiration and photosynthesis (previously parameterized and tested for the same tree canopy) coupled to a biophysically based model of carbon isotope discrimination. The importance of discrimination associated with CO 2 gradients encountered from the substomatal sites to the carboxylation sites was evaluated. We also assessed by simulation the effect of current irradiance on leaf gas exchange and the effect of long-term acclimation of photosynthetic capacity and stomatal and internal conductances to light regime on intracanopy gradients in D D D D . The main conclusions of this study are: (i) leaf D D D D can exhibit important variations (5 and 8‰ in total leaf material and soluble sugars, respectively) along light gradients within the foliage of an isolated tree; (ii) internal conductance must be taken into account to adequately predict leaf D D D D , and (iii) the spatial variations in D D D D
This article defends the appropriateness of tobacco-free school policies as an effective tool tow... more This article defends the appropriateness of tobacco-free school policies as an effective tool toward ensuring young people develop into healthy and intellectually strong adults, and demonstrates how such a policy can be introduced into a school district. Health education efforts to eliminate tobacco use are widely considered more effective when carried out in concert with school policies and adult role models offering the consistent message that tobacco use is unhealthy and unacceptable. Studies indicate students who attend schools with strict smoking policies are less likely to begin smoking than students who attend schools without such policies. Through research, support, and guidance, the Colorado Tobacco-Free Schools and Communities Project successfully has assisted almost half the 176 school districts in Colorado to adopt such policies.
Stable water isotopes are tracers of water movement within the soil-vegetation-atmosphere system.... more Stable water isotopes are tracers of water movement within the soil-vegetation-atmosphere system. They have the potential for a better understanding of water vapour transport within soils, evaporation and transpiration processes. To better understand those potentialities and possible lack of knowledge, a coupled heat-water and stable isotope transport model, called SiSPAT-Isotope was developed for bare soil. We presented the theoretical basis of the model in the first part of the paper, including a first validation of the likelihood of model results and a comparison with existing analytical solutions. In this companion paper, we go a step further by comparing the model results with two data sets collected on laboratory columns. In both cases, five soil columns were saturated and let drying during 173 and 253 days, respectively. At selected dates, one of the column was cut into slices and analysed to determine the volumetric water content, the deuterium and oxygen 18 concentrations profiles. The first data set was acquired on disturbed soil columns. The second one was collected on non-disturbed soil columns and it included a complete monitoring of atmospheric variables. It was not the case for the first one and a sensitivity analysis of model results to the air humidity was performed, showing its large influence on surface isotope concentrations. For both data sets, we also conducted a sensitivity analysis to the formulation of the kinetic fractionation factor, conditioning the resistance to isotope transport between the soil surface and the atmosphere, and to the value of soil tortuosity. The results showed that the model was able to reproduce the behaviour of the observed concentration profiles. A fair agreement between measured and calculated values was obtained for all profiles for the disturbed soil. Near surface concentrations were in general overestimated for the undisturbed soil, raising the question of possible influence of immobile water on concentrations values. We showed that soil tortuosity was mostly influential on the depth of the peak isotope concentration, which opens perspectives for its retrieval from the measurement of isotope concentration profiles. When only molecular diffusion was considered, the model was not able to 0022-1694/$ -see front matter q 2005 Published by Elsevier B.V. (I. Braud).
In this study, we performed a partitioning of evapotranspiration (ET) under fully controlled cond... more In this study, we performed a partitioning of evapotranspiration (ET) under fully controlled conditions (climatic chamber) along growth of a tall fescue cover (Festuca arundinacea) into soil evaporation (Ev) and plant transpiration (Tr) by measuring their stable oxygen isotopic compositions (υ ET , υ Ev and υ Tr ). We showed that it was possible, under the chamber's particular conditions, to realize the partition without (1) making the hypothesis of steady state transpiration usually done in the field, nor (2) calculating υ Ev as a function of air relative humidity, soil water and atmospheric vapour isotopic compositions. The contribution of Ev to total ET decreased over the experiment from 100% (bare soil) to 94% [16 days after the seeding (DAS), 83% (28 DAS), 70% (36 DAS) and 5% (43 DAS)].
1] Stable CO 2 isotope measurements are increasingly used to partition the net CO 2 exchange betw... more 1] Stable CO 2 isotope measurements are increasingly used to partition the net CO 2 exchange between terrestrial ecosystems and the atmosphere in terms of nonfoliar respiration (F R ) and net photosynthesis (F A ) in order to better understand the variations of this exchange. However, the accuracy of the partitioning strongly depends on the isotopic disequilibrium between these two gross fluxes, and a rigorous estimation of the errors on F A and F R is needed. In this study, we account for and propagate uncertainties on all terms in the mass balance and isotopic mass balance equations for CO 2 in order to get accurate estimates of the errors on F A and F R . We apply our method to a maritime pine forest in the southwest of France. Nighttime Keeling plots are used to estimate the 13 C and 18 O isotopic signature of F R (d R ), and for both isotopes the a priori uncertainty associated with this term is estimated to be around 2% at our site. Using d 13 C-CO 2 and [CO 2 ] measurements, we then show that the uncertainty on instantaneous values of F A and F R can be as large as 4 mmol m À2 s À1 . Even if we could get more accurate estimates of the net CO 2 flux, the isoflux, and the isotopic signatures of F A and F R , this uncertainty would not be significantly reduced because the isotopic disequilibrium between F A and F R is too small, around 2-3%. With d 18 O-CO 2 and [CO 2 ] measurements the uncertainty associated with the gross fluxes lies also around 4 mmol m À2 s À1 but could be dramatically reduced if we were able to get more accurate estimates of the CO 18 O isoflux and the associated discrimination during photosynthesis. This is because the isotopic disequilibrium between F A and F R is large, of the order of 12-17%. The isotopic disequilibrium between F A and F R and the uncertainty on d R vary among ecosystems and over the year. Our approach should help to choose the best strategy to study the carbon budget of a given ecosystem using stable isotopes.
The depth of water extraction is a key trait in grassland species. For comparing individual plant... more The depth of water extraction is a key trait in grassland species. For comparing individual plant forms in the field, different ecological studies have proven the efficiency of the isotopic signal of water (δ 18 O). To date, there has been no assessment of the aforementioned method within closely related grasslands species. For this study, Festuca arundinacea and Lolium multiflorum plants were grown in nutrient solution and the δ 18 O of different plant compartments were compared to the δ 18 O of the solution under both high and low transpiration conditions. In both species, the basal part of the tillers, excluding the eldest sheath, closely reflected the isotopic water composition of the nutrient solution, whereas other parts of the plant exhibited a significant enrichment in 18 O as a consequence of transpiration. In a second experiment, using the genetic variability within plants obtained from crossing F. arundinacea with Lolium multiflorum, the study explored the variability of the depth of water extraction and δ 18 O under conditions of soil water deficit. A plant nursery of hybrids was established in a greenhouse with plants sewn directly in the soil. Ten neutron probe access tubes were placed close to ten individual plants, which were sampled twice at 38 day intervals, following a period of 14 and 52 days of water deprivation. On these same dates, soil samples were extracted down to 1 meter in the vicinity of the 10 studied plants. Water was fully extracted from both plant and soil samples and the isotopic composition was measured. The predawn leaf water potential exhibited large variations in water status among the ten plants studied. The measured plant δ 18 O were similar to the values computed from estimated soil water extraction profiles (using neutron probe measurements) and soil δ 18 O of the soil water profile. On the second date of sampling, the δ 18 O and leaf water potentials of three individual plants (L. multiflorum) were observed throughout the day, revealing a gradual decrease in 18 O content as the plant water potential decreased. The role of short term changes in the soil-root hydraulic conductance on depth of water extraction is discussed in this study.
The spatial variations in the stable carbon isotope composition (d 13 C) of air and leaves (total... more The spatial variations in the stable carbon isotope composition (d 13 C) of air and leaves (total matter and soluble sugars) were quantified within the crown of a well-watered, 20-year-old walnut tree growing in a low-density orchard. The observed leaf carbon isotope discrimination (D) was compared with that computed by a three-dimensional model simulating the intracanopy distribution of irradiance, transpiration and photosynthesis (previously parameterized and tested for the same tree canopy) coupled to a biophysically based model of carbon isotope discrimination. The importance of discrimination associated with CO2 gradients encountered from the substomatal sites to the carboxylation sites was evaluated. We also assessed by simulation the effect of current irradiance on leaf gas exchange and the effect of long-term acclimation of photosynthetic capacity and stomatal and internal conductances to light regime on intracanopy gradients in D. The main conclusions of this study are: (i) leaf D can exhibit important variations (5 and 8‰ in total leaf material and soluble sugars, respectively) along light gradients within the foliage of an isolated tree; (ii) internal conductance must be taken into account to adequately predict leaf D , and (iii) the spatial variations in D and water-use efficiency resulted from the short-term response of leaf gas exchange to variations in local irradiance and, to a much lesser extent, from the long-term acclimation of leaf characteristics to the local light regime.
Concern exists about the suitability of laser spectroscopic instruments for the measurement of th... more 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.
Rainfall recycling by evapotranspiration from continental surfaces is certainly the most unknown ... more Rainfall recycling by evapotranspiration from continental surfaces is certainly the most unknown component of the global water cycle. This is due to the large variability of rainfall as well as the heterogeneity of these continental surfaces, both in time and space. Traditional measuring methods such as sap flow, micro lysimeter, water and energy balance estimation (Bowen ratio, eddy correlation) have been used since the 70s for a monitoring of real evapotranspiration fluxes over crops and others plant covers. A complementary method consists in using isotopic biogeochemistry. When making specific hypothesis, it is possible to identify and quantify the different sources of the atmospheric water vapour (vegetation and soil at different scales). Analysis of the heavy stable isotopic ratios of water in both liquid and vapour phases: 18O and 2H can allow determining the history of the water in the soil since the last rainfall event (infiltration, re-evaporation) or the root extraction de...
Recently available laser instruments can directly measure the isotopic composition of water vapou... more Recently available laser instruments can directly measure the isotopic composition of water vapour (delta18O, delta2H) in air. Here, we evaluate the calibration of a wavelength-scanned cavity ring-down spectrometer (CRDS) 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
The response of plants to increasing atmospheric CO2 concentrations will have an important influe... more The response of plants to increasing atmospheric CO2 concentrations will have an important influence on biogeochemical cycles in the future. These responses are physiologically driven, but have important feedbacks to water and nutrient cycles as the plants adjust photosynthetic activity within the constraints of these other cycles. Leaf scale manipulations of CO2 concentration provide us with a wealth of information on the short-term, biochemical response of leaf photosynthesis, but these responses do not translate to whole plant responses under sustained growth at higher CO2 concentrations, as has been evidenced by free air enrichment studies. Here we report on a study into whole plant responses to CO2 concentration using a new, 10m3 isotope-biogeochemistry growth chamber housing small (1.5m) potted Pinus nigra trees under controlled conditions. This chamber is capable of controlling the climatic and CO2 conditions and designed for sampling biogeochemical pools for isotopic analysi...
ABSTRACT The contribution of diffusion processes and leaf temperature variations to the spatial d... more ABSTRACT The contribution of diffusion processes and leaf temperature variations to the spatial distribution of leaf water isotopes.
Sustainable use of Genetic Diversity in Forage and Turf Breeding, 2010
ABSTRACT Depending on root depth and the vertical distribution of water, the soil contributes to ... more ABSTRACT Depending on root depth and the vertical distribution of water, the soil contributes to satisfy the transpiration demand of grass crops. Using soil humidity measurements repeated in time at different depth, the depth of water extraction (DWE) of a species can be measured in pure swards. However, that may be irrelevant in mixed swards because (i) roots of all species can be found in all horizons and (ii) the relationship between root density, soil humidity and water extraction is complex. To date, the use of natural abundance of 18O is the only way to precisely compare the DWE of plants sharing the same ground. The work reported here describes the results obtained in a mixture of Dactylis glomerata, Lolium perenne, Festuca arundinacea grown in dense sward and believed to exhibit various abilities to exploit soil resources. Pure stands were also analysed and compared to the mixed crop. The results showed that when the soil profile exhibited a monotonous gradient of natural 18O abundance in water, the ranking of the DWE of three species was possible and explained the differences in plant water status observed in summer. KeywordsPerennial ryegrass-Tall fescue-Cocksfoot-Water use-Isotopic signature- 18O
The response of plants to increasing atmospheric CO2 concentrations is a key element shaping futu... more The response of plants to increasing atmospheric CO2 concentrations is a key element shaping future biogeochemical cycles. While leaf scale manipulations of CO2 concentration provide us with a wealth of information on the biochemical response of leaf photosynthesis, these responses do not necessarily translate to whole plant responses at higher CO2 concentrations. Evidence from free air enrichment studies show different responses of plant gas exchange parameters in their degree of acclimation to long-term increases in atmospheric CO2, relative to those predicted from the instantaneous responses of leaf-level measurements. However, free air enrichment studies are also limited by the fact that they represent a single step change in CO2 and it is therefore of interest to understand how physiological responses derived from leaf-scale increases in CO2 compare to those from exposing the whole plant to increases CO2 across a range of elevated CO2 concentrations. Here we report on a study i...
The spatial variations in the stable carbon isotope composition ( d d d d 13 C) of air and leaves... more The spatial variations in the stable carbon isotope composition ( d d d d 13 C) of air and leaves (total matter and soluble sugars) were quantified within the crown of a well-watered, 20year-old walnut tree growing in a low-density orchard. The observed leaf carbon isotope discrimination ( D D D D ) was compared with that computed by a three-dimensional model simulating the intracanopy distribution of irradiance, transpiration and photosynthesis (previously parameterized and tested for the same tree canopy) coupled to a biophysically based model of carbon isotope discrimination. The importance of discrimination associated with CO 2 gradients encountered from the substomatal sites to the carboxylation sites was evaluated. We also assessed by simulation the effect of current irradiance on leaf gas exchange and the effect of long-term acclimation of photosynthetic capacity and stomatal and internal conductances to light regime on intracanopy gradients in D D D D . The main conclusions of this study are: (i) leaf D D D D can exhibit important variations (5 and 8‰ in total leaf material and soluble sugars, respectively) along light gradients within the foliage of an isolated tree; (ii) internal conductance must be taken into account to adequately predict leaf D D D D , and (iii) the spatial variations in D D D D
This article defends the appropriateness of tobacco-free school policies as an effective tool tow... more This article defends the appropriateness of tobacco-free school policies as an effective tool toward ensuring young people develop into healthy and intellectually strong adults, and demonstrates how such a policy can be introduced into a school district. Health education efforts to eliminate tobacco use are widely considered more effective when carried out in concert with school policies and adult role models offering the consistent message that tobacco use is unhealthy and unacceptable. Studies indicate students who attend schools with strict smoking policies are less likely to begin smoking than students who attend schools without such policies. Through research, support, and guidance, the Colorado Tobacco-Free Schools and Communities Project successfully has assisted almost half the 176 school districts in Colorado to adopt such policies.
Stable water isotopes are tracers of water movement within the soil-vegetation-atmosphere system.... more Stable water isotopes are tracers of water movement within the soil-vegetation-atmosphere system. They have the potential for a better understanding of water vapour transport within soils, evaporation and transpiration processes. To better understand those potentialities and possible lack of knowledge, a coupled heat-water and stable isotope transport model, called SiSPAT-Isotope was developed for bare soil. We presented the theoretical basis of the model in the first part of the paper, including a first validation of the likelihood of model results and a comparison with existing analytical solutions. In this companion paper, we go a step further by comparing the model results with two data sets collected on laboratory columns. In both cases, five soil columns were saturated and let drying during 173 and 253 days, respectively. At selected dates, one of the column was cut into slices and analysed to determine the volumetric water content, the deuterium and oxygen 18 concentrations profiles. The first data set was acquired on disturbed soil columns. The second one was collected on non-disturbed soil columns and it included a complete monitoring of atmospheric variables. It was not the case for the first one and a sensitivity analysis of model results to the air humidity was performed, showing its large influence on surface isotope concentrations. For both data sets, we also conducted a sensitivity analysis to the formulation of the kinetic fractionation factor, conditioning the resistance to isotope transport between the soil surface and the atmosphere, and to the value of soil tortuosity. The results showed that the model was able to reproduce the behaviour of the observed concentration profiles. A fair agreement between measured and calculated values was obtained for all profiles for the disturbed soil. Near surface concentrations were in general overestimated for the undisturbed soil, raising the question of possible influence of immobile water on concentrations values. We showed that soil tortuosity was mostly influential on the depth of the peak isotope concentration, which opens perspectives for its retrieval from the measurement of isotope concentration profiles. When only molecular diffusion was considered, the model was not able to 0022-1694/$ -see front matter q 2005 Published by Elsevier B.V. (I. Braud).
In this study, we performed a partitioning of evapotranspiration (ET) under fully controlled cond... more In this study, we performed a partitioning of evapotranspiration (ET) under fully controlled conditions (climatic chamber) along growth of a tall fescue cover (Festuca arundinacea) into soil evaporation (Ev) and plant transpiration (Tr) by measuring their stable oxygen isotopic compositions (υ ET , υ Ev and υ Tr ). We showed that it was possible, under the chamber's particular conditions, to realize the partition without (1) making the hypothesis of steady state transpiration usually done in the field, nor (2) calculating υ Ev as a function of air relative humidity, soil water and atmospheric vapour isotopic compositions. The contribution of Ev to total ET decreased over the experiment from 100% (bare soil) to 94% [16 days after the seeding (DAS), 83% (28 DAS), 70% (36 DAS) and 5% (43 DAS)].
1] Stable CO 2 isotope measurements are increasingly used to partition the net CO 2 exchange betw... more 1] Stable CO 2 isotope measurements are increasingly used to partition the net CO 2 exchange between terrestrial ecosystems and the atmosphere in terms of nonfoliar respiration (F R ) and net photosynthesis (F A ) in order to better understand the variations of this exchange. However, the accuracy of the partitioning strongly depends on the isotopic disequilibrium between these two gross fluxes, and a rigorous estimation of the errors on F A and F R is needed. In this study, we account for and propagate uncertainties on all terms in the mass balance and isotopic mass balance equations for CO 2 in order to get accurate estimates of the errors on F A and F R . We apply our method to a maritime pine forest in the southwest of France. Nighttime Keeling plots are used to estimate the 13 C and 18 O isotopic signature of F R (d R ), and for both isotopes the a priori uncertainty associated with this term is estimated to be around 2% at our site. Using d 13 C-CO 2 and [CO 2 ] measurements, we then show that the uncertainty on instantaneous values of F A and F R can be as large as 4 mmol m À2 s À1 . Even if we could get more accurate estimates of the net CO 2 flux, the isoflux, and the isotopic signatures of F A and F R , this uncertainty would not be significantly reduced because the isotopic disequilibrium between F A and F R is too small, around 2-3%. With d 18 O-CO 2 and [CO 2 ] measurements the uncertainty associated with the gross fluxes lies also around 4 mmol m À2 s À1 but could be dramatically reduced if we were able to get more accurate estimates of the CO 18 O isoflux and the associated discrimination during photosynthesis. This is because the isotopic disequilibrium between F A and F R is large, of the order of 12-17%. The isotopic disequilibrium between F A and F R and the uncertainty on d R vary among ecosystems and over the year. Our approach should help to choose the best strategy to study the carbon budget of a given ecosystem using stable isotopes.
The depth of water extraction is a key trait in grassland species. For comparing individual plant... more The depth of water extraction is a key trait in grassland species. For comparing individual plant forms in the field, different ecological studies have proven the efficiency of the isotopic signal of water (δ 18 O). To date, there has been no assessment of the aforementioned method within closely related grasslands species. For this study, Festuca arundinacea and Lolium multiflorum plants were grown in nutrient solution and the δ 18 O of different plant compartments were compared to the δ 18 O of the solution under both high and low transpiration conditions. In both species, the basal part of the tillers, excluding the eldest sheath, closely reflected the isotopic water composition of the nutrient solution, whereas other parts of the plant exhibited a significant enrichment in 18 O as a consequence of transpiration. In a second experiment, using the genetic variability within plants obtained from crossing F. arundinacea with Lolium multiflorum, the study explored the variability of the depth of water extraction and δ 18 O under conditions of soil water deficit. A plant nursery of hybrids was established in a greenhouse with plants sewn directly in the soil. Ten neutron probe access tubes were placed close to ten individual plants, which were sampled twice at 38 day intervals, following a period of 14 and 52 days of water deprivation. On these same dates, soil samples were extracted down to 1 meter in the vicinity of the 10 studied plants. Water was fully extracted from both plant and soil samples and the isotopic composition was measured. The predawn leaf water potential exhibited large variations in water status among the ten plants studied. The measured plant δ 18 O were similar to the values computed from estimated soil water extraction profiles (using neutron probe measurements) and soil δ 18 O of the soil water profile. On the second date of sampling, the δ 18 O and leaf water potentials of three individual plants (L. multiflorum) were observed throughout the day, revealing a gradual decrease in 18 O content as the plant water potential decreased. The role of short term changes in the soil-root hydraulic conductance on depth of water extraction is discussed in this study.
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