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1.
In this paper we describe measurements and modeling of 18O in CO2 and H2O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the δ18O value of leaf water, depth‐resolved soil water, atmospheric water vapor, and Keeling plot δ18O intercepts for net soil‐surface CO2 and ecosystem CO2 and H2O fluxes during three periods of the 2000 growing season. Daytime discrimination against C18OO, as calculated from measured above‐canopy CO2 and δ18O gradients, is also presented. To interpret the isotope measurements, we applied an integrated land‐surface and isotope model (ISOLSM) that simulates ecosystem H218O and C18OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the δ18O value of net ecosystem CO2 and H2O fluxes. Simulations indicate that incomplete equilibration between CO2 and H2O within C4 plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the δ18O value of above‐canopy vapor had a small impact on the predicted δ18O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the δ18O value of above‐canopy CO2 substantially affected the predicted ecosystem discrimination. Leaves dominate the ecosystem 18O‐isoflux in CO2 during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C18OO fluxes requires accurate predictions of both soil and leaf 18O‐isofluxes.  相似文献   

2.
In this paper, we present an integrated account of the diurnal variation in the stable isotopes of water (δD and δ18O) and dry matter (δ15N, δ13C, and δ18O) in the long‐distance transport fluids (xylem sap and phloem sap), leaves, pod walls, and seeds of Lupinus angustifolius under field conditions in Western Australia. The δD and δ18O of leaf water showed a pronounced diurnal variation, ranging from early morning minima near 0‰ for both δD and δ18O to early afternoon maxima of 62 and 23‰, respectively. Xylem sap water showed no diurnal variation in isotopic composition and had mean values of ?13·2 and ?2·3‰ for δD and δ18O. Phloem sap water collected from pod tips was intermediate in isotopic composition between xylem sap and leaf water and exhibited only a moderate diurnal fluctuation. Isotopic compositions of pod wall and seed water were intermediate between those of phloem and xylem sap water. A model of average leaf water enrichment in the steady state (Craig & Gordon, pp. 9–130 in Proceedings of a Conference on Stable Isotopes in Oceanographic Studies and Palaeotemperatures, Lischi and Figli, Pisa, Italy, 1965; Dongmann et al., Radiation and Environmental Biophysics 11, 41–52, 1974; Farquhar & Lloyd, pp. 47–70 in Stable Isotopes and Plant Carbon–Water Relations, Academic Press, San Diego, CA, USA, 1993) agreed closely with observed leaf water enrichment in the morning and early afternoon, but poorly during the night. A modified model taking into account non‐steady‐state effects (Farquhar and Cernusak, unpublished) gave better predictions of observed leaf water enrichments over a full diurnal cycle. The δ15N, δ13C, and δ18O of dry matter varied appreciably among components. Dry matter δ15N was highest in xylem sap and lowest in leaves, whereas dry matter δ13C was lowest in leaves and highest in phloem sap and seeds, and dry matter δ18O was lowest in leaves and highest in pod walls. Phloem sap, leaf, and fruit dry matter δ18O varied diurnally, as did phloem sap dry matter δ13C. These results demonstrate the importance of considering the non‐steady‐state when modelling biological fractionation of stable isotopes in the natural environment.  相似文献   

3.
Changes in the 2H and 18O of atmospheric water vapour provide information for integrating aspects of gas exchange within forest canopies. In this study, we show that diurnal fluctuations in the oxygen isotope ratio (δ18O) as high as 4‰ were observed for water vapour (δ18Ovp) above and within an old‐growth coniferous forest in the Pacific Northwest region of the United States. Values of δ18Ovp decreased in the morning, reached a minimum at midday, and recovered to early‐morning values in the late afternoon, creating a nearly symmetrical diurnal pattern for two consecutive summer days. A mass balance budget was derived and assessed for the 18O of canopy water vapour over a 2‐d period by considering the 18O‐isoflux of canopy transpiration, soil evaporation and the air entering the canopy column. The budget was used to address two questions: (1) do δ18O values of canopy water vapour reflect the biospheric influence, or are such signals swamped by atmospheric mixing? and (2) what mechanisms drive temporal variations of δ18Ovp? Model calculations show that the entry of air into the canopy column resulted in an isotopically depleted 18O‐isoflux in the morning of day 1, causing values of δ18Ovp to decrease. An isotopically enriched 18O‐isoflux resulting from transpiration then offset this decreased δ18Ovp later during the day. Contributions of 18O‐isoflux from soil evaporation were relatively small on day 1 but were more significant on day 2, despite the small H216O fluxes. From measurements of leaf water volume and sapflux, we determined the turnover time of leaf water in the needles of Douglas‐fir trees as ≈ 11 h at midday. Such an extended turnover time suggests that transpiration may not have occurred at the commonly assumed isotopic steady state. We tested a non‐steady state model for predicting δ18O of leaf water. Our model calculations show that assuming isotopic steady state increased isoflux of transpiration. The impact of this increase on the modelled δ 18Ovp was clearly detectable, suggesting the importance of considering isotopic non‐steady state of transpiration in studies of forest 18O water balance.  相似文献   

4.
The 18O signature of atmospheric water vapour (δ18OV) is known to be transferred via leaf water to assimilates. It remains, however, unclear how the 18O-signal transfer differs among plant species and growth forms. We performed a 9-hr greenhouse fog experiment (relative humidity ≥ 98%) with 18O-depleted water vapour (−106.7‰) on 140 plant species of eight different growth forms during daytime. We quantified the 18O-signal transfer by calculating the mean residence time of O in leaf water (MRTLW) and sugars (MRTSugars) and related it to leaf traits and physiological drivers. MRTLW increased with leaf succulence and thickness, varying between 1.4 and 10.8 hr. MRTSugars was shorter in C3 and C4 plants than in crassulacean acid metabolism (CAM) plants and highly variable among species and growth forms; MRTSugars was shortest for grasses and aquatic plants, intermediate for broadleaf trees, shrubs, and herbs, and longest for conifers, epiphytes, and succulents. Sucrose was more sensitive to δ18OV variations than other assimilates. Our comprehensive study shows that plant species and growth forms vary strongly in their sensitivity to δ18OV variations, which is important for the interpretation of δ18O values in plant organic material and compounds and thus for the reconstruction of climatic conditions and plant functional responses.  相似文献   

5.
The Craig-Gordon evaporative enrichment model of the hydrogen (δD) and oxygen (δ18O) isotopes of water was tested in a controlled-environment gas exchange cuvette over a wide range (400‰ δD and 40‰ δ18O) of leaf waters. (Throughout this paper we use the term “leaf water” to describe the site of evaporation, which should not be confused with “bulk leaf water” a term used exclusively for uncorrected measurements obtained from whole leaf water extractions.) Regardless of how the isotopic composition of leaf water was achieved (i.e. by changes in source water, atmospheric vapor δD or δ18O, vapor pressure gradients, or combinations of all three), a modified version of the Craig-Gordon model was shown to be sound in its ability to predict the δD and δ18O values of water at the site of evaporation. The isotopic composition of atmospheric vapor was shown to have profound effects on the δD and δ18O of leaf water and its influence was dependent on vapor pressure gradients. These results have implications for conditions in which the isotopic composition of atmospheric vapor is not in equilibrium with source water, such as experimental systems that grow plants under isotopically enriched water regimes. The assumptions of steady state were also tested and found not to be a major limitation for the utilization of the leaf water model under relatively stable environmental conditions. After a major perturbation in the δD and δ18O of atmospheric vapor, the leaf reached steady state in approximately 2 h, depending on vapor pressure gradients. Following a step change in source water, the leaf achieved steady state in 24 h, with the vast majority of changes occurring in the first 3 h. Therefore, the Craig-Gordon model is a useful tool for understanding the environmental factors that influence the hydrogen and oxygen isotopic composition of leaf water as well as the organic matter derived from leaf water.  相似文献   

6.
Silvicultural thinning usually improves the water status of remaining trees in water‐limited forests. We evaluated the usefulness of a dual stable isotope approach (δ13C, δ18O) for comparing the physiological performance of remaining trees between forest stands subjected to two different thinning intensities (moderate versus heavy) in a 60‐year‐old Pinus halepensis Mill. plantation in semiarid southeastern Spain. We measured bulk leaf δ13C and δ18O, foliar elemental concentrations, stem water content, stem water δ18O (δ18Ostem water), tree ring widths and leaf gas exchange rates to assess the influence of forest stand density on tree performance. Remaining trees in low‐density stands (heavily thinned) showed lower leaf δ18O, and higher stomatal conductance (gs), photosynthetic rate and radial growth than those in moderate‐density stands (moderately thinned). By contrast, leaf δ13C, intrinsic water‐use efficiency, foliar elemental concentrations and δ18Ostem water were unaffected by stand density. Lower foliar δ18O in heavily thinned stands reflected higher gs of remaining trees due to decreased inter‐tree competition for water, whereas higher photosynthetic rate was largely attributable to reduced stomatal limitation to CO2 uptake. The dual isotope approach provided insight into the early (12 months) effects of stand density manipulation on the physiological performance of remaining trees.  相似文献   

7.
The oxygen isotope signature of sulphate (δ18Osulphate) is increasingly used to study nutritional fluxes and sulphur transformation processes in a variety of natural environments. However, mechanisms controlling the δ18Osulphate signature in soil–plant systems are largely unknown. The objective of this study was to determine key factors, which affect δ18Osulphate values in soil and plants. The impact of an 18O‐water isotopic gradient and different types of fertilizers was investigated in a soil incubation study and a radish (Raphanus sativus L.) greenhouse growth experiment. Water provided 31–64% of oxygen atoms in soil sulphate formed via mineralization of organic residues (green and chicken manures) while 49% of oxygen atoms were derived from water during oxidation of elemental sulphur. In contrast, δ18Osulphate values of synthetic fertilizer were not affected by soil water. Correlations between soil and plant δ18Osulphate values were controlled by water δ18O values and fertilizer treatments. Additionally, plant δ34S data showed that the sulphate isotopic composition of plants is a function of S assimilation. This study documents the potential of using compound‐specific isotope ratio analysis for investigating and tracing fertilization strategies in agricultural and environmental studies.  相似文献   

8.
Previous mangrove tree ring studies attempted, unsuccessfully, to relate the δ18O of trunk cellulose (δ18OCELL) to the δ18O of source water (δ18OSW). Here, we tested whether biochemical fractionation associated with one of the oxygen in the cellulose glucose moiety or variation in leaf water oxygen isotope fractionation (ΔLW) can interfere with the δ18OSW signal as it is recorded in the δ18OCELL of mangrove (saltwater) and hammock (freshwater) plants. We selected two transects experiencing a salinity gradient, located in the Florida Keys, USA. The δ18OCELL throughout both transects did not show the pattern expected based on that of the δ18OSW. We found that in one of the transects, biochemical fractionation interfered with the δ18OSW signal, while in the other transect ΔLW differed between mangrove and hammock plants. Observed differences in ΔLW between mangroves and hammocks were caused by a longer effective leaf mixing length (L) of the water pathway in mangrove leaves compared to those of hammock leaves. Changes in L could have caused the δ18OCELL to record not only variations in the δ18OSW but also in ΔLW making it impossible to isolate the δ18OSW signal.  相似文献   

9.
The oxygen isotope composition (δ18O) of atmospheric CO2 is among a very limited number of tools available to constrain estimates of the biospheric gross CO2 fluxes, photosynthesis and respiration at large scales. However, the accuracy of the partitioning strongly depends on the extent of isotopic disequilibrium between the signals carried by these two gross fluxes. Chamber‐based field measurements of total CO2 and CO18O fluxes from foliage and soil can help evaluate and refine our models of isotopic fractionation by plants and soils and validate the extent and pattern of isotopic disequilibrium within terrestrial ecosystems. Owing to sampling limitations in the past, such measurements have been very rare and covered only a few days. In this study, we coupled automated branch and soil chambers with tuneable diode laser absorption spectroscopy techniques to continuously capture the δ18O signals of foliage and soil CO2 exchange in a Pinus pinaster Aït forest in France. Over the growing season, we observed a seasonally persistent isotopic disequilibrium between the δ18O signatures of net CO2 fluxes from leaves and soils, except during rain events when the isotopic imbalance became temporarily weaker. Variations in the δ18O of CO2 exchanged between leaves, soil and the atmosphere were well explained by theory describing changes in the oxygen isotope composition of ecosystem water pools in response to changes in leaf transpiration and soil evaporation.  相似文献   

10.
Non-climatic variations in the oxygen isotopic compositions of plants   总被引:4,自引:0,他引:4  
The 18O content of leaf water strongly influences the 18O contents of atmospheric CO2 and O2. The 18O signatures of these atmospheric gases, in turn, emerge as important indicators of large-scale gas exchange processes. Better understanding of the factors that influence the isotopic composition of leaf water is still required, however, for the quantitative utilization of these tracers. The 18O enrichment of leaf water relative to local meteoric water, is known to reflect climatic conditions. Less is known about the extent variations in the 18O content of leaf water are influenced by nonclimatic, species-specific characteristics. In a collection of 90 plant species from all continents grown under the same climatic conditions in the Jerusalem Botanical Garden we observed variations of about 9‰ in the δ18O values of stem water, δs, and of about 14‰ in the mid-day δ18O enrichment of bulk leaf water, δLW–δs. Differences between δ18O values predicted by a conventional evaporation model, δM, and δLW ranged between – 3.3‰ and + 11.8‰. The δ18O values of water in the chloroplasts (δch) in leaves of 10 selected plants were estimated from on-line CO2 discrimination measurements. Although much uncertainty is still involved in these estimates, the results indicated that δch can significantly deviate from δM in species with high leaf peclet number. The δ18O values of bulk leaf water significantly correlated with δ18O values of leaf cellulose (directly) and with instantaneous water use efficiency (A/E, inversely). Differences in isotopic characteristics among conventionally defined vegetation types were not significant, except for conifers that significantly differed from shrubs in δ18O and δ13C values of cellulose and in their peclet numbers, and from deciduous woodland species in their δ18O and δ13C values of cellulose. The results indicated that predictions of the δ18O values of leaf water (δLW, δM and δch) could be improved by considering plant species-specific characteristics.  相似文献   

11.
叶片水H218O富集的研究进展   总被引:1,自引:0,他引:1       下载免费PDF全文
 植物叶片水H218O富集对大气中O2和CO218O收支有着重要影响。蒸腾作用使植物叶片水H218O富集, 而植物叶片水H218O富集的程度主 要受大气水汽δ18O和植物蒸腾水汽δ18O的影响。过去, 通过引入稳态假设(蒸腾δ18O等于茎水δ18O)得到Craig-Gordon模型的闭合形式, 或 将植物整个叶片水δ18O经过Péclet效应校正后得到植物叶片水δ18O的富集程度。然而, 在几分钟到几小时的短时间尺度上, 植物叶片蒸腾 δ18O是变化的, 稳态假设是无法满足的。最近成功地实现了对大气水汽δ18O和δD的原位连续观测, 观测精度(小时尺度)可达到甚至优于稳定 同位素质谱仪的观测精度。在非破坏性条件下, 高时间分辨率和连续的大气水汽δ18O和蒸腾δ18O的动态观测, 将提高植物叶片水H218O富集的 预测能力。该文综述了植物叶片水H218O富集的理论研究的新进展、研究焦点和观测方法所存在的问题, 旨在进一步加深理解植物叶片水H218O 富集的过程及其机制。  相似文献   

12.
Rapid urbanization has greatly altered nitrogen (N) cycling from regional to global scales. Compared to natural forests, urban forests receive much more external N inputs with distinctive abundances of stable N isotope (δ15N). However, the large-scale pattern of soil δ15N and its imprint on plant δ15N remain less well understood in urban forests. By collecting topsoil (0–20 cm) and leaf samples from urban forest patches in nine large cities across a north–south transect in eastern China, we analyzed the latitudinal trends of topsoil C:N ratio and δ15N as well as the correlations between tree leaf δ15N and topsoil δ15N. We further explored the spatial variation of topsoil δ15N explained by corresponding climatic, edaphic, vegetation-associated, and anthropogenic drivers. Our results showed a significant increase of topsoil C:N ratio towards higher latitudes, suggesting lower N availability at higher latitudes. Topsoil δ15N also increased significantly at higher latitudes, being opposite to the latitudinal trend of soil N availability. The latitudinal trend of topsoil δ15N was mainly explained by mean annual temperature, mean annual precipitation, and atmospheric deposition of both ammonium and nitrate. Consequently, tree leaf δ15N showed significant positive correlations with topsoil δ15N across all sampled plant species and functional types. Our findings reveal a distinctive latitudinal trend of δ15N in urban forests and highlight an important role of anthropogenic N sources in shaping the large-scale pattern of urban forest 15N signature.  相似文献   

13.
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.  相似文献   

14.
The Péclet correction is often used to predict leaf evaporative enrichment and requires an estimate of effective path length (L). Studies to estimate L in conifer needles have produced unexpected patterns based on Péclet theory and leaf anatomy. We exposed seedlings of six conifer species to different vapour pressure deficits (VPD) in controlled climate chambers to produce steady‐state leaf water enrichment (in 18O). We measured leaf gas exchange, stable oxygen isotopic composition (δ18O) of input and plant waters as well as leaf anatomical characteristics. Variation in bulk needle water δ18O was strongly related to VPD. Conifer needles had large amounts of water within the vascular strand that was potentially unenriched (up to 40%). Both standard Craig–Gordon and Péclet models failed to accurately predict conifer leaf water δ18O without taking into consideration the unenriched water in the vascular strand and variable L. Although L was linearly related to mesophyll thickness, large within‐species variation prevented the development of generalizations that could allow a broader use of the Péclet effect in predictive models. Our results point to the importance of within needle water pools and isolating mechanisms that need further investigation in order to integrate Péclet corrections with ‘two compartment’ leaf water concepts.  相似文献   

15.
Kim K  Lee X 《Plant, cell & environment》2011,34(10):1790-1801
Dew formation, a common meteorological phenomenon, is expected to intensify in the future. Dew can influence the H218O and HDO isotopic compositions of leaf water (δL), but the phenomenon has been neglected in many experimental and modelling studies. In this study, the dew effect on δL was investigated with a dark plant chamber in which dew formation was introduced. The H218O and HDO compositions of water vapour, dew water and leaf water of five species were measured for up to 48 h of dew exposure. Our results show that the exchanges of H218O and HDO in leaf water with the air continued in the darkness when the net H216O flux was zero. Our estimates of the leaf conductance using the isotopic mass balance method ranged from 0.035 to 0.087 mol m?2 s?1, in broad agreement of the night‐time stomatal conductance reported in the literature. In our experiments, the conductance of the C4 species was 0.04 ± 0.01 mol m?2 s?1 and that of the C3 plants was 0.10 ± 0.04 mol m?2 s?1. At the end of 16 h dew exposure, 72 (±17) and 94 (±11)% of the leaf water came from dew according to the 18O and D tracer, respectively.  相似文献   

16.
The combined use of stable carbon and oxygen isotopes in plant matter is a tool of growing interest in cereal crop management and breeding, owing to its relevance for assessing the photosynthetic and transpirative performance under different growing conditions including water and N regimes. However, this method has not been applied to wheat grown under real field conditions. Here, plant growth, grain yield (GY) and the associated agronomic components, carbon isotope discrimination (Δ13C) plus oxygen isotope composition (δ18O) as well as leaf and canopy gas exchange were measured in field‐grown wheat subjected to different water and N availabilities. Water limitation was the main factor affecting yield, leaf and canopy gas exchange and Δ13C and δ18O, whereas N had a smaller effect on such traits. The combination of Δ13C and δ18O gave a clear advantage compared with gas exchange measurements, as it provides information on the instantaneous and the long‐term plant photosynthetic and transpirative performance and are less labour intensive than gas exchange measurements. In addition, the combination of plant Δ13C and δ18O predicted differences in GY and related agronomical parameters, providing agronomists and breeders with integrative traits for selecting crop management practices and/or genotypes with better performance under water‐limiting and N‐limiting conditions.  相似文献   

17.
It has been well-established that many epiphytic bromeliads of the atmospheric-type morphology, i.e., with leaf surfaces completely covered by large, overlapping, multicellular trichomes, are capable of absorbing water vapor from the atmosphere when air humidity increases. It is much less clear, however, whether this absorption of water vapor can hydrate the living cells of the leaves and, as a consequence, enhance physiological processes in such cells. The goal of this research was to determine if the absorption of atmospheric water vapor by the atmospheric epiphyte Tillandsia usneoides results in an increase in turgor pressure in leaf epidermal cells that subtend the large trichomes, and, by using chlorophyll fluorescence techniques, to determine if the absorption of atmospheric water vapor by leaves of this epiphyte results in increased photosynthetic activity. Results of measurements on living cells of attached leaves of this epiphytic bromeliad, using a pressure probe and of whole-shoot fluorescence imaging analyses clearly illustrated that the turgor pressure of leaf epidermal cells did not increase, and the photosynthetic activity of leaves did not increase, following exposure of the leaves to high humidity air. These results experimentally demonstrate, for the first time, that the absorption of water vapor following increases in atmospheric humidity in atmospheric epiphytic bromeliads is mostly likely a physical phenomenon resulting from hydration of non-living leaf structures, e.g., trichomes, and has no physiological significance for the plant's living tissues.  相似文献   

18.
Spatial variation in marine oxygen isotope ratios (δ18O) resulting from differential evaporation rates and precipitation inputs is potentially useful for characterizing marine mammal distributions and tracking movements across δ18O gradients. Dentine hydroxyapatite contains carbonate and phosphate that precipitate in oxygen isotopic equilibrium with body water, which in odontocetes closely tracks the isotopic composition of ambient water. To test whether dentine oxygen isotope composition reliably records that of ambient water and can therefore serve as a proxy for odontocete distribution and movement patterns, we measured δ18O values of dentine structural carbonate (δ18OSC) and phosphate (δ18OP) of seven odontocete species (n = 55 individuals) from regional marine water bodies spanning a surface water δ18O range of several per mil. Mean dentine δ18OSC (range +21.2 to +25.5‰ VSMOW) and δ18OP (+16.7 to +20.3‰) values were strongly correlated with marine surface water δ18O values, with lower dentine δ18OSC and δ18OP values in high‐latitude regions (Arctic and Eastern North Pacific) and higher values in the Gulf of California, Gulf of Mexico, and Mediterranean Sea. Correlations between dentine δ18OSC and δ18OP values with marine surface water δ18O values indicate that sequential δ18O measurements along dentine, which grows incrementally and archives intra‐ and interannual isotopic composition over the lifetime of the animal, would be useful for characterizing residency within and movements among water bodies with strong δ18O gradients, particularly between polar and lower latitudes, or between oceans and marginal basins.  相似文献   

19.
Almost no δ18O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ18O relationship between leaf water and cellulose. We measured δ18O values of bulk leaf water (δ18OLW) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ18O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally 18O‐enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (εbio) of more than 27‰ relative to δ18OLW, which might be explained by isotopic leaf water and sucrose synthesis gradients. δ18OLW and δ18O values of carbohydrates and cellulose in grasses were strongly related, indicating that the leaf water signal in carbohydrates was transferred to cellulose (εbio = 25.1‰). Interestingly, damping factor pexpx, which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ18OLW but not if modelled directly from δ18O of individual carbohydrates. We conclude that δ18OLW is not always a good substitute for δ18O of synthesis water due to isotopic leaf water gradients. Thus, compound‐specific δ18O analyses of individual carbohydrates are helpful to better constrain (post‐)photosynthetic isotope fractionation processes in plants.  相似文献   

20.
Alteration of the global nitrogen (N) cycle because of human‐enhanced N fixation is a major concern particularly for those ecosystems that are nutrient poor by nature. Because Sphagnum‐dominated mires are exclusively fed by wet and dry atmospheric deposition, they are assumed to be very sensitive to increased atmospheric N input. We assessed the consequences of increased atmospheric N deposition on total N concentration, N retention ability, and δ15N isotopic signature of Sphagnum plants collected in 16 ombrotrophic mires across 11 European countries. The mires spanned a gradient of atmospheric N deposition from about 0.1 up to about 2 g m?2 yr?1. Mean N concentration in Sphagnum capitula was about 6 mg g?1 in less polluted mires and about 13 mg g?1 in highly N‐polluted mires. The relative difference in N concentration between capitulum and stem decreased with increasing atmospheric N deposition, suggesting a possible metabolic mechanism that reduces excessive N accumulation in the capitulum. Sphagnum plants showed lower rates of N absorption under increasing atmospheric N deposition, indicating N saturation in Sphagnum tissues. The latter probably is related to a shift from N‐limited conditions to limitation by other nutrients. The capacity of the Sphagnum layer to filter atmospheric N deposition decreased exponentially along the depositional gradient resulting in enrichment of the mire pore water with inorganic N forms (i.e., NO3?+NH4+). Sphagnum plants had δ15N signatures ranging from about ?8‰ to about ?3‰. The isotopic signatures were rather related to the ratio of reduced to oxidized N forms in atmospheric deposition than to total amount of atmospheric N deposition, indicating that δ15N signature of Sphagnum plants can be used as an integrated measure of δ15N signature of atmospheric precipitation. Indeed, mires located in areas characterized by greater emissions of NH3 (i.e., mainly affected by agricultural activities) had Sphagnum plants with a lower δ15N signature compared with mires located in areas dominated by NOx emissions (i.e., mainly affected by industrial activities).  相似文献   

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