Relationships of grain δ13C and δ18O with wheat phenology and yield under water-limited conditions

Stable carbon isotope composition (δ¹³C) of dry matter has been widely investigated as a selection tool in cereal breeding programmes. However, reports on the possibilities of using stable oxygen isotope composition (δ¹⁸O) as a yield predictor are very scarce and only in the absence of water stress. Indeed, it remains to be tested whether changes in phenology and stomatal conductance in response to water stress overrule the use of either δ¹³C or δ¹⁸O when water is limited. To answer this question, a set of 24 genotypes of bread wheat ( Triticum aestivum ) were assayed in two trials with different levels of deficit irrigation and a third trial under rainfed conditions in a Mediterranean climate (northwest Syria). Grain yield (GY) and phenology (duration from planting to anthesis and from anthesis to maturity) were recorded, and the δ¹³C and δ¹⁸O of grains were analysed to assess their suitability as GY predictors. Both δ¹³C and δ¹⁸O showed higher broad-sense heritabilities ( H ²) than GY. Genotype means of GY across trials were negatively correlated with δ¹³C, as previously reported, but not with δ¹⁸O. Both isotopes were correlated with grain filling duration, whereas δ¹⁸O was also strongly affected by crop duration from planting to anthesis. We concluded that δ¹⁸O of grains is not a proper physiological trait to breed for suboptimal water conditions, as its variability is almost entirely determined by crop phenology. In contrast, δ¹³C of grains, despite being also affected by phenology, still provides complementary information associated with GY.     

Bulk leaf δ18O and δ13C reflect the intensity of intraspecific competition for water in a semi-arid tussock grassland

We investigated the extent to which plant water and nutrient status are affected by intraspecific competition intensity and microsite quality in a monodominant tussock grassland. Leaf gas exchange and stable isotope measurements were used to assess the water relations of Stipa tenacissima tussocks growing along a gradient of plant cover and soil depth in a semi-arid catchment of Southeast Spain. Stomatal conductance and photosynthetic rate decreased with increasing intensity of competition during the wet growing season, leading to foliar δ ¹⁸O and δ ¹³C enrichment. A high potential for runoff interception by upslope neighbours exerted strong detrimental effects on the water and phosphorus status of downslope S. tenacissima tussocks. Foliar δ ¹⁵N values became more enriched with increasing soil depth. Multiple stepwise regression showed that competition potential and/or rhizosphere soil depth accounted for large proportions of variance in foliar δ ¹³C, δ ¹⁸O and δ ¹⁵N among target tussocks (57, 37 and 64%, respectively). The results presented here highlight the key role that spatial redistribution of resources (water and nutrients) by runoff plays in semi-arid ecosystems. It is concluded that combined measurement of δ ¹³C, δ ¹⁸O and nutrient concentrations in bulk leaf tissue can provide insight into the intensity of competitive interactions occurring in natural plant communities.     

Absence or masking of metabolic fractionations of 13C in a freshwater benthic food web

1. Although marine research has indicated that metabolic fractionations of ¹³C due to differences in organismal trophic position and proximal composition can complicate the isotopic interpretation of energy flow pathways, such potentially confounding problems have never been examined in freshwater benthic food webs.
2. The δ¹³C values of animals comprising a littoral benthic food web composited from four Canadian Shield lakes showed no relationship with either individual trophic position (δ¹⁵N) or lipid content (C/N ratios).
3. Differences in the relative incorporation of autochthonous and allochthonous energy sources by freshwater benthic organisms will alter their δ¹³C and δ¹⁵N values, thereby masking any possibility of observing ¹³C trophic enrichment.
4. Removal of the possibly confounding influences of lipids through either empirical correction or by analytical extraction may be unnecessary in studies of freshwater benthic food webs. Likewise, a priori adjustments in δ¹³C for freshwater benthic organisms in order to accommodate trophic fractionations which are presumed to occur, based on data from marine offshore food webs, may also be inappropriate.     

Absence or masking of metabolic fractionations of 13C in a freshwater benthic food web

1. Although marine research has indicated that metabolic fractionations of ¹³C due to differences in organismal trophic position and proximal composition can complicate the isotopic interpretation of energy flow pathways, such potentially confounding problems have never been examined in freshwater benthic food webs.
2. The δ¹³C values of animals comprising a littoral benthic food web composited from four Canadian Shield lakes showed no relationship with either individual trophic position (δ¹⁵N) or lipid content (C/N ratios).
3. Differences in the relative incorporation of autochthonous and allochthonous energy sources by freshwater benthic organisms will alter their δ¹³C and δ¹⁵N values, thereby masking any possibility of observing ¹³C trophic enrichment.
4. Removal of the possibly confounding influences of lipids through either empirical correction or by analytical extraction may be unnecessary in studies of freshwater benthic food webs. Likewise, a priori adjustments in δ¹³C for freshwater benthic organisms in order to accommodate trophic fractionations which are presumed to occur, based on data from marine offshore food webs, may also be inappropriate.     

Diurnal and seasonal variation in the carbon isotope composition of leaf dark-respired CO2 in velvet mesquite (Prosopis velutina)

We evaluated diurnal and seasonal patterns of carbon isotope composition of leaf dark-respired CO₂ ( δ ¹³C_l) in the C₃ perennial shrub velvet mesquite ( Prosopis velutina ) across flood plain and upland savanna ecosystems in the south-western USA. δ ¹³C_l of darkened leaves increased to maximum values late during daytime periods and declined gradually over night-time periods to minimum values at pre-dawn. The magnitude of the diurnal shift in δ ¹³C_l was strongly influenced by seasonal and habitat-related differences in soil water availability and leaf surface vapour pressure deficit. δ ¹³C_l and the cumulative flux-weighted δ ¹³C value of photosynthates were positively correlated, suggesting that progressive ¹³C enrichment of the CO₂ evolved by darkened leaves during the daytime mainly resulted from short-term changes in photosynthetic ¹³C discrimination and associated shifts in the δ ¹³C signature of primary respiratory substrates. The ¹³C enrichment of dark-respired CO₂ relative to photosynthates across habitats and seasons was 4 to 6‰ at the end of the daytime period (1800 h), but progressively declined to 0‰ by pre-dawn (0300 h). The origin of night-time and daytime variations in δ ¹³C_l is discussed in terms of the carbon source(s) feeding respiration and the drought-induced changes in carbon metabolism.     

Trophic structure of a boreal forest arthropod community revealed by stable isotope (δ13C, δ15N) analyses

Terrestrial arthropods are important components of boreal ecosystems but relatively little is known about their trophic structure within communities. We measured δ¹³C and δ¹⁵N values in a broad range of arthropod taxa (Coleoptera, Diptera, Ephemeroptera, Homoptera, Hymenoptera, Lepidoptera, Odonata, Orthoptera, Araneae) from boreal forest in Prince Albert National Park, Saskatchewan, Canada. Isotopic measurements supported previous conventional investigations on foraging niches based on stomach content analysis and direct feeding observations but additional, new information was also obtained using the stable isotope approach. Significant differences were observed in both δ¹⁵N and δ¹³C values between various orders and families or superfamilies within orders. Higher variance in stable isotope values for scavengers (e.g. carrion beetles; Coleoptera, Silphidae) and generalists (e.g. ground beetles; Coleoptera, Carabidae) was found compared to specialists (e.g. grasshoppers; Orthoptera). Consistent isotopic differences between terrestrial and aquatic species were not found. However, aquatic insect δ¹³C values tended to be lower than those of their terrestrial counterparts. We discuss the potential for using stable isotope methods to reconstruct trophic linkages and interaction involving arthropods.     

Seasonal and annual changes in leaf δ13C in two co-occurring Mediterranean oaks: relations to leaf growth and drought progression

1. Changes of δ¹³C and its relation to leaf development, biochemical content and water stress were monitored over a 2 year period in two co-occurring Mediterranean oak species: the deciduous Quercus pubescens and the evergreen Quercus ilex .
2. The time course of leaf δ¹³C showed different patterns in the two species. Young Q. pubescens leaves had a high δ¹³C and a marked decrease occurred during leaf growth. In contrast, leaves at budburst and maturity did not differ significantly in the case of Q. ilex . We suggest that the difference between δ¹³C of young leaves was linked to differential use of reserves of carbon compounds in the two species.
3. δ¹³C values of mature leaves were negatively correlated with minimum seasonal values of predawn water potential, suggesting that a functional adjustment to water resources occurred.
4. There was a significant correlation between individual δ¹³C values for two successive years. This interannual dependence showed that δ¹³C rankings between trees were constant through time.     

Using the δ13C and δ15N of whitefish scales for retrospective ecological studies: changes in isotope signatures during the restoration of Lake Geneva, 1980–2001

The scales of whitefish Coregonus lavaretus were used in place of dorsal muscle, which necessitates killing the fish, to study food webs from the δ¹³C and δ¹⁵N isotopic ratios in the organic fraction. As scales are composed of both organic and calcified fractions, a protocol for scale decalcification was first devised. The δ¹³C and δ¹⁵N values of the decalcified scales were then shown to be closely correlated to those of the dorsal muscle, demonstrating that scales could be used in place of muscle to study food webs. Changes in the δ¹³C of whitefish were determined from a scale collection that extended over the period during which the trophic state of Lake Geneva was recovering.     

How to account for the lipid effect on carbon stable-isotope ratio (δ13C): sample treatment effects and model bias

This study investigated the impact of lipid extraction, CaCO₃ removal and of both treatments combined on fish tissue δ¹³C, δ¹⁵N and C:N ratio. Furthermore, the suitability of empirical δ¹³C lipid normalization and correction models was examined. δ¹⁵N was affected by lipid extraction (increase of up to 1·65‰) and by the combination of both treatments, while acidification alone showed no effect. The observed shift in δ¹⁵N represents a significant bias in trophic level estimates, i.e. lipid-extracted samples are not suitable for δ¹⁵N analysis. C:N and δ¹³C were significantly affected by lipid extraction, proportional to initial tissue lipid content. For both variables, rates of change with lipid content (ΔC:N and Δδ¹³C) were species specific. All tested lipid normalization and correction models produced biased estimates of fish tissue δ¹³C, probably due to a non-representative database and incorrect assumptions and generalizations the models were based on. Improved models need a priori more extensive and detailed studies of the relationships between lipid content, C:N and δ¹³C, as well as of the underlying biochemical processes.     

Starvation and low feeding levels result in an enrichment of 13C in lipids and 15N in protein of Nile tilapia Oreochromis niloticus L.

The influence of different feeding levels below and slightly above maintenance on whole body δ¹³C and δ¹⁵N values of Nile tilapia Oreochromis niloticus was examined. The energy budget of each fish was determined by indirect calorimetry. The δ¹³C values of the lipid-free material of Nile tilapia fed below and slightly above maintenance level did not differ between the feeding groups, but the δ¹³C values in the lipids and the δ¹⁵N values of the lipid-free material showed small but significant differences. Those fish with a negative lipid retention had significantly higher δ¹³C values in the lipid fraction compared to fish that synthesized fatty acids. There was a significant negative correlation between the amount of energy metabolized by the fish and both the δ¹³C values in the lipids and the δ¹⁵N values of the lipid-free material. Fasting and feeding below the maintenance level may influence the isotopic composition of animals and should therefore be considered in ecological and nutritional studies.     

Changes in feeding niche widths of perch and roach following biomanipulation, revealed by stable isotope analysis

1. We made an empirical test of a recent proposal that feeding niche widths might be determined as variance of stable isotope values. We determined δ ¹³C and δ ¹⁵N values of perch ( Perca fluviatilis ), roach ( Rutilus rutilus ) and their prey from a biomanipulated lake, when the mass removal of fish led to reduced inter- and intra-specific competition and increases in zooplankton abundance and body size.
2. After the first fish removals, both perch and roach mean δ ¹³C values decreased and mean δ ¹⁵N values increased, indicating a greater diet contribution from pelagic sources.
3. Variances of both δ ¹³C and δ ¹⁵N values first increased in both fish populations, indicating a wider food spectrum and expanded feeding niche width following reduced fish abundances. Observed changes were greater for the perch population than for roach.
4. In 2006, the perch population abruptly changed its diet so that most individuals were primarily consuming the abundant young-of-the-year fish, and this was reflected in significantly reduced variances of both δ ¹³C and δ ¹⁵N values.
5. We conclude that isotopic variance can indeed reflect changes in feeding niche width and offers a promising way to study such general ecological concepts.     

A new method to measure carbon isotope composition of CO2 respired by trees: stem CO2 equilibration

1.  Applying Keeling plot techniques to derive δ¹³C of respiratory input in a closed non-equilibrated chamber can lead to large errors because steady-state diffusion rules are violated in a non-steady-state environment. To avoid these errors, respiratory δ¹³C can be derived using equilibrated closed chambers.
2.  We introduce a new method to obtain stem respired CO₂δ¹³C (δ_{st - r}) with closed equilibrated stem chambers (E-SC). We present a theoretical model describing the equilibration process, test the model against field data and find excellent agreement. The method is further tested by comparing it with closed non-equilibrated stem chambers (NE-SC); we found no difference between these methods.
3.  Our theoretical model to describe CO₂ diffusion from the respiratory pool into the chamber and the equation to derive the δ¹³C of the efflux are general. They could be applied to other ecosystem components (e.g. soils).
4.  Our method is easy to implement, cost effective, minimizes sources of error and allows for rigorous leak detection. One major limitation is its inability to detect rapid change; the equilibration process requires 15 ± 2 h. A second limitation is that it cannot be used for species that produce abundant pitch at sites of stem wounding (e.g. Pseudotsuga menziesii ).
5.  Investigating δ¹³C of CO₂ respired by different ecosystem components is necessary to interpret δ¹³C of ecosystem respiration. This parameter has major implications with respect to global carbon cycle science.     

Leaf δ13C reflects ecosystem patterns and responses of alpine plants to the environments on the Tibetan Plateau

Leaf δ¹³C is an indicator of water-use efficiency and provides useful information on the carbon and water balance of plants over longer periods. Variation in leaf δ¹³C between or within species is determined by plant physiological characteristics and environmental factors. We hypothesized that variation in leaf δ¹³C values among dominant species reflected ecosystem patterns controlled by large-scale environmental gradients, and that within-species variation indicates plant adaptability to environmental conditions. To test these hypotheses, we collected leaves of dominant species from six ecosystems across a horizontal vegetation transect on the Tibetan Plateau, as well as leaves of Kobresia pygmaea (herbaceous) throughout its distribution and leaves of two coniferous tree species ( Picea crassifolia, Abies fabri ) along an elevation gradient throughout their distribution in the Qilian Mountains and Gongga Mountains, respectively. Leaf δ¹³C of dominant species in the six ecosystems differed significantly, with values for evergreen coniferous13C values of the dominant species and of K. pygmaea were negatively correlated with annual precipitation along a water gradient, but leaf δ¹³C of A. fabri was not significantly correlated with precipitation in habitats without water-stress. This confirms that variation of δ¹³C between or within species reflects plant responses to environmental conditions. Leaf δ¹³C of the dominant species also reflected water patterns on the Tibetan Plateau, providing evidence that precipitation plays a primary role in controlling ecosystem changes from southeast to northwest on the Tibetan Plateau.     

Traffic exposure increases natural 15N and heavy metal concentrations in mosses

Mosses have been used as biomonitors of atmospheric pollution for some years, but few studies have been carried out on the effect of NO_x emissions from traffic on moss tissue N. Eight species of moss (102 samples) growing on walls or roofs next to roads exposed to different traffic densities were collected from urban and rural sites in the UK. The shoots were sampled for total N, their stable isotope ¹⁵N/¹⁴N content (δ¹⁵N) and heavy metal content (Pb, Zn). There was a lack of correlation between tissue total N and traffic exposure, but a very good correlation between traffic exposure and tissue δ¹⁵N. Plants collected near motorways or busy urban roads had δ¹⁵N values ranging between +6 and −1‰, while in rural areas with hardly any traffic these ranged from −2 to −12‰. In a separate survey of mosses, the average δ¹⁵N of shoots from busy roadsides in London was +3.66‰, whereas from samples collected from farm buildings near poultry or cattle pens it was −7.8‰. This indicates that the two main atmospheric N sources, NO_x and NH_x, have different δ¹⁵N signatures, the former tending to be positive and the latter negative. Tissue concentrations of both Pb and Zn show a strong positive correlation with traffic exposure, with Zn in particular being greater than Pb. The results are discussed with regard to the use of moss tissue Zn as a means for monitoring or mapping pollution from vehicles, and of δ¹⁵N as an aid to distinguish between urban (NO_x) and rural (NH_x) forms of N pollution.     

Benthic algae support zooplankton growth during winter in a clear-water lake

We used stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes to assess the importance of benthic algae for the zooplankton individual growth in winter in a shallow, clear subarctic lake. The δ¹³C values of calanoid ( Eudiaptomus graciloides ) and cyclopoid ( Cyclops scutifer ) zooplankton in autumn suggest a food resource of pelagic origin during the ice-free period. The zooplankton δ¹³C values were high in spring compared to autumn. E. graciloides did not grow over winter and the change in δ¹³C was attributed to a decrease in lipid content during the winter. In contrast, the increase in δ¹³C values of C. scutifer over the winter was explained by their growth on organic carbon generated by benthic algae. The δ¹⁵N of the C. scutifer food resource during winter was low compared to δ¹⁵N of the benthic community, suggesting that organic matter generated by benthic algae was mainly channelled to zooplankton via ¹⁵N-depleted heterotrophic bacteria. The results demonstrate that benthic algae can sustain zooplankton metabolic demands and growth during long winters, which, in turn, may promote zooplankton growth on pelagic resources during the summer. Such multi-chain omnivory challenges the view of zooplankton as mainly dependent on internal primary production and stresses the importance of benthic resources for the productivity of plankton food webs in shallow lakes.     

Fractionation of δ15N and δ13C for Atlantic salmon and its intestinal cestode Eubothrium crassum

Stable isotopes of nitrogen (δ¹⁵N) and carbon (δ¹³C) were measured for Atlantic salmon Salmo salar and their intestinal cestode, Eubothrium crassum , sharing the same diet. Atlantic salmon muscle tissues were enriched in ¹⁵N and depleted in ¹³C compared to their prey (sprat Sprattus sprattus sprattus ) and their intestinal cestode. There was no significant difference in δ¹⁵N or δ¹³C between E. crassum and the sprat. Differences in nutrient uptake and intestine physiology between Atlantic salmon and E. crassum are discussed, as well as how these may give rise to different fractionations of stable isotopes between a host and its parasites. Furthermore, Atlantic salmon contained a significantly higher lipid content than their prey, which may partly explain differences in δ¹³C values between the host and its cestode. In addition, cestodes inhabiting lipid-rich hosts were also lipid rich. Larger Atlantic salmon were enriched in ¹⁵N compared to smaller fish. Cestodes inhabiting large hosts were also enriched in ¹⁵N compared to parasites living in smaller hosts. The last two results were explained by larger fish possibly feeding from a higher trophic level, or from larger and older prey, that resulted in both a higher lipid content and an enrichment in ¹⁵N.     

Correlations between net primary productivity and foliar carbon isotope ratio across a Tibetan ecosystem transect

Warming climate could affect leaf-level carbon isotope composition (δ¹³C) through variations in photosynthetic gas exchange. However, it is still unclear to what extent variations in foliar δ¹³C can be used to detect changes in net primary productivity (NPP) because leaf physiology is only one of many determinants of stand productivity. We aim to examine how well site-mean foliar δ¹³C and stand NPP co-vary across large resource gradients using data obtained from the Tibetan Alpine Vegetation Transects (1900–4900 m, TAVT). The TAVT data indicated a robust negative correlation between foliar δ¹³C and NPP across ecosystems (NPP=−2.7224δ¹³C-67.738, r²=0.60, p<0.001). The mean foliar δ¹³C decreased with increasing annual precipitation and its covariation with mean temperature and soil organic carbon and nitrogen contents. The results were further confirmed by global literature data. Pooled δ¹³C data from global literature and this study explained 60% of variations in annual NPP both from TAVT-measures and MODIS-estimates across 67 sites. Our results appear to support a conceptual model relating foliar δ¹³C and nitrogen concentration (N_mass) to NPP, suggesting that: 1) there is a general (negative) relationship between δ¹³C and NPP across different water availability conditions; 2) in water-limited conditions, water availability has greater effects on NPP than N_mass; 3) when water is not limiting, NPP increases with increasing N_mass.     

Stable isotope variability of meso-zooplankton along a gradient of dissolved organic carbon

1.  The δ¹³C and δ¹⁵N signatures of zooplankton vary with dissolved organic carbon (DOC), but inconsistent and limited taxonomic resolution of previous studies have masked differences that may exist among orders, genera or species and are attributable to dietary and/or habitat differences. Here we investigate differences among the isotopic signatures of five zooplankton taxa ( Daphnia , Holopedium , large Calanoida, small Calanoida and Cyclopoida) in Precambrian shield lakes with a sixfold range of DOC concentration.
2.  δ¹³C signatures of Daphnia , small calanoids and large calanoids became more depleted with increasing lake DOC, whereas Holopedium and cyclopoid δ¹³C became enriched with increasing DOC concentration.
3.  The variability of δ¹³C and δ¹⁵N isotopic signatures among zooplankton groups was reduced in high-DOC, compared to low-DOC lakes, especially for δ¹³C. Differences in δ¹³C and POM-corrected δ¹⁵N accounted for up to 33.7% and 19.5% of the variance, respectively, among lakes of varying DOC concentration.
4.  The narrow range of signatures found in higher DOC lakes suggests that different taxa have similar food sources and/or habitats. In contrast, the wide range of signatures in low-DOC lakes suggests that different taxa are exploiting different food sources and/or habitats. Together with the variable trends in zooplankton isotopic signatures along our DOC gradient, these results suggest that food web dynamics within the zooplankton community of temperate lakes will change as climate and lake DOC concentrations change.     

Unique perspectives on the influence of size and age on consumer δ15N from a rainbow smelt complex

Analysis of covariance indicated size had a greater influence than age on δ¹⁵N in three rainbow smelt Osmerus mordax morphotypes of Lake Utopia, New Brunswick, Canada. The distinction in size between the three forms allowed a unique analysis and separation of size and age effects, which is not commonly possible because of the strong correlation between size and age. These results provide support for the continued use of δ¹⁵N as a trophic level indicator in food web studies. While a slight increase in δ¹⁵N with age ( P ≤ 0·05) in intermediate-sized smelt raises questions about a potential accumulation of ¹⁵N with age, the effect of size was far stronger in governing δ¹⁵N values. The possible confounding influence of age when interpreting trophodynamics of short-lived consumers appears to be small.     

Seasonal Variation of δ^13C of Four Tree Species： A Biological Integrator of Environmental Variables

Foliar δ^13C values, an indicator of long-term intercellular carbon dioxide concentration and, thus, of long-term water use efficiency （WUE） in plants, were measured for Pinus massoniana Lamb., P. elliottii Engelm., Cunninghamia laceolata （Lamb.） Hook., and Schima superba Gardn. et Champ. in a restored forest ecosystem in the Jiazhu River Basin. Seasonal variation and the relationship between the foliar δ^13C values of the four species and environmental factors （monthly total precipitation, monthly average air temperature, relative humidity, atmospheric pressure, and monthly total solar radiation and evaporation） were investigated. The monthly δ^13C values and WUE of the four species increased with increasing precipitation, air temperature, solar radiation, and evaporation, whereas δ^13C values of the four species decreased with increasing relative humidity and atmospheric pressure. Despite significant differences in δ^13C seasonal means for the four species, our results demonstrate a significant convergence in the responses of δ^13C values and WUE to seasonal variations in environmental factors among the species investigated and that the δ^13C signature for each species gives a strong indication of environmental variables.