Altitudinal gradients of grassland carbon and nitrogen isotope composition are recorded in the hair of grazers

Aim The hair of grazers provides an isotopic record of environmental and nutritional signals. Here, we assess the effect of altitude on the carbon and nitrogen isotope composition of the hair of ruminant grazers and its relation to grassland vegetation, to evaluate the use of hair isotope data for ecosystem reconstruction, animal nutritional ecology and biogeochemical studies in montane environments. Location European Alps. Methods We sampled grassland vegetation (pure C₃) and the hair of ruminants along an altitudinal gradient (400–2500 m), and analysed their isotope composition (δ¹³C and δ¹⁵N). Results were compared with published effects of altitude on ¹³C in C₃ plants at the species level and on ¹⁵N at the community level. The study was complemented with a comparison of diet and hair isotope composition in ruminants held in confinement. Results δ¹³C of hair increased (c. 1.1‰ km⁻¹) and δ¹⁵N decreased (c. 1.1‰ km⁻¹) with altitude. The same changes occurred in local grassland vegetation, and in regional to global grassland data sets. Offsets between hair and vegetation ¹³C or ¹⁵N (‘diet–hair shift’) were independent of altitude. Sheep (Ovis aries) and cattle (Bos taurus) exhibited a ¹³C shift near +3‰, but that of goats (Capra hircus) was larger (+4.2‰) in alpine environments and in confinement. The diet–hair shift for ¹⁵N was more variable (+2.1 to +3.6‰). Main conclusions Grazer hair provides a faithful spatially and temporally integrated record of grassland isotope composition, useful for ecosystem and environment reconstruction. The effect of altitude on hair ¹⁵N is important for studies of trophic relationships: an altitude shift of 2000 m produced the same effect in hair ¹⁵N as would a shift from an animal tissue‐based to a plant‐based diet. The similarity of altitude effects on δ¹³C of individual plant species, vegetation and hair indicates that the effect of altitude on species‐level ‘intrinsic water use efficiency’ scales up linearly to the community and landscape level.     

Use of stable isotopes to understand food webs in Macao wetlands

In this study, components of the food-web in Macao wetlands were quantified using stable isotope ratio techniques based on carbon and nitrogen values. The δ¹³C and δ¹⁵N values of particulate organic matter (δ¹³C_POM and δ¹⁵N_POM, respectively) ranged from ?30.64 ± 1.0 to ?28.1 ± 0.7 ‰, and from ?1.11 ± 0.8 to 3.98 ± 0.7 ‰, respectively. The δ¹³C values of consumer species ranged from ?33.94 to ?16.92 ‰, showing a wide range from lower values in a freshwater lake and inner bay to higher values in a mangrove forest. The distinct dietary habits of consumer species and the location-specific food source composition were the main factors affecting the δ¹³C values. The consumer ¹⁵N-isotope enrichment values suggested that there were three trophic levels; primary, secondary, and tertiary. The primary consumer trophic level was represented by freshwater herbivorous gastropods, filter-feeding bivalves, and plankton-feeding fish, with a mean δ¹⁵N value of 5.052 ‰. The secondary consumer level included four deposit-feeding fish species distributed in Fai Chi Kei Bay and deposit-feeding gastropods in the Lotus Flower Bridge flat, with a mean δ¹⁵N value of 6.794 ‰. The tertiary consumers group consisted of four crab species, one shrimp species, and four fish species in the Lotus Flower Bridge Flat, with a mean δ¹⁵N value of 13.473 ‰. Their diet mainly comprised organic debris, bottom fauna, and rotten animal tissues. This study confirms the applicability of the isotopic approach in food web studies.     

Species-specific differences in temporal and spatial variation in δ13C of plant carbon pools and dark-respired CO2 under changing environmental conditions

Stable carbon isotope signatures are often used as tracers for environmentally driven changes in photosynthetic δ¹³C discrimination. However, carbon isotope signatures downstream from carboxylation by Rubisco are altered within metabolic pathways, transport and respiratory processes, leading to differences in δ¹³C between carbon pools along the plant axis and in respired CO₂. Little is known about the within-plant variation in δ¹³C under different environmental conditions or between species. We analyzed spatial, diurnal, and environmental variations in δ¹³C of water soluble organic matter (δ¹³C_WSOM) of leaves, phloem and roots, as well as dark-respired δ¹³CO₂ (δ¹³C_res) in leaves and roots. We selected distinct light environments (forest understory and an open area), seasons (Mediterranean spring and summer drought) and three functionally distinct understory species (two native shrubs—Halimium halimifolium and Rosmarinus officinalis—and a woody invader—Acacia longifolia). Spatial patterns in δ¹³C_WSOM along the plant vertical axis and between respired δ¹³CO₂ and its putative substrate were clearly species specific and the most δ¹³C-enriched and depleted values were found in δ¹³C of leaf dark-respired CO₂ and phloem sugars, ~?15 and ~?33 ‰, respectively. Comparisons between study sites and seasons revealed that spatial and diurnal patterns were influenced by environmental conditions. Within a species, phloem δ¹³C_WSOM and δ¹³C_res varied by up to 4 ‰ between seasons and sites. Thus, careful characterization of the magnitude and environmental dependence of apparent post-carboxylation fractionation is needed when using δ¹³C signatures to trace changes in photosynthetic discrimination.     

Within trophic level shifts in collagen–carbonate stable carbon isotope spacing are propagated by diet and digestive physiology in large mammal herbivores

Stable carbon isotope analyses of vertebrate hard tissues such as bones, teeth, and tusks provide information about animal diets in ecological, archeological, and paleontological contexts. There is debate about how carbon isotope compositions of collagen and apatite carbonate differ in terms of their relationship to diet, and to each other. We evaluated relationships between δ¹³C_collagen and δ¹³C_carbonate among free‐ranging southern African mammals to test predictions about the influences of dietary and physiological differences between species. Whereas the slopes of δ¹³C_collagen–δ¹³C_carbonate relationships among carnivores are ≤1, herbivore δ¹³C_collagen increases with increasing dietary δ¹³C at a slower rate than does δ¹³C_carbonate, resulting in regression slopes >1. This outcome is consistent with predictions that herbivore δ¹³C_collagen is biased against low protein diet components (¹³C‐enriched C₄ grasses in these environments), and δ¹³C_carbonate is ¹³C‐enriched due to release of ¹³C‐depleted methane as a by‐product of microbial fermentation in the digestive tract. As methane emission is constrained by plant secondary metabolites in browse, the latter effect becomes more pronounced with higher levels of C₄ grass in the diet. Increases in δ¹³C_carbonate are also larger in ruminants than nonruminants. Accordingly, we show that Δ¹³C_collagen‐_carbonate spacing is not constant within herbivores, but increases by up to 5 ‰ across species with different diets and physiologies. Such large variation, often assumed to be negligible within trophic levels, clearly cannot be ignored in carbon isotope‐based diet reconstructions.     

Estimating stable carbon isotope values of microphytobenthos in the Arctic for application to food web studies

Most studies on Arctic food webs have neglected microphytobenthos as a potential food source because we currently lack robust measurements of δ¹³C values for microphytobenthos from this environment. As a result, the role of microphytobenthos in high latitude marine food webs is not well understood. We combined field measurements of the concentration of aqueous carbon dioxide and the stable carbon isotopic composition of dissolved inorganic carbon (δ¹³C_DIC) from bottom water in the Beaufort and Chukchi seas with a set of stable carbon isotopic fractionation factors reflecting differences in algal taxonomy and physiology to estimate the stable carbon isotope composition of microphytobenthos-derived total organic carbon (δ¹³C_p). The δ¹³C_p for Phaeodactylum tricornutum, a pennate diatom likely to be a dominant microphytobenthos taxon, was estimated to be ?23.9 ± 0.4 ‰ as compared to a centric diatom (Porosira glacialis, δ¹³C_p = ?20.0 ± 1.6 ‰) and a marine haptophyte (Emiliana huxleyi, δ¹³C_p = ?22.7 ± 0.5 ‰) at a growth rate (µ) of 0.1 divisions per day (d^?1). δ¹³C_p values increased by ~2.5 ‰ when µ increased from 0.1 to a maximum growth rate of 1.4 d^?1. We compared our estimates of δ¹³C_p values for microphytobenthos with published measurements for other carbon sources in the Arctic and sub-Arctic. We found that microphytobenthos values overlapped with pelagic sources, yet differed from riverine and ice-derived carbon sources. These model results provide valuable insight into the range of possible isotopic values for microphytobenthos from this region, but we remain cautious in regard to the conclusiveness of these findings given the paucity of field measurements currently available for model validation.     

Stable isotopic (δ13C and δ18O) signatures of biogenic calcretes marking discontinuity surfaces: a case study from Upper Cretaceous carbonates of central Dalmatia and eastern Istria,Croatia

Biogenic calcretes associated with a regional Cretaceous to Paleogene subaerial unconformity and an intraformational composite (polygenic) surface in Upper Cretaceous intra-platform peritidal successions in central Dalmatia and eastern Istria, Croatia (Adriatic-Dinaridic Carbonate Platform), were analyzed for their δ¹³C and δ¹⁸O signatures in order to provide insight into the conditions of subaerial exposure and calcrete development. The distinctly negative δ¹³C signatures of biogenic calcretes marking the regional subaerial unconformity differ considerably from the δ¹³C values of the host marine limestones. This indicates carbon isotope exchange of primary marine CaCO₃ with CO₂ released by root and rhizomicrobial respiration and subsequent precipitation of pedogenic calcrete. The range of δ¹³C (from ?13.1 to ?8.2 ‰ Vienna PeeDee Belemnite standard, VPDB) and δ¹⁸O (from ?10.1 to ?6.1 ‰ VPDB) values of calcretes are similar to those reported from calcretes elsewhere, and the δ¹³C values of biogenic calcretes with typical Microcodium aggregates (?13.1 to ?12.3 ‰ VPDB) at the ?ibenik locality are very close to, or at the lower limit of, values for soil carbonates formed in isotopic equilibrium with soil CO₂. These values are expected for authigenic pedogenic carbonates formed under the influence of C₃ plant communities, without influence from heavier carbon from pre-existing carbonate and lack of input of atmospheric CO₂. Such low δ¹³C values support the interpretation of Microcodium aggregates as being precipitated under a direct biological control within the soil, although the relationship between formation mechanisms and stable isotope signatures of Microcodium needs further investigation. The δ¹³C values (?4.4 to ?3.6 ‰ VPDB) of rhizogenic calcretes formed inside firmground Thalassinoides burrows of the composite surface at the ?ibenik locality are more negative than the δ¹³C values of the host marine limestones, which confirms that the composite surface went through a phase of meteoric pedo(dia)genesis. However, the overall δ¹³C values of calcretes are less negative than expected, which might reflect contamination from associated primary marine carbonate. This study represents the first detailed stable isotope investigation of calcretes from carbonate successions of the External Dinarides, and the results may be applied to discontinuities present in other shallow-water carbonate rock successions.     

Trophic level delineation and resource partitioning in a South African afromontane forest bird community using carbon and nitrogen stable isotopes

Southern African forests are naturally fragmented yet hold a disproportionately high number of bird species. Carbon and nitrogen stable isotopes were measured in feathers from birds captured at Woodbush (n = 27 species), a large afromontane forest in the eastern escarpment of Limpopo province, South Africa. The δ¹³C signatures of a range of forest plants were measured to categorise the food base. Most plants sampled, including two of five grass species, had δ¹³C signatures typical of a C₃ photosynthetic pathway (?29.5 ± 1.9‰). Three grass species had a C₄ signature (?12.0 ± 0.6‰). Most bird species had δ¹³C values representing a predominantly C₃‐based diet (?24.8‰ to ?20.7‰). δ¹⁵N values were as expected, with higher levels of enrichment associated with a greater proportion of dietary animal matter. The cohesive isotopic niche defining most species (n = 22), where the ranges for δ¹³C and δ¹⁵N were 2.4‰ and 3.4‰, respectively, highlight the difficulties in understanding diets of birds in a predominantly C₃‐based ecosystem using carbon and nitrogen stable isotopes. However, variation in isotopic values between and within species provides insight into possible niche width and the use of resources by different birds within a forest environment.     

Reliability of δ13C and δ15N in faeces for reconstructing savanna herbivore diet

We tested the reliability of herbivore faecal δ¹³C and δ¹⁵N values for reconstructing diet through review of an extensive database derived from a 3-year study of ungulates in South Africa's Kruger National Park. Faeces are a useful material for stable isotope studies of diet because they record dietary turnover at very short time scales, and because sampling is non-invasive. However, the validity of faecal isotope proxies may be questioned because they represent only undigested food remains. Results from Kruger Park confirm that free-ranging browsers have faecal δ¹³C consistent with C₃ feeding, grazer faeces are C₄, and mixed-feeder faeces intermediate. Although the respective ranges do not overlap, there is significant variation in faecal δ¹³C of browsers and grazers (～2.0–4.0‰) across space and through time. We demonstrate that most (～70%) of this variation can be ascribed to corresponding patterns of variation in the δ¹³C of C₃ and C₄ plants, respectively, re-enforcing the fidelity of faecal isotope proxies for diet but highlighting a need for mixing models that control for variations in plant δ¹³C in order to achieve accurate diet reconstructions. Predictions for the effects of climate (rainfall) and ecophysiology on ¹⁵N-abundance variations in mammals do not persist in faeces. Rather, faecal δ¹⁵N tracks changes in plant δ¹⁵N, with further fractionation occurring primarily due to variations in dietary protein (reflected by %N). Controlling for these effects, we show that a dual-isotope multiple source mixing model (Isosource) can extend diet reconstructions for African savanna herbivores beyond simplified C₃/C₄ distinctions, although further understanding of variations in mammal δ¹⁵N are needed for greater confidence in this approach.     

Facies development and sequence stratigraphy of the Ludfordian (Upper Silurian) deposits in the Zbruch River Valley, Podolia, western Ukraine: local facies overprint on the δ13Ccarb record of a global stable carbon isotope excursion

The Ludfordian (Upper Silurian) succession in Podolia, western Ukraine, represents a Silurian carbonate platform developed in an epicontinental sea on the shelf of the paleocontinent of Baltica. Coeval deposits throughout this basin record a positive stable carbon isotope excursion known as the Lau excursion. The record of this excursion in Podolia exhibits an unusual amplitude from highly positive (+6.9 ‰) to highly negative (?5.0 ‰) δ¹³C_carb values. In order to link δ¹³C_carb development with facies, five sections in the Zbruch River Valley were examined, providing microfacies characterization and revised definitions of the Isakivtsy, Prygorodok, and Varnytsya Formations. The Isakivtsy Fm. is developed as dolosparite replacing originally bioclastic limestone. The Prygorodok Fm., recording strongly depleted (down to ?10.53 ‰) to near zero (0.12 ‰) δ¹³C_carb values is developed as laminated, organic-rich dolomicrite with metabentonite and quartz siltstone beds. The Varnytsya Fm. is characterized by peritidal deposition with consistent, slightly negative δ¹³C_carb values (?0.57 to ?3.20 ‰). It is proposed that dolomitization of the Isakivtsy Fm. is associated with a sequence boundary and erosional surface. The overlying Prygorodok Fm. represents the proximal part of a TST deposited in restricted and laterally extremely variable environments dominated by microbial carbonate production. The transition to the overlying Varnytsya Fm. facies is marked by a maximum flooding surface. The SB and MFS are potentially correlative within the basin and support a global rapid sea-level fall previously proposed for this interval. The interpretation of the Prygorodok Fm. as coastal lake deposits may explain the unusual δ¹³C_carb values and constitute one of the few records of this type of environment identified in the early Paleozoic.     

Characteristics of stable isotope signature of diet in tissues of captive Japanese macaques as revealed by controlled feeding

We determined the magnitude of isotopic fractionation of carbon and nitrogen stable isotope ratios (as enrichment factors, Δδ¹³C and Δδ¹⁵N, respectively) between the tissues and diets of captive Japanese macaques (Macaca fuscata) using a controlled feeding experiment, to provide basic data for reconstructing their feeding habits. The Δδ¹³C and Δδ¹⁵N values, respectively, were 0.9 ± 0.2 ‰ (mean ± standard deviation, SD) and 3.0 ± 0.3 ‰ for whole blood, 1.3 ± 0.2 ‰ and 4.3 ± 0.3 ‰ for plasma, and 0.8 ± 0.2 ‰ and 3.0 ± 0.2 ‰ for red blood cells. However, the Δδ¹³C and Δδ¹⁵N values for hair were 2.8 ± 0.3 ‰ and 3.4 ± 0.2 ‰, respectively. No difference was detected in the δ¹³C and δ¹⁵N values of hair sampled from different parts of the body. We investigated the effects of diet on δ¹³C in growing hair by alternating the diet of the macaques each month between two diets that differed markedly in δ¹³C. Hair regrown after shaving repeatedly recorded the δ¹³C of the diet consumed during the time of hair growth. On the other hand, hair naturally grown during the diet-change experiment did not show a clear pattern. One possible reason is that the hair had grown abnormally under unnatural indoor conditions and showed complicated isotope signatures. To reconstruct the long-term feeding history of Japanese macaques, we need to further clarify the relationships between the stable isotope signature of diet and various body tissues.     

13C isotope fractionation during rhizosphere respiration of C3 and C4 plants

Stable carbon isotopes are used extensively to partition total soil CO₂ efflux into root-derived rhizosphere respiration or autotrophic respiration and soil-derived heterotrophic respiration. However, it remains unclear whether CO₂ from rhizosphere respiration has the same δ¹³C value as root biomass. Here we investigated the magnitude of ¹³C isotope fractionation during rhizosphere respiration relative to root biomass in six plant species. Plants were grown in a carbon-free sand-perlite medium inoculated with microorganisms from a farm soil for 62 days inside a greenhouse. We measured the δ¹³C value of rhizosphere respiration using a closed-circulation 48-hour CO₂ trapping method during 40~42 and 60~62 days after sowing. We found a consistent depletion in ¹³C (0.9~1.7‰) of CO₂ from rhizosphere respiration relative to root biomass in three C₃ species (Glycine max L. Merr., Helianthus annuus L. and Triticum aestivum L.), but a relatively large depletion in ¹³C (3.7~7.0‰) in three C₄ species (Amaranthus tricolor L., Sorghum bicolor (L.) Moench and Zea mays L. ssp. mays). Overall, our results indicate that CO₂ from rhizosphere respiration is more ¹³C-depleted than root biomass. Therefore, accounting for this ¹³C fractionation is required for accurately partitioning total soil CO₂ efflux into root-derived and soil-derived components using natural abundance stable carbon isotope methods.     

Large 13C/12C and small 15N/14N isotope fractionation in an experimental detrital foodweb (litter–fungi–collembolans)

Correctly estimating the trophic fractionation factors (Δ¹⁵N and Δ¹³C) in controlled laboratory conditions is essential for the application of stable isotope analysis in studies on the trophic structure of soil communities. Laboratory experiments usually suggest large ¹⁵N/¹⁴N and small ¹³C/¹²C trophic fractionation, but in field studies litter-dwelling microarthropods and other invertebrates are consistently enriched in ¹³C relative to plant litter. In the present study, we report data from two laboratory experiments investigating both fungi–collembolans and litter–fungi–collembolans systems. In the fungi–collembolans system, Δ¹⁵N and Δ¹³C averaged 1.4 ± 0.1 and 1.0 ± 0.2 ‰, respectively. In microcosms with fungi-inoculated litter, the difference in δ¹⁵N between collembolans and plant litter averaged 1.5 ± 0.2 ‰, confirming the relatively small ¹⁵N/¹⁴N trophic fractionation at the basal level of detrital foodwebs reported in numerous field studies. In full agreement with field observations, the difference in δ¹³C between bulk litter and collembolans in laboratory microcosms averaged 3.6 ± 0.1 ‰ and only little depended on collembolan species identities or the presence of water-soluble compounds in the litter. We conclude that increased δ¹³C values typical of litter-dwelling decomposers are largely determined by an increased ¹³C content in saprotrophic microorganisms.     

Do grazer hair and faeces reflect the carbon isotope composition of semi-arid C3/C4 grassland?

The carbon isotope composition (δ¹³C) of C3/C4 mixed grassland is reflected in the δ¹³C of diet, hair or faeces of grazers, if ¹³C discrimination (¹³Δ) between grassland vegetation and these tissues is known and constant. However, these relationships could be modified by selective grazing or differential digestibility of the C3 and C4 components, potentially creating a bias between grassland and grazer tissue δ¹³C. Importantly, these factors have never been studied in detail. We investigated the relation between δ¹³C of C3/C4 grassland vegetation and that of faeces and hair of sheep in a 3-year (2005–2007) experiment in the Inner Mongolian semi-arid steppe. The experiment employed six stocking rates (0.375–2.25 sheep ha^?1 year^?1; four replications), which allowed for a large variation in species composition, digestibility, and diet selection. Faecal-nitrogen content, a proxy for digestibility, decreased from 1.9% to 1.5% during the grazing period due to aging of the herbage. At the same time, the C3/C4 ratio decreased due to the later growth initiation of C4 species. ¹³Δ between diet and faeces (¹³Δ_DF; 0.6‰) and between diet and hair (¹³Δ_DH; ?3.9‰) were not influenced by stocking rate, period in the season or C3/C4 ratio. Moreover, faeces–hair discrimination (¹³Δ_FH; ?4.3‰), which reflects differences between digestibility of the C3 and C4 components, did not vary along the different gradients. The δ¹³C of grassland vegetation can be estimated from the δ¹³C of sheep faeces and hair, provided that ¹³Δ was accounted for. This is useful for landscape- or regional-scale investigations or reconstruction of C3/C4 vegetation distribution from faeces and hair, which provide different temporal and spatial integration of grassland isotope signals.     

Evaluation of commercial and natural food in experimental cultures of white shrimp based on stable carbon isotopes

The farming of shrimp is developing quickly worldwide, and recently, ingredients such as seaweeds in low proportion (25 to 4 %), incorporated into the commercial food, have been shown to improve the shrimp productive variables. The change of commercial foods to commercial feed with a proportion of natural food, and finally, to natural food has been little and simultaneously evaluated. The aim of our study was to determine the relative contribution of dietary carbon to the growth of Litopenaeus vannamei fed with a proportion of 4 % Sargassum (δ¹³C = ?20.9?±?0.05?‰), 4 % Ulva (δ¹³C = ?20.6?±?0.6?‰) meal, and a control diet (δ¹³C?=??22.6?±?0.2?‰) in 60-L tanks for 30 days, and finally, with the green seaweed Ulva spp. (δ¹³C = ?13.2?±?0.25?‰) and Ulva meal (δ¹³C = ?14.5?±?0.6?‰) in open-air ponds for 120 days, by measuring δ¹³C for each of the foods and in the muscle of shrimp. After 15 days, the rates of metabolic turnover (Δ¹³C = δ¹³C_shrimp ? δ¹³C_food) were constant until the end of the experiment in the tanks. The muscle of shrimp assimilated carbon from all diets, which demonstrated the potential use of combined diets and the optimization of their use in diets containing seaweed. Our data will be useful in future interpretations of field and laboratory isotopic values for this species.     

Stable carbon and nitrogen isotope ratios of Eucalyptus and Acacia species along a seasonal rainfall gradient in Western Australia

Key message

Eucalyptus and Acacia species were surprisingly similar with respect to variations in δ ¹³ C, δ ¹⁵ N. Both genera respond with speciation and associated changes in leaf structure to drought.

Abstract

Stable carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N) in leaves of eucalypts (Corymbia and Eucalyptus) and Acacia (and some additional Fabaceae) species were investigated together with specific leaf area (SLA), leaf nitrogen (N) and leaf phosphorous (P) concentration along a north–south transect through Western Australia covering winter- and summer-dominated rainfall between 100 and 1,200 mm annually. We investigated 62 eucalypts and 78 woody Fabaceae species, mainly of the genus Acacia. Leaf δ¹³C values of Eucalyptus and Acacia species generally increased linearly with latitude from ?29.5 ± 1.3 ‰ in the summer-dominated rainfall zone (15°S–18°S) to about ?25.7 ± 1.1 ‰ in the winter-dominated rainfall zone (29°S–31°S). δ¹⁵N increased initially with southern latitudes (0.5 ± 1.6 ‰ at 15°S; 5.8 ± 3.3 ‰ at 24–29°S) but decreased again further South (4.6 ± 3.5 ‰ at 31°S). The variation in δ¹³C and δ¹⁵N was probably due to speciation of Eucalyptus and Acacia into very local populations. There were no species that were distributed over the whole sampling area. The variation in leaf traits was larger between species than within species. Average nitrogen concentrations were 11.9 ± 1.05 mg g^?1 in Eucalyptus, and were 18.7 ± 4.1 mg g^?1 in Acacia. Even though the average nitrogen concentration was higher in Acacia than Eucalyptus, δ¹⁵N gave no clear indication for N₂ fixation in Acacia. In a multiple regression, latitude (as a surrogate for rainfall seasonality), mean rainfall, leaf nitrogen concentration, specific leaf area and nitrogen fixation were significant and explained 69 % of the variation of δ¹³C, but only 36 % of the variation of δ¹⁵N. Higher nitrogen and phosphorus concentration could give Acacia an advantage over Eucalyptus in arid regions of undefined rainfall seasonality.     

Stromatolite records of environmental change in perennially ice-covered Lake Joyce,McMurdo Dry Valleys,Antarctica

Calcite-rich columnar stromatolites grew in perennially ice-covered Lake Joyce in the McMurdo Dry Valleys, Antarctica, during a period of environmental change associated with rising lake level. Stromatolite calcite contains carbon and oxygen isotope records of changes to microbial activity in response to variable light environments and water chemistry through time. The stromatolites grew synchronously with correlative calcite zones. The innermost (oldest) calcite zone has a wide range of δ¹³C_calcite values consistent with variable photosynthetic effects on local DIC ¹³C/¹²C. Subsequent calcite zones preserve a progressive enrichment in δ¹³C_calcite values of approximately + 2.6‰ through time, with δ¹³C_calcite values becoming less variable. This enrichment likely records the removal of ¹²C by photosynthesis from the DIC reservoir over decades, with photosynthetic effects decreasing as light levels became lower and more consistent through time. Mean δ¹⁸O_calcite values of the innermost calcified zone were at least 1‰ lower than those of the other calcified zones (t test p-level < 0.001). The significant difference in δ¹⁸O_calcite values between the innermost and other calcified zones could be a product of mixing source waters with different isotopic values associated with the initiation of lake stratification associated with rising lake level. Overall, Lake Joyce stromatolites record significant lateral variability in relative photosynthetic rate and long-lived lake water stratification with microbial modification of the DIC pool. Such processes provide criteria for interpreting microbial activity within polar paleolake deposits and may shed light on variability in lake environments associated with changing climate in the McMurdo Dry Valleys.     

Comparison of the exploitation of methane-derived carbon by tubicolous and non-tubicolous chironomid larvae in a temperate eutrophic lake

Carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) in three sympatric species of larval chironomids were analyzed in a temperate eutrophic shallow lake in Japan. Markedly lower δ¹³C values were reported in Chironomus plumosus (?51.2 ‰) and Tanypus sp. (?43.5 ‰) than those in photoautotrophic carbon sources [particulate organic matter (POM) and sediment]. There were positive correlations between δ¹³C and δ¹⁵N in the two chironomid species. These results indicated that they assimilated carbon derived from biogenic methane by exploiting methane-oxidizing bacteria (MOB). In contrast, Propsilocerus akamusi exhibited similar δ¹³C values to those of POM or sediment. A δ¹³C-based isotope mixing model was used to estimate the dietary contributions of MOB to each chironomid species. The mean contributions ranged from 11 to 15 % in C. plumosus, 13 to 19 % in Tanypus sp., but only up to 5 % in P. akamusi. In an aquarium, we observed that individuals of C. plumosus and Tanypus sp., which exhibited low δ¹³C values, built U-shaped larval tubes in the sediment, and an oxidized layer developed around these tubes. Propsilocerus akamusi did not exhibit this behavior. These results suggest that tube building may provide larval chironomids with greater access to methane-derived carbon through increased opportunities to feed on MOB.     

Stable isotope records of sei whale baleens from Chilean Patagonia as archives for feeding and migration behavior

Carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope variations in baleen plates of sei whales (Balaenoptera borealis) stranded after a mass mortality event in Chilean Patagonia were investigated to assess potential dietary and migratory patterns. Carbon and nitrogen isotope ratios of seven baleens from six individuals were analyzed. The δ¹³C values ranged from ? 19.1 to ? 15.9‰ and the δ¹⁵N values from 8.7 to 15.4‰. Variations of up to 2.9‰ for δ¹³C and 5.3‰ for δ¹⁵N were observed within one baleen. Carbon and nitrogen isotope records of each baleen were significantly correlated and showed recurring oscillations confirmed by wavelet analyses. Oscillations slightly differed in periodicity indicating variable baleen growth rates between 10.0 and 16.5 cm/year. Food sources of the whales are discussed in terms of available isotope data for potential prey taxa and potential migratory behavior on the basis of latitudinal isotope gradients of particulate organic matter. Cyclicity could be explained by regular migrations of the sei whales from subtropical calving areas to high‐latitude foraging grounds. δ¹⁵N records of baleens differed between individuals eventually pointing to diverse feeding and migratory preferences among sei whale individuals.     

Isotopic biosignatures in carbonate‐rich,cyanobacteria‐dominated microbial mats of the Cariboo Plateau,B.C.

Photosynthetic activity in carbonate‐rich benthic microbial mats located in saline, alkaline lakes on the Cariboo Plateau, B.C. resulted in pCO₂ below equilibrium and δ¹³C_DIC values up to +6.0‰ above predicted carbon dioxide (CO₂) equilibrium values, representing a biosignature of photosynthesis. Mat‐associated δ¹³C_carb values ranged from ~4 to 8‰ within any individual lake, with observations of both enrichments (up to 3.8‰) and depletions (up to 11.6‰) relative to the concurrent dissolved inorganic carbon (DIC). Seasonal and annual variations in δ¹³C values reflected the balance between photosynthetic ¹³C‐enrichment and heterotrophic inputs of ¹³C‐depleted DIC. Mat microelectrode profiles identified oxic zones where δ¹³C_carb was within 0.2‰ of surface DIC overlying anoxic zones associated with sulphate reduction where δ¹³C_carb was depleted by up to 5‰ relative to surface DIC reflecting inputs of ¹³C‐depleted DIC. δ¹³C values of sulphate reducing bacteria biomarker phospholipid fatty acids (PLFA) were depleted relative to the bulk organic matter by ~4‰, consistent with heterotrophic synthesis, while the majority of PLFA had larger offsets consistent with autotrophy. Mean δ¹³C_org values ranged from ?18.7 ± 0.1 to ?25.3 ± 1.0‰ with mean Δ¹³C_inorg‐org values ranging from 21.1 to 24.2‰, consistent with non‐CO₂‐limited photosynthesis, suggesting that Precambrian δ¹³C_org values of ~?26‰ do not necessitate higher atmospheric CO₂ concentrations. Rather, it is likely that the high DIC and carbonate content of these systems provide a non‐limiting carbon source allowing for expression of large photosynthetic offsets, in contrast to the smaller offsets observed in saline, organic‐rich and hot spring microbial mats.     

Foliar and litter needle carbon and oxygen isotope compositions relate to tree growth of an exotic pine plantation under different residue management regimes in subtropical Australia

Background and aim

Significant differences in tree growth were observed in an exotic pine plantation under different harvest residue management regimes at ages 2–10 years. However, the variations in tree growth between residue management treatments could not be explained by soil and foliar nutrient analyses, except by potassium (K) concentration. Therefore, this study determined the carbon isotope composition (δ¹³C) and oxygen isotope composition (δ¹⁸O) of current and archived foliar samples from the exotic pine plantation to establish relationships with foliar K concentration and tree growth indices as a means to determine changes in stomatal conductance (g_s) and photosynthetic rate (A_max) or water use efficiency (WUE), and therefore understand the variations in tree growth across treatments.

Methods

The harvest residue treatments were: (1) residue removal, RR₀; (2) single level residue retention, RR₁; and (3) double level residue retention, RR₂. Foliar δ¹³C and δ¹⁸O were determined for samples at ages 2, 4, 6 and 10 years, and the atmospheric ¹³C discrimination (Δ¹³C), intercellular CO₂ concentration (C_i) and WUE were determined from the δ¹³C data. Litter needle δ¹³C and δ¹⁸O were also determined over 15 months between ages 9 and 10 years. These parameters or variables where correlated to each other as well as to the periodic mean annual increment of basal area (PAIB) and the periodic mean annual increment of tree diameter at breast height (PAID) across the treatments and over time. Foliar δ¹³C and δ¹⁸O were also related to published data of foliar K concentrations of the same trees.

Results

Significant variations of foliar δ¹³C, and therefore WUE and Δ¹³C, across treatments were only observed at ages 4 and 10 years old, and foliar δ¹⁸O at age 4 years old only. The results showed increasing foliar δ¹³C, δ¹⁸O and WUE, and decreasing Δ¹³C and Ci, from RR₀ to RR₂ treatments. However, while the WUE was positively related to the PAID and PAIB at age 4 years, it was negatively related to PAID and PAIB at age 10 years old. Litter needle δ¹³C, indicative of WUE, was also negatively related to the PAID at age 10 years old. . At age 4 years, foliar δ¹³C and δ¹⁸O were positively related with a steep slope of 7.70 ‰ across treatments, and that both isotopes were positively related to foliar K concentrations. Similarly, δ¹⁸O was negatively related to the Δ¹³C. No significant relationship can be determined between foliar δ¹³C, or Δ¹³C, and δ¹⁸O at age 10 years old. In addition, WUE was increasing (p?<?0.001) and Δ¹³C and C_i decreasing (p?<?0.001) with decreasing PAID over time.

Conclusions

The variations at age 4 years in foliar δ¹³C or Δ¹³C and δ¹⁸O and increasing WUE with increasing growth rate suggest growth induced water-stress with increasing residue-loading rate as a result of the nutritional effect of the harvest residues on tree growth. At age 10 years, the negative relationships between WUE and PAID indicate nutrient limitation has an over-riding effect on δ¹³C variations rather than g_s. This was due to the lack of a significant relationship between foliar Δ¹³C and δ¹⁸O at this age, as well as over time.