Changes In δ13C Research Articles

Using DIC‐δ13C Pair to Constrain Anthropogenic Carbon Increase in the Southeastern Atlantic Ocean Over the Most Recent Decade (2010–2020)

AbstractThe southeastern Atlantic Ocean is a crucial yet understudied region for the ocean absorption of anthropogenic carbon (Canth). Data from the A12 (2020) and A13.5 (2010) cruises offer an opportunity to examine changes in dissolved inorganic carbon (DIC), its stable isotope (δ13C), and Canth over the past decade within a limited region (1∼3°E, 32∼42°S). For the decade of 2010–2020, Canth invasion was observed from the sea surface down to 1,200 m based on both DIC and δ13C data. The mean Canth increase rate (1.08 ± 0.26 mol m−2 yr−1) during this period accelerated from 0.87 ± 0.05 mol m−2 yr−1 during the previous period (1983/84–2010). The δ13C‐based Canth increase closely matches the DIC‐based estimation below 500 m but is 26% higher in the upper ocean. This discrepancy is likely due to δ13C's longer air‐sea exchange timescale, seasonal variability in the upper ocean, and the chosen ratio of anthropogenically induced changes in δ13C and DIC. Finally, column inventory changes based on the two methods also exhibit very similar mean Canth uptake rates. The paired DIC concentration and stable isotope dataset may enhance our ability to constrain Canth accumulation and its controlling mechanisms in the ocean.

Variation patterns in foliar δ13C and δ15N among plant life types along an altitudinal gradient on the Tibetan Plateau

Understanding how foliar δ13C and δ15N vary with the environment is crucial for elucidating the carbon and nitrogen cycle dynamics within ecosystems. Yet, there is limited knowledge about these variations among different plant life types along altitudinal gradients, particularly in subtropical forest ecosystems found in the southeastern Tibetan Plateau. Therefore, we collected leaves from 67 species, including 20 trees, 24 shrubs, and 23 herbs along an altitudinal gradient of 834 m to 3105 m in the subtropical forest of Medog, and investigated their patterns of variation along the altitudinal gradient. Our findings revealed a positive relationship between foliar δ13C and altitude, as well as a negative relationship between foliar δ13C and soil temperature. There was no significant correlation between foliar δ15N and altitude, but a hump-shaped relationship was observed between foliar δ15N and soil moisture. Among the different plant functional groups, the δ13C of shrubs is more sensitive and representative of environmental changes. In addition, we discovered no difference in the δ13C (δ15N) values of leaves among trees, shrubs, and herbs in this study. Still, there were significant differences when compared to the δ13C (δ15N) values of leaves in other typical climatic zones, indicating that climatic zones influenced the δ13C and δ15N values of leaves more than plant functional groups. In conclusion, our results provide new insights into the climatic drivers of the changes in δ13C and δ15N of the different plant life types along the altitudinal gradient in the southeastern Tibetan Plateau.

Nitrogen nutrition effects on δ13C of plant respired CO2 are mostly caused by concurrent changes in organic acid utilisation and remobilisation.

Nitrogen (N) nutrition impacts on primary carbon metabolism and can lead to changes in δ13C of respired CO2. However, uncertainty remains as to whether (1) the effect of N nutrition is observed in all species, (2) N source also impacts on respired CO2 in roots and (3) a metabolic model can be constructed to predict δ13C of respired CO2 under different N sources. Here, we carried out isotopic measurements of respired CO2 and various metabolites using two species (spinach, French bean) grown under different NH4 +:NO3 - ratios. Both species showed a similar pattern, with a progressive 13C-depletion in leaf-respired CO2 as the ammonium proportion increased, while δ13C in root-respired CO2 showed little change. Supervised multivariate analysis showed that δ13C of respired CO2 was mostly determined by organic acid (malate, citrate) metabolism, in both leaves and roots. We then took advantage of nonstationary, two-pool modelling that explained 73% of variance in δ13C in respired CO2. It demonstrates the critical role of the balance between the utilisation of respiratory intermediates and the remobilisation of stored organic acids, regardless of anaplerotic bicarbonate fixation by phosphoenolpyruvate carboxylase and the organ considered.

Carbon cycling through plant and fungal herbarium specimens tracks the Suess effect over more than a century of environmental change

Although the anthropogenic decline in atmospheric carbon stable isotope ratios (δ13C) over the last 150 years (termed the Suess effect) is well-studied, how different terrestrial trophic levels and modes reflect this decline remains unresolved. To evaluate the Suess effect as an opportunistic tracer of terrestrial forest carbon cycling, this study analyzed the δ13C in herbarium specimens collected in Minnesota, USA from 1877 to 2019. Our results suggest that both broadleaf trees and ectomycorrhizal fungi relied on recent photosynthate to produce leaves and sporocarps, while saprotrophic fungi on average used carbon fixed from the atmosphere 32–55 years ago for sporocarp construction. The δ13C values of saprotrophic fungal collections were also sensitive to the age of their plant carbon substrate, with sporocarps of twig specialists tracking changes in atmospheric δ13C more closely than saprotrophs growing on logs. Collectively, this study indicates that natural history collections can quantitatively track carbon cycling among plants and fungi over time.

Estimating ethanol correction factors for δ13C and δ15N isotopic signatures of freshwater zooplankton from multiple lakes

AbstractIn freshwater systems, δ13C and δ15N stable isotopes can be used to differentiate between pelagic and littoral energy sources and to quantify trophic position. In these ecosystems, crustacean zooplankton are frequently used to characterize the pelagic baseline. Zooplankton samples are often preserved prior to processing and analysis, which can affect isotopic signatures. Variability in preservation effects across studies make it difficult to determine if and how to correct for preservation effects. Here, we develop a correction factor for ethanol preservation and present a flexible statistical method that can be updated with additional data to increase its applicability. We collected zooplankton from five lakes in Minnesota, USA encompassing wide isotopic ranges (δ13C from −37.23‰ to −23.96‰; δ15N from 3.07‰ to 14.44‰). Changes in zooplankton δ13C and δ15N signatures were quantified using a Bayesian hierarchical model predicting fresh values from ethanol‐preserved values. Ethanol preservation increased δ13C by a factor of 1.158 (95% CI 0.866–1.441) and had a negligible effect on δ15N (slope = 1.077; 95% CI 0.833–1.359). Lake‐specific values did not differ from the overall relationship. K‐fold and leave‐one‐out cross validation tests verified that both models were accurate; RMSE of predicted δ13C = 0.701 and RMSE of predicted δ15N = 0.590. Our correction factors could be applied to other systems in which baseline δ13C and δ15N values fall within the range of our study, and this approach also enables the inclusion of data from additional lakes to estimate new corrections.

Carbon and nitrogen biogeochemistry of a high-altitude Himalayan lake sediment: Inferences for the late Holocene climate

A study was conducted to decipher changes in paleoenvironmental conditions of the Kashmir Valley (India) using stable isotopic compositions and elemental concentrations of total organic carbon (TOC) and total nitrogen (TN) in a sediment core from the Wular Lake. The Chronology of the core established through radiocarbon dating estimated the age of the core bottom to be 3752 Cal years BP, covering the late Holocene. Using carbon isotopic compositions of TOC (δ13C), nitrogen isotopic compositions of TN (δ15N), and TOC - TN contents, the study identified changes in biology and associated biogeochemical processes in the Wular Lake during the late Holocene. Changes in C and N biogeochemistry of the lake through the last 3752 Cal years BP suggested overall drier condition during 3752–1500 Cal years BP that transitioned into a wetter condition at around 1500 Cal years BP until at least 295 Cal years BP. Evidence for relatively intense drier events were observed within the dry and wet phases at around 2500 and 500 Cal years BP. Changes in δ13C and TOC contents in the sediment core revealed that the inorganic C dynamics and productivity (along with organic C contents) in the lake were largely regulated by variations in respired CO2 and HCO3− availability along with terrestrial matter supply through the Jhelum River. Similarly, variations in δ15N and TN contents showed changes in N dynamics of the lake with varying nitrification and decomposition throughout the studied period. Observed dry and wet phases in the region might be due to the weakening and strengthening of the precipitation, which was linked to negative and positive phases of the North Atlantic Oscillation, respectively.

Amino acid carbon isotope profiles provide insight into lability and origins of particulate organic matter

AbstractIdentifying the phytoplankton origin of particulate organic matter (POM) in the deep ocean is challenging due to the changing phytoplankton composition and varied extent of decomposition by heterotrophic bacteria and zooplankton. Here, we report vertical distributions of amino acid stable carbon isotope values (δ13C) measured in particles in the South China Sea. Carbon‐weighted amino acid δ13C values generally parallel bulk POM δ13C profiles with a positive offset from 2.4‰ in the surface water to 6.0‰ in deep. Accordingly, a lability model is introduced to describe the relative distribution and isotopic linkage of labile and refractory particulate organic carbon. Temporal changes of amino acid δ13C were observed during the microbial decomposition of two phytoplankton strains, the prokaryote cyanobacterium Synechococcus and the eukaryote diatom Thalassiosira weissflogii. Principal component analysis of individual amino acid δ13C values from the decomposing cells separate samples into two components, reflecting the influence of microbial decomposition and taxonomic differences, respectively. These two components were further applied to particles. A major contribution from decomposing phytoplankton to particles below the euphotic zone was observed, with more prokaryote organic matter in the basin and a larger contribution from eukaryotes at stations near the productive northern shelf. Our findings show the applicability of amino acid δ13C values in tracing the lability and phytoplankton origins of POM in samples with varied extents of decomposition in marine environments.

No detectable influence of the carbonate ion effect on changes in stable carbon isotope ratios (δ13C) of shallow dwelling planktic foraminifera over the past 160 kyr

Abstract. Laboratory experiments showed that the isotopic fractionation of δ13C and of δ18O during calcite formation of planktic foraminifera are species-specific functions of ambient CO32- concentration. This effect became known as the carbonate ion effect (CIE), whose role for the interpretation of marine sediment data will be investigated here in an in-depth analysis of the 13C cycle. For this investigation, we constructed new 160 kyr long mono-specific stacks of changes in both δ13C and δ18O from either the planktic foraminifera Globigerinoides ruber (rub) or Trilobatus sacculifer (sac) from 112 and 40 marine records, respectively, from the wider tropics (latitudes below 38°). Both mono-specific time series Δ(δ13Crub) and Δ(δ13Csac) are very similar to each other, and a linear regression through a scatter plot of both data sets has a slope of ∼ 0.99 – although the laboratory-based CIE for both species differs by a factor of nearly 2, implying that they should record distinctly different changes in δ13C, if we accept that the carbonate ion concentration changes on glacial–interglacial timescales. For a deeper understanding of the 13C cycle, we use the Solid Earth version of the Box model of the Isotopic Carbon cYCLE (BICYLE-SE) to calculate how surface-ocean CO32- should have varied over time in order to be able to calculate the potential offsets which would by caused by the CIE quantified in culture experiments. Our simulations are forced with atmospheric reconstructions of CO2 and δ13CO2 derived from ice cores to obtain a carbon cycle which should at least at the surface ocean be as close as possible to expected conditions and which in the deep ocean largely agrees with the carbon isotope ratio of dissolved inorganic carbon (DIC), δ13CDIC, as reconstructed from benthic foraminifera. We find that both Δ(δ13Crub) and Δ(δ13Csac) agree better with changes in simulated δ13CDIC when ignoring the CIE than those time series which were corrected for the CIE. The combination of data- and model-based evidence for the lack of a role for the CIE in Δ(δ13Crub) and Δ(δ13Csac) suggests that the CIE as measured in laboratory experiments is not directly transferable to the interpretation of marine sediment records. The much smaller CIE-to-glacial–interglacial-signal ratio in foraminifera δ18O, when compared to δ13C, prevents us from drawing robust conclusions on the role of the CIE in δ18O as recorded in the hard shells of both species. However, theories propose that the CIE in both δ13C and δ18O depends on the pH in the surrounding water, suggesting that the CIE should be detectable in neither or both of the isotopes. Whether this lack of role of the CIE in the interpretation of planktic paleo-data is a general feature or is restricted to the two species investigated here needs to be checked with further data from other planktic foraminiferal species.

Assessing transient changes in the ocean carbon cycle during the last deglaciation through carbon isotope modeling

Abstract. Atmospheric carbon dioxide concentration (pCO2) has increased by approximately 80 ppm from the Last Glacial Maximum (LGM) to the early Holocene. The change in this atmospheric greenhouse gas is recognized as a climate system response to gradual change in insolation. Previous modeling studies suggested that the deglacial increase in atmospheric pCO2 is primarily attributed to the release of CO2 from the ocean. Additionally, it has been suggested that abrupt change in the Atlantic meridional overturning circulation (AMOC) and associated interhemispheric climate changes are involved in the release of CO2. However, understanding remains limited regarding oceanic circulation changes and the factors responsible for changes in chemical tracers in the ocean during the last deglaciation and their impact on atmospheric pCO2. In this study, we investigate the evolution of the ocean carbon cycle during the last deglaciation (21 to 11 ka BP) using three-dimensional ocean fields from the transient simulation of the MIROC 4m climate model, which exhibits abrupt AMOC changes similar to those observed in reconstructions. We investigate the reliability of simulated changes in the ocean carbon cycle by comparing the simulated carbon isotope ratios with sediment core data, and we examine potential biases and overlooked or underestimated processes in the model. Qualitatively, the modeled changes in atmospheric pCO2 are consistent with ice core records. For example, during Heinrich Stadial 1 (HS1), atmospheric pCO2 increases by 10.2 ppm, followed by a reduction of 7.0 ppm during the Bølling–Allerød (BA) period and then by an increase of 6.8 ppm during the Younger Dryas (YD) period. However, the model underestimates the changes in atmospheric pCO2 during these events compared to values derived from ice core data. Radiocarbon and stable isotope signatures (Δ14C and δ13C) indicate that the model underestimates both the activated deep-ocean ventilation and reduced efficiency of biological carbon export in the Southern Ocean and the active ventilation in the North Pacific Intermediate Water (NPIW) during HS1. The relatively small changes in simulated atmospheric pCO2 during HS1 might be attributable to these underestimations of ocean circulation variation. The changes in Δ14C associated with strengthening and weakening of the AMOC during the BA and YD periods are generally consistent with values derived from sediment core records. However, although the data indicate continuous increase in δ13C in the deep ocean throughout the YD period, the model shows the opposite trend. It suggests that the model either simulates excessive weakening of the AMOC during the YD period or has limited representation of geochemical processes, including marine ecosystem response and terrestrial carbon storage. Decomposing the factors behind the changes in ocean pCO2 reveals that variations in temperature and alkalinity have the greatest impact on change in atmospheric pCO2. Compensation for the effects of temperature and alkalinity suggests that the AMOC changes and the associated bipolar climate changes contribute to the decrease in atmospheric pCO2 during the BA and the increase in atmospheric pCO2 during the YD period.

Source appointment of δ13C in sediments of a maar lake in southern China: Implications of fossil fuel CO2 emissions

Stable carbon isotopes (δ13C) in lake sediments can record changes in CO2 emissions from fossil fuels (FFs) combustion; however, the exact proportion of δ13C changes caused by FFs combustion remains unclear. Here, taking the Huguangyan Maar Lake (HGY) located in low latitude with high intensity of human activities in southern China as an example, we used the MixSIAR Bayesian stable isotope mixing model to trace the source change of δ13C in the HGY sediments over the past 130 years. The δ13C value, ranging from −20.23‰ to −24.29‰, with an average of −22.39 ± 1.5‰, displayed a continuous downward trend from the bottom to top throughout the core, with the fastest decline occurring between 1950AD and 1996AD. Quantitative source appointment indicates that the contribution of FFs sources to δ13C depletion in the HGY sediments has increased nearly three-fold over the past 130 years, from 18.6% to 67.8%, which may be related to the Suess Effect of the 13C-depleted FFs burning. The increase in δ13C in HGY sediments in the past decade was consistent with the increase in δ13C–CO2 emitted by global fossil-fuel consumption, and was also related to the decrease in fossil fuels consumption caused by China's Clean Air Act. This study demonstrates that the isotope traceability model is helpful for source appointment of δ13C in lake sediments and that the δ13C in maar lake sediments can be used to trace CO2 emissions from global FFs combustion. This therefore provides new insights on the carbon cycle during the Anthropocene.

Corals nitrogen and carbon isotopic signatures alters under Artificial Light at Night (ALAN)

This study examines the impact of Artificial Light at Night (ALAN) on two coral species, Acropora eurystoma and Pocillopora damicornis, in the Gulf of Aqaba/Eilat Red Sea, assessing their natural isotopic responses to highlight changes in energy and nutrient sourcing due to sensory light pollution. Our findings indicate significant disturbances in photosynthetic processes in Acropora eurystoma, as evidenced by shifts in δ13C values under ALAN, pointing to alterations in carbon distribution or utilization. In Pocillopora damicornis, similar trends were observed, with changes in δ13C and δ15N values suggesting a disruption in its nitrogen cycle and feeding strategies.The study also uncovers species-specific variations in heterotrophic feeding, a crucial factor in coral resilience under environmental stress, contributing to the corals' fixed carbon budget. Light measurements across the Gulf demonstrated a gradient of light pollution which possess the potential of affecting marine biology in the region. ALAN was found to disrupt natural diurnal tentacle behaviors in both coral species, crucial for prey capture and nutrient acquisition, thereby impacting their isotopic composition and health.Echoing previous research, our study underscores the need to consider each species' ecological and physiological contexts when assessing the impacts of anthropogenic changes. The findings offer important insights into the complexities of marine ecosystems under environmental stress and highlight the urgency of developing effective mitigation strategies.

Studies on the Correlation between δ13C and Nutrient Elements in Two Desert Plants

Stable carbon isotopes (δ13C) and elemental stoichiometry characteristics are important ways to research the water and nutrient use strategies of plants. Investigating the variation patterns inof δ13C and the major nutrient elements in different organs of plants and the correlation among them can reveal the ecological strategies of desert plants in extreme arid environments. In this study, two typical desert plants, Alhagi sparsifolia and Karelinia caspia, were studied in the Tarim Basin. By analyzing the changes in δ13C, carbon (C), nitrogen (N), and phosphorus (P) and the ecological stoichiometry of their roots, stems, and leaves, the distribution patterns among different organs and their correlation with soil environmental factors were revealed. The results showed the following: (1) The δ13C of the two plants differed significantly among different organs (p < 0.01). The root and stem of Alhagi sparsifolia had significantly greater δ13C than the leave, while the δ13C of Karelinia caspia showed a root > stem > leaf gradient; (2) the C content in the leaves of the two plants was significantly lower than that of the root (p < 0.01), whereas the N content showed the opposite trend (p < 0.01); (3) the average N:P of Alhagi sparsifolia was >16.00, indicating it was mainly limited by P elements, while the average N:P of Karelinia caspia was <14.00, suggesting it was mainly limited by N elements; (4) in the root, stem and leave of Alhagi sparsifolia and Karelinia caspia, the N content and C:N and the P content and C:P showed a significantly negative correlation (p < 0.01), and δ13C was negatively correlated with C:P; (5) soil total phosphorus (TP) is an important soil environmental factor affecting δ13C and the nutrient elements in Alhagi sparsifolia and Karelinia caspia. This study demonstrates that Alhagi sparsifolia and Karelinia caspia are able to effectively coordinate and regulate their water, N, and P use strategies in response to environmental stress. These results can provide scientific reference for the evaluation of plant physiological and ecological adaptations for ecological conservation in arid areas.

Diet of the Small Cave Bear Ursus (Spelaearctos) rossicus Borissak, 1930 (Mammalia, Carnivora, Ursidae) As Revealed by 13C and 15N Isotope Analyses in Bone Collagen.

The 13C and 15N isotope contents in bone collagen were analyzed using bones of the small cave bear Ursus (Spelaearctos) rossicus Borissak, 1930 from localities in the Middle and Southern Urals. The bones date from the last interglacial (MIS 5) and glacial (MIS 3) periods. The bones were from males and females aged 3, 4, and >4 years. Sexual, geographical, and chronological differences in 13C and 15N contents were studied. Notable gender, geographic, and chronological differences were observed between samples. In the Middle Urals, females led a more predatory lifestyle than males during the interglacial period, and the trophic niches of males and females converged due to an increase in herbivory during the transition to the glacial period. In the Southern Urals, males led a more predatory lifestyle than in the Middle Urals during the interglacial period. The extent of changes in δ13C and δ15N values in the Southern Urals during the transition was found to correspond to differences between trophic levels.

Temporal shifts in the marine feeding of individual Atlantic salmon inferred from scale isotope ratios.

Given the limited information on prey use during the marine residency period for Atlantic salmon, scales were collected from salmon at return to the River Namsen (Norway) for spawning after 1 year at sea, and scale material from the first and second summer marine feeding periods was analysed using stable isotope methods to understand dynamics of their trophic ecology. As the salmon increased in size from the first to second summer, they reduced their feeding niche and specialised more (narrowed the δ13C range) and increased their dependency on higher tropic level (δ15N) prey, likely fish. Changes in δ13C indicated a consistent pattern of movement towards the north and west between summer feeding periods. Hence, salmon during their first year at sea may have a migration route roughly resembling that of previous spawners, as inferred from earlier tagging studies. Feeding conditions and nutrient composition during the last summer at sea, i.e. in the months before returning to the river for spawning, impacted final body size and within-season timing of return. Fish undergoing the largest trophic niche shift (δ13C and δ15N combined) between summer feeding periods, returned earliest. The earliest returning fish had the fastest specific growth rates at sea. Hence, salmon encountering abundant high-quality fish food during the marine migration, particularly during the last months, may reach a size and energetic state whereby it is better to return early to a safer environment in freshwater than risk being eaten by a big predator at sea. Both trophic status (δ15N), resource use (δ13C) and growth rates were significantly correlated between feeding periods. Nutrient composition during the first summer at sea did not impact the fish body length after the following winter, but growth conditions during the first summer evidenced carry-over effects from the first to the second summer of feeding.

Glacial expansion of carbon-rich deep waters into the Southwestern Indian Ocean over the last 630 kyr

Oceanic carbon storage is one of the main sinks for atmospheric CO2, and thought to be the major contributing factor for CO2 drawdown during past glacial periods. Both physical and biogeochemical processes control the capacity of carbon storage in the ocean. During glacial periods of the Pleistocene the larger volume of deep-water masses of Southern Hemisphere origin in the Atlantic has been shown to promote carbon storage in the Southern Ocean. However, the latitudinal extension of this water mass in the Indian Ocean has been scarcely studied. In this study, we combine foraminiferal εNd and benthic δ13C of two sediment cores in the southwest Indian Ocean (MD96–2077, 33°S, 3781 m water depth; MD96–2052, 19°S, 2627 m water depth), to reconstruct the spatial and temporal evolution of glacial carbon-rich deep waters in the SW Indian over the last 630 kyr. The combined use of foraminiferal εNd and benthic δ13C allows to distinguish δ13C changes related to water mass mixing and from respired carbon accumulation within the water masses. Nutrient-rich deep waters, which cannot be explained by the enhanced proportion of southern-sourced waters, were present at core sites deeper than 2700 m during glacial periods and extended at least until 33°S into the SW Indian Ocean. From Marine Isotope Stage (MIS) 14 to MIS 10, glacial carbon storage increased gradually until reaching its highest capacity during the extreme glacial periods MIS 12 and 10. Orbital forcing (100-kyr eccentricity, 41-kyr obliquity), restricted air-sea exchange and enhanced ocean stratification, fostered higher carbon storage during periods of relatively lower eccentricity and obliquity. Furthermore, after MIS 10, a progressive transition was observed from 100-kyr eccentricity to 41-kyr obliquity cycles in benthic δ13C and δ18O records of core MD96–2077 and sea-ice cover changes derived from ice-rafted debris of the Agulhas Plateau composite core site.

Carbon dynamics of the plant-soil system during vegetation succession in dune-meadow cascade ecosystems in Horqin Sandy Land, China

Understanding the relationship between the plant-soil carbon dynamics and environment is important for the management of arid and semi-arid ecosystems. Under long-term overgrazing and reclamation, the landscape of the Horqin Sandy Land has transformed into a dune-meadow ecotone. However, the effects of landscape evolution on the carbon dynamics in this region remain poorly investigated. This study evaluated changes in plant-soil organic carbon content and stable carbon isotope ratios (δ13C) attributable to vegetation restoration in dune ecosystems and wetland degradation in meadow ecosystems, and evaluate the indicative implications and driving mechanisms of the 13C enrichment in the plant-soil system. The dune ecosystems comprised semimobile dune with 25- and 15-year-old Artemisia Halodendron communities (SMAH1, SMAH2), fixed dune with 30- and 40-year-old Caragana microphylla communities (FCM1, FCM2), semimobile dune with 40-year-old Salix gordejevii community (SMSG), and transitional zone with a 30-year-old artificial Populus forest (TPF) in the gentle lower part. The meadow ecosystems comprised a corn field that transformed from natural meadow over 40 years (MCF) and a natural meadow with Phragmites australis community (MPA). The results show that plant leaves in different landscapes in the cascade ecosystems had various δ13C. And the mechanism of the leaves carbon isotope fractionation with increasing leaf age was significant and consistent, but different vegetation communities had different nutrient utilization strategies during ontogeny. Landscape types and soil depths are the key factors affecting SOC content, its δ13C (δ13Cs) and SOC stock changes. And the overall soil carbon turnover was accelerated with vegetation restoration. Especially in TPF, vegetation restoration is particularly significant for SOC, which is mainly reflected in the upper 20 cm of soil profile. The degradation of MPA to MCF led to loss of SOC content and storage and accelerated the soil carbon turnover rate. The δ13Cs of the dune and meadow ecosystems were positively correlated with organic-rich litter nitrogen content and were significantly negatively correlated with SOC, soil moisture, clay content, and total organic carbon to total nitrogen ratio of organic-rich litter. The revegetation using native and introduced tree and shrub species increased SOC content in dune ecosystems and significantly promoted the degree of its coupling with soil nitrogen and water.

Analysis of omega-3 and omega-6 polyunsaturated fatty acid metabolism by compound-specific isotope analysis in humans

Natural variations in the 13C:12C ratio (carbon-13 isotopic abundance [δ13C]) of the food supply have been used to determine the dietary origin and metabolism of fatty acids, especially in the n-3 PUFA biosynthesis pathway. However, n-6 PUFA metabolism following linoleic acid (LNA) intake remains under investigation. Here, we sought to use natural variations in the δ13C signature of dietary oils and fatty fish to analyze n-3 and n-6 PUFA metabolism following dietary changes in LNA and eicosapentaenoic acid (EPA) + DHA in adult humans. Participants with migraine (aged 38.6 ± 2.3 years, 93% female, body mass index of 27.0 ± 1.1 kg/m2) were randomly assigned to one of three dietary groups for 16 weeks: 1) low omega-3, high omega-6 (H6), 2) high omega-3, high omega-6 (H3H6), or 3) high omega-3, low omega-6 (H3). Blood was collected at baseline, 4, 10, and 16 weeks. Plasma PUFA concentrations and δ13C were determined. The H6 intervention exhibited increases in plasma LNA δ13C signature over time; meanwhile, plasma LNA concentrations were unchanged. No changes in plasma arachidonic acid δ13C or concentration were observed. Participants on the H3H6 and H3 interventions demonstrated increases in plasma EPA and DHA concentration over time. Plasma δ13C-EPA increased in total lipids of the H3 group and phospholipids of the H3H6 group compared with baseline. Compound-specific isotope analysis supports a tracer-free technique that can track metabolism of dietary fatty acids in humans, provided that the isotopic signature of the dietary source is sufficiently different from plasma δ13C.

Ontogenetic changes in green turtle (Chelonia mydas) diet and home range in a tropical lagoon

Ontogenetic changes in habitat and diet are widespread among marine species. Most species of sea turtles are characterized by extreme ontogenetic changes in habitat use and diet, with large changes occurring in early developmental stages (e.g., neonates to juveniles). Changes can continue even after recruitment to shallow coastal habitats. In places where substantial transitions in habitat occur across short distances, it is possible that the distances of developmental movements from one habitat to another could be short. We investigated ontogenetic changes in home range size, home range location and diet of Chelonia mydas in a tropical coastal lagoon in north-western Australia by combining acoustic telemetry with stable isotope analysis. There was a substantial (but nonlinear) increase in home-range size (kernel utilization distribution: KUD) with length, and an increase in the average distance of the center of home ranges from shore with length: larger turtles tended to occupy larger areas further from the shore. These patterns were accompanied by complex nonlinear changes in δ13C, δ15N and δ34S of red blood cells and nails; changes were rapid from 36 cm (the length of the smallest individual captured) to 50 cm, before reversing more gradually with increasing size. δ15N and δ34S (but not δ13C) of red blood cells and nails increased monotonically with KUD and distance from shore. Seagrass was likely an important food for all sizes, macroalgae was potentially important for small (< 60 cm CCL) individuals, and the proportion of scyphozoan jellyfish in diet increased monotonically with size. The combination of acoustic telemetry and stable isotope analysis revealed ontogenetic shifts in use of space and diet across short distances in a tropical coastal lagoon.

How to use live sampling tissues and archived specimens in cetacean stable isotope research

Cetaceans are unique ecological engineers, and their restoration may have a crucial impact on the future structure of aquatic ecosystems, which calls for more investigations into their trophic ecology. Among current techniques, stable isotope analysis (SIA) has the advantages of non-invasive sampling and long timescales. However, the full benefits of SIA in cetacean research may not be achieved due to issues like different types of tissue between sampling methods and use of chemical preservation solutions in historical specimens. To address these challenges, we conducted a study on Narrow-ridged Finless Porpoises (Neophocaena asiaeorientalis). Multiple tissues from freshwater and marine subspecies, as well as tissues preserved using different solutions such as ethanol and formalin were collected for SIA. Linear mixed effects models were used for data analysis. Our results showed that, except for blubber, kidney, and stomach, differences between other tissues were correctable. In tissues from live sampling, we found no significant difference between blood and muscle, and skin could also be used for isotope analysis after proper correction. Ethanol preservation caused significant positive changes in δ13C and δ15N values of muscle, while formalin preservation caused negative changes in δ13C and δ15N. Our findings provide valuable insight into unifying data from stranded carcasses and live sampling, as well as correcting for the effect of chemical preservation on museum specimens. Findings from this research support further application of stable isotope analysis in the conservation of endangered finless porpoises, offer a reference for other similar cetaceans, and also provide guidance for chemical preservation when freezing conditions are not available.

Temporal dynamics of density separated soil organic carbon pools as revealed by δ13C changes under 17 years of straw return

Straw return is widely recommended as an effective measure to increase the amount of soil organic carbon (SOC) and improve soil fertility in agroecosystems. However, the temporal dynamics of the original and newly derived SOC during long-term straw return remain uncertain. This study aimed to (i) determine the temporal dynamics of original and newly derived SOC in various soil organic matter (SOM) fractions during cultivation and (ii) evaluate the effect of continuous straw return on the contents of original and newly derived SOC. These aims were achieved by analyzing topsoil samples (0–20 cm) in a 17-year field experiment using a continuous maize (Zea mays L.) cropping system. Three fertilizer treatments were tested: no fertilizer (NF), mineral fertilizer (NPK), and mineral fertilizer with straw return (NPKS). The SOC was physically separated into three density fractions: the free light fraction (fLFOC), occluded light fraction (oLFOC), and heavy fraction (HFOC), and their δ13C values were determined. The original and newly derived SOC of each fraction and the rate of conversion of exogenous C inputs were evaluated. The SOC content in the bulk soil and C fractions generally increased continually with cultivation time in the NPKS, indicating that the soil had not reached maximum C sequestration. NPKS significantly increased the C contents of fLFOC, oLFOC, and HFOC by 38.6%, 43.0%, and 11.6%, respectively, relative to the initial soil after the 17-year experiment. The newly derived SOC content was significantly higher in NPKS than in NF and NPK, and the rate of conversion of exogenous C inputs to SOC was the lowest in NPKS. The annual rate of loss of original SOC in NPKS was lowest for the light fractions (fLFOC and oLFOC) and highest for HFOC, implying that the return of straw moderated the mineralization of original SOC in the light fractions but increased the decomposition of original SOC in the heavy fraction via the priming effect (PE). The average rate of conversion of exogenous C inputs remained the highest in HFOC (23.5%), lowest in oLFOC (0.6%), and intermediate in fLFOC (4.1%) throughout the study. Thus, the stable HFOC pool retained more straw C, despite the stronger PE. Our results demonstrated that long-term straw return continuously increased the proportion of new SOC in the bulk soil and density fractions and increased the decomposition of the original SOC in the stable SOC fractions. These findings provide further detailed insights into the mechanisms of SOC stabilization in agricultural soils.