Variation In δ15N Research Articles

Microbial alteration in marine sediments: Insights from compound-specific isotopic compositions of amino acids in subseafloor environments

The proportion of amino acids (AAs) in sediment can be used as an indicator of microbial degradation, which is primarily the product of benthic prokaryote activity. The microbial activity would be reflected with the stable isotope ratio of nitrogen (δ15N) at the time of mineralization and resynthesis of AAs. In this study, the compound-specific isotope analysis of individual AAs was used to investigate δ15N variation associated with microbial processes in marine sediment samples. Our results showed a decrease in AA concentrations in core-top sediment was accompanied by an increase in δ15N values, suggesting large 15N enrichment in buried AAs. Phenylalanine displayed an increase in δ15N from the surface to depths greater than 2 cm, whereas relatively constant δ15N values at depths below 2 cm, suggesting that microbial utilization of phenylalanine varies with depth. Glycine showed the highest relative molar contribution (from 12.1 to 36.4%), with the largest δ15N increase (from 0.8 to 8.7) in deep sediment, implying that such information can serve as a measure of AA diagenesis in sedimentary environments. Our results also indicated that the δ15N values of individual AAs in sediment reflected the microbial alteration of organic matter at water-sediment interfaces and in sub-surface environments. These findings form an important basis for interpreting the δ15N values of AAs in sediment.

The relative importance of biological and environmental factors on the trophodynamics of a pelagic marine predator, the blue shark (Prionace glauca)

Marine ecosystems have been significantly altered by the cumulative impacts of human activities. Pelagic sharks have become vulnerable to increases in mortality rates caused by fishing. The decrease in number of these top predators could have substantial cascading effects on wider marine communities. Concerns about these potential impacts, and the critical need for effective management, have led to an increased interest in assessing the trophic ecology of sharks. While stable isotope analyses have been used to provide relevant insights about the trophic ecology of sharks, the causal factors leading to trophic variation between individuals has been largely overlooked. Here, we investigated the relative effect of biological factors, geographic location, and environmental factors on the spatial trophodynamics of the blue shark (Prionace glauca). Specifically, stable isotope values of δ15N and δ13C, and the estimated trophic position (TP) were analysed for 180 blue sharks collected from south of the Canary Islands in the Atlantic Ocean, to the north-western Mediterranean Sea. The results showed that models which included combined variables explained the variation in δ15N, TP and δ13C values better than models which considered only stand-alone predictors. The independent contributions of environmental variables and biological factors seemed to be more important than geographic location for δ15N and TP. δ15N and TP increased in a curvilinear fashion with body size, and TP was higher for females. In the case of δ13C values, only an effect from sex was observed. Among environmental variables, chlorophyll-a, pelagic productivity, and sea-surface temperature proved to be reliable predictors, particularly for δ15N and TP, most likely due to their relationship with productivity and prey availability. This study provides new information on ranking the factors that influence the trophodynamics of the blue shark, namely the environment, the geographic location, and the biological factors of the species.

Temporal and spatial variability in stable isotope values on seabird islands: What, where and when to sample

Invasive mammal eradications are widely used for managing island ecosystems. However, tracking the outcomes of such large-scale, whole ecosystem projects is challenging and costly, and monitoring all components of an ecosystem is near impossible. Instead, indicators of ecosystem change may provide more practical and integrated measures of ecosystem response to eradications. As high-order marine predators, seabirds subsidise island ecosystems with nutrients isotopically enriched in nitrogen. Invasive mammals have caused a global decline of seabirds on islands, reducing this nutrient subsidisation. Following eradications, nitrogen stable isotope analysis may provide a useful and resource-efficient indicator of ecosystem functional change on eradicated islands. However, isotope ratios are affected by a myriad of factors, with potential sources of variation being introduced by spatial and temporal variation in sampling, and within and between different taxa and ecosystem components. To correctly attribute isotopic change to post-eradication ecosystem function change, these confounding variables need to be understood. To address this need, we analysed stable isotopes of nitrogen in soil, plant, spider, and seabird guano samples collected at different distances from seabird colonies and at different stages of the short-tailed shearwater breeding cycle on six island sites around south-eastern Tasmania, Australia. Across these cool, temperate islands we detected no temporal variability in δ15N throughout the breeding season. However, there was notable spatial variability in δ15N values. The effects of seabird-derived nutrient subsidisation were highly localised with high δ15N values found inside seabird colonies and then rapidly decreasing from the colony boundary. Higher δ15N values also occurred in areas of higher burrow density within a colony. Variability in δ15N values also existed both within and between ecosystem components. Our results highlight the importance of context dependency when using ecological indicators and have important implications for the design, implementation and interpretation of studies employing stable isotopes as indicators for ecosystem change. We provide recommendations for designing future stable isotope studies on seabird islands.

Nitrogen Isotopes of Sinking Particles Reveal the Seasonal Transition of the Nitrogen Source for Phytoplankton

AbstractIn the high‐productivity subarctic ocean, phytoplankton growth is assumed to rely mainly on nitrate, based on snapshot observations conducted in spring or summer. However, the year‐round utilization of nitrate and ammonium by phytoplankton remains unexplored. Here, we show the seasonal transition of the dominant nitrogen source for phytoplankton by using the nitrogen isotopic composition (δ15N) of nitrate, ammonium, and sinking particles in the northwestern North Pacific. To decouple the combined effects of nitrogen sources for phytoplankton and subsequent trophic transfers in δ15N of sinking particles, we used compound‐specific δ15N analysis of amino acids (δ15NAA) in sinking particles. The seasonal change in δ15NAA revealed that most of the seasonal variation in δ15N of sinking particles arose from changes in the availability of nitrogenous nutrients in the euphotic layer. The δ15N of sinking particles suggested that the dominant nitrogen source for phytoplankton switches from nitrate to ammonium in autumn.

Can the stable isotope variability in a zooplankton time series be explained by its key species?

Stable isotope (SI) analysis is a standard tool to study marine food webs, usually based on the measurement of a few individuals from a small list of subjectively pre-defined species. The main objective of this study was to find out which species are significantly associated with the temporal variability of the SI composition of zooplankton in a tropical marine ecosystem. We investigated this by means of a novel species-biomass-isotopes-mixture (SBIM) approach that uses a relative biomass matrix to explain the SI signature of the zooplankton community. Furthermore, SBIM was applied to detect key taxa that can be considered bioindicators for important descriptors of ecosystem state (e.g., oligotrophy, carbon sources, mean trophic level). Plankton samples (64 μm mesh size) were obtained in Tamandaré Bay (northeastern Brazil) from June 2013 to August 2019. One aliquot of each sample was taken for stable isotope measurements and one for taxonomic identification and estimation of size and relative biomass. Total zooplankton biomass differed significantly between years, seasons and stations. Total zooplankton δ13C values ranged from −21.0 to −18.2‰ (mean ± standard deviation: −19.7 ± 0.7‰ in the dry season, and −19.4 ± 0.8‰ in the rainy season). Total zooplankton δ15N values ranged from 3.8 to 9.0‰ (7.0 ± 1.0‰ in the dry season, and 6.5 ± 1.2‰ rainy season). Total zooplankton C/N ratios ranged from 3.5 to 5.0 (4.2 ± 0.4 in the dry season and 4.2 ± 0.3 in the rainy season). The sparsely abundant and relatively large-sized copepod Pseudodiaptomus acutus was the most important species for explaining the variability in δ15N (22% of the total variability). Relative biomass (%) of P. acutus showed a strong positive correlation with δ15N, indicating a high trophic level (TL). Our results highlight the importance of less abundant taxa for marine food webs. Small-sized invertebrate larvae were negatively correlated with δ15N, indicating a TL below average. The copepod Dioithona oculata was the most important organism in explaining the δ13C of zooplankton (17.7% of the total variability, positive correlation with δ13C), indicating possible selective use of a13C-enriched food source (e.g., diatoms) by this cyclopoid copepod. Oithona spp. juveniles showed a negative relationship with zooplankton C/N ratio, which can be indicators of an oligotrophic ecosystem state and lipid-poor zooplankton. The tintinnid F. ehrenbergii showed a positive correlation with C/N, being an indicator for turbid “green waters'', during the rainy season, when the ecosystem was in a eutrophic state, with high lipid contents in the zooplankton community. The proposed SBIM approach opens up a novel pathway to understanding the factors and species that shape the temporal variability of food webs.

δ15N in Birch and Pine Leaves in the Vicinity of a Large Copper Smelter Indicating a Change in the Conditions of Their Soil Nutrition

δ13C and δ15N were analyzed in the leaves of Pinus sylvestris L. and Betula spp. under the conditions of severe heavy metal (Zn, Cu, Cd, and Pb) contamination. Twenty-seven plots located near the Karabash copper smelter (Russia) were studied. No reliable correlation of 13C in tree leaves with the level of pollution was observed. δ15N, both in Pinus sylvestris and Betula spp., increased similarly in polluted areas. δ15N was increased by 2.3‰ in the needles of Pinus sylvestris and by 1.6‰ in the leaves of Betula spp. in polluted plots compared to the background ones. The probable reasons for the increase in δ15N were estimated using multiple regression. The regression model, which includes two predictors: δ15N in the humus horizon and the occurrence of roots in the litter, explains 33% of the total variability of δ15N in leaves. Thus, in ecosystems polluted with heavy metals, the state of trees is determined not only by the direct toxic effects of heavy metals but also by indirect ones associated with the features of plant mineral nutrition. This fact opens the way to the search for opportunities to control the state of plants in disturbed ecosystems by regulating the content of mineral nutrition elements.

Histological and stable isotope analysis of archeological bones from St. Rombout's cemetery (Mechelen, Belgium): Intrasite, intraindividual, and intrabone variability

AbstractThis study compares histological preservation in archeological bones from different burial types to unravel the histotaphonomy‐to‐funerary practices relationship. Αn intraskeletal approach is also adopted to explore intraindividual (inner ear part of the petrous bone vs. upper/lower limb long bones) and intrabone (proximal vs. distal diaphysis) variability in bone collagen preservation, δ13C, and δ15N. The aim is to (a) target bones that likely retain higher amounts of collagen, (b) better understand the inner ear bone collagen isotopic signature and remodeling, and (c) assess intrabone isotopic and histological homogeneity. For the histological analysis, the data have been collected from 61 specimens (20 individuals) from the medieval/postmedieval cemetery of St. Rombout, Belgium. Thin sections have been studied using optical and scanning electron microscopy. For the collagen and isotopic data, 101 samples have been collected from 21 individuals. Distinct histological patterns are observed only in bones from single coffin burials; however, bone histology can display intraindividual and intrabone variability, which are important to account for interpretations. Collagen wt.%, δ13C, and δ15N show significant intraindividual differences but insignificant intrabone variability. This study also confirms the extraordinary nature of the petrous bone, as the inner ear bone collagen δ13C and δ15N values reflect the dietary input of the first approximately 2–3 years of life.

Climatic control effect on the soil nitrogen isotopic composition in Alisols across the physiographic regions of Pernambuco State, Northeast Brazil

Soil nitrogen dynamics in terrestrial ecosystems can be evaluated by analyzing the natural abundances of N isotope ratio mass spectrometry (δ15N) because δ15N integrates decades of organic matter transformations in the soil. On a large scale, the δ15N of soil organic matter across climate gradients may provide critical insights into understanding regional patterns of N cycling. However, the mechanisms that control N cycling in some tropical regions still present critical gaps, particularly in dry ecosystems. We evaluated the variation of soil δ15N in Alisols along the climatic gradient of three physiographic regions in Northeast Brazil: Zona da Mata (humid, rainfall well distributed throughout the year), Agreste (subhumid, rainfall concentrated in 5–7 months), and Sertão (semiarid, rainfall in highly intense events, concentrated over 3–5 months). Soil samples were collected at the 0–20 cm depth layer and analyzed for physical and chemical characteristics, including soil δ15N, by mass spectrometry. A linear model with physical biological, chemical, and climatic variables was generated to explain the spatial distribution of the soil δ15N. The map modeled values δ15N for Pernambuco State was produced using all observed values and the best-performing explicit spatial predictive variables in the model selection – pH, clay content, mean temperature, mean precipitation, and potential of evapotranspiration. Soil δ15N was positively related to clay content and negatively associated with the pH, temperature, rain, and evapotranspiration, expressing differences in nitrogen dynamics along the climatic gradient. In the Sertão, the low and erratic rainfall combined with a neutral pH and high temperature provide adequate conditions for nitrogen transformations and cause high soil δ15N.

Imprints of anthropogenic air pollution sources on nitrate isotopes in precipitation in a tropical metropolitan area

Identifying the origin of NOx emissions, their transformation into nitrate (NO3−) and its atmospheric deposition is important to better understand the impacts of air pollution on the environment and human health, on a local and regional scale. We measured nitrate isotopes (δ15N, δ18O of NO3−) in precipitation samples in a tropical station in Brazil in 2019–2020 and combined them with chemical, geospatial and meteorological data to trace NOx sources towards their wet deposition as NO3−. A total wet N deposition of ∼12 kg N/ha.year showed significant N air pollution, out of which the relative contribution of NO3−-N was ∼37% (4.6 kg NO3−-N/ha.year). The δ15N–NO3- values in precipitation ranged from −8.5 to +4.1‰, with a volume-weighted mean (VWM) value of −3.0 ± 2.7‰, whereas the δ18O–NO3- values ranged from +25.9 to +63.5‰ with a VWM value of +53.3 ± 7.3‰. The δ15N–NO3- results combined with air mass backward trajectory analysis and geospatial mapping revealed that the NOx emissions in Belo Horizonte originated from: vehicles and industries from the urban and industrialized areas, biomass burning related to wildfires, and ship emissions from the coastal areas. The temporal variation of δ15N, δ18O of NO3− values was attributed to NOx source variability and transformation processes. The use of integrated methods to evaluate the imprints of anthropogenic air pollution sources in Brazil will contribute to inform NOx source management strategies and improve air and water quality.

The variation and trends of nitrogen cycling and nitrogen isotope composition in tree rings: the potential for fingerprinting climate extremes and bushfires

PurposeClimate extremes, such as droughts and floods, have become intensified and more frequent due to intensifying climate change. Increased atmospheric carbon dioxide (CO2) and warming-induced water limitation, as well as climate extremes, may alter carbon (C) and nitrogen (N) cycling in forest ecosystems. This provides a brief review of stable nitrogen isotopic composition (δ15N) in tree ring in relation to climate extremes and bushfires in context of N availability and losses in forest ecosystems.Material and methodsTree rings were extracted from four Pinus sylvestris and four Larix gmelinii sample trees, located in a boreal plantation forest of Mohe City, Heilongjiang Province, China. Tree rings were measured to obtain mean annual basal area increment (BAI), while tree ring δ15N and total N concentrations were measured on mass spectrometer at 3-year intervals. The tree ring δ15N data were related to possible climate extremes and bushfires. A brief review of the relevant literature was also undertaken to support our preliminary research findings.Results and discussionGlobally, increasing atmospheric CO2 concentration and water limitations have led to a warmer-drier climate. This has also been associated with increases of climate extremes such as drought and floods as well as bushfires. These extremes have been recorded with detrimental effects on plant and soil structures within forest ecosystems and play an important role in regulating N availability and losses in forest ecosystems. Studies of N deposition within forest ecosystems using soil and plant δ15N also showed that N losses under various climate extremes can occur through direct changes in N cycling, such as increasing soil nitrification and denitrification or leaching. It is highlighted that tree rings δ15N has the potential to fingerprint the intensity and frequency of climate extremes and bushfires in the forest ecosystems, but more such tree ring δ15N research needs to be done in diversified forest ecosystems to confirm the potential of using tree ring δ15N for quantifying the frequency and intensity of climate extremes and bushfires at both regional and global scale.ConclusionThe variation and trend of δ15N in the soil–plant-climate systems are closely linked to the N cycling in forest ecosystems, and tree ring δ15N has the great potential to fingerprint both intensity and frequency of climate extremes such as drought and floods as well as bushfires.

Variability of the oxygen minimum zone associated with primary productivity and hydrographic conditions in the Eastern North Pacific during the last 1200 years

The expansion of the oxygen minimum zone (OMZ) associated with global warming has generated interest in its variability during the last two millennia. Several oceanographic mechanisms, as advection of dissolved oxygen and depletion of dissolved oxygen by oxidation of exported marine productivity, could explain the variability of δ15N in organic matter as a denitrification indicator of the water column in the Pacific Ocean. Our objective was to infer local or remote forcing mechanisms that lead to the strengthening or weakening of the OMZ in the Eastern Tropical North Pacific. A 42 cm sediment core was recovered at 680 m of depth in the Magdalena margin and sectioned every 1 cm for carbon and nitrogen stable isotope analysis. During the periods 1260–1420 CE and 1670–1800 CE, the desynchronization of δ15N between the Pescadero and Magdalena margins with respect to the Santa Barbara Basin confirms a decrease in denitrification in the Mexican Pacific. During the Little Ice Age (LIA: 1450 to 1800 CE), the δ15N indicated a reduction in denitrification in the Magdalena margin and the Santa Barbara Basin, but not in the Pescadero margin. This suggests an intense advection of dissolved oxygen from tropical waters during the LIA. In the Pescadero margin, denitrification conditions persisted over the same period, while exported productivity maintained a high oxygen demand.

Bulk and amino acid nitrogen isotopes suggest shifting nitrogen balance of pregnant sharks across gestation.

Nitrogen isotope (δ15N) analysis of bulk tissues and individual amino acids (AA) can be used to assess how consumers maintain nitrogen balance with broad implications for predicting individual fitness. For elasmobranchs, a ureotelic taxa thought to be constantly nitrogen limited, the isotopic effects associated with nitrogen-demanding events such as prolonged gestation remain unknown. Given the linkages between nitrogen isotope variation and consumer nitrogen balance, we used AA δ15N analysis of muscle and liver tissue collected from female bonnethead sharks (Sphyrna tiburo, n = 16) and their embryos (n = 14) to explore how nitrogen balance may vary across gestation. Gestational stage was a strong predictor of bulk tissue and AA δ15N values in pregnant shark tissues, decreasing as individuals neared parturition. This trend was observed in trophic (e.g., Glx, Ala, Val), source (e.g., Lys), and physiological (e.g., Gly) AAs. Several potential mechanisms may explain these results including nitrogen conservation, scavenging, and bacterially mediated breakdown of urea to free ammonia that is used to synthesize AAs. We observed contrasting patterns of isotopic discrimination in embryo tissues, which generally became enriched in 15N throughout development. This was attributed to greater excretion of nitrogenous waste in more developed embryos, and the role of physiologically sensitive AAs (i.e., Gly and Ser) to molecular processes such as nucleotide synthesis. These findings underscore how AA isotopes can quantify shifts in nitrogen balance, providing unequivocal evidence for the role of physiological condition in driving δ15N variation in both bulk tissues and individual AAs.

A multi-stable isotopic constraint on water column oxygen sinks in the Pearl River Estuary, South China

Bottom water oxygen depletion is a central concern in estuaries and coastal oceans worldwide. However, a mechanistic understanding and quantitative diagnosis of different oxygen-consuming processes are less clear. In this study, a multi-stable isotope approach is developed to delineate the role of oxygen respiration and nitrification contributing to total oxygen consumption in the Pearl River Estuary (PRE), a large eutrophic estuary in south China. The approach highly couples with analysis of the carbon isotope composition of dissolved inorganic carbon (δ13C-DIC) and with stable nitrogen isotope analysis in ammonium (δ15N-NH4+) and nitrate (δ15N-NO3−). In all seasons, relatively low DO concentrations were observed in the upper reach and, to some extent, in the outer estuary during summer, while high concentrations of DO were found in the transition zone between the inner and outer estuary. On the basis of isotopic differentiation, our data reveal that much more depleted δ13C-DIC is coincident with DIC additions and low oxygen in the upper reach and inner estuary during most seasons. This is most likely a consequence of organic carbon (OC) degradation via aerobic respiration. Based on the carbon isotopic mass balance of DIC and the stoichiometry ratio of −ΔDO/ΔDIC, we found that the OC degradation dominates the total oxygen consumption in the upper reach, as well as in the inner estuary during summer (48.3%–93.5%). In addition, nitrification is another key process in contributing to total oxygen loss in the upper reach, as supported by the well-coupled variations of δ15N of NH4+ and NO3− and apparent oxygen utilization (AOU). Using the formerly determined N isotopic fractionation and observed δ15N variation, we estimated that nitrification could account for 35.3%–44.1% and 28.5%–31.6% of the total oxygen consumption in the upper reach during winter and summer, respectively, while its contribution to total oxygen loss is minor in the inner and outer estuary. Overall, this study demonstrates the potential of the multi-stable isotopic approach to assess oxygen sink partitioning in large human-perturbed estuaries.

Relative importance of climatic and edaphic factors as drivers of plant δ15N along a longitudinal transect

ABSTRACT Many studies have shown that climatic and edaphic factors influence the variations in nitrogen isotopes (15N) in terrestrial ecosystems. However, the relative importance of co-varying climatic and edaphic controls on plant 15N remains somewhat unclear. To address this issue, regional-scale plant (L. chinensis) and soil samples were collected along a longitudinal transect across the temperate grasslands in northern China. The influences of climatic and edaphic factors on the plant 15N were disentangled using the variable importance in projection (VIP) approach. We found that the climatic and edaphic variables explained 52.4% and 31.2%, respectively, of the variation in plant 15N. The mean annual precipitation (MAP), soil N, and mean annual temperature (MAT) were the most important variables, but the soil water content (SWC), aridity index (AI), and soil clay content were also important variables. The soil pH and soil C: N ratios were identified as unimportant variables. Furthermore, a high-performance model for simulating plant 15N values was constructed based on the important variables (VIP > 0.8). Further investigations should focus on the effects of the interaction between the climatic and edaphic variables on the N cycles in the temperate grasslands to provide more reliable predictions of plant 15N.

Individual trophic niche specialization in American beaver (Castor canadensis)

The American beaver (Castor canadensis) has been described as a choosy generalist at the species/population scale, yet observational studies have shown little variation in diet among individuals. We compared isotopic values of δ13C or δ15N taken from hair of 32 beaver, representing seven colonies in northern Alabama, USA to determine 1) if colonies of beaver show overlap in isotopic niche width as a result of the similar use of food resources and 2) if individual trophic niche specialization occurs within colonies. Total Trophic Niche Width varied across the wetland with the widest being twice the narrowest. Each of the five niche ellipses overlapped with ≥ two other wetlands studied. The percentages of observed variance attributed to Within Individual Component, Between Individual Component, and Between Location Component for δ13C were 37%, 33%, 30%; and those for δ15N were 16%, 56%, and 28%. Dietary nitrogen differentiated the trophic niches of individual beavers. Our results revealed that colonies across the landscape showed separation in trophic niche, lending support that the choosy generalist classification is correct at the colony scale. Our results also support individual trophic niche specialization within colonies, as seen by the substantial amount variation in both δ13C (33%) and δ15N (56%) between individuals.

A Multi-Tissue, Multi-Species Assessment of Lipid and Urea Stable Isotope Biases in Mesopredator Elasmobranchs

The application of stable isotope analysis (SIA) is increasing in elasmobranch trophic ecology, but inconsistency remains in terms of the tissue pre-treatment methods chosen to remove biases introduced by lipids and urea. SIA of a range of non-lethally extracted tissues from a diverse group of elasmobranchs, including mesopredators, is increasing, yet most studies assume that isotope biases from lipid and urea are the same across tissues and species. To determine tissue- and species-specific isotope biases across treatment methods, three tissues and their components [muscle, fin, and blood separated into plasma and red blood cells (RBC)] were non-lethally extracted from three species of mesopredatory elasmobranchs and subjected to one of three treatment methods: (1) deionized water rinse [DW], (2) chloroform/methanol lipid extraction [LE], or (3) deionized water followed by chloroform/methanol [DW+LE]. In muscle δ13C, all treatments displayed minimal variation (∼ 0‰) but large increases in δ15N (∼ 1‰) indicated urea removal. Fin δ13C values decreased with DW but increased with LE and DW+LE, whilst all treatments increased fin δ15N (∼ 0.5‰), suggesting removal of both lipid and urea. Plasma δ13C and δ15N displayed high individual variation; large decreases in δ13C (∼−0.8‰) across all treatments, but particularly DW, suggested the removal of13C-enriched compounds while a small increase in δ15N (∼ 0.2‰) suggested minimal urea removal. In RBC, all treatments showed small δ13C declines (∼−0.5‰), with no difference in δ15N, suggesting minimal removal of13C-enriched compounds and urea. For muscle and fin, DW+LE is the most appropriate treatment to standardize δ13C and δ15N consistently across individuals and tissues. The large individual variation in treatment effects on plasma suggests it is unsuitable for current treatment methods. Consistent treatment effects for RBC allow for DW+LE standardization, however, broader species-specific effects are unknown. The importance of treatment choice for accurately estimating prey contributions to elasmobranch diet was highlighted using Bayesian stable isotope mixing model comparisons, with prey contributions varying significantly among treatments. This variability suggests that ecological inferences from elasmobranch tissue SIA are not robust to different treatment methods. It is recommended that studies employ standardized corrections using a combined DW+LE treatment where applicable.

Contrasting nitrogen cycling between herbaceous wetland and terrestrial ecosystems inferred from plant and soil nitrogen isotopes across China

Abstract Understanding nitrogen (N) cycling in different ecosystems is crucial to predicting and mitigating the global effects of altered N inputs. Although wetlands have always been assumed to differ largely from terrestrial ecosystems in N cycling, evidence from direct comparison from the field along wide environmental gradients is lacking. Here, we hypothesized strong coupling of plant and soil δ15N in terrestrial ecosystems due to lower N inputs and losses but weak coupling of plant and soil δ15N in wetlands because of higher N inputs and losses. We performed a large‐scale field investigation on 26 pairs of herbaceous wetland and terrestrial sites across China covering 21 degrees of latitude and determined natural abundance of nitrogen isotopes (δ15N) in soils and leaves of 346 dominant and subordinate plant species. We analysed the relationships between leaf and soil δ15N and their drivers including plant functional types in these two types of ecosystems. Plant functional types including mycorrhizal type and N2‐fixing status had consistently significant influences on leaf δ15N in herbaceous wetland and terrestrial ecosystems. Leaf δ15N increased significantly with soil δ15N within and across mycorrhizal types in both ecosystems, and, as hypothesized, the relationships were stronger and steeper in terrestrial than in wetland ecosystems. Moreover, leaf and soil δ15N were positively and significantly correlated within both N2‐fixers and non‐N2‐fixers in terrestrial ecosystems and within only non‐N2‐fixers in wetlands. At the community level, we also found more highly significant relationships between leaf and soil δ15N in terrestrial than in wetland ecosystems. Besides plant functional types, climatic and soil factors contributed to the variation in leaf δ15N in both ecosystems. Synthesis. Weaker relationships between plant and soil δ15N in wetlands at species and community levels support the hypothesis that larger N inputs and losses lead to weaker coupling in the plant–soil systems in wetlands than in terrestrial ecosystems. This provides strong evidence from a large spatial scale for contrasting N cycling in these two types of ecosystems regardless of plant functional type in terms of nutrient uptake strategy. Our findings add to our predictive power of ecosystem N dynamics under environmental changes, for example, land‐use changes and elevated N inputs.

Constructing an isotope ecology in southern Patagonia: Herbivore δ13C and δ15N variability from the Andes to Atlantic coast

The aim of this study is to assess the variability of the stable carbon and nitrogen isotope composition of herbivores -guanaco and huemul-in southern Patagonia. The study area extends from the eastern slopes of the Andes to the Atlantic Coast between 47° and 49° S. The ultimate aim of the research was to generate an isotopic ecology with an adequate spatial resolution that allows the incorporation and development of stable isotope studies in discussions of the population dynamics of hunter-gatherers in continental Patagonia. This paper presents the δ13C and δ15N values of 159 guanaco and huemul specimens, both modern and archaeological. This space was divided into three areas: Western area, Central Plateau and Eastern Area. Isotopic variability within groups was analyzed using the SIBER, from which the isotopic niches of the different groups are compared. The results show a spatial pattern of isotopic variability in herbivores, i.e. lower δ13C and δ15N values associated with forest environments in the western end of the transect and higher values, especially δ15N, towards the east, with maximum values observed near the Atlantic coast. Our results support the stated expectations on herbivore stable isotope composition variability and its association with environmental variables in a longitudinal transect across the southern Argentinean Patagonia.

Nitrate source apportionment and risk assessment: A study in the largest ion-adsorption rare earth mine in China

Nitrate (NO3−) pollution in water bodies has received widespread attention, but studies on nitrogen transformation and pollution risk assessment are still limited, especially in rare earth mining areas. In this study, surface and groundwater samples were collected from the largest rare earth mining site in southern China, and analyzed for the hydrochemical and stable isotopic characteristics. The results showed that the NO3− concentrations ranged from 1.61 to 453.11 mg/L, with 35% of surface water and 53.3% of groundwater samples exceeding the WHO standard (i.e., 50 mg/L). Health risk assessment showed that 31.4% of the water samples had a moderate to high non-carcinogenic risk, and the high-risk areas were concentrated in rare earth mining regions. Additionally, adults were more vulnerable to the non-carcinogenic health risks than children. The high variability of δ15N–NO3- (from −6.43 to 17.09‰) and δ18O–NO3- (from −7.91 to 22.79‰) showed that NO3− was influenced by multiple nitrogen sources and transformation processes. Hydrochemistry and isotopic evidence further indicated that NO3− was primarily influenced by nitrification and hydraulic connection between surface and groundwater. The results of the Bayesian mixing model showed that about 70% of NO3− originated from mine drainage and soil N in the rare earth mining area, while more than 90% of NO3− originated from fertilizer, soil N, and manure and sewage in rural and urban areas in the middle and downstream. This study suggests reducing anthropogenic nitrogen discharge (e.g., leaching agents and fertilizer inputs) as the primary means of NO3− pollution control with biogeochemical processes (e.g., denitrification) to further reduce its pollution.

Fish Exhibit Distinct Fluorochemical and δ15N Isotopic Signatures in the St. Lawrence River Impacted by Municipal Wastewater Effluents

We examined the influence of Montreal wastewater treatment plant (WWTP) effluents on two top predators, Walleye (Sander vitreus) and Sauger (Sander canadensis), with a focus on δ15N isotopic signatures and per- and polyfluoroalkyl substances (PFAS). These two fish species were collected in the summer 2013 in the St. Lawrence River upstream and downstream from a major WWTP, as well as in background sites (semi-remote lakes). Most of the δ15N variations for Sauger and Walleye are attributable to 1) δ15N values of the primary producers and sewage-derived particulate organic matter (SDPOM) at the base of the trophic food chain, 2) agricultural activities combined with biogeochemical processes, and 3) food web length. δ15N was significantly lower in fish collected in the effluent-mixed water masses than other sites of the St. Lawrence River, attributed to the SDPOM of the WWTP effluent. Relative to the background sites, certain PFAS were present at much higher levels in the St. Lawrence River, with profiles dominated by perfluoroalkyl sulfonates (PFSA). However, PFSA profiles generally remained consistent along the St. Lawrence River. PFOS levels in fish from the St. Lawrence exceeded the current Federal Environmental Quality Guidelines for protecting piscivorous mammals or birds. However, the human chronic daily intake of PFOS remained below current thresholds suggested by national agencies.