Triple Isotope Research Articles

Non-invasive determination of critical dissolved oxygen thresholds for stress physiology in fish using triple-oxygen stable isotopes and aquatic respirometry

ABSTRACT Understanding the critical thresholds of dissolved oxygen (O2) that trigger adaptive physiological responses in aquatic organisms is long hampered by a lack of robust, non-lethal or non-invasive methodologies. The isotope fractionation of triple O2 isotopes (18O/17O/16O) during respiration is linked to the amount of oxygen utilised, offering a potential avenue for new insights. Our experimental research involved measuring the oxygen isotope fractionation of dissolved O2 in closed-system aquatic respirometry experiments with wild sticklebacks (Gasterosteus aculeatus). These fish were either naturally adapted or experimentally acclimated to hypoxic and normoxic conditions. The aim was to observe their oxygen usage and isotope fractionation in response to increasingly severe hypoxia. Initial observations revealed a progressive 18O enrichment from the preferential uptake of 16O to a dissolved oxygen threshold of 3–5 mg O2 L–1, followed by an apparent reversal in oxygen isotope fractionation, which is mixing of 16O and 17O with the remaining O2 pool across all populations and indicative of a systematic change in oxygen metabolism among the fish. Unexpectedly, sticklebacks adapted to hypoxia but acclimated to normoxia exhibited stronger oxygen isotope fractionation compared to those adapted to normoxia and acclimated to hypoxia, contradicting the hypothesis that hypoxia adaptation would lead to reduced isotope discrimination due to more efficient oxygen uptake. These preliminary experimental results highlight the novel potential of using dissolved O2 isotopes as a non-invasive, non-lethal method to quantitatively assess metabolic thresholds in aquatic organisms. This approach could significantly improve our understanding of the critical oxygen responses and adaptation mechanisms in fish and other aquatic organisms across different oxygen environments, marking a significant step forward in aquatic ecological and physiological research.

Application of the adaptive Monte Carlo method for uncertainty evaluation in the determination of total testosterone in human serum by triple isotope dilution mass spectrometry

The measurement uncertainty is a crucial quantitative parameter for assessing the reliability of the result. The study aimed to propose a new budget for uncertainty evaluation of a reference measurement procedure for the determination of total testosterone in human serum. The adaptive Monte Carlo method (aMCM) was used for the propagation of probability distributions assigned to various input quantities to determine the uncertainty of the testosterone concentration. The basic principles of the propagation and the statistical analysis were described based on the experimental results of the quality control serum sample. The analysis of the number of Monte Carlo trials was discussed. The procedure of validation of the GUM uncertainty framework using the aMCM was also provided. The number of Monte Carlo trials was 2.974 × 106 when the results had stabilized. The total testosterone concentration was 16.02 nmol/L, and the standard uncertainty was 0.30 nmol/L. The coverage interval at coverage probability of 95% was 15.45 to 16.62 nmol/L, while the probability distribution for testosterone concentration was approximately described by a Gaussian distribution. The validation of results was not passed as the expanded uncertainty result obtained by the aMCM was slightly lower, about 7%, than that by the GUM uncertainty framework with consistent results of the concentration.Graphical

Triple isotopes (δD, δ18O, δ17O) characteristic of river water and groundwater in an arid watershed from Qaidam Basin, Northwestern China: implications for hydrological cycle

Combining traditional stable isotopes (δD and δ18O) and triple oxygen isotope (δ17O) is conducive to tracing hydrological cycle processes. The application of triple oxygen isotopes primarily focuses on precipitation, which is lacking in river water and groundwater. In this study, the spatial variations of δD, δ18O, δ17O, d-excess and 17O-excess of river water and groundwater in the Golmud River basin as well as the correlation between them were investigated to elucidate water origin and assess the evaporation influence on water bodies during flood season. Spatial changes in δD, δ18O and δ17O of river water exhibit a decrease-increase-stability pattern contrary to that observed for d-excess, 17O-excess has no distinct trend but is higher at both the source and downstream regions. The results show that river water and groundwater originate from precipitation in the mountainous area, and the meltwater in the source region also contribute to the river water with high d-excess and 17O-excess during flood season. The combination of d-excess and 17O-excess reveal that river water is also affected by evaporation and mixing of river water in tributaries. It was found that the river water is recharged in the mountains, undergoes evaporation in the upstream region and leaks into groundwater in the midstream region, which is recharged by the groundwater and evaporated again in the downstream region. This study could provide a more comprehensive understanding of the potential and value of triple oxygen isotopes in the hydrological cycle.

Identifying the external N and Hg inputs to the estuary ecosystem based on the triple isotopic information (δ15NNO3, Δ17ONO3 and δ18ONO3)

The supply and sources of N and Hg in the Geum estuary of the western coast of Korea were evaluated. Triple isotope proxies (δ15NNO3, Δ17ONO3 and δ18ONO3) of NO3− combined with conservative mixing between river and ocean waters were used to improve isotope finger-printing methods. The N pool in the Geum estuary was primarily influenced by the Yellow Sea water, followed by riverine discharge (821 × 106 mol yr−1) and atmospheric deposition (51 × 106 mol yr−1). The influence of the river was found to be greater for Hg than that of the atmosphere. The triple isotope proxies revealed that the riverine and atmospheric inputs of N have been affected by septic wastes and fossil fuel burning, respectively. From the inner estuary towards offshore region, the influence of the river diminishes, thus increasing the relative impact of the atmosphere. Moreover, the isotope proxies showed a significant influence of N assimilation in February and nitrification in May.

The triple oxygen isotope signature of uranium oxides in the nuclear fuel cycle

Uranium oxides, in the form of U3O8 and UO2, are principal compounds in the uranium nuclear fuel cycle and attract significant interest in the general field of nuclear forensics, for the development of new and credible signatures to support attribution. This study presents, for the first time, the analytical method and high-precision measurements of the triple oxygen isotope composition of U3O8 and UO2. We show that Δ’17O can be measured at the accuracy and reproducibility needed for nuclear forensics. Δ’17O exhibits a wide range, from -450 to -120 ppm, for both commercial and synthetic uranium oxides. The presentation of the data in Δ’17O-δ’18O space provides a two-dimension identification of uranium oxides for nuclear forensic characterization of samples. The relation between Δ’17O and the process from which the oxide was formed or manipulated, has the potential to trace the chemical history of the material. We determined the change in Δ’17O of the system U3O8 – atmospheric O2 resulting from isotope exchange at the temperature range of 298 K to 973 K. The dependency of the triple isotope exponent, θ, on temperature was calculated for this exchange reaction, concluding that the phase transition from pseudo-hexagonal to hexagonal structure plays a major role in determining Δ17O and θ. A set of reduction reactions of U3O8 to UO2 indicate that Δ’17O undergoes minimal change during this process.

Condensation of fallout glasses in the Hiroshima nuclear fireball resulting in oxygen mass-independent fractionation

A new kind of Hiroshima nuclear fallout, the Hiroshima glasses, was discovered around the Hiroshima Bay. Here, the chemical compositions and the silicon and oxygen triple isotope compositions were analyzed to understand the formation process of these new fallouts. The chemical analysis shows four different families of glasses: the melilitic glasses, the anorthositic glasses, the soda-lime glasses, and the silica glass. The silicon isotopic compositions show wide variations in the glasses, with δ30Si varying between -23.0 ± 1.8 ‰ and -1.5 ± 1.1 ‰. The oxygen isotopic compositions indicate the presence of mass-independent fractionation on ≈38 % of the analyses, reaching a Δ17O of -3.1 ± 0.6 ‰. The chemical and silicon isotopic compositions of the Hiroshima glasses show that these glasses were formed by condensation within the nuclear fireball. Our scenario for the Hiroshima glasses formation, tested by modeling (GGchem code), considers a rapid condensation (1.7–5.5 s) in the nuclear fireball (3200–1000 K) at atmospheric pressure with a gas resulting from a mixing between air, and vaporized water and city materials. Chemical reactions during the Hiroshima glasses condensation are the most probable source for the oxygen mass-independent fractionation. The formation of the Hiroshima glasses by condensation implies that they may be an analog to the first condensates in the solar system: Calcium-Aluminum-rich Inclusions (CAIs), which are found in chondrites. The Hiroshima glasses exhibit similarities with CAIs in their chemical and isotopic compositions (Si and O).

A New Method for Silicon Triple Isotope Analysis With Application to Siliceous Sponge Spicules

AbstractMultiple isotope ratios of elements disclose information on the fractionation mechanism that cannot be obtained from a single isotope ratio alone. We describe a laser fluorination technique in combination with gas source mass spectrometry for the measurement of triple Si isotope ratios with high precision that allows resolving ppm‐level variability in triple silicon isotope ratios (29Si/28Si, 30Si/28Si) Δ‘29Si due to purely mass‐dependent fractionation in Si isotopes. We demonstrate how the triple silicon isotope ratios can be used to characterize different mass‐dependent processes that fractionate silicon isotopes. We report new data on reference materials (NBS‐28 [RM8546], Diatomite, BigBatch) and on siliceous sponges. Our triple silicon isotope data resolve that kinetic fractionation, possibly related to breaking Si(OH)4 molecules, causes the low δ30Si of sponges. The data further suggest that the cause for fractionation may be the same for the groups of Demospongiae, Homoscleromorpha, and Hexactinellida and that triple silicon isotope ratios of sponges have potential as seawater silicon isotope proxy.

Triple oxygen stable isotope analysis of nitrite measured using continuous flow isotope ratio mass spectrometry

Oxygen stable isotopes (i.e., 16O, 17O, 18O) of nitrite (NO2−) are useful for investigating chemical processes and sources contributing to this important environmental contaminant and nutrient. However, it remains difficult to quantify the oxygen isotope compositions of NO2− due to the lack of internationally recognized NO2− reference materials with a well-known Δ(17O) value. Here we have adopted a combination of methodologies to develop a technique for measuring Δ(17O) of NO2− by reducing nitrate (NO3−) materials with internationally recognized Δ(17O) values to NO2− using activated cadmium catalyzed by chloride in a basic solution while conserving Δ(17O). The NO3− reference materials reduced to NO2− and sample NO2− unknowns are converted to N2O using sodium azide/acetic acid reagent and decomposed to O2 by passing through a heated gold tube and introduced into a continuous flow isotope ratio mass spectrometer for analysis at m/z 32, 33, and 34 for Δ(17O) quantification.The adapted method involves the following main points:•NO3− reference materials with internationally recognized oxygen isotope composition are reduced to NO2− under high pH conditions that conserve Δ(17O) values.•The NO3− reference materials reduced to NO2− and sample NO2− with unknown Δ(17O) values are reduced to N2O using chemical methods involving sodium azide/acetic acid.•The product N2O is extracted, purified, decomposed to O2, and analyzed for its isotope composition using a continuous flow isotope ratio mass spectrometer for Δ(17O) quantification. The Δ(17O) of NO2− samples are calibrated with respect to the NO3− reference materials with known Δ(17O) values.

Riverine nitrogen source and yield in urban systems

Although human reshaping of the nitrogen (N) cycle is well established, contributions of individual N sources to riverine and coastal eutrophication are less certain. Urban N fluxes are potentially substantial, particularly from sewer overflows. Results from four longitudinal surveys in rivers in and around the city of Pittsburgh, Pennsylvania, were used to characterize N chemistry and isotopic composition and were compared with LOADEST‐model‐derived total N (TN) flux budgets from three urban areas along the Ohio River (Pittsburgh, Pennsylvania; Cincinnati, Ohio; and Louisville, Kentucky). Triple nitrate isotopes reveal that riverine nitrate in the Pittsburgh region is dominated by wastewater inputs despite high atmospheric deposition rates. Our budget estimates demonstrate that the magnitude of urban N yields is comparable to yields reported for agricultural watersheds and that these high urban N yields cannot consist of permitted, point‐source discharges alone. Our results reveal that nonpoint sources in urban systems represent an important but overlooked source of TN to overall riverine budgets.

MALDI Mass Spectrometry Imaging and Semi-Quantification of Topically Delivered Lactic Acid.

Lactic acid is a common active ingredient in many topical skincare products; however, measuring its delivery into the skin is challenging due to the presence of a large level of endogenous lactic acid. In this study, matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) was used to quantitatively and qualitatively measure the delivery of lactic acid into the skin from a range of topical skincare products. Porcine skin samples were treated with various skincare products containing lactic acid. After 24h, skin samples were sectioned and treated via H&E staining or prepared for MALDI-MSI using chemical derivatization. Samples were then analyzed by MALDI-MSI imaging to obtain lactic acid distribution in the entire skin section. Due to the high level of endogenous lactic acid in the skin, a "triple isotope" of lactic acid (L-Lactic acid-13 C3 ), was needed to provide full resolution from the skin's background signal with MALDI-MSI. With this approach, the topically delivered lactic acid could be quantitatively and qualitatively analyzed from a variety of skincare products. The combination of L-Lactic acid-13 C3 and MALDI-MSI was successfully used to quantitatively and qualitatively measure the topical delivery of lactic acid from a variety of skincare products. This approach could be used in future work to better understand the mode of action of lactic acid as an active ingredient in skincare products.

Triple oxygen isotope composition of combustion sulfate

The estimated contribution of “primary” combustion sulfate to the total atmospheric sulfate flux is inconsistent, hindering our interpretation of the atmospheric “missing sulfate”. We expect combustion sulfate to inherit the characteristic triple oxygen isotope composition of air O2 to variable degrees. To test the hypothesis, we extracted sulfate from soot samples collected from 48 residential chimneys throughout China. The mean sulfate's Δ′17O0.5305 and δ18O values are −0.27 ± 0.06‰ and 12.8 ± 3.6‰, respectively, and the Δ′17O0.5305 and δ18O values correlate negatively and extend to air O2's triple oxygen isotope composition as an endmember, suggesting that combustion sulfate is generated dominantly by S(IV) multiphase oxidation via O2 in the plume, with possible additional pathways through •OH in aqueous films or droplets and NO2 at soot surface. Oxygen sources (i.e., air O2, fuel, ambient moisture) and oxidation kinetics determine the triple oxygen isotope composition of combustion sulfate. The characterization of combustion sulfate's triple isotope composition helps to improve global and regional atmospheric sulfur chemistry models.

Identification of food sources in tropical seagrass bed food web using triple stable isotopes and fatty acid signatures

Identification of food sources in tropical seagrass bed food web using triple stable isotopes and fatty acid signatures

Different flavours of oxygen help quantify seasonal variations of the biological carbon pump in the Celtic Sea

Shelf seas represent only 10% of the World’s Ocean by area but support up to 30% of its primary production. There are few measurements of biological production at high spatial and temporal resolution in these physically and biologically dynamic systems. Here, we use dissolved oxygen to-argon (O2/Ar) ratios and oxygen triple isotopes in O2 (16O, 17O, 18O) to estimate net community production, N(O2/Ar), and gross O2 production, G(17O), in summer and autumn 2014 and spring and summer 2015 in the Celtic Sea, as part of the UK Shelf-Sea Biogeochemistry Programme. Surface O2/Ar concentration ratios were measured continuously using a shipboard membrane inlet mass spectrometer. Additional depth profiles of O2/Ar concentration ratios, δ(17O) and δ(18O) were measured in discrete water samples from hydrocasts. The data were combined with wind-speed based gas exchange parameterisations to calculate biological air-sea oxygen fluxes. These fluxes were corrected for diapycnal diffusion, entrainment, production below the mixed layer, and changes over time to derive N(O2/Ar) and G(17O). The Celtic Sea showed the highest G(17O) in summer 2014 (825 mmol m–2 d–1) and lowest during autumn 2014 (153 mmol m–2 d–1). N(O2/Ar) was highest in spring 2015 (43 mmol m–2 d–1), followed by summer 2014 (42 mmol m–2 d–1), with a minimum in autumn 2014 (–24 mmol m–2 d–1). Dividing the survey region into three hydrographically distinct areas (Celtic Deep, Central Celtic Sea and Shelf Edge), we found that Celtic Deep and Shelf Edge had higher N(O2/Ar) in summer (71 and 63 mmol m–2 d–1, respectively) than in spring (49 and 22 mmol m–2 d–1). This study shows regional differences in the metabolic balance within the same season, as well as higher net community production in summer than in spring in some areas and years. The seasonal patterns in biological production rates and the export efficiency (f-ratio) identified the importance of biology for supporting the Celtic Sea’s ability to act as a net CO2 sink. Our measurements thus help improve our understanding of the biological carbon pump in temperate shelf seas.

Relative importance of marine and terrestrial organic matter in the trophic base of sandy beach communities revealed by a triple stable isotope approach.

砂浜海岸では，海藻や海洋生物遺骸などの有機物が漂着供給され，一方，陸からは海浜植物群落や沿岸林から有機物がもたらされる．したがって，砂浜海岸に生息する甲殻類・昆虫類などの節足動物は，海起源有機物と陸起源有機物の双方を直接あるいは間接的に餌資源として利用していると考えられるが，各有機物への寄与率は良くわかっていない．そこで，砂浜海岸に生息する節足動物の炭素・窒素・硫黄の安定同位体比を調べることで，砂浜生態系の栄養基盤としての海起源・陸起源有機物の相対的重要性やその空間構造への影響を明らかにすることを目的とした．調査は，2019 年 9 月に仙台湾の新浜海岸と閖上海岸で行った．汀線から陸にトランセクトを曳き，その線上の複数地点にベイトを入れたピットフォールトラップを設置して海浜動物を採集した．採集個体および打ち上げ海藻や海浜植物，後背地の内陸植物を加えて，炭素・窒素・硫黄安定同位体比を分析した．次いで，海浜動物に対する栄養源として，打ち上げ海藻，海浜植物，内陸植物への寄与率をベイズ統計による安定同位体混合モデル MixSIAR により推定した．安定同位体分析の結果，新浜海岸と閖上海岸ともに，節足動物は幅広い安定同位体比（炭素・窒素）を示したが，炭素安定同位体比は陸域植物と海藻類の間の値を示した．一方，硫黄安定同位体分析ではスナガニやハマトビムシ科が海藻と同様の高い安定同位体比を示したが，昆虫類では低く，生物群によって海起源有機物への依存度が異なった．MixSIARによる栄養寄与率は，新浜海岸と閖上海岸ともに，ゴミムシダマシ科とスナガニでは海起源有機物の寄与率が高かった．オオハサミムシでは，汀線からの距離にかかわりなく，防潮堤手前で採集された個体でも海起源有機物の寄与率が高い場合があった．このことから，砂浜に生息する節足動物は陸上起源の有機物だけでなく，栄養基盤として海起源有機物にも強く依存していることがわかった．

Triple silicon isotope insights into the formation of Precambrian cherts

Cherts are pervasive in the Precambrian and their O and Si isotopic compositions have been used extensively for reconstructing seawater paleo-temperatures. However, these reconstructions are obscured by the fact that cherts exhibit, at a given age, a large isotopic (O and Si) heterogeneity at bulk and micrometer scales. This is understood as reflecting the fact that cherts are an assemblage of silica precursors having different origins, and that diagenesis and metamorphism have modified the isotopic compositions acquired during the last equilibration with seawater. To test the contributions of the different processes that control the Si isotopic composition of cherts, we developed high-precision triple Si isotope measurements by Multicollector-Inductively Coupled-Plasma Mass-Spectrometer (MC-ICP-MS). This approach allows to identify deviations from the equilibrium Si isotopic fractionation at the level of a few ppm. Triple Si isotope data combined with trace elements and O isotope data in Precambrian cherts demonstrate that three different sources of silica were involved in the formation of cherts, and that both equilibrium and kinetic isotopic fractionation took place for Si. Archean seawater is predicted to have a triple Si isotopic composition kinetically fractionated relative to bulk silicate Earth. This requires a large kinetic isotopic fractionation during hydrothermalism of the oceanic crust and appears consistent with warm oceans in the Precambrian. However more work is required on the triple isotope systematics of Si inputs and output to seawater by rivers and authigenic clays formation. Triple Si isotope variations can thus be used as a new tool to unravel the origin of cherts and of their O and Si isotopic compositions. The present evidence of kinetic isotopic fractionations for Si isotopes also implies that Si isotopes should be associated to triple O isotope for paleotemperature reconstructions.

Climate impacts on tree-ring stable isotopes across the Northern Hemispheric boreal zone

Boreal regions are changing rapidly with anthropogenic global warming. In order to assess risks and impacts of this process, it is crucial to put these observed changes into a long-term perspective. Summer air temperature variability can be well reconstructed from conifer tree rings. While the application of stable isotopes can potentially provide complementary climatic information over different seasons.In this study, we developed new triple stable isotope chronologies in tree-ring cellulose (δ13Ctrc, δ18Otrc, δ2Htrc) from a study site in Canada. Additionally, we performed regional aggregated analysis of available stable isotope chronologies from 6 conifers' tree species across high-latitudinal (HL) and - altitudinal (HA) as well as Siberian (SIB) transects of the Northern Hemispheric boreal zone.Our results show that summer air temperature still plays an important role in determining tree-ring isotope variability at 11 out of 24 sites for δ13Ctrc, 6 out of 18 sites for δ18Otrc and 1 out of 6 sites for δ2Htrc. Precipitation, relative humidity and vapor pressure deficit are significantly and consistently recorded in both δ13Ctrc and δ18Otrc along HL.Summer sunshine duration is captured by all isotopes, mainly for HL and HA transects, indicating an indirect link with an increase in air and leaf temperature. A mixed temperature-precipitation signal is preserved in δ13Ctrc and δ18Otrc along SIB transect. The δ2Htrc data obtained for HL-transect provide information not only about growing seasonal moisture and temperature, but also capture autumn, winter and spring sunshine duration signals. We conclude that a combination of triple stable isotopes in tree-ring studies can provide a comprehensive description of climate variability across the boreal forest zone and improve ecohydrological reconstructions.

Stable strontium isotope fractionation in hydroponically grown mung and soy bean sprouts

This study investigated whether stable strontium isotope (88Sr/86Sr) fractionation occurs during nutrient uptake by bean plants. Mung and soy beans were cultivated hydroponically using two nutrient solutions with different strontium (Sr) isotope compositions. The Sr and calcium (Ca) concentrations, 87Sr/86Sr ratios and δ88/86Sr values of the seeds, organs, initial nutrient solutions, and residual nutrient solutions were measured. The triple Sr isotope plot illustrates that bean sprout organs deviated considerably from the binary end-member mixing lines between intact bean seeds and the initial nutrient solutions, indicating fractionation of δ88/86Sr. In addition, the residual nutrient solutions had heavier δ88/86Sr values than the initial nutrient solutions, further supporting preferential uptake of lighter Sr isotopes by bean sprouts. This study indicates that stable Sr isotope fractionation occurs during nutrient uptake by bean plants, and that controlled hydroponic experiments are highly effective for observing this process.

External fluid incursion during Cu-mineralization stage of Mina Justa iron oxide copper-gold (IOCG) deposit: Evidence from triple sulfur isotope geochemistry of chalcopyrite

IOCG deposits are major host for Cu, rare earth elements (REEs), U and other metals, and whose source(s) of ore-forming fluids are highly debated. Particularly, whether or not external fluid was involved during Cu-mineralization stage is of great interest and intense debate. In this contribution, we investigate this topic using triple sulfur isotope geochemistry of Cu-mineralization stage sulfides in the Mina Justa deposit. The results show that the chalcopyrite that intensely replaced early-stage pyrite has similar triple sulfur isotopic composition to early-stage pyrite, which largely in the range of magmatic sulfur (Δ33S = 0 ± 0.05 ‰, Δ33S = δ33S − 1000 × [(1 + δ34S/1000)0.515 – 1]). However, chalcopyrite that occurs alone or with only minimal replacement of coexisting pyrite shows triple isotope (Δ33S = 0.05 to 0.20 ‰) that deviates from magmatic sulfur. This indicates that the sulfur of chalcopyrite with intense replacement of early-stage pyrite mainly inherits sulfur from early-stage pyrite, while the sulfur of chalcopyrite that exist alone or with minimal replacement of pyrite obtained their sulfur from a non-magmatic source. Collectively, the triple sulfur isotope data indicates that external fluid is present in the Cu-mineralization stage.

Adsorption and Desorption Behaviors of Sr on Montmorillonite: A Triple Sr Isotope Perspective

Montmorillonite is a widespread mineral, which affects the migration of Sr in the critical zone. Hydrochemical factors influencing Sr adsorption have been evaluated in the literature through laboratory batch experiments. Since those experiments were conducted using high-concentration and high-purity standard solutions, the influences of chemical matrices or element relative concentrations in water remain unclear. Here, we reassessed the adsorption and desorption behaviors of Sr on montmorillonite using natural waters with various chemical matrices and ionic strengths. Triple Sr isotopes (87Sr/86Sr and δ88/86Sr) were used to trace the exchange processes between the minerals and the waters. Adsorption and desorption occurred in all processed samples, but to varying degrees. These phenomena (especially desorption) were principally controlled by the ionic competition of divalent ions (i.e., Ca and Mg), rather than by ionic strength. Sr isotope fractionation occurred during adsorption. Isotopically lighter Sr was preferentially adsorbed: the mean Δ88/86Srmineral–water (a value independent from the clay type) was −0.17‰. Consequently, water δ88/86Sr depended on both the mixing of mineral-bound and initial water Sr and on the degree of Sr adsorption; moreover, the exchange process was a reversible reaction and reached its chemical and isotopic equilibrium within a short time.

Modelling stream nutrient source and yield by coupling hydrological processes and nutrient budgets from an experimental headwater catchment with orchard expansion in the Changjiang River Delta area

AbstractAlthough watershed nutrient exports have been well studied and modelled, the dynamic changes in the contributions of individual nutrient sources to reservoirs are less certain under changing land use and climate. The intensification of agriculture has led to expansion in plantation areas and the corresponding increased use of chemical and organic fertilisers in many headwater catchments in recent decades in China. Nutrient yields are potentially substantial because of the expansion of agricultural ecosystems. Here, in combination with biogeochemical nutrient budgets, we developed a synthesis model to quantify daily nutrient yields in terms of dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP). We also identified their sources in an experimental headwater reservoir catchment based on high‐frequency observations of daily precipitation with nutrient concentrations in the stream outlets from January 2020 to December 2021. Annual DIN, DON, DIP and DOP yields were 31.68 ± 2.38, 5.98 ± 0.41, 1.12 ± 0.08 and 0.37 ± 0.27 kg ha−1 yr−1, respectively. All nutrient yields showed significant seasonal patterns, with over 60% of the total nutrient yield obtained from May to July. Our model suggests that over 70% of the annual nutrient yields were driven by rainfall exceeding 40 mm. Approximately 70% of catchment nutrient yields were from chemical and organic fertiliser applications. Triple nitrate isotopes (δ15N‐NO3−, δ18O‐NO3− and δ17O‐NO3−) revealed that stream nitrate sources and their contributions were almost consistent with the estimation from our model. The estimates demonstrated that orchard expansion by 25% of the total area led to a 63% increase in dissolved total nitrogen and an 88% increase in dissolved total phosphorus exports. Our results reveal that diffuse sources from orchard plantations are an important yet overlooked source of nutrients to the receiving headwater reservoir budget.