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The evolution and formation mechanisms of chemical components in surface water can reflect changes in the geological background of a basin and the extent of human interference. The Yangtze River basin is the largest water source area in China, yet its main ion sources and formation mechanisms are not fully understood. This study uses a combination of hydrochemistry, stable isotopes (δ 18 O, δD), the Craig-Gordon model, and the APCS-MLR model to quantitatively assess the water source replenishment and evaporation intensity of surface water in the Yangtze River. The study reveals the primary ion sources and controlling factors of surface water in the Yangtze River. The results show that the hydrochemical type in the upstream is mainly HCO 3 - -Ca 2+ and Na + -K + , while in the midstream and downstream it is primarily HCO 3 - -Ca 2+ and SO 4 2- -Ca 2+ . The evolution of hydrochemical types is mainly controlled by rock weathering and human inputs. The surface water sources in the Yangtze River are directly replenished by precipitation, with the evaporation ratio in the upstream (0.66) being higher than in the midstream (0.63) and downstream (0.47). The lc-excess in the upstream (-0.32 ‰) is lower than in the midstream (1.21 ‰) and downstream (-0.27 ‰), indicating more intense evaporation in the upstream. The hydrochemical composition of the Yangtze River surface water mainly comes from geological factors (80.5 %), industrial factors (11.1 %), agricultural factors (6.4 %), and unknown factors (2.0 %). This study enhances the understanding of the chemical composition, water source replenishment, ion sources, and evolution mechanisms of the Yangtze River surface water, providing a basis for maintaining water quality and sustainable development in the Yangtze River basin., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier B.V.)

Tamarix ramosissima has an important role in stabilizing sand dunes in desert ecosystems. Understanding the water use strategies of T. ramosissima is essential to understand its adaptations on coppice dunes. We utilized the stable isotopes δ 2 H and δ 18 O in soil water, groundwater, and xylem water to identify monthly differences in water sources. Additionally. we explored rooting depth using 2 H 2 O as an artificial tracer. In May, T. ramosissima derived 75% of its water from shallow and middle soil layers. In July, it absorbed 90% water from middle and deep soil layers. In August and September, it acquired approximately 80% of its water from deep soil layers. The labelling using 2 H as an artificial tracer indicated that the root system of T. ramosissima could reach depths >500 cm in the coppice dunes. 2 H absorption was observed at depths of 100, 200, 300 and 400 cm. Soil water is the dominant water source for T. ramosissima in the coppice dunes because groundwater is at depths >30 m. The flexible water-use strategies of T. ramosissima enable it to effectively utilize different available water sources to adapt to the arid environment. These findings improve our understanding of water uptake patterns and drought adaptation strategies of T. ramosissima in the coppice dunes of desert ecosystems., (© 2024 Wiley‐VCH GmbH. Published by John Wiley & Sons Ltd.)

To investigate the variation and fractionation of stable isotopes from irrigation water to soil, grapes, and wine, δ 2 H, δ 18 O, and δ 17 O in different samples from 10 regions in China were determined using a water isotope analyser. The values were significantly different among regions according to the chemometric analysis. All isotopes were significantly and positively correlated with irrigation water-soil and grape-wine. A significant water isotopic fractionation effect was observed from the irrigation water to the soil, grapes, and wine. Stable isotope distribution characteristics correlated with longitude, latitude, altitude, temperature, precipitation, station pressure and wind speed. The linear discriminant analysis (LDA), random forest (RF), support vector machine (SVM), and feed-forward neural network (FNN) models 58.33-100 %, 80-100 %, 53.33-100 %, and 73.33-100 % accurate for distinguishing the geographical origins of all samples from training and test data, respectively. These findings provide a theoretical basis for authenticating the geographic origin of Chinese wines using stable isotope analysis., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

North China type coalfield are gradually mining deep, and the mixing of groundwater is intensified. Hydrogen and oxygen isotopes are important elements for tracing groundwater movement. The fractionation response mechanism under mining conditions is not clear. In this paper, combined with numerical simulation, MixSIAR isotope mixing model and other methods, according to the δD, δ 18 O and hydrochemical information of various water bodies, the impact of coal mining on hydrogen and oxygen isotope fractionation is analyzed from multiple perspectives. The results show that summer soil water is the main source of recharge for limestone water, accounting for 30.7%-41.5%, and the Zhan River is the main source of recharge for limestone water. Before groundwater recharge, evaporation leads to the increase of δ 18 O in surface water by 0.31‰-5.58‰, water loss by 1.81%-28.00%, the increase of δ 18 O in soil water by 0.47‰-6.33‰, and water loss by 2.74%-35.80%. Compared with the coal mining layer, the degree of hydrogen and oxygen isotope drift and water-rock interaction in the coal mine stopping layer are significantly improved. The results of numerical simulation show that the pumping activity reduces the 18 O concentration in the mining layer. The ion ratio is used as a new variable to eliminate the influence of water-rock interaction when calculating the mixing ratio. The results show that the limestone water is in a state of receiving external recharge, and mixing effect increases the δ 18 O in limestone water by 0.86‰ on average, and the δD increases by 0.72‰ on average. The research results explain the controlled process of hydrogen and oxygen isotope fractionation under mining conditions, which is of great significance to coal mine safety production., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)

A "once-in-a-millennium" super rainstorm battered Zhengzhou, central China, from 07/17/2021 to 07/22/2021 (named "7.20" Zhengzhou rainstorm). It killed 398 people and caused billions of dollars in damage. A pressing question is whether rainstorms of this intensity can be effectively documented by geological archives to understand better their historical variabilities beyond the range of meteorological data. Here, four land snail shells were collected from Zhengzhou, and weekly to daily resolved snail shell δ 18 O records from June to September of 2021 were obtained by gas-source mass spectrometry and secondary ion mass spectrometry. The daily resolved records show a dramatic negative shift between 06/18/2021 and 09/18/2021, which has been attributed to the "7.20" Zhengzhou rainstorm. Moreover, the measured amplitude of this shift is consistent with the theoretical value estimated from the flux balance model and instrumental data for the "7.20" Zhengzhou rainstorm. Our results suggest that the ultra-high resolution δ 18 O of land snail shells have the potential to reconstruct local synoptic scale rainstorms quantitatively, and thus fossil snail shells in sedimentary strata can be valuable material for investigating the historical variability of local rainstorms under different climate backgrounds., (Copyright © 2024 Science China Press. Published by Elsevier B.V. All rights reserved.)

Identifying and quantifying water nitrate pollution is crucial for managing aquatic environment of a bay. Dongshan Bay, a significant semi-enclosed bay in the southeastern coastal area of Fujian Province, features mangrove and coral reef ecosystems at its estuary and bay mouth, respectively. Dongshan Bay is impacted by human activities such as mariculture. We quantified and analyzed nitrate pollution status in the surface waters of Dongshan Bay by measuring physicochemical parameters, stable isotopes (δ 15 N-NO 3 - , δ 18 O-NO 3 - and δ 15 N-NH 4 + ) of the surface waters, and using statistical methods including the MixSIAR isotope mixing model. The results showed that the concentrations of chlorophyll a and dissolved inorganic nitrogen in the surface waters exhibited a noticeable gradient change, decreasing from the estuary of the Zhangjiang River to the mouth of Dongshan Bay. The maximum concentrations of chlorophyll a, NH 4 + , NO 3 - and NO 2 - were 45.2 μg·L -1 , 52.67 μmol·L -1 , 379.2 μmol·L -1 and 3.93 μmol·L -1 , respectively. The nitrogen and oxygen isotope values of NH 4 + and NO 3 - in the surface waters showed significant spatial variations. According to the MixSIAR model results, nitrogen sources in the surface waters of Dongshan Bay were mainly freshwater inputs of the Zhangjiang River estuary, aquaculture wastewater, and groundwater. The freshwater input from the Zhangjiang River estuary contributed the most (25.2%), while aquaculture wastewater, groundwater and urban sewage accounted for 24.6%, 19.0%, and 15.1%, respectively. It is evident that freshwater input from the Zhangjiang River estuary is the primary source of nitrate in the surface waters of Dongshan Bay.

Nitrate (NO 3 - in a rural-urban canal in China. The NO 3 - in a rural-urban canal in China. The NO 3 - concentration (0.99-1.93 mg/L) of canal water increased along the flow direction and was higher than the internationally recognized eutrophication risk value in autumn and spring. The inputs of the Fuhe River, NH 4 + fertilizer, soil nitrogen, manure & sewage, and rainfall were the main driving factors of canal water NO 3 - based on principal component analysis and PMF, which was supported by evidence from δ 15 N/δ 18 O-NO 3 - . According to the chemical and isotopic analyses, nitrogen transformation was weak, highlighting the potential of δ 15 N/δ 18 O-NO 3 - to trace NO 3 - sources in canal water. The MixSIAR and PMF results with a <15% divergence emphasized the predominance of the Fuhe River (contributing >50%) and anthropogenic impacts (NH 4 + fertilizer plus manure & sewage, >37%) on NO 3 - in the entire canal, reflecting the effectiveness of the model analysis. According to the MixSIAR model, (1) higher NO 3 - concentration in canal water was caused by the general enhancement of human activities in spring and (2) NO 3 - source contributions were associated with land-use patterns. The high contributions of NH 4 + fertilizer and manure & sewage showed inverse spatial variations, suggesting the necessity of reducing excessive fertilizer use in the agricultural area and controlling blind wastewater release in the urban area. These findings provide valuable insights into NO 3 - dynamics and fate for sustainable management of canal water resources. Nevertheless, long-term chemical and isotopic monitoring with alternative modeling should be strengthened for the accurate evaluation of canal NO 3 - pollution in future studies., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)

To investigate the correlation between carbon and oxygen isotope compositions of plant cellulose and climatic factors as well as plant physiological indices on the southeastern margin of the Qinghai-Tibet Plateau, we examined plant species in eight sampling sites with similar latitudes and different longitudes in this region. Through the characteristics of δ 13 C and δ 18 O values, fractionation values (Δ 13 C and Δ 18 O) in leaf cellulose, we discussed water use efficiency (WUE) and the environmental factors, the variation of carbon and oxygen isotopes in the southeastern margin of the Qinghai-Tibet Plateau with elevation and longitude, and revealed the indication degrees of isotopic signals to different environments and vegetation physiology. By using the semi-quantitative model of carbon and oxygen dual isotopes, we investigated the physiological adaptation mechanisms of plants to varying environmental conditions. The results demonstrated that both Δ 13 C and Δ 18 O of cellulose decreased with increasing elevation and longitude, and Δ 13 C was more influenced by longitude, while Δ 18 O was more susceptible to elevation variation. Additionally, Δ 13 C and Δ 18 O were significantly and positively correlated with temperature (TEM), precipitation (PRE), potential evapotranspiration (PET), and relative humidity (RH). PRE was the dominant meteorological factor driving the variation of Δ 13 C, while RH was the dominant meteorological factor influencing Δ 18 O variation. In contrast to Δ 13 C, WUE showed a stronger correlation with elevation than with longitude, which increased as elevation and longitude increased. According to the carbon-oxygen model, plant stomatal conductance ( g s ) and photosynthetic capacity ( A max ) decreased with increasing precipitation and relative humidity, while the values increased with increasing elevation and longitude. The combined analysis of carbon and oxygen isotopes of organic matters would yield additional environmental and gas exchange information for studies on climate tracing and vegetation physiology studies on the southeastern margin of the Qinghai-Tibet Plateau.

Nitrate pollution in groundwater has become a global concern. One of the most important issues in controlling the nitrate pollution of groundwater is to identify the pollution source quickly and accurately. In this review, we firstly summarized the isotopic background values of potential sources of nitrate pollution in groundwater in 17 provinces (cities, autonomous regions) and 29 study areas in China, which could provide the fundamental database for subsequent research. Secondly, we reviewed the research progress of nitrate isotopes combined with multiple tracers for tracing nitrate in groundwater, and discussed their applicable conditions, advantages, and disadvantages. We found that halides and microorganisms combined with nitrate isotopes could accurately trace the pollution sources of domestic sewage, excrement and agricultural activities. The combination of Δ 17 O and nitrate isotopes could effectively distinguish the source of atmospheric deposition of nitrate in groundwater. The combination of groundwater age and nitrate isotopes could further determine the time scale of nitrate pollution. In addition, we summarized the application cases and compared the characteristics of mass balance mixing model, IsoSource model, Bayesian isotope mixing model, and EMMTE model for quantitative identification of nitrate pollution in groundwater. For the complexity and concealment of groundwater pollution sources, the coupling of nitrate isotopes with other chemical and biological tracing methods, as well as the application of nitrate isotope quantitative models, are effective tools for reliably identifying groundwater nitrate sources and transformation processes.

Precipitation in the plum rain period accounts for 40%-50% of annual precipitation in the monsoon region. To clarify the temporal variability of the isotopic composition of precipitation during the plum rain period from event to interannual time scale and identify the influencing factors, we analyzed the isotopic composition of precipitation and its influencing factors in Nanjing from 2015 to 2022. By using the Hybrid Single-particle Lagran-gian Integrated Trajectory (HYSPLIT) model with specific humidity analysis, we investigated the water vapor source and influencing factors. The results showed that 1) the isotopic abundance of atmospheric precipitation was depleted in the summer and enriched in winter. d x was lower in summer and higher in winter. The isotopic abundance of precipitation from the plum rain was depleted compared to mean value of the whole-year. 2) There was no significant correlation between δ 2 H and δ 1 8 was higher during plum rain period in years with more total plum rain precipitation. 3) The low-latitude South China Sea and the western Pacific Ocean source area provided water vapor for the plum rain. The shift of moisture source region led to abrupt changes in precipitation isotopes. Our results could provide data support for studies on precipitation isotopes in the monsoon region, as well as a reference point for further understanding the precipitation mechanism of the plum rain and stu-dying the seasonal variability of atmospheric circulation in the East Asian monsoon region.d x was lower in light rain, reflecting the effect of sub-cloud evaporation. The average d x was higher during plum rain period in years with more total plum rain precipitation. 3) The low-latitude South China Sea and the western Pacific Ocean source area provided water vapor for the plum rain. The shift of moisture source region led to abrupt changes in precipitation isotopes. Our results could provide data support for studies on precipitation isotopes in the monsoon region, as well as a reference point for further understanding the precipitation mechanism of the plum rain and stu-dying the seasonal variability of atmospheric circulation in the East Asian monsoon region.

The fate of riverine sulfate ion (SO 4 2- ) and its environmental effects in arid environment are difficult to evaluate due to its complicated sources and strongly coupled behaviors with water cycle which is significantly modified by humans. To understand the sulfur cycle in aquatic systems in arid environment, the chemical and sulfur and oxygen isotopic compositions (δ 34 S SO4 and δ 18 ) of major rivers around the Badain Jaran Desert, northwestern China, were investigated. These rivers had averaged SO SO4 ) of major rivers around the Badain Jaran Desert, northwestern China, were investigated. These rivers had averaged SO 4 2- content at 1336 µmol/L, over 10 times higher than the global average. The δ 34 S SO4 and δ 18 O SO4 values ranged from -5.3‰ to +11.8‰ and +1.6‰ to +12.8‰, respectively. The end-member analysis and the inverse model showed that riverine sulfate was mainly derived from evaporites dissolution (0-87%), sulfide oxidation (13%-100%) and precipitation (0-33%), indicating heterogeneity in sulfur sources and behaviors along the river drainage with the lithology variations and climate gradients. Multiple isotopic tools combining with hydro-chemistry compositions could be applied to reveal sulfur cycle in arid environment. Based on the calculation, sulfide oxidation plays the primary role in the headwater and upstream in the Qilian-Mountains area, where sulfide is widely exposed. While the proportion of evaporites dissolution contributing to riverine sulfate is much higher in downstream in a drier environment. Besides, less precipitation and higher temperature can lead to more intensive evaporation, affecting the process of sulfide oxidation and enhancing the rates of evaporites dissolution and sulfate precipitation in the basin., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article., (Copyright © 2023. Published by Elsevier B.V.)

The stable hydrogen and oxygen isotope technique is typically used to explore plant water uptake; however, the accuracy of the technique has been challenged by hydrogen isotope offsets between plant xylem water and its potential source water. In this study, the soil hydrogen and oxygen isotope waterline was used to correct the hydrogen isotope offsets for Salix psammophila and Caragana korshinskii, two typical shrub species on the Chinese Loess Plateau. Five different types of isotopic data [(i) δ18O, (ii) δ2H, (iii) combination δ18O with δ2H, (iv) corrected δ2H and (v) combination δ18O with corrected δ2H] were separately used to determine the water-use patterns of the two shrubs. The δ2H offset values of S. psammophila and C. korshinskii did not show significant temporal variation among the sampling months (May, July and September) but showed notable differences between the two shrubs (-0.4 ± 0.5‰ in S. psammophila vs -4.3 ± 0.9‰ in C. korshinskii). The obtained water absorption proportion (WAP) of S. psammophila in the different soil layers (0-20, 20-60 and 60-200 cm) did not differ significantly among the five different input data types. However, compared with the input data types (iii) and (v), the data types (i), (ii) and (iv) overestimated the WAP of C. korshinskii in the 0-20 cm soil layer and underestimated that in the 60-200 cm layer. The data type (iii) overestimated the WAP of C. korshinskii in 0-20 cm soil layer (25.9 ± 0.8%) in July in comparison with the WAP calculated based on data type (v) (19.1 ± 1.1%). The combination of δ18O and corrected δ2H, i.e., data type (v), was identified as the best data type to determine the water use patterns of C. korshinskii due to the strong correlation between the calculated WAP and soil water content and soil sand content. In general, S. psammophila mainly used (57.9-62.1%) shallow soil water (0-60 cm), whereas C. korshinskii mainly absorbed (52.7-63.5%) deep soil water (60-200 cm). We confirm that the hydrogen isotope offsets can cause significant errors in determining plant water uptake of C. korshinskii, and provide valuable insights for accurately quantifying plant water uptake in the presence of hydrogen isotope offsets between xylem and source water. This study is significant for facilitating the application of the stable hydrogen and oxygen isotope technique worldwide, and for revealing the response mechanism of shrub key ecohydrological and physiological processes to the drought environment in similar climate regions., (© The Author(s) 2023. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Sulfuric acid is involved in carbonate rock dissolution in karst areas and affects regional hydrogeochemical and carbon cycling processes. Sulfate sources and carbonate weathering at dripwater points of different hydrological types also differ significantly. Therefore, in this paper, three dripwater points (SQ2#, PB and MH6#) of different hydrological types in Dafeng Cave and Mahuang Cave were selected and systematically analyzed. The results show that: (1) ions in the cave dripwater mainly originated from rock weathering, and the water chemistry types were HCO 3 ·Ca-Mg and HCO 3 -SO 4 ·Ca-Mg. (2) Sulfur and oxygen isotopes reveal that sulfate in the cave water of Shuanghe Cave mainly came from a mixture of soil sulfate and chemical fertilizers. (3) The Simmr model shows that the main sources of sulfate at each dripwater point were natural sources, such as soil sulfate and gypsum dissolution. The natural sources contributed more than 80% of the sulfate in the cave water and were less affected by anthropogenic activities. (4) Carbonate weathering by sulfuric acid (CSW) can accelerate rock weathering and the prior calcite precipitation effect, involving regional CO 2 exchange through fissure pipes. The forward model results show that CSW reduced the karst carbon sinks at SQ2#, PB and MH6# by about 19.44%, 23.88% and 12.74%, respectively. Therefore, the impact cannot be ignored in assessing carbon source and sink processes in karst areas., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Inc. All rights reserved.)

The sustainability of water resources is a major challenge for the Ordos Basin and Loess Plateau of China. The basis of effective water management is an understanding of the water cycle process. This study investigated the surface water-groundwater origins and connectivity using stable isotopes (δD and δ 18 O) of surface water and groundwater in 11 river basins in the Ordos Basin. It was found that the surface water-groundwater origins and hydraulic connection were characterized by regional differences, mainly induced by climatic characteristics, hydrogeological conditions and human activities. Specifically, the impact of thick loess deposits caused surface water and groundwater to take long time to produce a hydraulic connection. In contrast, areas with thin loess deposits and frequent human activities showed a good connectivity between surface water and groundwater. As for water origins, summer precipitation was a common source of surface water and groundwater in the study area, and groundwater discharge was another source of surface water. However, surface water and groundwater were subjected to different degrees of evaporation during receiving precipitation recharge. Notably, thick loess deposits had an impact on groundwater evaporation because both the recharge of precipitation to groundwater and the discharge of groundwater to surface water took a long time. In addition, it was found that frequent human activities (mining, irrigation and urban construction) could weaken the impact of evaporation. This large-scale analysis provided new insights into the origins and connectivity of surface water and groundwater in areas with thick unsaturated zones for water resources management., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023. Published by Elsevier Inc.)

The subalpine shrub zone of the Qilian Mountains is an important water-retaining area, and it is crucial to clarify the processes of its hydrological cycle. Therefore, based on the stable isotope values of different water bodies in the subalpine shrub zone of the eastern Qilian Mountains from May to October 2019, the characteristics of δD and δ 18 O of different water bodies and their hydraulic relationships with each other were studied. The results showed that the stable isotope values of precipitation exhibited the largest fluctuations, while they were the most stable for groundwater. Plant transpiration was stronger than the evaporation of other water bodies. The stable isotope of precipitation was enriched in high temperature and low humidity environments. Isotopic values of plant and soil water were higher and more stable on the semi-sunny slope than on the semi-shady slope. According to the stable isotopes, there was a strong hydraulic relationship between the different water bodies in the study area, and precipitation was the ultimate source of all of them. Precipitation replenished soil water through infiltration. Part of the soil water was absorbed by plants, while the rest continued to infiltrate to replenish groundwater. Groundwater and precipitation replenished the river water.

A multi-isotopes approach involving the use of stable nitrate isotopes (δ 15 N-NO 3 - and δ 18 O-NO 3 - ) combined with stable water isotopes (δD-H 2 O and δ 18 O-H 2 O) and SIAR model as tracers can help identify the nitrogen source and understand the transformation process in a river-cascade reservoirs system. In this study, we identify the potential impact of the N source in the Lancang River basin, clarified the seasonal variations in the isotope values and estimated the probability distribution and proportional contribution of multi-terminal NO 3 - -N sources using Bayesian isotope mixing model. In addition, we investigate the factors that led to the seasonal variations of the stable isotopes and evaluated the relationship between the uncertainty of the contribution ratio of the N sources and isotopic variations in the river-cascade reservoirs system. The NO 3 - -N is the main component of DIN (dissolved inorganic nitrogen), accounting for 68.1 % of DIN. The ratios of δ 15 N-NO 3 - and δ 18 O-NO 3 - ranged from +4.2 ‰ to +10.3 ‰, and from +5.9 ‰ to +9.3 ‰ in the Lancang River. The δD-H 2 O and δ 18 O-H 2 O of the surface waters ranged from -109.47 ‰ to -76.44 ‰, and from -15.13 ‰ to -11.61 ‰, respectively. The SIAR model analysis results show that nitrification of livestock and poultry manure is the main source of NO 3 - -N in the upstream natural reach, accounting for 40.2 %. There is little difference between the wet season and the dry season. Nitrification of soil organic nitrogen is the main source of NO 3 - -N in the cascade development reach, accounting for 42.3 %. The contribution rate of atmospheric precipitation to nitrate concentration in both sampling periods is low (<5 %). This study provides a useful insight for reservoir water environmental managers to verify cascade development river pollution contributors and to better apply remedial solutions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Nitrate is a significant constituent of the total nitrogen pool in shallow aquifers and poses an escalating threat to groundwater resources, making it crucial to comprehend the source, conversion, and elimination of nitrogen using appropriate techniques. Although dual-isotope dynamics in nitrate have been widely used, uncertainties remain regarding the asynchronously temporal changes in δ 18 O-NO 3 - and δ 15 N-NO 3 - observed in hypoxic aquifers. This study aimed to investigate changes in nitrogen sources and transformations using temporal changes in field-based NO 3 - isotopic composition, hydro-chemical variables, and environmental DNA profiling, as the groundwater table varied. The results showed that the larger enrichment in δ 18 O-NO 3 - (+13‰) compared with δ 15 N-NO 3 - (-2‰) on average during groundwater table rise was due to a combination of factors, including high 18 O-based atmospheric N deposition, canopies nitrification, and soil nitrification transported vertically by rainfalls, and 18 O-enriched O 2 produced through microbial and root respiration within denitrification. The strong association between functional gene abundance and nitrogen-related indicators suggests that anammox was actively processed with nitrification but in small bacterial population during groundwater table rise. Furthermore, bacterial species associated with nitrogen-associated gradients provided insight into subsurface nitrogen transformation, with Burkholderiaceae species and Pseudorhodobacter potentially serving as bioindicators of denitrification, while Candidatus Nitrotogn represents soil nitrification. Fluctuating groundwater tables can cause shifts in hydro-chemical and isotopic composition, which in turn can indicate changes in nitrogen sources and transformations. These changes can be used to improve input sources for mixture models and aid in microbial remediation of nitrate., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023. Published by Elsevier B.V.)

Identifying phosphorus (P) sources is critical for solving eutrophication and controlling P in aquatic environments. Phosphate oxygen isotopes (δ 18 O p ) have been used to trace P sources. However, the application of this method has been greatly restricted due to δ 18 O P values from the potential source having wide and overlapping ranges. In this research, P sources were traced by combining δ 18 O p with multiple stable isotopes of nitrogen (δ 15 N), hydrogen (δD), and dissolved inorganic carbon (δ 13 C). Then, a Bayesian-based Stable Isotope Analysis in R (SIAR) model and IsoSource model were used to estimate the proportional contributions of the potential sources in the Tuojiang River. δ 18 O p was not in equilibrium with ambient water, and statistically significant differences in the δ 18 O p values were found between the potential sources, indicating that δ 18 O p can be used to trace the P sources. δ 15 N, δD, and δ 13 C could assist δ 18 O p in identifying the main sources of P. The SIAR and IsoSource models suggested that industrial and domestic sewage was the largest contributor, followed by phosphate rock and phosphogypsum and agricultural sewage. The uncertainty of the calculation results of the SIAR model was lower than that of the IsoSource model. These findings provide new insights into tracing P sources using multiple stable isotopes in watersheds.

Stable isotope ratios of nitrogen and oxygen ( 15 N/ 14 N and 18 O/ 16 O) of nitrate (NO 3 - ) are excellent tracers for developing systematic understanding of sources, conversions, and deposition of reactive atmospheric nitrogen (N r ) in the environment. Despite recent analytical advances, standardized sampling of NO 3 - ) isotopes in precipitation is still lacking. To advance atmospheric studies on N r species, we propose best-practice guidelines for accurate and precise sampling and analysis of NO 3 - isotopes in precipitation based on the experience obtained from an international research project coordinated by the International Atomic Energy Agency (IAEA). The precipitation sampling and preservation strategies yielded a good agreement between the NO 3 - concentrations measured at the laboratories of 16 countries and at the IAEA. Compared to conventional methods (e.g., bacterial denitrification), we confirmed the accurate performance of the lower cost Ti(III) reduction method for isotope analyses ( 15 N and 18 O) of NO 3 - in precipitation samples. These isotopic data depict different origins and oxidation pathways of inorganic nitrogen. This work emphasized the capability of NO 3 - isotopes to assess the origin and atmospheric oxidation of N r and outlined a pathway to improve laboratory capability and expertise at a global scale. The incorporation of other isotopes like 17 O in N r is recommended in future studies.

The precipitation oxygen isotopic composition is a useful environmental tracer for climatic and hydrological studies. However, accurate and high-resolution precipitation oxygen isoscapes are currently lacking in China. In this study, a precipitation oxygen isoscape in China for a period of 148 years is built by integrating observed and iGCMs-simulated isotope compositions using an optimal hybrid approach of three data fusion and two bias correction methods. The temporal and spatial resolutions of the isoscape are monthly and 50-60 km, respectively. Results show that the Convolutional Neural Networks (CNN) fusion method performs the best (correlation coefficient larger than 0.95 and root mean square error smaller than 1‰), and the other two data fusion methods perform slightly better than the bias correction methods. Thus, the isoscape is generated by using the CNN fusion method for the common 1969-2007 period and by using the bias correction methods for remaining years. The generated isoscape, which shows similar spatio-temporal distributions to observations, is reliable and useful for providing strong support for tracking atmospheric and hydrological processes., (© 2023. The Author(s).)

The local meteoric water line (LMWL) is an important basis for tracing the regional hydrological processes with stable isotopes. The establishment of LMWL, which can represent the overall characteristics of stable isotopes of local precipitation, is crucial for accurately revealing the hydrological processes. The influences of different temporal scales and regression methods on the established LMWL were analyzed and compared based on nine years of stable isotopic data of precipitation in Changwu Tableland, a typical area of the inland monsoon region of Northwest China. The results showed that, for different regression methods, the LMWL established by stable isotopes of annual precipitation was basically stable, whereas the LWML established by each precipitation event and the monthly precipitation data showed significant differences with different regression methods. The LMWL from the ordinary least squares regression (OLSR), major axis regression (MA) and reduced major axis regression (RMA) methods were significantly different based on the data of precipitation event, monthly precipitation data, and annual precipitation data, respectively. Only when OLSR, MA and RMA considering precipitation weighting were used, the LMWL established by these scale data was relatively close. This suggested that special attention should be paid to the selection of precipitation stable isotope data scale and regression method when LMWL was established in areas with temporal heterogeneity of precipitation and stable isotopic characteristics. For regions where stable isotope observations of precipitation were difficult and data were limited, the precipitation weighted major axis regression or reduced major axis regression methods are recommended during the establishment of representative LMWL.

The mountainous alpine area in western China is an Asian water tower and is an important ecological barrier area. Based on the previous research results, this study sorted out and summarized the isotopic ecohydrology of the alpine mountains in the western China and found that the local meteoric water line (LMWL) in the west of the alpine mountains is δD = 7.44δ 18 O + 5.23 (R 2 = 0.86). The temperature effect showed an increasing trend from the south to north, while the precipitation effect showed an opposite direction. The water vapor sources in the western alpine mountains are complex. When the temperature coefficient is <0, the water vapor source is dominated by the southwest monsoon; when the temperature coefficient is 0-0.3, the water vapor source is jointly dominated by the southwest monsoon and the westerly wind; when the temperature coefficient is more significant than 0.3, the water vapor source is dominated by the westerly wind. Different water bodies are affected by water supply and evaporation differences. The altitude effect of δ 18 O value in precipitation in the western alpine mountains is -1.3 ‰/100 m, and the altitude effect of δ 18 O value in river water is -0.17 ‰/100 m. The primary source of plant water in the study area is soil water, and the utilization rate of water is closely related to plant types and the regional environment. Water vapor recycling has become an essential part of regional precipitation water vapor sources. However, with the deepening of the major national strategy of ecological civilization construction, the western alpine mountainous area are undergoing profound and drastic changes in the ecological and hydrological processes under the changing environment, which brings great challenges to regional water resources security, ecological security and sustainable development. In the future, the comprehensive innovations in observation, sampling, modeling and theory are urgently needed., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this study., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Dissolved heavy metals (DHMs) contamination has raised global concern for ecological and human health development. Weathering of sulfide-bearing ore metals can produce acidic, sulfate-rich solutions in the presence of water and oxygen (O 2 ), and DHMs are released to deprave the river water quality. Sulfur and oxygen isotope signatures (δ 34 S SO4 and δ 18 O SO4 ) could identify this pyrite-derived sulfate; however, it is yet not well known whether the δ 34 S SO4 and δ 18 O SO4 values could limit the DHMs sources and illustrate anthropogenic impacts on DHMs along the river corridor. We tried to solve this problem through field works in the Luohe River and Yihe River, two tributaries of the Yellow River, China, where metal sulfide mine activities mostly occurred upstream, but agricultural and domestic behaviors concentrated in the lower plain reaches. In the Luohe River upper areas, δ 34 S SO4 values had negative correlations with concentrations of cadmium (Cd) (p < 0.01), nickel (Ni) (p < 0.05), molybdenum (Mo) (p < 0.01), uranium (U) (p < 0.01), and SO 4 2- (p < 0.01). However, as the δ 34 S SO4 values increased downstream in the Luohe River, concentrations of copper (Cu) (p < 0.05), mercury (Hg) (p < 0.05), Ni (p < 0.05), and SO 4 2- (p < 0.01) simultaneously elevated. The Bayesian Isotope Mixing Model (BIMM) results via δ 34 S SO4 values demonstrated 64.3%-65.3% of SO 4 2- from acid mine drainage (AMD) in the Luohe River's upper reaches and 63.5%-67.7% in the Yihe River's upper reaches, and about 33% from sewage and industrial effluents in the Luohe River's lower reaches and 27% in Yihe River's lower reaches. Our results confirmed the different anthropogenic impacts on the DHMs concentrations in Luohe River and Yihe River and provided a robust method for DHMs sources appointment and pollution management in river systems., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Watershed management practices (WMPs) alter the sources and transformation of reactive nitrogen (N) in peri-urban watersheds, but a precise description of how WMPs impact N cycling is still lacking. In this study, four sampling campaigns were conducted in the wet and dry seasons of 2019 (before WMPs) and 2020 (after WMPs) to determine the spatiotemporal variations in nitrate isotopes ( 15 N-NO 3 - and 18 O-NO 3 - ) and hydrochemical compositions in the Muli River watershed. The results showed that the WMPs could significantly reduce the N load in the middle and lower reaches, but substantial improvements were not observed in 2020. Manure and sewage (M&S, 36.2 ± 15.8-55.0 ± 19.4%) was the major source of nitrate (NO 3 - ) in the stream water, followed by smaller-scale wastewater treatment plants (WWTPs, 14.0 ± 10.9-25.6 ± 11.5%). The WMPs were effective in controlling M&S, resulting in an approximately 16.7% (p < 0.01) lower M&S contribution during the dry season in 2020 compared to that in 2019. However, the smaller-scale WWTP input increased by approximately 5.4% (p < 0.01) after the WMPs. During the study period, the assimilation of NO 3 - by phytoplankton was important for NO 3 - loss, but the WMPs promoted nitrification in the watershed because of the elevated redox potential (Eh). Overall, the present study provides a better estimate of the variations in nitrogen sources and transformation in a peri-urban watershed after WMPs and provides an approach for developing timely nitrogen management solutions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

The rapid increase in urbanization and intensive coal mining activities have accelerated the deterioration of surface water quality. Environmental problems caused by the accumulation of nitrate and sulfate from natural, urban, and agricultural sources have attracted extensive attention. Information on nitrate and sulfate sources and their transformations is crucial for understanding the nitrogen and sulfur cycles in surface water. In this study, we monitored nitrate and sulfate in three representative rivers in mining cities in northern China. The main pollution sources and biogeochemical processes were identified by using stable isotopes (δD, δ 18 O H2O , δ 15 N, δ 18 O NO3 , δ 34 S and δ 18 O SO4 ) and hydrochemistry. The contribution of natural and anthropogenic sources was quantitatively estimated based on a Bayesian mixed model. The results indicated a large variation in sulfate and nitrate sources between the different rivers. Nitrate in the Tuohe River mainly derived from manure/sewage (57.9%) and soil N (26.9%), while sulfate mainly derived from manure/sewage (41.7%) and evaporite dissolution (26.8%). For the Suihe River, nitrate was primarily sourced from chemical fertilizer (37.9%) and soil nitrogen (34.8%), while sulfate was mainly sourced from manure/sewage (33.1%) and chemical fertilizer (21.4%). For the Huihe River, nitrate mainly derived from mine drainage (56.6%) and manure/sewage (30.6%), while sulfate predominantly originated from mine drainage (58.3%) and evaporite dissolution (12.9%). Microbial nitrification was the major pathway for the migration and transformation of nitrate in the surface water. However, denitrification and bacterial sulfate reduction (BSR) did not play a significant role as aerobic conditions prevailed. In this study, we elucidated the sources and transformation mechanisms of nitrate and sulfate. Additionally, we provided a reference for formulating a comprehensive strategy for effective management and remediation of surface water contaminated with nitrate and sulfate in mining cities., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

The adverse effects of increased nitrate (NO 3 - ) pollution especially from the non-point source on the hydrosphere and anthroposphere are becoming more prominent. The non-point-derived NO 3 - in the rivers supplying the upstream threatens the aquatic ecosystem of Taihu Lake. Here, dual-stable isotopes (δ 15 N and δ 18 O) of NO 3 - were applied to the Bayesian model (SIAR) for quantitative source identification of reactive nitrogen (Nr) in a mixed agricultural and urban region along the complex river network of the Yangze River delta. The results showed that the NO 3 - concentrations in the rivers ranged from 1.09 to 4.44 mg L -1 and decreased from the highly urbanized areas to the lakeside rural areas. The specific isotopic characteristics of four sources (atmospheric deposition, AD; chemical fertilizer, CF; manure and sewage, MS; and soil leachate, SL) by the SIAR isotope model indicated that the MS source made the greatest contribution (46.56%) to the total NO 3 - load, followed by SL (27.86%), CF (23.77%), and AD (1.81%). The highly urbanized areas and the hybrid areas, which contained a mix of industrialized, populated, and agricultural areas, were identified as hotspot areas with heavy Nr pollution, responsible for spatial patterns of δ 15 N-NO 3 - and δ 18 O-NO 3 - . These hotspot areas were characterized by a less well-developed sewage pipeline system with high Nr emissions from cash crops. The changes in wastewater treatment level, the agricultural production structure, and meteorological changes were the main factors of spatial variation of Nr concentration and source in the upstream Taihu Lake Basin. The variation in Nr concentration across Taihu Lake would respond to these anthropogenic-driven Nr loads. These findings suggest that MS was the predominant source had the strongest effect on the overall riverine NO 3 - source which was the primary problem that needed to be solved., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

The authentication of geographical origin of food is important using stable isotope analysis. However, the isotopic databank is still short of comprehensive. The isoscapes model based on environmental similarity is used for the first time to predict the geospatial distribution of δ 13 C, δ 2 H and δ 18 O in Chinese rice in 2017 and 2018. 794 rice samples in 2017 were used to build isoscapes model. Independent verification shows that the predicted isotope distribution from this new approach is of high accuracy, with a root mean square error (RMSE) of 0.51 ‰, 7.09 ‰ and 2.06 ‰ for δ 13 C, δ 2 H and δ 18 O values for 2017, respectively. Our results indicate that it is possible to predict the spatial distribution of stable isotopes in rice using an isoscapes model based on environmental similarity. This novel strategy can enrich and complement a stable isotope reference database for rice origin identification at regional scale., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Phosphorus (P) is normally considered as the limited nutrient for shallow freshwater lakes and can potentially trigger eutrophication on account of high concentrations. Due to the various transportation and transformation processes, P source apportionment and management in lake ecosystems have become more and more difficult. Combining with sequential extraction of P fractions and mineralogical analysis, the isotopic compositions of oxygen in phosphate (δ 18 O P ) of resin-extractable P from the different samples including soil, estuary sediments, pond sediments, and lake sediments in the Shijiuhu Lake catchment, China, were investigated. The results showed that δ 18 O P values ranged from +15.23 to +21.92‰ in agricultural soil, +16.53 to +24.10‰ in estuary sediments, +18.90 to +20.90‰ in pond sediments, and +17.42 to +19.70‰ in lake sediments. Isotopic signatures indicated that chemical fertilizers with heavier δ 18 O P values (+20.70 to +26.50‰) were the predominant contributors of P in the soil. The river transportation together with Fe/Al-P desorption on anaerobic condition simultaneously stimulated the enrichment of P in the lake sediments, even though the biotic activity regulated the isotope values moving toward the equilibrium. Eroded soil was the important source of P in lake and pond sediments via drainage and runoff, and conserved the source isotope signal in the samples. Stronger biotic activity in the aquatic environments dragged δ 18 O P values toward the equilibrium. However, conspicuous off-equilibrium isotope signature suggested the terrestrial sources in the aquatic ecosystems. The calculation of two end-member linear mixing models suggested that soils also predominantly controlled the P occurrence in the lake sediments with contribution higher than 80%, indicating that decreasing inputs from the agricultural activities is important in P reduction on catchment scale. Generally, δ 18 O P from different sources can provide indirect and important evidences for the identification and management of P sources in the lake catchment., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Springs offer insight into the sources and mechanisms of groundwater recharge and can be used to characterize fluid migration during earthquakes. However, few reports provide sufficient annual hydrochemical and isotopic data to compare the variation characteristics and mechanisms with both atmospheric temperature and seismic effects. In this study, we used continuous δ 2 H, δ 18 O, and major ion data from four springs over 1 year to understand the groundwater origin, recharge sources, circulation characteristics, and coupling relationships with atmospheric temperature and earthquakes. We found that (1) atmospheric temperatures above and below 0 °C can cause significant changes in ion concentrations and water circulation depth, resulting in the mixing of fresh and old water in the aquifer, but it cannot cause changes in δ 2 H and δ 18 O. (2) Earthquakes of magnitude ≥ 4.8 within a 66 km epicentral distance can alter fault zone characteristics (e.g., permeability) and aggravate water-rock reactions, resulting in significant changes in δ 2 H, δ 18 O, and hydrochemical ion concentrations. (3) Hydrogen and oxygen isotopes are the most sensitive precursory seismic indicators. The results of this study offer a reference for the establishment of long-term hydrochemical and isotopic monitoring, with the potential for use in earthquake forecasting.

A clear understanding of the factors governing dual isotopes (δ 15 N-NO 3 - and δ 18 O-NO 3 - ) in typhoons is essential for understanding their NO 3 - sources and its formation mechanisms. In this study, sequential precipitation samples during typhoons, including In-fa and Chanthu, were collected from Ningbo, Hangzhou and Huzhou. The chemical compositions, nitrogen and oxygen isotopes of NO 3 - and oxygen isotopes of H 2 O (δ 18 O-H 2 O) were measured. The results showed that the δ 15 N-NO 3 - and δ 18 O-NO 3 - values ranged from -6.3‰ to 6.0‰, and 38.0‰ to 66.5‰, respectively. The lower δ 18 O-NO 3 - values (less than 52‰) indicated the importance of peroxy radicals (RO 2 or HO 2 ) in NO x oxidation to NO 3 - formation pathways. By the Monte Carlo simulation of δ 18 O-NO 3 - values of typhoons, the calculated oxidation proportions of NO by RO 2 (or HO 2 ) during the OH· pathway ranged from 0% to 27% of In-fa and from 0% to 32% of Chanthu, respectively, in the three cities. More NO x emissions from marine microbial processes caused the lower δ 15 N-NO 3 - values of typhoons in Ningbo than those in Hangzhou and Huzhou. The variation in δ 15 N-NO 3 - values in sequential samples in In-fa reflected the decreased marine sources (lightning) and the increased anthropogenic sources in land (coal combustion and microbial N cycle) from Phrase I to Phrase II and III. Based on the improved Bayesian model with nitrogen isotopic fractionation, the contributions of lightning + biomass burning, coal combustion, mobile sources and the microbial N cycle were 35.7%, 22.5%, 27.1% and 14.7% in In-fa, and 28.3%, 32.3%, 28.0% and 11.4% in Chanthu, respectively, in the three cities, emphasizing the influence of marine NO x sources (lightning). The results highlight the importance of RO 2 (or HO 2 ) in NO x oxidation pathways in typhoons and provide valuable insight into the NO x sources of typhoons., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Urban storm runoff, as the primary transport medium for nutrients entering urban rivers, contributes to urban water contamination. Accurate source identification is critical for controlling water pollution. Although some studies have used nitrate isotopic composition (δ 15 N-NO 3 - and δ 18 O-NO 3 - ) to identify nitrate (NO 3 -N sources in surface runoff in different urban functional areas, drainage pipeline runoff, and channels during the complete runoff process in Wuxi, east China. The results showed that N concentrations in pipeline runoff and channels were higher than those in surface runoff, indicating that high concentration of N pollutants were accumulated in drainage pipelines. Information of δ - -N) in urban storm runoff, the relatively low frequency of collecting samples in surface runoff within a single functional area hinders the understanding of spatial variations and dynamic process of NO 3 - -N sources over the runoff process. This study investigated the nitrogen (N) concentrations and analyzed dynamic changes of NO 3 - -N sources in surface runoff in different urban functional areas, drainage pipeline runoff, and channels during the complete runoff process in Wuxi, east China. The results showed that N concentrations in pipeline runoff and channels were higher than those in surface runoff, indicating that high concentration of N pollutants were accumulated in drainage pipelines. Information of δ 15 N-NO 3 - and δ 18 O-NO 3 - suggested that the main NO 3 - -N source varied between runoff stages. NO 3 - -N contribution from atmospheric deposition decreased in the order: surface runoff (57%) > residential pipeline runoff (25%) > channels (14%), while the opposite trend was observed for the contributions from sewage, increasing from 10%, 26% to 39%. In urban storm runoff, more sewage, fertilizers, and soil N were carried into the surface runoff after 30% of cumulative runoff ratio and carried into pipeline runoff in the initial 25% of cumulative runoff ratio in the residential area. As the first attempt to identify nitrate sources over the cumulative runoff in different urban functional areas, this work expands our understanding of the primary nitrate source in urban storm runoff. The findings provide important insights for developing strategies to mitigate non-point source water pollution., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Spatiotemporal variations in soil water content (SWC) and soil water stable isotopic compositions (SWSIC; 2 H/ 1 H (δD) and 18 O/ 16 O (δ 18 O)) provide critical information on elucidating land surface processes across scales. Meanwhile, little is known about the spatiotemporal characteristics of SWSIC and its driving factors. Therefore, it's necessary to improve tracer techniques of SWSIC by interpreting their spatiotemporal variability patterns as well as the correlations with other factors such as texture, soil depth and vegetation. To this end, the spatiotemporal variations in SWC and SWSIC along with their controlling factors were jointly investigated based on seven field campaigns over roughly a two-year period at an agricultural field in North China Plain. Two transects, vegetated and bared, were considered. The results of vegetated transect showed that both SWC and SWSIC exhibited considerable spatiotemporal variabilities at the field scale of ~100 m, with SWSIC displaying more complex patterns. Overall, the spatial variations in SWSIC were larger in wet seasons than in dry seasons, which decreased with increasing soil depth, largely due to less impacts of precipitation inputs and soil evaporation on SWSIC dynamics at deeper depths. The temporal stability analysis (TSA) showed that there existed temporal persistence of the spatial structure of SWSIC, particularly at deeper soil depths. Moreover, the SWSIC data in our study showed that the effect of vegetation on the SWSIC dynamics was noticeable with shading effects, root distribution and water uptake, which caused much lesser degrees of soil evaporation at the vegetated transect. What's more, the representative sites for monitoring spatial average δD values were identified, demonstrating the viability of using the TSA method to estimate the spatial average SWSIC values at field scales. These findings can improve the interpretation of SWSIC data for practical applications., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

The input of pollutants caused by human activities induces the deterioration of surface water quality. To reveal the characteristics of surface water quality in Chaohu Lake Basin and the influence of human activities, the hydrochemistry and stable isotope composition of hydrogen and oxygen in lake water and inflow river water were analyzed. The results show that the hydrochemical type of lake water is the Na-Cl type,while river water is the Na-Cl, Ca-Cl and mixed types. The ion proportional coefficient method and principal component analysis show that surface water is controlled by weathering of evaporated salt rocks and silicate rocks, in which Cl - and SO 4 2- are affected by fertilizers and sewage to some extent. There is a strong correlation between conventional ions and nutrient indexes, which indicates that dissolved ions are affected not only by rock weathering but also by human activities (such as the discharge of domestic sewage or nitrogen-containing wastewater and the use of fertilizers). The stable isotope values of hydrogen and oxygen in surface water are distributed at the lower right portion of the local precipitation line and are close to it, indicating that surface water mainly originates from precipitation. The high value of d-excess values in surface water indicates that evaporation is weak. As pollution indicators, EC, Cl - and NO 3 - indicates that the Nanfei River, Dianbu River, Shiwuli River and Pai River flow in northwestern of Chaohu Lake Basin through Hefei urban city are severely polluted, NO 3 - originates from manure and sewage. Rivers flowing through farmland areas are less polluted, and the use of agricultural fertilizer contributes greatly to NO 3 -., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Until now, there has been a lack of knowledge regarding the vertical profiles of nitrate formation in the urban boundary layer (BL) based on triple oxygen isotopes. Here, we conducted vertical measurements of the oxygen anomaly of nitrate (Δ 17 O-NO 3 - ) on a 325 m meteorological tower in urban Beijing during the winter and summer. The simultaneous vertical measurements suggested different formation mechanisms of nitrate aerosols at ground level and 120 and 260 m in the winter due to the less efficient vertical mixing under stable atmospheric conditions. Particularly, different chemical processes of nitrate aerosols at the three heights were found between clean days and polluted days in the winter. On clean days, nocturnal chemistry (NO 3 + HC and N 2 O 5 uptake) contributed to nitrate production equally with OH/H 2 O + NO 2 at ground level, while it dominated aloft (contributing 80% of nitrate production at 260 m), due to the higher aerosol liquid water content and O 3 concentration there. On polluted days, nocturnal reactions dominated the formation of nitrate at the three heights. Particularly, the contribution of the OH/H 2 O + NO 2 pathway to nitrate production increased from the ground level to 120 m might be attributed to the hydrolysis of NO 2 to HONO and then further photolysis to OH radicals in the day. In contrast, the proportion of N 2 O 5 + H 2 O decreased at 260 m, likely due to the low relative humidity aloft that inhibited the N 2 O 5 hydrolysis reactions in the residual layer. Our results highlighted that the differences between meteorology and gaseous precursors could largely affect particulate nitrate formation at different heights within the polluted urban BL.

Excessive quantities of nitrates in the aquatic environment can cause eutrophication and raise water safety concerns. Therefore, identification of the sources of nitrate is crucial to mitigate nitrate pollution and for better management of the water resources. Here, the spatiotemporal variations and sources of nitrate were investigated by stable isotopes (δ 15 N and δ 18 O), hydrogeochemical variables (e.g., NO 3 - ), and exogenous microbial signals (i.e., sediments, soils, domestic and swine sewage) in an agricultural watershed (Changle River watershed) in China. The concentration ranges of δ - ), and exogenous microbial signals (i.e., sediments, soils, domestic and swine sewage) in an agricultural watershed (Changle River watershed) in China. The concentration ranges of δ 15 N- and δ 18 O-NO 3 - between 3.03‰-18.97‰ and -1.55‰-16.47‰, respectively, suggested that soil nitrogen, chemical fertilizers, and manure and sewage (M&S) were the primary nitrate sources. Bayesian isotopic mixing model suggested that the major proportion of nitrate within the watershed (53.12 ± 10.40% and 63.81 ± 15.08%) and tributaries (64.43 ± 5.03% and 76.20 ± 4.34%) were contributed by M&S in dry and wet seasons, respectively. Community-based microbial source tracking (MST) showed that untreated and treated domestic wastewater was the major source (>70%) of river microbiota. Redundancy analysis with the incorporation of land use, hydrogeochemical variables, dual stable isotope, and exogenous microbial signals revealed domestic wastewater as the dominant cause of nitrate pollution. Altogether, this study not only identifies and quantifies the spatiotemporal variations in nitrate sources in the study area but also provides a new analytical framework by combining nitrate isotopic signatures and community-based MST approaches for source appointment of nitrate in other polluted watersheds., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

To explore the hydrogen and oxygen stable isotopes and vapor sources of atmospheric precipitation in the loess region of north Shaanxi, we collected 107 samples of precipitation during 2018 to 2020 in the Dingbian County, Shaanxi Province. After measuring δ 18 O and δ 2 H of precipitation, we analyzed the isotopic characteristics and vapor sources of precipitation in different seasons. There was obvious seasonal variability in δ 18 O and δ 2 H, in that they were more depleted in the wet season (June-September) but more enriched in the dry season (April-May, October-November). In addition, the deuterium excess values were high in the dry season but low in the wet season. The local meteoric water line was regressed as δ 2 H=7.35δ 18 O+4.19 ( R 2 =0.96, P <0.01) with slope and intercept lower than that of the global meteoric water line, indicating an impact of evaporation on isotope fractionation. There was temperature effect in the precipitation isotopes for the whole year and for the dry season, while the precipitation isotopes in the wet season were affected by the combined effects of temperature and rainfall amount. The HYSPLIT model showed that the dry season water vapor mainly came from the Atlantic Ocean and the polar Arctic Ocean, while the wet season vapor mainly came from the Indian Ocean and Pacific subject to the Westerly.

The demand for the effective traceability of hazardous chemicals is crucial for preventing and controlling chemical spills and other accidents involving hazardous chemicals. The aim of the study was to investigate the correlation between the geographical location of ethanol-producing industrial sites and the carbon, hydrogen, and oxygen stable isotope ratios of the Chinese-manufactured ethanol using statistical classification analysis to enable the traceability of the ethanol. The isotopic data of 54 ethanol samples obtained from 18 different ethanol manufacturing plants in China between 2019 and 2020. The results of the statistical analysis demonstrated that the δ 18 O values of the ethanol positively correlated with latitudes of the production plants but negatively correlated with the δ 13 C values of the ethanol. A small number of samples derived from sites that were geographically close to each other could not be visually distinguished by PCA and HCA. However, by applying and comparing the results of classification by LDA, K-NN and Ensemble, an optimal classification model was obtained. Upon application of these models, 96.3% of the ethanol samples were correctly classified based on their geographical origin, indicating that the combination of isotopic ratios and latitude data is practical and effective for measuring the traceability of ethanol., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

Formation pathways and sources of atmosphere nitrate (NO 3 - ) have attracted much attention as NO 3 - had detrimental effects on Earth's ecosystem and climate change. Here, we measured nitrogen (δ 15 N-NO 3 - ) and oxygen (δ 18 O-NO 3 - and Δ 17 O-NO 3 - ) isotope compositions in nitrate aerosols at the Qomolangma station (QOMS) over the Himalayan-Tibetan Plateau (HTP) to quantify the formation mechanisms and emission sources of nitrate at the background site. At QOMS, the enhanced NO 3 - concentrations were observed in the springtime. The average δ 15 N-NO 3 - , δ 18 O-NO 3 - , and Δ 17 O-NO 3 - values were 0.4 ± 4.9, 64.7 ± 11.5 and 27.6 ± 6.9‰, respectively. Seasonal variations of isotope ratios at QOMS can be explained by the different emissions and formation pathways to nitrate. The average fractions of NO 2 + OH and N 2 O 5 + H 2 O to nitrate production were estimated to be 43 and 52%, respectively, when the NO 3 + hydrocarbon (HC)/dimethyl sulfide (DMS) (NO 3 + HC/DMS) pathway was assumed to be 5%. Using stable isotope analysis in the R (SIAR) model, the relative contributions of biomass burning (BB), biogenic soil emission, traffic, and coal combustion to nitrate were estimated to be 28, 25, 24, and 23%, respectively, on yearly basis. By FLEXible PARTicle (FLEXPART) dispersion model, we highlighted that NO x from BB emission over South Asia that had undergone N 2 O 5 + H 2 O processes enhanced the nitrate concentrations in the springtime over the HTP region.

The variations of hydrogen and oxygen isotopes in rainfall are critical for understanding the sources of rainfall and the influence of local evaporation. Satellite precipitation products with high time resolution (for instance 1 h) could be helpful for testifying the accuracy of water sources, as it can clearly illustrate the route of cloud movement. In this study, we analyzed the composition of hydrogen and oxygen isotopes in different rainfall events in three stations from 2015 to 2018 along the transection of 38° N latitude from Taihang Mountains to the coastal region in North China, Taihang Mountain Station (mountainous area), Luancheng Station (pre-mountain plain) and Nanpi Station (coastal low plain). By selecting typical rainfall events, water vapor sources and its influence rainfall on hydrogen and oxygen isotopes were analyzed with hourly available CMORPH satellite precipitation products. Results showed that the hydrogen and oxygen isotopes of precipitation were cha-racterized by enrichment in the rainy season and depletion in the dry season. The hydrogen and oxygen isotopes in the rainy season showed a tendency of depletion with the increases of precipitation. The slope and intercept of the fitted relationship of hydrogen and oxygen isotopes in the piedmont region of the mountains were the lowest, indicating that precipitation in the piedmont plain was significantly affected by secondary evaporation fractionation. The effect of evaporation resulted in the largest variations of isotope ratio in the dry year. In the mountainous station, due to the heavy rainfall, large isotopic variation was found in rich precipitation year. Based on the route analysis of sate-llite precipitation products, dominant water vapor in the region was inland and northwest-oriented water vapor, while water vapor in the rainy season was from southwest and from the Pacific Ocean. There was a significant difference in the hydrogen and oxygen isotopes of precipitation in the mountainous and plain stations in 2016, owing to water vapor sources and effects of rainfall for the mountainous and evaporation for plain. The results from HYSPLIT model showed that during the rainstorm on 19th July in 2016, water vapor at the mountainous station was mainly from the southwest, while that in the coastal plain was a mixture of southwest and southeast sources. Overall, our results showed that spatial and temporal variations of hydrogen and oxygen isotopes were controlled by both water sources and evaporation processes along the transection of 38° north latitude in North China.

The increase of affected river reaches by reservoirs has drastically disturbed the original hydrological conditions, and subsequently influenced the nutrient biogeochemistry in the aquatic system, particularly in the cascade reservoir system. To understand the seasonal variation of nitrogen (N) behaviors in cascade reservoirs, hydrochemistry and nitrate dual isotopes (δ 15 N-NO 3 - and δ 18 ) were conducted in a karst watershed (Wujiang River) in southwest China. The results showed that NO 3 - ) were conducted in a karst watershed (Wujiang River) in southwest China. The results showed that NO 3 - -N accounted for almost 90% of the total dissolved nitrogen (TDN) concentration with high average concentration 3.8 ± 0.4 mg/L among four cascade reservoirs. Higher N concentration (4.0 ± 0.8 mg/L) and larger longitudinal variation were observed in summer than in other seasons. The relationship between the variation of NO 3 - -N and dual isotopes in the profiles demonstrated that nitrification was dominated transformation, while assimilation contributed significantly in the epilimnion during spring and summer. The high dissolved oxygen concentration in the present cascade reservoirs system prevented the occurrence of N depletion processes in most of the reservoirs. Denitrification occurred in the oldest reservoir during winter with a rate ranging from 18 to 28%. The long-term record of surface water TDN concentration in reservoirs demonstrated an increase from 2.0 to 3.6 mg/L during the past two decades (~ 0.1 mg/L per year). The seasonal nitrate isotopic signature and continuously increased fertilizer application demonstrated that chemical fertilizer contribution significantly influenced NO 3 - -N concentration in the karst cascade reservoirs. The research highlighted that the notable N increase in karst cascade reservoirs could influence the aquatic health in the region and further investigations were required.

We investigated the distributions of phosphate (PO 4 -P) and used the oxygen isotope composition of phosphate (δ 18 O P ) to quantify PO 4 -P sources in the waters of Huangbai River. According to the environmental characteristics of Huangbai River basin, the sampling stations in the Huangbai River were divided into three groups: sampling stations in the phosphate mining area, in the outcrop area of phosphate rock, and in the residential/agricultural area. The average PO 4 -P concentration was highest (2.34 ± 1.00 μmol/L) in the outcrop area of phosphate ore, intermediate in the residential/agricultural area (1.06 ± 1.21 μmol/L), and lowest in the phosphate mining area (0.58 ± 0.31 μmol/L). The δ 18 O P measured in the Huangbai River waters ranged from 6.0 to 20.9‰, with the highest average value in the outcrop area of phosphate rock (average: 14.6‰ ± 3.1‰). The majority of the measured δ 18 O P values in the Huangbai River deviated greatly from the expected equilibrium values, indicating that δ 18 O P in this area could be used to trace PO 4 -P sources. We used two end-member mixing models to quantify the contribution of PO 4 -P from different sources. In the phosphate mining area, the average fractions of PO 4 -P from phosphate ore and sewage were 49.5% ± 23.8% and 50.5% ± 23.8%, respectively. In the outcrop area of phosphate rock, the average fractions of PO 4 -P from phosphate ore and sewage were 60.1% ± 21.7% and 39.9% ± 21.7%, respectively. In the residential/agricultural area, the average fractions of PO4-P from fertilizer and sewage were 49.2% ± 23.2% and 50.8% ± 23.2%, respectively. These results indicate that phosphate mining activities was not an important source for PO 4 -P in the waters of Huangbai River. The natural weathering of phosphate rock, fertilization, and domestic sewage contributed more to the high PO 4 -P concentrations in the Huangbai River waters.

We examined the characteristics of water use in typical tree species of arbor and shrub in Hunshandake Sandy Land, Populus cathayana and Salix gordejevii , in the different seasons, with the aim to provide theoretical basis for the structural optimization of the artificial shelterbelt. Samples of precipitation, soil water, groundwater and stem water of the two vegetation were collected, and their distribution characteristics of δD-δ 18 O were analyzed by hydrogen and oxygen stable isotope technology. The contribution rate of these potential water source to the arbor and shrub species were calculated using multi-source linear mixing model. The precipitation equation line in the study area was δ D =7.84δ 18 O +9.12, while soil moisture lines in the dry and wet season were δ D =3.56δ 18 O -41.28 and δ D =4.30δ 18 O -42.02, respectively. The δD-δ 18 O of soil water and stem water in the two seasons were lower than the precipitation δD-δ 18 O, indicating that both of them were strongly affected by the evaporation. Soil water contents in the shallow layer were strongly affected by rainfall and evaporation, with substantial fluctuation. With the increases of soil depth, soil water content tended to be stable, and the hydrogen and oxygen isotope in each soil layer showed significant differences. In the dry season, P. cathayana mainly utilized soil water in 0-40 cm and 120-200 cm layers, with contribution rates of 50.2% and 31.5%, respectively. S. gordejevii mainly absorbed soil water in 20-40 cm and 60-100 cm layers, and the contribution rates were 53.2% and 22.9%, respectively. In the wet season, the greatest contribution of soil water to P. cathayana was mainly in the 0-40 cm soil layer, accounting for 72.8%. S. gordejevii was mainly in the 0-20 cm soil water, evenly utilized the deeper soil water and groundwater. Due to the differences in root depth and distribution of the arbor and shrub, their water use strategies differed in different seasons, which was conducive to the stability of the shelterbelt community and tree species coexistence in Hunshandake Sandy Land. We proposed that the mixed planting species with different root depth should be considered in the future planting of artificial shelterbelt, which would help rationally utilize water resources and maintain the stability of sandy land ecosystem.

Shanghai city has encountered possible food fraud regarding the geographical mislabeling of vegetables for economic gain. A combination of δ 13 C, δ 15 N, δ 2 H and δ 18 O values and partial least squares discrimination analysis and support vector machine (SVM) methods were used for the first time to assess farming methods and determine the origin of vegetables from Shanghai city, Anhui and Zhejiang provinces. The results showed that 65.8% of Shanghai vegetables, 38.2% of Anhui vegetables and 23.6% of Zhejiang vegetables appeared to be grown using green or organic farming methods. The optimal discriminant model was obtained using SVM with a predictive accuracy of 100% for Shanghai vegetables. Zhejiang vegetables had a predictive accuracy of 91.7%, while it was difficult to distinguish Anhui vegetables from Shanghai or Zhejiang vegetables. Therefore, this study provided a useful method to identify vegetable farming methods and discriminate vegetables from Shanghai and Zhejiang., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Inner Mongolian steppe is one of the ecological barriers in China. The variation of water resources is very important for the development of social-economy and the protection of eco-environment. We collected 254 water samples of precipitation, river, and shadow groundwater during wet-season and dry-season of 2018-2019 from Balaguer River watershed and meansured the physical-chemical indicators, δD and δ 18 O of water samples. The stable isotope technology, mathematical statistics, and the inverse distance weighting method were used to analyze the stable isotope composition, spatial-temporal variation, and impact factors. Moreover, the d -excess and the isotopic mixing ratio formula were used to analyze the conversion characteristics of different water and to identify their environment driving variables. The results showed that δD and δ 18 O of precipitation, river and shallow groundwater were higher in wet season than in dry season. The driving factors of different water transformation in the watershed were air temperature, altitude, and groundwater depth. Altitude was significantly negatively correlated with river δD, and the δD and δ 18 O of groundwater. δD and δ 18 O of groundwater fluctuated significantly in the area with groundwater depth less than 10 m, but were stable in other areas. There was a positive correlation between precipitation δ 18 O and air temperature. The d -excess in wet season was higher than that in dry season, with a decreasing distribution characteristic from southern to northern part in the study area. More than 50% river in upper stream came from precipitation, while more than half river water converted to groundwater, with different recharge-drainage relationships existed between surface water and groundwater in different river reaches.

Hydrogen and oxygen stable isotopes (δ 2 H and δ 18 O) in precipitation were analysed from June 2018 to May 2020 in Ningbo and were influenced by the subtropical monsoon climate in East China. The δ 2 H and δ 18 O values of precipitation in Ningbo varied from -90.0 to 6.0‰ and from -13.5 to -1.6‰, respectively. The local meteoric water line (LMWL) in Ningbo was obtained as δ 2 H = 9.27 δ 18 O + 35.95 and had a larger slope and intercept compared to the global meteoric water line (GMWL) because its water vapour sources were oceans. The more negative δ 18 O values and lower deuterium excess (D-excess) of precipitation in summer were due to water vapour sources from the East China Sea, the South Sea and the Western Pacific, which are controlled by the southeasterly monsoon. In contrast, the less negative δ 18 O and higher D-excess of precipitation in winter were influenced by water vapour sources from the North Asian continent and North China transported by the northwesterly monsoon. The precipitation amount effect was significant in Ningbo, especially in summer. The inverse temperature effect was appeared in Ningbo, except winter. These two effects may be caused mainly by the monsoon climate rather than by the secondary evaporation effect.

This study monitored the isotopic compositions of precipitation in Guilin as well as the influence of El Niño-Southern Oscillation (ENSO) on the isotope ratios and water vapour sources from 2015 to 2016. The results indicate that the lower isotope values of precipitation from ocean water vapour source are affected by long transport distance and fractionation during summer and autumn. In contrast, the isotope values of winter and spring precipitation are affected by continental air masses and their evaporation sources yielding higher values. The intercepts of the local meteoric water line in Guilin are larger than those of the global meteoric water line, which is typical for subtropical monsoon climate. During the El Niño event, development of anomalous anticyclonic circulation enhances the northbound transport over the western Pacific and brings abundant water vapour to the southern part of China. During El Niño event prevailing period, precipitations exhibit a lower δ 18 O value and low d (deuterium excess) value, indicating that the 2015/2016 ENSO event had a significant effect on the precipitation distribution, precipitation amount, and isotope ratios in regional precipitation.

Stable isotope technology has been widely used to trace sources and evolution of water bodies, relationships between different water bodies and pollution sources. Based on δ 18 O and δD data from the Yarlung Tsangpo River in 2017, this paper analyzes the composition characteristics of δ 18 O and δD in the river water during the low-flow, high-flow and normal-flow seasons of the entire Yarlung Tsangpo River and further reveals the spatial evolution and influencing factors. The results show that the values of δ 18 O, δD and d-excess were different in time and space. The δ 18 O, δD and d-excess values of the Yarlung Tsangpo River in the low-flow season were significantly higher than in the high-flow season. This was mainly due to weak evaporation enrichment and precipitation supply effects on the river water during the wet season. From the perspective of season change, the d-excess in the low-season is significantly higher than in the high-flow season and the normal-flow season; from the perspective of spatial change, the d-excess shows the same spatial variation trend as the δ 18 O, which first decreases and then increases. Based on the investigation of δ18O and δD in the main and Branch rivers of the Yarlung Tsangpo River Basin, it was found that the upper reaches of the basin were mainly supplied by snowmelt, the lower reaches were mainly supplied by rainfall., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

The high frequency of acid rain in southern China has captured public and official concern since 1980s. Subsequently, gas emission reduction measures have been implemented to improve the air quality. Variations in SO 2 emission intensities can influence the sulfur and oxygen isotopic compositions of sulfate in rainwater, since atmospheric sulfate is mainly formed via the oxidation of sulfur gases from natural and anthropogenic sources. To evaluate the impacts of emission reduction measures on atmospheric sulfate, the seasonal and long-term trends in stable isotopic compositions of sulfate in rainwater in Guizhou province, southwestern China have been investigated based on rainwater samples collected from June 2016 to June 2018 and literature investigation (2000-2010).The results reveal that coal combustion remains a major contributor to sulfate in rainwater, although its SO 2 emission has significantly decreased over the past two decades. The δ 34 S sulfate and δ 18 O sulfate values in rainwater are negatively correlated and have significant seasonal changes. The seasonality in δ 34 S sulfate has been interpreted as due to the changes in contributions of dimethyl sulfide and coal combustion, while the seasonal pattern of δ 18 O sulfate is consistent with that of δ 18 O water values, indicating sulfate in rainwater is mainly formed by heterogeneous oxidation of SO 2 . Combined with the data from previous studies (Xiao and Liu, 2002; Liu, 2007; Xiao et al., 2009; Xiao et al., 2014), we found that the volume weighted mean δ 34 S values of sulfate in rainwater in Guizhou province show a marked increase between 2001 and 2018, indicating that the 34 S-depleted SO 2 emission from coal combustion has declined during this period. Furthermore, the synchronous changes in δ 34 S values, sulfate concentration and pH values of rainwater suggest that the frequency of acid rain in Guizhou province has dropped over the past two decades, which is likely to result from the emission reduction measures taken in Guizhou province., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

The Lancang River in China is the headwater of the Mekong River. The impacts of reservoirs on the water, sediment and nutrient trapping in the Lancang River have attracted considerable attention, both locally and abroad. In this research, watershed-scale nitrogen load and nitrate sources along the Lancang River upstream in free-flowing reaches (FFRs) and downstream regulated reaches (RRs) were analyzed using stable nitrogen and oxygen isotopes. The results showed that the nitrogen nutrient (TN, NO 3 - ) concentration increased from upstream to downstream along the Lancang River, and the highest values come from large-scale urban samples rather than the reservoirs. Compared to other large rivers in China, such as the Yangtze River, Yellow River and Yalu Tsangpo River, nitrogen nutrient content in the Lancang River is at low level. The nitrate concentration ranged from 0.14 mg/L to 0.63mg/Land increased significantly downstream. The isotopic values ranged from 2.8‰ to 5.2‰ for δ 4 + ) concentration increased from upstream to downstream along the Lancang River, and the highest values come from large-scale urban samples rather than the reservoirs. Compared to other large rivers in China, such as the Yangtze River, Yellow River and Yalu Tsangpo River, nitrogen nutrient content in the Lancang River is at low level. The nitrate concentration ranged from 0.14 mg/L to 0.63mg/Land increased significantly downstream. The isotopic values ranged from 2.8‰ to 5.2‰ for δ 15 N-NO 3 - and from 4‰ to 8.5‰ for δ 18 O-NO 3 - along the river, and the δ 15 N-NO 3 - value rose significantly downstream. According to the nitrogen and oxygen isotope approach, soil organic nitrogen mineralization was the main source of the nitrate with an average of 51% contribution; domestic sewage was the second largest contributor with an average of 33% but increase downstream, likely due to the significantly larger population in the downstream region. Furthermore, the nitrate concentration decreased and δ 15 N- and δ 18 O-NO 3 - enriched in the Nuozhadu reservoir, indicating that the reservoir may enhance nitrate consumption and reduce nitrogen pollution to downstream reaches. The results provide a perspective of nitrogen nutrient for the trans-border river management and more insight researches are called for understanding the controversial nutrient transport topic in this region., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

During atmospheric precipitation, the evaporation of raindrops falling from the bottom of cloud layer to the ground and passing through unsaturated air, a process was called sub-cloud secondary evaporation, which will change the isotopic composition of precipitation. Using the hydrogen and oxygen stable isotope method to understand the temporal and spatial variation of secondary evaporation effect under clouds and its causes is important to understand regional water cycle process. Based on hourly meteorological data of 187 meteorological stations in Shaanxi-Gansu-Ningxia region from March 2018 to February 2019, the spatial and temporal variations of evaporation surplus ratio ( f ) and precipitation excess deuterium variation (Δ d ) were analyzed using the improved Ste-wart model, and the relationships between f and meteorological elements and Δ d were examined. The results showed that, at the hourly scale, the minimum values of f and Δ d in all provinces of the region appeared in the daytime, and the maximum values appeared in the night, indicating that the secondary evaporation effect under the cloud was more obvious in the daytime. At the monthly scale, the monthly variation trend of f and Δ d in each province was relatively consistent, with the minimum value appearing in the summer half year, and the maximum value appearing in the winter half year, indicating that the second evaporation effect under cloud was more significant in the summer half year. From the spatial perspective, the spatial variation of f and Δ d values in the region was consistent with that at the seasonal scale. In spring, the eastern and western regions were larger while the central part was smaller. In summer, the northwest region was smaller, and other regions were larger. In autumn, it decreased from south to north. In winter, the central and southern regions were smaller, and the western and northeast regions were larger. The spatial differences of secondary evaporation effects under clouds in different seasons was significant. The slopes of the linear relationship between f and Δ d in Shaanxi, Gansu and Ningxia provinces were all less than 1‰·% -1 , which may be caused by the arid and semi-arid climate in this area. When air temperature was higher and the relative humidity, vapor pressure, precipitation and raindrop diameter were smaller, the value of Δ d was smaller, and the secondary evaporation effect under the cloud was more obvious.