Metagenomics and Stable Isotopes Uncover the Augmented Sulfide-Driven Autotrophic Denitrification in a Seasonally Hypoxic, Sulfate-Abundant Reservoir.

The mechanism governing sulfur cycling in nitrate reduction within sulfate-rich reservoirs during seasonal hypoxic conditions remains poorly understood. This study employs nitrogen and oxygen isotope fractionation in nitrate, along with metagenomic sequencing to elucidate the intricacies of the coupled sulfur oxidation and nitrate reduction process in the water column. In the Aha reservoir, a typical seasonally stratified water body, we observed the coexistence of denitrification, bacterial sulfide oxidation, and bacterial sulfate reduction in hypoxic conditions. This is substantiated by the presence of abundant N/S-related genes (nosZ and aprAB/dsrAB) and fluctuations in N/S species. The lower 15εNO3/18εNO3 ratio (0.60) observed in this study, compared to heterotrophic denitrification, strongly supports the occurrence of sulfur-driven denitrification. Furthermore, we found a robust positive correlation between the metabolic potential of bacterial sulfide oxidation and denitrification (p < 0.05), emphasizing the role of sulfide produced via sulfate reduction in enhancing denitrification. Sulfide-driven denitrification relied on ∑S2- as the primary electron donor preferentially oxidized by denitrification. The pivotal genus, Sulfuritalea, emerged as a central player in both denitrification and sulfide oxidation processes in hypoxic water bodies. Our study provides compelling evidence that sulfides assume a critical role in regulating denitrification in hypoxic water within an ecosystem where their contribution to the overall nitrogen cycle was previously underestimated.

Changes in source composition of wet nitrate deposition after air pollution control in a typical area of Southeast China.

In recent years, China has adopted numerous policies and regulations to control NOx emissions to further alleviate the adverse impacts of NO3--N deposition. However, the variation in wet NO3--N deposition under such policies is not clear. In this study, the southeastern area, with highly developed industries and traditional agriculture, was selected to explore the variation in NO3--N deposition and its sources changes after such air pollution control through field observation and isotope tracing. Results showed that the annual mean concentrations of NO3--N in precipitation were 0.67 mg L-1 and 0.54 mg L-1 in 2014-2015 and 2021-2022, respectively. The average wet NO3--N depositions in 2014-2015 and 2021-2022 was 7.76 kg N ha-1 yr-1 and 5.03 kg N ha-1 yr-1, respectively, indicating a 35% decrease. The δ15N-NO3- and δ18O-NO3- values were lower in warm seasons and higher in cold seasons, and both showed a lower trend in 2021-2022 compared with 2014-2015. The Bayesian model results showed that the NOx emitted from coal-powered plants contributed 53.6% to wet NO3--N deposition, followed by vehicle exhaust (22.9%), other sources (17.1%), and soil emissions (6.4%) during 2014-2015. However, the contribution of vehicle exhaust (33.3%) overpassed the coal combustion (32.3%) and followed by other sources (25.4%) and soil emissions (9.0%) in 2021-2022. Apart from the control of air pollution, meteorological factors such as temperature, precipitation, and solar radiation are closely related to the changes in atmospheric N transformation and deposition. The results suggest phased achievements in air pollution control and that more attention should be paid to the control of motor vehicle exhaust pollution in the future, at the same time maintaining current actions and supervision of coal-powered plants.

Molecular and Dual-Isotopic Profiling of the Microbial Controls on Nitrogen Leaching in Agricultural Soils under Managed Aquifer Recharge.

Nitrate (NO3-) leaching is a serious health and ecological concern in global agroecosystems, particularly those under the application of agricultural-managed aquifer recharge (Ag-MAR); however, there is an absence of information on microbial controls affecting NO3- leaching outcomes. We combine natural dual isotopes of NO3- (15N/14N and 18O/16O) with metagenomics, quantitative polymerase chain reaction (PCR), and a threshold indicator taxa analysis (TITAN) to investigate the activities, taxon profiles, and environmental controls of soil microbiome associated with NO3- leaching at different depths from Californian vineyards under Ag-MAR application. The isotopic signatures demonstrated a significant priming effect (P < 0.01) of Ag-MAR on denitrification activities in the topsoil (0-10 cm), with a 12-25-fold increase of 15N-NO3- and 18O-NO3- after the first 24 h of flooding, followed by a sharp decrease in the enrichment of both isotopes with ∼80% decline in denitrification activities thereafter. In contrast, deeper soils (60-100 cm) showed minimal or no denitrification activities over the course of Ag-MAR application, thus resulting in 10-20-fold of residual NO3- being leached. Metagenomic profiling and laboratory microcosm demonstrated that both nitrifying and denitrifying groups, responsible for controlling NO3- leaching, decreased in abundance and potential activity rates with soil depth. TITAN suggested that Nitrosocosmicus and Bradyrhizobium, as the major nitrifier and denitrifier, had the highest and lowest tipping points with regard to the NO3- changes (P < 0.05), respectively. Overall, our study provides new insight into specific depth limitations of microbial controls on soil NO3- leaching in agroecosystems.

Riverine nitrate source and transformation as affected by land use and land cover.

A mixed land use/land cover (LULC) catchment increases the complexity of sources and transformations of nitrate in rivers. Spatial paucity of sampling particularly low-resolution sampling in tributaries can result in a bias for identifying nitrate sources and transformations. In this study, high spatial resolution sampling campaigns covering mainstream and tributaries in combination with hydro-chemical parameters and dual isotopes of nitrate were performed to reveal spatio-temporal variations of nitrate sources and transformations in a river draining a mixed LULC catchment. This study suggested that point sources dominated the nitrate in the summer and winter, while non-point sources dominated the nitrate in the spring and autumn. A positive correlation was observed between proportions from sewage and land use index (LUI). However, negative correlations between soil nitrogen/nitrogen fertilizer and LUI were observed. With an increase of urban areas, the increased contribution from domestic sewage resulted in an increase of NO3- concentrations in rivers. Both urban and agricultural inputs should be considered in nitrate pollution management in a mixed LULC catchment. We concluded that the seasonal variations of nitrate sources were mainly affected by flow velocity conditions and agricultural activities, while spatial variations were mainly affected by LULC. In addition, we found a novel underestimation of dominated sources from Bayesian model because of mixing effect of isotope values from the tributaries to mainstream, however, high spatial resolution sampling can make up for this shortcoming. δ15N and δ18O values of nitrate indicated that nitrate originated from nitrification in soils. The nitrate concentrations and correlation between δ15N and 1/[NO3-] suggested little contribution of nitrate removal by denitrification. Thus, the nitrate reduction in the Yuehe River basin needs to be strengthened. The study provides new implications for estimation of nitrate sources and transformations and basis for nitrate reduction in the river with mixed LULC catchment.

Reservoir NO3- pollution and chemical weathering: by dual isotopes of δ15N-NO3-, δ18O-NO3- and geochemical constraints.

Reservoir dams alter the nutrient composition and biogeochemical cycle. Thus, dual isotopes of δ18O-NO3- and δ15N-NO-3 and geochemical signatures were employed to study the NO3- pollution and chemical weathering in the Three Gorges Reservoir (TGR), China. This study found that the TGR dam alters the δ15N-NO3- composition and is enriched in the recharge period. Values of δ15N-NO3- varied from 4.5 to 12.9‰ with an average of 9.8‰ in the recharge period, while discharge period δ15N-NO3- ranged from 3.2 to 12.5‰, with an average of 9.3‰. δ18O-NO3- varies (1.2-11.3‰) with an average of 6.5‰ and (2.4-12.4‰) with an average of 7.5‰, in the recharge and discharge periods, respectively. Stable isotopic values sharply decreased from upstream to downstream, indicating the damming effects. δ18O-NO3- and δ15N NO3- confirm that sewage effluents, nitrification of soil organic material, and NH4+ fertilizers were the primary sources of NO3- in the reservoir. Carbonate weathering mainly provides ions to the reservoir. HCO3- + SO42- and Ca2+ + Mg2+ represent 90% of major ions in the TGR. Downstream sampling sites showed low solute concentration during the recharge period, indicating the dam effect on solute concentration. Ca-Mg-Cl-, Ca-HCO3- and Ca-Cl- were the main water types in the TGR. The average percentage of solutes contribution revealed the carbonate weathering, evaporites dissolution, silicate weathering, and atmospheric input were 51.9%, 41%, 7.8%, and 1.7% for the recharge period. In contrast, the discharge period contributed 66.4%, 29.2%, 10%, and 4.3%, respectively. TGR water is moderately suitable for irrigation, and hardness is high in drinking water. This study provides new insight into the dual isotopic approach and geochemical signatures to interpret the NO3- cycle and chemical weathering process under dam effects in the TGR. However, this isotopic application has some limitations in source identification, isotope fractionation, and transformation mechanisms of nitrate. Thus, further studies need to be done on these topics for a better undestanding.

Seasonal differences in sources and formation processes of PM2.5 nitrate in an urban environment of North China.

Nitrate (NO3-) has been the dominant ion of secondary inorganic aerosols (SIAs) in PM2.5 in North China. Tracking the formation mechanisms and sources of particulate nitrate are vital to mitigate air pollution. In this study, PM2.5 samples in winter (January 2020) and in summer (June 2020) were collected in Jiaozuo, China, and water-soluble ions and (δ15N, δ18O)-NO3- were analyzed. The results showed that the increase of NO3- concentrations was the most remarkable with increasing PM2.5 pollution level. δ18O-NO3- values for winter samples (82.7‰ to 103.9‰) were close to calculated δ18O-HNO3 (103‰ ± 0.8‰) values by N2O5 pathway, while δ18O-NO3- values (67.8‰ to 85.7‰) for summer samples were close to calculated δ18O-HNO3 values (61‰ ± 0.8‰) by OH oxidation pathway, suggesting that PM2.5 nitrate is largely from N2O5 pathway in winter, while is largely from OH pathway in summer. Averaged fractional contributions of PN2O5+H2O were 70% and 39% in winter and summer sampling periods, respectively, those of POH were 30% and 61%, respectively. Higher δ15N-NO3- values for winter samples (3.0‰ to 14.4‰) than those for summer samples (-3.7‰ to 8.6‰) might be due to more contributions from coal combustion in winter. Coal combustion (31% ± 9%, 25% ± 9% in winter and summer, respectively) and biomass burning (30% ± 12%, 36% ± 12% in winter and summer, respectively) were the main sources using Bayesian mixing model. These results provided clear evidence of particulate nitrate formation and sources under different PM2.5 levels, and aided in reducing atmospheric nitrate in urban environments.

Determination of Dietary Amino Acid Digestibility in Humans.

Dietary amino acid digestibility is a fundamental measure of importance in protein quality evaluation. Determining amino acid digestibility in humans, as the disappearance of an amino acid across the total digestive tract, has been discredited. Extensive cecal and colonic microbial metabolism renders fecal estimates of amino acids misleading. True ileal amino acid digestibility determined at the end of the small intestine predicts amino acid uptake more accurately. Given that ileal digestibility determination cannot be undertaken routinely in humans, a pig-based assay has been developed and validated. The growing pig values for digestibility, however, relate to healthy adult humans and there is a need to be able to determine amino acid digestibility for humans with specific physiological states. To this end, isotope-based methods for determining dietary amino acid digestibility indirectly show promise but remain to be fully validated.

Deciphering and quantifying nitrate sources and processes in the central Yellow Sea using dual isotopes of nitrate.

Anthropogenic activities pose significant challenges to the accumulation of coastal nitrogen (N). Accurate identification of nitrate (NO3-) sources is thus essential for mitigating excessive N in many marginal seas. We investigated the dual isotopes of NO3- in the central Yellow Sea to elucidate the sources and cycling processes of NO3-. The results revealed significant spatial variability in NO3- concentrations among the Yellow Sea Surface Water (YSSW), Changjiang Diluted Water (CDW), Yellow Sea Cold Water Mass (YSCWM), and Taiwan Warm Current Water (TWCW). Stratification played a crucial role in restricting vertical nutrient transport, leading to distinct nutrient sources and concentrations in different water masses. The dual NO3- isotopic signature indicated that atmospheric deposition was the primary source of surface NO3-, contributing approximately 30 % to the NO3- in the YSSW. In the NO3--rich CDW, the heavier δ15N-NO3- and δ18O-NO3- suggested incomplete NO3- assimilation. Organic matter mineralization and water stratification played crucial roles in the accumulation of nutrients within the YSCWM and TWCW. Notably, regenerated NO3- accounted for approximately half of the NO3- stored in the YSCWM. A synthesis of NO3- dual isotope data across the coastal China seas revealed significant spatial and seasonal variations in the N source. The study emphasized the dynamics of coastal NO3- supply, which are shaped by the complex interconnections among marine, terrestrial, and atmospheric processes. Our approach is a feasible method for exploring the origins of N amidst the escalating pressures of anthropogenic nutrient pollution in coastal waters.

Changes in fronts regulate nitrate cycling in Zhanjiang Bay: A comparative study during the normal wet season, rainstorm, and typhoon periods.

Typhoons and rainstorms (>250 mm/day) are extreme weather events changing hydrological characteristics and thus nitrogen (N) cycle in coastal waters. However, responses of N cycle to rainstorms and typhoons and their underlying mechanisms need to be elucidated. In this study, we conducted an analysis of a comparative dataset encompassing concentrations of nitrate (NO3-), ammonium (NH4+), dissolved oxygen (DO), chlorophyll a (Chl a), hydrological parameters, dual isotopic composition of NO3- (δ15N-NO3- and δ18O-NO3-) in Zhanjiang Bay during three distinct periods: the normal wet season, rainstorm, and typhoon periods. After the rainstorm, the salinity front in Zhanjiang Bay was more weakened and steadier than that during the normal wet season, mainly because onshore wind and a large amount of freshwater was inputted into the ocean surface. This weakened and steady salinity front strengthened water stratification and provided a favorable condition for phytoplankton blooms. Correspondingly, evident NO3- deficits coincided with elevated δ15N-NO3- and δ18O-NO3- values indicated that sufficient NO3- sustained phytoplankton blooms, leading to NO3- assimilation during the rainstorm period. By contrast, due to the onshore wind induced by the typhoon, the salinity front in Zhanjiang Bay was more intensified and unsteady after the typhoon than the normal wet season. The salinity front after the typhoon was unsteady enough to enhance vertical mixing in the water column. Relatively high DO concentrations suggested that enhanced vertical mixing after the typhoon support freshly organic matter decomposition and nitrification via oxygen injection from the air into the water column. In addition, NO3- deficits coincided with elevated δ15N-NO3- values and δ18O-NO3- values demonstrated the coexistence of NO3- assimilation during the typhoon period. This study suggests that the changing processes involved in NO3- cycling after typhoons and rainstorms are associated with the stability and intensity of the salinity front altered by these weather events.

Quantification of NOx sources contribution to ambient nitrate aerosol, uncertainty analysis and sensitivity analysis in a megacity.

Dual isotopes of nitrogen and oxygen of NO3- are crucial tools for quantifying the formation pathways and precursor NOx sources contributing to atmospheric nitrate. However, further research is needed to reduce the uncertainty associated with NOx proportional contributions. The acquisition of nitrogen isotopic composition from NOx emission sources lacks regulation, and its impact on the accuracy of contribution results remains unexplored. This study identifies key influencing factors of source isotopic composition through statistical methods, based on a detailed summary of δ15N-NOx values from various sources. NOx emission sources are classified considering these factors, and representative means, standard deviations, and 95 % confidence intervals are determined using the bootstrap method. During the sampling period in Tianjin in 2022, the proportional nitrate formation pathways varied between sites. For suburban and coastal sites, the ranking was [Formula: see text] (NO2 + OH radical) > [Formula: see text] (N2O5 + H2O) > [Formula: see text] (NO3 + DMS/HC), while the rural site exhibited similar fractional contributions from all three formation pathways. Fossil fuel NOx sources consistently contributed more than non-fossil NOx sources in each season among three sites. The uncertainties in proportional contributions varied among different sources, with coal combustion and biogenic soil emission showing lower uncertainties, suggesting more stable proportional contributions than other sources. The sensitivity analysis clearly identifies that the isotopic composition of 15N-enriched and 15N-reduced sources significantly influences source contribution results, emphasizing the importance of accurately characterizing the localized and time-efficient nitrogen isotopic composition of NOx emission sources. In conclusion, this research sheds light on the importance of addressing uncertainties in NOx proportional contributions and emphasizes the need for further exploration of nitrogen isotopic composition from NOx emission sources for accurate atmospheric nitrate studies.

Dual isotopic evidence of δ15N and δ18O for priority control of vehicle emissions in a megacity of East China: Insight from measurements in summer and winter.

The source apportionment and main formation pathway of nitrate aerosols in China are not yet fully understood. In this study, PM2.5 samples were collected in Shanghai in the summer and winter of 2019. Water-soluble inorganic ions and isotopic signatures of stable nitrogen (δ15N-NO3-) and stable oxygen (δ18O-NO3-) in PM2.5 were determined. The results showed that NO3- was less important in summer (NO3-/SO42- = 0.4 ± 0.8), while it became the dominant species in winter (52.1 %). The average values of δ15N-NO3- and δ18O-NO3- in summer were + 2.0 ± 6.1 ‰ and 63.3 ± 9.4 ‰ respectively, which were significantly lower than those in winter (+7.2 ± 3.4 ‰ and 88.3 ± 12.1 ‰), indicating discrepancies between NOx sources and nitrate formation pathways. Both δ15N-NO3- and δ18O-NO3- were elevated at night, demonstrating that N2O5 hydrolysis contributed to the nocturnal nitrate increase even in summer. The contribution of the OH oxidation pathway to nitrate aerosols averaged at 70.5 ± 17.0 % in summer and N2O5 hydrolysis dominated the nitrate production in winter (approximately 80 %). On average, vehicle exhaust, coal combustion, natural gas burning, and soil emission contributed 50.7 %, 21.5 %, 15.9 %, and 11.9 %, respectively, to nitrate aerosols in summer, and contributed 56.8 %, 23.9 %, 13.6 %, and 5.7 %, respectively, to nitrate production in winter. Notably, natural gas burning is a non-negligible source of nitrate aerosols in Shanghai. In contrast to an inverse correlation between δ15N-NO3- and PM2.5, the value of δ18O-NO3- was positively correlated with nitrate concentration and aerosol liquid water content (ALWC) in winter, suggesting that explosive growth of nitrate was driven by continuous accumulation of N-depleted NOx and rapid N2O5 hydrolysis under calm and humid conditions. To continuously improve air quality, priority control should be given to vehicle emissions as the dominant source of NOx and volatile organic compounds (VOCs) in Shanghai.

Disentangling external loadings, hydrodynamics and biogeochemical controls on the fate of nitrate in a coastal embayment.

Nitrogen, as an essential nutrient, largely contributes to the coastal eutrophication. However, the accurate depiction and evaluation of how external loadings, hydrodynamics, and biogeochemical reactions mediate the occurrence, transport, and transformation of nitrate (NO3-) within coastal embayment still pose ongoing challenges to date. In this study, we took advantage of dual isotopes of NO3- to track external NO3- loadings, radium and dual isotopes of H2O to characterize the influences of hydrodynamic on NO3- transport, δ18O-NO3- and δ18O-H2O along with microbial analysis to explore major NO3- biogeochemical reactions in Tolo Harbour, Hong Kong. The multiple isotopic evidence showed that NO3- in surface harbour water was predominantly contributed by precipitation in wet season and its impact was strengthened by stratification. In dry season, NO3- in the surface harbour water became largely influenced by benthic input and biogeochemical reactions due to intensified vertical mixing. Based on NO3- mass balance model, biogeochemical reaction, especially nitrification, was found to be the major process to secure the closure of NO3- budget and increase NO3- inventory from wet to dry season. Hydrodynamics redistributed the external NO3- loadings and mediated nitrogen biogeochemical reactions, both of which further synergistically regulated the fate of NO3- in the embayment.

Revealing the ammonia oxidation process and shortcut nitrification performance using nitrogen and oxygen isotope fractionation effect.

Natural abundance isotope fractionation properties have become the most effective way to explore nitrogen transformations of biological nitrogen removal from wastewater. The migration and transformation characteristics of N and O elements in the shortcut nitrification were analyzed using the N and O dual isotopic fractionation technique. The effects of dissolved oxygen (DO) and temperature changes on the performance of shortcut nitrification and isotopic fractionation were investigated. The fractionation characteristics of N and O elements during shortcut nitrification were explored by adjusting DO concentration (0.2-0.4, 1-1.2 and 3-4 mg/L) and temperature (33 ± 1 °C, 25 ± 1 °C and 18 ± 1 °C). Both δ15NNO2 and δ18ONO2 showed a gradually increasing trend with the accumulation of NO2--N, and the fractionation effects induced by temperature were significantly higher than those by DO. The higher the temperature, the more significant the increase in δ15NNO2; the higher the DO, the more remarkable the increase in δ18ONO2, while δ15NNO2: δ18ONO2 was maintained at 0.77-6.45. The 18O-labeled H2O was successfully transferred to NO2--N, and the replacement of O element was as high as 100 %, indicating that DO and H2O simultaneously participated in the shortcut nitrification process. The dynamic changes in isotope fractionation effects can be successfully applied to reveal the performance and mechanism of shortcut nitrification.

Analysis of nitrite oxidation process and nitrification performance by nitrogen and oxygen isotope fractionation effect.

The N and O isotope fractionation effects in NO2--N oxidation and nitrification performance of an activated sludge system treating municipal wastewater are unknown. The nitrifying sludge was cultured under different temperature (33 ± 1 °C, 25 ± 1 °C,and 18 ± 1 °C) and dissolved oxygen (DO: 0.5-1 mg/L, 3-4 mg/L, and 7-8 mg/L). The inverse kinetic isotope effects of N and O (15εNO2 and 18εNO2) were -0.62‰ to -7.08‰ and -0.87‰ to -1.68‰ in the process of NO2--N oxidation, respectively. 15εNO3 gradually increased with increasing of temperature (15εNO3-33°C (14.49‰) > 15εNO3-25°C (10.43‰) > 15εNO3-18°C (7.3‰)), while the 15εNO3:18εNO3 was maintained at 1.02-5.32. The increase of temperature improved the nitrification activity, which promoted the fractionation effect, but the change of DO did not highlight this difference. The exchange of NO2--N and H2O (XNOB) was 32.5 ± 1.5%, and the kinetic isotope effect of H2O participating in the reaction (18εk, H2O, 2) was 22.57 ± 1.79‰, indicating that H2O was involved in the NO2--N oxidation rather than DO. In summary, the elevated temperature enhanced the fractionation effect of NO2--N oxidation. This study provides a new perspective to reveal the mechanism of NO2--N oxidation, optimize the process of nitrogen removal from wastewater and further control water eutrophication.

Stepwise Approach for Tracing the Geographical Origins of the Manila Clam Ruditapes philippinarum Using Dual-Element Isotopes and Carbon Isotopes of Fatty Acids.

While there are many studies that have reported methods for tracing the geographical origin of seafoods, most of them have focused on identifying parameters that can be used effectively and not the direct application of these methods. In this study, we attempted to differentiate the geographical origins of the Manila clam R. philippinarum collected from different sites in Korea, the Democratic People's Republic of Korea, and China using a combination of analyses based on dual-element isotopes, fatty acids (FAs), and compound-specific isotopic analysis of FAs. We hypothesized that a stepwise application of new parameters to unclassified samples could achieve this objective by integrating new information while reducing time and labor. The FA profiles and compound-specific carbon isotopic values of FAs were found to enhance the discrimination power of determining the geographic origin up to 100%. Our findings demonstrate the advantageousness of using several parameters simultaneously over the conventional method of employing individual analytical methods when identifying geographic origins of the Manila clam, which could have implications for tracing the origins of different shellfish species or other food products as well.

Watershed scale spatiotemporal nitrogen transport and source tracing using dual isotopes among surface water, sediments and groundwater in the Yiluo River Watershed, Middle of China.

Nitrogen pollution has been shown to have strong potential threaten to the human drinking water and agriculture. However, identifying the nitrogen and spatial-temporal variation and nitrogen pollution sources among surface water, sediments and groundwater at the watershed scale is still of insufficient understanding. In this study, multi-methods (dual isotopes, hydraulic, hydrogeochemical methods) have been used and 400 samplings (40 sediments, 20 shallow groundwater and 40 surface waters in four periods in dry and wet seasons) were collected from 2018 to 2020. The results showed that the concentration of NO3--N, NH4+-N, NO2--N and total nitrogen (TN) had variable spatial and temporal changes in whole watershed. The concentration of TN, NO3--N, NH4+-N and NO2--N in downstream was higher than midstream and upstream both in dry and wet seasons. The concentration of TN, NO3--N, NH4+-N and NO2--N of the whole watershed in wet season was higher than dry season. The dual isotope values indicate that nitrogen sources were mainly derived from manure and sewage waste input (MSI), agriculture chemical fertilizers (ACFI) and sediments nitrogen input (SNI). Those nitrogen sources have different proportion in downstream, midstream and upstream in dry and wet seasons (the largest proportion: MSI 95.24% in downstream and ACFI 86.26% in upstream both in dry season, SNI 31.75% in midstream in wet season). Water exchange has positive correlation with the nitrogen concentration. High level of nitrogen in river also can be a diver in different location and seasons. Those results can be useful for developing regional management strategies and plans for water pollution control and treatment at watershed-scale.

Coupling stable isotopes to evaluate sources and transformations of nitrate in groundwater and inflowing rivers around the Caohai karst wetland, Southwest China.

Nitrate is one of the most common pollutants in aquatic ecosystems, particularly in highly vulnerable karst aquifers. In Caohai Lake, an important karst wetland in southwestern China, karst surface water and groundwater are important recharge water sources, and nitrates flow into the wetland along with the surface water and groundwater, degrading the wetland water quality. Therefore, identifying the sources of nitrate in the surface water and groundwater in the Caohai catchment is of great significance to the protection of the wetland water environment. In this study, the nitrate concentrations, hydrochemistry and multiple stable isotope ratios (δ18O-H2O, δD-H2O, δ15N-NO3- and δ18O-NO3-) were used to identify the sources and fate of the NO3- in the groundwater and inflowing rivers around the Caohai wetland. The results showed that the NO3- concentrations in the groundwater samples from the southern side exceeded the WHO limit during the wet season, while other samples did not exceed the limit. The mean concentrations of NO3- in groundwater were higher than those in the inflowing river water, and NO3- concentrations decreased in the order of wet season>dry season>normal season in the groundwater and inflowing rivers. The hydrochemistry and multiple isotope ratios suggest that the nitrate transformation was dominated by nitrification processes, while denitrification had an influence on the transformation of NO3- (as evidenced by isotopes) in groundwater during the dry season. According to the analyses based on a stable isotope analysis in R (SIAR), sewage and manure were the main sources of NO3- in the groundwater, while sewage, manure and chemical fertilizer were the major sources of NO3- in the inflowing river water; therefore, the scientific use of farmland fertilizers and the treatment of domestic sewage should be strengthened to safeguard groundwater quality and control the NO3- concentrations in rivers.

Multi-element isotopic evidence for monochlorobenzene and benzene degradation under anaerobic conditions in contaminated sediments.

Industrial chemicals are frequently detected in sediments due to a legacy of chemical spills. Globally, site remedies for groundwater and sediment decontamination include natural attenuation by in situ abiotic and biotic processes. Compound-specific isotope analysis (CSIA) is a diagnostic tool to identify, quantify, and characterize degradation processes in situ, and in some cases can differentiate between abiotic degradation and biodegradation. This study reports high-resolution carbon, chlorine, and hydrogen stable isotope profiles for monochlorobenzene (MCB), and carbon and hydrogen stable isotope profiles for benzene, coupled with measurements of pore water concentrations in contaminated sediments. Multi-element isotopic analysis of δ13C and δ37Cl for MCB were used to generate dual-isotope plots, which for 2 locations at the study site resulted in ΛC/Cl(130) values of 1.42 ± 0.19 and ΛC/Cl(131) values of 1.70 ± 0.15, consistent with theoretical calculations for carbon-chlorine bond cleavage (ΛT = 1.80 ± 0.31) via microbial reductive dechlorination. For benzene, significant δ2H (122‰) and δ13C (6‰) depletion trends, followed by enrichment trends in δ13C (1.6‰) in the upper part of the sediment, were observed at the same location, indicating not only production of benzene due to biodegradation of MCB, but subsequent biotransformation of benzene itself to nontoxic end-products. Degradation rate constants calculated independently using chlorine isotopic data and carbon isotopic data, respectively, agreed within uncertainty thus providing multiple lines of evidence for in situ contaminant degradation via reductive dechlorination and providing the foundation for a novel approach to determine site-specific in situ rate estimates essential for the prediction of remediation outcomes and timelines.

Seasonal variation of nitrogen biogeochemical processes constrained by nitrate dual isotopes in cascade reservoirs, Southwestern 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 (δ15N-NO3- and δ18O-NO3-) were conducted in a karst watershed (Wujiang River) in southwest China. The results showed that NO3--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 NO3--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 NO3--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.

Nitrogen removal performance of sulfur autotrophic denitrification under different S2O32- additions using isotopic fractionation of nitrogen and oxygen.

The dual isotope fractionation of nitrogen (N) and oxygen (O) is an effective way to track the transformation of NO3--N in biological denitrification process. The Sulfur autotrophic denitrification combined with the different concentrations of S2O32- was investigated using the dual isotope fractionation of nitrogen (N) and oxygen (O) to reveal the nitrogen removal mechanism of the activated sludge. Based on successful autotrophic denitrification incubation, the modified Logistic model responded to the short-term effects of S2O32- addition on NO3--N removal and SO42- generation. Under the S2O32- addition of 0.5, 1, 2 and 4 times of the incubation stage (49.29 mg/L-394.32 mg/L), the fractionation effect of N in NO3--N (15εNO3) decreased from 8.74 ± 1.81‰ to 2.08 ± 0.06‰, and the fractionation effect of O in NO3--N (18εNO3) declined from 11.34 ± 0.46‰ to 5.48 ± 0.46‰. The 15εNO3/18εNO3 was maintained at 0.46-0.94, indicating a negative correlation between addition amount and isotope effect, and the addition of high concentrations of S2O32- was not suitable for system stabilization. Moreover, the 18O-labeled H2O (δ18OH2O) tests significantly proved the presence of O exchange between NO2--N/NO3--N and H2O (67%/97%) during the nitrogen removal process, while the reoxidation of NO2--N was explored in the autotrophic denitrification. The kinetic models coupled with isotope fractionation effectively revealed the nitrogen removal characteristics in the autotrophic denitrification systems, and verified the difference between the activated sludge-based wastewater treatment process and the natural ecosystem.