Search Results (165)

This study investigated spring water in the core area and buffer zone of the Shibing Dolomite Karst World Heritage Site using one-year monthly monitoring, hydrochemistry, nitrate dual isotopes, and the MixSIAR model. The buffer zone spring exhibits shallow fissure-conduit flow with rapid hydrological response, anthropogenic nitrate dominance (>62%), nitrification as the main process, and limited denitrification. Its nitrate concentration shows seasonal peaks. In contrast, the core area spring is recharged by deep fissure water, with natural nitrate sources (>80%), stable nitrate levels (5–7.4 mg/L), and potential local denitrification. Nitrogen export in the buffer zone increases 4.5 times in the rainy season (NO₃⁻ accounting for 93% of TN). The core area shows higher TN export flux per unit area (3.34 vs. 0.4 g/m²/a) and greater DON proportion. Nitrogen export far exceeds that from rocky desertified areas, suggesting that dissolved nitrogen leaching drives karst rocky desertification evolution. Full article

Hangzhou Bay has long experienced excessive nitrogen loading coupled with limited hydrodynamic exchange, leading to some of the highest nitrogen concentrations in China’s coastal waters. As critical land-sea ecotones, intertidal wetlands play a crucial role in mitigating nitrogen pollution across the bay. However, rapid urbanization and extensive reclamation since 1990 have led to a loss of over 50% of the intertidal wetlands in southern Hangzhou Bay. In this study we measured sediment denitrification and anammox potentials across key habitats: salt marshes (vegetated by Spartina alterniflora, Phragmites australis, and Scirpus mariqueter), bare mudflats, and shellfish aquaculture zones. We used ¹⁵N isotope tracing techniques coupled with slurry incubation experiments. Analysis of sediment physicochemical properties was conducted to elucidate the driving mechanisms of nitrogen removal. By integrating wetland landscape evolution with regional nitrogen budgets, we evaluated the nitrogen sink function of these intertidal wetlands. Our results revealed a distinct spatial hierarchy in denitrification potential, decreasing in the order: S. alterniflora (13.02 ± 3.54 μmol·N·kg⁻¹·h⁻¹) > shellfish aquaculture zones (12.86 ± 7.50 μmol·N·kg⁻¹·h⁻¹) > P. australis (11.54 ± 1.80 μmol·N·kg⁻¹·h⁻¹) > S. mariqueter (7.33 ± 2.08 μmol·N·kg⁻¹·h⁻¹) > bare mudflats (5.99 ± 1.62 μmol·N·kg⁻¹·h⁻¹). S. alterniflora has higher primary productivity, biomass accumulation, and a more robust root system structure. It regulates the content and availability of sediment organic carbon, the supply of nitrate nitrogen, pH, and water content. These regulations subsequently enhance denitrification. In contrast, shellfish aquaculture zones enhance denitrification potential primarily through bioturbation, which increases water content and lowers pH conditions. An integrated assessment of denitrification potential and landscape patterns revealed that, despite ongoing habitat loss, the remaining intertidal wetlands in southern Hangzhou Bay still remove about 30.65% of exogenous inorganic nitrogen. This finding underscores their critical role as effective pollution buffers under high nitrogen loading. Full article

The Yangtze River, the largest river system in Asia, continues to receive substantial nitrogen loads despite the implementation of management measures. Within this vast and complex system, the spatial patterns and drivers of key nitrogen transformation processes, such as nitrification and denitrification, remain poorly constrained. In particular, systematic isotopic evidence from studies spanning the entire upstream–midstream–downstream continuum remains scarce. This study integrates multiple isotopes (δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻, δ¹⁵N-NH₄⁺) with hydrochemical techniques to elucidate the dominant controls on nitrogen transport and transformation and their spatial heterogeneity across the Yangtze River Basin. Results indicate that dissolved inorganic nitrogen (DIN) is the dominant form of nitrogen pollution in the basin. NO₃⁻ concentrations exhibited significant spatial variability, following the pattern downstream (2.86 mg/L) > upstream (1.83 mg/L) > midstream (1.75 mg/L). Isotopic signatures revealed that nitrification is the dominant process controlling the formation and transformation of NO₃⁻ throughout the basin. Most δ¹⁸O-NO₃⁻ values (−5.20‰ to +12.78‰) fell within or close to the theoretical range for nitrification, and a strong positive correlation was observed between δ¹⁵N-NO₃⁻ and δ¹⁵N-NH₄⁺ (R² = 0.72, p < 0.01), collectively confirming that the conversion of NH₄⁺ to NO₃⁻ is the primary pathway. Conversely, denitrification was significantly suppressed under the prevailing high dissolved oxygen conditions (mean 9.78 ± 2.46 mg/L), as further evidenced by the lack of a significant correlation between δ¹⁵N-NO₃⁻ and ln(NO₃⁻). Furthermore, preferential assimilation of NH₄⁺ by phytoplankton reduced the efficiency of nitrate removal via biological assimilation and influenced isotopic composition. These findings provide a scientific basis for identifying priority nitrogen sources and optimizing targeted nitrogen management strategies in the Yangtze River Basin. Full article

Nitrate (NO₃⁻) serves as a critical nitrogen source and signaling molecule essential for its growth and quality formation. Although substantial genetic variation in nitrogen use efficiency (NUE) has been documented among tea cultivars, a systematic characterization of nitrate (NO₃⁻) absorption kinetics and the associated genome-wide transcriptional regulatory networks across varying nitrate concentrations remains lacking. This study employed ¹⁵N isotope labeling and transcriptome sequencing to systematically analyze the absorption characteristics and molecular response mechanisms of the cultivars ‘Longjing 43’ and ‘Zhongming 6 hao’ under varying NO₃⁻ concentrations. Results revealed significant differentiation in absorption strategies: ‘Zhongming 6 hao’ exhibited a significantly higher absorption rate at low concentrations, whereas ‘Longjing 43’ demonstrated enhanced performance at high concentrations. Transcriptome analysis indicated that both cultivars shared coordinated regulation of ‘photosynthesis’ and ‘nitrogen metabolism’ pathways. Furthermore, 14 nitrogen metabolism genes and 64 differentially expressed transcription factors (including MYB, NAC, and LBD families) were identified. Specifically, the CsNiR gene (encoding nitrite reductase) was functionally validated; silencing of CsNiR significantly reduced nitrite reductase activity, confirming its positive regulatory role. This study provided a theoretical framework and key candidate genes for breeding nitrogen-use-efficient varieties, which is essential for sustainable tea production. Full article

Despite a plethora of studies in recent years focusing on the impact of nitrogen source addition on plant responses, there remains a lack of clarity regarding the differential utilization and distribution patterns of various nitrogen sources by Larix olgensis A. Henry seedlings. Specifically, the mechanisms by which ammonium nitrogen, nitrate nitrogen, and urea are differentially absorbed and distributed among different organs within the plant, as well as how these processes couple with rhizosphere soil microbial processes, still await elucidation. This study, conducted under field experimental conditions, employed a combination of ¹⁵N isotopic tracing, soil physicochemical property measurements, enzyme activity analysis, and microbial community functional analysis to investigate the effects of three nitrogen sources (NH₄⁺, NO₃⁻, and urea) and their varying addition levels on nitrogen absorption and distribution in Larix olgensis A. Henry seedlings. The results indicate that nitrogen source type significantly influences the nitrogen absorption rate and internal distribution patterns of plants. Within 24 h, seedlings preferentially absorb ammonium nitrogen and retain a higher proportion of newly absorbed nitrogen in their roots. The high ammonium chloride (GN) treatment group exhibited the highest ¹⁵N abundance in the root region, suggesting rapid root assimilation and short-term underground retention. By 48 h, the ¹⁵N abundance and AT% values in most organs across different treatment groups were significantly higher than those at 24 h, facilitating the transport of nitrate nitrogen and urea to stems and leaves, indicating a gradual shift in nitrogen distribution towards the aboveground parts. Moderate nitrogen addition improved soil nutrient conditions, altered pH and conductivity, enhanced nitrogen transformation processes related to urease and nitrate reductase, and increased microbial diversity and metabolic functions related to carbon metabolism, nitrogen metabolism, and energy metabolism. Soil pH, total nitrogen (TN), ammonium nitrogen (NH₄⁺-N), and organic carbon (OC) are core environmental factors driving the differentiation of soil microbial community structure, with distinct specificity in the response of microbial groups across different taxonomic levels to soil physicochemical properties. Full article

Nitrate pollution in freshwater has become an increasing concern for both environmental sustainability and human health, especially in water reuse systems and intensive aquaculture. Photocatalytic reduction in nitrate to nitrogen gas (N₂) represents a promising low-chemical treatment strategy that can operate under sunlight or LED irradiation, and in general, enable nitrate removal without generating concentrated waste streams. Over the past decade, the development of advanced photocatalytic materials, including heterojunction semiconductors, plasmonic catalysts, and single-atom co-catalysts, has significantly enhanced visible-light absorption and overall photocatalytic performance. Despite these advances in photocatalyst design and synthesis, several critical challenges still limit the large-scale implementation of photocatalytic nitrate reduction to N₂. First, selectivity toward N₂ remains limited, as competing reaction pathways often lead to the formation of undesirable byproducts, such as nitrite (NO₂⁻), ammonium (NH₄⁺), and nitrous oxide (N₂O). Second, nitrogen reaction pathways are often uncertain, because many studies lack isotopic labeling or nitrogen mass balances, making it difficult to verify that the detected N₂ originates from nitrate reduction. Third, practical implementation is restricted by several technical challenges, including catalyst fouling or leaching, limitations in reactor design, excessive addition of hole scavengers, and the relatively high energy demand associated with indoor LED-driven systems. This review critically surveys advances from 2015 to 2025 in photocatalytic materials and reaction mechanisms for nitrate conversion to N₂. It highlights best practices for reliable product quantification and reaction pathway validation, and evaluates the feasibility of integrating these systems into recirculating aquaculture systems (RAS), where effective nitrate management is essential. In addition, the potential role of modern inline nitrate sensors (optical and electrochemical) and automated process control is discussed, outlining pathways toward hybrid photocatalytic–biological nitrate removal systems for sustainable aquaculture applications. Full article

Groundwater nitrate (NO₃⁻) pollution, caused by anthropogenic activities, poses a global threat to water security. The mixing of multiple nitrate pollution sources and the associated biogeochemical reactions may create a complex chemical background, which renders traditional hydrochemical methods and single δ¹⁵N isotope analysis approaches limited in accurately identifying pollution sources and quantifying their contribution ratios. Accordingly, we adopted an integrated framework incorporating hydrochemistry, isotopes, and the MixSIAR model. Within this framework, results from different components mutually validate each other, helping to achieve more accurate source identification and contribution quantification. Results revealed severe nitrate contamination with striking spatial heterogeneity: concentrations were significantly higher in the eastern region (9.3–1890.7 mg·L⁻¹, Mean: 472.8 mg·L⁻¹) than in the western region (8.5–204.1 mg·L⁻¹, Mean: 52.0 mg·L⁻¹). Hydrochemical and δ¹⁸O-NO₃⁻ evidence identified nitrification as the dominant nitrogen transformation process. Critically, the MixSIAR model quantified drastically different source contributions between the two regions. In the eastern industrial zone, industrial wastewater was the predominant source (61.3%), followed by manure and sewage (18.5%). In contrast, in the western agricultural area, natural and agricultural sources dominated, with soil nitrogen contributing 43.9% and chemical fertilizer 31.7%. The findings pinpoint specific pollution drivers for each region, offering a robust scientific basis for formulating differentiated and effective nitrate pollution control strategies. Full article

The North China Plain is one of the largest plains in China, where domestic water supply, agricultural irrigation, and industrial production rely on groundwater resources. Groundwater quality is increasingly affected by the combined effects of intense human activity and geological conditions. To ensure sustainable groundwater utilization, it is crucial to investigate the hydrogeochemical processes linked to hydrogeological conditions. In this study, 85 samples were collected from cold wells and 56 samples from geothermal wells in North China. By integrating self-organizing mapping (SOM), hydrochemical and isotopic analysis, nitrate distribution, water quality index (WQI), and human health risk assessment (HHRA) methodologies, we systematically evaluated the spatial variability of groundwater quality and the associated health risks in the region. Hydrochemical analysis indicates that groundwater recharge is primarily driven by atmospheric precipitation. Shallow cold groundwater in Cluster 1 exhibited a mixed phase, whereas geothermal water in Clusters 2 and 3 and cold groundwater in Cluster 4 predominantly displayed a Na-Cl type. Cation exchange processes are the primary factors controlling ion composition. Water quality assessment studies indicate that 75.15% of the groundwater is suitable for drinking. The average water quality index of the geothermal water was higher than that of the cold water. Shallow groundwater in plains is significantly affected by agricultural activities, typically manifested as elevated NO₃⁻ concentrations. Arsenic and boron are the primary non-carcinogenic risk pollutants in geothermal water, and children are more vulnerable than adults. The non-carcinogenic risk zones for cold wells were primarily distributed in Shijiazhuang, Baoding, and the coastal areas downstream of the Yellow River. Tianjin has high-risk geothermal water. Therefore, effective strategies must be implemented to protect this valuable water resource and achieve sustainable development in the region. Full article

Bacterial sourcing in urban watersheds is a critical water quality concern because elevated index bacteria concentrations routinely trigger beach advisories and closures in coastal Southern California and elsewhere. This study evaluates diurnal controls on dry-weather bacterial loading in a groundwater-fed storm drain within the Malibu Creek watershed using a 24 h monitoring campaign. Discharge, nutrients, major ions, stable water isotopes, and index bacteria (total coliforms and Escherichia coli) were measured at six time intervals. Storm drain discharge varied by more than an order of magnitude, with rapid nighttime increases of up to +91 L/min during irrigation periods. Total Dissolved Solids ranged from 1276 to 2175 mg/L, peaking during groundwater-dominated low-flow conditions. Nitrate–N ranged from 1.08 to 2.96 mg/L, and orthophosphate from 0.44 to 2.16 mg/L, with nutrient concentrations increasing as irrigation inputs increased. Total coliform concentrations ranged from 13,000 to 670,000 MPN/100 mL, and E. coli ranged from 300 to 120,000 MPN/100 mL, exceeding concentrations in tap water and recycled water runoff by up to two orders of magnitude. End member mixing analysis showed that storm drain flow consisted of approximately 45% groundwater, 23–26% tap water, and 30–33% recycled water during early morning peak flow, shifting to ~56% groundwater and <12% recycled water by mid-morning. The lowest bacterial concentrations occurred during groundwater-only flow, while the largest bacterial increases coincided with the greatest positive changes in discharge rather than with maximum absolute flow. These results support an irrigation-driven biofilm stripping mechanism as the dominant control on dry-weather bacterial loading, with groundwater seepage sustaining biofilm persistence but not peak bacterial release. The findings highlight the importance of internal storm drain processes for managing coastal bacterial exceedances and protecting beach health. Full article

Selenium contamination in arid agricultural basins remains a key ecological concern, yet the Wister Unit of the Imperial Wildlife Area has received comparatively little hydrochemical study. This investigation provides the most integrated assessment to date of selenium, salinity, nitrate, stable water isotopes (δ²H and δ¹⁸O), and selected redox-sensitive trace elements within the Wister Unit and its contributing open agricultural drains, with the goal of identifying controls on selenium concentrations and mobility. Water samples from open agricultural drains, shallow groundwater tile drains, canal project water, and tailwater return flow were analyzed for Total Dissolved Solids (TDS), major ions, nutrients, selenium, and stable water isotopes. A subset of samples was anlayzed for iron, manganese, and vanadium. Overall, 71% of open drain and tile drain samples collected in this study exceeded the U.S. Environmental Protection Agency aquatic-life criterion of 5 µg/L, indicating persistent ecological risk. All waters plotted along an evaporation trajectory originating from imported Colorado River irrigation water; however, isotopic enrichment did not scale directly with salinity. Pure evaporation models predicted much lower TDS values than observed, and the most evaporated samples were not the most saline or selenium-rich. These results demonstrate that simple soil water evaporation alone cannot explain the data. Instead, the broad isotopic range at similar salinities reflects a secondary process in which salts that accumulated in soils during dry or average years are later mobilized and flushed during periods of surplus water and heavy irrigation. Low dissolved iron, manganese, and vanadium concentrations in a subset of water samples indicate predominantly oxidizing conditions, under which selenium behaves conservatively during salt leaching, producing a strong correlation with TDS. Selenium levels measured in Wister Unit are generally lower than those reported in nearby areas during the 1990s–2000s, implying changes in salt accumulation, hydrologic routing, or agricultural practices. These results refine the conceptual model for the Wister Unit and motivate future work on selenium speciation, nitrate isotope tracing, time series monitoring, and soil-salt interactions. Full article

Nitrate in Baiyangdian Lake is directly linked to the sustainability of watershed ecological functions, acting as a key priority for regional ecological protection. Subsequent to the completion of a series of ecological restoration projects, its sources have undergone inevitable shifts, rendering the original pollution control framework incompatible with the new context. Thus, accurate identification of nitrate sources and their seasonal variation characteristics constitutes a core prerequisite for enhancing the targeting of pollution management. This study integrated dual stable isotopes (δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻) in water and potential source samples, along with hydrochemical data, and applied the Bayesian stable isotope mixing model (MixSIAR) to elucidate the sources of NO₃⁻ in Baiyangdian Lake. The results indicated that denitrification exerted a weak influence on the isotopic composition of NO₃⁻ in Baiyangdian Lake. Plots of the NO₃⁻/Cl⁻ versus Cl⁻ ratios for water samples and δ¹⁵N-NO₃⁻ versus δ¹⁸O-NO₃⁻ ratios for both water samples and potential sources confirmed anthropogenic sources as the primary nitrate contributors. The δ¹⁵N-NO₃⁻ vs. 1/[NO₃⁻] plot revealed that the number of NO₃⁻ sources exceeded two. The MixSIAR model demonstrated that wastewater treatment plant (WWTP) discharge was the dominant source throughout the four seasons, accounting for 49–62% with the highest contribution in winter and the lowest in summer. Soil nitrogen release contributed 19–32%, reaching its annual peak in summer. Sediment release accounted for 11–13%, maintaining a relatively low contribution across all seasons. Chemical fertilizer, manure, and sewage (M&S), and atmospheric deposition each contributed less than 6.5%, with negligible contributions. A significant reduction in the contributions of sediment release and M&S reflected the optimization effect of long-term regional ecological restoration efforts. WWTPs point source discharge and seasonal non-point source input from soil nitrogen collectively constituted the core sources of nitrate in Baiyangdian Lake. These findings provide crucial scientific support for the precise source apportionment and differentiated management of nitrate pollution in the basin. Full article

Thorium carbide (${\mathrm{ThC}}_{2\pm x}$) nano-structured thin disc-like pellets were produced from thoria nanoparticles (${\mathrm{ThO}}_2$-NP) and multi-walled carbon nanotubes (MWCNT). These composites are to be studied as a target material candidate for radioactive ion beam (RIB) production via nuclear reactions upon impact with high-energy proton beams on a stack of solid pellets. The ${\mathrm{ThO}}_2$-NP precursor was produced via precipitation of thorium oxalate from a thorium nitrate solution with oxalic acid and subsequent hydrothermal oxidation of the oxalate, creating the thoria nanoparticles. The ${\mathrm{ThO}}_2$-NP were then mixed with MWCNT in isopropyl alcohol and sonicated by two different methods to create a nanoparticle dispersion. This dispersion was then heated under medium vacuum to evaporate the solvent; the resulting powder was pressed into pellets and taken to an inert-atmosphere oven, where it was heated to 1650 °C and carbothermally reduced to ${\mathrm{ThC}}_{2\pm x}$. The resulting pellets were characterized via XRD, SEM-EDS, and Raman spectroscopy. The resulting thorium pellets exhibited, at most, trace levels of the oxide precursor. Furthermore, the nanotube structures were still present in the final product and are expected to contribute positively towards faster radioisotope release times by lowering isotope diffusion times, which is required for the efficient extraction of the shortest-lived (<1 s half-life) radioisotopes. Full article

Atmospheric nitrogen deposition is increasing worldwide, with profound implications for plant nitrogen acquisition and ecosystem nutrient cycling, particularly in nitrogen-limited systems. In this study, we investigated how inorganic nitrogen form regulates nitrogen uptake in H. altissima through pot experiments by applying ammonium nitrogen, nitrate nitrogen, mixed nitrogen, and a nitrogen-free control in Songnen grassland ecosystems at the eastern end of Eurasia. Soil abiotic properties, root morphological traits, and nitrogen uptake dynamics were jointly quantified using integrative modeling in combination with ¹⁵N stable isotope tracing. Relative to the no-nitrogen control, both ammonium and nitrate nitrogen significantly altered soil physicochemical conditions and stimulated root development, with ammonium consistently exhibiting stronger effects. Ammonium and nitrate applications reduced soil pH by 4.83% and 6.25%, increased electrical conductivity by 2.01% and 1.17%, and enhanced inorganic nitrogen pools by 115.84% and 45.69%, respectively. Root morphological traits were significantly enhanced under ammonium, nitrate, and mixed nitrogen treatments. ¹⁵N tracing further demonstrated that ammonium nitrogen significantly increased root ¹⁵N uptake compared with the no-nitrogen control (p < 0.05) and promoted a 20.10% greater allocation of absorbed nitrogen to aboveground biomass than nitrate nitrogen. Collectively, these findings highlight nitrogen form as a key regulator of soil–plant nitrogen coupling, with ammonium nitrogen more effectively enhancing nitrogen acquisition and internal translocation than nitrate. Full article

Rapid urbanization has intensified nitrate pollution in megacity rivers, posing severe challenges to urban water governance and sustainable nitrate management. This study presents nitrate dual-isotope signatures (δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻) from surface water samples collected during the wet season from the Yongding River (YDR) and Chaobai River (CBR) in the Beijing–Tianjin–Hebei megacity region of North China. Average concentrations of nitrate (as NO₃⁻) were 8.5 mg/L in YDR and 12.7 mg/L in CBR. The δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ values varied from 6.1‰ to 19.1‰ and −1.1‰ to 10.6‰, respectively. The spatial distribution of NO₃⁻/Cl⁻ ratios and isotopic data indicated mixed sources, primarily sewage and manure in downstream sections and agricultural inputs in upstream areas. Isotopic evidence revealed widespread nitrification processes and could have potentially localized denitrification under low-oxygen conditions in the lower YDR. Bayesian mixing model (MixSIAR) results indicated that sewage and manure constituted the main nitrate sources (49.4%), followed by soil nitrogen (23.7%), chemical fertilizers (19.2%), and atmospheric deposition from rainfall (7.7%). The self-organizing map (SOM) further revealed three nitrate regimes, including natural and agricultural, mixed, and sewage dominated conditions, indicating a clear downstream gradient of increasing anthropogenic influence. The results suggest that efficient nitrogen management in megacity rivers requires improving biological nutrient removal in wastewater treatment, regulating fertilizer application in upstream areas, and maintaining ecological base flow for natural denitrification. This integrated framework provides a quantitative basis for nitrate control and supports sustainable water governance in highly urbanized watersheds. Full article

Understanding how supplemental nitrogen (N) interacts with biological N₂ fixation (BNF) in modern soybean cultivars is essential for designing fertilization strategies that avoid unnecessary N inputs. We investigated N partitioning among soil, fertilizer and symbiotic sources in soybean grown in a greenhouse pot experiment on a tropical Oxisol. Plants were inoculated with Bradyrhizobium and subjected to four N managements: no external N, soil-applied ¹⁵N-urea (20 kg N ha⁻¹), foliar ¹⁵N-urea (2 kg N ha⁻¹, 0.7% w/v), and the combination of soil + foliar N. Using ¹⁵N isotope dilution, we quantified N derived from the atmosphere (NDFA), fertilizer (NDFF) and soil (NDFS) at organ and whole-plant scales, and related these fractions to nodulation, nitrogenase activity and yield. In the absence of external N, NDFA exceeded 97% in all organs, indicating a strong reliance on BNF and efficient internal N remobilization during grain filling, accompanied by higher leaf nitrate reductase activity. Soil and soil + foliar N markedly increased NDFF and NDFS while suppressing nodulation (particularly at V4) and reducing nitrogenase activity, yet they did not improve grain yield or vegetative biomass. Foliar N alone had only modest effects on N partitioning and did not enhance yield. Under these tropical soil conditions, symbiotic fixation and internal N remobilization were sufficient to meet grain N demand, highlighting the limited agronomic benefit and potential ecological cost of supplemental N during reproductive growth. Full article