Nitrogen wet deposition in the Three Gorges Reservoir area: Characteristics, fluxes, and contributions to the aquatic environment

Atmospheric organic nitrogen deposition around the Danjiangkou Reservoir: Fluxes, characteristics and evidence of agricultural source

Dissolved organic nitrogen (DON) deposition was the substantial component of dissolved total nitrogen (DTN) deposition in the world's nitrogen deposition hot spots areas. However, the information on the importance for DON deposition and its sources was still scarce, which limited the comprehensive assessment of the ecological threat from nitrogen deposition. Six sampling sites around the Danjiangkou Reservoir were set up to collect the dry and wet deposition samples from October 2017 to September 2021. The results showed that dry and wet DTN deposition averaged 34.72 kg ha-1 yr-1 and 22.27 kg ha-1 yr-1, respectively. Dry NH4+-N, NO3--N and DON deposition averaged 14.28 kg ha-1 yr-1, 5.91 kg ha-1 yr-1 and 14.53 kg ha-1 yr-1, respectively. Wet NH4+-N, NO3--N and DON deposition averaged 11.14 kg ha-1 yr-1, 3.89 kg ha-1 yr-1and 7.24 kg ha-1 yr-1, respectively. The contributions of DON to DTN were 41.85% (in dry deposition) and 32.50% (in wet deposition), respectively. Dry DON deposition varied between 26.44 kg ha-1 yr-1 and 9.11 kg ha-1 yr-1, and significantly differed among six sampling sites (P < 0.05). The different intensity of agricultural activities disturbance at the sampling sites was the important reason for the spatial variations of DON deposition. DON deposition was significantly correlated with ammonium nitrogen (NH4+-N) deposition (P < 0.05). According to the results of positive matrix factorization (PMF) model, agriculture source contributed significantly to the DON deposition, the contributions at six sampling sites ranged from 45.8% to 73.7% in dry deposition, and from 56.8% to 81.6% in wet deposition. In summary, our findings found that agricultural activities were the important factors influencing the spatial patterns of DON deposition around Danjiangkou Reservoir and provided new evidence for the anthropogenic source of DON deposition in China.

Atmospheric Reactive Nitrogen Deposition from 2010 to 2021 in Lake Taihu and the Effects on Phytoplankton

The effects of nitrogen deposition reduction on nutrient loading in freshwaters have been widely studied, especially in remote regions. However, understanding of the ecological effects is still rather limited. Herein, we re-estimated nitrogen deposition, both of wet and dry deposition, in Lake Taihu with monthly monitoring data from 2010 to 2021. Our results showed that the atmospheric deposition of reactive nitrogen (namely NH₄⁺ and NO₃^-) in Lake Taihu was 4.94-11.49 kton/yr, which equaled 13.9%-27.3% of the riverine loading. Dry deposition of NH₄⁺ and NO₃^- contributed 53.1% of the bulk deposition in Lake Taihu. Ammonium was the main component of both wet and dry deposition, which may have been due to the strong agriculture-related activities around Lake Taihu. Nitrogen deposition explained 24.9% of the variation in phytoplankton community succession from 2010 to 2021 and was the highest among all the environmental factors. Atmospheric deposition offset the effects of external nitrogen reduction during the early years and delayed the emergence of nitrogen-fixing cyanobacterial dominance in Lake Taihu. Our results implied that a decrease in nitrogen deposition due to a reduction in fertilizer use, especially a decrease in NH₄⁺ deposition, could limit diatoms and promote non-nitrogen-fixing cyanobacterial dominance, followed by nitrogen-fixing taxa. This result was also applied to other shallow eutrophic lakes around the middle and lower reaches of the Yangtze River, where significant reduction of fertilizer use recorded during the last decades.

Morphological characteristics of amino acids in wet deposition of Danjiangkou Reservoir in China's South-to-North Water Diversion Project

Amino acids are an important constituent in organic nitrogen deposition, and changes in the content of their components have a direct impact on the nitrogen input to the ecosystem. From December 2018 to November 2019, 176 precipitation samples were collected at Danjiangkou Reservoir, the source of the middle line of the South-to-North Water Diversion Project, and the variation characteristics of dissolved free amino acids (DFAA) and dissolved combined amino acids (DCAA) were analyzed. The volume-weighted value concentration ranges of DFAA and DCAA were 0.159-1.136 μmol/L and 1.603-7.044 μmol/L, respectively, and amino acids were dominated by DCAA in wet deposition. Our results showed that glutamic acid (Glu), glycine (Gly), and aspartic acid (Asp) were the dominant amino acids in both DFAA and DCAA. The concentration of DFAA was highest in winter, while the concentration of DCAA was in autumn. Dissolved total amino acids (DTAA) were insignificantly correlated with DFAA, whereas they were linearly correlated with DCAA, indicating a significant influence of agricultural activities on DTAA. The analysis of the backward trajectory of air masses showed that amino acids were mainly influenced by proximity inputs around the reservoir. The bioavailability of organic matter was higher in the southeastern of the reservoir than in the northwestern. The wet deposition flux of TDN was 14.096 kg N/ha/year, and the potential ecological impact on water bodies cannot be ignored. This study was conducted to clarify the variation characteristics of amino acids fractions in wet deposition and to provide parameters for regional assessment of amino acids wet deposition. The ecological impact of nitrogen wet deposition on water bodies will be explored to provide a basis for nitrogen pollution control and water quality protection in the middle line of the South-to-North Water Diversion Project.

Fluxes, characteristics and influence on the aquatic environment of inorganic nitrogen deposition in the Danjiangkou reservoir

Atmospheric reactive nitrogen (Nr) deposition has caused serious damage to the terrestrial and freshwater ecosystems and also affected human health. Measuring temporal and spatial characteristics of Nr deposition is critical for proposing control strategy to decrease negative effects. We investigated the fluxes of ammonia nitrogen (NH₄-N) and nitrate nitrogen (NO₃-N) in both dry and wet deposition from October 2017 to September 2020 at six sites around the Danjiangkou reservoir. The results showed that the fluxes of dissolved inorganic nitrogen (DIN) decreased from 24.39 kg ha^-1 yr^-1 (2017-2018) to 16.11 kg ha^-1 yr^-1 (2019-2020) for dry deposition, and from 19.71 kg ha^-1 yr^-1 (2017-2018) to 12.29 kg ha^-1 yr^-1 (2019-2020) for wet deposition. Both NH₄-N and NO₃-N in wet deposition exhibited significant (P < 0.01) differences among four seasons, and were markedly influenced by the precipitation. The fluxes of NO₃-N deposition showed significant (P < 0.05) difference among six samples. Dry component contributed more to total DIN deposition, and NH₄-N was the dominant species in DIN deposition. The ratios of NH₄-N to NO₃-N in four seasons were higher than 2. A positive matrix factorization (PMF) model estimated that the factors of agriculture and fossil fuel combustion accounted for 77.1 % and 17.0 %, respectively, to the dry NH₄-N deposition; and that the factors of agricultural source and biomass burning accounted for 56.2 % and 21.1 %, respectively, to the wet NH₄-N deposition. The DIN deposition contributed to 7.7 % of the total Nr input into the reservoir, and the contribution of DIN deposition to the increase in the nitrogen concentration (ΔN) of the Danjiangkou reservoir was 0.13 mg L^-1 yr^-1. The dry DIN deposition was significantly correlated with the concentration of nitrogen in Danjiangkou reservoir (P < 0.01). This study suggested that the control measures of agricultural activity were essential to reduce Nr deposition, and to decrease the potential risks of water pollution in the reservoir. Furthermore, more long-term study is necessary to understand the relation between control measures, Nr deposition and water quality.

Characteristics and health risks of benzene series and halocarbons near a typical chemical industrial park

Health risks of typical benzene series and halocarbons (BSHs) in a densely populated area near a large-scale chemical industrial park were investigated. Ambient and indoor air and tap water samples were collected in summer and winter; and the concentration characteristics, sources, and exposure risks of typical BSH species, including five benzene series (benzene, toluene, ethylbenzene, o-xylene, m,p-xylene) and five halocarbons (dichloromethane, trichloromethane, trichloroethylene, tetrachloromethane, and tetrachloroethylene), were analysed. The total mean concentrations of BSHs were 53.32 μg m^-3, 36.29 μg m^-3, and 26.88 μg L^-1 in indoor air, ambient air, and tap water, respectively. Halocarbons dominated the total BSHs with concentrations relatively higher than those in many other industrial areas. Industrial solvent use, industrial processes, and vehicle exhaust emissions were the principal sources of BSHs in ambient air. The use of household products (e.g., detergents and pesticides) was the principal source of indoor BSHs. Inhalation is the primary human exposure route. Ingestion of drinking water was also an important exposure route but had less impact than inhalation. Lifetime non-cancer risks of individual and cumulative BSHs were below the threshold (HQ = 1), indicating no significant lifetime non-cancer risks in the study area. However, tetrachloromethane, benzene, trichloromethane, ethylbenzene, and trichloroethylene showed potential lifetime cancer risk. The cumulative lifetime cancer risks exceeded the tolerable benchmark (1 × 10^-4), indicating a lifetime cancer risk of BSHs to residents near the chemical industry park. This study provides valuable information for the management of public health in chemical industrial parks.

Revisiting seasonal dynamics of total nitrogen in reservoirs with a systematic framework for mining data from existing publications

Investigation of seasonal variations of water quality parameters is essential for understanding the mechanisms of structural changes in aquatic ecosystems and their pollution control. Despite the ongoing rise in scientific production on spatiotemporal distribution characteristics of water quality parameters, such as total nitrogen (TN) in reservoirs, attempts to use published data and incorporate them into a large-scale comparison and trends analyses are lacking. Here, we propose a framework of Data extraction, Data grouping and Statistical analysis (DDS) and illustrate application of this DDS framework with the example of TN in reservoirs. Among 1722 publications related to TN in reservoirs, 58 TN time-series data from 19 reservoirs met the analysis requirements and were extracted using the DDS framework. We performed statistical analysis on these time-series data using Dynamic Time Warping (DTW) combined with agglomerative hierarchical clustering as well as Generalized Additive Models for Location, Scale, and Shape (GAMLSS). Three patterns of seasonal TN dynamics were identified. In Pattern V-Sum, TN concentrations change in a "V" shape, dropping to its lowest value in summer; in Pattern P-Sum, TN increases in late summer/early fall before decreasing again; and in Pattern P-Spr, TN peaks in spring. Identified patterns were driven by phytoplankton growth and precipitation (Pattern V-Sum), nitrate wet deposition and agricultural runoff (Pattern P-Sum), and anthropogenic discharges (Pattern P-Spr). Application of the DDS framework has identified a key bottleneck in assessing the dynamics of TN - low data accessibility and availability. Providing an easily accessible data sharing platform and increasing the accessibility and availability of raw data for research will facilitate improvements and expand the applicability of the DDS framework. Identification of additional spatiotemporal patterns of water quality parameters can provide new insights for more comprehensive pollution control and management of aquatic ecosystems.

Migration and Transformation of Heavy Metals in the Soil of the Water-Level Fluctuation Zone in the Three Gorges Reservoir under Simulated Nitrogen Deposition

The accumulation of heavy metals (HMs) in the water-level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) area is potentially harmful to the water environment. In order to reveal whether nitrogen (N) deposition is a potential driving factor for the migration and transformation of HMs (Cd, Cr, Cu, Ni, and Pb), a simulated N deposition experiment was performed on the soil in the WFLZ of the TGR. The results showed that the accumulative release amounts of HMs increased with the increase of N deposition. It was found that the Elovich equation, double-constant equation, and parabolic diffusion equation could well describe the release process of Cu, Cd, Cr, and Ni, while the double-constant equation, parabolic diffusion equation, and first-order equation could be applicable for Pb. The exchangeable fractions of HMs increased to varying degrees after the N deposition treatment, wherein Ni was most significant, indicating that N deposition could increase the ecological risk of HM pollution in the TGR area. The results provide insight into the major factors affecting the release of different HMs under N deposition in this vulnerable region ecologically.

Wet Inorganic Nitrogen Deposition at the Daheitin Reservoir in North China: Temporal Variation, Sources, and Biomass Burning Influences

Atmospheric nitrogen deposition is of great concern to both air quality and the ecosystem, particularly in northern China, which covers one-quarter of China’s cultivated land and has many heavily air polluted cities. To understand the characteristics of wet N deposition at rural sites in northern China, one-year wet deposition samples were collected in the Daheitin reservoir region. Due to the intense emissions of gaseous nitrogen compounds from heating activities during cold seasons and distinct dilution effects under different rainfall intensities and frequencies, the volume weighted mean concentrations of wet N deposition showed higher levels in dry seasons but lower levels in wet seasons. In contrast, the wet N deposition rates varied consistently with precipitation, i.e., high during the wet season and lower during the dry season. The annual wet deposition rate of total inorganic ions (the sum of NO3−–N and NH4+–N) at the rural site in North China from July 2019 to June 2020 was observed at 18.9 kg N ha−1 yr−1, still remained at a relatively high level. In addition, biomass burning activities are ubiquitous in China, especially in northern China; however, studies on its impact on wet N deposition are limited. Non-sea salt potassium ion (nss-K+) was employed as a molecular tracer to investigate the characteristics of biomass burning activities as well as their impact on the chemical properties of wet N deposition. Three precipitation events with high nss-K+ levels were captured during the harvest season (June to July). The variations in the patterns of nss-K+, deposited N species, and ratios of nss-K+ to nitrogen species as well as their relationships all indicated that biomass burning emissions contributed remarkably to NO3−–N but had a minor influence on NH4+–N.