Direct measurement of dissolved N₂ and denitrification along a subtropical river-estuary gradient, China

Divergent drivers of the spatial variation in greenhouse gas concentrations and fluxes along the Rhine River and the Mittelland Canal in Germany

Lotic ecosystems are sources of greenhouse gases (GHGs) to the atmosphere, but their emissions are uncertain due to longitudinal GHG heterogeneities associated with point source pollution from anthropogenic activities. In this study, we quantified summer concentrations and fluxes of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and dinitrogen (N2), as well as several water quality parameters along the Rhine River and the Mittelland Canal, two critical inland waterways in Germany. Our main objectives were to compare GHG concentrations and fluxes along the two ecosystems and to determine the main driving factors responsible for their longitudinal GHG heterogeneities. The results indicated that the two ecosystems were sources of GHG fluxes to the atmosphere, with the Mittelland Canal being a hotspot for CH4 and N2O fluxes. We also found significant longitudinal GHG flux discontinuities along the mainstems of both ecosystems, which were mainly driven by divergent drivers. Along the Mittelland Canal, peak CO2 and CH4 fluxes coincided with point pollution sources such as a joining river tributary or the presence of harbors, while harbors and in-situ biogeochemical processes such as methanogenesis and respiration mainly explained CH4 and CO2 hotspots along the Rhine River. In contrast to CO2 and CH4 fluxes, N2O longitudinal trends along the two lotic ecosystems were better predicted by in-situ parameters such as chlorophyll-a concentrations and N2 fluxes. Based on a positive relationship with N2 fluxes, we hypothesized that in-situ denitrification was driving N2O hotspots in the Canal, while a negative relationship with N2 in the Rhine River suggested that coupled biological N2 fixation and nitrification accounted for N2O hotspots. These findings stress the need to include N2 flux estimates in GHG studies, as it can potentially improve our understanding of whether nitrogen is fixed through N2 fixation or lost through denitrification.

Impacts of a subtropical storm on nitrogen functional microbes and associated cycling processes in a river-estuary continuum

Storms, in subtropical regions such as S.E. China, cause major changes in the physical and biogeochemical fluxes of anthropogenic N species through the river-estuary continuum to the coast. Two weeks continuous observations at a sampling station (Station E) in the upper Jiulong River Estuary (S.E. China) were conducted to track the changes of physical and biogeochemical parameters together with genomic identification of nitrogen cycling microbes through a complete storm event in June 2019. In conjunction with previous N flux measurements, it was found that there was greatly increased flux of N to and through the upper estuary during the storm. During the storm, the freshwater/brackish water boundary moved downstream, and previously deposited organic rich sediment was resuspended. During baseflow, anthropogenically derived ammonium was oxidised dominantly by the marine nitrifying (AOA) microbe Nitrosopelagicus. However, during the storm, the dominant ammonia-oxidizing archaea (AOA) at Station E changed to the riverine genus (Nitrosotenuis) while the marine genus, Nitrosopumilus decreased. At the same time the dominant ammonia-oxidizing bacteria (AOB) was still the marine genus (Nitrosomanas). Estuarine nitrifiers had higher abundance, weighted entropy and diversity during the Flood, suggesting that the high NH₄-N and DO during the Rising period of the Flood resulted in a bloom of nitrifiers. The changing gene abundances of nitrifiers were reflected in changes in the concentration and isotopic composition of DIN confirming active nitrification in the oxygen-rich water column. During the storm the numbers of denitrifiers (narG, nirS and nod), DNRA (nrfA) and anammox (hzsB) were found in the water column increased, and the larger fraction was associated with the <22 μm free-living fraction. However it was not possible with the data obtained to estimate what fraction of these anaerobic bacteria were active in the dominantly oxic water column.

Nitrogen and phosphorus spatio-temporal distribution and fluxes intensifying eutrophication in three tropical rivers of Côte d'Ivoire (West Africa)

Nutrient contamination assessments in the three West African tropical Comoé, Bandama, and Bia Rivers (Côte d'Ivoire) were performed from March 2016 to March 2018. Five stations per river were sampled. Nutrients spatio-temporal distributions were mapped and showed nitrogen concentrations (nitrite 0.001 to 0.025 mg/L NO₂^--N, and nitrate 0.26 to 3.60 mg/L NO₃^--N) increased significantly with rainfall contrary to phosphorus (0.01 to 0.12 mg/L P). The Chl-a and TSI_tsr data revealed the hypereutrophic status of rivers. Moreover, N:P mass ratio suggests nitrogen as the main limiting factor of primary production during the low (March) and high flow periods (October-November), while phosphorus is the limiting factor in June, at the high flow beginning. The land uses around watersheds were the main sources of phosphorus and nitrogen enhancing the rivers' eutrophication. Phosphorus and nitrogen fluxes were related to leaching river catchments and were significant sources of nutrients to the Atlantic Ocean.

Direct measurements of dissolved N2 and N2O highlight the strong nitrogen (N) removal potential of riverine wetlands in a headwater stream

Increasing levels of nitrogen (N) in aquatic ecosystems due to intensified human activities is focusing attention on N removal mechanisms as a means to mitigate environmental damage. Important N removal processes such as denitrification can resolve this issue by converting N to gaseous emissions. Here, the spatiotemporal variability of N removal rates in China's Zhongtian River, a headwater stream that contains wetlands, was investigated by quantifying gaseous emissions of the main end products, N₂ and N₂O, using the water-air exchange model. Excess concentrations of these gases relative to their saturations in the water column generally varied within 1.4-8.7 μmol L^-1 and 8.7-20.3 nmol L^-1, with mean values of 4.5 μmol L^-1 and 13.7 nmol L^-1, respectively, demonstrating significant N removal in the river. The reach with wetlands was characterized by higher in-stream N₂ production than the non-wetland reach, especially in July, when aquatic vegetation is most abundant. High N₂O emissions during the same period in the non-wetland reach indicate that environmental conditions associated with vegetation are conducive to N₂ production and likely constrain N₂O emission. Changes in dissolved oxygen, pH, temperature, and carbon to nitrogen ratios are correlated with the observed spatiotemporal variabilities in gaseous N production. The mean N removal rate in the wetland reach was roughly twice that in the non-wetland reach, i.e., 22.4 vs. 10.3 mmol N m^-2 d^-1, while the corresponding efficiency was about five times as high, i.e., 15 % vs. 3 %. This study reveals the spatiotemporal patterns of in-stream N removal in a headwater stream and highlights the efficacy of wetlands in N removal. The data provide a strong rationale for constructing artificial wetlands as a means to mitigate N pollution and thereby optimize riverine environmental conditions.

The Coupling Response between Different Bacterial Metabolic Functions in Water and Sediment Improve the Ability to Mitigate Climate Change

Extreme climatic events, such as heat wave and large temperature fluctuations, are predicted to increase in frequency and intensity during the next hundred years, which may rapidly alter the composition and function of lake bacterial communities. Here, we conducted a year-long experiment to explore the effect of warming on bacterial metabolic function of lake water and sediment. Predictions of the metabolic capabilities of these communities were performed with FAPROTAX using 16S rRNA sequencing data. The results indicated that the increase in temperature changed the structure of bacterial metabolic functional groups in water and sediment. During periods of low temperature, the carbon degradation pathway decreased, and the synthesis pathway increased, under the stimulation of warming, especially under the conditions temperature fluctuation. We also observed that nitrogen fixation ability was especially important in the warming treatments during the summer season. However, an elevated temperature significantly led to reduced nitrogen fixation abilities in winter. Compared with the water column, the most predominant functional groups of nitrogen cycle in sediment were nitrite oxidation and nitrification. Variable warming significantly promoted nitrite oxidation and nitrification function in winter, and constant warming was significantly inhibited in spring, with control in sediments. Co-occurrence network results showed that warming, especially variable warming, made microbial co-occurrence networks larger, more connected and less modular, and eventually functional groups in the water column and sediment cooperated to resist warming. We concluded that warming changed bacterial functional potentials important to the biogeochemical cycling in the experimental mesocosms in winter and spring with low temperature. The effect of different bacteria metabolism functions in water column and sediment may change the carbon and nitrogen fluxes in aquatic ecosystems. In conclusion, the coupling response between different bacterial metabolic functions in water and sediment may improve the ability to mitigate climate change.

Spatial and seasonal change in algal community structure and its interaction with nutrient dynamics in a gravel-bed urban river

Harmful algal blooms frequently occur in urban rivers due to intense human activities. However, little is known about the change in algal community structure and its interactions with nutrient dynamics in gravel-bed urban rivers. In present study, water samples were collected from a gravel-bed River Xin'an, China for five months over four seasons and a rainy month to measure algal community structure, dissolved nitrogen gas (N₂) and Argon (Ar) concentrations, and other water quality parameters. The results showed that the harmful Cyanophyta accounted for 31.6 ± 24.1% of the total community in the hot season while Bacillariophyta contributed more than 60% to the community in the other three seasons. The N₂ was supersaturated in the moderate and cold seasons but it was unsaturated in the hot season, along with high concentrations of nitrogen-fixing cyanobacteria (Anabaena), indicating that the nitrogen fixation capacity was strong and even stronger than denitrification and anammox in the hot season. However, nitrogen fixation was not the main source of nitrogen in the water column. The concentrations of nutrients and Chla in the downstream river were significantly higher than those in the upstream river (p < 0.001 for nutrients and p = 0.029 for Chla), suggesting that human activities along the river greatly affected nutrient concentrations, as well as algal growth. Our study provides new insights into the algal community succession in a gravel-bed urban river and puts forward effective measures such as controlling exogenous nutrient input and dredging organic sediment for mitigating the harmful algal blooms in urban rivers.

New insights into eutrophication management: Importance of temperature and water residence time

Eutrophication and harmful cyanobacterial blooms threaten water resources all over the world. There is a great controversy about controlling only phosphorus or controlling both nitrogen and phosphorus in the management of lake eutrophication. The primary argument against the dual nutrients control of eutrophication is that nitrogen fixation can compensate the nitrogen deficits. Thus, it is of great necessary to study the factors that can significantly affect the nitrogen fixation. Due to the difference of climate and human influence, the water quality of different lakes (such as water temperature, N:P ratio and water residence time) is also quite different. Numerous studies have reported that the low N:P ratio can intensify the nitrogen fixation capacities. However, the effects of temperature and water residence time on the nitrogen fixation remain unclear. Thus, 30 shallows freshwater lakes in the eastern plain of China were selected to measure dissolved N₂ and Ar concentrations through N₂: Ar method using a membrane inlet mass spectrometer to quantify the nitrogen fixation capacities and investigate whether the temperature and water residence time have a great impact on nitrogen fixation. The results have shown that the short lake water residence time can severely inhibit the nitrogen fixation capacities through inhibiting the growth of nitrogen-fixing cyanobacteria, changing the N:P ratio and resuspending the solids from sediments. Similarly, lakes with low water temperature also have a low nitrogen fixation capacity, suggesting that controlling nitrogen in such lakes is feasible if the growth of cyanobacteria is limited by nitrogen.

Low diffusive nitrogen loss of urban inland waters with high nitrogen loading

Little is known about the exchange of gaseous nitrogen (N2) with the atmosphere from urban inland waters, which are characterized by low carbon-to‑nitrogen ratios and low nitrogen-to‑phosphorus ratios. Here, we studied diffusive nitrogen loss based on the measurement of dissolved N2 concentrations and related gene abundance of N2 production and fixation in rivers and lakes in the megacity of Beijing, China, between 2018 and 2020. The excess dissolved N2 (△N2) ranged from -51.2 to 56.8 μmol L-1 (average - 0.03 ± 13.8 μmol L-1), and approximately 43% of the river samples and 72% of the lake samples being undersaturated with N2, suggesting that the lakes mainly acted as a role of N2 sink. The N2 removal fraction (△N2/DIN, average 3.5 ± 4.3%) at the sites of rivers with positive △N2 was lower than that in other rivers around the world. The average N2 flux (0.8 ± 23.9 mmol m-2 d-1) in the urban rivers was also lower than that in other rivers. The low carbon-to‑nitrogen ratios in Beijing inland waters are not beneficial for N2 production during denitrification, and low nitrogen-to‑phosphorus ratios potentially favor N2 fixation with a high abundance of the nitrogenase nifH gene in the sediment, resulting in low net N2 production. The traditional paradigm is that rivers constantly lose vast N to the atmosphere via denitrification and anammox, but this study indicates that urban inland rivers emit negligible N even under high nitrogen loading.

Nitrogen loss from a turbid river network based on N2 and N2O fluxes: Importance of suspended sediment

Riverine nitrogen loss makes a large contribution to the global nitrogen budget. However, little research has focused on nitrogen loss from large turbid rivers with high suspended sediment (SPS) concentrations. In this work, nitrogen loss amounts and related drivers were studied across fluvial networks of the Yellow River, the largest turbid river in the world, based on in situ measurement of nitrogen gas (N₂) and nitrous oxide (N₂O) fluxes at the water-air interface via the diffusion model and floating chamber methods, respectively. The results showed that N₂ and N₂O fluxes from the Yellow River ranged from -2.93 to 48.54 mmol m^-2 d^-1 and from 2.42 to 712.23 μmol m^-2 d^-1, respectively, with the nitrogen loss amount estimated to be 5.56 × 10⁷ kg N yr^-1 for the Yellow River, including the mainstem and main tributaries. Other than nitrogen compounds and water temperature, nitrogen loss from the Yellow River was also affected by SPS. Both N₂ flux: DIN and N₂O flux: DIN ratios increased remarkably in the middle reaches, probably due to a sharp increase of SPS concentration in this section. Furthermore, greater SPS concentrations were a main cause for the higher N₂O flux in the middle reaches than those in the other reaches of the Yellow River, and the possible effect of SPS was stronger on N₂O flux than on N₂ flux. This study demonstrates the importance of SPS in nitrogen loss from large turbid rivers, and more research is demanded to further clarify the role of SPS in riverine nitrogen cycle.

Genomic Characteristics of a Novel Species of Ammonia-Oxidizing Archaea from the Jiulong River Estuary

Ammonia-oxidizing archaea (AOA) are ubiquitous in diverse ecosystems and play a pivotal role in global nitrogen and carbon cycling. Although AOA diversity and distribution are widely studied, mainly based on the amoA (alpha subunit of ammonia monooxygenase) genotypes, only limited investigations have addressed the relationship between AOA genetic adaptation, metabolic features, and ecological niches, especially in estuaries. Here, we describe the AOA communities along the Jiulong River estuary in southern China. Nine high-quality AOA metagenome-assembled genomes (MAGs) were obtained by metagenomics. Five of the MAGs are proposed to constitute a new species, "Candidatus Nitrosopumilus aestuariumsis" sp. nov., based on the phylogenies of the 16S and 23S rRNA genes and concatenated ribosomal proteins, as well as the average amino acid identity. Comparative genomic analysis revealed unique features of the new species, including a high number of genes related to diverse carbohydrate-active enzymes, phosphatases, heavy-metal transport systems, flagellation, and chemotaxis. These genes may be crucial for AOA adaptation to the eutrophic and heavy-metal-contaminated Jiulong River estuary. The uncovered detailed genomic characteristics of the new estuarine AOA species highlight AOA contributions to ammonia oxidation in the Jiulong River estuary.IMPORTANCE In this study, AOA communities along a river in southern China were characterized, and metagenome-assembled genomes (MAGs) of a novel AOA clade were also obtained. Based on the characterization of AOA genomes, the study suggests adaptation of the novel AOAs to estuarine environments, providing new information on the ecology of estuarine AOA and the nitrogen cycle in contaminated estuarine environments.

Spatial-temporal characteristics of nitrogen degradation in typical Rivers of Taihu Lake Basin, China

In this study, we focus on the measurement of different nitrogen (N) forms and investigate the spatial-temporal variability of degradation coefficient in river channels. We aim to provide a new approach of deriving in-situ degradation coefficients of different N forms, and highlight factors that determine the spatial-temporal variability of degradation coefficients. Our results are based on a two-year field survey in 34 channels around the Taihu Lake Basin, eastern China. The derived degradation coefficients of different N forms based our newly-developed experimental device are: degradation coefficients of TN, NH₄⁺-N and NO₃-N range from 0.006-0.449 d^-1, 0.022-1.175 d^-1 and -0.096-2.402 d^-1, respectively. The degradation coefficients of N show strong dependence on N concentration and water temperature. The seasonal difference of water temperature and N concentration leads to spatial-temporal variability of degradation coefficients. The derived degradation coefficients of N are further verified through one-dimensional water quality model simulations. The degradation coefficient obtained in this study and the influencing factors of its spatial-temporal variability provide invaluable reference for studies in aquatic environment.

Suspended particles potentially enhance nitrous oxide (N2O) emissions in the oxic estuarine waters of eutrophic lakes: Field and experimental evidence

Estuaries are considered hot spots for the production and emissions of nitrous oxide (N₂O) and easily occur suspended particles (SPS), however, current understanding about the role of SPS in the N₂O emissions from the oxic estuarine waters of lacustrine ecosystems is still limited. In this study, field investigations were performed in the estuaries of hypereutrophic Taihu Lake, and laboratory simulations were simultaneously conducted to ascertain the characteristics of N₂O emissions with different SPS concentrations. The results showed that the N₂O emission fluxes ranged from 9.75 to 118.38 μg m^-2 h^-1, indicating a high spatial heterogeneity for the N₂O emissions from the estuaries of Taihu Lake. Although the dissolved oxygen (DO) concentrations were up to 7.85 mg L^-1 in the estuarine waters, from where the N₂O emissions fluxes were approximately three times that of the lake regions. Multiple regression model selected the total nitrogen (TN), SPS, and DO concentrations as the crucial factors influencing the N₂O emission fluxes. Particularly for SPS, the simulation results showed that the N₂O concentrations increased gradually with the increase in the SPS concentrations of an oxic water column containing 4 mg L^-1 of NO₃^--N, indicating that a high SPS concentration can accelerate the N₂O emissions. It was related to the change of denitrifying bacteria population in the SPS, as evidenced by its significantly positive correlation with N₂O emissions (p < 0.01). Our findings will draw attentions to the role of SPS playing in the N₂O productions and emissions in eutrophic lakes, and its effect on nitrogen cycle should be considered in the future study.

Riverine nutrient fluxes and environmental effects on China's estuaries

An increase in riverine nutrient fluxes significantly influences the estuarine ecosystem. This study collected nutrient data in most of China's rivers from 1963 to 2015 to estimate the nutrient fluxes from major rivers and analyze interannual variability of nutrient fluxes and estuarine environmental effects. The results showed that the nutrient fluxes from the Yangtze River increased annually from 1963 to 2012. The trend of nutrient fluxes from the Yellow River was consistent with that from the Jiulong River, i.e., nutrient fluxes increased from 1998 to 2007 and then decreased. The areal nutrient fluxes from China's major rivers were higher than those from major world rivers, while the areal nutrient yield rates per capita were lower than those from major world rivers. We also found that China's estuaries were predominantly phosphorus-limited and slowly moving toward lower dissolved silica over dissolved inorganic nitrogen (DSi:DIN) ratios with time. Meanwhile, the nutrient limitation of phytoplankton growth in most of China's estuary systems was moving toward a higher incidence of phosphorus and silicon limitations as a result of increased DIN fluxes, and this would likely alter phytoplankton communities. Furthermore, the decreases in the DSi:DIN ratio and dissolved silica over dissolved inorganic phosphate (DSi:DIP) ratio, and the increases in both DIN and DIP fluxes, caused increased red tide blooms.

Nitrogen removal rates in a frigid high-altitude river estimated by measuring dissolved N2 and N2O

Rivers are important sites of both nitrogen removal and emission of nitrous oxide (N₂O), a powerful greenhouse gas. Previous measurements have focused on nitrogen-rich temperate rivers, with cold, low-nitrogen river systems at high-altitude receiving less attention. Here, nitrogen removal rates were estimated by directly measuring dissolved N₂ and N₂O of the Yellow River in its source region of the Tibetan Plateau, a frigid high-altitude environment. We measured the dissolved N₂ and N₂O using N₂:Ar ratio method and headspace equilibrium technique, respectively. Dissolved N₂ in the river water ranged from 337 to 513 μmol N₂ L^-1, and dissolved N₂O ranged from 10.4 to 15.4 nmol N₂O L^-1. Excess dissolved N₂ (△N₂) ranged from -8.6 to 10.5 μmol N₂ L^-1, while excess dissolved N₂O (△N₂O) ranged from 2.1 to 8.3 nmol N₂O L^-1; they were relatively low compared with most other rivers in the world. However, N₂ removal fraction (△N₂/DIN, average 21.6%) and EF_5r values (N₂O - N/NO₃ - N, range 1.6 × 10^-4-5.0 × 10^-2) were comparable with many other rivers despite the high altitude for the Yellow River source region. Furthermore, the EF_5r values increased with altitude. Estimated fluxes of N₂ and N₂O to the atmosphere from the river surface ranged from -67.5 to 93.5 mmol N m^-2 d^-1 and from 4.8 to 93.8 μmol N m^-2 d^-1, respectively, and the nitrogen removal from rivers was estimated to be 1.87 × 10⁷ kg N yr^-1 for the Yellow River source region. This is the first report of nitrogen removal for a frigid high-altitude river; the results suggest that N removal and N₂O emission from cold high-altitude rivers should be considered in the global nitrogen budget.

Decadal changes in nutrient fluxes and environmental effects in the Jiulong River Estuary

Estuaries are areas of both freshwater and seawater that are partially enclosed with contact to the open sea and a flow of fresh water. Although the Jiulong River Estuary has a relatively small catchment, this area was found to exhibit high nutrient fluxes. The nutrient fluxes showed obvious fluctuations for different years. The Jiulong River Estuary was predominantly P-limited, and was slowly moving towards higher DIN:DIP and DSi:DIP ratios as the nitrate concentrations increased. The high nutrient fluxes into the estuary may affect estuarine ecosystems by the alteration of DO concentrations in bottom waters, causing harm to benthic fauna due to a lack of oxygen, triggering algal blooms. Additionally, the Jiulong River Estuary was slowly moving towards lower DSi:DIN and DSi:DIP ratios along with the change of time scales, which caused nutrient limitation of phytoplankton growth as P and Si levels decreased and became more limiting.

Abnormal pH elevation in the Chaobai River, a reclaimed water intake area

The pH is a primary index reflecting water quality in rivers. The Jian River and Chaobai River are two reclaimed water intake areas which have elevated pH. This elevated pH has a marked effect on both the phytoplankton, species in water and vegetation on the shore. Understanding the main reasons causing pH elevation in river water has important implications for river ecosystem management and the improvement of water quality and can provide a theoretical basis for the direction of water quality improvement. For this reason, each biogeochemical and physical process influencing pH changes in water was quantified along the flow direction in the Wenyu to Chaobai reclaimed water diversion project, in which proton consumption and production by such processes were monitored and calculated at five monitoring sections. The calculations indicated that photosynthesis and denitrification were the primary reasons for the increase of pH in the Jian River and Chaobai River. Oppositely, carbonate precipitation and sediment decomposition restricted the increase of pH in both rivers. In addition, CO₂ emission to the air also promoted a increase of pH in the Jian River, while CO₂ absorption from the air restricted the increase of pH in the Chaobai River. NO₃^- nitrogen in reclaimed water was not efficiently removed and the reclaimed water flow condition in the intake area created favorable conditions for photosynthesis of algae breeding and denitrification by microorganisms. Therefore, biogeochemical and physical processes that promoted the increase of pH were greater than inhibiting processes and the pH gradually increased along the flow direction. The contribution rates of photosynthesis and denitrification for the increase of pH were 55.48 and 27.09%, respectively, in the Jian River and 78.08 and 21.92%, respectively, in the Chaobai River. In addition, CO₂ emission contributed 17.43% of the increase in pH in the Jian River.