Unexpected low CO2 emission from highly disturbed urban inland waters.

Constituents and functionality of urban inland waters are significantly perturbed by municipal sewage inputs and tailwater discharge from wastewater treatment plants. However, large knowledge gaps persist in understanding greenhouse gas dynamics in urban inland waters due to a lack of in situ measurements. Herein, via a 3-year field campaign (2018-2020), we report river and lake CO2 emission and related aquatic factors regulating the emission in the municipality of Beijing. Mean pCO2 (546 ± 481 µatm) in the two urban lakes was lower than global non-tropical freshwater lakes and CO2 flux in 47% of the lake observations was negative. Though average pCO2 in urban rivers (3124 ± 3846 µatm) was among the higher range of global rivers (1300-4300 µatm), average CO2 flux was much lower than the global river average (99.7 ± 147.5 versus 358.4 mmol m-2 d-1). The high pCO2 cannot release to the atmosphere due to the low gas exchange rate in urban rivers (average k600 of 1.3 ± 1.3 m d-1), resulting in low CO2 flux in urban rivers. Additionally, eutrophication promotes photosynthetic uptake and aquatic organic substrate production, leading to no clear relationships observed between pCO2 and phytoplankton photosynthesis or dissolved organic carbon. In consistence with the findings, CO2 emission accounted for only 32% of the total greenhouse gas (GHG) emission equivalence (CO2, CH4 and N2O) in Beijing waters, in contrast to a major role of anthropogenic CO2 to anthropogenic GHG in the atmosphere in terms of radiative forcing (66%). These results pointed to unique GHG emission profiles and the need for a special account of urban inland waters in terms of aquatic GHG emissions.

Indirect nitrous oxide emission factors of fluvial networks can be predicted by dissolved organic carbon and nitrate from local to global scales.

Streams and rivers are important sources of nitrous oxide (N2 O), a powerful greenhouse gas. Estimating global riverine N2 O emissions is critical for the assessment of anthropogenic N2 O emission inventories. The indirect N2 O emission factor (EF5r ) model, one of the bottom-up approaches, adopts a fixed EF5r value to estimate riverine N2 O emissions based on IPCC methodology. However, the estimates have considerable uncertainty due to the large spatiotemporal variations in EF5r values. Factors regulating EF5r are poorly understood at the global scale. Here, we combine 4-year in situ observations across rivers of different land use types in China, with a global meta-analysis over six continents, to explore the spatiotemporal variations and controls on EF5r values. Our results show that the EF5r values in China and other regions with high N loads are lower than those for regions with lower N loads. Although the global mean EF5r value is comparable to the IPCC default value, the global EF5r values are highly skewed with large variations, indicating that adopting region-specific EF5r values rather than revising the fixed default value is more appropriate for the estimation of regional and global riverine N2 O emissions. The ratio of dissolved organic carbon to nitrate (DOC/NO3 - ) and NO3 - concentration are identified as the dominant predictors of region-specific EF5r values at both regional and global scales because stoichiometry and nutrients strictly regulate denitrification and N2 O production efficiency in rivers. A multiple linear regression model using DOC/NO3 - and NO3 - is proposed to predict region-specific EF5r values. The good fit of the model associated with easily obtained water quality variables allows its widespread application. This study fills a key knowledge gap in predicting region-specific EF5r values at the global scale and provides a pathway to estimate global riverine N2 O emissions more accurately based on IPCC methodology.

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-to­nitrogen ratios and low nitrogen-to­phosphorus 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-to­nitrogen ratios in Beijing inland waters are not beneficial for N2 production during denitrification, and low nitrogen-to­phosphorus 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.

Distinctive Patterns and Controls of Nitrous Oxide Concentrations and Fluxes from Urban Inland Waters.

Inland waters are significant sources of nitrous oxide (N2O), a powerful greenhouse gas. However, considerable uncertainty exists in the estimates of N2O efflux from global inland waters due to a lack of direct measurements in urban inland waters, which are generally characterized by high carbon and nitrogen concentrations and low carbon-to-nitrogen ratios. Herein, we present direct measurements of N2O concentrations and fluxes in lakes and rivers of Beijing, China, during 2018-2020. N2O concentrations and fluxes in the waters of Beijing exceeded previous estimates of global rivers due to the high carbon and nutrient concentrations and high aquatic productivity. In contrast, the N2O emission factor (N2O-N/DIN, median 0.0005) was lower than global medians and the N2O yield (ΔN2O/(ΔN2O + ΔN2), average 1.6%) was higher than those typically observed in rivers and streams. The positive relationship between N2O emissions and denitrifying bacteria as well as the Michaelis-Menten relationship between N2O emissions and NO3--N concentrations suggested that bacteria control the net production of N2O in waters of Beijing with N saturation, leading to a low N2O emission factor. However, low carbon-to-nitrogen ratios are beneficial for N2O accumulation during denitrification, resulting in high N2O yields. This study demonstrates the significant N2O emissions and their distinctive patterns and controls in urban inland waters and suggests that N2O emission estimates based on nitrogen loads and simple emission factor values are not appropriate for urban inland water systems.

Intense methane ebullition from urban inland waters and its significant contribution to greenhouse gas emissions.

The evasions of methane (CH4) and carbon dioxide (CO2) from inland waters represent substantial fluxes of greenhouse gases into the atmosphere, offsetting a large part of the continental carbon sink. However, the CH4 and CO2 emissions from urban inland waters are less constrained. In particular, ebullitive CH4 emissions from these waters are poorly understood. Here, we measured the concentrations and fluxes of CH4 and CO2 in rivers and lakes in the megacity of Beijing, China, between 2018 and 2019. The CH4 concentration ranged from 0.08 to 70.2 µmol L-1 with an average of 2.5 ± 5.9 µmol L-1. The average CH4 ebullition was 11.3 ± 30.4 mmol m-2 d-1 and was approximately 6 times higher than the global average. The average total CH4 flux (14.2 ± 35.1 mmol m-2 d-1) was 3 times higher than the global average, with ebullition accounting for 80% of the flux. The high surface water CH4 concentrations and ebullitive fluxes were caused by high sediment organic carbon/dissolved organic carbon contents, high aquatic primary productivity and shallow water depths in the urban inland waters. The CH4 emissions accounted for 20% of CO2 emissions in terms of the carbon release and were 1.7 times higher in terms of CO2 equivalent emissions from Beijing inland waters. Furthermore, the CH4 ebullition and its contribution to the total carbon gas emissions increased exponentially with the water temperature, suggesting a positive feedback probably occurs between the greenhouse gas emissions from urban inland waters and climate warming. This study confirms the major role of CH4 ebullition from urban inland waters in the global carbon budget under the rapid progress of global urbanization.

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.

Pore-scale model and dewatering performance of municipal sludge by ultrahigh pressurized electro-dewatering with constant voltage gradient.

The disposal of huge municipal sludge with high moisture content has led to numerous energy consumption and brought extensive concerns in the world. In this paper, three dewatering modes, ultrahigh-pressure mechanical dewatering mode (UMDW), pressurized electro-dewatering (PEDW) with constant voltage mode (U-PEDW) and constant voltage gradient mode (G-PEDW) were performed on a self-designed pressurized electro-dewatering apparatus for municipal sludge. The pore structures and moisture distributions were detected by low-field nuclear magnetic resonance technology. Meanwhile, the moisture distributions and quantitative bound strength were analyzed by the thermogravimetric differential scanning calorimetry test. Moreover, a pore-scale electro-osmosis model was accordingly developed based on the fractal characteristics of pore size distribution. Finally, the effect of dewatering modes and operating parameters on moisture content and energy consumption were examined in detail. The results indicate that the pore-scale electro-osmosis model show good consistency with experimental data. The electric field can drive the middle-layers-water to overcome the capillary pressure, and make G-PEDW removing more water than UMDW. The moisture content of dewatered municipal sludge by G-PEDW and U-PEDW reaches to 28.41% and 27.33%, respectively. Furthermore, the energy consumption of G-PEDW is 189.62Wh/kg.H2O, it is much lower than that of U-PEDW. Therefore, the G-PEDW mode with low moisture content and less energy consumption indicates best dewatering performance compared with UMDW and U-PEDW modes. The present work is helpful to understand the dewatering mechanisms of G-PEDW and provides useful guidelines for G-PEDW dewatering technology.

Both microbial abundance and community composition mattered for N2 production rates of the overlying water in one high-elevation river.

Overlying water is another potential hotspot of nitrogen removal through anammox and denitrification reactions in river systems. However, N2 production and the controlling factors have rarely been investigated in the overlying water of high-elevation rivers. This study analyzed the abundance and community of denitrifying and anammox bacteria as well as their effects on N2 production rates in the overlying water of the Yellow River source region (elevation range: 2687-4223 m). Higher suspended particle concentrations remarkably promoted functional gene abundances of both denitrifying and anammox bacteria (r > 0.9, p < 0.01). N2 production rates in overlying water samples ranged from 0.25 to 4.22 µmol N2 L-1 d-1. The overlying water was estimated to contribute to 36.8% (on average) of riverine N2 emission flux. Higher temperatures markedly accelerated N2 production rates (p = 0.051). Moreover, N2 production rates were positively related to both anammox and denitrifying bacterial abundances (p < 0.05), and such relationships were markedly affected by corresponding community compositions. The explanatory power of denitrifier abundance (R2 = 0.56) for N2 production rate variations was greatly elevated when it was integrated with community composition (R2 = 0.92). This study highlights the significance of overlying water nitrogen removal in the Yellow River source region; moreover, the effects of both microbial abundance and community composition on riverine N2 production rates should be considered in future research.

Ammonia oxidizers in river sediments of the Qinghai-Tibet Plateau and their adaptations to high-elevation conditions.

Ammonia-oxidizing bacteria (AOB) and archaea (AOA) as well as complete ammonia oxidizers (comammox) aerobically catalyze ammonia oxidation which plays essential roles in riverine nitrogen cycle. However, performances of these ammonia oxidizers in high-elevation river sediments have rarely been documented. This study investigated the abundance, community, and activity of ammonia oxidizers in five high-elevation rivers of the Qinghai-Tibet Plateau (QTP). Comammox were dominant ammonia oxidizers in 23% of studied samples and the clade B was principal comammox type. amoA gene abundances of AOA and AOB in these high-elevation rivers were comparable to those in low-elevation rivers. However, in contrast to most studied low-elevation rivers, AOB amoA gene abundance outnumbered AOA in 92% samples, which might be caused by the lower temperature and more intense solar radiation of the QTP. Potential nitrification rates (PNRs) ranged from 0.02 to 2.95 nmol-N h-1 g-1 dry sediment. Ammonia concentration was the limiting factor to PNRs at some sites, and when ammonia was not limiting, the PNR: ammonia ratio was greater at higher temperatures. There was no apparent variation in ammonia oxidizer community compositions along the elevation gradient due to the high elevation (2687 to 4223 m) of our entire study area. However, compared with low-elevation rivers, the lower temperature, huge diurnal temperature change, and lower nutrient conditions in the QTP rivers shaped distinctive communities for ammonia oxidizers; the unique community characteristics were significantly correlated to PNRs. These results suggest that ammonia oxidizers in the five high-elevation rivers have adapted to high-elevation conditions; more research should be conducted to study their adaptation mechanisms and their roles in riverine nitrogen cycle.

Ammonia Oxidizers in High-Elevation Rivers of the Qinghai-Tibet Plateau Display Distinctive Distribution Patterns.

Ammonia-oxidizing bacteria (AOB) and archaea (AOA) as well as comammox catalyze ammonia oxidation. The distribution and biogeography of these ammonia oxidizers might be distinctive in high-elevation rivers, which are generally characterized by low temperature and low ammonium concentration but strong solar radiation; however, these characteristics have rarely been documented. This study explored the abundance, community, and activity of ammonia oxidizers in the overlying water of five rivers in the Qinghai-Tibet Plateau (QTP). Potential nitrification rates in these rivers ranged from 5.4 to 38.4 nmol N liter-1 h-1, and they were significantly correlated with ammonium concentration rather than temperature. Comammox were found in 25 of the total 28 samples, and they outnumbered AOA in three samples. Contrary to most studied low-elevation rivers, average AOB amoA gene abundance was significantly higher than that of AOA, and AOB/AOA ratios increased with decreasing water temperature. The Simpson index of the AOA community increased with elevation (P < 0.05), and AOA and AOB communities exhibited high dissimilarities with low-elevation rivers. Cold-adapted (Nitrosospira amoA cluster 1, 33.6%) and oligotrophic (Nitrosomonas amoA cluster 6a, 31.7%) groups accounted for large proportions in the AOB community. Suspended sediment concentration exerted significant effects on ammonia oxidizer abundance (r > 0.56), and owing to their elevational variations in source and concentration, suspended sediments facilitated distance-decay patterns for AOA and AOB community similarities. This study demonstrates distinctive biogeography and distribution patterns for ammonia oxidizers in high-elevation rivers of the QTP. Extensive research should be conducted to explore the role of these microbes in the nitrogen cycle of this zone.IMPORTANCE Ammonia-oxidizing archaea (AOA) and bacteria (AOB) as well as comammox contribute to ammonia oxidation, which plays significant roles in riverine nitrogen cycle and N2O production. Source regions of numerous rivers in the world lie in high-elevation zones, but the abundance, community, and activity of ammonia oxidizers in rivers in high-elevation regions have rarely been investigated. This study revealed distinctive distribution patterns and community structures for ammonia oxidizers in five high-elevation rivers of the Qinghai-Tibet Plateau, and the individual and combined effects of low temperature, low nutrients, and strong solar radiation on ammonia oxidizers were elucidated. The findings of this study are helpful to broaden our knowledge on the biogeography and distribution pattern of ammonia oxidizers in river systems. Moreover, this study provides some implications to predict the performance of ammonia oxidizers in high-elevation rivers and its variations under global climate warming.

Occurrence of anammox on suspended sediment (SPS) in oxic river water: Effect of the SPS particle size.

Anammox is a newly discovered nitrogen transformation process. However, its role in nitrogen removal in fresh water is far from understood. Here, we hypothesized that anammox could occur on suspended sediment in oxic river water. To test this hypothesis, simulation experiments with a nitrogen stable (15N) isotopic tracer technique were conducted to study the occurrence of anammox on suspended sediment (SPS) in oxic river water, and the effects of the SPS particle size, including <20 µm, 20-63 µm, 63-100 µm, 100-200 µm, and <200 µm (original SPS) size fractions, were investigated. The results showed that anammox occurred in oxic water with SPS due to the existence of low oxygen microsites around/on SPS, and the anammox rate was even higher than the denitrification rate. The anammox rate increased with the SPS concentration, and it was negatively correlated with the particle size and was positively correlated with the organic carbon content of SPS (p < 0.05). The 29N2 produced by anammox in a system containing 1.0 g L-1 SPS with a particle size below 20 µm was 0.27 mg-N/m3·d, which was 5.3 times higher than that produced with a particle size of 100-200 µm. The anammox rate was significantly positively correlated with the anammox bacterial abundance (p < 0.01), and Ca. Brocadia was the dominant species. This study suggests that the SPS in oxic water may be a 'hotspot' for the anammox process and that its role in nitrogen removal should be considered in future studies.

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 EF5r 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 EF5r 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.

The cycle of nitrogen in river systems: sources, transformation, and flux.

Nitrogen is a requisite and highly demanded element for living organisms on Earth. However, increasing human activities have greatly altered the global nitrogen cycle, especially in rivers and streams, resulting in eutrophication, formation of hypoxic zones, and increased production of N2O, a powerful greenhouse gas. This review focuses on three aspects of the nitrogen cycle in streams and rivers. We firstly introduce the distributions and concentrations of nitrogen compounds in streams and rivers as well as the techniques for tracing the sources of nitrogen pollution. Secondly, the overall picture of nitrogen transformations in rivers and streams conducted by organisms is described, especially focusing on the roles of suspended particle-water surfaces in overlying water, sediment-water interfaces, and riparian zones in the nitrogen cycle of streams and rivers. The coupling of nitrogen and other element (C, S, and Fe) cycles in streams and rivers is also briefly covered. Finally, we analyze the nitrogen budget of river systems as well as nitrogen loss as N2O and N2 through the fluvial network and give a summary of the effects and consequences of human activities and climate change on the riverine nitrogen cycle. In addition, future directions for the research on the nitrogen cycle in river systems are outlined.