Distinctive Microbial Processes and Controlling Factors Related to Indirect N2O Emission from Agricultural and Urban Rivers in Taihu Watershed

Microbial mechanisms underlying the reduction of N2O emissions from submerged plant covered system

Submerged plant (SP) restoration is a crucial strategy for restoring aquatic ecosystem. However, the effect of SP on nitrous oxide (N2O) emissions remains controversial, and the impact of SP-attached biofilms on N2O emissions is often overlooked. In this study, SP and non-submerged plant (NSP) systems were set up and operated continuously for 189 days, revealing that SP reduced N2O flux by 42.4 %. By comparing the N2O net emission rates from water, sediment, and biofilms, we identified biofilms as the primary medium responsible for the reduction in N2O emissions in both SP and NSP systems. Further analysis of N2O metabolic rates from nitrification, denitrification, and abiotic processes under light and dark conditions confirmed that counter-diffusion of dissolved oxygen and nutrients in SP biofilms plays a key role in reducing denitrification-driven N2O emissions. Additionally, SP-attached biofilms increased nosZII-type denitrifiers (e.g., Bacillus) and reduced N2O production potential ((nirS+nirK)/(nosZI+nosZII)). Notably, the establishment of a SP restoration project in a typical eutrophic freshwater lake demonstrated that SP could reduce N2O fluxes by 61.5 %. This study provides significant insights for strategies aimed at mitigating N2O emissions.

Stocking filter-feeder in fed fish aquaculture pond: Unexpected Aggravation of nitrous oxide emission

Intensive farming of fed fish could produce large amounts of uneaten feed and feces, potentially leading to increased nitrous oxide (N2O) emissions. Filter-feeding fish can ingest residual feed and feces, but it is unclear whether introducing them into fed fish farming ponds could reduce N2O emissions. This study employed monoculture of fed largemouth bass (Micropterus salmoides, LB) and polyculture of LB with filter-feeding silver carp (Hypophthalmichthys molitrix, SC) at density ratios of 18:1, 9:1 and 4.5:1 to compare the N2O emission characteristics. The results showed that silver carp could indeed feed on largemouth bass feces, and isotope mixing model indicated that feces was the second largest contributor to the food of silver carp, reaching 14.75 %-15.56 %. However, polyculture of the two species did not or even increased N2O emission flux at water-air interface and its release potential in sediment. Increased mineralization, nitrification and denitrification rates were observed in polyculture systems, particularly at high stocking densities of silver carp. Also, the higher NH4+ accumulation were found across sediment-water interface within polyculture systems. Metagenome revealed that polyculture disturbed the microbial community structure and increased the abundance of Burkholderiales and Steroidobacteraceae. Moreover, polyculture increased the abundance of nitrogen-cycling functional genes, including gdhA, hao, nirB and norB, potentially contributing to the elevated N2O emissions. Structural equation model highlighted that polyculture of largemouth bass and silver carp could drive N2O emissions, mainly through increased sedimental NH4+ concentration and microbial activity. These findings indicate that the introduction of extractive filter-feeding fish into fed fish farming ponds could not reduce N2O emissions, implying the need for optimized management strategies to balance aquaculture productivity with environmental sustainability.

Microplastic diversity stimulates N2O emission during NO3--N transformation by altering microbial interaction and electron consumption in eutrophic water

Microplastic mixtures, consisting of various types, are widespread in aquatic ecosystems. However, the role of microplastic diversity in influencing N2O emission during NO3--N transformation remains unclear, particularly in eutrophic water bodies. To address this, we established 10 microcosms with microplastic diversity of 0, 1, 3, and 5 and explored the effects of microplastic diversity on NO3--N transformation, N2O emission, microbial communities, co-occurrence networks, and electron transfer. Results showed that microplastic diversity slightly impacted NO3--N transformation rates, but remarkably enhanced N2O emission. Although elevated microplastic diversity caused notable variations in microbial community, bacterial abundance had insignificant correlations with NO3--N transformation or N2O emission rates. Notably, the increased microplastic diversity made microbial networks more complex and stable, indirectly promoting N2O emission by altering electron transfer and consumption during NO3--N transformation. Especially, electron consumption had the most direct effect on N2O emission. Furthermore, the increasing microplastic diversity slightly affected NOR activity, while significantly decreasing NOS activity and raising (nirK+nirS)/nosZ ratio, which suggested that microplastic diversity primarily enhanced N2O emission by inhibiting its further reduction. Our findings provide deeper insight into the nitrogen transformation and greenhouse gas emission influenced by microplastic mixtures in eutrophic aquatic environments.

Impact of particle-attached microbial denitrification on N2O production in an agricultural-urban watershed

Anthropogenically influenced rivers are key hotspots for nitrous oxide (N2O) emissions. However, the seasonal and spatial heterogeneity of N2O emissions in subtropical riverine systems, particularly the role of particle-attached microbes (PAM) in regulating N2O production, remains poorly understood, contributing to uncertainties in global N2O estimates. This study investigates the potential impacts of PAM-driven nitrogen transformations on N2O production in the Dongjiang River under agricultural and urban influences. Water samples collected during the wet and dry seasons were analyzed for N2O concentrations, dual nitrogen-oxygen isotopes (δ15N-NO3-, δ18O-NO3-), and metagenomic sequencing of PAM. All samples exhibited N2O supersaturation, with emissions significantly higher in the dry season than in the wet season. A linearly positive δ15N-δ18O correlation, accompanied by lower NO3- in the bottom layers than the surface layers in the dry season indicates active denitrification, leading to elevated N2O concentrations. PAM-driven denitrification was identified as the dominant nitrogen transformation process, supported by higher abundances of denitrification genes (nirKS, norBC, nosZ) relative to nitrification genes (amoABC). Despite aerobic water column conditions, low-oxygen microhabitats around suspended particles facilitated N2O production. A significantly positive correlation (p < 0.05, R2 = 0.42) between N2O concentrations and the nirK/nosZ gene ratio suggests that gene expression imbalances contributed to net N2O accumulation. Additionally, the downstream urban area exhibited lower DO and higher DOC levels, enhancing denitrification and increasing N2O production by 4.7 % compared to the upstream agricultural region. Seasonal differences further influenced N2O dynamics: higher DOC/NO3- ratios in the dry season promoted heterotrophic denitrification, while elevated temperatures in the wet season favored complete denitrification, reducing N2O emissions. These findings provide critical insights into PAM-driven nitrogen cycling, informing strategies for mitigating N2O emissions and managing nitrogen pollution in subtropical riverine systems.

Are we underestimating the driving factors and potential risks of freshwater microplastics from in situ and in silico perspective?

The high loads of heterogeneous microplastics (MPs) in water system sparked the exploration of MPs source and impact in the environment. However, the contributions of driving factors to MPs contamination and the potential risks posed by multidimensional characteristics are still poorly understood. By incorporating in situ investigation with machine learning predictions, this study reported widespread MPs contamination in both textile upstream and receiving watershed in the Yangtze River Delta. The dominant MPs categories were fibers (0.1-0.5 mm in size), transparent in color, and composed of polyethylene terephthalate. These morphological characteristics indicated a conditional fragmentation process, suggesting that larger MPs are more prone to fragmentation. Multivariable analysis revealed significant correlations between MPs occurrence and factors of metal concentrations, geographic locations, and water qualities, highlighting the roles of textile production and automotive tire wear in determining MPs abundance. Among five machine learning models, Random Forest outperformed others in predicting MPs abundance. The interpretable analysis indicated that longitude (35.3 %), TN (13.8 %) and Sb (13.4 %) were pivotal nodes in shaping the MPs abundance. Emission point sources from express, autotire and textile yield feature importance from 6.60 % to 7.88 %. A total 12.39 % of the predicted variability can be further explained by interaction effects. Besides, MPERI and MultiMP indices based on abundance, size, color, shape, and polymer distributions suggested that most sampling sites fell within moderate to high-risk categories. Artificial neural network-based assessment results are suitable for explaining the MPs induced risks and polymer type was the most influential variable in determining the risk values. These quantitative insights into the driving factors and potential risks behind MPs occurrence improve our knowledge to manage MPs pollution in large-scale watersheds, providing crucial information for the development of effective mitigation strategies.

Human Activities Reshape Greenhouse Gas Emissions From Inland Waters

Inland waters are significant sources of greenhouse gases (GHGs) in an increasingly human-dominated world, yet the mechanisms by which human activities reshape GHG emissions from these systems remain poorly understood. Here, we synthesized research from three human-dominated landscapes-agricultural, urban, and impounded river systems-to demonstrate that inland waters within these systems exhibit significantly higher GHG emissions compared to their natural or seminatural counterparts. This is particularly evident for CH4 and N2O emissions, which show median enhancement ratios of 2.0-10 and 2.4-13 across the systems, respectively. In contrast, CO2 emissions exhibit overall lower enhancement (median enhancement ratios of < 2.0-3.1), largely due to simultaneously increased photosynthetic uptake from aquatic eutrophication. These observations underscore a clear human footprint on aquatic GHG emissions and the underlying biogeochemical processes. The observed changes in GHG emissions are driven by increased inputs of sediments, carbon, and nutrients from human-disturbed landscapes, coupled with the expansion of aquatic anoxia resulting from increased aquatic metabolism, fine sediment deposition, and eutrophication. Beyond altering emission rates, human activities also modify the abundance and distribution of inland waters, potentially exerting substantial, yet unquantified, effects on landscape-scale GHG emissions. We highlight the importance of understanding these processes for accurately quantifying and mitigating the human footprint on aquatic GHG emissions. Future research and mitigation efforts should account for the variability and mechanisms discussed in this review to effectively address human-induced GHG emissions from inland waters.

Manganese-modified reed biochar decreased nutrients and methane release from algae debris-contaminated sediments

Biochar is one of the ways for carbon storage, pollution control and biosolid reuse. Aquatic plant reeds are widely used in nutrient removal in wetlands and have huge biomass. Nonetheless, little is known regarding the effects of reed-based biochar on sediments. In this study, reed-based biochar (BC) modified with or without Mn-oxidizers (MBC) was prepared to investigate their impacts on nutrient removal, methane (CH4) emission fluxes, and CH4 concentration and microbial community in sediments for 20 days. We found that BC and MBC significantly reduced CH4 emission fluxes by 56.84 ± 10.47% and 69.95 ± 0.76% (p < 0.05) compared to control (CK), respectively. In addition, BC and MBC had a higher efficiency of nutrient removal, and the removal rate increased by 4.4% for NH4+-N and 10.13% for TN in BC and by 3.21%, 8.43% and 18.29% for NH4+-N, TN and TP in MBC, respectively. Proteobacteria, Chloroflexi, Bacteroidota, Firmicutes, Desulfobacterota and Acidobacteriota were the predominant phyla in sediments and might contribute to nutrient removal. Network analysis revealed that biochar addition promoted interspecific competition in sediments, which could be more beneficial for enhancing the stability of microbial community structures. The decreased mcrA (CH4 bioproduction) abundance but increased pomA (CH4 oxidation) abundance was detected in BC and MBC compared to CK, explaining biochar-reduced CH4 emissions. This study highlights that reed straw-based biochar can be used in the in-situ remediation of polluted sediments and provides a choice for carbon storage and pollution control for managers.

Freshwater Salinization Mitigated N2O Emissions in Submerged Plant-Covered Systems: Insights from Attached Biofilms

Submerged plants (SMPs) play a critical role in improving water quality and reducing N2O greenhouse gas emissions. However, freshwater salinization represents a major environmental challenge in aquatic systems. To investigate the impact of salinization on N2O emissions, this study conducted indoor mesocosm experiments simulating SMP and nonsubmerged plant (Non_SMP) areas in freshwater lakes. The objective was to explore the effects and microbial mechanisms of the attached biofilm on N2O emission in freshwater salinization. Salinization systems (700-1500 μS cm-1) reduced N2O flux by 37.0 and 40.5% compared to freshwater systems (<700 μS cm-1) of SMPs and Non_SMPs, respectively. Kinetic experiments showed that the reduction in N2O emissions was mainly attributed to the attached biofilm rather than the sediment or water. The N2O net emission rates of the attached biofilm decreased by 47.1 and 71.8% in salinization systems of SMPs and Non_SMPs, respectively, compared with freshwater systems. Additionally, biofilms in salinization systems exhibited lower denitrification rates. Furthermore, salinization reduced the N2O production potential ((nirS + nirK)/(nosZI + nosZII)), thereby further decreasing N2O emissions. This study provides valuable insights into the role and mechanisms of biofilms in mitigating N2O emissions in salinized freshwater lakes.

Sustainable management of riverine N2O emission baselines

The riverine N2O fluxes are assumed to linearly increase with nitrate loading. However, this linear relationship with a uniform EF5r is poorly constrained, which impedes the N2O estimation and mitigation. Our meta-analysis discovered a universal N2O emission baseline (EF5r = k/[NO3 -], k = 0.02) for natural rivers. Anthropogenic impacts caused an overall increase in baselines and the emergence of hotspots, which constitute two typical patterns of anthropogenic sources. The k values of agricultural and urban rivers increased to 0.09 and 0.05, respectively, with 11% and 14% of points becoming N2O hotspots. Priority control of organic and NH4 + pollution could eliminate hotspots and reduce emissions by 51.6% and 63.7%, respectively. Further restoration of baseline emissions on nitrate removal is a long-term challenge considering population growth and declining unit benefits (ΔN-N2O/N-NO3 -). The discovery of EF lines emphasized the importance of targeting hotspots and managing baseline emissions sustainably to balance social and environmental benefits.

Anthropogenic pressure induced discontinuities of microbial communities along the river

Microorganisms play a fundamental role in driving biogeochemical functions within rivers. Theoretically, the directional flowing nature of river contributes to the continuous downstream change pattern of microbial communities. This continuity is anticipated to be influenced by human activities as anthropogenic materials lead to the mixing of environmental substances and their resident microorganisms with local communities. Here, we conducted a field investigation along the Beiyun River, which successively flows through pristine forest areas, artificial urban and agricultural areas with a length of 184 km, to explore the influence of anthropogenic events on microbial similarity, diversity, composition, co-occurrence, and assembly mechanisms in sediments along the river. Piecewise linear regression tests showed that discontinuities of microbial similarity occurred following the transitions from low to high anthropogenic pressure. LEfSe analysis illustrated that microorganisms associated with wastewater treatment plants and gut were differentially abundant in urban and agricultural streams. By quantifying contributions of ecological assembly processes, we found that the dominant role shifted from variable selection (60.78% in forest group) to homogenous selection (79.52% in urban group and 57.14% in agriculture group) as the differences in NH4+-N, NO3--N and NO2--N content decreased. Moreover, the complexity and stability of microbial networks were reduced from upstream forest streams to downstream urban and agricultural streams, indicating more fragmented networks. Our study provides enhanced knowledge about the factors controlling the microbial community assembly in rivers under increasing human pressure through the integration of physical, environmental, and ecological mechanisms, which can serve as a basis for predicting and responding to changes in ecosystem function under the intensified human pressure.

Terrestrial dissolved organic matter inputs affect the nitrous oxide emission revealed by FT-ICR MS

Nitrous oxide (N2O) emission from lake systems could be affected via intrusion of terrestrial organic matter, causing impairment in biogeochemical cycling. The sources and mechanisms by which DOM (Dissolved organic matter) alters emissions of N2O are poorly understood. Here, we simulate different terrestrial DOM (anthropogenic sources, natural sources, and surface runoff) to assess the mechanisms affecting N2O emissions with variations of DOM. We used a combination of absorption spectroscopy, excitation-emission matrix fluorescence, and Fourier transform ion cyclotron resonance mass spectrometry to characterize DOM comprehensively. For the characterization of DOM, a combination of absorption spectroscopy, excitation-emission matrix fluorescence, and Fourier transform ion cyclotron resonance mass spectrometry was used. Microbial analysis was conducted to identify the potential microbial mechanisms. Different terrestrial DOM inputs primarily impact N2O emissions through the denitrification process (14.52 %, p < 0.05), with significant effects on the abundance of narG (12.97 %, p < 0.05) and nirK+S (10.13 %, p < 0.05). The biodegradable components in sediments directly promote N2O emissions, while in aquatic systems, the labile components (proteins, sugars, and lipids-like) were preferentially metabolized, producing reluctant derivatives. The biodegradable components (i.e., protein-like) from anthropogenic sources rapidly facilitate N2O production. Natural and surface runoff sources were the significant drivers for the continuous release and metabolism of DOM. N2O Loss emissions are negatively influenced by the regulation of carbon and nitrogen metabolism by nitrifiers and denitrifies in the sediment (p < 0.001). Metabolism of carbon and nitrogen regulated by nitrifier and denitrifies in the sediments negatively influences N2O flux (p < 0.001). N2O emissions were mainly influenced by bioavailability of inputs: DOM and varying terrestrial conditions. The results provide a theoretical base for the management of greenhouse gas emissions from lakes.

Pollution characterization and multi-index ecological risk assessment of microplastics in urban rivers from a Chinese megacity

Currently, a comprehensive understanding of the pollution risks of microplastics (MPs) in urban river ecosystems is still lacking. This study investigated the spatial distribution and morphological characteristics of MPs in surface waters of major rivers in Shenzhen, a megacity in China, using laser direct infrared (LDIR) imaging. A promisingly comprehensive risk assessment method, MultiMP, was first proposed in this study, taking into account the multidimensional characteristics of MPs including abundance, size, shapes, and polymer types. The results showed that MPs were widespread and highly heterogeneous, and the abundance of MPs ranged from 38 to 18380 particles/L, with an average of 2305 particles/L. Morphologically, polyamide (PA) (average 53.7 %), 30-50 µm (73.8 %), and pellet (65.7 %) were the predominant MPs types. Driving factors analyses revealed geographical distance, salinity, water temperature, and total nitrogen had relatively higher impacts on the abundance and morphology of MPs. The MultiMP results indicated that most of the river sampling sites and five major basins in Shenzhen were at moderate to high-risk levels. Polymer type and abundance had a relatively high impact on the environmental risk of MPs in the region. These findings contribute to improving the insights and management of the MPs pollution risks in megacity water bodies.

Variability in N2O emission controls among different ponds within a hilly watershed

While it is well established that small water bodies like ponds play a disproportionately large role in contributing to N2O emissions, few studies have focused on lowland ponds in hilly watersheds. Here, we explored the characteristics of N2O concentrations and emissions from various typical ponds (village, tea, forested, and aquaculture ponds) in a hilly watershed and examined the specific controls influencing N2O production. Our findings revealed that tea ponds exhibited the highest N2O flux (8.42 ± 8.23 μmol m-2 d-1), which was 2.8 to 3.3 times greater than other types of ponds. Remarkable seasonal variations were observed in tea and forested ponds due to the seasonality of nutrient-enriched runoff, whereas such variations were less pronounced in village and aquaculture ponds. Key factors such as nitrogen levels, temperature, and dissolved oxygen (DO) emerged as the primary controls of N2O concentrations in ponds, heavily influenced by land use and human activities in their drainage areas. Specifically, N2O production in tea and aquaculture ponds was driven by N inputs from fertilization and feed, respectively, while DO levels governed the process in village and forested ponds, influenced by abundant algae and forest vegetation. This study emphasizes that environmental factors predominantly drive N2O production in ponds within hilly watersheds, but land use in the pond drainages acts as an indirect yet crucial influence. This highlights the need for future research to develop targeted emission reduction strategies based on land use to effectively mitigate N2O emissions, promising a path toward more sustainable and climate-friendly watershed management.

Water periods impact the structure and metabolic potential of the nitrogen-cycling microbial communities in rivers of arid and semi-arid regions

This study examined the influence of water periods on river nitrogen cycling by analysing nitrogen functional genes and bacterial communities in the Qingshui River, an upstream tributary of the Yellow River in China. Nitrate nitrogen predominated as inorganic nitrogen during the low-flow seasons, whereas salinity was highest during the high-flow seasons. Overall, the functional gene abundance increased with decreasing water volume, and nitrogen concentrations were determined by various specific gene groups. The relative abundance of bacteria carrying these genes varied significantly across water periods. The abundance of Pseudomona, Hydrogenophaga (carrying narGHI and nirB genes), and Flavobacterium (carrying nirK, norBC, and nosZ genes) significantly increased during the low-flow seasons. Nitrogen transformation bacteria exhibited both symbiotic and mutualistic relationships. Microbial network nodes and sizes decreased with decreasing water volume, whereas modularity increased. Additionally, the water period affected the functional microbial community structure by influencing specific environmental factors. Among them, SO42- primarily determined the denitrification, dissimilatory nitrate reduction to ammonium, and assimilatory nitrate reduction to ammonium communities, whereas NO2--N and Mg2+ were the main driving factors for the nitrogen-fixing and nitrifying communities, respectively. These findings have substantial implications for better understanding the reduction in river nitrogen loads in arid and semi-arid regions during different water periods.

Microbial nitrogen cycling in Microcystis colonies and its contribution to nitrogen removal in eutrophic Lake Taihu, China

Cyanobacterial blooms induced by excessive loadings of nitrogen (N) and other nutrients are a severe ecological problem in aquatic ecosystems. Previous studies of N removal have primarily focused on sediment-water interface, yet the role of cyanobacterial colonies has recently been attracting more research attention. In this study, N cycling processes were quantified for cyanobacterial colonies (primarily Microcystis colonies) and their contribution to N removal was estimated for a large, shallow eutrophic lake in China, Lake Taihu. Various N cycling processes were determined via stable 15N isotope, together with 16S rRNA gene sequencing and quantitative microbial element cycling (QMEC) chip. Denitrification was found to be the most prominent process, estimated to be 36.63, 9.85, 3.35, and 3.15 times higher than dissimilatory nitrate reduction to ammonium (DNRA), nitrification, ammonium (NH4+) uptake and nitrate (NO3-) uptake rates, respectively. Denitrifiers accounted for a large part of the bacterial taxa (35.50 ± 24.65%), and the nirS gene was the most abundant among N cycling-related genes, with (2.54 ± 0.51) × 109 copies g-1Microcystis colonies. A field investigation revealed a positive correlation between the potential denitrification rate and the Chl-a concentration (mostly derived from Microcystis colonies). Based on a multiple stepwise regression model and historical data from 2007 to 2015 for Lake Taihu, the total amount of N removed via denitrification by Microcystis colonies was estimated at 171.72 ± 49.74 t yr-1; this suggests that Microcystis colonies have played an important role in N removal in Lake Taihu since the drinking water crisis in 2007. Overall, this study revealed the importance of denitrification within Microcystis colonies for N removal in eutrophic lakes, like Lake Taihu.

Significant diurnal variations in nitrous oxide (N2O) emissions from two contrasting habitats in a large eutrophic lake (Lake Taihu, China)

Algae and macrophytes in lake ecosystems regulate nitrous oxide (N2O) emissions from eutrophic lakes. However, knowledge of diurnal N2O emission patterns from different habitats remains limited. To understand the diurnal patterns and driving mechanisms of N2O emissions from contrasting habitats, continuous in situ observations (72 h) of N2O fluxes from an algae-dominated zone (ADZ) and reed-dominated zone (RDZ) in Lake Taihu were conducted using the Floating Chamber method. The results showed average N2O emission fluxes of 0.15 ± 0.06 and 0.02 ± 0.04 μmol m-2 h-1 in the ADZ and RDZ in autumn, respectively. The significantly higher (p < 0.05) N2O fluxes in the ADZ were mainly attributed to differences in nitrogen (N) levels. The results also showed significant diurnal differences (p < 0.05) in the N2O emission fluxes within the ADZ and RDZ, and daytime fluxes were significantly higher (p < 0.05) than nighttime fluxes. The statistical results indicated that N2O emissions from the ADZ were mainly driven by diurnal variations in N loading and the dissolved oxygen (DO) concentration, and those from the RDZ were more influenced by DO, redox potential, and pH. Finally, we determined the proper time for routine monitoring of N2O flux in the two habitats. Our results highlight the importance of considering diverse habitats and diurnal variations when estimating N2O budgets at a whole-lake scale.

Wastewater treatment plant effluents increase the global warming potential in a subtropical urbanized river

Rivers play a pivotal role in global carbon (C) and nitrogen (N) biogeochemical cycles. Urbanized rivers are significant hotspots of greenhouse gases (GHGs, N2O, CO2 and CH4) emissions. This study examined the GHGs distributions in the Guanxun River, an effluents-receiving subtropical urbanized river, as well as the key environmental factors and processes affecting the pattern and emission characteristics of GHGs. Dissolved N2O, CO2, and CH4 concentrations reached 228.0 nmol L-1, 0.44 mmol L-1, and 5.2 μmol L-1 during the wet period, and 929.8 nmol L-1, 0.7 mmol L-1, and 4.6 μmol L-1 during the dry period, respectively. Effluents inputs increased C and N loadings, reduced C/N ratios, and promoted further methanogenesis and N2O production dominated by incomplete denitrification after the outfall. Increased urbanization in the far downstream, high hydraulic residence time, low DO and high organic C environment promoted methanogenesis. The strong CH4 oxidation and methanogenic reactions inhibited by the effluents combined to suppress CH4 emissions in downstream near the outfall, and the process also contributed to CO2 production. The carbon fixation downstream from the outfall were inhibited by effluents. Ultimately, it promoted CO2 emissions downstream from the outfall. The continuous C, N, and chlorine inputs maintained the high saturation and production potential of GHGs, and altered microbial community structure and functional genes abundance. Ultimately, the global warming potential downstream increased by 186 % and 84 % during wet and dry periods on the 20-year scale, and increased by 91 % and 49 % during wet and dry periods on the 100-year scale, respectively, compared with upstream from the outfall. In urbanized rivers with sufficient C and N source supply from WWTP effluents, the large effluent equivalently transformed the natural water within the channel into a subsequent "reactor". Furthermore, the IPCC recommended EF5r values appear to underestimate the N2O emission potential of urbanized rivers with high pollution loading that receiving WWTP effluents. The findings of this study might aid the development of effective strategies for mitigating global climate change.

IPCC Emission Factor Overestimates N2O Emissions from Agricultural Ditches

Agricultural ditches emit disproportionate amounts of nitrous oxide (N2O), but their contributions to regional or global N2O emissions remain unclear due to limited data. The Intergovernmental Panel on Climate Change (IPCC) recommends using emission factors (EFs) to estimate indirect N2O emission, but the EF for ditches (EF5g) is categorized as groundwater, which potentially introduces a significant bias. This study conducted a regional-scale campaign in the North China Plain, one of the world's most intensive agricultural regions, and calculated the EF5g values from agricultural ditches by the concentration method (N2O-N/NO3--N). The results found that the regional-scale mean EF5g value (0.0028) was less than half of the IPCC default value (0.006), illustrating that the current IPCC methodology significantly overestimates N2O emissions from agricultural ditches. Despite the relatively small EF5g values, agricultural ditches exhibited a high mean N2O concentration (3.36 μg L-1) and a large regional emission (1.14 ± 0.86 Gg N2O-N yr-1), which is equal to 3.8 ± 2.9% of direct N2O emission from the croplands in the North China Plain. Since ditches are ubiquitous in agricultural regions and are likely to expand under climate change, refining EF5g is crucial to accurately assess their contribution to global N2O budgets.

Evaluating the role of high N2O affinity complete denitrifiers and non-denitrifying N2O reducing bacteria in reducing N2O emissions in river

Freshwater rivers are hotspots of N2O greenhouse gas emissions. Dissolved organic carbon (DOC) is the dominant electron donor for microbial N2O reduction, which can reduce N2O emission through enriching high N2O affinity denitrifiers or enriching non-denitrifying N2O-reducing bacteria (N2ORB), but the primary regulatory pathway remains unclear. Here, field study indicated that high DOC concentration in rivers enhanced denitrification rate but reduced N2O flux by improving nosZ gene abundance. Then, four N2O-fed membrane aeration biofilm reactors inoculated with river sediments from river channel, estuary, adjacent lake, and a mixture were continuously performed for 360 days, including low, high, and mixed DOC stages. During enrichment stages, the (nirS+nirK)/nosZ ratio showed no significant difference, but the community structure of denitrifiers and N2ORB changed significantly (p < 0.05). In addition, N2ORB strains isolated from different enrichment stages positioned in different branches of the phylogenetic tree. N2ORB strains isolated during high DOC stage showed significant higher maximum N2O-reducing capability (Vmax: 0.6 ± 0.4 ×10-4 pmol h-1 cell-1) and N2O affinity (a0: 7.8 ± 7.7 ×10-12 L cell-1 h-1) than strains isolated during low (Vmax: 0.1 ± 0.1 ×10-4 pmol h-1 cell-1, a0: 0.7 ± 0.4 ×10-12 L cell-1 h-1) and mixed DOC stages (Vmax: 0.1 ± 0.1 ×10-4 pmol h-1 cell-1, a0: 0.9 ± 0.9 ×10-12 L cell-1 h-1) (p < 0.05). Hence, under high DOC concentration conditions, the primary factor in reducing N2O emissions in rivers is the enrichment of complete denitrifiers with high N2O affinity, rather than non-denitrifying N2ORB.

Unveiling riverine N2O dynamics along urbanization gradients by integrating hydrological, biogeochemical and microbial processes

Human-disturbed rivers are globally significant sources of atmospheric nitrous oxide (N2O). Yet, the underlying mechanisms of urbanization impact on riverine N2O dynamics are not well understood. This study unveiled the effects of urbanization on N2O dynamics by integrating hydrological, biogeochemical and microbial processes in a river with various urbanization intensities. Riverine NO3- concentration enhanced with increasing urban land percentage, primarily because of the increased proportional contribution of sewage & manure source. The 15N site preference and relevant isotopic evidences revealed that the proportion of denitrification derived N2O increased from 60 % to 76 %, with the urban land percentage increasing from 〈 5 % to 〉 22 %, which was caused by decreases in flow velocity and dissolved oxygen saturation, increases in NO3- concentration and N2O-denitrifying genes. The non-negligible contribution of nitrification to N2O production (∼ 40 %) in lower-urbanized river stretches may be attributed to aerobic conditions and lower impermeable riparian zone facilitating the occurrence of in-river nitrification and the access of in-soil nitrification to river. Urbanization-mediated decreases in flow velocity and dissolved oxygen and increases in nitrogen availability and denitrification process resulted in an increase in N2O concentration and flux, with N2O concentration approximately four times higher in higher-urbanized river reaches (50.7 ± 26.3 nmol/L) than in lower-urbanized river reaches (14.4 ± 2.5 nmol/L). In addition, increased proportional contribution of sewage & manure source also provides the possibility for exogenous N2O inputs with urban expansion. These findings contribute to deepening our understanding of how urbanization drives N2O dynamics in river systems.

Urbanization significantly increases greenhouse gas emissions from a subtropical headwater stream in Southeast China

Streams are disproportionately significant contributors to increases in greenhouse gas (GHG) effluxes in river networks. In the context of global urbanization, a growing number of streams are affected by urbanization, which has been suggested to stimulate the water-air GHG emissions from fluvial systems. This study investigated the seasonal and longitudinal profiles of GHG (N2O, CH4, and CO2) concentrations of Jiuxianghe Stream, a headwater stream undergoing urbanization, and estimated its GHG diffusive fluxes and global warming potentials (GWPs) using the boundary layer method. The results showed that N2O, CH4, and CO2 concentrations in Jiuxianghe Stream were 0.45-7.19 μg L-1, 0.31-586.85 μg L-1, and 0.16-11.60 mg L-1, respectively. N2O, CH4, and CO2 concentrations in the stream showed 4.55-, 23.70-, and 7.68-fold increases from headwaters to downstream, respectively, corresponding to the forest-urban transition within the watershed. Multiple linear regression indicated that NO3--N, NH4+-N, and DOC:NO3--N accurately predicted N2O and CO2 concentrations, indicating that N nutrients were the driving factors. The Jiuxianghe Stream was a source of atmospheric GHGs with a daily GWP of 7.31 g CO2-eq m-2 d-1 on average and was significantly positively correlated with the ratio of construction land and forest in the sub-watershed. This study highlights the critical role of urbanization in amplifying GHG emissions from streams, thereby augmenting our understanding of GHG emissions from river networks. With global urbanization on the rise, streams experiencing urbanization are expected to make an unprecedentedly significant contribution to riverine GHG budgets in the future.

Recent advances on greenhouse gas emissions from wetlands: Mechanism, global warming potential, and environmental drivers

Greenhouse gas (GHG) emissions from wetlands have exacerbated global warming, attracting worldwide attention. However, the research process and development trends in this field remain unknown. Herein, 1865 papers related to wetlands GHG emissions published from January 2000 to December 2023 were selected, and CiteSpace and VOSviewer were used for bibliometric analysis to visually analyze the publications distribution, research authors, organizations and countries, core journal and keywords, and discussed the research progress, trends and hotspots in the fields. Over the past 24 years, the research has gone through three phases: the "embryonic" stage (2000-2006), the accumulation stage (2007-2014), and the acceleration stage (2015-2023). China has played a pivotal role in this domain, publishing the most papers and working closely with the United States, United Kingdom, Canada, Germany, and Australia. In addition, this study synthesized 311 field observations from 123 publications to analyze the variability in GHG emissions and their driving factors in four different types of natural wetlands. The results suggested that the average carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes in different wetlands were significantly different. River wetlands exhibited the highest GHG fluxes, while marsh wetlands demonstrated greater global warming potential (GWP). The average CO2, CH4 and N2O fluxes were 60.41 mg m-2·h-1, 2.52 mg m-2·h-1 and 0.05 mg m-2·h-1, respectively. The GWP of Chinese natural wetlands was estimated as 648.72 Tg·CO2-eq·yr-1, and CH4 contributed the largest warming effect, accounting for 57.43%. Correlation analysis showed that geographical location, climate factors, and soil conditions collectively regulated GHG emissions from wetlands. The findings provide a new perspective on sustainable wetland management and reducing GHG emissions.

Using the Potential Transformation of Dissolved Organic Matter to Understand Carbon Emissions from Inland Rivers

Understanding the transformation of river dissolved organic matter (DOM) is important for assessing the emissions of greenhouse gases (GHGs) in inland waters. However, the relationships between the variations in DOM components and GHGs remain largely unknown. Here, parallel factor analysis (PARAFAC) was applied to investigate the DOM components in 46 inland rivers in China. We found that the GHG emissions in peri-urban rivers were 1.10-2.15 times greater than those in urban rivers. Microbial and environmental factors (e.g., living cell numbers, microbial activity and pH) explained more than 70% of the total variance in GHG emissions in rivers. DOM variations relationships between different components ware revealed based on compositional data principal component analysis (CoDA-PCA). Microbial-mediated DOM production and degradation were quantified, and the degradation levels in peri-urban rivers were 11.8-25.2% greater than those in urban rivers. Differences in carbon emission potential between urban and peri-urban rivers were related to DOM variances and transformations and were affected by water chemistry (e.g., NH4-N and As). This study clarifies the regulatory effects of DOM composition variations and transformations on GHG emissions, and enhances the understanding of the DOM biogeochemical cycle.

Benthic clade II-type nitrous oxide reducers suppress nitrous oxide emissions in shallow lakes

Shallow lakes, recognized as hotspots for nitrogen cycling, contribute to the emission of the potent greenhouse gas nitrous oxide (N2O), but the current emission estimates for this gas have a high degree of uncertainty. However, the role of N2O-reducing bacteria (N2ORB) as N2O sinks and their contribution to N2O reduction in aquatic ecosystems in response to N2O dynamics have not been determined. Here, we investigated the N2O dynamics and microbial processes in the nitrogen cycle, which included both N2O production and consumption, in five shallow lakes spanning approximately 500 km. The investigated sites exhibited N2O oversaturation, with excess dissolved N2O concentrations (ΔN2O) ranging from 0.55 ± 0.61 to 53.17 ± 15.75 nM. Sediment-bound N2O (sN2O) was significantly positively correlated with the nitrate concentration in the overlying water (p < 0.05), suggesting that nitrate accumulation contributes to benthic N2O generation. High N2O consumption activity (RN2O) corresponded to low ΔN2O. In addition, a significant negative correlation was found between RN2O and nir/nosZ, showing that bacteria encoding nosZ contributed to N2O consumption in the benthic sediments. Redundancy analysis indicated that benthic functional genes effectively reflected the variations in RN2O and ∆N2O. qPCR analysis revealed that the clade II nosZ gene was more sensitive to ΔN2O than the clade I nosZ gene. Furthermore, four novel genera of potential nondenitrifying N2ORB were identified based on metagenome-assembled genome analysis. These genera, which are affiliated with clade II, lack genes responsible for N2O production. Collectively, benthic N2ORB, especially for clade II-type N2ORB, harnesses N2O consumption activity leading to low N2O emissions from shallow lakes. This study advances our knowledge of the role of benthic clade II-type N2ORB in regulating N2O emissions in shallow lakes.

Long-neglected contribution of nitrification to N2O emissions in the Yellow River

Rivers play a significant role in the global nitrous oxide (N2O) budget. However, the microbial sources and sinks of N2O in river systems are not well understood or quantified, resulting in the prolonged neglect of nitrification. This study investigated the isotopic signatures of N2O, thereby quantifying the microbial source of N2O production and the degree of N2O reduction in the Yellow River. Although denitrification has long been considered to be the dominant pathway of N2O production in rivers, our findings indicated that denitrification only accounted for 18.3% (8.2%-43.0%) of the total contribution to N2O production in the Yellow River, with 50.2%-80.2% being concurrently reduced. The denitrification contribution to N2O production (R2 = 0.44, p < 0.01) and N2O reduction degree (R2 = 0.70, p < 0.01) were positively related to the dissolved organic carbon (DOC) content. Similar to urban rivers and eutrophic lakes, denitrification was the primary process responsible for N2O production (43.0%) in certain reaches with high organic content (DOC = 5.29 mg/L). Nevertheless, the denitrification activity was generally constrained by the availability of electron donors (average DOC = 2.51 mg/L) throughout the Yellow River basin. Consequently, nitrification emerged as the primary contributor in the well-oxygenated Yellow River. Additionally, our findings further distinguished the respective contribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to N2O emissions. Although AOB dominated the N2O production in the Yellow River, the AOA specie abundance (AOA/(AOA + AOB)) contributed up to 32.6%, which resulted in 25.6% of the total nitrifier-produced N2O, suggesting a significant occurrence of AOA in the oligotrophic Yellow River. Overall, this study provided a non-invasive approach for quantifying the microbial sources and sinks to N2O emissions, and demonstrated the substantial role of nitrification in the large oligotrophic rivers.

High-Resolution Estimates of N2O Emissions from Inland Waters and Wetlands in China

Inland waters (rivers, lakes, and reservoirs) and wetlands (marshes and coastal wetlands) represent large and continuous sources of nitrous oxide (N2O) emissions, in view of adequate biomass and anaerobic conditions. Considerable uncertainties remain in quantifying spatially explicit N2O emissions from aquatic systems, attributable to the limitations of models and a lack of comprehensive data sets. Herein, we conducted a synthesis of 1659 observations of N2O emission rates to determine the major environmental drivers across five aquatic systems. A framework for spatially explicit estimates of N2O emissions in China was established, employing a data-driven approach that upscaled from site-specific N2O fluxes to robust multiple-regression models. Results revealed the effectiveness of models incorporating soil organic carbon and water content for marshes and coastal wetlands, as well as water nitrate concentration and dissolved organic carbon for lakes, rivers, and reservoirs for predicting emissions. Total national N2O emissions from inland waters and wetlands were 1.02 × 105 t N2O yr-1, with contributions from marshes (36.33%), rivers (27.77%), lakes (25.27%), reservoirs (6.47%), and coastal wetlands (4.16%). Spatially, larger emissions occurred in the Songliao River Basin and Continental River Basin, primarily due to their substantial terrestrial biomass. This study offers a vital national inventory of N2O emissions from inland waters and wetlands in China, providing paradigms for the inventorying work in other countries and insights to formulate effective mitigation strategies for climate change.

Greenhouse gases emissions from aquaculture ponds: Different emission patterns and key microbial processes affected by increased nitrogen loading

Global aquaculture production is expected to rise to meet the growing demand for food worldwide, potentially leading to increased anthropogenic greenhouse gases (GHG) emissions. As the demand for fish protein increases, so will stocking density, feeding amounts, and nitrogen loading in aquaculture ponds. However, the impact of GHG emissions and the underlying microbial processes remain poorly understood. This study investigated the GHG emission characteristics, key microbial processes, and environmental drivers underlying GHG emissions in low and high nitrogen loading aquaculture ponds (LNP and HNP). The N2O flux in HNP (43.1 ± 11.3 μmol m-2 d-1) was significantly higher than in LNP (-11.3 ± 25.1 μmol m-2 d-1), while the dissolved N2O concentration in HNP (52.8 ± 7.1 nmol L-1) was 150 % higher than in LNP (p < 0.01). However, the methane (CH4) and carbon dioxide (CO2) fluxes and concentrations showed no significant differences (p > 0.05). N2O replaced CH4 as the main source of Global Warming Potential in HNP. Pond sediments acted as a sink for N2O but a source for CH4 and CO2. The △N2O/(△N2O + △N2) in HNP (0.015 ± 0.007 %) was 7.7-fold higher than in LNP (0.002 ± 0.001 %) (p < 0.05). The chemical oxygen demand to NO2-N ratio was the most important environmental factor explaining the variability of N2O fluxes. Ammonia-oxidizing bacteria driven nitrification in water was the predominant N2O source, while comammox-driven nitrification and nosZII-driven N2O reduction in water were key processes for reducing N2O emission in LNP but decreased in HNP. The strong CH4 oxidization by Methylocystis and CO2 assimilation by algae resulted in low CH4 emissions and CO2 sink in the aquaculture pond. The Mantel test indicated that HNP increased N2O fluxes mainly through altering functional genes composition in water and sediment. Our findings suggest that there is a significant underestimation of N2O emissions without considering the significantly increased △N2O/(△N2O + △N2) caused by increased nitrogen loading.

Unveiling Microbial Nitrogen Metabolism in Rivers using a Machine Learning Approach

Microbial nitrogen metabolism is a complicated and key process in mediating environmental pollution and greenhouse gas emissions in rivers. However, the interactive drivers of microbial nitrogen metabolism in rivers have not been identified. Here, we analyze the microbial nitrogen metabolism patterns in 105 rivers in China driven by 26 environmental and socioeconomic factors using an interpretable causal machine learning (ICML) framework. ICML better recognizes the complex relationships between factors and microbial nitrogen metabolism than traditional linear regression models. Furthermore, tipping points and concentration windows were proposed to precisely regulate microbial nitrogen metabolism. For example, concentrations of dissolved organic carbon (DOC) below tipping points of 6.2 and 4.2 mg/L easily reduce bacterial denitrification and nitrification, respectively. The concentration windows for NO3--N (15.9-18.0 mg/L) and DOC (9.1-10.8 mg/L) enabled the highest abundance of denitrifying bacteria on a national scale. The integration of ICML models and field data clarifies the important drivers of microbial nitrogen metabolism, supporting the precise regulation of nitrogen pollution and river ecological management.

Labile dissolved organic matter (DOM) and nitrogen inputs modified greenhouse gas dynamics: A source-to-estuary study of the Yangtze River

Although rivers are increasingly recognized as essential sources of greenhouse gases (GHG) to the atmosphere, few systematic efforts have been made to reveal the drivers of spatiotemporal variations of dissolved GHG (dGHG) in large rivers under increasing anthropogenic stress and intensified hydrological cycling. Here, through a source-to-estuary survey of the Yangtze River in March (spring) and October (autumn) of 2018, we revealed that labile dissolved organic matter (DOM) and nitrogen inputs remarkably modified the spatiotemporal distribution of dGHG. The average partial pressure of CO2 (pCO2), CH4 and N2O concentrations of all sampling sites in the Yangtze River were 1015 ± 225 μatm, and 87.5± 36.5 nmol L-1, and 20.3 ± 6.6 nmol L-1, respectively, significantly lower than the global average. In terms of longitudinal and seasonal variations, higher GHG concentrations were observed in the middle-lower reach in spring. The dominant drivers of spatiotemporal variations in dGHG were labile, protein-like DOM components and nitrogen level. Compared with the historical data of dGHG from published literature, we found a significant increase in N2O concentrations in the Yangtze River during 2004-2018, and the increasing trend was consistent with the rising riverine nitrogen concentrations. Our study emphasized the critical roles of labile DOM and nitrogen inputs in driving the spatial hotspots, seasonal variations and annual trends of dGHG. These findings can contribute to constraining the global GHG budget estimations and controls of GHG emission in large rivers in response to global change.

Elucidating the links between N2O dynamics and changes in microbial communities following saltwater intrusions

Saltwater intrusion in estuarine ecosystems alters microbial communities as well as biogeochemical cycling processes and has become a worldwide problem. However, the impact of salinity intrusion on the dynamics of nitrous oxide (N2O) and associated microbial community are understudied. Here, we conducted field microcosms in a tidal estuary during different months (December, April and August) using dialysis bags, and microbes inside the bags encountered a change in salinity in natural setting. We then compared N2O dynamics in the microcosms with that in natural water. Regardless of incubation environment, saltwater intrusion altered the dissolved N2O depending on the initial saturation rates of N2O. While the impact of saltwater intrusion on N2O dynamics was consistent across months, the dissolved N2O was higher in summer than in winter. The N-related microbial communities following saltwater intrusion were dominated by denitrifers, with fewer nitrifiers and bacterial taxa involved in dissimilatory nitrate reduction to ammonium. While denitrification was a significant driver of N2O dynamics in the studied estuary, nitrifier-involved denitrification contributed to the additional production of N2O, evidenced by the strong associations with amoA genes and the abundance of Nitrospira. Higher N2O concentrations in the field microcosms than in natural water limited N2O consumption in the former, given the lack of an association with nosZ gene abundance. The differences in the N2O dynamics observed between the microcosms and natural water could be that the latter comprised not only indigenous microbes but also those accompanied with saltwater intrusion, and that immigrants might be functionally rich individuals and able to perform N transformation in multiple pathways. Our work provides the first quantitative assessment of in situ N2O concentrations in an estuary subjected to a saltwater intrusion. The results highlight the importance of ecosystem size and microbial connectivity in the source-sink dynamics of N2O in changing environments.

Drainage ditches are significant sources of indirect N2O emissions regulated by available carbon to nitrogen substrates in salt-affected farmlands

Agriculture is a main source of nitrous oxide (N2O) emissions. In agricultural systems, direct N2O emissions from nitrogen (N) addition to soils have been widely investigated, whereas indirect emissions from aquatic ecosystems such as ditches are poorly known, with insufficient data available to refine the IPCC emission factor. In this contribution, in situ N2O emissions from two ditch water‒air interfaces based on a diffusion model were investigated (almost once per month) from June 2021 to December 2022 in an intensive arable catchment with high N inputs and salt-affected conditions in the Qingtongxia Irrigation District, northwestern China. Our results implied that agricultural ditches (mean 148 μg N m-2 h-1) were significant sources for N2O emissions, and were approximately 2.1 times greater than those of the Yellow River directly connected to ditches. Agronomic management strategies increased N2O fluxes in summer, while precipitation events decreased N2O fluxes. Agronomic management strategies, including fertilization (294--540 kg N hm-2) and irrigation on farmland, resulted in enhanced diffuse N loads in drain water, whereas precipitation diluted the dissolved N2O concentration in ditches and accelerated the ditch flow rate, leading to changes in the residence time of N-containing substances in water. The spatial analysis showed that N2O fluxes (202-233 μg N m-2 h-1) in the headstream and upstream regions of ditches due to livestock and aquaculture pollution sources were relatively high compared to those in the midstream and downstream regions (100-114 μg N m-2 h-1). Furthermore, high available carbon (C) relative to N reduced N2O fluxes at low DOC:DIN ratio levels by inhibiting nitrification. Spatiotemporal variations in the N2O emission factor (EF5) across ditches with higher N resulted in lower EF5 and a large coefficient of variation (CV) range. EF5 was 0.0011 for the ditches in this region, while the EF5 (0.0025) currently adopted by the IPCC is relatively high. The EF5 variation was strongly controlled by the DOC:DIN ratio, TN, and NO3--N, while salinity was also a nonnegligible factor regulating the EF5 variation. The regression model incorporating NO3--N and the DOC:DIN ratio could greatly enhance the predictions of EF5 for agricultural ditches. Our study filled a key knowledge gap regarding EF5 from agricultural ditches in salt-affected farmland and offered a field investigation for refining the EF5 currently used by the IPCC.

Denitrification regulates spatiotemporal pattern of N2O emission in an interconnected urban river-lake network

Urban rivers are hotspots of N2O production and emission. Interconnected river-lake networks are constructed to improve the water quality and hydrodynamic conditions of urban rivers in many cities of China. However, the impact of the river-lake connectivity project on N2O production and emission remains unclear. This study investigated dissolved N2O and emission of the river-lake network in Wuhan City, China from March 2021 to December 2021. The results showed that river-lake connection greatly decreased riverine Nitrogen (N) concentration and increased dissolved oxygen (DO) concentration compare to traditional urban rivers. N2O emissions from the urban river interconnected with lakes (LUR: 67.3 ± 92.6 μmol/m2/d) were much lower than those from the traditional urban rivers (UR: 467.3 ± 1075.7 μmol/m2/d) and agricultural rivers (AR: 20.4 ± 15.3μmol/m2/d). Regression tree analysis suggested that the N2O concentrations were extremely high when hypoxia exists (DO < 1.6 mg/L), and TDN was the primary factor regulating N2O concentrations when hypoxia does not occur. Thus, we ascribe the low N2O emission in the LUR and AR to the lower N contents and higher DO concentrations. The microbial process of N2O production and consumption were quantitatively estimated by isotopic models. The mean proportion of denitrification derived N2O (fbD) was 63.5 %, 55.6 %, 42.3 % and 42.7 % in the UR, LUR, lakes and AR, suggested denitrification dominated N2O production in the urban rivers, but nitrification dominated N2O production in the lakes and AR. The positive correlation between logN2O and fbD suggested that denitrification is the key process to regulate the N2O production and emission. The abundance of denitrification genes (nirS and nirK) was much higher than that of nitrification genes (amoA and amoB), also evidenced that denitrification was the main N2O source. Therefore, river-lake interconnected projects changed the nutrients level and hypoxic condition, leading to the inhibition of denitrification and nitrification, and ultimately resulting in a decrease of N2O production and emission. These results advance the knowledge on the microbial processes that regulate N2O emissions in inland waters and illustrate the integrated management of water quality and N2O emission.

Microbial interaction patterns and nitrogen cycling regularities in lake sediments under different trophic conditions

Exploring how nitrogen (N) cycling microbes interact in eutrophic lake sediments and how biogenic elements influence the nitrogen cycle is crucial for understanding biogeochemical cycles and nitrogen accumulation mechanisms. In this study, sediment samples were collected from various areas of Taihu Lake with different trophic conditions in all four seasons from 2015 to 2017. Using high-throughput sequencing and molecular ecological network analysis, we investigated the microbial interaction patterns and the role of nitrogen cycling in sediments from lakes with different trophic conditions. The results showed distinct structures of sediment microbial networks between lake areas with different trophic conditions. In the more eutrophic region, network indices indicate higher transfer efficiency of energy, material, and information, more significant competition, and weaker niche differentiation of the microbial community. The sedimentary environment in the moderately eutrophic area exhibited greater potential for denitrification, nitrification, and anammox compared to the mesotrophic area, but the inhibition between N functional microbes and limitations in N removal processes were also more likely to occur. The topological structure of the networks showed that the carbon (C), sulfur (S), and iron (Fe) cycles had a strong influence on the nitrogen cycle in both lake areas. In the moderately eutrophic lake area, C- and S-cycling functional bacteria facilitated a closed cycle of the coupled N fixation-nitrification-DNRA (dissimilatory nitrate reduction to ammonium) process and reduced N removal. In the mesotrophic lake area, C- and S-cycling functional bacteria promoted both N fixation and mineralization, and Fe-cycling functional bacteria coupled with denitrifiers enhanced the nitrogen removal process of products from nitrogen fixation and mineralization. This study improved the understanding of the nitrogen cycling mechanism in lake sediments under different trophic conditions.

Estimating Yangtze River basin's riverine N2O emissions through hybrid modeling of land-river-atmosphere nitrogen flows

Riverine ecosystems are a significant source of nitrous oxide (N2O) worldwide, but how they respond to human and natural changes remains unknown. In this study, we developed a compound model chain that integrates mechanism-based modeling and machine learning to understand N2O transfer patterns within land, rivers, and the atmosphere. The findings reveal a decrease in N2O emissions in the Yangtze River basin from 4.7 Gg yr-1 in 2000 to 2.8 Gg yr-1 in 2019, with riverine emissions accounting for 0.28% of anthropogenic nitrogen discharges from land. This unexpected reduction is primarily attributed to improved water quality from human-driven nitrogen control, while natural factors contributed to a 0.23 Gg yr-1 increase. Notably, urban rivers exhibited a more rapid N2O efflux ( [Formula: see text] ), with upstream levels nearly 3.1 times higher than rural areas. We also observed nonlinear increases in [Formula: see text] with nitrogen discharge intensity, with urban areas showing a gradual and broader range of increase compared to rural areas, which exhibited a sharper but narrower increase. These nonlinearities imply that nitrogen control measures in urban areas lead to stable reductions in N2O emissions, while rural areas require innovative nitrogen source management solutions for greater benefits. Our assessment offers fresh insights into interpreting riverine N2O emissions and the potential for driving regionally differentiated emission reductions.

Effect and microbial mechanism of suspended sediments particle size on nitrous oxide emission in eutrophic lakes

Suspended sediment (SPS) is an important environmental factor in eutrophic lakes, where they may play a significant role in the microbial nitrogen cycle and thus affect the N2O source and sink function. This study investigated the correlation and corresponding microbial mechanisms between N2O emission fluxes and SPS particle sizes. N2O emission characteristics were investigated in four parallel operated lab-scale microcosmic systems, in which different sizes of SPS particles were inoculated (i.e., <75, 75-150, 150-300, and >300 μm). The results show that, N2O emission fluxes in the eutrophic lakes were exponentially correlated with the lake trophic level index (TLI) (R2 = 0.94, p < 0.01) and the specific surface area of the SPS (R2 = 0.38, p < 0.05). In the microcosmic systems, SPS with 75-150 μm particles had the highest N2O emission rate of 5.94 ± 0.007 μg N/L/d, which was 2.6 times that of the <75 μm particle size system. The microcosmic system with particle size >300 μm had the highest N2O reduction rate (Vmax) of 6.776 μmol/L/h, which was 16-50 times that of the other three groups. Larger particle size SPS have a smaller specific surface area, which could affect the microenvironment on SPS surface and thus affect the microbe functions. The microbial community structure results indicated that the dominant microorganisms on the SPS surface were denitrifying bacteria. The maximum (nirS + nirK)/nosZ ratio was 30.2 for the 75-150 μm system, which was nearly 2 times higher than the other systems. The >300 μm system had the highest nosZ abundance, indicating a strong ability to reduce N2O. The co-occurrence networks analysis indicated that the cooperation and competition among nitrifiers and denitrifiers determined N2O emissions. These results provide fundamental insights into the influence of SPS size on N2O emissions in eutrophic lakes.

Shifts in the high-resolution spatial distribution of dissolved N2O and the underlying microbial communities and processes in the Pearl River Estuary

Estuaries are significant sources of the ozone-depleting greenhouse gas N2O. However, owing to large spatial heterogeneity and discrete measurements, N2O emissions from estuaries are considerably uncertain. Microbial processes are disputed in terms of the dominant N2O production under severe human disturbance. Herein, combining real-time and high-resolution measurements with bioinformatics analysis, we accurately mapped the consecutive two-dimensional N2O distribution in the Pearl River Estuary (PRE), China, and revealed its underlying microbial mechanisms. Both the horizontal and vertical distributions of N2O concentrations varied greatly at fine scales. Supersaturated N2O concentrations (9.1 to 132.2 nmol/L) in the surface water decreased along the estuarine salinity gradient, with several emission hotspots scattering upstream. The vertical N2O distribution showed marked differences from complete mixing upstream to incomplete mixing downstream, with constant or changeable concentrations with increasing depth. Furthermore, spatially varied denitrifying and nitrifying microorganisms controlled the N2O production and distribution in the PRE, with denitrification playing the dominant role. The nirK-type and nirS-type denitrifying bacteria were the primary producers of N2O in the water and sediment columns, respectively. In addition, substrate concentration (NO3- and DOC) regulated N2O production by affecting key microbial processes, while physical influences (water-mass mixing and salt wedges) reshaped N2O distribution. With these information, a conceptual model of estuarine N2O production and distribution was constructed to generalize the possible biochemical processes under environmental constraints, which could provide insights into the N2O biogeochemical cycle and emission mitigation from a mechanistic perspective.

Pollution abatement reducing the river N2O emissions although it is partially offset by a warming climate: Insights from an urbanized watershed study

Global nitrogen (N) pollution has resulted in increased river nitrous oxide (N₂O) emissions, which contribute to climate change. However, little is known about how pollution abatement conversely reduces river N₂O production in a warming climate. Here, field observations and microcosmic experiments were conducted in a coastal urbanized watershed (S.E. China) to explore the interactive effect of changing nitrate and temperature on river sediment denitrification (DNF) and N₂O production. The results showed that urban river reaches (UR) with higher organic carbon content and denitrifying gene abundance in sediments have a greater DNF rate, nitrate removal efficiency (NRE), and N₂O concentration than agricultural river reaches (AR). Microcosmic incubation suggested that the DNF rate and associated N₂O production decreased under low nitrate addition, wherein the NRE increased. The scenario simulation illustrated a nonlinear response of N₂O production to nitrate removal (i.e., ΔN₂O/ΔNO₃-N) from both UR and AR sediments at a given temperature, and the DNF rate and N₂O production increased with increasing temperature. An increase in temperature by 1 degree Celsius would offset 18.75% of the N₂O reduction by nitrate removal via DNF. These findings implied that watershed pollution abatement undoubtedly contributes to the reduction in global river N₂O emissions although it is partially offset by extra N₂O production caused by global warming.

Impacts of high-quality coal mine drainage recycling for replenishment of aquatic ecosystems in arid regions of China: Bacterial community responses

Coal mine water is usually recycled as supplementary water for aquatic ecosystems in arid and semiarid mining regions of China. To ensure ecosystem health, the coal mine water is rigorously treated using several processes, including reverse osmosis, to meet surface water quality standards. However, the potential environmental impacts of this management pattern on the ecological function of receiving water bodies are unclear. In this study, we built several microcosm water ecosystems to simulate the receiving water bodies. High-quality treated coal mine drainage was mixed into the model water bodies at different concentrations, and the sediment bacterial community response and functional changes were systematically investigated. The results showed that the high-quality coal mine drainage could still shape bacterial taxonomic diversity, community composition and structure, with a concentration threshold of approximately 50%. Moreover, both the Mantel test and the structural equation model indicated that the salinity fluctuation caused by the receiving of coal mine drainage was the primary factor shaping the bacterial communities. 10 core taxa in the molecular ecological network influenced by coal mine drainage were identified, with the most critical taxa being patescibacteria and g_Geothermobacter. Furthermore, the pathway of carbohydrate metabolism as well as signaling molecules and interactions was up-regulated, whereas amino acid metabolism showed the opposite trend. All results suggested that the complex physical-chemical and biochemical processes in water ecosystems may be affected by the coal mine drainage. The bacterial community response and underlying functional changes may accelerate internal nutrient cycling, which may have a potential impact on algal bloom outbreaks.

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

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

Shifts in composition and function of bacterial communities reveal the effect of small barriers on nitrous oxide and methane accumulation in fragmented rivers

Rivers are often blocked by barriers to form different habitats, but it is not clear whether this change will affect the accumulation of N₂O and CH₄ in rivers. Here, low barriers (less than 2 m, LB) increased N₂O concentration by 1.13 times and CH₄ decreased by 0.118 times, while high barriers (higher than 2 m, less than 5 m high, HB) increased N₂O concentration by 1.19 times and CH₄ by 2.76 times. Co-occurrence network analysis indicated LB and HB can promote the enrichment of Cyanobium and Chloroflexi, further limiting complete denitrification and increasing N₂O accumulation. The LB promotes methanotrophs (Methylocystis, Methylophilus, and Methylotenera) to compete with denitrifiers (Pseudomonas) in water, and reduce CH₄ accumulation. While the HB can promote the methanotrophs to compete with nitrifiers (Nitrosospira) in sediment, thus reducing the consumption of CH₄. LB and HB reduce river velocity, increase water depth, and reduce dissolved oxygen (DO), leading to enrichment of nirS-type denitrifiers and the increase of N₂O concentration in water. Moreover, the HB reduces DO concentration and pmoA gene abundance in water, which can increase the accumulation of CH₄. In light of the changes in the microbial community and variation in N₂O and CH₄ accumulation, the impact of fragmented rivers on global greenhouse gas emissions merits further study.

Sources and migration similarly determine nitrate concentrations: Integrating isotopic, landscape, and biological approaches

Rapid land use change has significantly increased nitrate (NO₃^-) loading to rivers, leading to eutrophication, and posing water security problems. Determining the sources of NO₃^- to waters and the underlying influential factors is critical for effectively reducing pollution and better managing water resources. Here, we identified the sources and influencing mechanisms of NO₃^- in a mixed land-use watershed by integrating stable isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), molecular biology, water chemistry, and landscape metrics measurements. Weak transformation processes of NO₃^- were identified in the river, as evinced by water chemistry, isotopes, species compositions, and predicted microbial genes related to nitrogen metabolism. NO₃^- concentrations were primarily influenced by exogenous inputs (i.e., from soil nitrogen (NS), nitrogen fertilizer (NF), and manure & sewage (MS)). The proportions of NO₃^- sources seasonally varied. In the wet season, the source contributions followed the order of NS (38.6 %) > NF (31.4 %) > atmospheric deposition (ND, 16.2 %) > MS (13.8 %). In the dry season, the contributions were in the order of MS (39.2 %) > NS (29.2 %) > NF (29 %) > ND (2.6 %). Farmland and construction land were the original factors influencing the spatial distribution of NO₃^- in the wet and dry seasons, respectively, while slope, basin relief (HD), hypsometric integral (HI), and COHESION, HD were the primary indicators associated with NO₃^- transport in the wet and dry seasons, respectively. Additionally, spatial scale differences were observed for the effects of landscape structure on NO₃^- concentrations, with the greatest effect at the 1000-m buffer zone scale in the wet season and at the sub-basin scale in the dry season. This study overcomes the limitation of isotopes in identifying nitrate sources by combining multiple approaches and provides new research perspectives for the determination of nitrate sources and migration in other watersheds.

Nitrification Regulates the Spatiotemporal Variability of N2O Emissions in a Eutrophic Lake

Nitrous oxide (N₂O) emissions from lakes exhibit significant spatiotemporal heterogeneity, and quantitative identification of the different N₂O production processes is greatly limited, causing the role of nitrification to be undervalued or ignored in models of a lake's N₂O emissions. Here, the contributions of nitrification and denitrification to N₂O production were quantitatively assessed in the eutrophic Lake Taihu using molecular biology and isotope mapping techniques. The N₂O fluxes ranged from -41.48 to 28.84 μmol m^-2 d^-1 in the lake, with lower N₂O concentrations being observed in spring and summer and significantly higher N₂O emissions being observed in autumn and winter. The ¹⁵N site preference and relevant isotopic evidence demonstrated that denitrification contributed approximately 90% of the lake's gross N₂O production during summer and autumn, 27-83% of which was simultaneously eliminated via N₂O reduction. Surprisingly, nitrification seemed to act as a key process promoting N₂O production and contributing to the lake as a source of N₂O emissions. A combination of N₂O isotopocule-based approaches and molecular techniques can be used to determine the precise characteristics of microbial N₂O production and consumption in eutrophic lakes. The results of this study provide a basis for accurately assessing N₂O emissions from lakes at the regional and global scales.

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 (EF_5r ) model, one of the bottom-up approaches, adopts a fixed EF_5r value to estimate riverine N₂ O emissions based on IPCC methodology. However, the estimates have considerable uncertainty due to the large spatiotemporal variations in EF_5r values. Factors regulating EF_5r 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 EF_5r values. Our results show that the EF_5r values in China and other regions with high N loads are lower than those for regions with lower N loads. Although the global mean EF_5r value is comparable to the IPCC default value, the global EF_5r values are highly skewed with large variations, indicating that adopting region-specific EF_5r 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 EF_5r 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 EF_5r 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 EF_5r values at the global scale and provides a pathway to estimate global riverine N₂ O emissions more accurately based on IPCC methodology.

Microplastics distribution characteristics in typical inflow rivers of Taihu lake: Linking to nitrous oxide emission and microbial analysis

The microplastics in nature water are important for the environmental fate of nitrous oxide (N₂O). This study investigated the influence and microbial mechanism of microplastic abundance to the N₂O flux in typical inflow rivers of Taihu lake. The microplastic abundance were in a range of 160-700 particles/m³ surface water, and 514-3018 particles/kg dry sediment. The highest percentage of microplastic color was transparent, significantly higher than other color (p<0.0001) in both surface water and sediment. The dominant microplastic size was 500-5000 μm in surface water, while size lower than 1000 μm was dominant in sediment. The microplastic abundance in sediment was negatively correlated with the concentration of suspended sediments (SPS) (p<0.05), Chl-a (p<0.05), NH₄⁺-N (p<0.05) and TP (p<0.01) in inflow river surface water. The dissolved N₂O concentration were 45.71-132.42 nmol/L, and the N₂O fluxes were 29.85-276.60 μmol/m²/d. The dissolved N₂O concentration was significantly correlated with the nirK abundance and nirK/nosZI ratio negatively (p<0.05), revealed that sediment nirK-type denitrification was the main driver of dissolved N₂O. Meanwhile, the N₂O flux (water-air interface) was significantly correlated with nosZI, napA, narG and nirS negatively, implied that nitrification and denitrification interaction in sediment is the main influence factor. The denitrification process in sediment was the main driven factor of N₂O releasing. Mantel-test shows that microplastic abundance in surface water was significantly correlated with nitrification (p = 0.001∼0.01) and denitrification (p = 0.01∼0.05) genera in water. The dominant denitrification microorganism was Dechloromonas in sediment and Flavobacterium in surface water. These results provided new insight into the fact that plastisphere which comprises microbial community on microplastic could affect the N₂O emission in aquatic system.

Functional keystone drive nitrogen conversion during different animal manures composting

In this study, nitrogen transformation of chicken manure (CM) and cattle dung (CD) during composting was analyzed and its related functional keystones were identified. The results showed that chicken manure showed more severe nitrogen conversion during composting. The main N conversion factors in cattle dung were nitrite nitrogen (NO₂^--N) and ammonium nitrogen (NH₄⁺-N), while the main N conversion factors in chicken manure were NH₄⁺-N and nitrate nitrogen (NO₃^--N). The nitrogen-transforming bacterial community in chicken manure was more diverse. Variations in functional keystone abundances in cattle dung tended to be confined to the cooling and maturation periods, whereas changes in chicken manure persisted throughout the composting process. Environmental factors affected the functional keystones of nitrogen transformation. This study may provide directions for regulating nitrogen conversion in animal manure composting.

Enhanced microbial nitrification-denitrification processes in a subtropical metropolitan river network

Urban rivers are hotspots of regional nitrogen (N) pollution and N transformations. Previous studies have reported that the microbial community of urban rivers was different from that of natural rivers. However, how microbial community affects N transformations in the urban rivers is still unclear. In this study, we employed N nutrients-related isotope technology (includes natural-abundance isotopes survey and isotope-labeling method) and bioinformatics methods (includes 16S rRNA high-throughput sequencing and quantitative PCR analysis) to investigate the major N transformations, microbial communities as well as functional gene abundances in a metropolitan river network. Our results suggested that the bacterial community structure in the highly urbanized rivers was characterized by higher richness, less complexity and increased abundances of nitrification and denitrifying bacterium compared to those in the suburban rivers. These differences were mainly caused by high sewage discharge and N loadings. In addition, the abundances of nitrifier gene (amoA) and denitrifier genes (nirK and nirS) were significantly higher in the highly urbanized rivers (2.36 × 10³, 7.43 × 10⁷ and 2.28 × 10⁷ copies·mL^-1) than that in the suburban rivers (0.43 × 10³, 2.18 × 10⁷ and 0.99 × 10⁷ copies·mL^-1). These changes in microbes have accelerated nitrification-denitrification processes in the highly urbanized rivers as compared to those in the suburban rivers, which was evidenced by environmental isotopes and the rates of nitrification (10.52 vs. 0.03 nmol·L^-1·h^-1) and denitrification (83.31 vs. 22.49 nmol·g^-1·h^-1). Overall, this study concluded that the excess exogenous N has significantly shaped the specific aquatic bacterial communities, which had a potential for enhancing nitrification-denitrification processes in the highly urbanized river network. This study provides a further understanding of microbial N cycling in urban river ecosystems and expands the combined application of isotopic technology and bioinformatics methods in studying biogeochemical cycling.