Hot spots and hot moments of nitrogen removal from hyporheic and riparian zones: A review

Distinct vertical profiles of microbial communities and functional genes between different lake sediment layers mediated by nutrients in the sediments and pore waters

Lakes, as vital wetland ecosystems, face increasing anthropogenic pressures, including nutrient pollution, which significantly exacerbates the threat of eutrophication. Microorganisms inhabiting lake sediments are key drivers of biogeochemical cycles of essential elements in aquatic ecosystems. While their roles in eutrophic lakes have been explored, their vertical profiles of community compositions and functional genes in the sediments of eutrophic lakes remain poorly resolved. This study aimed to investigate the spatial distribution patterns of and correlations between nutrient concentrations, microbial communities, and element-cycling-related functional genes in the sediment of Honghu Lake, a representative of shallow lakes located in China's Hubei province. 16S rRNA sequencing was used to explore the microbial community structure, and a high-throughput quantitative-PCR-based gene chip, quantitative microbial element cycling (QMEC), was employed to assess the abundance of functional genes for C, N, P, and S metabolisms. A series of bioinformatic analyses were orchestrated to explore the functional differences between sediment layers. The results indicate that nutrient concentrations, functional gene abundance, and alpha diversity of microbial communities generally decrease with depth from the surface sediment to deeper layers. The main environmental variables correlating with the microbial communities included total phosphorus and total nitrogen in the sediments, and NH4+-N in the pore waters. In the co-occurrence networks, different highly connected species were identified as key members in different sediment layers. Some functional genes were exclusively detected in specific locations and layers, increasing the heterogeneity of the biogeochemical functions, and weakening the functional redundancy of microbial communities. This study showed the connections between environmental variables, microbial community compositions, and element cycling functions in a typical shallow lake, and emphasized the heterogeneity of nutrients, microbial communities, and functional genes in lake sediments and other aquatic ecosystems.

Denitrifying anaerobic methane oxidation activity and microbial mechanisms in Riparian zone soils of the Yulin River, a tributary of the Three Gorges Reservoir

Riparian zones are recognized as major sources of greenhouse gas emissions, particularly methane (CH4). Denitrifying anaerobic methane oxidation (DAMO) has garnered growing attention due to its significant contribution to mitigating CH4 emissions in wetland environments. Nonetheless, the specific role and microbial mechanisms of DAMO in controlling CH4 release within riparian zones are still not well comprehended. This study employed isotopic labeling experiments to measure the nitrate-dependent anaerobic methane oxidation (NaDAMO) and nitrite-dependent anaerobic methane oxidation (NiDAMO) potential of soil samples from riparian zones that were collected during different hydrological cycles. Moreover, soil physicochemical properties, DAMO activity, and microbial abundance were integrated to analyze the key factors and mechanisms influencing DAMO in riparian zone soils. The isotope tracer results showed that NaDAMO activities (1.41-11.93 nmol 13CO2 g-1day-1) were significantly higher than NiDAMO activities (0.66-9.19 nmol 13CO2 g-1day-1) in the riparian zone (p < 0.05). NiDAMO activities were more strongly influenced by hydrological variations compared to NaDAMO activities, exhibiting higher levels during the discharge period (2.78-9.19 nmol 13CO2 g-1day-1) compared to the impoundment period (0.66-4.10 nmol 13CO2 g-1day-1). The qPCR analysis showed that the gene copies of NaDAMO archaeal mcrA (107 copies g-1) were approximately ten times greater than those of NiDAMO bacterial pmoA (106 copies g-1) in the majority of the sampling sites. Correlation analyses revealed that NiDAMO activity was influenced by soil pH (p < 0.05), while NaDAMO microbes were influenced by temperature, organic carbon, and ammonia nitrogen concentrations (p < 0.05). In summary, this research explored how hydrological changes in the riparian zone influence DAMO activities and their underlying mechanisms, providing a theoretical basis for mitigating CH4 emissions in riparian zones of reservoir regions.

Amplified impacts of human activities: Non-linear responses of riverine microbial communities to distribution of land use

Rapid global urbanization poses considerable ecological risks to freshwater systems, notably leading to substantial reductions in microbial communities. To assess the impacts of human activities on these communities, we applied the high-throughput amplicon DNA sequencing to examine spatial variations in riverine microbial communities within an urbanized watershed. Coupled with the Geographical Detector Model, the effects of the land use were identified across the watershed. Results show that microbial communities were closely linked to the human-impacted land use patterns. The upstream region, dominated by forest cover (71%), exhibited the highest microbial population (3384 OTUs), whereas the urbanized downstream outlet (91% urban land) showed the lowest microbial population (471 OTUs). Additionally, the spatial distribution of the human-impacted land use appears to abruptly alter microbial pathways along the river. The spatial threshold effect of human-impacted land use is indicated by a Moran's I value exceeding 0.80. Notably, a 300-m buffer zone around different land uses seems to significantly influenced sediment microbial communities. Besides, the influence of land use on microbial communities is intensified by spatial drivers. For instance, agricultural land use was found to impact riverine Parcubacteria communities, with factor detector values increasing by over 30% in 400-500 m buffer zones. These findings provide new insights into the complex relationship between human activity and riverine microbial communities, highlighting important implications for ecosystem management in rapidly urbanization regions.

Sediment grain size regulates the biogeochemical processes of nitrate in the riparian zone by influencing nutrient concentrations and microbial abundance

Riparian zones play a crucial role in reducing nitrate pollution in both terrestrial and aquatic environments. Complex deposition action and dynamic hydrological processes will change the grain size distribution of riparian sediments, affect the residence time of substances, and have a cascade effect on the biogeochemical process of nitrate nitrogen (NO3--N). However, simultaneous studies on NO3--N transformation and the potential drivers in riparian zones are still lacking, especially neglecting the effect of sediment grain size (SGS). To fill this knowledge gap, we first systematically identified and quantified NO3--N biogeochemical processes in the riparian zone by integrating molecular biotechnology, 15N stable isotope tracing, and microcosmic incubation experiments. We then evaluated the combined effects of environmental variables (including pH, dissolved organic carbon (DOC), oxidation reduction potential, SGS, etc.) on NO3--N transformation through Random Forest and Structural Equation Models. The results demonstrated that NO3--N underwent five microbial-mediated processes, with denitrification, dissimilatory nitrate reduction to ammonium (DNRA) dominated the NO3--N attenuation (69.4 % and 20.1 %, respectively), followed by anaerobic ammonia oxidation (anammox) and nitrate-dependent ferric oxidation (NDFO) (8.4 % and 2.1 %, respectively), while nitrification dominated the NO3--N production. SGS emerged as the most critical factor influencing NO3--N transformation (24.96 %, p < 0.01), followed by functional genes (nirS, nrfA) abundance, DOC, and ammonia concentrations (14.12 %, 16.40 %, 13.08 %, p < 0.01). SGS influenced NO3--N transformation by regulating microbial abundance and nutrient concentrations. RF predicted that a 5 % increase in the proportion of fine grains (diameter < 50 μm) may increase the NO3--N transformation rate by 3.8 %. This work highlights the significance of integrating machine learning and geochemical analysis for a comprehensive understanding of nitrate biogeochemical processes in riparian zones, contributing valuable references for future nitrogen management strategies.

Uncovering the hidden world of riverbed sediments: The role of sediment heterogeneity and cross-bar channel fills in the hydrogeochemical dynamics of the hyporheic zone

Groundwater-surface water interaction (hyporheic exchange) is critical in numerous hydrogeochemical processes; however, hyporheic exchange is difficult to characterize due to the various spatial (e.g., sedimentary architecture) and temporal (e.g., stage fluctuations) variables that influence it. This interdisciplinary study brings forth novel insights by integrating various methodologies including geophysical surveys, physical and chemical sediment characterization, and water chemistry analysis to explore the interplay of the numerous facets governing hyporheic zone processes within a compound bar deposit. The findings reveal distinct sedimentary facies and geochemical zones within the compound bar, driven by the sedimentary architecture. Cross-bar channel fills are identified as critical structures influencing hydrogeochemical dynamics, acting as baffles to groundwater flow and modulating nutrient transformations. Geophysical imaging and hydrogeochemical analyses highlight the complex interplay between sediment characteristics and subsurface hydraulic connectivity, emphasizing the role of sediment heterogeneity in controlling hyporheic exchange and solute mixing. The study concludes that sediment heterogeneity, particularly the presence of cross-bar channel fills, plays a pivotal role in the hydrogeochemical dynamics of the hyporheic zone. These structures significantly influence hyporheic flow paths, solute residence times, and nutrient cycling, underscoring the necessity to consider the fine-scale sedimentary architecture in models of hyporheic exchange. The findings contribute to a deeper understanding of riverine ecosystem processes, offering insights that can inform management strategies for water quality and ecological integrity.

The competition of heavy metals between hyporheic sediments and microplastics of driving factors in the Beiluo River Basin

Both sediments and microplastics (MPs) are medias of heavy metals (HMs) in river ecosystems. This study investigated HMs (Mn, Cr, V, As, Cu, Co, Cd, Pb, and Ni) concentration and driving factors for competitive enrichment between hyporheic sediments versus MPs. The medias basic characteristics indicated that the sediments were mostly sand and rich in Fe2O3; three polymer types were identified, with blue, fragment, less than 500 µm being the main types of MPs. The results have shown that the average content of extracted HMs in MPs was much higher than that of the same metals accumulated in sediments. HMs in sediments and MPs reached heavily polluted at some points, among which As and Cd were ecological risks. Electrostatic adsorption and surface complexation, and biofilm-mediated and organic matter complexation were the interaction mechanism of HMs with sediments and MPs. Further, the driving factors affecting the distribution of HMs in the two carriers were analyzed by multivariate statistical analysis. The results demonstrated that carrier characteristics, hydrochemical factors, and the inherent metal load of MPs were the main causes of the high HMs content. These findings improved our understanding of HMs fate and environmental risks across multiple medias.

Nitrogen in landfills: Sources, environmental impacts and novel treatment approaches

Nitrogen (N) accumulation in landfills is a pressing environmental concern due to its diverse sources and significant environmental impacts. However, there is relatively limited attention and research focus on N in landfills as it is overshadowed by other more prominent pollutants. This study comprehensively examines the sources of N in landfills, including food waste contributing to 390 million tons of N annually, industrial discharges, and sewage treatment plant effluents. The environmental impacts of N in landfills are primarily manifested in N2O emissions and leachate with high N concentrations. To address these challenges, this study presents various mitigation and management strategies, including N2O reduction measures and novel NH4+ removal techniques, such as electrochemical technologies, membrane separation processes, algae-based process, and other advanced oxidation processes. However, a more in-depth understanding of the complexities of N cycling in landfills is required, due to the lack of long-term monitoring data and the presence of intricate interactions and feedback mechanisms. To ultimately achieve optimized N management and minimized adverse environmental impacts in landfill settings, future prospects should emphasize advancements in monitoring and modeling technologies, enhanced understanding of microbial ecology, implementation of circular economy principles, application of innovative treatment technologies, and comprehensive landfill design and planning.

Elucidation of the dominant factors influencing N2O emission in water-level fluctuation zones in a karst canyon reservoir, southwest China

The water-level fluctuations zones (WLFZs) are crucial transitional interfaces within river-reservoir systems, serving as hotspots for N2O emission. However, the comprehension of response patterns and mechanisms governing N2O emission under hydrological fluctuation remains limited, especially in karstic canyon reservoirs, which introduces significant uncertainty to N2O flux assessments. Soil samples were collected from the WLFZs of the Hongjiadu (HJD) Reservoir along the water flow direction from transition zone (T1 and T2) to lacustrine zone (T3, T4 and T5) at three elevations for each site. These soil columns were used to conduct simulation experiments under various water-filled pore space gradients (WFPSs) to investigate the potential N2O flux pattern and elucidate the underlying mechanism. Our results showed that nutrient distribution and N2O flux pattern differed significantly between two zones, with the highest N2O fluxes in the transition zone sites and lacustrine zone sites were found at 75 % and 95 % WFPS, respectively. Soil nutrient loss in lower elevation areas is influenced by prolonged impoundment durations. The higher N2O fluxes in the lacustrine zone can be attributed to increased nutrient levels resulting from anthropogenic activities. Furthermore, correlation analysis revealed that soil bulk density significantly impacted N2O fluxes across all sites, while NO3-and SOC facilitated N2O emissions in T1-T2 and T4-T5, respectively. It was evident that N2O production primarily contributed to nitrification in the transition zone and was constrained by the mineralization process, whereas denitrification dominated in the lacustrine zone. Notably, the annual N2O efflux from WLFZs accounted for 27 % of that from the water-air interface in HJD Reservoir, indicating a considerably lower contribution than anticipated. Nevertheless, this study highlights the significance of WLFZs as a vital potential source of N2O emission, particularly under the influence of anthropogenic activities and high WFPS gradient.

Insights into biogeochemistry and hot spots distribution characteristics of redox-sensitive elements in the hyporheic zone: Transformation mechanisms and contributing factors

Biogeochemical hot spots play a crucial role in the cycling and transport of redox-sensitive elements (RSEs) in the hyporheic zone (HZ). However, the transformation mechanisms of RSEs and patterns of RSEs hot spots in the HZ remain poorly understood. In this study, hydrochemistry and multi-isotope (N/C/S/O) datasets were collected to investigate the transformation mechanisms of RSEs, and explore the distribution characteristics of RSEs transformation hot spots. The results showed that spatial variability in key drivers was evident, while temporal change in RSEs concentration was not significant, except for dissolved organic carbon. Bacterial sulfate reduction (BSR) was the primary biogeochemical process for sulfate and occurred throughout the area. Ammonium enrichment was mainly caused by the mineralization of nitrogenous organic matter and anthropogenic inputs, with adsorption serving as the primary attenuation mechanism. Carbon dynamics were influenced by various biogeochemical processes, with dissolved organic carbon mainly derived from C3 plants and dissolved inorganic carbon from weathering of carbonate rocks and decomposition of organic matter. The peak contribution of dissolved organic carbon decomposition to the DIC pool was 46.44 %. The concentration thresholds for the ammonium enrichment and BSR hot spots were identified as 1.5 mg/L and 8.84 mg/L, respectively. The distribution pattern of RSEs hot spots was closely related to the hydrogeological conditions. Our findings reveal the complex evolution mechanisms and hot spots distribution characteristics of RSEs in the HZ, providing a basis for the safe utilization and protection of groundwater resources.

Unveiling the dynamic of nitrogen through migration and transformation patterns in the groundwater level fluctuation zone of a different hyporheic zone sediment

This study investigates the impact of water levels and soil texture on the migration and transformation of nitrate (NO3--N) and ammonium (NH4+-N) within a soil column. The concentrations of NO3--N gradually decreased from an initial concentration of 34.19 ± 0.86 mg/L to 14.33 ± 0.77 mg/L on day 70, exhibiting fluctuations and migration influenced by water levels and soil texture. Higher water levels were associated with decreased NO3--N concentrations, while lower water levels resulted in increased concentrations. The retention and absorption capacity for NO3--N were highest in fine sand soil, followed by medium sand and coarse sand, highlighting the significance of soil texture in nitrate movement and retention. The analysis of variance (ANOVA) confirmed statistically significant variations in pH, dissolve oxygen and oxidation-reduction potential across the soil columns (p < 0.05). Fluctuating water levels influenced the migration and transformation of NO3--N, with distinct patterns observed in different soil textures. Water level fluctuations also impacted the migration and transformation of NH4+-N, with higher water levels associated with increased concentrations and lower water levels resulting in decreased concentrations. Among the soil types considered, medium sand exhibited the highest absorption capacity for NH4+-N. These findings underscore the significant roles of water levels, soil texture, and soil type in the migration, transformation, and absorption of nitrogen compounds within soil columns. The results contribute to a better understanding of nitrogen dynamics under varying water levels and environmental conditions, providing valuable insights into the patterns of nitrogen migration and transformation in small-scale soil column experiments.

With spatial distribution, risk evaluation of heavy metals and microplastics to emphasize the composite mechanism in hyporheic sediments of Beiluo River

This study aimed to assess the hazardous impacts of heavy metals (HMs) enrichment on the surface of microplastics (MPs) in the hyporheic zone. The present work analyzed the spatial distribution and risk evaluation of HMs (V, Cr, Mn, Co, Ni, Cu, Zn, As, Cd, and Pb) and MPs and the mechanism of HMs enrichment on MPs in the sediments. The highest rates of contamination were for Cd, Pb, and As. The main types of MPs were fiber, blue, and a size smaller than 500 µm. The lower reaches of the Beiluo River had the most serious HMs and MPs pollution, especially BL-10 (HMs: CF-Cd, 41.91; EF-Cd, 50.87; Igeo-Cd, 4.80; RI, 1291; PN, 29.83; MPs: abundance, 890 ± 18 items/kg). Meanwhile, the principal component analysis showed that natural, industrial activities, and agricultural production and transportation were primary HMs sources in sediments, and Cd, Co, and Pb were the main enriched metals on the surface of MPs. More importantly, regarding the interaction mechanism of these composite pollutants, we concluded that electrostatic adsorption and biofilm mediation were the main mechanisms of the synergistic effect. Overall, our findings provide a theoretical basis for further research on the ecotoxicity of composite pollutants in aquatic environments.

Redox gradients drive microbial community assembly patterns and molecular ecological networks in the hyporheic zone of effluent-dominated rivers

The impacts of effluent discharge on receiving waterbodies have been a research hotspot. Nonetheless, limited information is available on the microbial community assembly patterns in the hyporheic zones (HZ) responding to the changes in the microenvironments, e.g., solute transport and redox gradient variations. Using two representative effluent-dominated rivers as model systems, the spatio-temporal bacterial community dynamics and assembly patterns in oxic and suboxic zones in the shallow riverbed sediments were disentangled via null model- and neutral model-based approaches. Bacterial dynamics in community composition were observed driven by environmental filtering, i.e., impacts of environmental variables, more than geographic distances, i.e., the depths of sediments. The communities in samples collected in summer were largely shaped by stochasticity, in which homogeneous selection occupied a higher proportion in oxic (∼39%) than in suboxic zone (∼23%). Deterministic processes contributed to a more complex community structure for samples from oxic zones, whereas weakened the interspecies interactions in suboxic zones. The richness and abundances of non-neutral community were confirmed governing the deterministic assembly in oxic zones. Key species ascribed to 'connectors' and 'network hubs' dominated the community assembly variations in samples collected in winter, and in oxic zones, respectively. Significant positive relationships between β-nearest taxon index and dissolved organic nitrogen (DON) and nitrate highlighted their vital roles in community assembly via deterministic selective pressures in oxic zones. The significance thresholds of nitrogen species for community transition in winter (ΔDON: 2.81 mg-N/L, ΔNO3-: 1.09 mg-N/L) were lower than in summer, probably implying that stricter effluent quality standards should be established in colder seasons. Combined, our work poses first insights on the roles of redox zonation in driving microbial community assembly in HZ, which is of significance in guiding ecological remediation processes in effluent-dominated rivers.

Non-rhizosphere reinforces the contributions of Feammox and anammox to nitrogen loss than rhizosphere in riparian zones

The emergence of anaerobic ammonium oxidation (anammox) coupled to iron reduction (named Feammox) refreshes the microbial pathways for nitrogen (N) loss. However, the ecological role of Feammox, compared with conventional denitrification and anammox, in microbial N attenuation in ecosystems remains unclear. Here, the specific contribution of Feammox to N loss and the underlying microbiome interactive characteristics in a riparian ecosystem were investigated through 15N isotope tracing and molecular analysis. Feammox was highlighted in the riparian interface soils and maximally contributed 14.2% of N loss. Denitrification remained the dominant contributor to N loss (68.0%-95.3%), followed by anammox (5.7%-19.1%) and Feammox (0-14.2%). The rates of Feammox and anammox significantly decreased in rhizosphere soils (0.15 ± 0.08 μg N g-1 d -1 for Feammox, 0.80 ± 0.39 μg N g-1 d -1 for anammox) compared with those in non-rhizosphere soils; however, the activities of denitrification remarkably increased in the rhizosphere (13.17 ± 3.71 μg N g-1 d -1). In rhizosphere soils, the competition between bioavailable organic matter (e.g., amino acids and carbohydrates) and ammonium for electron acceptor [i.e., Fe(III)] was the vital inducement for restricted Feammox, while the nitrite consumption boosted by heterotrophic denitrifiers was responsible for weakened anammox. The functional gene of autotrophic Acidimicrobiaceae bacterium A6, instead of heterotrophic Geobacteraceae spp., was significantly positively correlated with Feammox activity. Rare iron-reducing bacteria showed higher node degrees in the non-rhizosphere network than in the rhizosphere network. A syntrophic relationship was found between iron-reducing bacteria (e.g., Anaeromyxobacter, Geobacter) and iron-oxidizing bacteria (e.g., Sideroxydans) in the non-rhizosphere network and facilitated the Feammox pathway. This study provides an in-depth exploration of microbial driven N loss in a riparian ecosystem and introduces new insights into riparian management practices toward high-efficient N pollution alleviation.

The effect of social media environmental information exposure on the intention to participate in pro-environmental behavior

With the threat of global warming, countries worldwide have enhanced their environmental campaigns on social media to increase users' willingness to take pro-environmental actions. In this study, we examined the direct and indirect effects of exposure to environmental information on Chinese young adults' (18-25 years old) intention to participate in environmental protection actions (e.g., recycling, using public transportation, involvement in an environmental group, and participation in eco-friendly events). Data were collected from a sample of 291 Chinese young adults using a web-based survey and a thoroughly designed questionnaire. The accumulated data were analyzed using SPSS version 20. Hierarchical regression and mediation analysis were performed for testing hypotheses. The results indicated that exposure to environmental information on Chinese social media platforms (WeChat and Xiaohongshu) positively affected individuals' intention to participate in pro-environmental behavior, perceived pro-environmental behavior control, pro-environmental attitude, and fear of victimization. The indirect effect demonstrated that pro-environmental behavior control and attitude mediated the relationship between exposure to environmental information on both WeChat and Xiaohongshu and the intention to participate in pro-environmental behavior. Extending the existing literature, this study provides empirical evidence on the influence of environmental information exposure on the intention to participate in environmental protection among Chinese adults. In addition, it provides valuable insights into the mediating mechanisms involving cognitive, psychological, and emotional factors in this relationship. Policy makers should implement effective pro-environmental promotions on social media to inspire individuals to engage in environmentally friendly actions. In addition, social media managers should strictly authenticate and remove misleading environmental content.

Synergistic effect of zero-valent iron and static magnetic field on wastewater purification and bioelectricity generation in electroactive constructed wetlands

This study investigated the enhancement effect of zero-valent iron and static magnetic field on the pollutant removal and power generation of electroactive constructed wetland. As demonstration, a conventional wetland was systematically modified by introducing zero-valent iron and then a static magnetic field, leading to progressive increases in pollutant (namely NH4+-N and chemical oxygen demand) removal efficiencies. By introducing both zero-valent iron and a static magnetic field, the power density increased four-fold to 9.2 mW/m2 and the internal resistance decreased by 26.7% to 467.4 Ω. Notably, static magnetic field decreased the relative abundance of electrochemically active bacteria (such as Romboutsia), while significantly enhancing species diversity. The permeability of the microbial cell membrane was improved, leading to a reduction in activation loss and internal resistance, thereby enhancing power generation capacity. Results showed that the addition of zero-valent iron and the applied magnetic field were beneficial to the pollutants removal and bioelectricity generation.

Effects of benthic bioturbation on anammox in nitrogen removal at the sediment-water interface in eutrophic surface waters

Anaerobic ammonium oxidation (anammox) significantly contributes to nitrogen loss in freshwater ecosystems. The sediment-water interface (SWI), known as a "hot spot" for anammox, also harbors numerous macroinvertebrates. However, the impact of their bioturbation on anammox has generally been overlooked. This study compared the effects of three representative macroinvertebrates (i.e., Propsilocerus akamusi, Branchiura sowerbyi and Radix swinhoei) with different bioturbation modes on anammox and the N-removal processes at the SWI by using a microcosmic system. The results demonstrated that all three benthic macroinvertebrates promoted anammox in addition to denitrification processes. The highest N-removal was achieved in the presence of P. akamusi considered as a gallery-diffuser, where the relative abundance of Planctomycetes (to which the anammox bacteria belong) increased by 70%. P. akamusi increased the abundance of anammox hzsB gene by 2.58-fold and promoted potential anammox rate by 12.79 nmol N g-1 h-1, which in turn facilitated total N-removal mass increased by 2.42-fold. In the presence of B. sowerbyi and R. swinhoei, the potential anammox rates increased by 4.81 and 5.57 nmol N g-1 h-1, respectively. These results underscore the substantial impact of macroinvertebrates on anammox and N-removal processes, highlighting their crucial role in N pollution control, and sustaining the overall health and stability of eutrophic water bodies.

Effects of substrate improvement on winter nitrogen removal in riparian reed (Phragmites australis) wetlands: rhizospheric crosstalk between plants and microbes

With continued anthropogenic inputs of nitrogen (N) into the environment, non-point source N pollutants produced in winter cannot be ignored. As the water-soil interface zones, riparian wetlands play important roles in intercepting and buffering N pollutants. However, winter has the antagonistic effect on the N removal. Substrate improvement has been suggested as a strategy to optimize wetland performance and there remain many uncertainties about the inner mechanism. This study explores the effects of substrate improvement on N removal in winter and rhizospheric crosstalk between reed (Phragmites australis) and microbes in subtropical riparian reed wetlands. The rates of wetland N removal in winter, root metabolite profiles, and rhizosphere soil microbial community compositions were determined following the addition of different substrates (gravel, gravel + biochar, ceramsite + biochar, and modified ceramsite + biochar) to natural riparian soil. The results showed that the addition of different substrates to initial soil enhanced N removal from the microcosms in winter. Gravel addition increased NH4+-N removal by 8.3% (P < 0.05). Gravel + biochar addition increased both TN and NH4+-N removals by 8.9% (P < 0.05). The root metabolite characteristics and microbial community compositions showed some variations under different substrate additions compared to the initial soil. The three treatments involving biochar addition decreased lipid metabolites and enhanced the contents and variety of carbon sources in rhizosphere soil, while modified ceramsite + biochar addition treatment had a greater impact on the microbial community structure. There was evidence for a complex crosstalk between plants and microbes in the rhizosphere, and some rhizosphere metabolites were seen to be significantly correlated with the bacterial composition of the rhizospheric microbial community. These results highlighted the importance of rhizospheric crosstalk in regulating winter N removal in riparian reed wetland, provided a scientific reference for the protection and restoration of riparian reed areas and the prevention and control of non-point source pollution.

Nitrogen removal in freshwater sediments of riparian zone: N-loss pathways and environmental controls

The riparian zone is an important location of nitrogen removal in the terrestrial and aquatic ecosystems. Many studies have focused on the nitrogen removal efficiency and one or two nitrogen removal processes in the riparian zone, and less attention has been paid to the interaction of different nitrogen transformation processes and the impact of in situ environmental conditions. The molecular biotechnology, microcosm culture experiments and 15N stable isotope tracing techniques were used in this research at the riparian zone in Weinan section of the Wei River, to reveal the nitrogen removal mechanism of riparian zone with multi-layer lithologic structure. The results showed that the nitrogen removal rate in the riparian zone was 4.14-35.19 μmol·N·kg-1·h-1. Denitrification, dissimilatory reduction to ammonium (DNRA) and anaerobic ammonium oxidation (anammox) jointly achieved the natural attenuation process of nitrogen in the riparian zone, and denitrification was the dominant process (accounting for 59.6%). High dissolved organic nitrogen and nitrate ratio (DOC:NO3-) would promote denitrification, but when the NO3- content was less than 0.06 mg/kg, DNRA would occur in preference to denitrification. Furthermore, the abundances of functional genes (norB, nirS, nrfA) and anammox bacterial 16S rRNA gene showed similar distribution patterns with the corresponding nitrogen transformation rates. Sedimentary NOX-, Fe(II), dissolved organic carbon (DOC) and the nitrogen transformation functional microbial abundance were the main factors affecting nitrogen removal in the riparian zone. Fe (II) promoted NO3- attenuation through nitrate dependent ferrous oxidation process under microbial mediation, and DOC promotes NO3- attenuation through enhancing DNRA effect. The results of this study can be used for the management of the riparian zone and the prevention and control of global nitrogen pollution.

Does the aging behavior of microplastics affect the process of denitrification by the difference of copper ion adsorption?

Riparian sediment is a hot zone for denitrification that can withhold copper and microplastics (MPs) from outside. It has been proven that MPs affect denitrification and the existing forms of copper in the environment. However, the impact of copper on sediment denitrification under exposure to MPs remains unclear. This study revealed the response of sediment denitrification to copper availability under the adsorption of MPs and the complexation of MP-derived dissolved organic matter (DOM). These results showed that MP accumulation inhibited denitrification. However, aged MPs increased the activity of nitrite reductase (12.64%), nitrogen dioxide reductase (37.68%), and electron transport (28.93%) compared with pristine MPs. The aging behavior of MPs alleviated 28.18% nitrite accumulation and 16.41-118.35% nitrous oxide emissions. Thus, the aging behavior of MPs alleviated the inhibition of denitrification. Notably, we resolved the copper ion adsorption and complexation by MPs, MP-derived DOM contributed to the denitrification process, and we found that the key nitrogen removal factors were affected by K_L, K_M, and K₂. These results fill a gap in our understanding of biochemical synthesis of MPs during denitrification. Furthermore, it can be used to build a predictive understanding of the long-term effects of MPs on the sediment nitrogen cycle.

Prediction of spatial distribution characteristics of ecosystem functions based on a minimum data set of functional traits of desert plants

The relationship between plant functional traits and ecosystem function is a hot topic in current ecological research, and community-level traits based on individual plant functional traits play important roles in ecosystem function. In temperate desert ecosystems, which functional trait to use to predict ecosystem function is an important scientific question. In this study, the minimum data sets of functional traits of woody (wMDS) and herbaceous (hMDS) plants were constructed and used to predict the spatial distribution of C, N, and P cycling in ecosystems. The results showed that the wMDS included plant height, specific leaf area, leaf dry weight, leaf water content, diameter at breast height (DBH), leaf width, and leaf thickness, and the hMDS included plant height, specific leaf area, leaf fresh weight, leaf length, and leaf width. The linear regression results based on the cross-validations (FTEI_{W - L}, FTEI_{A - L}, FTEI_{W - NL}, and FTEI_{A - NL}) for the MDS and TDS (total data set) showed that the R² (coefficients of determination) for wMDS were 0.29, 0.34, 0.75, and 0.57, respectively, and those for hMDS were 0.82, 0.75, 0.76, and 0.68, respectively, proving that the MDSs can replace the TDS in predicting ecosystem function. Then, the MDSs were used to predict the C, N, and P cycling in the ecosystem. The results showed that non-linear models RF and BPNN were able to predict the spatial distributions of C, N and P cycling, and the distributions showed inconsistent patterns between different life forms under moisture restrictions. The C, N, and P cycling showed strong spatial autocorrelation and were mainly influenced by structural factors. Based on the non-linear models, the MDSs can be used to accurately predict the C, N, and P cycling, and the predicted values of woody plant functional traits visualized by regression kriging were closer to the kriging results based on raw values. This study provides a new perspective for exploring the relationship between biodiversity and ecosystem function.

Insight into the microbial nitrogen cycle in riparian soils in an agricultural region

Riparian zones are considered as an effective measure on preventing agricultural non-point source nitrogen (N) pollution. However, the mechanism underlying microbial N removal and the characteristics of N-cycle in riparian soils remain elusive. In this study, we systematically monitored the soil potential nitrification rate (PNR), denitrification potential (DP), as well as net N₂O production rate, and further used metagenomic sequencing to elucidate the mechanism underlying microbial N removal. As a whole, the riparian soil had a very strong denitrification, with the DP 3.17 times higher than the PNR and 13.82 times higher than the net N₂O production rate. This was closely related to the high soil NO₃^--N content. In different profiles, due to the influence of extensive agricultural activities, the soil DP, PNR, and net N₂O production rate near the farmland edge were relatively low. In terms of N-cycling microbial community composition, the taxa of denitrification, dissimilatory nitrate reduction, and assimilatory nitrate reduction accounted for a large proportion, all related to NO₃^--N reduction. The N-cycling microbial community in waterside zone showed obvious differences to the landside zone. The abundances of N-fixation and anammox genes were significantly higher in the waterside zone, while the abundances of nitrification (amoA&B&C) and urease genes were significantly higher in the landside zone. Furthermore, the groundwater table was an important biogeochemical hotspot in the waterside zone, the abundance of N-cycle genes near the groundwater table was at a relative higher level. In addition, compared to different soil depths, greater variation in N-cycling microbial community composition was observed between different profiles. These results reveal some characteristics of the soil microbial N-cycle in the riparian zone in an agricultural region and are helpful for restoration and management of the riparian zone.

Water and heat exchange responses to flooding and local storm events in the hyporheic zone driven by a meandering bend

The hyporheic zone, i.e. the groundwater-surface water interface within riverine/riparian ecosystems, plays a key role in water transport, energy flow and biogeochemical cycling at watershed scales. Water and heat exchange are fundamental processes regulating biogeochemical cycles in the hyporheic zones. To improve the understanding of hyporheic flow and heat transport in meandering streams, high-resolution measurements of water level and temperature, combined with a 3-D coupled model of flow and heat transport in the hyporheic zone of a meandering bend, were carried out during a summer flood season. Results show the distinct spatio-temporal variations of hyporheic water and heat exchange. Flooding events (the incoming flood water generated by the upstream rainfall) and local rainstorm events (the storm or rainfall occurring over the local study area) are major drivers for the coupled processes. Incoming flooding from the upper stream increases the hyporheic water and heat exchange in the riverbed and inner bank leading to the longer intra-meander residence times, and warms the riverbed and riverbanks due to the post-rainfall thermal recovery. Local rainstorm event increases hyporheic water and heat exchange flux both laterally and vertically and cools down the riverbed and riverbanks. The water exchange and thermal regimes in the intra-meander seems more driven by the local exchange flows, while the counterparts in the outer bank are dominated by the regional groundwater flow. The temperatures in the inner banks are 1 to 3 °C higher than those in the outer banks, indicating the better hydrological connectivity between river water and groundwater in the intra-meander. The meander apex is a hot spot for hyporheic water and heat exchange. The results highlight the close coupling among river morphology, hyporheic flow, and thermal heterogeneity in a meander system.

Groundwater Quality and Health: Making the Invisible Visible

Linking groundwater quality to health will make the invisible groundwater visible, but there are knowledge gaps to understand the linkage which requires cross-disciplinary convergent research. The substances in groundwater that are critical to health can be classified into five types according to the sources and characteristics: geogenic substances, biogenic elements, anthropogenic contaminants, emerging contaminants, and pathogens. The most intriguing questions are related to quantitative assessment of human health and ecological risks of exposure to the critical substances via natural or induced artificial groundwater discharge: What is the list of critical substances released from discharging groundwater, and what are the pathways of the receptors' exposure to the critical substances? How to quantify the flux of critical substances during groundwater discharge? What procedures can we follow to assess human health and ecological risks of groundwater discharge? Answering these questions is fundamental for humans to deal with the challenges of water security and health risks related to groundwater quality. This perspective provides recent progresses, knowledge gaps, and future trends in understanding the linkage between groundwater quality and health.

Nitrogen transformation promotes the anaerobic degradation of PAHs in water level fluctuation zone of the Three Gorges Reservoir in Yangtze River, China: Evidences derived from in-situ experiment

Polycyclic aromatic hydrocarbons (PAHs) pose a great threat to human health and ecological system safety. The interception of nitrogen is common found in the riparian zone. However, there is no evidence on how nitrogen addition affects the anaerobic degradation of PAHs in soil of the water-level-fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) in Yangtze River, China. Here, we investigated the PAHs degradation rate, the variation of key functional genes and microbial communities after nitrogen addition in soil that experienced a flooding period of water-level-fluctuation. The results revealed that the ∑16PAHs were decreased 16.19 %-36.65 % and more 3-5-rings PAHs were biodegraded with nitrogen addition in WLFZ. The most genes involved in PAHs-anaerobic degradation and denitrification were up-regulated by nitrate addition, and phyla Firmicutes, Actinobacteria and Proteobacteria were more advantages in nitrogen addition groups. The Tax4Fun based genome function analysis revealed that the microbial activity of PAHs-degradation increased with nitrate addition. The co-occurrence network analysis indicated that nitrogen addition accelerated the metabolism of nitrogen and PAHs. It is the first time to provide the direct experimental evidences that nitrogen transformation in the WLFZ soil promotes anaerobic PAHs degradation. This work is of importance to understand the effect of nitrogen intercepted in the WLFZ soil of TGR in Yangtze River, China.

Hydrological connectivity affects nitrogen migration and retention in the land‒river continuum

Land use change and excessive nitrogen (N) loading threaten the health of receiving water bodies worldwide. However, the role of hydrological connectivity in linking watershed land use, N biogeochemistry and river water quality remain unclear. In this study, we investigated 15 subwatersheds in the Jiulong River watershed (southeastern China) during a dry baseflow period in 2018, combined with 3‒year (2017-2019) nutrient monitoring in 5 subwatersheds to explore river N dynamics (dissolved nutrients, dissolved gases and functional genes) and their controlling factors at three hydrological connectivity scales, i.e., watershed, hydrologically sensitive areas (HSAs) and riparian zone. The results show that land use at HSAs (less than 20% of watershed area) and watershed scales contributed similarly to river N variation, indicating that HSAs are hotspots for transporting land N into river channels. In particular, the agricultural land was the main factor affecting river nitrate and nitrous oxide (N₂O) concentrations, while the built-up land significantly affected river ammonium and nitrite. At the riparian zone scale, soils and sediments substantially influenced river N retention processes (i.e., nitrification and denitrification). Management and protection measures targeting HSAs and riparian zones are expected to efficiently reduce river N loading and improve water quality.

Microbially mediated Fe-N coupled cycling at different hydrological regimes in riparian wetland

Although the significance of the coupled Fe- and N- cycling processes on biogeochemical transformation in riparian wetlands is well-known, the regulation associated with the changes on the microbiotas during different hydrological regimes remains unclear. This study performed field investigations on the bacterial community compositions (BCC) and specific genera associated to Fe- and N- cycling in the rhizosphere soil and sediments in a riparian wetland in Poyang lake, China. The predominant phyla Proteobacteria, Acidobacteria, and Nitrospirae from all the samples remarkably decreased after long-term continuous flooding, while Actinobacteria, Firmicutes and Bacteroidetes were enriched. For the family level, the relative abundances of iron-oxidizing bacteria (FeOB) Gallionellaceae, and N fixing bacteria Nitrospiraceae and Bradyrhizobiaceae significantly declined upon the long-term flooding and then increased with dewatering, which were consistent with the functional genes sequencing analysis. In which, the Bradyrhizobiaceae (RA 2.0 %-34.6 %) was the dominant nirS denitrifier and potential iron-reducing bacteria (FeRB), Sideroxydans lithotrophicus was one of the dominant FeOB (RA 1.7 %-23 %), which was also identified to be the nirS dentrifier (RA 0.2 %-4.3 %). The absolute quantification of the functional genes levels including nirS, nirK, FeRB (Geobacter spp.) showed their significant increases by 3-7 times upon desiccation compared to that under post-CF. The PCA and RDA results indicated the linkage between redox changes of N and Fe during inundation mediated by FeRB, NOB, and FeOB, which were closely related to hydrochemical indices NO₃^-, Fe²⁺ and SO₄^2-. These evidences all implied the likely occurrence of nitrate reduction coupled to Fe(II) oxidation (NRFeOx) under oligotrophic conditions, which was potentially facilitated by metabolizers consisting of highly correlated Bradyrhizobiaceae and Sideroxydans (rho = 0.86, p < 0.01). These findings provide an interpretation of the biological reactions in the microbially mediated NRFeOx processes driven by hydrological change, which could assist the mechanistic understanding of the global biogeochemical cycles of iron and nitrogen in riparian wetlands.

Hydrobiogechemical interactions in the hyporheic zone of a sulfate-impacted, freshwater stream and riparian wetland ecosystem

Coupled abiotic and biotic processes in the hyporheic zone, where surface water and groundwater mix, play a critical role in the biogeochemical cycling of carbon, nutrients, and trace elements in streams and wetlands. Dynamic hydrologic conditions and anthropogenic pollution can impact redox gradients and biogeochemical response, although few studies examine the resulting hydrobiogeochemical interactions generated within the hyporheic zone. This study examines the effect of hyporheic flux dynamics and anthropogenic sulfate loading on the biogeochemistry of a riparian wetland and stream system. The hydrologic gradient as well as sediment, surface water, and porewater geochemistry chemistry was characterized at multiple points throughout the 2017 spring-summer-fall season at a sulfate-impacted stream flanked by wetlands in northern Minnesota. Results show that organic-rich sediments largely buffer the geochemical responses to brief or low magnitude changes in hydrologic gradient, but sustained or higher magnitude fluxes may variably alter the redox regime and, ultimately, the environmental geochemistry. This has implications for a changing climate that is expected to dramatically alter the hydrological cycle. Further, increased sulfate loading and dissolved or adsorbed ferric iron complexes in the hyporheic zone may induce a cryptic sulfur cycle linked to iron and carbon cycling, as indicated by the abundance of intermediate valence sulfur compounds (e.g., polysulfide, elemental sulfur, thiosulfate) throughout the anoxic wetland and stream-channel sediment column. The observed deviation from a classical redox tower coupled with potential changes in hydraulic gradient in these organic-rich wetland and stream hyporheic zones has implications for nutrient, trace element, and greenhouse gas fluxes into surface water and groundwater, ultimately influencing water quality and global climate.

Regulating autogenic vegetation in the riparian zone reduces carbon emissions: Evidence from a microcosm study

Riparian zones have been found to be hot spots of greenhouse gas (GHG) production and have attracted increasing attention in recent decades. The occurrence of autogenic vegetation in riparian zones is prevalent, but little information is available concerning the influence of the occurrence and decomposition of this vegetation on carbon mitigation. We conducted a 220-day (110 days for the dry season and 110 days for the flooded season) microcosm experiment to study the mitigation and transformation of carbon regulated by the vegetation. The results revealed that there was a carbon dioxide (CO₂) flux in the treatment with vegetation, and that without vegetation harvesting (835.58 mg/m²/h) was close to that with vegetation harvesting (796.22 mg/m²/h) under the simulated dry season conditions, but it was significantly higher than that without vegetation seedlings (411.55 mg/m²/h). After being flooded, the decomposition of the vegetation residues increased the total organic carbon (TOC) content of the sediment, the dissolved organic carbon (DOC) concentration of the water, and the dissolved CO₂ and methane (CH₄) contents of the sediment. This effect was reversed by harvesting the vegetation biomass. During the flooded season, the CO₂ flux reached 222.95 mg/m²/h in the vegetation seeded treatment, but it decreased to -53.71 mg/m²/h when the vegetation biomass was harvested before being submerged. This was due to the decrease in the substrate available for CO₂ production, the altered microorganism communities, and the decrease in the abundance of carbon metabolizing related enzymes. As a result, vegetation harvesting reduced the net carbon emissions by 48 % compared to that without vegetation regulation during the 220-day incubation period. The results of this study are significant to implementing measures to reduce GHG emissions from the riparian zone.

Reclaimed Water Reuse for Groundwater Recharge: A Review of Hot Spots and Hot Moments in the Hyporheic Zone

As an alternative resource, reclaimed water is rich in the various nutrients and organic matter that may irreparably endanger groundwater quality through the recharging process. During groundwater recharge with reclaimed water, hot spots and hot moments (HSHMs) in the hyporheic zones, located at the groundwater–reclaimed water interface, play vital roles in cycling and processing energy, carbon, and nutrients, drawing increasing concern in the fields of biogeochemistry, environmental chemistry, and pollution treatment and prevention engineering. This paper aims to review these recent advances and the current state of knowledge of HSHMs in the hyporheic zone with regard to groundwater recharge using reclaimed water, including the generation mechanisms, temporal and spatial characteristics, influencing factors, and identification indicators and methods of HSHMs in the materials cycle. Finally, the development prospects of HSHMs are discussed. It is hoped that this review will lead to a clearer understanding of the processes controlling water flow and pollutant flux, and that further management and control of HSHMs can be achieved, resulting in the development of a more accurate and safer approach to groundwater recharge with reclaimed water.

Dissimilatory nitrate reduction in urban lake ecosystems: A comparison study between closed and open lakes in Chengdu, China

Urban lake ecosystems play important roles in nitrogen cycling, yet the occurrence, contribution and mechanism of nitrate reduction in urban closed and open lakes (UCL and UOL) remain unclear. On November - December of 2020, the potential rates of denitrification (DEN), anammox (ANA), and dissimilatory nitrate reduction to ammonium (DNRA) were quantified using slurries incubations in six urban lakes of Chengdu, China. The environmental variables, genes abundance (nirS, hzsB and nrfA), bacterial 16S rRNA gene were also measured. UOL had higher water ammonium (NH4+), nitrate (NO3-) and nitrite (NO2-), and sediment NH4+, NO3-, total organic carbon (TOC) and ferrous iron (Fe2+) content than UCL. The potential rates of DEN and anammox in UOL were 2.16- and 3.45-times more than in UCL, respectively. Conversely, the DNRA rate in UCL was 1.20-fold higher than UOL. Higher nirS and hzsB abundance were found in UOL, while higher nrfA abundance occurred in UCL. High-throughput sequencing analysis showed that the relative abundance of DEN bacteria was higher in UOL (2.59-12.30%) than in UCL (1.96-6.70%) at the genus level, while the relative abundance of DNRA bacteria was higher in UCL (2.02-4.19%) than in UOL (1.14-2.31%). The difference in the relative abundance of anammox bacteria at the genus level was not significant. Multiple linear regression showed that the physicochemical properties and nitrate reduction bacteria together control the potential nitrate reduction rates. Since a higher nitrogen retention capability appears in UCL, according to the nitrogen retention index (NRI), further management should be focused on urban closed lakes to avoid the potential for eutrophication.

Submerged macrophytes regulate diurnal nitrous oxide emissions from a shallow eutrophic lake: A case study of Lake Wuliangsuhai in the temperate arid region of China

Submerged macrophytes can increase oxygen concentrations of water and promote diel oxygen fluctuations, and this phenomenon is hypothesized to play a vital role in regulating nitrous oxide (N₂O) emissions from eutrophic lakes. However, the effects of submerged macrophytes on N₂O emissions in shallow eutrophic lakes remain poorly investigated. In this study, Lake Wuliangsuhai, a typical shallow eutrophic lake, was investigated to study the role of submerged macrophytes in regulating N₂O emissions. We measured the N₂O fluxes and related parameters through continual 72-h in situ diel monitoring in two sampling sections that covered dense submerged macrophyte areas and open water. In this study, the dissolved oxygen (DO) concentration of the water in the submerged macrophyte area exhibited significant diurnal variations, with significantly higher water oxygen concentrations than the open water area during the daytime. The N₂O fluxes of Lake Wuliangsuhai ranged from 0.01 to 0.24 μmol m^-2 h^-1, with an average value of 0.11 μmol m^-2 h^-1. Moreover, significant diel variations in the N₂O flux and net N₂O production were observed in the submerged macrophyte areas, where the maximum N₂O flux occurred at midday. The molar ratios of NH₄⁺-N to oxygen (N/O ratio) of the water were responsible for the diel variations in the N₂O production in the lake. However, the high oxygen concentration of the water was the major regulator of the N₂O flux of Lake Wuliangsuhai. Therefore, submerged macrophyte restoration is significant not only for water quality improvement in shallow eutrophic lakes but also for N₂O emission mitigation by increasing the DO concentration of the water.

Antibiotic-accelerated cyanobacterial growth and aquatic community succession towards the formation of cyanobacterial bloom in eutrophic lake water

Antibiotics can stimulate the growth of model cyanobacterial species under pure culture conditions, but their influence on cyanobacterial blooms in natural aquatic ecosystems remains unclear. In this study, three commonly detected antibiotics (sulfamethoxazole, tetracycline, and ciprofloxacin) and their ternary mixture were proved to selectively stimulate (p < 0.05) the growth and photosynthetic activity of cyanobacteria in an aquatic microcosm at an environmentally relevant exposure dose of 300 ng/L under both oligotrophic and eutrophic conditions. Under the eutrophic condition, cyanobacteria reached a bloom density of 1.61 × 10⁶ cells/mL in 15 days without antibiotics, while the cyanobacteria exposed to tetracycline, sulfamethoxazole, ciprofloxacin, and their ternary mixture exceeded this bloom density within only 10, 8, 7, and 6 days, respectively. Principal coordinate analysis indicated that the antibiotic contaminants accelerated the prokaryotic community succession towards the formation of a cyanobacterial bloom by promoting the dominance of Microcystis, Synechococcus, and Oscillatoria under the eutrophic condition. After 15 days of culture, the antibiotic exposure increased the density of cyanobacteria by 1.38-2.31-fold and 2.28-3.94-fold under eutrophic and oligotrophic conditions, respectively. Antibiotic exposure generated higher stimulatory effects on cyanobacterial growth under the oligotrophic condition, but the antibiotic(s)-treated cyanobacteria did not form a bloom due to nutrient limitation. Redundancy analysis indicated that the three target antibiotics and their ternary mixture affected the prokaryotic community structure in a similar manner, while tetracycline showed some differences compared to sulfamethoxazole, ciprofloxacin, and the ternary antibiotic mixture with regard to the regulation of the eukaryotic community structure. This study demonstrates that antibiotic contaminants accelerate the formation of cyanobacterial blooms in eutrophic lake water and provides insights into the ecological effects of antibiotics on aquatic microbial communities.

Experimental analysis of natural organic matter controls on nitrogen reduction during bank storage

Particulate organic carbon (POC) significantly influences nitrogen processes in riparian zones. However, the role of different types of natural organic matter (e.g., plant leaves and mud deposits) in nitrate reduction during the hydraulically driven mixing between rivers and groundwater within bank storage (BS) is not well known. Here, we used laboratory columns filled with 30 cm of riparian soil and buried with POC of varying quantity and quality (leaf litter, mud deposit, a mixture of both and a sediment control) on the soil surface in the column to investigate the effects of POC addition on nitrate reduction in low-permeable media. Pore waters were collected and measured periodically to compare the physicochemical differences among these treatments over time during scenarios of downward and upward flow. The leaf litter treatment had a larger amount of POC and higher reactivity, driving pore water to be in suboxic conditions with lower Eh and pH values. Nitrate reduction occurred immediately with downward surface water, and NO₃^- was removed in the POC buried layer. Due to the low POC content and low reactivity of the carbon source in the mud deposit, denitrification primarily occurred in the deeper sediment during the downwelling stage, as well as when groundwater returned to the POC buried layer with longer travel times. Both POC quantity and POC quality had strong effects on nitrate reduction. Our results suggested that the leaf litter treatment was preferential for nitrate reduction over the mud deposit treatment, with a higher NO₃^- reduction rate and less NH₄⁺ accumulation during the complete BS process.