Dual roles of dissolved organic nitrogen in groundwater nitrogen cycling: Nitrate precursor and denitrification promoter

Sources and fate of nitrate in the unsaturated zone in an alluvial-lacustrine plain

Nitrate pollution in terrestrial and aquatic ecosystems in global agricultural areas poses an environmental concern. However, there is limited understanding of hydrogeological controls on the behavior of nitrogen compounds in unsaturated zones. Here, Self-Organizing Map and multiple isotopes approaches (δ15N-NO3-, δ18O-NO3-, and δ15N-NH4+) were used to investigate the sources, transport and transformation of N-species in the unsaturated zone in an alluvial-lacustrine plain, southeast China. The results revealed significant spatial heterogeneity in soil texture and physicochemical properties with vertically four soil geochemical and N-species zones (high NO₃⁻, high Fe(Ⅲ) and Mn, low ionic, and high NH₄⁺ contents), dominated by agricultural input, soil minerals and redox conditions. Nitrate in the unsaturated zone primarily originated from fertilizers and soil nitrogen. Excess nitrogen fertilizers infiltrated into the soil, where mineralization, nitrification, and dissimilatory nitrate reduction to ammonium (DNRA) acted as key mechanisms for nitrogen transformation. The change in the depositional environment from the plain to the lakeshore area led to nitrification gradual decrease and DNRA significant increase. Consequently, a conceptual model of reactive transport of N-species, influenced by hydrogeologic conditions and biogeochemical processes, was proposed. This study provides a new insight into the nitrate behaviors in unsaturated zone and contributes to groundwater nitrogen management strategies.

Reactive transport of different dissolved organic nitrogen components in an unconfined aquifer

Dissolved organic nitrogen (DON) is often an overlooked form of nitrogen that can leach from the soil into aquifers. The reactive transport and dispersion of DON in aquifers can contribute to regional nitrogen contamination. The current body of research has primarily focused on the vertical leaching process of DON through the vadose zone. However, these studies have largely ignored the broader reactive transport of DON within aquifers under the influence of groundwater flow. In this study, we investigate the reactive transport of DON under groundwater flow conditions. Utilizing molecular biological technologies, we aim to reveal DON's intrinsic role in the nitrogen cycle within aquifers. Our findings reveal that urea exhibits greater mobility compared to amino acids and proteins. The transport of amino acids and proteins reduces the NO3--N concentrations (44.6 % and 89.6 %) compared to the blank control, while urea leads to the accumulation of NO3--N in groundwater (10.1 %). Amino acid and protein columns show higher relative abundances of Pseudomonas (10.1 % and 7.3 %) and Thermomonas (3.9 % and 5.1 %) with denitrification functions, facilitating denitrification in groundwater. Conversely, the presence of urea increases the relative abundances of Nitrosomonadaceae and Nitrophilus (0.33 % and 0.67 %), posing a potential NO3--N contamination risk. Biotransformation has the greatest effect on protein transport (19.6 %), while adsorption mainly influences amino acid transport (12.4 %). The study provides fundamental insights into the reactive transport of different DON components in aquifers, which holds important implications for regional groundwater environment protection.

Synchronous production of bioethanol and short-chain fatty acids associated with microbial mechanisms via the short-term cultivation of waste molasses inoculated with Aspergillus oryzae

This study proposes a novel approach for the concurrent production of short-chain fatty acids (SCFAs) and bioethanol, during a 96 h cultivation of waste molasses (with direct air exposure) inoculated with Aspergillus oryzae using vermiculite as a carrier. Results showed that fungal-bacterial symbiotes were formed by enriching acidogens for the bioconversion of SCFAs, such as Enterococcus, Bacillus and Pseudomonas and bioethanol producers, like Klebsiella, Candida tropicalis, Aspergillus oryzae and Barnettozyma californica. The Aspergillus oryzae was found to play various diverse roles during the short-term cultivation process, secreting hydrolase (i.e. α-amylase) for the hydrolysis of waste molasses and contributing to the formation of fungal-bacterial symbiotes. Furthermore, the mechanisms of fungal-bacterial interaction related to simultaneous generation of SCFAs and bioethanol were investigated. Consequently, the cultivation liquid obtained could feasibly be used as a low-cost carbon source for enhancing total nitrogen (TN) removal in the process of low C/N ratio wastewater treatment.

Deciphering denitrification drivers in a high‑nitrogen estuary: Insights from stable isotope analysis and microbial molecular techniques

Coastal estuaries are increasingly impacted by anthropogenic nitrogen inputs, disrupting nitrogen cycling and posing significant threats to ecosystem health. This study investigates nitrogen sources and transformation processes in the Jiulong River Estuary (JRE), a highly eutrophic subtropical estuary in Southeast China. By analyzing and comparing samples from groundwater, surface water, and sediment, this study reveals distinct nitrogen transformation dynamics across interconnected environmental compartments. A comprehensive framework integrating stable isotope analysis, sediment incubation experiments, and microbial molecular techniques was employed to characterize nitrogen dynamics both regionally and at the sediment-water interface within diverse wetland types. Manure and sewage were identified as the primary nitrogen sources. Salinity emerged as a key regulator of nitrogen transformations, with freshwater wetlands exhibiting the highest denitrification potential, followed by mudflats, aquaculture ponds, and mangroves. Abiotic factors, including hydrological conditions and wetland types, were found to predominantly drive nitrogen transformations, while biotic factors, such as microbial community composition and functional gene abundances, played a secondary but interconnected role under the influence of abiotic drivers. These findings offer valuable insights into nitrogen cycling in estuarine ecosystems and propose a robust framework for mitigating nitrogen pollution and managing eutrophication in coastal regions.

Metabolic coupling of aerobic methane oxidation and short-cut nitrification and denitrification for anaerobic effluent treatment in photo-sequencing batch biofilm reactor

This study explored the use of algae to supply oxygen in situ as an alternative to mechanical aeration for anaerobic effluent treatment in a photo-sequencing batch biofilm reactor (PSBBR). By establishing alternating aerobic (dissolved oxygen (DO) > 2 mg /L)/anoxic conditions (<0.5 mg-DO/L) through a 6-h off/6-h on biogas sparging cycle and continuous illumination (1500-3000 lux), the PSBBR achieved a significant ammonia removal rate of 15-25 mg N L-1d-1. This system demonstrated robust partial nitrification and nitrite reduction activities, coupled with aerobic methane oxidation. Metagenomic analysis revealed the enrichment of key microbial groups, including Leptolyngbyaceae, Methylocystis, Nitrosomonas and Hyphomicrobium. The key functional genes of methane (mmo, mdh, gfa, frm and fdh) and nitrogen (amo, hao, narGHI, and napAB) metabolisms were identified, while notably lacking nitrite oxidation genes. In conclusion, this study provides a promising post-treatment approach for anaerobic effluent through integrating biogas utilization with efficient nitrogen removal.

Using surface runoff to reveal the mechanisms of landscape patterns driving on various forms of nitrogen in non-point source pollution

Non-point source (NPS) pollution directly threatens river water quality, constrains sustainable economic development, and poses hazards to human health. Comprehension of the impact factors on NPS pollution is essential for scientific river water quality management. Despite the landscape pattern being considered to have a significant impact on NPS pollution, the driving mechanism of landscape patterns on NPS pollution remains unclear. Therefore, this study coupled multi-models including the Soil and Water Assessment Tool (SWAT), Random Forest, and Partial Least Squares Structural Equation Modeling (PLS-SEM) to construct the connection between landscape patterns, NPS pollution, and surface runoff. The results suggested that increased runoff during the wet season enhances the link between landscape patterns and NPS pollution, and the explained NPS pollution variation by landscape pattern increased from 59.6 % (dry season) to 84.9 % (wet season). Furthermore, from the impact pathways, we find that the sink landscape pattern can significantly and indirectly influence NPS pollution by regulating surface runoff during the wet season (0.301*). Meanwhile, the sink and source landscape patterns significantly and directly impact NPS pollution during different seasons. Moreover, we further find that the percentage of paddy land use (Pad_PLAND) and grassland patch density (Gra_PD) metrics can significantly predict the dissolved total nitrogen (DTN) and nitrate nitrogen (NO3--N) variation. Thus, controlling the runoff migration process by guiding the rational evolution of watershed landscape patterns is an important development direction for watershed NPS pollution management.

Potential effects of Cu2+ stress on nitrogen removal performance, microbial characteristics, and metabolism pathways of biofilm reactor

Sequencing batch biofilm reactors (SBBR) were utilized to investigate the impact of Cu2+ on nitrogen (N) removal and microbial characteristics. The result indicated that the low concentration of Cu2+ (0.5 mg L-1) facilitated the removal of ammonia nitrogen (NH4+-N), total nitrogen (TN), nitrate nitrogen (NO3--N), and chemical oxygen demand (COD). In comparison to the average effluent concentration of the control group, the average effluent concentrations of NH4+-N, NO3--N, COD, and TN were found to decrease by 40.53%, 17.02%, 10.73%, and 15.86%, respectively. Conversely, the high concentration of Cu2+ (5 mg L-1) resulted in an increase of 94.27%, 55.47%, 22.22%, and 14.23% in the aforementioned parameters, compared to the control group. Low concentrations of Cu2+ increased the abundance of nitrifying bacteria (Rhodanobacter, unclassified-o-Sacharimonadales), denitrifying bacteria (Thermomonas, Comamonas), denitrification-associated genes (hao, nosZ, norC, nffA, nirB, nick, and nifD), and heavy-metal-resistant genes related to Cu2+ (pcoB, cutM, cutC, pcoA, copZ) to promote nitrification and denitrification. Conversely, high concentration Cu2+ hindered the interspecies relationship among denitrifying bacteria genera, nitrifying bacteria genera, and other genera, reducing denitrification and nitrification efficiency. Cu2+ involved in the N and tricarboxylic acid (TCA) cycles, as evidenced by changes in the abundance of key enzymes, such as (EC:1.7.99.1), (EC:1.7.2.4), and (EC:1.1.1.42), which initially increased and then decreased with varying concentrations of Cu2+. Conversely, the abundance of EC1.7.2.1, associated with the accumulation of nitrite nitrogen (NO2--N), gradually declined. These findings provided insights into the impact of Cu2+ on biological N removal.

The impact of dissolved organic nitrogen (DON) retention in the vadose zone on nitrogen leaching losses

Leaching of dissolved organic nitrogen (DON) is a significant pathway for nitrogen (N) loss in agricultural ecosystems. The excessive application of N for enhanceing agricultural productivity often results in the leaching of N into groundwater. Yet not well understood, the extent of retention in the vadose zone has critical implications for risk management and remediation strategies. This study aims to advance simulation techniques for modelling the transport process of reactive DON within a heterogeneous vadose zone. Through a combination of laboratory experiments and numerical simulations, the study firstly examines the extent of DON retention in the vadose zone and quantitatively analyse groundwater contamination risk from this kind of accumulation. Our findings indicate that heavy N fertilizer application and high-intensity rainfall events led to elevated contents of DON in the vadose zone and increased DON leaching fluxes into groundwater. Besides, intensifier rainfall reduced the N storage more quickly in scenarios devoid of DON application with higher mineralization rate, while DON slowly mineralized to other forms, largely accumulated in the top layer and migrated deeper with intensifier rainfall after input of urea. In our scenarios, DON accounted for a substantial portion (33-68%) of the total dissolved nitrogen (TDN) leaching fluxes, with exogenous DON content contributing significantly (25-85%) to the overall DON leaching into the aquifer. These results underscore the need for effective strategies to mitigate groundwater contamination risks associated with agricultural N use.

Spectral and molecular insights into the characteristics of dissolved organic matter in nitrate-contaminated groundwater

The characteristics of dissolved organic matter (DOM) serve as indicators of nitrate pollution in groundwater. However, the specific DOM components associated with nitrate in groundwater systems remain unclear. In this study, dual isotopes of nitrate, three-dimensional Excitation emission matrices (EEMs) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were utilized to uncover the sources of nitrate and their associations with DOM characteristics. The predominant nitrate in the targeted aquifer was derived from soil organic nitrogen (mean 46.0%) and manure &sewage (mean 34.3%). The DOM in nitrate-contaminated groundwater (nitrate-nitrogen >20 mg/L) exhibited evident exogenous characteristics, with a bioavailable content 2.58 times greater than that of uncontaminated groundwater. Regarding the molecular characteristics, DOM molecules characterized by CHO + 3N, featuring lower molecular weights and H/C ratios, indicated potential for mineralization, while CHONS formulas indicated the exogenous features, providing the potential for accurate traceability. These findings provided insights at the molecular level into the characterization of DOM in nitrate-contaminated groundwater and offer scientific guidance for decision-making regarding the remediation of groundwater nitrate pollution.

Bioremediation of trichloroethylene-contaminated groundwater using green carbon-releasing substrate with pH control capability

In this research, a sustainable substrate, termed green and long-lasting substrate (GLS), featuring a blend of emulsified substrate (ES) and modified rice husk ash (m-RHA) was devised. The primary objective was to facilitate the bioremediation of groundwater contaminated with trichloroethylene (TCE) using innovative GLS for slow carbon release and pH control. The GLS was concocted by homogenizing a mixture of soybean oil, surfactants (Simple Green™ and soya lecithin), and m-RHA, ensuring a gradual release of carbon sources. The hydrothermal synthesis was applied for the production of m-RHA production. The analyses demonstrate that m-RHA were uniform sphere-shape granules with diameters in micro-scale ranges. Results from the microcosm study show that approximately 83% of TCE could be removed (initial TCE concentration = 7.6 mg/L) with GLS supplement after 60 days of operation. Compared to other substrates without RHA addition, higher TCE removal efficiency was obtained, and higher Dehalococcoides sp. (DHC) population and hydA gene (hydrogen-producing gene) copy number were also detected in microcosms with GLS addition. Higher hydrogen concentrations enhanced the DHC growth, which corresponded to the increased DHC populations. The addition of the GLS could provide alkalinity at the initial stage to neutralize the acidified groundwater caused by the produced organic acids after substrate biodegradation, which was advantageous to DHC growth and TCE dechlorination. The addition of m-RHA reached an increased TCE removal efficiency, which was due to the fact that the m-RHA had the zeolite-like structure with a higher surface area and lower granular diameter, and thus, it resulted in a more effective initial adsorption effect. Therefore, a significant amount of TCE could be adsorbed onto the surface of m-RHA, which caused a rapid TCE removal through adsorption. The carbon substrates released from m-RHA could then enhance the subsequent dechlorination. The developed GLS is an environmentally-friendly and green substrate.

Higher stochasticity in comammox Nitrospira community assembly in upland soils than the adjacent paddy soils at a regional scale

Understanding the assembly mechanisms of microbial communities, particularly comammox Nitrospira, in agroecosystems is crucial for sustainable agriculture. However, the large-scale distribution and assembly processes of comammox Nitrospira in agricultural soils remain largely elusive. We investigated comammox Nitrospira abundance, community structure, and assembly processes in 16 paired upland peanuts and water-logged paddy soils in south China. Higher abundance, richness, and network complexity of comammox Nitrospira were observed in upland soils than in paddy soils, indicating a preference for upland soils over paddy soils among comammox Nitrospira taxa in agricultural environments. Clade A.2.1 and clade A.1 were the predominant comammox Nitrospira taxa in upland and paddy soils, respectively. Soil pH was the most crucial factor shaping comammox Nitrospira community structure. Stochastic processes were found to predominantly drive comammox Nitrospira community assembly in both upland and paddy soils, with deterministic processes playing a more important role in paddy soils than in upland soils. Overall, our findings demonstrate the higher stochasticity of comammox Nitrospira in upland soils than in the adjacent paddy soils, which may have implications for autotrophic nitrification in acidic agricultural soils.

Nanoscale zero-valent iron alleviate antibiotic resistance risk during managed aquifer recharge (MAR) by regulating denitrifying bacterial network

The frequent occurrence of antibiotics in reclaimed water is concerning, in the case of managed aquifer recharge (MAR), it inevitably hinders further water purification and accelerates the evolutionary resistance in indigenous bacteria. In this study, we constructed two column reactors and nanoscale zero-valent iron (nZVI) amendment was applied for its effects on water quality variation, microbial community succession, and antibiotic resistance genes (ARGs) dissemination, deciphered the underlying mechanism of resistance risk reduction. Results showed that nZVI was oxidized to iron oxides in the sediment column, and total effluent iron concentration was within permissible limits. nZVI enhanced NO3--N removal by 15.5% through enriching denitrifying bacteria and genes, whereas made no effects on oxacillin (OXA) removal. In addition, nZVI exhibited a pivotal impact on ARGs and plasmids decreasing. Network analysis elucidated that the diversity and richness of ARG host declined with nZVI amendment. Denitrifying bacteria play a key role in suppressing horizontal gene transfer (HGT). The underlying mechanisms of inhibited HGT included the downregulated SOS response, the inhibited Type-Ⅳ secretion system and the weakened driving force. This study afforded vital insights into ARG spread control, providing a reference for future applications of nZVI in MAR.

Combined effects of aquifer heterogeneity and subsurface dam on nitrate contamination in coastal aquifers

Subsurface dams are effective for seawater intrusion mitigation, yet they can cause upstream nitrate accumulation. This research examines the interplay between subsurface dam construction and aquifer layering on nitrate pollution in coastal settings, employing numerical models to simulate density-driven flow and reactive transport. The study reveals that while subsurface dams are adept at curbing seawater intrusion, they inadvertently broaden the nitrate accumulation zone, especially when a low-K layer is present. Heterogeneous aquifers see more pronounced nitrate accumulation from subsurface dams. This effect is pronounced as it influences dissolved organic carbon dynamics, with a notable retreat inland correlating with the expansion of the nitrate pollution plume. A critical finding is that controlling seawater intrusion via dam height adjustment within the Effective Damming Region effectively reduces nitrate levels and bolsters freshwater output. However, exceeding the critical threshold-where the dam surpasses the low-K layer's bottom-results in a substantial shift in nitrate concentration, underscoring the need for precise dam height calibration to avoid aggravating nitrate pollution. This study's innovative contribution lies in its quantification of the nuanced effects of subsurface dams in stratified aquifers, providing an empirical basis for dam design that considers the layered complexities of coastal aquifers. The insights offer a valuable framework for managing nitrate contamination, thus informing sustainable coastal groundwater management and protection strategies.

Review of soil dissolved organic nitrogen cycling: Implication for groundwater nitrogen contamination

Dissolved organic nitrogen (DON) in groundwater is derived from soil DON transformation and migration processes, which has been considered an emerging nitrogen (N) pollutant. However, due to the limitations of the analytical methods and the complexity of the involved transformation process, the role of DON in soil N cycling remains unclear. Therefore, this review aims to critically examine previous related studies on DON and highlight the knowledge gaps related to DON transformations and molecular characteristics in soils. In addition, the DON distributions and key transformation processes, as well as their influencing factors, were summarized. About 60% of DON components have not been determined due to the limited analytical techniques and methodologies. The depolymerization process of polymers into DON is the rate-limiting step of N mineralization. Furthermore, DON leaching amounts accounted for 7-1500% of soil nitrate (NO3--N) amounts, becoming the dominate pathway of N loss. Further studies are required to provide accurate information on DON compositions and transformation mechanisms, as well as their influencing factors, in soils. The suggested studies can provide further insights into the role of DON in soil N cycling, thereby controlling effectively groundwater N contamination.

Unveiling the biogeochemical mechanism of nitrate in the vadose zone-groundwater system: Insights from integrated microbiology, isotope techniques, and hydrogeochemistry

Clarifying the biogeochemical mechanism of nitrate (NO3-) in the vadose zone-groundwater system, particularly in agricultural contexts, is crucial for mitigating groundwater NO3- pollution. However, comprehensive studies on the impacts of changes in chemical indicators and microbial communities on NO3- are still lacking. This paper aims to address this gap by employing hydrogeochemistry, stable isotopes, and microbial techniques to assess the NO3- biogeochemical processes in the vadose zone-groundwater system. The findings suggested that NO3- in upper soil layers was primarily influenced by plant root absorption, assimilation, and nitrification processes. The oxygen contents gradually decreased with the nitrification process, resulting in the occurrence of the denitrification. However, denitrification predominantly occurred in the 60-80 cm soil layer in the study area. The limited thickness of the denitrification layer results in less NO3- consumption, leading to increased NO3- leaching into groundwater. Hydrochemical and isotopic characteristics further indicated that groundwater NO3- concentrations were mainly controlled by nitrification, followed by denitrification and mixing processes. The 16S rRNA sequencing analysis revealed great influences of soil sampling depths and groundwater NO3- concentrations on the microbial community structure. Additionally, the PICRUSt2-based prediction results demonstrated a stronger potential for dissimilatory reduction of NO3- to ammonium (DNRA) in both soil and groundwater compared to the other processes, potentially due to the widespread presence of the nrfH functional genes. However, the chemical indicators and isotopes used in this study did not support the occurrence of DNRA process in the vadose zone-groundwater system. This finding highlights the importance of an integrated approach combining microbiological, isotopic, and hydrogeochemical data to comprehensive understanding biogeochemical processes. The study developed a conceptual model elucidating the NO3- biogeochemical processes in the vadose zone-groundwater system within an agricultural area, contributing to enhanced comprehension and advancement of sustainable management practices for groundwater nitrogen.

Enhancing nitrate attenuation in groundwater via selectively applying surface agricultural practices: A novel and sustainable strategy for non-point source pollution mitigation

Non-point nitrate pollution in groundwater has been accelerated by agricultural development, but sustainable nitrogen removal is a challenge because of its wide distribution and negative side effects. Surface agricultural practices (SAPs), which are demonstrably effective in driving the downward infiltration of dissolved organic carbon (DOC), have not been well explored for their potential to enhance nitrate attenuation in groundwater. Therefore, a combination of soil column and groundwater incubation experiments was performed to investigate the carbon and nitrogen responses to different SAPs (manure fertilization, lucerne planting, and straw return). The soil column experiment showed that SAPs promoted DOC and reduced nitrate leaching into groundwater, and straw treatment witnessed the highest DOC leaching flux (252.71 g m^-2 yr^-1) and lowest nitrate leaching flux (9.51 g m^-2 yr^-1). The groundwater incubation experiment showed that leachates from the straw treatment displayed the best denitrification-enhancement performance, with the highest NO₃^--N reduction efficiency (92.93%) and rate (1.627 mg/day), N₂ selectivity (99.78%), and net nitrogen removal (0.09 mg). Furthermore, Fourier transform ion cyclotron resonance mass spectrometry confirmed that CHOS molecules with lower double bond equivalents (0-5) and larger carbon numbers (10-15) were more accessible to denitrifiers. This study provides a new path for the sustainable control of non-point source nitrate pollution.

The role of anthraquinone-2-sulfonate on intra/extracellular electron transfer of anaerobic nitrate reduction

To improve the electron (e^-) transfer efficiency, exogenous redox mediators (RMs) were usually employed to enhance the denitrification efficiency due to the electron shuttling. Previous studies were mainly focused on how to improve the extracellular electron transfer (EET) by exogenous RMs. However, the intracellular electron transfer (IET), another crucial e^- transfer pathway, of biological denitrification was scarcely reported, especially for the relationship between the denitrification and IET. In this study, Coenzyme Q, Complexes I, II and III were determined as the core components in the IET chain of denitrification by using four specific respiration chain inhibitors (RCIs). Anthraquinone-2-sulfonate (AQS) partially recovered the IET of denitrification from NO₃^--N to N₂ gas when the RCIs were added. Specifically, the generations of N₂ gas were improved by 9.68%-18.25% in the experiments with RCIs and AQS, comparing to that with RCIs. nrfA gene was not detected by reverse transcription-polymerase chain reaction, suggesting that Klebsiella oxytoca strain could not conduct dissimilatory nitrate reduction to ammonium. Nitrate assimilation was considered as the main NH₄⁺-N formation way of K. oxytoca strain. The two e^- transfer pathways of denitrification were constructed and the roles of AQS on the IET and EET of denitrification were specifically discussed. The results of this study provided a better understanding of the e^- transfer pathways of denitrification, and suggested a potential practical use of exogenous RM on bio-treatment of nitrate-containing wastewater.

Bidirectional potential effects of DON transformation in vadose zones on groundwater nitrate contamination: Different contributions to nitrification and denitrification

The main cause of groundwater nitrate contamination is the continual downward migration of dissolved nitrogen (N) in vadose zone with leachate. In recent years it has been found that dissolved organic N (DON) rise to forefront due to its great migration capacity and environmental effects. However, it remains unknown how the transformation behaviors of DONs with different properties in vadose zone profile may impact N forms distribution and groundwater nitrate contamination. To address the issue, we conducted a series of 60-day microcosm incubation experiments to investigate the effects of various DONs transformation behaviors on the distribution of N forms, microbial communities, and functional genes. The results revealed that urea and amino acids mineralized immediately after substrates addition. By contrast, amino sugars and proteins caused less dissolved N throughout entire incubation period. The transformation behaviors could substantially alter the microbial communities. Moreover, we discovered that amino sugars remarkably increased the absolute abundances of denitrification function genes. These results delineated that DONs with unique characteristics (such as amino sugar) promoted different N geochemical processes in distinct ways: different contributions to nitrification and denitrification. This can provide new insights for nitrate non-point source pollution control in groundwater.

Seawater intrusion alters nitrogen cycling patterns through hydrodynamic behavior and biochemical reactions: Based on Bayesian isotope mixing model and microbial functional network

Seawater intrusion is a global coastal environmental issue of great concern and significantly impacts the regional biogeochemical environment and material cycles, including nitrogen cycling. To reveal the mechanism of seawater intrusion altering nitrogen cycling patterns through hydrodynamic behavior and biochemical reactions, the Bayesian mixing model (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) and 16S rDNA gene amplicon sequencing are used to establish nitrogen cycling pathways and microbial functional network. The results show that the nitrate in the coastal groundwater is from manure and septic waste (M&S, over 44 %), soil organic nitrogen (SON, over 20 %), and nitrogen fertilizer (FN, over 16 %). The hydrological interaction has promoted the coupling between material cycling and microbial community in the coastal groundwater systems. Among them, precipitation infiltration has caused the gradual decrease of specific microbes along the flow direction, such as Lactobacillus, Acinetobacter, Bifidobacterium, etc. And seawater intrusion has caused the mutations of specific microbes (Planktomarina, Clade_Ia, Wenyingzhuangia, Glaciecola, etc.) and convergence of microbial community at the salt-freshwater interface in the aquifer. In the coastal intruded aquifer systems, the nitrogen cycling pattern can be divided into oxidation and reduction processes. The oxidation process involves the enhancement of nitrification while the weakening of denitrification and anammox with the increase of aquifer depth. The reduction process consists of the enhancement of denitrification and anammox while the erosion of nitrification and ammonification with increased seawater intrusion. In addition, seawater intrusion can mitigate nitrate contamination by promoting denitrification and anammox in coastal areas.

Microencapsulated reactor for simultaneous removal of calcium, fluoride and phenol using microbially induced calcium precipitation: Mechanism and functional characterization

Fluoride ions (F^-) and phenol in groundwater have become a great hurdle to the pursuit of a healthy drinking water source. This study established a microencapsulated immobilization reactor with Aquabacterium sp. CZ3 for the simultaneous removal of nitrate (NO₃^--N), calcium (Ca²⁺), F^-, and phenol from groundwater with 100%, 67.84%, 88.67%, and 100% removal efficiencies, respectively. The three-dimensional mesh structure of microcapsules facilitated the transport and metabolism of substances, while their synergistic effect with bacteria promoted the removal of contaminants. F^- was removed by co-precipitation to generate Ca₅(PO₄)₃F and CaF₂ and adsorption. On one hand, the phenol toxicity promoted the production of extracellular polymers and improved the tolerance of bacteria; on the other hand, the degradation of phenol provided a carbon source for bacteria and promoted the denitrification. The development of microencapsulated immobilized reactor provided a clear mechanism for phenol and F^- removal under the microbially induced calcium precipitation (MICP) technique, while providing a valuable solution for the treatment of complex groundwater resources.

Mainstream partial Anammox for improving nitrogen removal from municipal wastewater after organic recovery via magnetic separation

Total nitrogen (TN) removal from municipal wastewater after organic recovery is challenging because of the low ratio of chemical oxygen demand (COD) to TN. Anaerobic ammonium oxidation (Anammox) is promising because it has no organic requirement, but its performance in treating effluents following COD captured remains unclear. This study used mainstream partial Anammox to remove nitrogen from effluent following magnetic separation within a continuous-flow anoxic-oxic reactor. Compared with traditional nitrification and denitrification, partial Anammox increased TN removal efficiency by 15.0% and contributed 23.6% of TN removal. Quantitative polymerase chain reaction revealed that the copy number of the Anammox gene (hzsB) increased substantially, while those of the nitrite oxidation (nxrA) and denitrification (narG and nirS) genes decreased. High-throughput sequencing identified Candidatus Brocadia as the dominant genus of anaerobic ammonium-oxidizing bacteria. These findings demonstrate the effectiveness of mainstream partial Anammox for treating COD-captured effluents and its potential in municipal wastewater treatment.

Analysis of urea nitrogen leaching under high-intensity rainfall using HYDRUS-1D

The leaching of nitrogen in farmland could lead to groundwater nitrogen pollution, while precipitation is the driving factor. Nevertheless, the influences of extreme precipitation on soil organic nitrogen leaching, such as urea, is not clear. In this study, we used soil column experimental data to construct a nitrogen migration and transformation model. The constructed model was applied to unsaturated zones in farmlands in the Dagu River Basin of China. Three rainfall scenarios, namely rainstorms, heavy rainstorms, and extreme rainstorms, were simulated to analyze the variation in urea nitrogen leaching to groundwater. As the rainfall intensity increased, the quantity of urea nitrogen leaching increased from 0.04% to 18.09%. The cumulative urea nitrogen leaching flux is related to the initial soil moisture content. The time interval between fertilization inversely influenced the urea nitrogen leaching flux. Applying urea-based fertilizers shortly before high-intensity rainfall significantly increases the soil-crop system urea leaching loss and decreases crop nitrogen utilization. These findings imply that urea leaching under extreme rainstorms should not be ignored, and based on weather forecasting, fertilizer application in farmland should be avoided before high-intensity rainfall.