Nitrate reduction in the reed rhizosphere of a riparian zone: From functional genes to activity and contribution

Dissimilatory nitrate reduction pathways drive high nitrous oxide emissions and nitrogen retention under the flash drought in the largest freshwater lake in China

Flash drought (FD) events induced by climate change may disrupt the normal hydrological regimes of floodplain lakes and affect the plant-microbe mediated dissimilatory nitrate reduction (DNR), i.e., denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA), thus having important consequences for nitrous oxide (N2O) emissions and nitrogen (N) retention. However, the responses of the DNR pathways in the floodplain lake to the record-breaking FD in 2022 in Yangtze River of China, as well as the underlying microbial mechanisms and feedbacks to climate change remain poorly understood. Here, we collected exposed sediments and Carex cinerascens-associated soils in the littoral wetlands of Poyang Lake during 2022 FD and the dry seasons prior to and after this event. The potential DNR rates and the synergistic metabolism of microbial guilds involved in DNR were investigated using 15N isotope pairing technique, high-throughput and metagenomic sequencing. We found that the in situ N2O fluxes in the littoral wetlands were highest during the flash drought, especially in the exposed sediments. The potential DNRA rates were highest under flash drought conditions, and DNRA dominated the DNR for both exposed sediments (80.4 %) and Carex cinerascens-associated soils (57.5 %). Nutrients (i.e., N and P) and DNRA bacterial communities played a key role in producing the extremely high N2O fluxes from exposed sediments, which could be explained by the synergistic metabolism of DNRA bacteria and denitrifiers through the exchange of the key intermediates in DNR. Therefore, the climate change-induced flash drought promoted greater nitrous oxide emissions and N retention in the littoral wetlands of Poyang Lake, producing a greater flux of greenhouse gas emissions and elevating the risk of lake eutrophication. Hence, flash droughts reinforce a positive feedback between climate change and nitrous oxide emission from these aquatic ecosystems.

Groundwater nitrate response to hydrogeological conditions and socioeconomic load in an agriculture dominated area

Nitrate pollution is widespread environmental concern in most shallow groundwater systems. This study conducts a comprehensive investigation of shallow groundwater, deep groundwater, and surface water in a region of the Chinese Loess Plateau. Nitrate pollution in this area is severe with more than half of the shallow groundwater samples exceeding the limit of nitrate for drinking water (50 mg/L). Temporal variation of nitrate reveals a continued increase of nitrate in shallow groundwater, although the fertilizer use has been reduced, recently. Spatially, deep groundwater nitrate is much lower than that in shallow groundwater, supporting the anthropogenic origin of nitrate from surface. The intrinsic vulnerability index indicates less susceptibility of loess aquifer to pollution in comparison to alluvial aquifer. However, high levels of nitrate are observed in both alluvial and loess aquifers. The pollution risk assessment combined with anthropogenic loads explains the occurrence of nitrate more precisely. Agricultural inputs and release of sewage-effluents are the major contributions of nitrate from hydrogeochemical evidence. The high nitrate in loess aquifer, which is supposed to be less susceptible to contamination, indicates anthropogenic loads to be a non-negligible factor. Control of nitrate pollution in loess area is long-standing issue and will require sustained monitoring.

Electrocatalytic nitrate reduction using iron single atoms for sustainable ammonium supplies to increase rice yield

Increasing food production and ensuring drinking water safety have always been a focus of attention, especially for people in underdeveloped regions of the world. Traditional excessive fertilizer applications have increased crop yield but also caused groundwater nitrate pollution. Agricultural irrigating water is an important reservoir for nitrogen (N) (e.g., nitrate) accumulation after fertilization. Ammonium (NH4+-N) is a more readily absorbed N form by rice than nitrate (NO3--N). In this study, we proposed a strategy using iron single-atom catalysts (Fe-SAC) to selectively reduce NO3--N to NH4+-N from the real paddy field irrigating water to provide sustainable NH4+-N supplies for rice uptakes, thereby highlighting decreasing N fertilizer applications and mitigating NO3--N pollution. Then, we constructed a solar-energy-driven electrochemical reactor for NO3--N reduction, with the Fe single atom as the core catalyst, and achieved an average NH4+-N selectivity of 80.2 ± 2.6% with no additional energy input. Sustainable NH4+-N supplies resulted in a 30.4 % increase in the 100-grain weight of the cultivated rice and a 50% decrease of fertilizer application than those of the fertilization group in the pot experiment, which were one of the best values ever reported. Furthermore, the 15N isotope tracing results indicated a N use efficiency (NUE) from 15NO3--N of 71.2 ± 3.2%. Sustainable NH4+-N supplies played a key role in promoting rice root development which contributed to the high NUE. Our study shares unique insights in increasing grain yield, reducing fertilizer applications, and preventing nitrate leaching into groundwater.

Does invasive submerged macrophyte diversity affect dissimilatory nitrate reduction processes in sediments with varying microplastics?

Nitrogen removal is essential for restoring eutrophic lakes. Microorganisms and aquatic plants in lakes are both crucial for removing excess nitrogen. However, microplastic (MP) pollution and the invasion of exotic aquatic plants have become increasingly serious in lake ecosystems due to human activity and plant-dominant traits. This field mesocosm study explored how the diversity of invasive submerged macrophytes affects denitrification (DNF), anammox (ANA), and dissimilatory nitrate reduction to ammonium (DNRA) in lake sediments with varying MPs. Results showed that invasive macrophytes suppressed DNF rates, but DNRA and ANA were less sensitive than DNF to the diversity of invasive species. Sediment MPs increased the biomass of invasive species more than native species, but did not affect microbial processes. The effects of MPs on nitrate dissimilatory reduction were process-specific. MPs increased DNF rates and the competitive advantage of DNF over DNRA by changing the sediment environment. The decoupling of DNF and ANA was also observed, with increased DNF rates and decreased ANA rates. The study findings suggested new insights into how the invasion of exotic submerged macrophytes affects the sediment nitrogen cycle complex environments.

Influence of crop residue-induced Fe-DOC complexation on nitrate reduction in paddy soil

Although complexation between dissolved organic matter (DOM) and ubiquitous Fe is known to have a major influence on electron transferring ability in redoximorphic soil, it was unclear whether and how this complexation affected nitrate reduction and N2O productivity. The nitrate reduction of paddy soil in the presence of crop residues returning under flooding conditions was explored in this study. The rate of nitrate reduction in control soil was 0.0677 d-1, while it improved 1.99 times in treatment soil with Chinese milk vetch (CMV) straw returning. During a 28-day incubation period, N2O productivity decreased 0.08-0.91 ppb in CMV soil and 0.43-0.50 ppb in rice straw soil compared with control. The presence of crop residue increased DOC content and Fe (III) reduction rate, which aided in the formation of Fe (II)-DOC complexation. Meanwhile, the addition of CMV increased the content of DOC by 5.14-78.77 mg/kg and HCl extractable Fe (II) by 35.12-1221.03 mg/kg. Crop residues returning to soil increased the relative abundance of iron reductive and electroactive genera, as well as denitrifying genera with more copies of denitrification genes (Archangiaceae, Gemmatimonadaceae, and Burkholderiaceae). The synergistic effect of Fe-DOC complexation, electroactive genera, and denitrifying genera contributed to up-regulated expression of napA and narG (5.84 × 106 and 3.39 × 107 copies increased in the CMV soil compared to the control) numbers and equally accelerated reduction of nitrate to nitrite, while further nitrite reduction was primarily attributed to the abiotic reaction by Fe (II). From a bio-electrochemical point of view, this work provided new insight into the nitrate reduction of paddy soil impacted by Fe-DOC complexation.

Nitrite-dependent anaerobic methane oxidation (N-DAMO) in global aquatic environments: A review

The vast majority of processes in the carbon and nitrogen cycles are driven by microorganisms. The nitrite-dependent anaerobic oxidation of methane (N-DAMO) process links carbon and nitrogen cycles, offering a novel approach for the simultaneous reduction of methane emissions and nitrite pollution. However, there is currently no comprehensive summary of the current status of the N-DAMO process in natural aquatic environments. Therefore, our study aims to fill this knowledge gap by conducting a comprehensive review of the global research trends in N-DAMO processes in various aquatic environments (excluding artificial bioreactors). Our review mainly focused on molecular identification, global study sites, and their interactions with other elemental cycling processes. Furthermore, we performed a data integration analysis to unveil the effects of key environmental factors on the abundance of N-DAMO bacteria and the rate of N-DAMO process. By combining the findings from the literature review and data integration analysis, we proposed future research perspectives on N-DAMO processes in global aquatic environments. Our overarching goal is to advance the understanding of the N-DAMO process and its role in synergistically reducing carbon emissions and removing nitrogen. By doing so, we aim to make a significant contribution to the timely achievement of China's carbon peak and carbon neutrality targets.

Rhizosphere-Associated Anammox Bacterial Diversity and Abundance of Nitrogen Cycle-Related Functional Genes of Emergent Macrophytes in Eutrophic Wetlands

Rhizospheric microbial community of emergent macrophytes plays an important role in nitrogen removal, especially in the eutrophic wetlands. The objective of this study was to identify the differences in anammox bacterial community composition among different emergent macrophytes and investigate revealed the the main factors affecting on the composition, diversity, and abundance of anammox bacterial community. Results showed that the composition, diversity, and abundance of the anammox community were significantly different between the vegetated sediments of three emergent macrophytes and unvegetated sediment. The composition of the anammox bacterial community was different in the vegetated sediments of different emergent macrophytes. Also, the abundance of nitrogen cycle-related functional genes in the vegetated sediments was found to be higher than that in the unvegetated sediment. Canonical correspondence analysis (CCA) and structural equation models analysis (SEM) showed that salinity and pH were the main environmental factors influencing the composition and diversity of the anammox bacterial community and NO2--N indirectly affected anammox bacterial community diversity by affecting TOC. nirK-type denitrifying bacteria abundance had significant effects on the bacterial community composition, diversity, and abundance of anammox bacteria. The community composition of anammox bacteria varies with emergent macrophyte species. The rhizosphere of emergent macrophytes provides a favorable environment and promotes the growth of nitrogen cycling-related microorganisms that likely accelerate nitrogen removal in eutrophic wetlands.

Denitrification dominates dissimilatory nitrate reduction across global natural ecosystems

Denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) are three competing processes of microbial nitrate reduction that determine the degree of ecosystem nitrogen (N) loss versus recycling. However, the global patterns and drivers of relative contributions of these N cycling processes to soil or sediment nitrate reduction remain unknown, limiting our understanding of the global N balance and management. Here, we compiled a global dataset of 1570 observations from a wide range of terrestrial and aquatic ecosystems. We found that denitrification contributed up to 66.1% of total nitrate reduction globally, being significantly greater in estuarine and coastal ecosystems. Anammox and DNRA could account for 12.7% and 21.2% of total nitrate reduction, respectively. The contribution of denitrification to nitrate reduction increased with longitude, while the contribution of anammox and DNRA decreased. The local environmental factors controlling the relative contributions of the three N cycling processes to nitrate reduction included the concentrations of soil organic carbon, ammonium, nitrate, and ferrous iron. Our results underline the dominant role of denitrification over anammox and DNRA in ecosystem nitrate transformation, which is crucial to improving the current global soil N cycle model and achieving sustainable N management.

Microbial community structure and functional prediction in five full-scale industrial park wastewater treatment plants

The development of industrial parks has become an important global trend contributing significantly to economic and industrial growth. However, this growth comes at a cost, as the treatment of multisource industrial wastewater generated in these parks can be difficult owing to its complex composition. Microorganisms play a critical role in pollutant removal during industrial park wastewater treatment. Therefore, our study focused on the microbial communities in five full-scale industrial park wastewater treatment plants (WWTPs) with similar treatment processes and capacities. The results showed that denitrifying bacteria were dominant in almost every process section of all the plants, with heterotrophic denitrification being the main pathway. Moreover, autotrophic sulfur denitrification and methane oxidation denitrification may contribute to total nitrogen (TN) removal. In plants where the influent had low levels of COD and TN, dominant bacteria included oligotrophic microorganisms like Prosthecobacter (2.88 % ~ 10.02 %) and hgcI_clade (2.05 % ~ 9.49 %). Heavy metal metabolizing microorganisms, such as Norank_f__PHOS-HE36 (3.96 % ~ 5.36 %) and Sediminibacterium (1.86 % ~ 5.34 %), were prevalent in oxidation ditch and secondary settling tanks in certain plants. Functional Annotation of Prokaryotic Taxa (FAPROTAX) revealed that microbial communities in the regulation and hydrolysis tanks exhibited higher potential activity in the nitrogen (N) and sulfur (S) cycles than those in the oxidation ditch. Sulfate/sulfite reduction was common in most plants, whereas the potential occurrence of sulfide compounds and thiosulfate oxidation tended to be higher in plants with a relatively high sulfate concentration and low COD content in their influent. Our study provides a new understanding of the microbial community in full-scale industrial park WWTPs and highlights the critical role of microorganisms in the treatment of industrial wastewater.

Salinity change induces distinct climate feedbacks of nitrogen removal in saline lakes

Current estimations of nitrogen biogeochemical cycling and N2O emissions in global lakes as well as predictions of their future changes are overrepresented by freshwater datasets, while less consideration is given to widespread saline lakes with different salinity (representing salinization or desalinization). Here, we show that N2O production by denitrification is the main process of reactive nitrogen (Nr, the general abbreviations of NH4+-N, NO2--N and NO3--N) removal in hypersaline lake sediments (e.g. Lake Chaka). The integration of our field measurements and literature data shows that in response to natural salinity decrease, potential Nr removal increases while N2O production decreases. Furthermore, denitrification-induced N2 production exhibits higher salinity sensitivity than denitrification-induced N2O production, suggesting that the contribution of N2O to Nr removal decreases with decreasing salinity. This field-investigation-based salinity response model of Nr removal indicates that under global climate change, saline lakes in the process of salinization or desalination may have distinct Nr removal and climate feedback effects: salinized lakes tend to generate a positive climate feedback, while desalinated lakes show a negative feedback. Therefore, salinity change should be considered as an important factor in assessing future trend of N2O emissions from lakes under climate change.

Role of Lake Aquatic-Terrestrial Ecotones in the Ecological Restoration of Eutrophic Water Bodies

Freshwater lake eutrophication is a global concern causing adverse effects on aquatic ecosystems. The degradation of lake aquatic-terrestrial ecotones, which are the transitional zones between terrestrial and water ecosystems, contributes to eutrophication. These ecotones play vital roles in nutrient cycling, runoff control, biodiversity conservation, and habitat provision. In the past three decades, the research on lake aquatic-terrestrial ecotones has focused on techniques for managing contaminants and runoff purification. This paper reviews the recent studies on the restoration ability of eutrophic water bodies in lake aquatic-terrestrial ecotones in recent years regarding three aspects: the establishment, restoration mechanism, and improvement of restoration function. In addition, ecological factors such as lakeshore height, water level, surface runoff, shallow groundwater level, and rainfall intensity have impacts on the restoration capacity of lake aquatic-terrestrial ecotones.

Patterns and drivers of anaerobic nitrogen transformations in sediments of thermokarst lakes

Significant attention has been given to the way in which the soil nitrogen (N) cycle responds to permafrost thaw in recent years, yet little is known about anaerobic N transformations in thermokarst lakes, which account for more than one-third of thermokarst landforms across permafrost regions. Based on the N isotope dilution and tracing technique, combined with qPCR and high-throughput sequencing, we presented large-scale measurements of anaerobic N transformations of sediments across 30 thermokarst lakes over the Tibetan alpine permafrost region. Our results showed that gross N mineralization, ammonium immobilization, and dissimilatory nitrate reduction rates in thermokarst lakes were higher in the eastern part of our study area than in the west. Denitrification dominated in the dissimilatory nitrate reduction processes, being two and one orders of magnitude higher than anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA), respectively. The abundances of the dissimilatory nitrate reduction genes (nirK, nirS, hzsB, and nrfA) exhibited patterns consistent with sediment N transformation rates, while α diversity did not. The inter-lake variability in gross N mineralization and ammonium immobilization was dominantly driven by microbial biomass, while the variability in anammox and DNRA was driven by substrate supply and organic carbon content, respectively. Denitrification was jointly affected by nirS abundance and organic carbon content. Overall, the patterns and drivers of anaerobic N transformation rates detected in this study provide a new perspective on potential N release, retention, and removal upon the formation and development of thermokarst lakes.

Insights into microbial interactive mechanism regulating dissimilatory nitrate reduction processes in riparian freshwater aquaculture sediments

Aquaculture can substantially alter the accumulation and cycling of nutrients in sediments. However, the microbial mechanisms mediating sediment dissimilatory nitrate (NO₃^-) reduction in freshwater aquaculture ponds are still unclear, which rule the removal and retention of N element. In the present study, three microbial NO₃^- reduction processes in riparian aquaculture pond sediments (i.e., crab, shrimp and fish ponds) and natural freshwater sediments (i.e., lakes and rivers) were investigated via isotopic tracing and molecular analyses. The potential rates of denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) significantly increased in the aquaculture ponds compared with the natural freshwaters. Denitrification contributed 90.40-94.22% to the total NO₃^- reduction (product as N₂), followed by 2.49-5.82% of anammox (product as N₂) and 2.09-5.18% of DRNA (product as NH₄⁺). The availability of C and N substrates, rather than functional gene abundance, regulated the activities of NO₃^- reductions and microbiome composition. Microbial mechanism based on network analysis indicated that heterotrophic denitrifiers and DNRA bacteria (e.g., Bacillus, Micromonospora, Mycobacterium and Brachybacterium) determined the community structure and function for N conversions in aquaculture ponds, whereas the such microbial network in natural freshwater sediments was manipulated by autotrophic denitrifiers (e.g., Desulfuromonas, Polaromonas, Solitalea). Collectively, this study provides an in-depth exploration of microbial nitrogen removal in freshwater aquaculture areas and supports management strategies for N pollution caused by reclamation for aquaculture in riparian zones.

Molecular analysis of microbial nitrogen transformation and removal potential in the plant rhizosphere of artificial tidal wetlands across salinity gradients

This study explored the microbial nitrogen transformation and removal potential in the plant rhizosphere of seven artificial tidal wetlands under different salinity gradients (0-30‰). Molecular biological and stable isotopic analyses revealed the existence of simultaneous anammox (anaerobic ammonium oxidation), nitrification, DNRA (dissimilatory nitrate reduction to ammonium) and denitrification processes, contributing to nitrogen loss in rhizosphere soil. The microbial abundances were 2.87 × 10³-9.12 × 10⁸ (nitrogen functional genes) and 1.24 × 10⁸-8.43 × 10⁹ copies/g (16S rRNA gene), and the relative abundances of dissimilatory nitrate reduction and nitrification genera ranged from 6.75% to 24.41% and from 0.77% to 1.81%, respectively. The bacterial 16S rRNA high-throughput sequencing indicated that Bacillus, Zobellella and Paracoccus had obvious effects on nitrogen removal by heterotrophic nitrifying/aerobic denitrifying process (HN-AD), and autotrophic nitrification (Nitrosomonas, Nitrospira and Nitrospina), conventional denitrification (Bradyrhizobium, Burkholderia and Flavobacterium), anammox (Candidatus Brocadia and Candidatus Scalindua) and DNRA (Clostridium, Desulfovibrio and Photobacterium) organisms co-existed with HN-AD bacteria. The potential activities of DNRA, nitrification, anammox and denitrification were 1.23-9.23, 400.03-755.91, 3.12-35.24 and 30.51-300.04 nmolN₂·g^-1·d^-1, respectively. The denitrification process contributed to 73.59-88.65% of NO_x^- reduction, compared to 0.71-13.20% and 8.20-15.42% via DNRA and anammox, as 83.83-90.74% of N₂ production was conducted by denitrification, with the rest through anammox. Meanwhile, the nitrification pathway accounted for 95.28-99.23% of NH₄⁺ oxidation, with the rest completed by anammox bacteria. Collectively, these findings improved our understanding on global nitrogen cycles, and provided a new idea for the removal of contaminants in saline water treatment.

The difference between drainage channels and sewers in rural areas: from sewage quality to bacterial characteristics

Channels and sewers are commonly used to collect sewage during extensively rural areas. The sewage and bacterial characteristics of rural sewage collection systems can influence their operation and maintenance performance which further affect appropriate system decision. In this study, eight rural sewage collection systems (four each of channels and sewers) were applied to evaluate the sewage quality, bacterial characteristics, and their differences of two kinds of collection systems. The results indicate that significantly distinction existed between the rural sewage collection systems of channels and sewers. Sewage in channels had higher suspended solid (SS) concentration but lower sulfide concentration than that in sewers. The SS, sulfate, and chemical oxygen demand (COD) removal capacity in channels was nearly 3.5, 4.0, and 0.6 times than those in sewers. At least 14 genera and 18 species of bacteria showed significantly distinction between channels and sewers even their main phylum, genus, and species of bacteria communities was Proteobacteria (∼70.3%), Acinetobacter (∼22.3%), and Pseudomonas fragi (∼13.8%), respectively. The structural characteristics and bacterial function caused the difference between channels and sewers. Overall, this study revealed the intrinsic and essential differences of channels and sewers, providing basic and meaningful data for rural sewage collection systems decision.

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.

The Effects of Edaphic Factors on Riparian Plants in the Middle and Lower Reaches of the Hanjiang River, China

It is important to understand the interactions between soil and plant in riparian zones to ensure ecosystem function. The effects of edaphic factors on plant composition and species diversity were investigated in the middle and lower reaches of the Hanjiang River (MLHR), China. A total of 154 species of herbs were recorded, and vegetation was divided into 32 clusters according to a two-way indicator species analysis (TWINSPAN). Cynodon dactylon and Paspalum distichum were the most common clusters, accounting for 22.7% and 12.5% of total samples, respectively. Hydric and mesic species were dominant in the first and second transects, with mesophytes dominating the third and fourth transects. First transects situated were mainly composed of perennials and annuals, respectively. Perennials in second transects were present in 83% of sites, and perennials in the third and fourth transects were present in 86% of the sites. Margalef richness index (Dma), Simpson dominance diversity index (D), Shannon–Wiener diversity index (H) and Pielou evenness index (Jsw) were higher in the first transects in some sites. The redundancy analysis (RDA) results indicated that soil moisture content was the dominant factor affecting hydrophilic vegetation types, and total nitrogen (TN) and soil organic matter (SOM) were the most critical factors affecting plant life history types and species diversities in this area.

High efficient bio-denitrification of nitrate contaminated water with low ammonium and sulfate production by a sulfur/pyrite-based bioreactor

Sulfur-based autotrophic denitrification (SAD) and pyrite-based autotrophic denitrification (PAD) are important technologies that address nitrate pollution, but high sulfate production and low denitrification efficiency, respectively, limit their application in engineering. A bio-denitrification reactor with sulfur and pyrite as filler materials was studied to remove NO₃^--N from nitrate contaminated water. At an influent NO₃^--N concentration of 50 mg/L, NO₃^--N removal efficiency of the sulfur/pyrite-based bioreactor was 99.2%, producing less NH₄⁺-N and SO₄^2- than the sulfur-based bioreactor, even after long-term operation. Denitrification performance was significantly related to environmental variable, especially dissolved oxygen. Proteobacteria and Epsilonbacteraeota were the predominant phyla in the sulfur/pyrite-based bioreactor, and fewer dissimilatory nitrate reductions to ammonia process-related bacteria were enriched compared to those in the sulfur-based bioreactor. Sulfur-pyrite bio-denitrification provides an efficient alternative method for treatment of nitrate contaminated water.

Nitrate removal by alkali-resistant Pseudomonas sp. XS-18 under aerobic conditions: Performance and mechanism

To improve poor nitrate removal by microorganisms under strong alkaline conditions, a new type of aerobic nitrification-reducing bacteria was isolated in this study. Using nitrogen balance and genome information, the capacity of Pseudomonas XS-18 to remove nitrate and the mechanism of alkali tolerance were analyzed. At pH 11.0, XS-18 could remove 12.17 mg N/(L·h) nitrate. At C/N ratios of 13.0 and 25 °C, nitrite and ammonia nitrogen were barely enriched. XS-18 could reduce nitrate through dissimilation and assimilation, and 21.74% and 77.39% of nitrate was converted into cellular components and organic nitrogen, respectively. Meanwhile, functional genes (nirBD, nasAB, gdhA, glnA, and gltBD) associated with nitrogen metabolism were determined. In addition, Na⁺/H⁺ antiporters (MnhACDEFG, PhaACDEFG, NhaCD and TrkAH) and a cell surface protein (SlpA) from the XS-18 genome, as well as compatible solutes that help stabilize intracellular pH, were also characterized. XS-18 possessed significant potential in alkaline wastewater treatment.

Stimulation of N2 O emission via bacterial denitrification driven by acidification in estuarine sediments

Ocean acidification in nitrogen-enriched estuaries has raised global concerns. For decades, biotic and abiotic denitrification in estuarine sediments has been regarded as the major ways to remove reactive nitrogen, but they occur at the expense of releasing greenhouse gas nitrous oxide (N₂ O). However, how these pathways respond to acidification remains poorly understood. Here we performed a N₂ O isotopocules analysis coupled with respiration inhibition and molecular approaches to investigate the impacts of acidification on bacterial, fungal, and chemo-denitrification, as well as N₂ O emission, in estuarine sediments through a series of anoxic incubations. Results showed that acidification stimulated N₂ O release from sediments, which was mainly mediated by the activity of bacterial denitrifiers, whereas in neutral environments, N₂ O production was dominated by fungi. We also found that the contribution of chemo-denitrification to N₂ O production cannot be ignored, but was not significantly affected by acidification. The mechanistic investigation further demonstrated that acidification changed the keystone taxa of sedimentary denitrifiers from N₂ O-reducing to N₂ O-producing ones and reduced microbial electron-transfer efficiency during denitrification. These findings provide novel insights into how acidification stimulates N₂ O emission and modulates its pathways in estuarine sediments, and how it may contribute to the acceleration of global climate change in the Anthropocene.

Submerged plants alleviated the impacts of increased ammonium pollution on anammox bacteria and nirS denitrifiers in the rhizosphere

Excess nitrogen input into water bodies can cause eutrophication and affect the community structure and abundance of the nitrogen-transforming microorganisms; thus, it is essential to remove nitrogen from eutrophic water bodies. Aquatic plants can facilitate the growth of rhizosphere microorganisms. This study investigated the impact of ammonium pollution on the anammox and denitrifying bacteria in the rhizosphere of a cultivated submerged macrophyte, Potamogeton crispus (P. crispus) by adding three different concentrations of slow-release urea (0, 400, 600 mg per kg sediment) to the sediment to simulate different levels of nitrogen pollution in the lake. Results showed that the ammonium concentrations in the interstitial water under three pollution treatments were significantly different, but the nitrate concentration remained stable. The abundance of anammox 16S rRNA and nitrite reductase (nirS) gene in rhizosphere sediments exhibited no significant differences under the three pollution conditions. The increase in the nitrogen pollution levels did not significantly affect the growth of anammox bacteria and nirS denitrifying bacteria (denitrifiers). The change trend of the abundance ratio of (anammox 16S rRNA)/nirS in different nitrogen treatment groups on the same sampling date was very close, indicating that this ratio was not affected by ammonium pollution levels when P. crispus existed. The redundancy analysis showed that there was a positive correlation between the abundance of anammox 16S rRNA and nirS gene and that the abundance of these bacteria was significantly affected by the mole ratio of NH₄⁺/NO₃^-. This study reveals that submerged plants weaken the environmental changes caused by ammonia pollution in the rhizosphere, thereby avoiding strong fluctuation of anammox bacteria and nirS denitrifiers.

Phytodepuration of Nitrate Contaminated Water Using Four Different Tree Species

Water pollution by excessive amounts of nitrate (NO₃⁻) has become a global issue. Technologies to clean up nitrate-contaminated water bodies include phytoremediation. In this context, this research aimed to evaluate four tree species (Salix alba L., Populus alba L., Corylus avellana L. and Sambucus nigra L.) to remediate nitrate-contaminated waters (100 and 300 mg L⁻¹). Some physiological parameters showed that S. alba L. and P. alba L. increased particularly photosynthetic activity, chlorophyll content, dry weight, and transpired water, following the treatments with the above NO₃⁻ concentrations. Furthermore, these species were more efficient than the others studied in the phytodepuration of water contaminated by the two NO₃⁻ levels. In particular, within 15 days of treatment, S. alba L. and P. alba L. removed nitrate quantities ranging from 39 to 78%. Differently, C. avellana L. and S. nigra L. did not show particular responses regarding the physiological traits studied. Nonetheless, these species removed up to 30% of nitrate from water. In conclusion, these data provide exciting indications on the chance of using S. alba L. and P. alba L. to populate buffer strips to avoid NO₃⁻ environmental dispersion in agricultural areas.

Fundamentals and potential environmental significance of denitrifying anaerobic methane oxidizing archaea

Many properties of denitrifying anaerobic methane oxidation (DAMO) bacteria have been explored since their first discovery, while DAMO archaea have attracted less attention. Since nitrate is more abundant than nitrite not only in wastewater but also in the natural environment, in depth investigations of the nitrate-DAMO process should be conducted to determine its environmental significance in the global carbon and nitrogen cycles. This review summarizes the status of research on DAMO archaea and the catalyzed nitrate-dependent anaerobic methane oxidation, including such aspects as laboratory enrichment, environmental distribution, and metabolic mechanism. It is shown that appropriate inocula and enrichment parameters are important for the culture enrichment and thus the subsequent DAMO activity, but there are still relatively few studies on the environmental distribution and physiological metabolism of DAMO archaea. Finally, some hypotheses and directions for future research on DAMO archaea, anaerobic methanotrophic archaea, and even anaerobically metabolizing archaea are also discussed.

Highly efficient ammonium removal through nitrogen assimilation by a hydrogen-oxidizing bacterium, Ideonella sp. TH17

Ideonella sp. TH17, an autotrophic hydrogen-oxidizing bacterium (HOB), was successfully enriched and isolated from activated sludge in a domestic wastewater treatment plant (WWTP). Batch experiments were conducted to identify the cell growth and ammonium (NH₄⁺-N) removal, and to verify the pathways of nitrogen utilization under different conditions. At a representative NH₄⁺-N concentration of 100 mg/L in domestic wastewater, it was the first time that a HOB strain achieved a nearly 100% ammonium removal. More than 90% of NH₄⁺-N was assimilated to biomass nitrogen by strain TH17. Only a little of N₂ (<10% of initial NH₄⁺-N) was detected without N₂O emission in aerobic denitrification process. Autotrophic NH₄⁺-N assimilation contributed predominantly to biomass nitrogen production, supplemented by assimilatory nitrate (NO₃^--N) reduction under aerobic conditions. A total of 17 amino acids, accounting for 54.25 ± 1.98% of the dry biomass, were detected in the bacterial biomass harvested at 72 h. These results demonstrated that the newly isolated strain TH17 was capable of removing NH₄⁺-N and recovering nutrients from wastewater efficiently. A new solution was thus provided by this HOB strain for ammonium treatment in sustainable WWTPs of future.

NH4+-N/NO3--N ratio controlling nitrogen transformation accompanied with NO2--N accumulation in the oxic-anoxic transition zone

Although nitrogen (N) transformations have been widely studied under oxic or anoxic condition, few studies have been carried out to analyze the transformation accompanied with NO₂^--N accumulation. Particularly, the control of mixed N species in N-transformation remains unclear in an oxic-anoxic transition zone (OATZ), a unique and ubiquitous redox environment. To bridge the gap, in this study, OATZ microcosms were simulated by surface water and sediments of a shallow lake. The N-transformation processes and rates at different NH₄⁺-N/NO₃^--N ratios, and NO₂^--N accumulations in these processes were evaluated. N-transformation process exhibited a turning point. Simultaneous nitrification and denitrification occurred in its early stage (first 10 days, dissolved oxygen (DO) ≥ 2 mg/L) and then denitrification dominated (after 10 days, DO < 2 mg/L), which were not greatly affected by the NH₄⁺-N/NO₃^--N ratio, on the contrary, the transformation rates of NH₄⁺-N and NO₃^--N were distinctly affected. The NH₄⁺-N transformation rates were positively correlated with the NH₄⁺-N/NO₃^--N ratio. The highest NO₃^--N transformation rate was observed at an NH₄⁺-N/NO₃^--N ratio of 1:1 with organic carbon/NO₃^--N of 3.09. The NO₂^--N accumulation, which increased with the decrease in NH₄⁺-N/NO₃^--N ratio, was also controlled by organic carbon concentration and type. The peak concentration of NO₂^--N accumulation occurred only when the NO₃^--N transformation rate was particularly low. Thus, NO₂^--N accumulation may be reduced by adjusting the control parameters related to N and organic carbon sources, which enhances the theoretical insights for N-polluted aquatic ecosystem bioremediation.

Succession of functional bacteria in a denitrification desulphurisation system under mixotrophic conditions

Large-scale use of ammonia, sulphate, and nitrate in industrial manufacturing has resulted in the generation of industrial wastewater pollutants. However, approaches to eliminate such contamination have not been extensively studied. Accordingly, in this study, we investigated the succession of bacteria under different influent loadings in a mixotrophic denitrification desulphurisation system. Four expanded granular sludge bed reactors were operated simultaneously. The sulphide loading of reactor I was 1.2 kg/m³‧day, the sulphide load of reactor II was 2.4 kg/m³‧day, and the sulphide load of reactor III was 3.6 kg/m³‧day. The molar ratio of carbon versus nitrogen in the influent under each condition was fixed at 1.26:1, and the molar ratio of sulphur versus nitrogen was fixed at 5:6; each reactor was operated for 90 days. Reactor IV was a verification reactor. The three conditions were repeated, and each condition was operated for 90 days. Middle- and late-stage samples under each condition were sequenced using a high-throughput sequencer. Azoarcus, Thauera, Arcobacter, and Pseudomonas were the core genera of the denitrification desulphurisation system under mixotrophic conditions. The genus Azoarcus was a cornerstone genus of mixotrophic conditions, as demonstrated using the random forest model and correlation network analysis.

Dissimilatory nitrate/nitrite reduction to ammonium (DNRA) pathway dominates nitrate reduction processes in rhizosphere and non-rhizosphere of four fertilized farmland soil

Nitrate (NO₃^-) reduction partitioning between denitrification, anaerobic ammonium oxidation (anammox), denitrifying anaerobic methane oxidation (DAMO), and dissimilatory nitrate reduction to ammonium (DNRA), can influence the nitrogen (N) use efficiency and crop production in arid farmland. The microbial structure, function and potential rates of denitrification, anammox, DAMO and DNRA, and their respective contributions to total NO₃^- reduction were investigated in rhizosphere and non-rhizosphere soil of four typical crops in north China by functional gene amplification, high-throughput sequencing, network analysis and isotopic tracing technique. The measured denitrification and DNRA rate varied from 0.0294 to 20.769 nmol N g^-1 h^-1and 2.4125-58.682 nmol N g^-1 h^-1, respectively, based on which DNRA pathway contributed to 84.44 ± 14.40% of dissimilatory NO₃^- reduction, hence dominated NO₃^- reduction processes compared to denitrification. Anammox and DAMO were not detected. High-throughput sequencing analysis on DNRA nrfA gene, and denitrification nirS and nirK genes demonstrated that these two processes did not correlate to corresponding gene abundance or dominant genus. RDA and Pearson's correlation analysis illustrated that DNRA rate was significantly correlated with the abundance of Chthiniobacter, as well as total organic matter (TOM); denitrification rate was significantly correlated with the abundance of Lautropia, so did TOM. Network analysis showed that the genus performed DNRA was the key connector in the microbial community of dissimilatory nitrate reducers. This study simultaneously investigated the dissimilatory nitrate reduction processes in rhizosphere and non-rhizosphere soils in arid farmland, highlighting that DNRA dominated NO₃^- reduction processes against denitrification. As denitrification results in N loss, whereas DNRA contributes to N retention, the relative contributions of DNRA versus denitrification activities should be considered appropriately when assessing N transformation processes and N fertilizer management in arid farmland fields.

Hotspot of dissimilatory nitrate reduction to ammonium (DNRA) process in freshwater sediments of riparian zones

Dissimilatory nitrate reduction to ammonium (DNRA), an important intermediate process in the N-cycle, links N-compound oxidation and reduction processes. Hence, the oxic-anoxic interface would be the hotspot of the DNRA process. In freshwater ecosystems, the riparian zone is the most typical carrier of the oxic-anoxic interface. Here we report spatio-temporal evidence of a higher abundance and rate of DNRA in the riparian zone than in the open water sediments based on molecular and ¹⁵N isotopic-tracing technologies, hence signifying a hotspot for the DNRA process. These abudance and rates were significantly higher than those in open water sediments. ¹⁵N isotopic paring technology revealed that the DNRA hotspot promoted higher rates of N-compound oxidation (NO₂^-), reduction (NO₃^- and DNRA), and N₂ production (anammox and denitrification) in the riparian zone than those in open water sediment. However, high-through sequencing analysis showed that the DNRA bacteria in the riparian zone and openwater sediments were insignificantly different. Network and correlation analysis showed that the DNRA abundance and rates were significantly positively correlated with TOM, TC/NH₄⁺, and TC/NO₂^-, but not with the dominant genera (Anaeromyxobacter, Lacunisphaera, and Sorangium), which played different roles on the connection in the respective community networks. The DNRA process in the riparian zone could be driven mainly by the related environmental biogeochemical characteristics induced by anthropogenic changes, followed by microbial processes. This result provides valuable information for the management of riparian zones because anthropogenic changes in the riparian water table are expected to increase, inducing consequent changes in the reduction from NO₃^- to NH₄⁺.