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Acidification of coastal waters, synergistically driven by increasing atmospheric carbon dioxide (CO 2 ) and intensive land-derived nutrient inputs, exerts significant stresses on the biogeochemical cycles of coastal ecosystem. However, the combined effects of anthropogenic nitrogen (N) inputs and aquatic acidification on nitrification, a critical process of N cycling, remains unclear in estuarine and coastal ecosystems. Here, we showed that increased loading of ammonium (NH 4 ) in estuarine and coastal waters alleviated the inhibitory effect of acidification on nitrification rates but intensified the production of the potent greenhouse gas nitrous oxide (N + ) in estuarine and coastal waters alleviated the inhibitory effect of acidification on nitrification rates but intensified the production of the potent greenhouse gas nitrous oxide (N 2 O), thus accelerating global climate change. Metatranscriptomes and natural N 2 O isotopic signatures further suggested that the enhanced emission of N 2 O may mainly source from hydroxylamine (NH 2 ) reduction pathway of nitrifying microbes. This study elucidates how anthropogenic N inputs regulate the effects of coastal acidification on nitrification and associated N 2 - ) reduction pathway of nitrifying microbes. This study elucidates how anthropogenic N inputs regulate the effects of coastal acidification on nitrification and associated N 2 O emissions, thereby enhancing our ability to predict the feedbacks of estuarine and coastal ecosystems to climate change and human perturbations., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Largely removed from anthropogenic delivery of nitrogen (N), Antarctica has notably low levels of nitrogen. Though our understanding of biological sources of ammonia have been elucidated, the microbial drivers of nitrate (NO 3 - ) cycling in coastal Antarctica remains poorly understood. Here, we explore microbial N cycling in coastal Antarctica, unraveling the biological origin of NO 3 fixation, nitrification, and denitrification, but not for anaerobic ammonium oxidation, signifying a unique microbial N-cycling dynamic. We identify the predominance of complete ammonia oxidizing (comammox) Nitrospira, capable of performing the entire nitrification process. Their adaptive strategies to the Antarctic environment likely include synthesis of trehalose for cold stress, high substrate affinity for resource utilization, and alternate metabolic pathways for nutrient-scarce conditions. We confirm the significant role of comammox Nitrospira in the autotrophic, nitrification process via - C-DNA-based stable isotope probing. This research highlights the crucial contribution of nitrification to the N budget in coastal Antarctica, identifying comammox Nitrospira clade B as a nitrification driver.2 fixation, nitrification, and denitrification, but not for anaerobic ammonium oxidation, signifying a unique microbial N-cycling dynamic. We identify the predominance of complete ammonia oxidizing (comammox) Nitrospira, capable of performing the entire nitrification process. Their adaptive strategies to the Antarctic environment likely include synthesis of trehalose for cold stress, high substrate affinity for resource utilization, and alternate metabolic pathways for nutrient-scarce conditions. We confirm the significant role of comammox Nitrospira in the autotrophic, nitrification process via 13 C-DNA-based stable isotope probing. This research highlights the crucial contribution of nitrification to the N budget in coastal Antarctica, identifying comammox Nitrospira clade B as a nitrification driver., (© 2024. The Author(s).)

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

The coupling between nitrification and denitrification/anammox (nitrate/nitrite used in denitrification/anammox derives from nitrification) is a significant process of reactive nitrogen (N) removal that has attracted much attention. However, the dynamics of coupled nitrification-denitrification/anammox in the periodically low-oxygen estuaries and coasts remain unclear. Here, continuous-flow experiments combined with isotope tracing techniques were conducted in periodically low-oxygen areas of the Yangtze Estuary to reveal the changes in benthic sediment denitrification and anammox as well as their coupling with nitrification. Our results showed that denitrification increased but anammox decreased during low-oxygen summer. The occurrence of low oxygen also promoted coupled nitrification-denitrification but decreased coupled nitrification-anammox. These results implied that decreased dissolved oxygen in summer did not largely restrict nitrification activity, and anaerobic denitrification/anammox regulated the magnitude of coupled nitrification-denitrification/anammox rates. Denitrification (74.95-100 %) was the dominant process in total N removal, while coupled nitrification-denitrification accounted for a higher proportion (45.68-97.05 %) of denitrification, indicating that coupling between nitrification and denitrification played a dominant role in N removal. In addition to dissolved oxygen levels, carbon and N substrate availabilities were also important variables to regulate N transformations. Overall, this study advanced our knowledge of the distribution patterns and controlling factors of N removal processes and highlighted that coupled nitrification-denitrification might have a significant but neglected role in N removal from periodically low-oxygen estuaries., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

In this study, we evaluated the biotransformation mechanisms of lincomycin (LIN) and three fluoroquinolone antibiotics (FQs), ciprofloxacin (CFX), norfloxacin (NFX), and ofloxacin (OFX), which regularly enter aquatic environments through human activities, by different ammonia-oxidizing microorganisms (AOM). The organisms included a pure culture of the complete ammonia oxidizer (comammox) Nitrospira inopinata, an ammonia oxidizing archaeon (AOA) Nitrososphaera gargensis, and an ammonia-oxidizing bacterium (AOB) Nitrosomonas nitrosa Nm90. The removal of these antibiotics by the pure microbial cultures and the protein-normalized biotransformation rate constants indicated that LIN was significantly co-metabolically biotransformed by AOA and comammox, but not by AOB. CFX and NFX were significantly co-metabolized by AOA and AOB, but not by comammox. None of the tested cultures transformed OFX effectively. Generally, AOA showed the best biotransformation capability for LIN and FQs, followed by comammox and AOB. The transformation products and their related biotransformation mechanisms were also elucidated. i) The AOA performed hydroxylation, S-oxidation, and demethylation of LIN, as well as nitrosation and cleavage of the piperazine moiety of CFX and NFX; ii) the AOB utilized nitrosation to biotransform CFX and NFX; and iii) the comammox carried out hydroxylation, demethylation, and demethylthioation of LIN. Hydroxylamine, an intermediate of ammonia oxidation, chemically reacted with LIN and the selected FQs, with removals exceeding 90%. Collectively, these findings provide important fundamental insights into the roles of different ammonia oxidizers and their intermediates on LIN and FQ biotransformation in nitrifying environments including wastewater treatment systems., Competing Interests: Declaration of Competing Interest The authors declare no conflict of interest., (Copyright © 2021. Published by Elsevier Ltd.)

Coupling of nitrogen removal processes with nitrification (NR n ) are vital synergistic nitrogen elimination mechanisms in aquatic environments. However, the effects of antibiotics on NR n are not well known. In the present work, 20-day continuous-flow experiments combined with 15 N tracing techniques and quantitative PCR were performed to simulate the impact of sulfamethoxazole (SMX, a sulfonamide antibiotic) with near in situ concentration on NR n processes in sediments of Yangtze Estuary. Results showed that SMX with near in situ concentration significantly decreased NR n , NR w (uncoupling of nitrogen removal processes with nitrification) and actual nitrogen removal rates via inhibiting nitrogen transformation functional genes (AOB, narG, nirS, nosZ) and anammox 16S rRNA gene, while the coupling links between nitrification and nitrogen removal processes were not broken by the exposure. The proportion of NR n in total nitrogen removal processes decreased by approximately 10% with SMX addition, due to the different inhibition on AOB, denitrifying genes and anammox 16S rRNA gene. N 2 O production and nitrite accumulation remarkably increased with SMX addition under simultaneous nitrification and denitrification, and they strongly correlated with each other. The more severely inhibition on nirS gene (13.6-19.8%) than Nitrospira nxrB gene (0.3-8.2%) revealed that the increased nitrite accumulation with SMX addition mainly occurred in heterotrophic denitrification, suggesting that the increased N 2 O production was dominated by the heterotrophic nitrite reduction. Moreover, we estimated that the ratio of external inorganic N eliminated by actual nitrogen removal can upgrade to 6.4-7.4% under circumstances of no inhibition by SMX. This study revealed the effects of SMX with near in situ concentration on NR n processes and illustrated the microbial mechanism on functional genes level. Our results highlighted the inhibitory effects of SMX on NR n may contribute to reactive N retention and N 2 O production in estuarine and coastal ecosystems., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Nitrous oxide (N 2 O) emissions show large variability among dam reservoirs, which makes it difficult to estimate N 2 O contributions to global greenhouse gases (GHGs). Because river damming alters hydraulic residence time and water depth, the hydraulic load (i.e., the ratio of the mean water depth to the residence time) was hypothesized to control N 2 O emissions from dam reservoirs. To test this hypothesis, we investigated N 2 O fluxes and related parameters in the cascade reservoirs along the Wujiang River in Southwest China. The N 2 O fluxes showed obvious temporal and spatial variations, ranging from -7.86 to 337.22 μmol m -2 d -1 , with an average of 12.76 μmol m -2 d -1 . Nitrification was the main pathway of N 2 O production in these reservoirs, and seasonal dissolved oxygen (DO) stratification played an important role in regulating the N 2 O production. The reservoir N 2 O flux had a significant negative logarithmic relationship with the hydraulic load, suggesting its control of the N 2 O emission. This was because the hydraulic load was a prerequisite for regulating the nitrification-denitrification and the DO stratification in the dam reservoirs. This empirical relationship will help to estimate the contribution of reservoir N 2 O emissions to global GHGs.

The recently discovered complete ammonia-oxidizing (comammox) bacteria occur in various environments, including wastewater treatment plants. To better understand their role in micropollutant biotransformation in comparison with ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA), we investigated the biotransformation capability of Nitrospira inopinata (the only comammox isolate) for 17 micropollutants. Asulam, fenhexamid, mianserin, and ranitidine were biotransformed by N. inopinata , Nitrososphaera gargensis (AOA), and Nitrosomonas nitrosa Nm90 (AOB). More distinctively, carbendazim, a benzimidazole fungicide, was exclusively biotransformed by N. inopinata . The biotransformation of carbendazim only occurred when N. inopinata was supplied with ammonia but not nitrite as the energy source. The exclusive biotransformation of carbendazim by N. inopinata was likely enabled by an enhanced substrate promiscuity of its unique AMO and its much higher substrate (for ammonia) affinity compared with the other two ammonia oxidizers. One major plausible transformation product (TP) of carbendazim is a hydroxylated form at the aromatic ring, which is consistent with the function of AMO. These findings provide fundamental knowledge on the micropollutant degradation potential of a comammox bacterium to better understand the fate of micropollutants in nitrifying environments.

Complete ammonia oxidizers (comammox), as novel microbial communities, are predicted to play an important role in the nitrogen cycle. Here we reported the presence of complete nitrification in tidal sediments and examined the diversity and abundance of comammox in natural ecosystems. Metagenome and metatranscriptome of the enrichment culture from tidal sediments harbored the genes of comammox. Near-complete comammox AmoA/B/C- and Hao-like sequences showed close relationships to the known comammox (with sequence identity from 79 to 99%) rather than classical betaproteobacterial ammonia-oxidizing bacteria (β-AOB) (57 to 66%) and ammonia-oxidizing archaea (AOA) (24 to 38%). To analyze the diversity of comammox in natural environments, a new primer set targeting clade A comammox Nitrospira (COM-A) amoA genes was designed based on sequences obtained in this study and sequences from published database. In silico evaluation of the primers showed the high coverage of 89 and 100% in the COM-A amoA database. Application of the primers in six different ecosystems proved their strong availability. Community composition of COM-A suggested a relatively higher diversity than β-AOB in similar environments. Quantification results showed that COM-A amoA genes accounted for about 0.4-5.6% in total amoA genes. These results provide novel insight into our perception of the enigmatic comammox and have significant implications for profound understanding of complex nitrification process.

In this study, we evaluated the biotransformation mechanisms of lincomycin (LIN) and three fluoroquinolone antibiotics (FQs), ciprofloxacin (CFX), norfloxacin (NFX), and ofloxacin (OFX), which regularly enter aquatic environments through human activities, by different ammonia-oxidizing microorganisms (AOM). The organisms included a pure culture of the complete ammonia oxidizer (comammox) Nitrospira inopinata, an ammonia oxidizing archaeon (AOA) Nitrososphaera gargensis, and an ammonia-oxidizing bacterium (AOB) Nitrosomonas nitrosa Nm90. The removal of these antibiotics by the pure microbial cultures and the protein-normalized biotransformation rate constants indicated that LIN was significantly co-metabolically biotransformed by AOA and comammox, but not by AOB. CFX and NFX were significantly co-metabolized by AOA and AOB, but not by comammox. None of the tested cultures transformed OFX effectively. Generally, AOA showed the best biotransformation capability for LIN and FQs, followed by comammox and AOB. The transformation products and their related biotransformation mechanisms were also elucidated. i) The AOA performed hydroxylation, S-oxidation, and demethylation of LIN, as well as nitrosation and cleavage of the piperazine moiety of CFX and NFX; ii) the AOB utilized nitrosation to biotransform CFX and NFX; and iii) the comammox carried out hydroxylation, demethylation, and demethylthioation of LIN. Hydroxylamine, an intermediate of ammonia oxidation, chemically reacted with LIN and the selected FQs, with removals exceeding 90%. Collectively, these findings provide important fundamental insights into the roles of different ammonia oxidizers and their intermediates on LIN and FQ biotransformation in nitrifying environments including wastewater treatment systems.

Variations of N transformation processes, N2O release rates, and N2O emission pathways were investigated at different levels of salinity (stage 1, low salinity inhibition stage; stage 2, medium salinity promotion stage; and stage 3, high salinity promotion stage) using 15 N-18O dual-isotope labeling technique. Potential nitrification rates were reduced by saltwater incursion in stage 1, increased markedly in stage 3, and significantly inhibited potential denitrification rate under higher salinity. N2O emission significantly increased along salinity gradient in stage 3 due to the changes of potential nitrification rates. Saltwater incursion significantly increased the contribution of heterotrophic denitrification to N2O emission in stage 1. In stages 2 and 3, contributions of nitrifier denitrification and nitrification-coupled denitrification to N2O emission increased gradually, and nitrifier denitrification became the dominant pathway of N2O emission under high salinity. Changes of N transformations, N2O emission rates, and their pathways were regulated both by composition of microbial community and physicochemical properties of the sediment. Due to increased ammonification rate, organic N decomposition accelerated by saltwater can reduce the wetland N sink and may turn coastal wetland to significant reactive N source. [ABSTRACT FROM AUTHOR]

Purpose: Denitrification and anaerobic ammonia oxidation (anammox) play key roles in nitrogen (N) loss, and nitrification can supply substrates of NO2– and NO3– for denitrification and anammox. Coupled nitrification-denitrification/anammox processes are thus crucial for N removal in coastal ecosystems. This study aims to examine the spatial-temporal variations of ambient, coupled, and uncoupled N removal rates in the coastal sediments off the north East China Sea, and to clarify the controlling factors and microbial mechanisms of coupled nitrification-denitrification/anammox. Materials and methods: The rates of ambient, coupled, and uncoupled denitrification and anammox in coastal sediments off the north East China Sea were quantified using the continuous-flow experiments combined with 15N isotope pairing technique. The quantitative polymerase chain reaction method was used to determine the abundances of nitrifiers, denitrifiers, and anammox bacteria, with the functional genes of amoA and nirS, and 16S rRNA gene, respectively. Results and discussion: Ambient denitrification rates varied between 0.43 and 7.39 μmol N m−2 h−1, and ambient anammox rates ranged from 0.05 to 0.62 μmol N m−2 h−1. Coupled nitrification-denitrification was the dominant N removal pathway. The rates and coupling of N removal processes with nitrification varied distinctly between nearshore and offshore sites, which were driven by diverse environmental factors. Redundancy analysis suggested that nitrate and sulfide were important factors controlling the coupled and uncoupled N removal rates, and nitrate was proved to be the key factor influencing the ratio between coupled and uncoupled N removal via an integrated analysis. Abundances of ammonia oxidizing bacteria (AOB) correlated significantly with coupled denitrification rates and abundances of denitrifiers, suggesting the importance of AOB in coupled nitrification-denitrification. Conclusions: This study investigated the ambient, coupled, and uncoupled denitrification and anammox rates in coastal sediments off the north East China Sea. Nitrate was proved to be the critical factor influencing the ratio between coupled and uncoupled N removal, and AOB may play important role in coupled nitrification-denitrification. These results emphasized that nitrification is crucial for N removal with important implications on N loss in coastal ecosystems. [ABSTRACT FROM AUTHOR]

Estuarine and intertidal wetlands are important sites for nitrogen transformation and elimination. However, the factors controlling nitrogen removal processes remain largely uncertain in the highly dynamic environments. In this study, continuous-flow experiment combined with 15N isotope pairing technique was used to investigate in situ rates of denitrification and anaerobic ammonium oxidation (anammox) and their coupling with nitrification in intertidal wetlands of the Yangtze Estuary. The measured rates varied from below the detection limit to 152.39 µmol N/(m2·hr) for denitrification and from below the detection limit to 43.06 µmol N/(m2·hr) for anammox. The coupling links of nitrogen removal processes with nitrification were mainly dependent on nitrate, organic carbon, sulfide, dissolved oxygen and ferric iron in the estuarine and intertidal wetlands. Additionally, it was estimated that the actual nitrogen removal processes annually removed approximately 5% of the terrigenous inorganic nitrogen discharged into the Yangtze Estuary. This study gives new insights into nitrogen transformation and fate in the estuarine and intertidal wetlands. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract Coupling of denitrification and anaerobic ammonium oxidation (anammox) with nitrification is an important mechanism for controlling nitrogen removal from aquatic ecosystems. In this work, we determined ambient rates of denitrification and anammox and their coupling links to nitrification in sediments of the Yangtze Estuary along a salinity gradient, using continuous-flow experiments combined with 15N isotope pairing technique. The rates of total denitrification and coupled nitrification-denitrification ranged from 0.44 to 7.03 μmol N m−2 h−1 and from below the detection limit to 4.26 μmol N m−2 h−1, respectively. The rates of total anammox and coupled nitrification-anammox varied from 0.09 to 1.21 μmol N m−2 h−1 and from below the detection limit to 0.73 μmol N m−2 h−1, respectively. It is shown that high availability of NO 3 − in bottom water as well as ferrous iron and organic carbon in sediments was beneficial to the uncoupled nitrogen removal processes, while low NO 3 − concentration in bottom water promoted the coupled nitrification-nitrogen removal processes. Along the salinity gradient, the coupled nitrification-nitrogen removal processes were the main nitrogen removal pathway in the Yangtze Estuary, and their contribution to total nitrogen removal was affected by NO 3 − concentrations, dissolved oxygen and salinity of sampling sites. Overall, this study improves the understanding of nitrate transformation processes in the estuarine and coastal environments. Highlights • Coupled nitrification-N removal processes were first examined in the Yangtze Estuary. • The coupling of N removal processes to nitrification depended tightly on NO 3 − content. • Coupled nitrification-N removal processes were the primary pathways removing N. [ABSTRACT FROM AUTHOR]

Ammonia oxidation, the first and rate‐limiting step in nitrification, plays a critical role in the nitrogen cycle. However, the links between the dynamics of ammonia‐oxidizing communities and ecosystem processes along the estuarine salinity gradient remain uncertain. In this study, we examined the diversity, abundance, and community structure of ammonia‐oxidizing prokaryotes, and the potential nitrification rates along the Yangtze estuarine salinity gradient. Phylogenetic analysis showed that the predominant ammonia‐oxidizing bacteria (AOB) and ammonia‐oxidizing archaea (AOA) fell within the Nitrosospira and Nitrosopumilus clusters, respectively. The AOB amoA gene abundance (4.67 × 105 to 3.90 × 107 copies per gram of dry sediment) outnumbered AOA (5.14 × 104 to 8.88 × 106 copies per gram of dry sediment). The potential nitrification rates varied between 0.13 and 0.63 μg N·g−1·day−1 and related only to AOA amoA gene abundance. Salinity had significant effects on AOA amoA gene abundance, nitrification rates, and the community structure of ammonia‐oxidizing prokaryotes. Principal coordinate analysis showed that the AOB amoA gene clones derived from the middle‐ and high‐salinity regions behaved as a cohesive group, while all the low‐salinity clone libraries were grouped together. Moreover, the distribution of AOA communities showed a distinct salinity differentiation. Overall, this study improves the understanding of the dynamic shifts in ammonia‐oxidizing microorganisms in the Yangtze Estuary. Key Points: Significant shifts in ammonia‐oxidizing community structure were found along the salinity gradientBoth of AOA amoA gene abundance and potential nitrification rates were significantly and negatively correlated with salinityPotential nitrification rates were significantly related to AOA amoA gene abundance, but not with AOB [ABSTRACT FROM AUTHOR]

Silver nanoparticles (AgNPs) are the most common materials in nanotechnology-based consumer products. Due to wide application of AgNPs, their potential environmental impact is a current and highly topical focus of concern. Nitrification is one of the most susceptible processes to AgNPs in the nitrogen cycle, but effects of AgNPs on nitrification in aquatic environments are not well understood. Here, we report the AgNP impacts on nitrification and associated nitrous oxide (N2O) production in estuarine sediments. AgNPs inhibited nitrification rates, which decreased exponentially with increasing AgNP concentrations. The response of nitrifier N2O production to AgNPs exhibited low-dose (<534, 1476, 2473 μg L-1 for 10, 30, and 100 nm AgNPs, respectively) stimulation and high-dose inhibition (hormesis effect). Compared with control, N2O production could be enhanced by >100% at low doses. This result was confirmed by metatranscriptome studies showing upregulation of nitric oxide reductase (norQ) gene expression in the low-dose treatment. Isotopomer analysis revealed that hydroxylamine oxidation was the main N2O production pathway, and its contribution to N2O emission was enhanced when exposed to low-dose AgNPs. This study highlights molecular underpinnings of AgNP effects on nitrification activity and demonstrates that the release of AgNPs into the environment should be controlled because they interfere with nitrifying communities and stimulate N2O emission. [ABSTRACT FROM AUTHOR]

• Dissimilatory nitrate reduction rates increased along reclamation chronosequences. • The rates varied due to the altered soil physicochemical characteristics. • Denitrification was the dominant pathway contributing to total nitrate reduction. • N 2 O emission was stimulated significantly along reclamation chronosequences. • More reactive nitrogen was exported in converted paddy soil than in coastal wetland. Conversion of coastal wetland to paddy soil can significantly affect soil biogeochemistry cycling. However, the impacts of coastal wetland reclamation on dissimilatory nitrate reduction processes and nitrogen fate remain poorly understood. In this study, the effects of coastal wetland reclamation on the rates and related functional gene abundances of dissimilatory nitrate reduction processes, N 2 O emission, and the fate of reactive nitrogen were investigated in Chongming Island, China. Conversion of coastal wetland to paddy soil can significantly increase dissimilatory nitrate reduction rates. Denitrification was the dominant pathway contributing to the total nitrate reduction, while anammox and DNRA played an important role. Conversion of coastal wetland to paddy soil can significantly affect soil physicochemical characteristics and abundances of related functional genes, leading to the variations of nitrate reduction rates. Although the nitrate removal rates increased along the reclamation chronosequences, due to the intensive N fertilizer application and increased nitrification and ammonification rates, more reactive N can be exported in paddy soil than in wetland, and N 2 O emission was stimulated significantly along reclamation chronosequences. This study can provide valuable insights into the mechanisms of reclamation influences on soil N recycling. [ABSTRACT FROM AUTHOR]

Wetland ecosystems are hotspots for nitrogen cycling, while the community dynamics of nitrification-driving microbes include ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), complete ammonia oxidizer (comammox) Nitrospira, and nitrite-oxidizing bacteria were not well-studied. In this study, we investigated the nitrification process in typical inland and coastal wetlands in northern China and found a high diversity of ammonia oxidizers. To simulate the freezing-thawing state under natural environment, the soil samples were first frozen at −20 °C for two months, and then gradually thawed at 4 °C. Further microcosm incubation combined with 13C-DNA-stable isotope probing analyses showed that after the freeze–thaw cycle, only the activities of AOA and comammox Nitrospira clade B recovered. The results indicated that these two groups of nitrifiers were more freeze–thaw resistant, which is in line with literature showing that AOA and comammox Nitrospira clade B were relatively more abundant than their counterparts in low-temperature environments. We tested, for the first time, the cold-shock resilience of nitrifying microbes in wetland soils and spotted candidates driving nitrification under fluctuating temperatures. [Display omitted] • Wetland ecosystems are hotspots for nitrogen cycling. • Nitrification process in inland and coastal wetlands in northern China is studied. • Ammonia-oxidizing archaea & comammox Nitrospira clade B are active ammonia oxidizers. • These two groups of nitrifiers are more adapted to low-temperature environments. [ABSTRACT FROM AUTHOR]

Autotrophic ammonia oxidation, the first and rate limiting step of the nitrification process, is catalyzed by three phylogenetically different groups of microorganisms: ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA) and the recently discovered complete ammonia oxidizers (Comammox) that as single organism oxidizes ammonia to nitrate via nitrite. Studies have shown that all sorts of ammonia-oxidizing microorganisms could produce nitrous oxide (N2O), biologically or via abiotic reactions during ammonia oxidation. Here we conducted a detailed study regarding the N2O production by ammonia oxidizers under various conditions. Results showed that among the three tested ammonia oxidizers, Comammox Nitropsira released the highest amount of N2O, suggesting that Comammox might be a quite overlooked contributor to the gross N2O production in terrestrial environment. Unexpectedly, the tested AOB strain produced the least N2O, which could be explained by the absence of genes for "nitrifier denitrification" in its genome. Results collectively indicated a more dynamic and complex mechanism of ammonia-oxidizer mediated N2O production than previously reported. [ABSTRACT FROM AUTHOR]