The impact of commonly-used livestock antibiotics on soil nitrogen transformations under varying redox conditions is largely unknown. Soil column incubations were conducted using three livestock antibiotics (monensin, lincomycin and sulfamethazine) to better understand the fate of the antibiotics, their effect on nitrogen transformation, and their impact on soil microbial communities under aerobic, anoxic, and denitrifying conditions. While monensin was not recovered in the effluent, lincomycin and sulfamethazine concentrations decreased slightly during transport through the columns. Sorption, and to a limited extent degradation, are likely to be the primary processes leading to antibiotic attenuation during leaching. Antibiotics also affected microbial respiration and clearly impacted nitrogen transformation. The occurrence of the three antibiotics as a mixture, as well as the occurrence of lincomycin alone affected, by inhibiting any nitrite reduction, the denitrification process. Discontinuing antibiotics additions restored microbial denitrification. Metagenomic analysis indicated that Proteobacteria, Bacteroidetes, Actinobacteria, and Chloroflexi were the predominant phyla observed throughout the study. Results suggested that episodic occurrence of antibiotics led to a temporal change in microbial community composition in the upper portion of the columns while only transient changes occurred in the lower portion. Thus, the occurrence of high concentrations of veterinary antibiotic residues could impact nitrogen cycling in soils receiving wastewater runoff or manure applications with potential longer-term microbial community changes possible at higher antibiotic concentrations.

Microaerobic and hypoxic methane oxidation coupled to denitrification (MAME-D and HYME-D) occur in stabilized landfills with leachate recirculation when biological denitrification is limited by lack of organics. To evaluate nitrate denitrification efficiency and culture MAME-D/HYME-D involved bacteria, a leach bed bioreactor semi-continuous experiment was conducted for 60 days in 5 runs, under nitrate concentrations ranging of 20 mg/L–55 mg/L, wherein 5% sterile leachate was added during runs 4 and 5. Although the HYME-D system demonstrated high denitrification efficiency (74.93%) and nitrate removal rate reached 2.62 mmol N/(L⋅d), the MAME-D system exhibited a denitrification efficiency of almost 100% and nitrate removal rate of 4.37 mmol N/(L⋅d). The addition of sterile leachate increased the nitrate removal rate in both systems, but caused the decrease of methane consumption in HYME-D. A stable isotope batch experiment was carried out to investigate the metabolic products by monitoring the 13CO2 and 15N2O production. The production of organic intermediates such as citrate, lactic acid, acetate, and propionic acid were also observed, which exhibited a higher yield in HYME-D. Variations in the microbial communities were analyzed during the semi-continuous experiment. MAME-D was mainly conducted by the association of type Ⅰ methanotroph Methylomonas and the methylotrophic denitrifier Methylotenera. Methane fermentation processed by Methylomonas under hypoxic conditions produced more complex organic intermediates and increased the diversity of related heterotrophic denitrifiers. The addition of sterile real leachate, resulting in increase of COD/N, influenced the microbial community of HYME-D system significantly.

Dissimilatory nitrate reduction to ammonia (DNRA) is an important nitrate reduction pathway in lake sediments; however, little is known about the biotic factors driving the DNRA potential rates and contributions to the fate of nitrate. This study reports the first investigation of DNRA potential rates and contributions in lake sediments linked to DNRA community structures. The results of ¹⁵N isotope-tracing incubation experiments showed that 12 lakes had distinct DNRA potentials, which could be clustered into 2 groups, one with higher DNRA potentials (rates varied from 2.7 to 5.0 nmol N g⁻¹ h⁻¹ and contributions varied from 27.5% to 35.4%) and another with lower potentials (rates varied from 0.6 to 2.3 nmol N g⁻¹ h⁻¹ and contributions varied from 8.1% to 22.8%). Sediment C/N and the abundance of the nrfA gene were the key abiotic and biotic factors accounting for the distinct DNRA potential rates, respectively. A high-throughput sequencing analysis of the nrfA gene revealed that the sediment C/N could also affect the DNRA potential rates by altering the ecological patterns of the DNRA community composition. In addition, the interactions between the DNRA community and the denitrifying community were found to be obviously different in the two groups. In the higher DNRA potential group, the DNRA community mainly interacted with heterotrophic denitrifiers, while in the lower DNRA potential group, both heterotrophic and sulfur-driven autotrophic denitrifiers might cooperate with the DNRA community. The present study highlighted the role of the sulfur-driven nitrate reduction pathway in C-limited sediments, which has always been overlooked in freshwater environments, and gave new insights into the molecular mechanism influencing the fate of nitrate.

The conversion of natural saltmarshes to shrimp aquaculture ponds can potentially influence the biogeochemical cycling of nutrients in coastal wetlands, but its impact on the dynamics of sediment dissimilatory nitrate (NO3−) reduction remains poorly understood. In this study, three sediment NO3− reduction processes including denitrification (DNF), anaerobic ammonium oxidation (ANAMMOX), and dissimilatory NO3− reduction to ammonium (DNRA) were examined simultaneously in a natural saltmarsh and two shrimp culture ponds (5- and 18-year-old) in July and November, using nitrogen (N) isotope-tracing experiments. Our results showed that sediment potential DNF, ANAMMOX and DNRA rates were generally higher in the shrimp culture ponds than the natural saltmarsh in the two seasons. The rates of all three processes generally increased with the age of shrimp ponds, with the magnitude of increase being less pronounced for DNF and ANAMMOX than DNRA. The contribution of DNRA to total NO3− reduction increased significantly following saltmarsh conversion to shrimp ponds, suggesting that DNRA became an increasingly important biogeochemical process under shrimp culture. DNRA competed with DNF and limited reactive N loss to some extent after natural saltmarshes converted to shrimp culture ponds. The results of redundancy analysis revealed that the availability of substrates and sulfides in sediments, rather than the bacteria gene abundance, were the most important factor influencing the NO3− reduction processes. Overall, our findings highlighted that shrimp-aquaculture reclamation may aggravate nitrogen loading in coastal wetlands by promoting the production of bioavailable ammonium.

Thiocyanate (SCN⁻)-based autotrophic denitrification (AD) has recently been demonstrated as a promising technology that could be integrated with anaerobic ammonium oxidation (Anammox) to achieve simultaneous removal of nitrogen and SCN⁻. However, there is still a lack of a complete SCN⁻-based AD model, and the potential microbial competition/synergy between AD bacteria and Anammox bacteria under different operating conditions remains unknown, which significantly hinders the possible application of coupling SCN⁻-based AD with Anammox. To this end, a complete SCN⁻-based AD model was firstly developed and reliably calibrated/validated using experimental datasets. The obtained SCN⁻-based AD model was then integrated with the well-established Anammox model and satisfactorily verified with experimental data from a system coupling AD with Anammox. The integrated model was lastly applied to investigate the impacts of influent NH₄⁺-N/NO₂⁻-N ratio and SCN⁻ concentration on the steady-state microbial composition as well as the removal of nitrogen and SCN⁻. The results showed that the NH₄⁺-N/NO₂⁻-N ratio in the presence of a certain SCN⁻ level should be controlled at a proper value so that the maximum synergy between AD bacteria and Anammox bacteria could be achieved while their competition for NO₂⁻ would be minimized. For the simultaneous maximum removal (>95%) of nitrogen and SCN⁻, there existed a negative relationship between the influent SCN⁻ concentration and the optimal NH₄⁺-N/NO₂⁻-N ratio needed.

The Yangtze River, which is the largest in Euro-Asian, receives tremendous anthropogenic nitrogen input and is typically characterized by severe eutrophication and hypoxia. Two major processes, denitrification and anaerobic ammonium oxidation (anammox), play vital roles for removing nitrogen global in nitrogen cycling. In the current study, sediment samples were collected from both latitudinal and longitudinal transects along the coastal Yangtze River and the East China Sea (ECS). We investigated community composition and distributions of nosZ gene-encoded denitrifiers by high throughput sequencing, and also quantified the relative abundances of both denitrifying and anammox bacteria by q-PCR analysis. Denitrifying communities showed distinct spatial distribution patterns that were impacted by physical (water current and river runoffs) and chemical (nutrient availability and organic content) processes. Both denitrifying and anammox bacteria contributed to the nitrogen removal in Yangtze Estuary and the adjacent ECS, and these two processes shifted from coastal to open ocean with reverse trends: the abundance of nosZ gene decreased from coastal to open ocean while anammox exhibited an increasing trend based on quantifications of hzsB and 16S rRNA genes. Further correspondence correlation analysis revealed that salinity and nutrients were the main factors in structuring composition and distribution of denitrifying and anammox bacteria. This study improved our understanding of dynamic processes in nitrogen removal from estuarine to open ocean. We hypothesize that denitrification is the major nitrogen removal pathway in estuaries, but in open oceans, low nutrient and organic matter concentrations restrict denitrification, thus increasing the importance of anammox as a nitrogen removal process.

Estuaries are considered hot spots for the production and emissions of nitrous oxide (N2O) and easily occur suspended particles (SPS), however, current understanding about the role of SPS in the N2O emissions from the oxic estuarine waters of lacustrine ecosystems is still limited. In this study, field investigations were performed in the estuaries of hypereutrophic Taihu Lake, and laboratory simulations were simultaneously conducted to ascertain the characteristics of N2O emissions with different SPS concentrations. The results showed that the N2O emission fluxes ranged from 9.75 to 118.38 μg m−2 h−1, indicating a high spatial heterogeneity for the N2O emissions from the estuaries of Taihu Lake. Although the dissolved oxygen (DO) concentrations were up to 7.85 mg L−1 in the estuarine waters, from where the N2O emissions fluxes were approximately three times that of the lake regions. Multiple regression model selected the total nitrogen (TN), SPS, and DO concentrations as the crucial factors influencing the N2O emission fluxes. Particularly for SPS, the simulation results showed that the N2O concentrations increased gradually with the increase in the SPS concentrations of an oxic water column containing 4 mg L−1 of NO3−-N, indicating that a high SPS concentration can accelerate the N2O emissions. It was related to the change of denitrifying bacteria population in the SPS, as evidenced by its significantly positive correlation with N2O emissions (p < 0.01). Our findings will draw attentions to the role of SPS playing in the N2O productions and emissions in eutrophic lakes, and its effect on nitrogen cycle should be considered in the future study.

Denitrification is considered as the dominant nitrogen (N) removing pathway, however, anaerobic oxidation of ammonium (anammox) also plays a significant part in N loss in agricultural ecosystems. Large N inputs into agricultural soils may stimulate the growth of anammox bacteria, resulting in high activity and diversity of anammox bacteria and subsequent more N loss. In some specific niches, like oxic-anoxic interface, three processes, nitrification, anammox and denitrification couple with each other, and significant anammox reaction could be observed. Soil parameters like pH, dissolved oxygen, salinity, oxidation-reduction potential (ORP), and substrate concentrations impact the anammox process. Here we summarize the current knowledge on anammox activity and contribution to N loss, abundance and diversity of anammox bacteria, factors affecting anammox, and the relationship between anammox and other N loss pathways in agricultural soils. We propose that more investigations are required for (1) the role of anammox to N loss with different agricultural management strategies; (2) microscale research on the coupling of nitrification-anammox-denitrification, that might be a very complex process but ideal model for further studies responsible for N cycling in terrestrial ecosystems; and (3) new methods to estimate differential contributions of anammox, codenitrification and denitrification in total N loss in agricultural ecosystems. New research will provide much needed information to quantify the contribution of anammox in N loss from soils at landscape, ecosystem and global scales.

In this study, we develop a new composite material of Fe-Cu/D407 composite via using nanoscale zero-valent iron (nZVI) with copper deposited on chelating resin (D407) to remove nitrate from the water. The experimental results show that a remarkable nitrate removal and the selectivity of N₂ are 99.9% and 89.7%, respectively, under the anaerobic conditions of Cu/Fe molar ratio of 1:2, pH = 3.0. Even without of inert gas and adjusting the initial pH of the solution, the removal rate of nitrate by Fe-Cu/D407 reached to 85% and the selectivity of nitrogen reached to 55%. Meanwhile, the Fe-Cu/D407 maintained preferable removal efficiency of nitrate (100% - 92%) over a wide pH range of 3–11. In addition, the removal rate of the drinking water, lake water and wastewater from the Fe-Cu/D407 is still very high and the reactivity of Fe-Cu/D407 was relatively unaffected by the presence of dissolved ions in the waters tested. Moreover, the synergetic effect of Fe, Cu and D407 in the composite Fe-Cu/D407 were well investigated for the first time according to the analyses of TPR, XPS and EIS. The catalytic mechanism and denitrification routes were also proposed.

Feammox is a newly discovered and important anaerobic nitrogen (N) loss pathway, and its variation and role in removing N following the application of N fertilizer and its migration from paddies to other land use types and from surface soils to deep soils have not been thoroughly elucidated to date. In this study, field sampling and slurry incubation experiments were performed to evaluate the Feammox rate between different land use types (paddy, irrigation ditch, riparian zone and lake, 0–10 cm) and different paddy soil depths (0–70 cm) in a wheat-rice rotation area in China. Based on a ¹⁵N-labelled isotope-tracing technique and analysis of microbial communities, it was estimated that the potential Feammox rate ranged from 0.031 to 0.42 mg N kg⁻¹ d⁻¹ in this area. In the soil profile of the paddy, the depth of 20–30 cm was the active region of Feammox, with a value of 0.37 ± 0.057 mg N kg⁻¹ d⁻¹. Compared with the surface soil (0–10 cm) of the paddy (0.18 ± 0.031 mg N kg⁻¹ d⁻¹), the potential Feammox rate of the irrigation ditch soil was not significantly different, but that of the lake riparian soil and lake sediment were decreased by 27.27% and 32.11%, respectively (p < 0.01). Fe(III) content was the best predictor of the Feammox rate and explained the variation of the Feammox rate by 36.00% in the surface soil. At the genus level, the paddy soil at a depth of 20–30 cm had the greatest abundance of the genera in which the Fe reduction bacteria were distributed; and where Bacillus, Geobacter and Anaeromyxobacter had higher proportions. It was estimated that the potential N loss by Feammox was in the range of 7.36 (the lake) ∼43.35 (the paddy) kg N ha⁻¹ year⁻¹ in the surface soil of this area. Considering denitrification and the Feammox rate as a whole, we found that denitrification remained to be the main contributor to N loss in the surface soil (94.72–96.89% of N loss), although Feammox dominated N loss in the deep soil (below 0–10 cm).