N2O Accumulation Research Articles

A novel method based on hydrodynamic cavitation to effectively remove NO2

Marine ships’ exhaust causes serious air pollution, among which NO is the most difficult to control. However, when a wet scrubber is used to oxidize and remove NO, a large amount of NO2 is produced, which makes it difficult to improve the NOx removal efficiency. We propose a new method for NO2 removal based on hydrodynamic cavitation (HC). Compared with the bubbling method, the HC method could be used to improve mass transfer by increasing the gas–liquid contact area and generating microjets, thereby improving NO2 removal efficiency. A 12-h long continuous experiment was conducted which revealed that HC could delay but not prevent NO production. Experiments using HCl, NaOH, and HNO3 to adjust the pH of the solution showed that the change in pH or the accumulation of NO3- had little effect on the formation of NO. The results obtained using ion chromatography analysis further revealed that the accumulation and decomposition of HNO2 were the primary causes of NO formation. Following this, we carried out experiments to investigate possible enhancement of NO2 removal efficiency by H2O2, Na2S2O8, NaClO2, and ClO2 under conditions of HC. The results showed that ClO2, H2O2, and Na2S2O8 increased the NO2 removal efficiency by only 2.29%–4.79% compared to water. However, the NO2 removal efficiency of NaClO2 was 0.54% lower than that of water, and it gradually decreased. The results of this study provide new ideas for improving NO removal efficiency using oxidants.

Model-based evaluation of N2O recovery as an energy source in sulfur-driven NO-based autotrophic denitrification

Instead of the conventional perception of nitrous oxide (N2O) as a potent greenhouse gas whose production should be minimized, this work aimed to assess N2O recovery as a potential energy source from nitric oxide (NO) in the form of Fe(II)EDTA-NO through element sulfur (S0) or thiosulfate (S2O32−)-driven NO-based autotrophic denitrification (SNADS0 or SNADS2O3). A mathematical model was proposed to describe substrate dynamics related to N2O production and reduction and was successfully calibrated and validated using batch experimental data from lab-scale SNADS0 and SNADS2O3 systems under different substrates conditions. The model was subsequently employed to assess the potential of N2O accumulation and recovery by altering the S/N mass ratio and the ratio of gas volume to liquid volume of the system. The simulation results suggested that with a S/N mass ratio of nearly 1.0, high-purity N2O could be more rapidly and efficiently recovered from Fe(II)EDTA-NO in the SNADS0 and SNADS2O3 systems with a higher ratio of gas volume to liquid volume (i.e., a N2O recovery efficiency of up to 80.2%−84.9% reached within 3.1 h−3.5 h under the studied conditions). Comparatively, the SNADS0 process showed an economic and viable advantage for practical applications to the efficient treatment and resource utilization of NO-containing flue gas.

Stable and rapid partial nitrification achieved by boron stimulating autoinducer-2 mediated quorum sensing at room & low temperature.

The stable and rapid initiation of partial nitrification is a long-term interest of researchers. In this study, boron, an activator of autoinducer-2 (AI-2), was first proposed to promote partial nitrification and improve sludge properties. Two concentrations of boron were added to six laboratory-scale nitrification reactors to study the effects of boron stimulated AI-2-mediated quorum sensing on nitrification at room and low temperature. Boron could significantly increase the concentration of AI-2 in the reactor (p<0.05), and the high concentration of boron showed better performance at two temperatures. The realization of partial nitrification was mainly due to the accumulation of NO2-- N promoted by the increase of AI-2 concentration. NO2-- N accumulation occurred in all the boron-treated reactors, while no NO2-- N accumulation occurred in the reactor without boron at room temperature or the accumulation efficiency was 14% lower at low temperature. Community analysis and qPCR further confirmed that exogenous boron had no significant effect on AOB, but inhibited NOB, especially at room temperature. The changes of EPS were slightly different at the two temperatures: at room temperature, boron increased the total EPS production, while at low temperature it mainly changed the PN/PS value of EPS. It was worth noting that the effect of boron on partial nitrification depended not only on its concentration, but also on NH4+-N concentration and temperature. This study provided a new strategy for the rapid and stable initiation of partial nitrification process.

Nitrogen removal performance and influencing factors of in situ utilization of soluble microbial products (SMPs) by an anammox and denitrification immobilized filler coupling system

An anaerobic ammonia oxidation (anammox) and denitrifying immobilized filler coupling reactor (RAD) was established to systematically evaluate its performance for enhancing nitrogen removal using endogenous soluble microbial products (SMPs) in the absence of exogenous COD, and was compared with the coupling mode with exogenous COD addition. The results showed that compared with independent anammox reactor (RA), the RAD can effectively use SMPs to reduce the NO3−–N yield and improve the total nitrogen removal efficiency. The NO3−–N yield of the RAD decreased over time during the cycle, decreasing by up to 70 % compared to the RA. 3D-excitation emission matrix showed that from the beginning to the end of the cycle, the SMP components changed from available tryptophan to difficult-to-use humic acids. In addition, the temperature and reaction cycle affected NOx−–N transformation, while low temperature and a long cycle were not conducive to the complete reduction of NO3−–N, leading to the accumulation of NO2−–N. In contrast, adding exogenous COD accelerated NO3−–N removal by the enhancing denitrification activity, but posed a potential threat to anammox activity. High-throughput sequencing analysis showed that Candidatus Kuenenia and Halomonas were the dominant species of the anammox and denitrifying immobilized fillers, respectively, which supported good coupling effect. These results provide valuable information for the optimization of anammox systems and the reduction of organic carbon consumption.

Interactive effect of simulated acid rain, calcium silicate, and γ-aminobutyric acid on physiological processes in corn and wheat

The effect of calcium silicate, γ-aminobutyric acid (GABA), and their mixture on the adaptation of wheat and corn to simulated acid rain has been evaluated in the pot experiments. Acid rain was simulated by watering twice with distilled water acidified with sulfuric acid to pH = 2. Test plants were grown in a plant growth chamber under controlled conditions of temperature, illumination and relative humidity for 18 days. The physiological state of the test plants was assessed by characteristics of growth (shoot height, root length, dry weight of shoots and roots), the content of photosynthetic pigments, flavonoids, and proline in leaves. For the corn, the content of anthocyanins in shoots and roots was also evaluated. In parallel, the physical and chemical characteristics of the soil (pH, electrical conductivity, redox potential, content of soluble carbonates, and nitrates) were determined.It was established that simulated acid rain inhibited the growth and accumulation of photosynthetic pigments in the leaves of wheat and corn. The content of protective metabolites (proline, flavonoids, and anthocyanins) increased. Wheat showed greater sensitivity to the inhibiting effect of acidification compared to corn. The application of CaSiO3 was more effective than GABA in restoring pH value and HCO3- concentration in soil, while the application of GABA more effectively promoted the accumulation of NO3- anions in soil. Combining CaSiO3 with GABA was the most effective in restoring soil physical and chemical properties altered by simulated acidification and stimulating the growth and photosynthesis in the test-plants. Thus, the mixture of CaSiO3 with GABA is promising for further studies of the possibility of its application to mitigate the negative impact of acid depositions on vegetation and soil.

Gross N transformations and plant N use efficiency in intensive vegetable production soils

Long-term application of high doses of N fertilizer to achieve high yields has led to a significant accumulation of soil N (largely in the form of NO3−) in greenhouse vegetable production (GVP). At present, it is not clear how efficiently vegetables can use the NO3− accumulated in soil and from which source the N taken up by vegetables in GVP originates. In this study, three greenhouse soils with various degrees of soil NO3− accumulation (N1 <N2 <N3) and one abandoned land soil (AL) adjacent to greenhouses were analysed by 15N tracing pot experiments. The amount of N derived from soil (ANDFS), N use efficiency (NUE) of cucumbers, and gross N transformation rates were investigated to explore processes leading to the accumulation of NO3− in soil and how available this N is for N uptake by cucumbers. The results showed that the percentage of N derived from soil of cucumbers were 72–89% in greenhouse soils. ANDFS and NUE values of cucumbers decreased with NO3− accumulation. Plant NH4+ uptake (UNH4) was one of the main factors regulating NUE and ANDFS. Soil gross N mineralisation rates (M) were 2.5- and 3.0-fold higher in N1 than in N2 and N3, respectively, while autotrophic nitrification rates (ONH4) were significantly lower in N1 and N2 (by 4.31 and 4.25 mg N kg−1 day−1) than N3 (9.44 mg N kg−1 day−1). This resulted in significantly higher ONH4/M ratios in soils with relatively higher NO3− accumulation, which reduced the amount of NH4+ available to cucumbers and inhibited UNH4, in turn, reducing ANDFS and NUE. Higher ONH4 in soils with relatively high NO3− accumulation could result in fertilizer-derived NH4+ being rapidly oxidised to NO3−, leading not only to lower NUE but also to greater NO3− accumulated in soil due to a lower plant NO3− uptake rate. High soil electrical conductivity (EC) in N2 and N3 caused salt stress in cucumber plants and inhibited plant N uptake, resulting in low ANDFS and NUE. These results demonstrate that NO3− that is accumulated and newly mineralized in soil is available for cucumbers, and should be considered together with the prevailing soil N transformations in N fertilizer recommendations for GVP. Combined application of NH4+ fertilizer (e.g. in form of urea) with nitrification inhibitors is recommended to increase NH4+ retention and enhance NUE and utilisation of soil accumulated and newly mineralized NO3− by cucumbers.

Insights into N2O turnovers under polyethylene terephthalate microplastics stress in mainstream biological nitrogen removal process

The ubiquitous microplastics in wastewater have raised growing concerns due to their unintended effects on microbial activities. However, whether and how microplastics affect nitrous oxide (N2O) (a potent greenhouse gas) turnovers in mainstream biological nitrogen removal (BNR) process remain unclear. This work therefore aimed to fill such knowledge gap by conducting both long-term and batch tests. After over 100 days of feeding with wastewater containing polyethylene terephthalate (PET) microplastics (0–500 μg/L), the long-term results showed that both production and reduction of N2O during denitrification were reduced, as well as the N2O production during nitrification. Accordingly, 60% reduction in N2O accumulation and 70% reduction in N2O production were observed in the denitrification and nitrification batch tests, respectively. Nevertheless, the long-term N2O emission factors under PET microplastics stress were comparable to that in the control reactor, mainly because PET microplastics led to more nitrite accumulation in anoxic period. With the aid of online N2O sensors and site-preference analysis, it was demonstrated that the heterotrophic bacteria pathway and ammonia oxidizing bacteria denitrification pathway for N2O production were negatively affected by PET microplastics, whereas a clear increase in the contribution of hydroxylamine pathway (+ 22.9%) was observed. Further investigation revealed that PET microplastics even at environmental level (i.e. 10 μg/L) significantly reshaped the BNR sludge characteristics (e.g. much larger particle size) and microbial communities (e.g. Thauera, Rhodobacte and Nitrospira) as well as the nitrogen metabolism pathways, which were chiefly responsible for the changes of N2O turnovers and N2O production pathways under the PET microplastics stress.

Metabolism transformation and free nitrous acid tolerance of denitrifying polyphosphate accumulating organisms under different intracellular polyphosphate levels

Polyphosphate (Poly-P) is a key compound in the known metabolism of denitrifying polyphosphate organisms (DPAOs). This study investigated the efficiency of endogenous denitrification, metabolic changes, and tolerance to free nitrous acid (FNA) of DPAOs at different levels of Poly-P. A high C/P ratio (100:1) made DPAOs transform from polyphosphate accumulating metabolism (PAM) to glycogen accumulating metabolism (GAM), and the nitrate (NO3--N) reduction rate of DPAOs increased significantly in GAM mode. Accumulibacter IA, IIC, and IID were the main dominant DPAOs, and their synergy and the increase of PHA may be responsible for the high NO3--N reduction efficiency. The DPAOs under GAM mode had better tolerance to FNA, and N2O reductase was destroyed due to the FNA influence, leading to a large accumulation of N2O. Flow cytometry results showed that FNA caused DPAOs to over-consume ATP and depolarize, thereby inhibiting their growth. And FNA disrupted the membrane integrity and esterase activity of DPAOs, which may result in early apoptosis. The metabolic transformation and FNA tolerance of DPAOs under GAM mode ensure the stable operation and performance recovery of denitrifying phosphorus removal systems, which demonstrates the DPAOs-dominated biological phosphorus removal has wide application prospects.

Sulfur cycle contributes to stable autotrophic denitrification and lower N2O accumulation in electrochemically integrated constructed wetlands: Electron transfers patterns and metagenome insights

Excessive discharge of nutrients from wastewater treatment plants (WWTPs) is an important pollutant source of eutrophic water bodies. In this work, three electrochemically integrated horizontal flow constructed wetlands (E-HFCWs) were developed for advanced nutrients removal from WWTPs effluent with different S/N ratios. In E-HFCWs, PO43−-P, NO3−-N and TN removal percentages at current of 0.2 A and hydraulic retention time (HRT) of 24 h did not differ significantly as S/N ratios altered. When fed with low, middle and high concentrations of SO42−-S wastewater during this period, PO43−-P removal percentages respectively reached 99.3 %±0.9 %, 99.2 %±1.1 % and 99.0 %±1.4 %, NO3−-N removal percentages respectively reached 99.5 %±0.5 %, 99.6 %±0.4 % and 99.4 %±0.8 %, and TN removal percentages respectively reached 92.0 %±2.5 %, 90.8 %±3.4 % and 91.2 %±2.7 %. This work highlighted that sulfur cycle played crucial roles in improving nitrogen removal stability as current or HRT decreased in higher S/N ratio groups. The formed sulfur ferrites under higher current or HRT condition served as “electron reservoir”, and would resupply electron for denitrification when electron supplied by electrolysis was deficient. In addition, the higher S/N ratio groups allowed significantly lower N2O accumulation, which was accordance with the concept of carbon neutral. Based on metagenome results, the occurrence of more abundant sulfur-oxidizing denitrifying genes and bacteria (e.g., Thiobacillus) in higher S/N ratio groups under lower current or HRT further demonstrated the significant roles of sulfur cycle in stable autotrophic denitrification performance in E-HFCWs. Overall, this work provides perspective on the future practical application for the regulation of nitrogen removal stability enhancement and N2O emission reduction in electrochemically integrated bioreactors.

Effect of electric current intensity on performance of polycaprolactone/FeS2-based mixotrophic biofilm–electrode reactor

In this study, a bioelectrochemical system consisting of pyrite-based autotrophic denitrification (PAD) and heterotrophic denitrification (HD) was established to polish nitrate wastewater. The loading of electric current (EC) could stimulate the dissolution of pyrite. Appropriate EC (I ≤ 30 mA) was conducive to nitrate removal, too high EC (I = 40 mA) would inhibit nitrate removal and lead to an obvious accumulation of NO2–-N and NH4+-N. Microbial analysis revealed that the increase of EC could inhibit the diversity of heterotrophic microbes, but appropriate EC (I = 10 mA) could increase the diversity of autotrophic microbes. The EC loading was conducive to the enrichment of iron autotrophic denitrifiers (Ferritrophicum), pyrite-oxidizing bacteria (Thiobacillus, Sulfurimonas), and sulfur autotrophic denitrifiers (Dechloromonas, Thiobacillus, and Arenimonas). The EC loading enlarged the contribution of PAD, making PAD a dominant pathway in denitrification.

Treatment Effect of Corncob and Rice Straw Enhanced Subsurface Flow Constructed Wetland on Low C/N Ratio Wastewater

The lack of carbon sources severely inhibits denitrification in wastewater with a low C/N ratio. Corncob and rice straw were chosen as supplementary carbon sources to bring into the wetland system to supplement the carbon sources needed for denitrification, and the enhancing effects of the two carbon sources on nitrogen removal from the wetland were studied. The cumulative release of carbon was in the order of rice straw[(145.17±9.44) mg·g-1]>corncob[(57.41±5.04) mg·g-1] based on the 11-day pure water extraction and release experiment, whereas the cumulative release of nitrogen was in the order of rice straw[(2.31±0.09) mg·g-1]>corncob[(0.66±0.08) mg·g-1]. The average carbon/nitrogen ratios released and accumulated by corncob and rice straw during the observation period were 94.78 and 63.64, respectively. Corncob was more suited as an additional carbon source than rice straw. COD concentrations in the effluent from the corncob and straw constructed wetlands were found to be below 50 mg·L-1 for the 58-day pilot test of subsurface flow constructed wetlands, except on days 8 to 12. The NO3--N removal rates of the corncob-added built wetlands were 93%-99% over the observation period, with good denitrification performance. In comparison, the lowest NO3--N removal rate of the constructed wetland with the addition of rice straw was only 76.8% at the late stage of operation, and the denitrification rate dropped dramatically. The control group removal rates of NO3--N were only 76.2%-77.7%, indicating a clear lack of carbon sources. The accumulation of NO2--N was also induced by a lack of carbon supply. NO2--N effluent concentrations were 2.5-6 times and 6-26 times higher in the constructed wetlands with rice straw and the control groups, respectively, than those in the wetlands constructed with corncob. The addition of corncob resulted in a more substantial reduction in NO2--N content in the constructed wetland than the addition of rice straw (P<0.05). The TN removal rates of wetlands constructed with corncob and rice straw and the control group were 83.75%-93.49%, 76.59%-78.85%, and 67.85%-72.56%, respectively, with significant differences among the three (P<0.01). Finally, pretreatment with dilute alkali heating raised the cumulative carbon release of corncob to (93.73±17.49) mg·g-1 and the carbon/nitrogen ratio to 175.8, significantly improving the carbon release performance of corncob and demonstrating that it is a suitable source of extra carbon.

Distinct Denitrifying Phenotypes of Predominant Bacteria Modulate Nitrous Oxide Metabolism in Two Typical Cropland Soils.

Denitrifying nitrous oxide (N2O) emissions in agroecosystems result from variations in microbial composition and soil properties. However, the microbial mechanisms of differential N2O emissions in agricultural soils are less understood. In this study, microcosm experiments using two main types of Chinese cropland soil were conducted with different supplements of nitrate and glucose to simulate the varying nitrogen and carbon conditions. The results show that N2O accumulation in black soil (BF) was significantly higher than that in fluvo-aquic soil (FF) independent of nitrogen and carbon. The abundance of most denitrifying genes was significantly higher in FF, but the ratios of genes responsible for N2O production (nirS and nirK) to the gene responsible for N2O reduction (nosZ) did not significantly differ between the two soils. However, the soils showed obvious discrepancies in denitrifying bacterial communities, with a higher abundance of N2O-generating bacteria in BF and a higher abundance of N2O-reducing bacteria in FF. High accumulation of N2O was verified by the bacterial isolates of Rhodanobacter predominated in BF due to a lack of N2O reduction capacity. The dominance of Castellaniella and others in FF led to a rapid reduction in N2O and thus less N2O accumulation, as demonstrated when the corresponding isolate was inoculated into the studied soils. Therefore, the different phenotypes of N2O metabolism of the distinct denitrifiers predominantly colonized the two soils, causing differing N2O accumulation. This knowledge would help to develop a strategy for mitigating N2O emissions in agricultural soils by regulating the phenotypes of N2O metabolism.

Fabrication and characterization of Ti/polyaniline-Co/PbO2–Co for efficient electrochemical degradation of cephalexin in secondary effluents

The traditional interlayer of PbO2 electrode possessed many problems, such as short service lifetime and limited specific surface area. Herein, a novel and efficient Ti/polyaniline-Co/PbO2–Co electrode was conctructed employing cyclic voltammetry to introduce a Co-doped polyaniline interlayer and anodic electrodeposition to synthetize a β-PbO2-Co active layer. Compared with pristine PbO2 electrode, Ti/polyaniline-Co/PbO2–Co exhibited more compact crystalline shape and higher active sites amounts. Pratically, the electrochemical degradation of 5 mg L−1 cephalexin in real secondary effluents was effectively achieved by the novel anode with 87.42% cephalexin removal and 71.8% COD mineralization after 120 min of 15 mA cm−2 electrolysis. The hydroxyl radical production and electrochemical stability were increased by 3.16 and 3.27 times respectively. The cephalexin degradation pathway was investigated by combining a density functional theory-based theoretical approach and LC-QTrap-MS/MS. The most likely cleavage point of the β-lactam ring was the O=C–N bond, whose attack would produce small molecular compounds containing the thiazole and 4, 6-thiazine rings. Further oxidation produced inorganic ions; quantitative investigations indicated the amino groups to undergo decomposition to form aqueous NH4+, which was further oxidized to NO3−. The accumulation of NO3− and SO42−, combined with a decrease in toxicity toward Escherichia coli, demonstrated the efficient mineralization of cephalexin on the Ti/polyaniline-Co/PbO2–Co electrode.

Cassava (Manihot esculenta) Slow Anion Channel (MeSLAH4) Gene Overexpression Enhances Nitrogen Assimilation, Growth, and Yield in Rice.

Nitrogen is one of the most important nutrient elements required for plant growth and development, which is also immensely related to the efficient use of nitrogen by crop plants. Therefore, plants evolved sophisticated mechanisms and anion channels to extract inorganic nitrogen (nitrate) from the soil or nutrient solutions, assimilate, and recycle the organic nitrogen. Hence, developing crop plants with a greater capability of using nitrogen efficiently is the fundamental research objective for attaining better agricultural productivity and environmental sustainability. In this context, an in-depth investigation has been conducted into the cassava slow type anion channels (SLAHs) gene family, including genome-wide expression analysis, phylogenetic relationships with other related organisms, chromosome localization, and functional analysis. A potential and nitrogen-responsive gene of cassava (MeSLAH4) was identified and selected for overexpression (OE) analysis in rice, which increased the grain yield and root growth related performance. The morpho-physiological response of OE lines was better under low nitrogen (0.01 mm NH4NO3) conditions compared to the wild type (WT) and OE lines under normal nitrogen (0.5 mm NH4NO3) conditions. The relative expression of the MeSLAH4 gene was higher (about 80-fold) in the OE line than in the wild type. The accumulation and flux assay showed higher accumulation of and more expansion of root cells and grain dimension of OE lines compared to the wild type plants. The results of this experiment demonstrated that the MeSLAH4 gene may play a vital role in enhancing the efficient use of nitrogen in rice, which could be utilized for high-yielding crop production.

Impacts of engineered nanoparticles and antibiotics on denitrification: Element cycling functional genes and antibiotic resistance genes

The wide presence of antibiotics and minerals warrants their combined effects on the denitrification in natural aquatic environment. Herein, we investigated the effects of two antibiotics, sulfamethazine (SMZ) and chlortetracycline (CTC), on the reduction of NO3--N and accumulation of NO2--N in the absence and presence of engineered nanoparticles (NPs) (Al2O3, SiO2, and geothite) using 16 S rRNA sequencing and high-throughput quantitative PCR. The results showed that the addition of antibiotics inhibited the reduction of NO3--N by changing the bacterial community structure and reducing the abundance of denitrification genes, while engineered NPs promoted the denitrification by increasing the abundance of denitrification genes. In the binary systems, engineered NPs alleviated the inhibitory effect of antibiotics through enriching the denitrification genes and adsorbing antibiotics. Antibiotics and its combination with engineered NPs changed the composition of functional genes related to C, N, P, S metabolisms (p < 0.01). The addition of antibiotics and/or engineered NPs altered the bacterial community structure, which is dominated by the genera of Enterobacter (40.7–90.5%), Bacillus (4.9–58.5%), and Pseudomonas (0.21–12.7%). The significant relationship between denitrification, carbon metabolism genes, and antibiotic resistance genes revealed that the heterotrophic denitrifying bacteria may host the antibiotic resistance genes and denitrification genes simultaneously. The findings underscore the significance of engineered NPs in the toxicity assessment of pollutants, and provide a more realistic insight into the toxicity of antibiotics in the natural aquatic environment.

Distribution and Production of N2O in the Subtropical Western North Pacific Ocean During the Spring of 2020

Nitrous oxide (N2O) is an important greenhouse gas emitted in significant volumes by the Pacific Ocean. However, the relationship between N2O dynamics and environmental drivers in the subtropical western North Pacific Ocean (STWNPO) remains poorly understood. We investigated the distribution of N2O and its production as well as the related mechanisms at the surface (0–200 m), intermediate (200–1500 m), and deep (1500–5774 m) layers of the STWNPO, which were divided according to the distribution of water masses. We applied the transit time distribution (TTD) method to determine the ventilation times, and to estimate the N2O equilibrium concentration of water parcels last in contact with the atmosphere prior to being ventilated. In the surface layer, biologically derived N2O (ΔN2O) was positively correlated with the apparent oxygen utilization (AOU) (R2 = 0.48), suggesting that surface N2O may be produced by nitrification. In the intermediate layer, ΔN2O was positively correlated with AOU and NO3− (R2 = 0.92 and R2 = 0.91, respectively) and negatively correlated with nitrogen sinks (N*) (R2 = 0.60). Hence, the highest ΔN2O value in the oxygen minimum layer suggested N2O production through nitrification and potential denitrification (up to 51% and 25% of measured N2O, respectively). In contrast, the deep layer exhibited a positive correlation between ΔN2O and AOU (R2 = 0.92), suggesting that the N2O accumulation in this layer may be caused by nitrification. Our results demonstrate that the STWNPO serves as an apparent source of atmospheric N2O (mean air−sea flux 2.0 ± 0.3 μmol m-2 d-1), and that nitrification and potential denitrification may be the primary mechanisms of N2O production in the STWNPO. We predict that ongoing ocean warming, deoxygenation, acidification, and anthropogenic nitrogen deposition in the STWNPO may elevate N2O emissions in the future. Therefore, the results obtained here are important for elucidating the relationships between N2O dynamics and environmental changes in the STWNPO and the global ocean.

Enhanced denitrification of dispersed swine wastewater using Ca(OH)2-pretreated rice straw as a solid carbon source

In a pilot-scale packed bed reactor, the denitrification performance and microbial community structure of the dispersed swine wastewater treatment using calcium hydroxide (Ca(OH)2) pretreated rice straw as a carbon source were investigated. In a Ca(OH)2-pretreated rice straw supported denitrification system (Ca(OH)2-RS), the removal efficiency of NO3−-N was 96.39% at an influent NO3−-N load of 0.04 kg/(m3•d). Meanwhile, there was no obvious accumulation of NO2−-N or chemical oxygen demand (COD) in the effluent of Ca(OH)2-RS. The contents of soluble microbial byproduct-like substances and tryptophan-like substances in the effluent of Ca(OH)2-RS were reduced by 46.2% and 43.4%, respectively, compared with the influent. Overall, the Ca(OH)2-pretreated rice straw system had a strong resistance to fluctuations in water quality conditions, such as influent NO3−-N and COD concentrations. According to the microbial assay results, the Ca(OH)2 pretreatment enriched more denitrifying bacteria. Among them, Proteobacteria (42.33%) and Bacteroidetes (35.28%) were the dominant bacteria. Moreover, the main denitrifying functional bacteria, generanorank_f_Saprospiraceae (13.32%), norank_f_Porphyromonadaceae (4.22%), and Flavobacterium (3.25%), were enriched in Ca(OH)2-RS. This suggested that using Ca(OH)2-pretreated rice straw as a carbon source was a stable and efficient technology to enhance the denitrification performance of dispersed swine wastewater.

Nitrate Removal from Groundwater by Heterotrophic and Electro-Autotrophic Denitrification

A heterotrophic and autotrophic denitrification (HAD) system shows satisfactory performance for groundwater with nitrate contamination. In this study, an HAD system combining solid-phase heterotrophic denitrification and electrochemical hydrogen autotrophic denitrification (SHD-EHD) was developed for the treatment of nitrate-contaminated groundwater, in which polycaprolactone (PCL) was used as the carbon source to enhance the nitrate removal performance and prevent secondary pollution of the electrochemical hydrogen autotrophic denitrification (EHD) system. The denitrification performance, microbial community structure and nitrogen metabolism were investigated. The results showed that a high nitrate removal rate of 99.04% was achieved with an influent nitrate concentration of 40 mg/L, a current of 40 mA and a hydraulic retention time (HRT) of 4 h. By comparing the performance with the EHD system, it was found that the HAD system with PCL promoted the complete denitrification and reduced the accumulation of NO2−-N. Analysis of the microbial community structure identified the key denitrifying bacteria: Dechloromonas, Thauera and Hydrogenophaga. A comparison of microbial communities from SHD-EHD and solid-phase heterotrophic denitrification (SHD) demonstrated that electrical stimulation promoted the abundance of the dominant denitrifying bacteria and the electroactive bacteria. Analysis of the nitrogen metabolic pathway revealed that the conversion of NO to N2O was the rate-limiting step in the overall denitrification pathway. The SHD-EHD developed in this study showed great potential for groundwater nitrate removal.

Nitrite oxidizing bacteria, Nitrobacter and Nitrospira, are differently influenced by season, fertilizer, and tillage in long-term maize culture

Nitrite (NO2−) oxidation is the second step in nitrification, following ammonia (NH3) oxidation, and catalyzed by nitrite oxidizing bacteria (NOB). Their activity is critical in preventing toxic accumulation of NO2−. The two major NOB genera in soil are Nitrobacter and Nitrospira. This study investigated how N fertilization and tillage management influenced these two NOB communities in long-term (>40 years) maize (Zea mays L.) cropping. To evaluate NOB community changes we used PCR and denaturing gradient gel electrophoresis (DGGE) to analyze Nitrobacter and Nitrospira NO2− oxidoreductase genes (nxr). Season, fertilizer rate, and tillage all influenced Nitrobacter and Nitrospira communities, but differentially for the two genera. Nitrobacter was more diverse in summer, whereas Nitrospira was more diverse in winter. Nitrobacter was more diverse in N-fertilized samples, whereas Nitrospira diversity decreased with increasing fertilizer rate in winter but not summer. Nitrobacter diversity was not significantly influenced by tillage, whereas no-tillage samples had more diverse Nitrospira, compared to plow tillage samples. In addition to providing evidence for better understanding the relationship between soil management and NOB communities, this study also helped to suggest linkages between ammonia oxidizing bacteria and Nitrobacter and between ammonia oxidizing archaea and Nitrospira that may facilitate future studies concerning ammonia-oxidizing nitrifiers and NOB.

Environmental impacts of antibiotics addition to algal-bacterial-based aquaponic system.

Antibiotics usage is a double-edged sword among the production promotion and environmental aggravation of aquaculture system. In this study, the effects of sulfadiazine addition on algal-bacterial-based aquaponic (AA) system were thoroughly investigated. Results showed that sulfadiazine addition increased the nitrogen (N) and carbon (C) recovery of AA system by 1.3 times and 2.9 times, respectively. Meanwhile, the global warming potential was increased by 63% due to aggravated nitrous oxide (N2O) emission. This was mainly because sulfadiazine increased the abundance of nirS genes and decreased the abundance of nosZ genes, which subsequently led to higher N2O accumulation. Furthermore, resistance gene (sul-1, sul-2, and intI-1) abundance in the treatment group was an order higher than that of the control group, which would give rise to the environmental risk for agroecological system. KEY POINTS: • Sulfadiazine addition increased NUE at expense of aggravated GHG emissions. • Sulfadiazine disrupted the balance between the abundance of nirS and nosZ genes. • Sulfadiazine addition increased the resistance gene abundance of AA system.