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Oxygen minimum zones (OMZs), such as those found in the eastern South Pacific (ESP), are the most important N.sub.2 O sources in the global ocean relative to their volume. N.sub.2 O production is related to low O.sub.2 concentrations and high primary productivity. However, when O.sub.2 is sufficiently low, canonical denitrification takes place and N.sub.2 O consumption can be expected. N.sub.2 O distribution in the ESP was analyzed over a wide latitudinal and longitudinal range (from 5° to 30° S and from 71-76° to ~ 84° W) based on ~ 890 N.sub.2 O measurements. Intense N.sub.2 O consumption, driving undersaturations as low as 40%, was always associated with secondary NO.sub.2 .sup.-- accumulation (SNM), a good indicator of suboxic/anoxic O.sub.2 levels. First, we explore relationships between ΔN.sub.2 O and O.sub.2 based on existing data of denitrifying bacteria cultures and field observations. Given the uncertainties in the O.sub.2 measurements, a second relationship between ΔN.sub.2 O and NO.sub.2 .sup.-- (> 0.75 μM) was established for suboxic waters (O.sub.2 < 8 μM). We reproduced the apparent N.sub.2 O production (ΔN.sub.2 O) along the OMZ in ESP with high reliability (r.sup.2 = 0.73 p = 0.01). Our results will contribute to the quantification of the N.sub.2 O that is recycled in O.sub.2 deficient waters, and improve the prediction of N.sub.2 O behavior under future scenarios of OMZ expansion and intensification.

Mn/TiO.sub.2 and Fe-Mn/TiO.sub.2 catalysts were prepared using the impregnation method to explore the effect of iron doping at low temperature on the catalytic oxidation of high concentration NO by manganese-based catalysts and the catalytic reaction mechanism. This study reveals that the Fe-Mn/TiO.sub.2 catalyst performed exceptionally well in catalyzing high concentration NO smelting flue gas within a temperature range of 30-170 °C. At 150 °C, the denitrification efficiency of Fe-Mn/TiO.sub.2 (0.3) catalyst was 97.7%. Characterization analysis revealed that the incorporation of iron increased the ratio of high valence manganese and surface chemisorbed oxygen, which promoted the dispersion of surface-active substances, ultimately leading to an increase in the specific surface area of the catalyst. These factors facilitated the adsorption and activation of NH.sub.3 as well as the oxidation of NO to NO.sub.2, thus increasing the low-temperature redox capacity of the catalyst. Meanwhile, the ammonia selective catalytic reduction (NH.sub.3-SCR) denitrification mechanism over Fe-Mn/TiO.sub.2 catalysts was consistent with the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms under low-temperature reaction conditions.

Nitrogen (N) is a crucial limiting nutrient for phytoplankton growth in the ocean. The main source of bioavailable N in the ocean is delivered by N.sub.2 -fixing diazotrophs in the surface layer. Since field observations of N.sub.2 fixation are spatially and temporally sparse, the fundamental processes and mechanisms controlling N.sub.2 fixation are not well understood and constrained. Here, we implement benthic denitrification in an Earth system model (ESM) of intermediate complexity (UVic ESCM 2.9) coupled to an optimality-based plankton-ecosystem model (OPEM v1.1). Benthic denitrification occurs mostly in coastal upwelling regions and on shallow continental shelves, and it is the largest N loss process in the global ocean. We calibrate our model against three different combinations of observed Chl, NO3-, PO43-, O.sub.2, and N*=NO3--16PO43-+2.9. The inclusion of N* provides a powerful constraint on biogeochemical model behavior. Our new model version including benthic denitrification simulates higher global rates of N.sub.2 fixation with a more realistic distribution extending to higher latitudes that are supported by independent estimates based on geochemical data. The volume and water-column denitrification rates of the oxygen-deficient zone (ODZ) are reduced in the new version, indicating that including benthic denitrification may improve global biogeochemical models that commonly overestimate anoxic zones. With the improved representation of the ocean N cycle, our new model configuration also yields better global net primary production (NPP) when compared to the independent datasets not included in the calibration. Benthic denitrification plays an important role shaping N.sub.2 fixation and NPP throughout the global ocean in our model, and it should be considered when evaluating and predicting their response to environmental change.

An ex-situ 15 N–18O tracing experiment with soils collected from the valley and slope, respectively, of a subtropical secondary karst forest with three N addition levels, i.e., 0, 50, and 100 kg N ha−1 year−1 for each topographic position to investigate N2O production pathways. Autotrophic nitrification pathways (ammonia oxidation, nitrifier denitrification, and nitrification-coupled denitrification) accounted for > 70% of total N2O production, but denitrification pathways (heterotrophic denitrification and co-denitrification) were the minor source of N2O at both topographic positions. In the valley, chronic N addition stimulated ammonia oxidation-derived N2O, which was paralleled by increased ammonia-oxidizing archaea (AOA) amoA gene transcript abundance, but inhibited nitrifier denitrification- and nitrification-coupled denitrification–derived N2O along with suppressed ammonia-oxidizing bacteria (AOB) amoA gene transcript abundance and stimulated nosZII gene transcript abundance, respectively. On the slope, chronic N addition stimulated ammonia oxidation-derived N2O along with increased AOB amoA gene transcript abundance, and enhanced nitrifier denitrification-derived N2O congruent with increased AOB amoA and decreased nirK gene transcript abundances. In addition, chronic N addition reduced the relative contribution of heterotrophic denitrification to N2O production but had no significant influence on heterotrophic denitrification-derived N2O on the slope. Overall, our results provide a comprehensive view in terms of how topography-driven soil properties regulate N2O production and its pathways in a subtropical forest.

The elimination of these two pollutants is crucial since NO.sub.x and VOCs have recently been found to be the main atmospheric pollutants in solid material combustion procedures. The synergistic removal of pollutants within the denitrification zone of SCR suffered from narrow temperature window and low activity. In this study, co-precipitation was employed to create [gamma]-Al.sub.2O.sub.3 doped Mn-based composite catalyst materials. The link between [gamma]-Al.sub.2O.sub.3 addition and catalytic performance was explored using XRD, SEM, and other characterisation means. The results of the activity tests demonstrated that the MnCoAlO.sub.x-7% catalyst was more than 90% effective at denitrification over an extensive temperature spectrum (95-310 °C) and 90% effective for benzene conversion at 195 °C. In the synergistic elimination process, benzene promoted the NH.sub.3-SCR reaction, whereas NO.sub.x enhances benzene oxidation at low temperatures, inhibits it at middle and high temperatures, and has no effect at high temperatures. Reaction mechanism studies had shown that benzene facilitated the formation of amides from NH.sub.3 and promoted nitrites conversion to improve denitrification efficiency during synergistic removal; NO.sub.x reduced the intermediates produced by benzene oxidation at medium and high temperatures, leading to a decrease in benzene removal.

Microplastics are ubiquitous in estuarine, coastal, and deep sea sediments. The impacts of microplastics on sedimentary microbial ecosystems and biogeochemical carbon and nitrogen cycles, however, have not been well reported. To evaluate if microplastics influence the composition and function of sedimentary microbial communities, we conducted a microcosm experiment using salt marsh sediment amended with polyethylene (PE), polyvinyl chloride (PVC), polyurethane foam (PUF) or polylactic acid (PLA) microplastics. We report that the presence of microplastics alters sediment microbial community composition and nitrogen cycling processes. Compared to control sediments without microplastic, PUF- and PLA-amended sediments promote nitrification and denitrification, while PVC amendment inhibits both processes. These results indicate that nitrogen cycling processes in sediments can be significantly affected by different microplastics, which may serve as organic carbon substrates for microbial communities. Considering this evidence and increasing microplastic pollution, the impact of plastics on global ecosystems and biogeochemical cycling merits critical investigation. Plastic pollution has infiltrated every ecosystem, but few studies have quantified the biogeochemical or ecological effects of plastic. Here the authors show that microplastics in ocean sediment can significantly alter microbial community structure and nitrogen cycling.

We have shown that many fungi (eukaryotes) exhibit distinct denitrifying activities, although occurrence of denitrification was previously thought to be restricted to bacteria (prokaryotes), and have characterized the fungal denitrification system. It comprises NirK (copper-containing nitrite reductase) and P450nor (a cytochrome P450 nitric oxide (NO) reductase (Nor)) to reduce nitrite to nitrous oxide (N2O). The system is localized in mitochondria functioning during anaerobic respiration. Some fungal systems further contain and use dissimilatory and assimilatory nitrate reductases to denitrify nitrate. Phylogenetic analysis of nirK genes showed that the fungal-denitrifying system has the same ancestor as the bacterial counterpart and suggested a possibility of its proto-mitochondrial origin. By contrast, fungi that have acquired a P450 from bacteria by horizontal transfer of the gene, modulated its function to give a Nor activity replacing the original Nor with P450nor. P450nor receives electrons directly from nicotinamide adenine dinucleotide to reduce NO to N2O. The mechanism of this unprecedented electron transfer has been extensively studied and thoroughly elucidated. Fungal denitrification is often accompanied by a unique phenomenon, co-denitrification, in which a hybrid N2 or N2O species is formed upon the combination of nitrogen atoms of nitrite with a nitrogen donor (amines and imines). Possible involvement of NirK and P450nor is suggested.

Recent research has highlighted the existence of some bacteria that are capable of performing heterotrophic nitrification and have a phenomenal ability to denitrify their nitrification products under aerobic conditions. A high-salinity-tolerant strain ADN-42 was isolated from Hymeniacidon perleve and found to display high heterotrophic ammonium removal capability. This strain was identified as Pseudomonas sp. via 16S rRNA gene sequence analysis. Gene cloning and sequencing analysis indicated that the bacterial genome contains N 2 O reductase function ( nos Z) gene. NH 3 -N removal rate of ADN-42 was very high. And the highest removal rate was 6.52 mg/L · h in the presence of 40 g/L NaCl. Under the condition of pure oxygen (DO >8 mg/L), NH 3 -N removal efficiency was 56.9 %. Moreover, 38.4 % of oxygen remained in the upper gas space during 72 h without greenhouse gas N 2 O production. Keeping continuous and low level of dissolved oxygen (DO <3 mg/L) was helpful for better denitrification performance. All these results indicated that the strain has heterotrophic nitrification and aerobic denitrification abilities, which guarantee future application in wastewater treatment.

The continuous increase of the greenhouse gas nitrous oxide (N2O) in the atmosphere due to increasing anthropogenic nitrogen input in agriculture has become a global concern. In recent years, identification of the microbial assemblages responsible for soil N2O production has substantially advanced with the development of molecular technologies and the discoveries of novel functional guilds and new types of metabolism. However, few practical tools are available to effectively reduce in situ soil N2O flux. Combating the negative impacts of increasing N2O fluxes poses considerable challenges and will be ineffective without successfully incorporating microbially regulated N2O processes into ecosystem modeling and mitigation strategies. Here, we synthesize the latest knowledge of (i) the key microbial pathways regulating N2O production and consumption processes in terrestrial ecosystems and the critical environmental factors influencing their occurrence, and (ii) the relative contributions of major biological pathways to soil N2O emissions by analyzing available natural isotopic signatures of N2O and by using stable isotope enrichment and inhibition techniques. We argue that it is urgently necessary to incorporate microbial traits into biogeochemical ecosystem modeling in order to increase the estimation reliability of N2O emissions. We further propose a molecular methodology oriented framework from gene to ecosystem scales for more robust prediction and mitigation of future N2O emissions. This review summarizes the major microbial pathways of soil N2O production, and key environmental factors modulating their relative contributions, and further proposes to use a combination of state-of-the-art approaches for better source partitioning and incorporation of microbial datasets to achieve better predictive ecosystem models.

Background and aims Plants can directly affect nitrogen (N) transformation processes at the micro-ecological scale when soil comes into contact with roots. Due to the methodological limitations in measuring direct N.sub.2 losses in plant-soil systems, however, the effect of rhizosphere processes on N.sub.2O production and reduction to N.sub.2 has rarely been quantified. Methods For the first time, we developed a robotic continuous flow plant-soil incubation system (using a He+O.sub.2 + CO.sub.2) combined with N.sub.2O .sup.15N site preference approach to examine the effect of plant root activity (barley - Hordeum vulgare L.) on: i) soil-borne N.sub.2O and N.sub.2 emissions, ii) the specific contribution of different pathways to N.sub.2O fluxes in moist soils (85% water holding capacity) receiving different inorganic N forms. Results Our results showed that when a nitrate-based N fertiliser was applied, the presence of plants tripled both N.sub.2O and N.sub.2 losses during the growth period but did not alter the N.sub.2O/(N.sub.2O + N.sub.2) product ratio. The .sup.15N site preference data indicated that bacterial denitrification was the dominant source contributing to the observed N.sub.2O fluxes in both nitrate and ammonium treated soils, whereas the presence of barley increased the contribution of fungal N.sub.2O in the nitrate treated soils. During the post-harvest period, N.sub.2O and N.sub.2 emissions significantly increased in the ammonium-fertilised treatment, being more pronounced in the soil with a senescing root system. Conclusion Overall, our study showed a significant interaction between rhizosphere processes and N forms on the magnitude, patterns, and sources of soil borne N.sub.2O and N.sub.2 emissions in moist agricultural soils.