Ammonia Oxidation Research Articles

Full-scale simultaneous partial nitrification, anammox, and denitrification for the efficient treatment of carbon and nitrogen in low-C/N wastewater

A full-scale simultaneous partial nitrification, anaerobic ammonia oxidation (anammox), and denitrification (SNAD) reactor was initiated to address the problem of high energy consumption for the treatment of low C/N wastewater. The SNAD system achieved a nitrogen removal rate of 0.9 kg/(m3·d) at an influent NH₄+–N concentration of 500 mg/L after 450 days of stable operation. Partial nitrification was achieved by maintaining free ammonia levels at 0.8 ± 0.3 mg/L and dissolved oxygen concentrations between 0.3 mg/L and 1.2 mg/L, which resulted in synergistic nitrogen removal, with anammox contributing 61 % and denitrification contributing 39 %. Microbiological analyses indicated that the dominant microorganisms were Candidatus Brocadia, Thauera, Denitratisoma, and Nitrosomonas. In conclusion, study provides a solid foundation for the broader implementation of the SNAD process in wastewater treatment systems.

Rational desig of hierarchical defect-rich NiSe2/Se-NiCuOx heterostructures: Modulating electronic structure for exceptional ammonia oxidation performance

Direct ammonia oxidation reaction (AOR) is a promising route for efficiently removing ammonia–nitrogen from wastewater. However, the sluggish kinetics and poor N2 selectivity present significant challenges for AOR catalysts. Herein, a novel cost-effective hierarchical catalyst was constructed by selenizing NiCuOx nanorods on nickel foam support (NiSe2/Se-NiCuOx/NF) via the hydrothermal-calcination selenization method. The selenizing procedure radically manipulates the surface structure of NiSe2/Se-NiCuOx/NF, enriching it with oxygen vacancy defects and heterostructures to achieve high efficiency towards AOR. Consequently, the optimal NiSe2/Se-NiCuOx/NF only requires 1.50 V to achieve a current density of 65mA cm−2. Furthermore, the N2 selectivity was up to 98.18 % after 5 h of electrolysis. Density functional theory (DFT) calculation elucidates that the defects and heterostructures effectively facilitates electron transfer between NiSe2 and Cu2O, significantly reduces the energy barrier of the rate-determining step (RDS) of O-S mechanism. This work demonstrates the feasibility of metallic selenides as efficient electrocatalysts for AOR.

Drivers of denitrification and nitrification in a dryland agroecosystem: The role of abiotic and biotic factors

Agricultural practices such as tillage and fertilization impact soil nitrogen (N) cycling processes, but how they alter the coupling between the activity, abundance and diversity of N-cycling microbes remains to be understood. Here, we used a fifteen-year trial in a dryland agroecosystem on the Loess Plateau of China (two tillage regimes crossed with six fertilization treatments) to understand how (de)nitrification potentials are determined by soil abiotic conditions and the abundances and compositions of the (de)nitrifier communities. We measured the abundances of bacterial (AOB) and archaeal (AOA) ammonia oxidizers and nirK- and nirS-nitrite reducers, their community compositions, potential nitrification (PNA) and denitrification (PDA), and soil abiotic conditions. PNA and PDA across the 12 treatments were positively correlated to AOB abundance and nirS abundance, respectively. Co-occurrence network analysis revealed the presence of dominant ecological modules of (de)nitrifiers sensitive to agricultural treatments, and more complex network under no-tilled than tilled conditions as well as under multiple fertilizers than unfertilized conditions. Path analysis and random forest analysis both showed that PNA was explained by AOB abundances and the relative abundance of one module of (de)nitrifiers driven by soil ammonium concentration, while PDA was most related to soil organic carbon concentration, pH and to a lesser extent nirS abundance. These findings demonstrate that, in agricultural soils, the potential of denitrification –a facultative activity for denitrifiers– is mainly predicted by abiotic conditions, while the potential of nitrification –an obligate activity for nitrifiers– is determined by biotic variables, here AOB abundances and a particular cluster of microbial populations.

Simulation of rapid sand filters to understand and design sequential iron and manganese removal using reactive transport modelling

Iron (Fe2+), manganese (Mn2+), and ammonium (NH4+) oxidation processes were studied in three single media and three dual media full-scale rapid sand filters (RSFs) using reactive transport modelling (RTM) in PHREEQC and parameter estimation using PEST. Here, we present the insights gained into the spatial distribution of Fe and Mn mineral coatings in RSFs and its influence on the oxidation sequence and rates. Fe2+ and Mn2+ oxidation predominantly occurred simultaneously in the RSFs, contrary to the expected sequential oxidation based on Gibbs free energy calculations. During backwashing, RSF grains become fully mixed, which initiates heterogeneous Mn2+ oxidation on Mn-coated grains that end up in the top layer. The resulting grains have a mixed Fe/Mn mineral coating, which is limiting heterogeneous Mn2+ oxidation due to the limited Mn mineral surface available. Mixed coatings did not seem to affect Fe2+ oxidation rates, instead oxidation rates were increasing at lower pH. We found that RSFs can be designed to spatially separate Fe2+ and Mn2+ oxidation, which results in optimal conditions for Mn2+ oxidation. The RSF needs to consist of two layers with varying density to inhibit mixing and complete Fe2+ oxidation should occur in the top layer. The developed RTM can be used to estimate the depth at which Fe2+ oxidation is complete, and thus the ideal intersection depth of the two layers. A novel perspective is provided on how mineral coating distribution in single and dual media filters influence removal rates and the sequence of oxidation, which contributes to the design of more efficient groundwater filters.

Patterns of nitrate load variability under surface water-groundwater interactions in agriculturally intensive valley watersheds

Nitrate pollution is a significant environmental issue closely related to human activities, complicated hydrological interactions and nitrate fate in the valley watershed strongly affects nitrate load in hydrological systems. In this study, a nitrate reactive transport model by coupling SWAT-MODFLOW-RT3D between surface water and groundwater interactions at the watershed scale was developed, which was used to reproduce the interaction between surface water and groundwater in the basin from 2016 to 2019 and to reveal the nitrogen transformation process and the evolving trend of nitrate load within the hydrological system of the valley watershed. The results showed that the basin exhibited groundwater recharge to surface water in 2016-2019, particularly in the northwestern and northeastern mountainous regions of the valley watershed and the southern Beishan Reservoir vicinity. Groundwater recharge to surface water declined by 20.17% from 2016 to 2019 due to precipitation. Nitrate loads in the hydrologic system of the watershed are primarily derived from human activities (including fertilizer application from agricultural activities and residential wastewater discharges) and the nitrogen cycle. Nitrate loads in surface water declined 16.05% from 2016 to 2019. Nitrate levels are higher in agricultural farming and residential areas on the eastern and northern sides of the watershed. Additionally, hydrological interactions are usually accompanied by material accumulation and environmental changes. Nitrate levels tend to rise with converging water flows, a process that becomes more pronounced during precipitation events and cropping seasons in agriculturally intensive valley watersheds. However, environmental changes alter nitrogen transformation processes. Nitrogen fixation, nitrification, and ammonification intensify nitrogen inputs during river pooling, enhancing nitrogen cycling fluxes and elevating nitrate loads. These processes are further enhanced during groundwater recharge to surface water, leading to evaluated nitrate load. Enhanced denitrification, dissimilatory nitrate reduction to ammonium (DNRA), anaerobic ammonia oxidation, and assimilation promote the nitrogen export from the system and reduce the nitrate load during surface water recharge to groundwater.

Nitrogen, sulfur, iron, and microbial communities co-shape the seasonal biogeochemical behaviors of As and Sb in coastal tidal flat wetlands associated with rivers

Arsenic (As) and antimony (Sb) are affected by complex biogeochemical processes in coastal ecosystems. However, the influence of N, S, Fe, and microbial communities on the biogeochemistry of As and Sb in coastal tidal flat wetlands remain uncertain, particularly when rivers flow into these areas. This study combined diffusive gradients in the thin-film technique with high-throughput sequencing to investigate the release and vertical distribution of As and Sb in river and coastal tidal flat wetland sediments. The results indicated a distinct stratification phenomenon in the As release at depths ranging from 20mm to −150mm. At river sites, the release of As occurred in the upper layer (above −40mm), with peak values of 4.3 and 9.3μg/L at HS and SY sites in summer, respectively, likely due to anaerobic ammonium oxidation. In the lower layer (below −40mm), both As and Sb were released, and this was possibly due to Fe reduction. However, at the coastal tidal flat sites, the release of As and Sb may have been driven by anaerobic ammonium oxidation, dissimilatory nitrate reduction to ammonium, sulfate reduction, and Fe reduction. At the river sites, As exhibited increased activity during the summer, and the residual forms were converted more easily into mobile forms. Sb remained relatively stable during both winter and summer. Conversely, both As and Sb primarily existed in residual forms and exhibited higher stability during summer in the coastal tidal flat sites. The microbial phyla Nitrospirota (3.6−7.0%) and Acidobacteriota (9.5−10.2%) were more prevalent at the river sites, whereas Desulfobacterota (8.8−12.0%) and Firmicutes (0.13−27.9%) were more prevalent at the coastal tidal flat sites. The bacterial genera involved in the N, S, and Fe transformation processes differed between the two sites, and they were primarily Thiobacillus, Limnobacter, and Sulfurovum at the river sites and Sva1033, Anaeromyxobacter, and Sva0485 at the coastal tidal flat sites. In this study, the microorganisms that mediated N, S, and Fe complex processes at various depths in the sediment–water interface were decoupled to elucidate the effect of these processes on the biogeochemical behavior of As and Sb as they move from rivers to coastal tidal flat wetlands. Environmental ImplicationCoastal wetlands have been experiencing heavy metal contamination, particularly in the estuarine regions affected by industrial activities. It is still not well understood how As and Sb behave biogeochemically in river and coastal tidal flat wetlands. This study provided an in-depth understanding of the effect of nitrogen, sulfur, iron, and microbial communities on the behavior of As and Sb in these areas. This study also highlights the eutrophication may increase As/Sb pollution in river areas. And these heavy metals may accumulate in seafood, potentially leading to heightened health risks of As and Sb poisoning in coastal tidal flat wetlands.

Unveiling the mechanism underlying in−situ enhancement on anammox system by sulfide: Integration of biological and isotope analysis

The in−situ utilization of sulfide to remove the nitrate produced during the anaerobic ammonium oxidation (anammox) process can avoid prolonged sludge acclimatization, facilitating the rapid initiation of coupled nitrogen removal processes. However, the understanding of in−situ enhancement on anammox system by sulfide remains unclear. Herein, sulfide (Na2S) was introduced as an additional electron donor to remove the nitrate derived from the anammox under varying sulfide/nitrogen (S/N, S2−-S/NO3–-N, molar ratio) ratios (0.004–4.375). The underlying mechanisms were elucidated by molecular biology techniques including flow cytometry, quantitative polymerase chain reaction, and 16S rRNA amplicon sequencing, alongside isotope tracer analysis. Results revealed that anammox reactors, when operated with in−situ sulfide addition, exhibited a significant enhancement in total nitrogen removal efficiency (NRE) ranging from 11.5%−41.7% (achieved 96%), with the optimal S/N ratios of 0.01−0.8. Isotope tracer analysis indicated the successful coupling of the anammox, sulfur autotrophic denitrification (SADN), and dissimilatory nitrate reduction to ammonium (DNRA) processes within the system, with their contributions to nitrogen removal being 46%−50%, 24%−30%, and 20%−22%, respectively. Moreover, a notable increase in the abundance of sulfur–oxidizing bacteria (SOB) (20%−40% increase) and DNRA bacteria (10%−20% increase) were observed. Effective collaboration was further supported by the sustained viability of microbial communities. It is speculated that the heightened presence of SOB and DNRA bacteria created a low toxicity environment by converting sulfide to biogenic sulfur, thereby promoting the well−being of anammox bacteria. However, the excessive dosage of sulfide (S/N=1.8) intensified the DNRA process (contribution>35%) and weakened the anammox process, leading to an increase in effluent NH4+-N concentration and a decline in NRE. This study confirms that the in−situ adding an appropriate amount of sulfide favors achieving complete nitrogen removal in anammox system, which provides a novel avenue to resolve the issue of the residual nitrate in anammox process.

Plant litter crust enhances nitrogen accumulation by regulating microbial diversity and urease activity in semi-arid sandy soils

Plant litter plays a crucial role in regulating soil nitrogen (N) cycling in dryland ecosystems. However, the mechanisms by which plant litter crust drives microbial community composition to influence N levels in sandy soils remain unclear. In this study, we examined the effects of litter crust on sandy soil microhabitat characteristics (temperature, moisture, and porosity), microbial diversity and composition, urease activity, and soil N variables (total-N, nitrate-N, and ammonium-N) across different stages of litter crust development (including early-, mid-, and post-term) in a field setting. We found that litter crust promotes bacterial and fungal alpha diversity in sandy soils, leading to a shift in bacterial community composition from oligotrophs (i.e., Actinobacteria) to copiotrophs (i.e., Proteobacteria and Bacteroidetes), and a shift in fungal community composition dominance from Ascomycota to Basidiomycota. Litter crust enhances the complexity and stability of bacterial and fungal co-occurrence networks in sandy soils, especially in early- and mid-term stages of crust development. Additionally, litter crust increases nitrification (aerobic ammonia oxidation) and decreases denitrification (nitrate reduction) in these soils. Notably, the increase in soil moisture and urease activity, along with the decrease in soil temperature due to litter crust, effectively promotes N accumulation in sandy soils. Our results demonstrate that N levels during the litter crust period are mainly influenced by a combination of bacterial beta diversity and fungal alpha diversity at the 0–5 cm depth, and bacterial alpha diversity along with soil properties (i.e., soil variables and urease activity) at the 5–10 cm depth. Overall, our results reveal that litter crust contributes to N accumulation in sandy soils by regulating bacterial and fungal community composition and diversity, as well as by buffering soil temperature and soil moisture, and enhancing urease activity. These findings provide new insights into the critical role of soil microbes in maintaining N functions during litter crust development in semi-arid sandy ecosystems.

Integrated impacts of mariculture on nitrogen cycling processes in the coastal groundwater of Beihai, southern China

Groundwater nitrogen (N) contamination in coastal zones is becoming an increasingly serious global issue. Mariculture, as a major anthropogenic activity, has profound impacts on coastal groundwater and constitutes an important source of coastal N contamination. However, a comprehensive understanding of the impact of mariculture on N cycling (especially N removal) is still lacking. Taking the Daguansha mariculture region in southern China as the study area, we aimed to investigate the environmental impact of mariculture on coastal groundwater and identify N cycling processes influenced by mariculture using hydrogeochemistry, multiple isotopes, coupled with 16S rRNA gene sequencing, and the quantitative polymerase chain reaction (qPCR) experiments. The results showed that the combined effects of seawater intrusion and seepage from land-based mariculture ponds have led to localized groundwater salinization in the region. Meanwhile, mariculture promotes nitrification and anammox processes in groundwater. The dominance of ammonia-oxidizing and anammox bacteria in the upper aquifer is attributable to local salinization, N and organic carbon input, as well as anoxic to suboxic conditions induced by seepage from aquaculture ponds. In addition, the gene abundances of ammonia oxidation (dominated by AOA) and denitrification were positively correlated, indicating their cooperative interaction. This study provides deeper insight into N cycling in coastal groundwater systems affected by extensive mariculture.

Efficient ammonia oxidation by Pseudomonas citronellolis strain YN-21 under strongly acidic conditions: Performance and mechanism

Ammonia oxidation microorganisms generally tend to have low rates of ammonia oxidation under acidic conditions, as the protonated ammonia is not a substrate for ammonia monooxygenase. In this work, heterotrophic ammonia oxidation bacteria (HAOB) Pseudomonas citronellolis strain YN-21 showed high efficiency in removing NH4+ (12.7 mg/L/h) even at initial pH 4.5. The potential acid resistance mechanisms (H+ efflux, H+ consumption, and production of alkaline substances) maintained intracellular pH neutrality. Transcriptome analysis showed that genes involved in amino acid metabolism, carbohydrate metabolism, ABC transporter and nitrogen metabolism were significantly up-regulated, which facilitated the rapid removal of NH4+ in an acidic environment. Moreover, urea could be used as an alternative nitrogen source for YN-21 in a strongly acidic environment, and the production of NH3 from urea hydrolysis provided a substrate for ammonia oxidation. These results provide new insights into efficient ammonia oxidation in acidic environments.

Cooperative Active Sites on Ag2Pt3TiS6 for Enhanced Low-Temperature Ammonia Fuel Cell Electrocatalysis.

Ammonia has attracted considerable interest as a hydrogen carrier that can help decarbonize global energy networks. Key to realizing this is the development of low temperature ammonia fuel cells for the on-demand generation of electricity. However, the efficiency of such systems is significantly impaired by the sluggish ammonia oxidation reaction (AOR) and oxygen reduction reaction (ORR). Here, we report the design of a bifunctional Ag2Pt3TiS6 electrocatalyst that facilitates both reactions at mass activities exceeding that of commercial Pt/C. Through comprehensive density functional theory calculations, we identify that active site motifs composed of Pt and Ti atoms work cooperatively to catalyze ORR and AOR. Notably, in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) experiments indicate a decreased propensity for *NOx formation and hence an increased resistance toward catalyst poisoning for AOR. Employing Ag2Pt3TiS6 as both the cathode and anode, we constructed a low temperature ammonia fuel cell with a high peak power density of 8.71 mW cm-2 and low Pt loading of 0.45 mg cm-2. Our findings demonstrate a pathway towards the rational design of effective electrocatalysts with multi-element active sites that work cooperatively.

Biocrusts Mediate the Niche Distribution and Diversity of Ammonia-Oxidizing Microorganisms in the Gurbantunggut Desert, Northwestern China

Biological soil crusts (biocrusts) play pivotal ecological roles in regulating nitrogen cycling within desert ecosystems. While acknowledging the essential role played by ammonia-oxidizing microorganisms in nitrogen transformation, there remains a paucity of understanding concerning how disturbances to biocrusts impact the diversity and spatial distribution patterns among ammonia oxidizer communities within temperate deserts. This investigation delved into assessing how 4 years’ worth of removing biocrust influenced niche differentiation between nitrifying archaea and bacteria while also examining its effects on shaping community structures of predominant ammonia-oxidizing archaea (AOA) within the Gurbantunggut Desert soils. Despite notable variations in abundance of ammonia-oxidizing microbes across distinct soil depths throughout different seasons, it became apparent that removing biocrust significantly altered both the abundance and niche pattern for AOA alongside their bacterial counterparts during winter and summer periods. Notably dominating over their bacterial counterparts within desert soils, AOA displayed their highest archaeal to bacterial amoA gene copy ratio (6549-fold higher) at a soil depth of 5–10 cm during summer. Moreover, substantial impacts were observed upon AOA diversity along with compositional changes following such perturbation events. The aftermath saw an emergence of more diffuse yet dynamic AOA communities, especially noticeable amidst winter when nitrogen and water limitations were relatively alleviated. In summary, our findings underscore how interactions between biocrust coverages alongside factors like soil temperature, total carbon content, or NO3−_N concentrations govern niches occupied by ammoxidation communities whilst influencing assemblage processes too. The sensitivity shown by dominant AOAs towards biocrust removal further underscores how biocrust coverage influences nitrogen transformation processes while potentially involving other communities and functions in desert ecosystems.

Selective Electrocatalytic Oxidation of Ammonia by Ru-dpp Complexes Containing Aromatic Nitrogen Donor as Axial Ligand.

Inspired by the rapid growth of Ru-based complexes as molecular ammonia oxidation catalysts, we propose novel Ru-dpp complexes bearing a nitrogen donor as the axial ligand into the ammonia oxidation catalysts family. Herein, a series of Ru-dpp complexes [Ru(K3-N,N',N″-dpp)(bpy)(L)]·PF6 (where Hdpp = 2-[5-(pyridin-2-yl)-1H-pyrrol-2-yl]pyridine; bpy = 2,2'-bipyridine; L = pyridine (Ru-py); 4-methylpyridine (Ru-pic); pyrimidine (Ru-pmd); isoquinoline (Ru-isoq)) containing aromatic nitrogen donor axial ligand are synthesized and fully characterized by NMR, IR, and ESI-MS. The structural analysis displays that dpp- as a pincer ligand coordinates to ruthenium, and nitrogen donor L binds to ruthenium at an axial ligand. The reaction of Ru-L with ammonia generates ammonia-ligated Ru-L-NH3 complexes, which are monitored by NMR and UV-vis spectra. The electrochemical properties are studied by cyclic voltammetry and density functional theory calculation. These titled complexes have good electrocatalytic performances for selective oxidation of ammonia into hydrazine at 0.6 V versus Cp2Fe+/0 onset potential. The turnover frequency of N2H4 formation is 326.4-393.4 h-1, and the faraday efficiency of generating hydrazine exceeds 97%. The N-N formation mechanism via bimolecular coupling of ruthenium amide/imide is proposed. The aminyl character of ruthenium amide intermediate is confirmed by EPR spectra.

How Ir–Rh Alloys Improve Electrochemical Ammonia Oxidation Activity Studied by Density Functional Theory

AbstractThe electrochemical ammonia oxidation reaction (AOR) has applications in hydrogen storage and ammonia waste remediation. Using density functional theory, we investigate the mechanism of AOR on Ir, Rh, and their alloys at varied atomic ratios (, , and ) toward (g), (aq), and (aq) formation. This work introduces a method for computational alloy design by considering both electronic energy and configurational entropy. The structures considered are selective to (g) formation and all favored *N–N coupling. An alloy was found to reduce the theoretical onset potential for (g) formation relative to pure Ir while not exhibiting a downhill coupling step corresponding to catalyst poisoning by *N as shown for pure Rh, consistent with previous experimental work. The formation of (aq) and (aq) demand significantly higher potentials, typically limited by the final hydroxylation step before desorption.

Biochar and neem seed cake co-amendment effects on soil nitrogen cycling and NH3 volatilization in contrasting soils

In a 28-day incubation study, ball milling technologies were applied to enhance the sorptive and functional properties of pristine biochar. The effect of ball-milled (BM) biochar, neem seed cake, and their co-amendment was evaluated on nitrification, ammonia (NH3) volatilization, and the abundance of nitrifying microbial communities in three contrasting tropical soils of different pH (acidic, neutral, and alkaline). The amendments were applied at 2% dry w/w to soils fertilized with ammonium chloride (NH4Cl). Results showed that in neutral and alkaline soils, the co-amendment led to a 40% and 64% increase in NH3 volatilization, respectively, compared to control due to significant ammonium (NH4+) retention and temporary nitrification inhibition. Conversely, in acidic soil, BM biochar and neem seed cake amplified nitrification by 23% and 62%, respectively, compared to sole amendments, while neem seed cake increased NH3 volatilization by 56% compared to BM biochar + neem seed cake due to NH4+ retention, altered soil pH, and changes in nitrifying microbial community. The abundance of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), and complete ammonia oxidizers (Comammox) was altered by changes in soil pH and N availability modulated by BM biochar and neem seed cake. Correlation analysis revealed significant relationships between soil organic matter (SOM), NH4+, and NO3− on the abundance of nitrifying microorganisms. The study affirms the efficacy of BM biochar-neem seed cake co-amendment on nitrification inhibition but indicates potential N losses by NH3 volatilization depending on soil type, highlighting the need for soil type-specific management strategies to optimize N retention while minimizing environmental impacts.

Phenacetin enhanced the inorganic nitrogen removal performance of anammox bacteria naturally in-situ enriched system

Among the earliest synthetic antipyretic drugs, phenacetin (PNCT) could be used as the novel partial nitrification (PN) inhibitor to effectively inhibit nitrite-oxidizing bacteria (NOB). In practical application, the rapidly starting of PN could provide stable source of nitrite for anaerobic ammonium oxidation (anammox) process. However, impact of PNCT on anaerobic ammonia oxidizing bacteria (AnAOB) and its underlying mechanisms were not clear. In this research, totally 14 times of PNCT aerobic soaking treatment were performed in the AnAOB naturally enrichment system to improve total inorganic nitrogen removal efficiency (TINRE). After once of PNCT treatment, TINRE rose from 61.89 % to 79.93 %. After 14 times of PNCT treatment, NOB Nitrospira relative abundance decreased from 9.82 % to 0.71 %, though Candidatus Brocadia relative abundance also declined, it might gradually adjust to PNCT by converting the leading oligotype species. The activity and relative abundances of NOB were reduced by PNCT via decreasing the abundances of genes amoA and nxrB, enzymes NxrA and NxrB. Moreover, Candidatus Jettenia and Ca. Brocadia might be the potential host of qacH-01 and they played the crucial role in the shaping profile of antibiotic resistance genes (ARGs). The explosive propagation or transmission of ARGs might not take place after PNCT treatment.

Both AOA and AOB contribute to nitrification and show linear correlation with nitrate leaching in purple soils with a wide nitrogen gradient

Ammonia oxidizers play an important role in nitrification that forms nitrate, the main form of leaching nitrogen (N). However, little is known about how ammonia oxidizers bridge long-term N fertilization levels and soil nitrate leaching. We conducted a field experiment in purple soil, investigating the interactions among soil physico-chemical parameters, ammonia-oxidizing microbial communities, and N leaching under 0, 90, 180, 270, and 360 kg N ha−1 yr−1 fertilization levels. We found that soil inorganic N leaching increased exponentially with increasing N application rate. N fertilization enhanced the abundances of the amoA gene in ammonia-oxidizing archaea (AOA) and bacteria (AOB), while partial least squares regression analysis revealed that AOA and AOB abundances were correlated with pH and soil organic carbon (SOC). Compared with no N fertilization, N application reduced AOA alpha diversity and increased AOB alpha diversity. AOA alpha diversity was associated with pH and bulk density, whereas soil SOC and inorganic N content were more important in predicting changes in AOB alpha diversity. A linear relationship was established between soil NO3−-N leaching, the potential nitrification rate (PNR), and the abundances of AOA and AOB. The association of soil NO3−-N leaching and PNR with both AOA and AOB abundances were further corroborated by Mantel test, random forest regression, and partial least squares path modelling. Furthermore, alterations in the AOB alpha diversity, soil pH and NH4+-N content also contribute to the increasing soil NO3−-N leaching along the N application rate. Our results suggest that AOA, which previous studies have found to be active only under low N conditions, can also contribute to nitrification and support soil NO3−-N leaching at a wide range of N gradients. Overall, this finding advances the current understanding of the relationship between soil N leaching and microbial functional properties to some extent.

Biological Nitrification Inhibitors with Antagonistic and Synergistic Effects on Growth of Ammonia Oxidisers and Soil Nitrification

Biological nitrification inhibition (BNI) refers to the plant-mediated process in which nitrification is inhibited through rhizospheric release of diverse metabolites. While it has been assumed that interactive effects of these metabolites shape rhizosphere processes, including BNI, there is scant evidence supporting this claim. Hence, it was a primary objective to assess the interactive effects of selected metabolites, including caffeic acid (CA), vanillic acid (VA), vanillin (VAN), syringic acid (SA), and phenylalanine (PHE), applied as single and combined compounds, against pure cultures of various ammonia-oxidising bacteria (AOB, Nitrosomonas europaea, Nitrosospira multiformis, Nitrosospira tenuis, Nitrosospira briensis) and archaea (AOA, Nitrososphaera viennensis), as well as soil nitrification. Additionally, benzoic acid (BA) was examined as a novel biological nitrification inhibitor. All metabolites, except SA, tested as single compounds, achieved varied levels of inhibition of microbial growth, with CA exhibiting the highest inhibitory potential. Similarly, all metabolites applied as single compounds, except PHE, inhibited soil nitrification by up to 62%, with BA being the most potent. Inhibition of tested nitrifying microbes was also observed when compounds were assessed in combination. The combinations VA + PH, VA + CA, and VA + VAN exhibited synergism against N. tenuis and N. briensis, while others showed antagonism against N. europaea, N. multiformis, and N. viennensis. Although all combinations suppressed soil nitrification, their interactions against soil nitrification revealed antagonism. Our findings indicate that both antagonism and synergism are possible in rhizospheric interactions involving BNI metabolites, resulting in growth inhibition of nitrifiers and suppression of soil nitrification.

Role of Hydroxyl Radical in the Degradation of ATO: DFT Study.

Large-scale implementation of NTO (5-nitro-1,2,4-triazol-3-one), an energetic material used in military applications, causes its discharge to the environment. Reduction of NTO with bacterial nitroreductase or iron-containing minerals results in the formation of ATO (5-amino-1,2,4-triazol-3-one), which is an important intermediate in the process of NTO degradation in the environment. ATO may be dissolved in surface water and groundwater due to its good water solubility. Many organic pollutants in surface water can be decomposed under the action of the hydroxyl radical, an important reactive oxygen species produced under sunlight irradiation. A detailed investigation of possible mechanisms for ATO decomposition in water induced by the hydroxyl radical as one of the pathways for ATO environmental degradation was performed by computational study at the PCM(Pauling)/M06-2X/6-311++G(d,p) level. Decomposition of ATO was found to be a multistep process that can begin with a hydrogen atom abstraction from ATO. Formed intermediates undergo further H atom abstraction, hydroxyl radical attachment to carbon atoms, and rupture of C-N bonds, leading to low-weight inorganic species such as nitrogen gas, ammonia, nitric acid, and carbon(IV) oxide. The calculated activation energy and exergonicity of the studied reactions support the contribution of hydroxyl radical to ATO degradation in the environment.

N-H Bond Activation of Ammonia by a Redox-Active Carboranyl Diphosphine.

In this work, we report the room-temperature N-H bond activation of ammonia by the carboranyl diphosphine 1-PtBu2-2-PiPr2-closo-C2B10H10 (1) resulting in the formation of zwitterionic 7-P(NH2)tBu2-10-P(H)iPr2-nido-C2B10H10 (2). Unlike the other phosphorus-based ambiphiles that require geometric constraints to enhance electrophilicity, the new mode of bond activation in this main-group system is based on the cooperation between electron-rich trigonal phosphine centers and the electron-accepting carborane cluster. As an exception among many other metal-based and metal-free systems, the N-H bond activation of gaseous ammonia or aqueous ammonium hydroxide by carboranyl diphosphine 1 proceeds with tolerance of air and water. Mechanistic details of ammonia activation were explored computationally by DFT methods, demonstrating an electrophilic activation of ammonia by the phosphine center. This process is driven by the reduction of the boron cluster followed by an ammonia-assisted deprotonation and proton transfer. A subsequent reaction of 2 and TEMPO results in the cleavage of all N-H and P-H bonds with the formation of a cyclic phosphazenium cation supported by an anionic cluster N(7-PtBu2-8-PiPr2)-nido-C2B9H10 (3). Transformations reported herein represent the first example of ammonia oxidation via triple hydrogen atom abstraction facilitated by a metal-free system.