Ammonium-derived nitrous oxide is a global source in streams.

Global riverine nitrous oxide (N2O) emissions have increased more than 4-fold in the last century. It has been estimated that the hyporheic zones in small streams alone may contribute approximately 85% of these N2O emissions. However, the mechanisms and pathways controlling hyporheic N2O production in stream ecosystems remain unknown. Here, we report that ammonia-derived pathways, rather than the nitrate-derived pathways, are the dominant hyporheic N2O sources (69.6 ± 2.1%) in agricultural streams around the world. The N2O fluxes are mainly in positive correlation with ammonia. The potential N2O metabolic pathways of metagenome-assembled genomes (MAGs) provides evidence that nitrifying bacteria contain greater abundances of N2O production-related genes than denitrifying bacteria. Taken together, this study highlights the importance of mitigating agriculturally derived ammonium in low-order agricultural streams in controlling N2O emissions. Global models of riverine ecosystems need to better represent ammonia-derived pathways for accurately estimating and predicting riverine N2O emissions.

Hot moment of N2O emissions in seasonally frozen peatlands.

Since the start of the Anthropocene, northern seasonally frozen peatlands have been warming at a rate of 0.6 °C per decade, twice that of the Earth's average rate, thereby triggering increased nitrogen mineralization with subsequent potentially large losses of nitrous oxide (N2O) to the atmosphere. Here we provide evidence that seasonally frozen peatlands are important N2O emission sources in the Northern Hemisphere and the thawing periods are the hot moment of annual N2O emissions. The flux during the hot moment of thawing in spring was 1.20 ± 0.82 mg N2O m-2 d-1, significantly higher than that during the other periods (freezing, -0.12 ± 0.02 mg N2O m-2 d-1; frozen, 0.04 ± 0.04 mg N2O m-2 d-1; thawed, 0.09 ± 0.01 mg N2O m-2 d-1) or observed for other ecosystems at the same latitude in previous studies. The observed emission flux is even higher than those of tropical forests, the World's largest natural terrestrial N2O source. Furthermore, based on soil incubation with 15N and 18O isotope tracing and differential inhibitors, heterotrophic bacterial and fungal denitrification was revealed as the main source of N2O in peatland profiles (0-200 cm). Metagenomic, metatranscriptomic, and qPCR assays further revealed that seasonally frozen peatlands have high N2O emission potential, but thawing significantly stimulates expression of genes encoding N2O-producing protein complexes (hydroxylamine dehydrogenase (hao) and nitric oxide reductase (nor)), resulting in high N2O emissions during spring. This hot moment converts seasonally frozen peatlands into an important N2O emission source when it is otherwise a sink. Extrapolation of our data to all northern peatland areas reveals that the hot moment emissions could amount to approximately 0.17 Tg of N2O yr-1. However, these N2O emissions are still not routinely included in Earth system models and global IPCC assessments.

Complete ammonia oxidization in agricultural soils: High ammonia fertilizer loss but low N2 O production.

The contribution of agriculture to the sustainable development goals requires climate-smart and profitable farm innovations. Increasing the ammonia fertilizer applications to meet the global food demands results in high agricultural costs, environmental quality deterioration, and global warming, without a significant increase in crop yield. Here, we reported that a third microbial ammonia oxidation process, complete ammonia oxidation (comammox), is contributing to a significant ammonia fertilizer loss (41.9 ± 4.8%) at the rate of 3.53 ± 0.55 mg N kg-1 day-1 in agricultural soils around the world. The contribution of comammox to ammonia fertilizer loss, occurring mainly in surface agricultural soil profiles (0-0.2 m), was equivalent to that of bacterial ammonia oxidation (48.6 ± 4.5%); both processes were significantly more important than archaeal ammonia oxidation (9.5 ± 3.6%). In contrast, comammox produced less N2 O (0.98 ± 0.44 µg N kg-1 day-1 , 11.7 ± 3.1%), comparable to that produced by archaeal ammonia oxidation (16.4 ± 4.4%) but significantly lower than that of bacterial ammonia oxidation (72.0 ± 5.1%). The efficiency of ammonia conversion to N2 O by comammox (0.02 ± 0.01%) was evidently lower than that of bacterial (0.24 ± 0.06%) and archaeal (0.16 ± 0.04%) ammonia oxidation. The comammox rate increased with increasing soil pH values, which is the only physicochemical characteristic that significantly influenced both comammox bacterial abundance and rates. Ammonia fertilizer loss, dominated by comammox and bacterial ammonia oxidation, was more intense in soils with pH >6.5 than in soils with pH <6.5. Our results revealed that comammox plays a vital role in ammonia fertilizer loss and sustainable development in agroecosystems that have been previously overlooked for a long term.

Contribution of Ammonium-Induced Nitrifier Denitrification to N2O in Paddy Fields.

Paddy fields are one of the most important sources of nitrous oxide (N2O), but biogeochemical N2O production mechanisms in the soil profile remain unclear. Our study used incubation, dual-isotope (15N-18O) labeling methods, and molecular techniques to elucidate N2O production characteristics and mechanisms in the soil profile (0-60 cm) during summer fallow, rice cropping, and winter fallow periods. The results pointed out that biotic processes dominated N2O production (72.2-100%) and N2O from the tillage layer accounted for 91.0-98.5% of total N2O in the soil profile. Heterotrophic denitrification (HD) was the main process generating N2O, contributing between 53.4 and 96.6%, the remainder being due to ammonia oxidation pathways, which was further confirmed by metagenomics and quantitative polymerase chain reaction (qPCR) assays. Nitrifier denitrification (ND) was an important N2O production source, contributing 0-46.6% of total N2O production, which showed similar trends with N2O emissions. Among physicochemical and biological factors, ammonium content and the ratio of total organic matter to nitrate were the main driving factors affecting the contribution ratios of the ammonia oxidation pathways and HD pathway, respectively. Moisture content and pH affect norC-carrying Spirochetes and thus the N2O production rate. These findings confirm the importance of ND to N2O production and help to elucidate the impact of anthropogenic activities, including tillage, fertilization, and irrigation, on N2O production.

Nitrifiers Cooperate to Produce Nitrous Oxide in Plateau Wetland Sediments.

The thawing of dormant plateau permafrost emits nitrous oxide (N2O) through wetlands; however, the N2O production mechanism in plateau wetlands is still unclear. Here, we used the 15N-18O double tracer technique and metagenomic sequencing to analyze the N2O production mechanism in the Yunnan-Kweichow and Qinghai-Tibet plateau wetlands during the summer of 2020. N2O production activity was detected in all 16 sediment samples (elevation 1020-4601 m: 2.55 ± 0.42-26.38 ± 3.25 ng N g-1 d-1) and was promoted by nitrifier denitrification (ND). The key functional genes of ND (amoA, hao, and nirK) belonged to complete ammonia oxidizing (comammox) bacteria, and the key ND species was the comammox bacterium Nitrospira nitrificans. We found that the comammox bacterial species N. nitrificans and the ammonia oxidizing bacterial (AOB) species Nitrosomonas europaea cooperate to produce N2O in the plateau wetland sediments. Furthermore, we inferred that environmental factors (elevation and total organic matter (TOM)) influence the cooperation pattern via N. nitrificans, thus affecting the N2O production activity in the plateau wetland sediments. Our findings advance the mechanistic understanding of nitrifiers in biogeochemical cycles and global climate change.

Towards a more labor-saving way in microbial ammonium oxidation: A review on complete ammonia oxidization (comammox).

In the Anthropocene, nitrogen pollution is becoming an increasing challenge for both mankind and the Earth system. Microbial nitrogen cycling begins with aerobic nitrification, which is also the key rate-limiting step. For over a century, it has been accepted that nitrification occurs sequentially involving ammonia oxidation, which produces nitrite followed by nitrite oxidation, generating nitrate. This perception was changed by the discovery of comammox Nitrospira bacteria and their metabolic pathway. In addition, this also provided us with new knowledge concerning the complex nitrogen cycle network. In the comammox process, ammonia can be completely oxidized to nitrate in one cell via the subsequent activity of the enzyme complexes, ammonia monooxygenase, hydroxylamine dehydrogenase, and nitrite oxidodreductase. Over the past five years, research on comammox made great progress. However, there still exist a lot of questions, including how much does comammox contribute to nitrification? How large is the diversity and are there new strains to be discovered? Do comammox bacteria produce the greenhouse gas N2O, and how or to which extent may they contribute to global climate change? The above four aspects are of great significance on the farmland nitrogen management, aquatic environment restoration, and mitigation of global climate change. As large number of comammox bacteria and pathways have been detected in various terrestrial and aquatic ecosystems, indicating that the comammox process may exert an important role in the global nitrogen cycle.

Comammox activity dominates nitrification process in the sediments of plateau wetland.

The recent discovery of complete ammonia oxidation (comammox) has increased our understanding of nitrification. Although comammox has been shown to play an important role in plain wetland ecosystems, studies of comammox contribution are still limited in plateau wetland ecosystems. Here, we analyzed the abundance, activity, community and biogeochemical mechanisms of the comammox bacteria in Yunnan-kweichow and Qinghai-Tibet plateau wetlands from elevations of 1000-5000 m. Comammox bacteria were widely distributed in all 16 sediment samples with abundances higher than 0.96 ± 0.26 × 107 copies g-1 (n = 16). Comammox showed high activity (1.18 ± 0.17 to 1.98 ± 0.08 mg N kg-1 d-1) at high-elevation (3000-5000 m) and dominated the nitrification process (activity contribution: 37.20 - 60.62%). The activity contribution of ammonia-oxidizing bacteria (1.07 ± 0.08 to 2.79 ± 0.35 mg N kg-1 d-1) dominated the nitrification process (44.55 - 64.15%) in low-elevation (1000-3000 m) samples. All detected comammox Nitrospira belonged to clade A, while clade B was not detected. Elevation always had a strongest effect on key comammox species. Thus, we infer that elevation may drive the high relative abundance of the species Candidatus Nitrospira nitrificans (avg. 12.40%) and the low relative abundance of the species Nitrospira sp. SG-bin2 (avg. 4.75%) in high-elevation samples that showed a high comammox activity (avg. 1.62 mg N kg-1 d-1) and high contribution (avg. 46.08%) to the nitrification process. These results indicate that comammox may be an important and currently underestimated microbial nitrification process in plateau wetland ecosystems.

Heavy metal could drive co-selection of antibiotic resistance in terrestrial subsurface soils.

Terrestrial surface ecosystems are important sinks for antibiotic resistance genes (ARGs) due to the continuous discharge of contaminants from human-impacted ecosystems. However, the abundance and resistance types of ARGs and their influencing factors in terrestrial subsurface soils are not well known. In this study, we investigated the abundance and diversity of ARGs, and their correlations with metal resistance genes (MRGs), mobile genetic elements (MGEs), bacteria, and heavy metals in subsurface soils using high throughput quantitative PCR and metagenomic sequencing approaches. Abundant and diverse ARGs were detected with high spatial heterogeneity among sampling sites. Vertically, there was no significant difference in ARG profiles between the aquifer and non-aquifer soils. Heavy metals were key factors shaping ARG profiles in soils with high heavy metal contents, while they showed no significant effect in low contents. Moreover, heavy metals could trigger the proliferation of antibiotic resistance by increasing MGE abundance or influencing bacterial communities. Metagenomic analysis also revealed the widespread co-occurrence of ARGs and MRGs, with heavy metals possibly enhancing the co-selection of ARGs and MRGs in soils with high heavy metal contents. This study highlighted the heavy metal-driven co-selection of ARGs and revealed the occurrence of ARG pollution in terrestrial subsurface soils.

Abundance and Functional Importance of Complete Ammonia Oxidizers and Other Nitrifiers in a Riparian Ecosystem.

The discovery of complete ammonia oxidation (comammox) has altered our understanding of nitrification, which is the rate-limiting process in the global nitrogen cycle. However, understanding the ecological role of comammox or its contribution to nitrification in both natural and artificial ecosystems is still in its infancy. Here, we investigated the community distribution and function of comammox bacteria in riparian ecosystems and analyzed interactions between comammox and other nitrogen cycling microorganisms. The comammox bacterial abundance and rate were higher in summer than in winter and higher in nonrhizosphere soils than in the rhizosphere. Fringe soils in the riparian zone comprise a comammox hotspot, where the abundance (2.58 × 108 copies g-1) and rate (0.86 mg N kg-1 d-1) of comammox were not only higher than at other sampling sites but also higher than those of other ammonia oxidation processes. The comammox rate correlated significantly positively with relative abundance of the comammox species Candidatus Nitrospira nitrificans but not with that of the species Candidatus Nitrospira nitrosa. Analysis of comammox interaction with other ammonia-oxidizing processes revealed ammonia-oxidizing archaea to dominate interface soils, comammox to dominate in fringe soils, and anaerobic ammonium oxidation (anammox) to dominate in interface sediments of the riparian zone. These results indicate that comammox may constitute an important and currently underestimated process of microbial nitrification in riparian zone ecosystems.

Air pollution could drive global dissemination of antibiotic resistance genes.

Antibiotic-resistant pathogens pose a significant threat to human health. Several dispersal mechanisms have been described, but transport of both microbes and antibiotic resistance genes (ARGs) via atmospheric particles has received little attention as a pathway for global dissemination. These atmospheric particles can return to the Earth's surface via rain or snowfall, and thus promote long-distance spread of ARGs. However, the diversity and abundance of ARGs in fresh snow has not been studied and their potential correlation with particulate air pollution is not well explored. Here, we characterized ARGs in 44 samples of fresh snow from major cities in China, three in North America, and one in Europe, spanning a gradient from pristine to heavily anthropogenically influenced ecosystems. High-throughput qPCR analysis of ARGs and mobile genetic elements (MGEs) provided strong indications that dissemination of ARGs in fresh snow could be exacerbated by air pollution, severely increasing the health risks of both air pollution and ARGs. We showed that snowfall did effectively spread ARGs from point sources over the Earth surface. Together our findings urge for better pollution control to reduce the risk of global dissemination of antibiotic resistance genes.

Dissimilatory nitrate reduction to ammonium (DNRA) in traditional municipal wastewater treatment plants in China: Widespread but low contribution.

Recent reports on the occurrence and contribution of dissimilatory nitrate reduction to ammonium (DNRA) in marine, inland water, and soil systems have greatly improved our understanding of the global nitrogen (N) cycle. This also promoted the investigation of the role and ecological features of DNRA in anthropogenic ecosystems. However, so far, the use of DNRA in municipal wastewater treatment plants (WWTPs), which are one of the most common and largest biotechnologically artificial water ecosystems, has not been investigated. Accordingly, this study focused on the abundance, activity, community structure, and diversity of DNRA bacteria in full-scale WWTPs. DNRA bacteria were detected in all treatment units in six tested municipal WWTPs, even in aerobic zones (dissolved oxygen > 2 mg L-1). Although the relative abundance of DNRA bacteria (0.2-4.0%) was less than that of denitrifying bacteria (0.7-10.1%) among all investigated samples, the abundance of DNRA bacteria still reaches 109 gene copies g-1. However, 15N-isotope tracing indicated that the potential DNRA rates were significantly lower (0.4-2.1 nmol N g-1 h-1) than those of denitrification (9.5-15.7 nmol N g-1 h-1), but higher than anammox rate (0.3-1.3 nmol N g-1 h-1). The DNRA bacterial community structure was primarily affected by temperature gradient despite the treatment process. High-throughput sequencing analysis targeting the DNRA nrfA gene showed that Nitrospira accounted for the largest proportion of nrfA genes among all samples (6.2-36.3%), followed by Brocadia (5.9-22.1%). Network analysis further indicated that Nitrospira played an important role in both the DNRA bacterial community and entire bacterial community in municipal WWTPs. These results suggest that the ecological habitats of DNRA bacteria in anthropogenic ecosystems were far more abundant than previously assumed. However, the contribution to N transformation by the widespread DNRA was not significant in traditional municipal WWTPs.

Applying constructed wetland-microbial electrochemical system to enhance NH4+ removal at low temperature.

NH4+ removal at low temperature (<10 °C) has baffled researchers and engineers for decades. Bioelectrochemical process has been increasingly valued as a promising approach to enhance NH4+ removal by both electrochemical and stimulated microbial processes. The feasibility and the mechanism of enhanced NH4+ removal were investigated in Constructed Wetland-Microbial Electrochemical System (CW-MES) with different electrode spacings including Constructed Wetland-Microbial Fuel Cell (CW-MFC) and Constructed Wetland-Microbial Electrolysis Cell (CW-MEC) at low temperature. Solar cell panel was firstly implemented in CW-MEC to enhance NH4+ removal. The low-temperature operation lasted for about four months, CW-MEC successfully enhanced NH4+ removal while CW-MFC did not exhibit positive effect. The NH4+-N removal efficiency of CW-MEC achieved 88.2 ± 7.0%, which was 11.7 ± 6.5% higher than conventional constructed wetland (CCW). The maximum NH4+-N removal efficiency of CW-MEC achieved 100%. The average NH4+-N mass removal rate was 436.02 mg m-2 d-1. It was found that NH4+ was mainly removed by the nitrification-autotrophic denitrification process in CW-MES while it was mainly converted to NO3- in CCW. Ammoxidation and denitrification were both enhanced by electricity while NH4+ was used as the main substrate for electricity generation. AOA (Candidatus Nitrosocosmicus) and NOB (Nitrospira) were the main contributors to nitrification. This study provided a cost-effective and sustainable method for electrochemically enhanced microbial NH4+ removal at low-temperature and revealed the relevant mechanism.

Application of the Cobas 4800 System for the Detection of High-Risk Human Papillomavirus in 5650 Asymptomatic Women.

High-risk papillomavirus (HR-HPV) testing combined with cytology improves the detection of cervical lesions and increases length of screening intervals. For a population-based HR-HPV survey, testing automation is in great need. The Cobas 4800 HPV Test System is a fully automated assay that can simultaneously detect HPV16, HPV18, and other 12 pooled HR-HPV genotypes. This system has been employed for HR-HPV screening in a number of countries; however, such application in a large population in China has not been documented. In this study, we employed the Cobas 4800 HPV Test System to detect HR-HPV in cervical cytology specimens collected from a total of 5650 asymptomatic women from a region of South China. We reported the following: (1) the prevalence of the 14 genotypes of HR-HPV was 12.96%; (2) for those with HR-HPV infection, 2.25% were positive for HPV16, 0.50% for HPV18, 9.15% for pooled 12 HPV types, and 1.06% for multiple HPV infection; and (3) there was no significant difference in the HR-HPV prevalence among different age groups. HPV16 and HPV18 have been shown to be the predominant HPV types found in cervical cancer patients in some regions in China, indicating that a fully automated assay like the Cobas 4800 HPV Test System is especially valuable for population-based HR-HPV screening in these regions as this assay can concurrently detect HPV16 and HPV18.

Recovering partial nitritation in a PN/A system during mainstream wastewater treatment by reviving AOB activity after thoroughly inhibiting AOB and NOB with free nitrous acid.

Starting up or recovering partial nitritation is a major challenge for achieving or maintaining stable partial nitritation/anammox (PN/A) during mainstream wastewater treatment. This study presents a novel strategy for recovering the nitrite pathway by selectively reviving ammonium oxidizing bacteria (AOB) after thoroughly inhibiting AOB and nitrite oxidizing bacteria (NOB) using free nitrous acid (FNA). A sequencing batch reactor was operated for PN/A to treat real domestic wastewater for 423 days, during which twice FNA treatment was temporarily implemented. Results showed that with a single 0.45 mg/L FNA treatment on flocculent sludge, the NO3--N concentration during the aerobic period showed an uptrend again and the partial nitritation performance was deteriorated. In contrast, 1.35 mg/L FNA treatment induced the inhibition of both AOB and NOB leading to regressive ammonium oxidation, but a subsequently higher DO (1.5 mg/L) and longer aeration duration recovered partial nitritation. For the relative abundances of the acquired biomass related to nitrogen conversion, Nitrosomonas, Nitrospira and Nitrolancea increased to 9.65%, 10.27% and 4.35%, respectively, at the beginning of the 1.35 mg/L FNA treatment, and Nitrospira and Nitrolancea decreased to 2.80% and 0.03% whereas Nitrosomonas declined to 8.71% after 76 days. Ca. Brocadia showed less resilience after the 1.35 mg/L FNA treatment, with the relative abundance decreasing from 13.38% to 0.62% due to insufficient nitrite. Molecular ecological network analysis indicates that among anammox taxa, Ca. Kuenenia and Ca. Brocadia formed important links with other N cycle processes. Moreover, the proposed strategy shows operational flexibility because it can be easily used to control NOB in mainstream PN/A applications offered by flocculent sludge systems.

Comammox bacterial abundance, activity, and contribution in agricultural rhizosphere soils.

The newly identified complete ammonia oxidation (comammox), which is capable of oxidizing ammonia directly to nitrate, has complemented our knowledge of nitrification in the global nitrogen (N) cycle. However, understanding the contribution and ecological roles of comammox in complex soil environments is still in its infancy. Here, the community structure and function of comammox and the interactions with other ammonia oxidation processes in rhizosphere and non-rhizosphere soils of four different crop fields (maize, cotton, soybean, and millet) were investigated in summer and winter. The only identified comammox species Candidatus Nitrospira nitrificans was widely distributed in all sampled soils. Comammox bacterial abundance was lower than that of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). The measured comammox potential rate ranged from 0.01 ± 0.002 to 0.40 ± 0.02 mg N kg-1 d-1, contributing <19.2 and 22.1% to ammonia oxidation in summer and winter, the remainder being due to AOA and AOB. The potential rate and community composition of comammox bacteria were significantly different on a temporal scale, while crop species and soil types (rhizosphere and non-rhizosphere) showed no obvious influences. In terms of oxidation rates, AOA (1.2 ± 0.7 mg N kg-1 d-1) dominated the ammonia oxidation in agricultural soils over AOB (0.31 ± 0.1 mg N kg-1 d-1) and comammox (0.2 ± 0.1 mg N kg-1 d-1). Both anammox bacterial abundance and activity were below the detection limits, indicating a negligible contribution of anammox in agricultural rhizosphere soils. The identification of comammox bacterial abundance and activity in situ enriches our knowledge of nitrification in agricultural systems.

Biogeographic distribution of comammox bacteria in diverse terrestrial habitats.

Comammox, the microbial group capable of completely oxidizing ammonia to nitrate, challenged the traditional two-step nitrification process where ammonia is oxidized by ammonia-oxidizing archaea (AOA) and bacteria (AOB), and nitrite by nitrite-oxidizing bacteria (NOB). However, the distribution of comammox bacteria in various habitats and their potential environmental drivers remain poorly understood. Using qPCR and high-throughput sequencing approach, we analyzed the abundance and community patterns of comammox from 38 samples taken from five different habitat types including paddy fields in Shaoguan and Antu, the wheat fields, river, and grassland in the Qinghai-Tibet Plateau, and the fringe and central riparian zones of Chaohu Lake of China during winter and summer. Comammox bacteria were detected in all samples, with Ca. N. nitrificans dominating the community, followed by Ca. N. nitrosa. Generally, in paddy fields of Shaoguan and Antu, ammonia (NH4+) was the key factor affecting comammox bacteria. However, in wheat fields, river and grassland of the Qinghai-Tibet Plateau, altitude was the strongest factor affecting comammox bacteria. In Chaohu Lake, comammox bacteria showed temporal heterogeneity, being higher in winter than summer, especially in the fringe riparian zone. Our results suggest that comammox is widespread in diverse habitats and exhibit niche partitioning, and can be affected by different environmental factors that may vary by habitat.

Hotspot of dissimilatory nitrate reduction to ammonium (DNRA) process in freshwater sediments of riparian zones.

Dissimilatory nitrate reduction to ammonium (DNRA), an important intermediate process in the N-cycle, links N-compound oxidation and reduction processes. Hence, the oxic-anoxic interface would be the hotspot of the DNRA process. In freshwater ecosystems, the riparian zone is the most typical carrier of the oxic-anoxic interface. Here we report spatio-temporal evidence of a higher abundance and rate of DNRA in the riparian zone than in the open water sediments based on molecular and 15N isotopic-tracing technologies, hence signifying a hotspot for the DNRA process. These abudance and rates were significantly higher than those in open water sediments. 15N isotopic paring technology revealed that the DNRA hotspot promoted higher rates of N-compound oxidation (NO2-), reduction (NO3- and DNRA), and N2 production (anammox and denitrification) in the riparian zone than those in open water sediment. However, high-through sequencing analysis showed that the DNRA bacteria in the riparian zone and openwater sediments were insignificantly different. Network and correlation analysis showed that the DNRA abundance and rates were significantly positively correlated with TOM, TC/NH4+, and TC/NO2-, but not with the dominant genera (Anaeromyxobacter, Lacunisphaera, and Sorangium), which played different roles on the connection in the respective community networks. The DNRA process in the riparian zone could be driven mainly by the related environmental biogeochemical characteristics induced by anthropogenic changes, followed by microbial processes. This result provides valuable information for the management of riparian zones because anthropogenic changes in the riparian water table are expected to increase, inducing consequent changes in the reduction from NO3- to NH4+.

Source, contribution and microbial N-cycle of N-compounds in China fresh snow.

The importance and contribution of nitrogen compounds and the related microbial nitrogen cycling processes in fresh snow are not well understood under the current research background. We collected fresh snow samples from 21 cities that 80% are from China during 2016 and 2017. Principal component analysis showed that SO42- were in the first principal component, and N-compounds were the second. Furthermore, the main pollutant ions SO42- and NO3- were from anthropogenic sources, and SO42- contributed (61%) more to the pollution load than NO3- (29%), which were confirmed through a series of precipitation mechanism analysis. We selected five N-cycle processes (consist of oxidation and reduction processes) for molecular biology experiments, including Ammonia-oxidation process, Nitrite-oxidation process, Denitrification process, Anaerobic-ammoxidation process (Anammox) and Dissimilatory nitrate reduction to ammonium process (DNRA). Except ammonia-oxidizing archaeal (AOA) and bacterial (AOB) amoA genes (above 107 copies g-1), molecular assays of key functional genes in various nitrogen conversion processes showed a belowed detection limit number, and AOB abundance was always higher than AOA. The determination of the microbial transformation rate using the 15N-isotope tracer technique showed that the potential rate of five N-conversion processes was very low, which is basically consistent with the results from molecular biology studies. Taken together, our results illustrated that microbial nitrogen cycle processes are not the primary biological processes causing the pollution in China fresh snow.

Anaerobic ammonium oxidation is a major N-sink in aquifer systems around the world.

Global-scale N-oxide contamination of groundwater within aquifers occurs due to the widespread use of N-bearing fertilizers and chemicals, threatening both human and environmental health. However, the conversion of these pollutants in active nitrogen (N) cycling processes in the subsurface biosphere still remains unclear. This study investigates the global occurrence of anaerobic ammonium oxidation (anammox) in aquifers, where anammox was found to be turned on and off between saturated and unsaturated soil horizons, and contributed 36.8-79.5% to N loss in saturated soil horizons, the remainder being due to denitrification which has traditionally been considered the main pathway for removal of N-pollutants from aquifers. Although anammox activity was undetectable in the unsaturated soil horizons, it could potentially be activated by contact with ascending groundwater. High-throughput pyrosequencing analysis identified Candidatus Brocadia anammoxidans as being the most abundant anammox bacterium in the saturated soils investigated. However, the anammox bacterial abundance was determined by the relative richness of Candidatus Jettenia asiatica. Isotopic pairing experiments revealed that coupling anammox with ammonium oxidation and respiratory ammonification enabled the formation of a revised N cycle in aquifer systems, in which respiratory ammonification acted as an important coordinator. Anammox can therefore contribute substantially to aquifer N cycling and its role in remediation of aquifers contaminated with N-oxides may be of global importance.