Nitrogen loss by anaerobic ammonium oxidation in unconfined aquifer soils

Effect of a Prolonged-Release System of Urea on Nitrogen Losses and Microbial Population Changes in Two Types of Agricultural Soil

Urea is the nitrogen-containing fertilizer most used in agricultural fields; however, the nutrient given by the urea is lost into the environment. The aim of this research was to determine the effect of two soil textures by applying a prolonged-release system of urea (PRSU) on the N losses. This research shows an important decrease of the nitrate and ammonium losses from 24.91 to 87.94%. Also, the microbiological population increases after the application of the PRSU. It was concluded that both soil textures presented the same loss-reduction pattern, where the N from the nitrates and ammonium was reduced in the leachates, increasing the quality of the soil and the microbial population in both soil textures after the PRSU application.

Nitrogen fixation and diazotroph diversity in groundwater systems

Biological nitrogen fixation (BNF), the conversion of N2 into bioavailable nitrogen (N), is the main process for replenishing N loss in the biosphere. However, BNF in groundwater systems remains poorly understood. In this study, we examined the activity, abundance, and community composition of diazotrophs in groundwater in the Hetao Plain of Inner Mongolia using 15N tracing methods, reverse transcription qPCR (RT-qPCR), and metagenomic/metatranscriptomic analyses. 15N2 tracing incubation of near in situ groundwater (9.5-585.4 nmol N L-1 h-1) and N2-fixer enrichment and isolates (13.2-1728.4 nmol N g-1 h-1, as directly verified by single-cell resonance Raman spectroscopy), suggested that BNF is a non-negligible source of N in groundwater in this region. The expression of nifH genes ranged from 3.4 × 103 to 1.2 × 106 copies L-1 and was tightly correlated with dissolved oxygen (DO), Fe(II), and NH4+. Diazotrophs in groundwater were chiefly aerobes or facultative anaerobes, dominated by Stutzerimonas, Pseudomonas, Paraburkholderia, Klebsiella, Rhodopseudomonas, Azoarcus, and additional uncultured populations. Active diazotrophs, which prefer reducing conditions, were more metabolically diverse and potentially associated with nitrification, sulfur/arsenic mobilization, Fe(II) transport, and CH4 oxidation. Our results highlight the importance of diazotrophs in subsurface geochemical cycles.

Response of microbial taxonomic and nitrogen functional attributes to elevated nitrate in suburban groundwater

Anthropogenic nitrogen (N) input has led to elevated levels of nitrate nitrogen (NO₃^--N) in the groundwater. However, insights into the responses of the microbial community and its N metabolic functionality to elevated NO₃^--N in suburban groundwater are still limited. Here, we explored the microbial taxonomy, N metabolic attributes, and their responses to NO₃^--N pollution in groundwaters from Chaobai River catchment (CR) and Huai River catchment (HR) in Beijing, China. Results showed that average NO₃^--N and NH₄⁺-N concentrations in CR groundwater were 1.7 and 3.0 folds of those in HR. NO₃^--N was the dominant nitrogen specie both in HR and CR groundwater (over 80 %). Significantly different structures and compositions of the microbial communities and N cycling gene profiles were found between CR groundwater and HR groundwater (p < 0.05), with CR groundwater harboring significantly lower microbial richness and abundance of N metabolic genes. However, denitrification was the dominant microbial N cycling process in both CR and HR groundwater. Strong associations among NO₃^--N, NH₄⁺-N, microbial taxonomic, and N functional attributes were found (p < 0.05), suggesting denitrifiers and Candidatus_Brocadia might serve as potential featured biomarkers for the elevated NO₃^--N and NH₄⁺-N concentration in groundwater. Path analysis further revealed the significant effect of NO₃^--N on the overall microbial N functionality and microbial denitrification (p < 0.05). Collectively, our results provide field evidence that elevated levels of NO₃^--N and NH₄⁺-N under different hydrogeologic conditions had a significant effect on the microbial taxonomic and N functional attributes in groundwater, with potential implications for improving sustainable N management and risk assessment of groundwater.

Characterization of Enrichment Cultures of Anammox, Nitrifying and Denitrifying Bacteria Obtained from a Cold, Heavily Nitrogen-Polluted Aquifer

Anammox bacteria related to Candidatus Scalindua were recently discovered in a cold (7.5 °C) aquifer near sludge repositories containing solid wastes of uranium and processed polymetallic concentrate. Groundwater has a very high level of nitrate and ammonia pollution (up to 10 and 0.5 g/L, respectively) and a very low content of organic carbon (2.5 mg/L). To assess the potential for bioremediation of polluted groundwater in situ, enrichment cultures of anammox, nitrifying, and denitrifying bacteria were obtained and analyzed. Fed-batch enrichment of anammox bacteria was not successful. Stable removal of ammonium and nitrite (up to 100%) was achieved in a continuous-flow reactor packed with a nonwoven fabric at 15 °C, and enrichment in anammox bacteria was confirmed by FISH and qPCR assays. The relatively low total N removal efficiency (up to 55%) was due to nonstoichiometric nitrate buildup. This phenomenon can be explained by a shift in the metabolism of anammox bacteria towards the production of more nitrates and less N₂ at low temperatures compared to the canonical stoichiometry. In addition, the too high an estimate of specific anammox activity suggests that N cycle microbial groups other than anammox bacteria may have contributed significantly to N removal. Stable nitrite production was observed in the denitrifying enrichment culture, while no "conventional" nitrifiers were found in the corresponding enrichment cultures. Xanthomonadaceae was a common taxon for all microbial communities, indicating its exclusive role in this ecosystem. This study opens up new knowledge about the metabolic capabilities of N cycle bacteria and potential approaches for sustainable bioremediation of heavily N-polluted cold ecosystems.

Spatial-Temporal Distribution, Morphological Transformation, and Potential Risk of Dissolved Inorganic Nitrogen in the Contaminated Unconfined Aquifer from a Retired Nitrogenous Fertilizer Plant

The accumulation of nitrogen in groundwater in the industrial plots, especially the high ammonium, can result in a serious threat to the groundwater system in the urban area. This study monitored the dissolved inorganic nitrogen (DIN) of the polluted groundwater four times in one year in a retired nitrogenous fertilizer plant site with a production history of nearly 40 years, to analyze the spatial-temporal characteristics of DIN species (NH4+-N, NO3−-N, and NO2−-N) and the effects of groundwater environment on their transfer and transformation. The results showed that NH4+-N (<0.025 to 1310 mg/L) was the main DIN species (61.38−76.80%) with low mobility, whereas the concentration of NO3−-N and NO2−-N was 0.15−146 mg/L and <0.001−12.4 mg/L, accounting for 22.34−36.07% and 0.53−2.83% of total DIN, respectively. The concentration and proportion of NO3−-N and NO2−-N showed an upward trend with time, posing a threat to the safety of surrounding groundwater, and their high spatial-temporal variation was related to the morphological transformation and the transport. In the wet season, the pH and redox condition benefited the nitrification, and NO3−-N easily migrated from the deep soil solution to groundwater, hence the NO3−-N can be accumulated. Therefore, the analysis of species and behaviors of DIN in shallow groundwater is indispensable for environmental risk assessment.

Bacteria Community Vertical Distribution and Its Response Characteristics to Waste Degradation Degree in a Closed Landfill

The diversity, community structure and vertical distribution characteristics of bacteria in the surface and subsurface soil and water samples of a closed landfill in Shanghai Jiading District were investigated to reveal the relationships between natural waste degradation degree and the succession of bacterial community composition. High-throughput sequencing of bacteria 16S rDNA genes was used to analyze the bacterial community structure and diversity. The results showed that the diversity of bacteria in the surface samples was higher than that in the deep samples. Proteobacteria was the dominant phylum in all the samples, and the percentage increased with depth. At the genus level, Thiobacillus, Pseudomonas, Aquabacterium, and Hydrogenophaga were the dominant genera in surface, medium, deep and ultra-deep soils, respectively. The Bray–Curtis dissimilarity of the soil bacterial communities in the same layer was small, indicating that the community composition of the samples in the same layer was similar. The RDA result showed that ammonium, nitrate, pH and C/N significantly influenced the community structure of soil bacteria. This is of great relevance to understand the effect of natural waste degradation on bacterial communities in closed landfills.

Metabolic Diversity and Aero-Tolerance in Anammox Bacteria from Geochemically Distinct Aquifers

Anaerobic ammonium oxidation (anammox) is important for converting bioavailable nitrogen into dinitrogen gas, particularly in carbon-poor environments. However, the diversity and prevalence of anammox bacteria in the terrestrial subsurface-a typically oligotrophic environment-are little understood. To determine the distribution and activity of anammox bacteria across a range of aquifer lithologies and physicochemistries, we analyzed 16S rRNA genes and quantified hydrazine synthase genes and transcripts sampled from 59 groundwater wells and metagenomes and metatranscriptomes from an oxic-to-dysoxic subset. Data indicate that anammox and anammox-associated bacteria (class "Candidatus Brocadiae") are prevalent in the aquifers studied, and that anammox community composition is strongly differentiated by dissolved oxygen (DO), but not ammonia/nitrite. While "Candidatus Brocadiae" diversity decreased with increasing DO, "Candidatus Brocadiae" 16S rRNA genes and hydrazine synthase (hzsB) genes and transcripts were detected across a wide range of bulk groundwater DO concentrations (0 to 10 mg/L). Anammox genes and transcripts correlated significantly with those involved in aerobic ammonia oxidation (amoA), potentially representing a major source of nitrite for anammox. Eight "Candidatus Brocadiae" genomes (63 to 95% complete), representing 2 uncharacterized families and 6 novel species, were reconstructed. Six genomes have genes characteristic of anammox, all for chemolithoautotrophy. Anammox and aerotolerance genes of up to four "Candidatus Brocadiae" genomes were transcriptionally active under oxic and dysoxic conditions, although activity was highest in dysoxic groundwater. The coexpression of nrfAH nitrite reductase genes by "Candidatus Brocadiae" suggests active regeneration of ammonia for anammox. Our findings indicate that anammox bacteria contribute to loss of fixed N across diverse anoxic-to-oxic aquifer conditions, which is likely supported by nitrite from aerobic ammonia oxidation. IMPORTANCE Anammox is increasingly shown to play a major role in the aquatic nitrogen cycle and can outcompete heterotrophic denitrification in environments low in organic carbon. Given that aquifers are characteristically oligotrophic, anammox may represent a major route for the removal of fixed nitrogen in these environments, including agricultural nitrogen, a common groundwater contaminant. Our research confirms that anammox bacteria and the anammox process are prevalent in aquifers and occur across diverse lithologies (e.g., sandy gravel, sand-silt, and volcanic) and groundwater physicochemistries (e.g., various oxygen, carbon, nitrate, and ammonium concentrations). Results reveal niche differentiation among anammox bacteria largely driven by groundwater oxygen contents and provide evidence that anammox is supported by proximity to oxic niches and handoffs from aerobic ammonia oxidizers. We further show that this process, while anaerobic, is active in groundwater characterized as oxic, likely due to the availability of anoxic niches.

Dissimilatory nitrate reduction and functional genes in two subtropical rivers, China

Dissimilatory nitrate reduction processes, including denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA), are important pathways of nitrate transformation in the aquatic environments. In this study, we investigated potential rates of denitrification, anammox, and DNRA in the sediments of two subtropical rivers, Jinshui River and Qi River, with different intensities of human activities in their respective catchment, China. Our objectives were to assess the seasonality of dissimilatory nitrate reduction rates, quantify their respective contributions to nitrate reduction, and reveal the relationship between dissimilatory nitrate reduction rates, functional gene abundances, and physicochemicals in the river ecosystems. Our results showed higher rates of denitrification and anammox in the intensively disturbed areas in autumn and spring, and higher potential DNRA in the slightly disturbed areas in summer. Generally, denitrification, anammox, and DNRA were higher in summer, autumn, and spring, respectively. Relative contributions of nitrate reduction from denitrification, anammox, and DNRA were quite different in different seasons. Dissimilatory nitrate reduction rates and gene abundances correlated significantly with water temperature, dissolved organic carbon (DOC), sediment total organic carbon (SOC), NO₃^-, NH₄⁺, DOC/NO₃^-, iron ions, and sulfide. Understanding dissimilatory nitrate reduction is essential for restoring nitrate reduction capacity and improving and sustaining ecohealth of the river ecosystems.

Soil microbial community composition in a paddy field with different fertilization managements

Microbes play vital roles in soil quality; however, their response to N (nitrogen) and P (phosphorus) fertilization in acidic paddy soils of subtropical China remains poorly understood. Here, a 10-year field experiment was conducted to evaluate the effects of different fertilization treatments on microbial communities by Illumina MiSeq sequencing. The results showed that different fertilization treatments did not exert a significant effect on microbial alpha diversity, but altered soil properties, and thus affected microbial community composition. The microbial communities in the T₁ (optimized N and P fertilizer) and T₂ (excessive N fertilizer) treated soils differed from those in the T₀ (no N and P fertilizer) and T₃ (excessive P fertilizer) treated soils. In addition, the bacterial phyla Proteobacteria, Chloroflexi, and Acidobacteria, and the fungal phyla Ascomycota and Basidiomycota dominated all the fertilized treatments. Soil total potassium (TK) concentration was the most important factor driving the variation in bacterial community structure under different fertilization regimes, while the major factors shaping fungal community structure were soil TN and NO₃^--N (nitrate N). These findings indicate that optimization of N and P application rates might result in variations in soil properties, which changed the microbial community structure in the present study.

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.

Bias of marker genes in PCR of anammox bacteria in natural habitats

The identification of anammox bacteria is mostly relied on PCR with various marker genes. However, the community composition revealed by different marker genes and whether the marker genes influence the resulted community composition remain unclear. We compared the community structure of anammox bacteria in enriched and natural environments revealed by 16S rRNA and functional genes (hzo, hzsA and hzsB) from public database and published papers. The genus of Ca. Scalindua showed the lowest similarities with other genera, especially for the hzsA gene (66.9%-68.6%). The 16S rRNA gene is the most commonly used marker gene in natural habitats with 151 out 221 papers in total. The anammox bacterial community composition is distributed according to the source of habitat regardless the use of various marker genes. The role of marker gene is limited with explanatory of 5.4% for variance of community composition, versus 20.5% of habitat. The effect of marker gene is mainly acted on freshwater habitat, which shows significant different community composition revealed by 16S rRNA and hzo, with Ca. Brocadia and Ca. Jettenia as dominant genus, respectively.

Temporal analysis of the microbial communities in a nitrate-contaminated aquifer and the co-occurrence of anammox, n-damo and nitrous-oxide reducing bacteria

Groundwater-N pollution derives from agricultural and urban activities, and compromises water quality in shallow aquifers, putting human and environmental health at risk. Nonetheless, subsurface microbiota can transform dissolved inorganic nitrogen into N₂. In this study, we surveyed the microbial community of a shallow aquifer by sampling one well, one piezometer and a spring within an agricultural area that receives N-inputs of more than 700 kg/ha per year through irrigation with wastewater. The survey was conducted during a year with a 16S rRNA next-gen approach. In parallel, we quantified the number of gene copies and transcripts related to anaerobic ammonium oxidation (anammox, hzo), nitrite-dependent anaerobic methane oxidation (n-damo, nod and pmoA) and nitrous oxide reduction (last step of denitrification, nosZ), during the dry and rainy seasons. Our results showed that the groundwater samples had 17.7 to 22.5 mg/L of NO₃^--N. The bacterial and archaeal community structure was distinctive at each site, and it remained relatively stable over time. We verified the co-occurrence of N-transforming bacteria, which was correlated with the concentration of NO₂^-/NO₃^- and ORP/DO values (DO: ~3.0 mg/L). Our analyses suggest that these conditions may allow the presence of nitrifying microorganisms which can couple with anammox, n-damo and denitrifying bacteria in interrelated biogeochemical pathways. Gene density (as the number of gene copies per litre) was lower in the rainy season than in the dry season, possibly due to dilution by rainwater infiltration. Yet, the numbers of hzo gene copies here found were similar to those reported in oceanic oxygen minimum zones and in a carbonate-rock aquifer. The transcript sequences showed that Candidatus Brocadia spp. (anammox), Candidatus Methylomirabilis spp. (n-damo) and autotrophic denitrifying Betaproteobacteria coexist in the groundwater environment, with the potential to attenuate the concentration of dissolved inorganic nitrogen by reducing it to N₂ rather than N₂O; delivering thus, an important ecosystem service to remove contaminants.

Nitrate reduction in the reed rhizosphere of a riparian zone: From functional genes to activity and contribution

The increased nitrogen (N) fertilizer usage caused substantial nitrate (NO₃^-) leaching into groundwater and eutrophication in downstream aquatic systems. Riparian zones positioned as the link interfaces of terrestrial and aquatic environments are effective in NO₃^- removal. However, the microbial mechanisms regulating NO₃^- reduction in riparian zones are still unclear. In this study, four microbial NO₃^- reduction processes were explored in fine-scale riparian soil horizons by isotopic tracing technique, qPCR of functional gene, high-throughput amplicon sequencing, and phylogenetic molecular ecological network analysis. Interestingly, anaerobic ammonium oxidation (anammox) contributed to NO₃^- removal of up to 48.2% only in waterward sediments but not in landward soil. Denitrification was still the most significant contributor to NO₃^- reduction (32.0-91.8%) and N-losses (51.7-100%). Additionally, dissimilatory nitrate reduction to ammonium (DNRA) played a key role in NO₃^- reduction (4.4-67.5%) and was even comparable to denitrification. Community structure analysis of denitrifying, anammox, and DNRA bacterial communities targeting the related functional gene showed that spatial heterogeneity played a greater role than both temporal and soil type (rhizosphere and non-rhizosphere soil) variability in microbial community structuring. Denitrification and DNRA communities were diverse, and their activities did not depend on gene abundance but were significantly related to organic matter, suggesting that gene abundance alone was insufficient in assessing their activity in riparian zones. Based on networks, DNRA plays a keystone role among the microbial NO₃^- reducers. As the last line of defense in the interception of terrestrial NO₃^-, these findings contribute to our understanding of NO₃^- removal mechanisms in riparian zones, and could potentially be exploited to reduce the diffusion of NO₃^- pollution.

The missing nitrogen pieces: A critical review on the distribution, transformation, and budget of nitrogen in the vadose zone-groundwater system

Intensive agriculture and urbanization have led to the excessive and repeated input of nitrogen (N) into soil and further increased the amount of nitrate (NO₃^-) leaching into groundwater, which has become an environmental problem of widespread concern. This review critically examines both the recent advances and remaining knowledge gaps with respect to the N cycle in the vadose zone-groundwater system. The key aspects regarding the N distribution, transformation, and budget in this system are summarized. Three major missing N pieces (N in dissolved organic form, N in the deep vadose zone, and N in the nonagricultural system), which are crucial for closing the N cycle yet has been previously assumed to be insignificant, are put forward and discussed. More work is anticipated to obtain accurate information on the chemical composition, transformation mechanism, and leaching flux of these missing N pieces in the vadose zone-groundwater system. These are essential to support the assessment of global N stocks and management of N contamination risks.

Global Distribution of Anaerobic Ammonia Oxidation (Anammox) Bacteria - Field Surveys in Wetland, Dryland, Groundwater Aquifer and Snow

The discovery of anaerobic ammonia oxidation (anammox) expanded our knowledge on the microbial nitrogen cycle. Previous studies report that anammox bacteria are distributed in a wide range of habitats and plays significant roles in the global nitrogen cycle. However, most studies focus only on individual ecosystems or datasets from public databases. To date, our understanding of how anammox bacteria respond to environmental properties and are distributed in different habitats on a global scale, remain unclear. To explore the global distribution of anammox bacteria, samples were collected from different habitats at different locations globally, including wetlands, drylands, groundwater aquifers and snow from 10 countries across six continents. We then used high-throughput amplicon sequencing targeting the functional gene hydrazine synthase (HZS) and generated community profiles. Results showed that Candidatus Brocadia is detected as the dominant genus on a global scale, accounting for 80.0% to 99.9% of the retrieved sequences in different habitats. The Jettenia-like sequences were the second most abundant group, accounting for no more than 19.9% of the retrieved sequences in all sites. The samples in drylands, wetlands and groundwater aquifers showed similar community composition and diversity, with the snow samples being the most different. Deterministic processes seem stronger in regulating the community composition of anammox bacteria, which is supported by the higher proportion explained by local-scale factors. Groundwater aquifers showed high gene abundance and the most complex co-occurrence network among the four habitat types, suggesting that it might be the preferred habitat of anammox bacteria. There is little competition between anammox bacterial species based on co-occurrence analysis. Hence, we could infer that environmental factors such as anaerobic and stable conditions, instead of substrate limitations, may be vital factors determining the anammox bacteria community. These results provide a better understanding of the global distribution of anammox bacteria and the ecological factors that affect their community structuring in diverse habitats.

Dynamics and environmental importance of anaerobic ammonium oxidation (anammox) bacteria in urban river networks

Anaerobic ammonium oxidation (anammox) is recognized as an important bioprocess for nitrogen removal, yet little is known about the associated microbial communities in urban river networks which are intensively disturbed by human activity. In the present study, we investigated the community composition and abundance of anammox bacteria in the urban river network of Shanghai, and explored their potential correlations with nitrogen removal activities and the environmental parameters. High biodiversity of anammox bacteria was detected in the sediment of urban river networks, including Candidatus Brocadia, Scalindua, Jettenia, and Kuenenia. Anammox bacterial abundance ranged from 3.7 × 10⁶ to 3.9 × 10⁷ copies g^-1 dry sediment based on 16S rRNA gene, which was strongly correlated to the metabolic activity of anammox bacteria (P < 0.01). A strong linkage between anammox bacteria and denitrifiers was detected (P < 0.05), implying a potential metabolic interdependence between these two nitrogen-removing microbes was existed in urban river networks. Sediment ammonium (NH₄⁺) made a significant contribution to the anammox bacterial community-environment relationship, while anammox bacterial abundance related significantly with sediment total organic carbon (TOC) and silt contents (P < 0.05). However, no statistically significant correlation was observed between cell-specific anammox rate and the measured environmental factors (P > 0.05). In general, the community composition and abundance of anammox bacteria in different hierarchies of the river network was homogeneous, without significant spatial variations (P > 0.05). These results provided an opportunity to further understand the microbial mechanism of nitrogen removal bioprocesses in urban river networks.

Spatial Distribution and Health Risk Assessment of Potentially Toxic Elements in Surface Soils of Bosten Lake Basin, Central Asia

A geographically weighted regression and classical linear model were applied to quantitatively reveal the factors influencing the spatial distribution of potentially toxic elements of forty-eight surface soils from Bosten Lake basin in Central Asia. At the basin scale, the spatial distribution of the majority of potentially toxic elements, including: cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), thallium (Tl), vanadium (V), and zinc (Zn), had been significantly influenced by the geochemical characteristics of the soil parent material. However, the arsenic (As), cadmium (Cd), antimony (Sb), and mercury (Hg) have been influenced by the total organic matter in soils. Compared with the results of the classical linear model, the geographically weighted regression can significantly increase the level of simulation at the basin spatial scale. The fitting coefficients of the predicted values and the actual measured values significantly increased from the classical linear model (Hg: r² = 0.31; Sb: r² = 0.64; Cd: r² = 0.81; and As: r² = 0.68) to the geographically weighted regression (Hg: r² = 0.56; Sb: r² = 0.74; Cd: r² = 0.89; and As: r² = 0.85). Based on the results of the geographically weighted regression, the average values of the total organic matter for As (28.7%), Cd (39.2%), Hg (46.5%), and Sb (26.6%) were higher than those for the other potentially toxic elements: Cr (0.1%), Co (4.0%), Ni (5.3%), V (0.7%), Cu (18.0%), Pb (7.8%), Tl (14.4%), and Zn (21.4%). There were no significant non-carcinogenic risks to human health, however, the results suggested that the spatial distribution of potentially toxic elements had significant differences.

Anammox and denitrification separately dominate microbial N-loss in water saturated and unsaturated soils horizons of riparian zones

Fertilized agroecosystems may show considerable leaching of the mobile nitrogen (N) compound NO₃^-, which pollutes groundwater and causes eutrophication of downstream waterbodies. Riparian buffer zones, positioned between terrestrial and aquatic environments, effectively remove NO₃^- and serve as a hotspot for N₂O emissions. However, microbial processes governing NO₃^- reduction in riparian zones still remain largely unclear. This study explored the underlying mechanisms of various N-loss processes in riparian soil horizons using isotopic tracing techniques, molecular assays, and high-throughput sequencing. Both anaerobic ammonium oxidation (anammox) and denitrification activity were maximized in the riparian fringe rather than in the central zones. Denitrifying anaerobic methane oxidation (damo) process was not detected. Interestingly, both contrasting microbial habitats were separated by a groundwater table, which forms an important biogeochemical interface. Denitrification dominated cumulative N-losses in the upper unsaturated soil, while anammox dominated the lower oxic saturated soil horizons. Archaeal and bacterial ammonium oxidation that couple dissimilatory nitrate reduction to ammonium (DNRA) with a high cell-specific rate promoted anammox even further in oxic subsurface horizons. High-throughput sequencing and network analysis showed that the anammox rate positively correlated with Candidatus 'Kuenenia' (4%), rather than with the dominant Candidatus 'Brocadia'. The contribution to N-loss via anammox increased significantly with the water level, which was accompanied by a significant reduction of N₂O emission (∼39.3 ± 10.6%) since N-loss by anammox does not cause N₂O emissions. Hence, water table management in riparian ecotones can be optimized to reduce NO₃^- pollution by shifting from denitrification to the environmentally friendly anammox pathway to mitigate greenhouse gas emissions.

Resuscitation of anammox bacteria after >10,000 years of dormancy

Water is essential for life on Earth, and an important medium for microbial energy and metabolism. Dormancy is a state of low metabolic activity upon unfavorable conditions. Many microorganisms can switch to a metabolically inactive state after water shortage, and recover once the environmental conditions become favorable again. Here, we resuscitated dormant anammox bacteria from dry terrestrial ecosystems after a resting period of >10 ka by addition of water without any other substrates. Isotopic-tracer analysis showed that water induced nitrate reduction yielding sufficient nitrite as substrate and energy for activating anammox bacteria. Subsequently, dissimilatory nitrate reduction to ammonium (DNRA) provided the substrate ammonium for anammox bacteria. The ammonium and nitrite formed were used to produce dinitrogen gas. High throughput sequencing and network analysis identified Brocadia as the dominant anammox species and a Jettenia species seemed to connect the other community members. Under global climate change, increasing precipitation and soil moisture may revive dormant anammox bacteria in arid soils and thereby impact global nitrogen and carbon cycles.

Microbial Nitrogen Cycle Hotspots in the Plant-Bed/Ditch System of a Constructed Wetland with N2O Mitigation

Artificial microbial nitrogen (N) cycle hotspots in the plant-bed/ditch system were developed and investigated based on intact core and slurry assays measurement using isotopic tracing technology, quantitative PCR and high-throughput sequencing. By increasing hydraulic retention time and periodically fluctuating water level in heterogeneous riparian zones, hotspots of anammox, nitrification, denitrification, ammonium (NH₄⁺) oxidation, nitrite (NO₂^-) oxidation, nitrate (NO₃^-) reduction and DNRA were all stimulated at the interface sediments, with the abundance and activity being about 1-3 orders of magnitude higher than those in nonhotspots. Isotopic pairing experiments revealed that in microbial hotspots, nitrite sources were higher than the sinks, and both NH₄⁺ oxidation (55.8%) and NO₃^- reduction (44.2%) provided nitrite for anammox, which accounted for 43.0% of N-loss and 44.4% of NH₄⁺ removal in riparian zones but did not involve nitrous oxide (N₂O) emission risks. High-throughput analysis identified that bacterial quorum sensing mediated this anammox hotspot with B.fulgida dominating the anammox community, but it was B. anammoxidans and Jettenia sp. that contributed more to anammox activity. In the nonhotspot zones, the NO₂^- source (NO₃^- reduction dominated) was lower than the sink, limiting the effects on anammox. The in situ N₂O flux measurement showed that the microbial hotspot had a 27.1% reduced N₂O emission flux compared with the nonhotspot zones.