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

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.

Evaluation of ecological impacts with ferrous iron addition in constructed wetland under perfluorooctanoic acid stress

In this study, ferrous ion (Fe(II)) had the potential to promote ecological functions in constructed wetlands (CWs) under perfluorooctanoic acid (PFOA) stress. Concretely, Fe(II) at 30 mg/L and 20-30 mg/L even led to 11.37% increase of urease and 93.15-243.61% increase of nitrite oxidoreductase respectively compared to the control. Fe(II) promotion was also observed on Nitrosomonas, Nitrospira, Azospira, and Zoogloea by 1.00-6.50 folds, which might result from higher expression of nitrogen fixation and nitrite redox genes. These findings could be explanation for increase of ammonium removal by 7.47-8.75% with Fe(II) addition, and reduction of nitrate accumulation with 30 mg/L Fe(II). Meanwhile, both Fe(II) stimulation on PAOs like Dechloromonas, Rhodococcus, Mesorhizobium, and Methylobacterium by 1.58-2.00 folds, and improvement on chemical phosphorus removal contributed to higher total phosphorus removal efficiency under high-level PFOA exposure. Moreover, Fe(II) raised chlorophyll content and reduced the oxidative damage brought by PFOA, especially at lower dosage. Nevertheless, combination of Fe(II) and high-level PFOA caused inhibition on microbial alpha diversity, which could result in decline of PFOA removal (by 4.29-12.83%). Besides, decrease of genes related to nitrate reduction demonstrated that enhancement on denitrification was due to nitrite reduction to N2 pathways rather than the first step of denitrifying process.

Nitrogen removal performance and mechanisms of three aquatic plants for farmland tail water purification

Constructed wetlands (CWs) are commonly used to control excessive nitrogen from farmlands; however, the interactions between vegetation and microorganisms, nitrogen removal performance, and the mechanisms involved remain unclear in subtropical areas. This study aimed to investigate the nitrogen removal performance and mechanism of CWs containing Canna indica, Acorus calamus, and Thalia dealbata. The results show that CWs with plants had significantly higher nitrogen removal efficiencies than those without, with those planted with T. dealbata having the highest efficiency. T. dealbata performed better than the other two plants due to its high biomass and excellent nitrogen uptake capacity; more importantly, CWs with it had the highest abundance of nitrogen functional genes. Microbial nitrification-denitrification, the primary process of nitrogen removal in CWs, contributed to 88 %, 91 %, and 84 % of the TN removal in the CWs with C. indica, A. calamus, and T. dealbata, respectively, 29 %-158 % higher than that in CWs without plants. Microorganisms played a crucial role in nitrogen removal in the CWs, while plants significantly stimulated microbial activity by enhancing microbial abundance and creating a suitable environment for growth and metabolism. These results can help in understanding the contribution of plants in cleaning farmland tailwater and further optimization of plant configuration and management strategies in wetland ecosystems to improve nitrogen removal efficiency.

Tracing and utilizing nitrogen loss in wastewater treatment: The trade-off between performance improvement, energy saving, and carbon footprint reduction

Biological nitrogen removal is widely applied to reduce the discharge of inorganic nitrogen and mitigate the eutrophication of receiving water. However, nitrogen loss is frequently observed in wastewater treatment systems, yet the underlying principle and potential enlightenment is still lacking a comprehensive discussion. With the development and application of novel biological technologies, there are increasing achievement in the deep understanding and mechanisms of nitrogen loss processes. This article reviews the potential and novel pathways of nitrogen loss, occurrence mechanisms, influential factors, and control strategies. A survey of recent literature showed that 3%∼73% of nitrogen loss beyond the nitrogen budget can be ascribed to the unintentional presence of simultaneous nitrification/denitrification, partial nitrification/anammox, and endogenous denitrification processes, under low dissolved oxygen (DO) and limited available organic carbon source at aerobic conditions. Key influential parameters, including DO, aeration strategies, solid retention time (SRT), hydraulic retention time (HRT), temperature and pH, significantly affect both the potential pathways of nitrogen loss and its quantitative contribution. Notably, the widespread and spontaneous growth of anammox bacteria is an important reason for ammonia escape at anaerobic/anoxic conditions, leading to 7%∼78% of nitrogen loss through anammox pathway. Moreover, the unwanted nitrous oxide (N2O) emission should also be considered as a key pathway in nitrogen loss. Future development of new nitrogen removal technologies is proposed to suppress the generation of harmful nitrogen losses and reduce the carbon footprint of wastewater treatment by controlling key influential parameters. Transforming "unintentional observation" to "intentional action" as high-efficiency and energy-efficient nitrogen removal process provides a new approach for the development of wastewater treatment.

Light-Driven Electron Uptake from Nonfermentative Organic Matter to Expedite Nitrogen Dissimilation by Chemolithotrophic Anammox Consortia

Nonphotosynthetic microorganisms are typically unable to directly utilize light energy, but light might change the metabolic pathway of these bacteria indirectly by forming intermediates such as reactive oxygen species (ROS). This work investigated the role of light on nitrogen conversion by anaerobic ammonium oxidation (anammox) consortia. The results showed that high intensity light (>20000 lx) caused ca. 50% inhibition of anammox activity, and total ROS reached 167% at 60,000 lx. Surprisingly, 200 lx light was found to induce unexpected promotion of the nitrogen conversion rate, and ultraviolet light (<420 nm) was identified as the main contributor. Metagenomic and metatranscriptomic analyses revealed that the gene encoding cytochrome c peroxidase was highly expressed only under 200 lx light. 15N isotope tracing, gene abundance quantification, and external H2O2 addition experiments showed that photoinduced trace H2O2 triggered cytochrome c peroxidase expression to take up electrons from extracellular nonfermentative organics to synthesize NADH and ATP, thereby expediting nitrogen dissimulation of anammox consortia. External supplying reduced humic acid into a low-intensity light exposure system would result in a maximal 1.7-fold increase in the nitrogen conversion rate. These interesting findings may provide insight into the niche differentiation and widespread nature of anammox bacteria in natural ecotopes.

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.

Land-Water Transport and Sources of Nitrogen Pollution Affecting the Structure and Function of Riverine Microbial Communities

The characterization of variations in riverine microbiota that stem from contaminant sources and transport modes is important for understanding biogeochemical processes. However, the association between complex anthropogenic nitrogen pollution and bacteria has not been extensively investigated owing to the difficulties faced while determining the distribution of nitrogen contaminants in watersheds. Here, we employed the Soil and Water Assessment Tool alongside microbiological analysis to explore microbial characteristics and their responses to complex nitrogen pollution patterns. Significant variations in microbial communities were observed in sub-basins with distinct land-water pollution transport modes. Point source-dominated areas (PSDAs) exhibited reduced microbial diversity, high number of denitrification groups, and increased nitrogen cycling compared with others. The negative relative deviations (-3.38) between the measured and simulated nitrate concentrations in PSDAs indicated that nitrate removal was more effective in PSDAs. Pollution sources were also closely associated with microbiota. Effluents from concentrated animal feeding operations were the primary factors relating to the microbiota compositions in PSDAs and balanced areas. In nonpoint source-dominated areas, contaminants from septic tanks become the most relevant sources to microbial community structures. Overall, this study expands our knowledge regarding microbial biogeochemistry in catchments and beyond by linking specific nitrogen pollution scenarios to microorganisms.

Recent Advances in Autotrophic Biological Nitrogen Removal for Low Carbon Wastewater: A Review

Due to carbon source dependence, conventional biological nitrogen removal (BNR) processes based on heterotrophic denitrification are suffering from great bottlenecks. The autotrophic BNR process represented by sulfur-driven autotrophic denitrification (SDAD) and anaerobic ammonium oxidation (anammox) provides a viable alternative for addressing low carbon wastewater. Whether for low carbon municipal wastewater or industrial wastewater with high nitrogen, the SDAD and anammox process can be suitably positioned accordingly. Herein, the recent advances and challenges to autotrophic BNR process guided by SDAD and anammox are systematically reviewed. Specifically, the present applications and crucial operation factors were discussed in detail. Besides, the microscopic interpretation of the process was deepened in the viewpoint of functional microbial species and their physiological characteristics. Furthermore, the current limitations and some future research priorities over the applications were identified and discussed from multiple perspectives. The obtained knowledge would provide insights into the application and optimization of the autotrophic BNR process, which will contribute to the establishment of a new generation of efficient and energy-saving wastewater nitrogen removal systems.

Specific Denitrifying and Dissimilatory Nitrate Reduction to Ammonium Bacteria Assisted the Recovery of Anammox Community From Nitrite Inhibition

The anaerobic ammonium oxidation (anammox) by autotrophic anaerobic ammonia-oxidizing bacteria (AnAOB) is a biological process used to remove reactive nitrogen from wastewater. It has been repeatedly reported that elevated nitrite concentrations can severely inhibit the growth of AnAOB, which renders the anammox process challenging for industrial-scale applications. Both denitrifying (DN) and dissimilatory nitrate reduction to ammonium (DNRA) bacteria can potentially consume excess nitrite in an anammox system to prevent its inhibitory effect on AnAOB. However, metabolic interactions among DN, DNRA, and AnAOB bacteria under elevated nitrite conditions remain to be elucidated at metabolic resolutions. In this study, a laboratory-scale anammox bioreactor was used to conduct an investigation of the microbial shift and functional interactions of AnAOB, DN, and DNRA bacteria during a long-term nitrite inhibition to eventual self-recovery episode. The relative abundance of AnAOB first decreased due to high nitrite concentration, which lowered the system’s nitrogen removal efficiency, but then recovered automatically without any external interference. Based on the relative abundance variations of genomes in the inhibition, adaptation, and recovery periods, we found that DN and DNRA bacteria could be divided into three niche groups: type I (types Ia and Ib) that includes mainly DN bacteria and type II and type III that include primarily DNRA bacteria. Type Ia and type II bacteria outcompeted other bacteria in the inhibition and adaptation periods, respectively. They were recognized as potential nitrite scavengers at high nitrite concentrations, contributing to stabilizing the nitrite concentration and the eventual recovery of the anammox system. These findings shed light on the potential engineering solutions to maintain a robust and efficient industrial-scale anammox process.

Functional genera for efficient nitrogen removal under low C/N ratio influent at low temperatures in a two-stage tidal flow constructed wetland

A two-stage tidal flow constructed wetland (referred to as TFCW-A and TFCW-B) was used to treat low chemical oxygen demand/total nitrogen (COD/TN or simply C/N) ratio influent at low temperatures (<15 °C). The influence of the flooding-resting time (A: 8 h-4 h, B: 4 h-8 h) and effluent recirculation on nitrogen removal and microbial community characteristics were explored. TFCW-B achieved optimal average nitrogen removal efficiency with effluent recirculation (96.05% ammonium nitrogen (NH₄⁺-N); 78.43% TN) and led to nitrate nitrogen (NO₃^--N) accumulation due to the lack of a carbon source and longer resting time. Ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) were inhibited at low temperatures. Except for nrfA, AOA, AOB, narG and nirS were separated by the flooding-resting time rather than by spatial position. Furthermore, the dominant genera in TFCW-A were Arthrobacter, Rhodobacter, Pseudomonas, and Solitalea, whereas prolonging resting time promoted the growth of Thauera and Zoogloea in TFCW-B. Spearman correlation analysis showed that Zoogloea and Rhodobacter had the strongest correlations with other genera. Moreover, the NH₄⁺-N concentration was significantly positively influenced by Arthrobacter, Rhodobacter, Pseudomonas, and Solitalea but negatively influenced by Thauera and Zoogloea. There was no significant correlation between TN and the dominant genera. This study not only provides a practicable system for wastewater treatment with a low C/N ratio but also presents a theoretical basis for the regulation of microbial communities in nitrogen removal systems at low temperatures.

Ecological wetland paradigm drives water source improvement in the stream network of Yangtze River Delta

Jiaxing created a precedent using bypass riparian marshes to purify micro-polluted water sources in China. Pond-wetland complex with constructed root channel technology becomes a paradigm which can be analogized as "human-body wetland model" based on bionics or biomimetics. Heterogeneous plant-bed/ditch system with highly active land/water ecotone interfaces, especially meandering boundaries, breeds many biochemical reactions "living areas". Optimization of hydraulic regulation promotes redox environment alternations and wetland treatment efficiency. Here we reported a series of upgrades and performances in Guanjinggang wetland after the Shijiuyang prototype. Morphological reform of plant-bed/ditch system played a vital role. Spatially root channel zone was main force of wetland purification, and temporally the treatment effect was higher in low-temperature seasons indicating non-temperature dependent mechanisms worked. Water pollution comprehensive index improved steadily from IV to III, and comprehensive pollution load was reduced by ca. 40%-60%. Comprehensive evaluation function value further showed the gradients purification effect of the upgraded wetland. Ecological wetlands ameliorated source water quality, and reduced drinking water treatment reagents, thereby bringing about economic benefits. Through wetlands operation, people can see how the micro-polluted surface water becomes clear and clean, so promoting a significant social benefit. As a viable component of urban green space, wetlands could beautify regional eco-environment, freshen the air, increase urban ecological taste, and enhance the eco-environmental protection publicity. Thus, the multifunctional service values and indirect benefits are substantial. Jiaxing ecological wetlands provide a typical paradigm for water pollution remediation in developing countries and plays a leading role in technology engineering radiation effect.

Effect of the Influent Substrate Concentration on Nitrogen Removal from Summer to Winter in Field Pilot-Scale Multistage Constructed Wetland–Pond Systems for Treating Low-C/N River Water

The quality of micropolluted water is unstable and its substrate concentration fluctuates greatly. The goal is to predict the concentration effect on the treatment of nitrogen in a river with an actual low C/N ratio for the proposed full-scale Xiaoyi River estuary wetland, so that the wetland project can operate stably and perform the water purification function effectively in the long term. Two pilot-scale multistage constructed wetland–pond (MCWP) systems (S1 and S2, respectively) based on actual engineering with the same “front ecological oxidation ponds, two-stage horizontal subsurface flow constructed wetlands and surface flow constructed wetlands (SFCWs) as the core and postsubmerged plant ponds” as the planned process were constructed to investigate the effect of different influent permanganate indexes (CODMn) and total nitrogen (TN) contents on nitrogen removal from micropolluted river water with a fixed C/N ratio from summer to winter in the field. The results indicate that the TN removal rate in the S1 and S2 systems was significant (19.56% and 34.84%, respectively). During the process of treating this micropolluted water with a fixed C/N ratio, the influent of S2 with a higher CODMn concentration was conducive to the removal of TN. The TN removal rate in S2 was significantly affected by the daily highest temperature. There was significant nitrogen removal efficiency in the SFCWs. The C/N ratio was a major determinant influencing the nitrogen removal rate in the SFCWs. The organic matter release phenomenon in SFCWs with high-density planting played an essential role in alleviating the lack of carbon sources in the influent. This research strongly supports the rule that there is seasonal nitrogen removal in the MCWPs under different influent substrate concentrations, which is of guiding significance for practical engineering.

Performance of Canna Indica based microscale vertical flow constructed wetland under tropical conditions for domestic wastewater treatment

Constructed wetlands (CWs) have great potential as low-cost natural wastewater treatment in developing countries. The present study appraises the performance of the vertical flow constructed wetland for domestic wastewater treatment. More specifically, the potential of Canna Indica in the removal of carbon, nitrogen, and phosphorus (CNP) from wastewater under tropical conditions. CW cell was fabricated with a vegetative layer of Canna Indica and tested with domestic wastewater. Based on the test results, Canna Indica shows a high Removal Efficiency (RE) of Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD₅) on the order of 87% and 91%, respectively. Similarly, nutrients removal efficiency for total nitrogen (TN), total phosphorus (TP) was found to be 97% and 98%, respectively. The investigation also revealed that there is considerable removal of sulfates with efficiency equal to 78.4%. Overall, the Canna Indica based CWs were found to be suitable for wastewater treatment in the tropical regions, provided a viable medium for treating the wastewater in peri-urban and rural areas of developing countries.

An integrated migration and transformation model to evaluate the occurrence characteristics and environmental risks of Nitrogen and phosphorus in constructed wetland

In this study, an integrated migration and transformation (IMT) model based on microbial action, plant absorption, sediment release and substrate adsorption was firstly established to evaluate the temporal-spatial distribution of N and P in Lingang hybrid constructed wetland (CW), Tianjin. Compared to the conventional transformation model that only considers the microbial action, the IMT model could accurately predict the occurrence characteristics of N and P. In Lingang CW, NO₃^--N (0.56-3.63 mg/L) was the most important form of N, and the TP was at a relatively low concentration level (0.04-0.07 mg/L). The spatial distribution results showed that a certain amount of N and P could be removed by CW. Form the temporal perspective, the N and P concentrations were greatly affected by the dissolved oxygen (DO). The simulated values obtained by IMT model indicated that the distribution of N and P was more affected by the temporality compared with the spatiality, which was consistent with measured values. Besides, the PCA indicated that TN, NO₃^--N and DO were important factors, which affected the water quality of CW. The Nemerow pollution index method based on the simulated values indicated that Lingang CW was overall moderately polluted, and the subsurface area was the main functional unit of pollutants removal in CW. This work provides a new model for accurately predicting the occurrence characteristics of N and P pollutants in CW, which is of great significance for identifying its environmental risks and optimizing the construction of wetlands.

High denitrification potential but low nitrous oxide emission in a constructed wetland treating nitrate-polluted agricultural run-off

Constructed wetlands (CW) can efficiently remove nitrogen from polluted agricultural run-off, however, a potential caveat is nitrous oxide (N₂O), a harmful greenhouse gas and stratospheric ozone depleter. During five sampling campaigns, we measured N₂O fluxes from a 0.53 ha off-stream CW treating nitrate-rich water from the intensively fertilized watershed in Rampillon, France, using automated chambers with a quantum cascade laser system, and manual chambers. Sediment samples were analysed for potential N₂ flux using the HeO₂ incubation method. Both inlet nitrate (NO₃^-) concentrations and N₂O emission varied significantly between the seasons. In the Autumn and Winter inlet concentrations were about 11 mg NO₃^--N L^-1, and < 6.5 mg NO₃^--N L^-1 in the Spring and Summer. N₂O emission was highest in the Autumn (mean ± standard error: 9.7 ± 0.2 μg N m^-2 h^-1) and lowest in the Summer (wet period: 0.2 ± 0.3 μg N m^-2 h^-1). The CW was a very weak source of N₂O emitting 0.32 kg N₂O-N ha^-1 yr^-1 and removing around 938 kg NO₃^--N ha^-1 yr^-1, the ratio of N₂O-N emitted to NO₃^--N removed was 0.033%. The automated and manual chambers gave similar results. From the potential N₂O formation in the sediment, only 9% was emitted to the atmosphere, the average N₂ N ₂O ratio was high: 89:1 for N₂-N_potential: N₂O-N_potential and 1353:1 for N₂-N_potential: N₂O-N_emitted. These results indicate complete denitrification. The focused principal component analysis showed strong positive correlation between the gaseous N₂O fluxes and the following environmental factors: NO₃^--N concentrations in inlet water, streamflow, and nitrate reduction rate. Water temperature, TOC and DOC in the water and hydraulic residence time showed negative correlations with N₂O emissions. Shallow off-stream CWs such as Rampillon may have good nitrate removal capacity with low N₂O emissions.

Coupling transformation of carbon, nitrogen and sulfur in a long-term operated full-scale constructed wetland

The coupling transformation of carbon, nitrogen and sulfur compounds has been studied in lab-scale and pilot-scale constructed wetlands (CWs), but few studies investigated full-scale CW. In this study, we used batch experiments to investigate the potentials of carbon, nitrogen and sulfur transformation in a long-term operated, full-scale horizontal subsurface flow wetland. The sediments collected from the HSFW were incubated for 48 h in the laboratory with supplying various dosages of carbon, nitrogen and sulfur compounds. The results showed that heterotrophic denitrification was the main pathway. At the same time, the sulfide (S^2-)-based autotrophic denitrification was also present. Increasing TOC concentration or NO₃^- concentration could promote heterotrophic denitrification but did not inhibit the sulfide-based autotrophic denitrification. In our experiment, the highest NO₃^- removal via autotrophic denitrification was 25.23% while that via heterotrophic denitrification was 73.66%, leading to the total NO₃^- removal of 98.89%. The results also demonstrated that NO₃^- rather than NO₂^- was the preferable electron acceptor for both heterotrophic and sulfide-based autotrophic denitrification in the CW. Increasing S^2- concentrations promote NO₃^- removal from 12.99% to 25.23% without organic carbon, but varying NO₃^- or NO₂^- has no effects. These results indicated that concentrations of S^2-, instead of NO₃^- or NO₂^-, was the limiting factor for sulfide-based autotrophic denitrification in the studied CW. The microbial community analysis and correlation analysis between the transformation of carbon, nitrogen and sulfur compounds and relative abundance of bacteria further confirmed that in the CW, the key pathways coupling transformation were heterotrophic denitrification and sulfide-based autotrophic denitrification. Overall, the current study will enhance understanding of carbon, nitrogen, and sulfur transformation in CW and support better design and treatment efficiency.

A comparison of the mechanisms and performances of Acorus calamus, Pontederia cordata and Alisma plantagoaquatica in removing nitrogen from farmland wastewater

This study examined the performances of Acorus calamus, Pontederia cordata, and Alisma plantagoaquatica in removing nitrogen (N) from farmland wastewater. P. cordata showed the fastest rate of N removal, followed by A. plantagoaquatica, whereas that of A. calamus was slowest. P. cordata and A. plantagoaquatica achieving a greater rate of TN reduction in soil than that by A. calamus. A. plantagoaquatica demonstrated the highest N adsorption capacity, 32.6% and 392.1% higher than that of P. cordata and A. calamus, respectively. The higher potential nitrification and denitrification rate, and abundance of functional genes in the P. cordata microcosm resulted in a stronger process of nitrification-denitrification, which accounted for 65.99% of TN loss. Plant uptake and nitrification-denitrification were responsible for 50.06% and 49.94% of TN removed within the A. plantagoaquatica. Nitrification-denitrification accounted for 86.35% of TN loss in A. calamus. These findings helped to insight into N removal mechanisms in different plants.

A review on China's constructed wetlands in recent three decades: Application and practice

Constructed wetlands (CWs) have been introduced to and developed in China for environmental engineering over the most prosperous three decades (1990-2020). To study the origin, development process, and future trend of CWs, this review summarized a wide range of literatures between 1990 and 2020 by Chinese authors. Firstly, the publication number over years, research highlights, and the author contributions with the most published papers in this field were conducted through bibliometric analysis. Secondly, the most principal components of CWs, substrates and macrophytes were summarized and analyzed. Thirdly, the typical application cases from traditional CWs, pond systems to combined pond-wetland systems were presented. In China, CWs were predominately distributed in the east of the so-called 'Hu Huanyong Line'. Therefore CWs were limited by the socio-economic level and climatic conditions. It is unquestionable that the overall level of China's CWs has improved significantly, and one of the most prominent features has started towards the plural pattern development. There has been a trend of large-scale or low-cost CW application in the recent years. However, lifecycle research and management are required for better strategies in the future.

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.

The bacterial community structure and N-cycling gene abundance in response to dam construction in a riparian zone

Dam construction has significantly altered riparian hydrological regime and environmental conditions in the reservoir region, yet knowledge concerning how bacterial community and N-cycling genes respond to these changes remains limited. In this study, we investigated the bacterial community composition, network structure and N-cycling genes in the water level fluctuation zones (WLFZs) of the Three Gorges Reservoir (TGR). Here, samples collected from five different water levels were divided into three groups: waterward sediments, interface sediments, and landward soils. Our results show that higher contents of NO₂^--N, SOC, DOC, NH₄⁺-N, and TP were characterized in waterward and interface sediments whereas higherNO₃^--N content was observed in landward soils. The α-diversity of bacterial community decreased gradually from waterward sediments to landward soils. Compared with waterward sediments and landward soils, the interface sediments showed a unique bacterial community pattern with diverse primary producers as well as N-cycling microbes. The interface sediments also had a much more complex co-occurrence network and a higher possible community stability. Among all of N-cycling genes, higher abundances of nrfA and AOA amoA genes were observed in interface sediments. The dissimilarity in bacterial community composition and N-cycling gene abundance was mainly driven by water-level. Moreover, random forest model revealed that AOA amoA and nirS genes were the most sensitive indicators in response to water level fluctuations. Overall, this study suggests distinct abundance, diversity, and network structure of microbes in riparian sediments and soils across the gradient of water levels and enhances our understanding with respect to comprehensive effects of dam construction on nitrogen cycle.

Numerical assessment of a soil moisture controlled wastewater SDI disposal system in Alabama Black Belt Prairie

To promote the environmental sustainability of rural sanitation, a soil moisture controlled wastewater subsurface drip irrigation (SDI) dispersal system was field tested in the Black Belt Prairie of Alabama, USA. The soil moisture control strategy was designed to regulate wastewater disposal timing according to drain field conditions to prevent hydraulic overloading and corresponding environmental hazard. CW2D/HYDRUS simulation modeling was utilized to explore difficult-to-measure aspects of system performance. While the control system successfully adapted hydraulic loading rate to changing drain field conditions, saturated field conditions during the dormant season presented practical application challenges. The paired field experiment and simulation model demonstrate that soil biofilm growth was stimulated in the vicinity of drip emitters. Although biofilm growth is critical in maintaining adequate COD and NH₄⁺-N removal efficiencies, the efficient removal of biodegradable COD itself by soil biofilm limits denitrification of formed NO₃^--N . Furthermore, stimulated soil biofilm growth can create soil clogging around drip emitters, which was discerned in the field experiment along with salt accumulation, both of which were verified by simulation. Comparable modeling of system performance in sand and clay media demonstrate that the placement of soil moisture sensors within the drain field can have pronounced impacts on system hydraulic performance, depending on the soil permeability. Overall, the soil moisture control strategy tested is shown as a viable supplemental technology to promote the environmental sustainability of rural sanitation systems.

Microbial coupling mechanisms of nitrogen removal in constructed wetlands: A review

Nitrogen removal through microorganisms is the most important pathway in constructed wetlands (CWs). In this review, we summarize the microbial coupling mechanisms of nitrogen removal, which are the common methods of nitrogen transformation. The electron pathways are shortened and consumption of oxygen and energy is reduced during the coupling of nitrogen transformation functional microorganisms. The highly efficient nitrogen removal mechanisms are cultivated from the design conditions in CWs, such as intermittent aeration and tidal flow. The coupling of microorganisms and substrates enhances nitrogen removal mainly by supplying electrons, and plants affect nitrogen transformation functional microorganisms by the release of oxygen and exudates from root systems as well as providing carriers for microbial attachment. In addition, inorganic elements such as Fe, S and H act as electron donors to drive the autotrophic denitrification process in CWs.

Insight into the nitrogen accumulation in urban center river from functional genes and bacterial community

Along with urbanization, the intensified nitrogen pollution in urban rivers and the form of black-odor rivers has become one of the biggest concerns. Better understanding of the nitrogen transformations and microbial mechanisms occurring within urban rivers could help to manage their water quality. In this study, pollution characteristics, potential nitrogen removal rate, composition and function of bacterial community, and abundance of functional genes associated with nitrogen transformation were comparatively investigated in a typical urban river (FC) and a suburban river (LH). Compared with LH, FC was characterized by higher content of nutrients, lower potential nitrogen removal rate and lower abundance of functional genes associated with nitrogen transformation in both overlying water and sediment, especially in summer. Sediment dissolved organic matter characterized by excitation−emission matrix (EEM) showed that FC was more severely polluted by high nitrogen organic matter. Our results revealed that anammox was the main nitrogen removal pathway in both rivers and potential nitrogen removal rates decreased significantly in summer. Bacterial community analysis showed that the benthic communities were more severely influenced by the pollutant than aquatic ones in both rivers. Furthermore, the FC benthic community was dominated by anaerobic respiring, fermentative, sulfate reduction bacteria. Quantitatively, the denitrification rate showed a significant positive correlation with the abundance of denitrification genes, whilst the anammox rate was significantly negatively correlated with bacterial diversity. Meanwhile, NH₄⁺-N had a significant negative correlation to both denitrification and anammox in sediment. Taken together, the results indicated that the increased nitrogen pollutants in an urban river altered nitrogen removal pathways and bacterial communities, which could in turn exacerbate the nitrogen pollution to this river.

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.

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 SO₄^2- were in the first principal component, and N-compounds were the second. Furthermore, the main pollutant ions SO₄^2- and NO₃^- were from anthropogenic sources, and SO₄^2- 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.

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.

Nitrous Oxide and Methane Dynamics in Woodchip Bioreactors: Effects of Water Level Fluctuations on Partitioning into Trapped Gas Phases

Woodchip bioreactors (WBRs) are low-cost, passive systems for nonpoint source nitrogen removal at terrestrial-aquatic interfaces. The greenhouse gases nitrous oxide (N₂O) and methane (CH₄) can be produced within WBRs, and efforts to reduce N₂O and CH₄ emissions from WBR systems require improved understanding of the biogeochemical and physical-chemical mechanisms regulating their production, transport, and release. This study evaluates the impact of trapped gas-filled void volumes as sinks of dissolved gases from water and as sources of episodic fluxes when water levels fall. Dissolved gas tracer experiments in a laboratory bioreactor were used to parameterize nonequilibrium advection-dispersion-gas transfer models and quantify trapping of gas-filled voids as a function of antecedent hydrological conditions. Experiments following a water-level rise revealed that up to 24% of the WBR pore volume was occupied by trapped gas phases, which were primarily located in pore spaces inside woodchips. This finding was confirmed with X-ray-computed microtomography. N₂O (3.3-10%) and CH₄ (4.3-14%) injected into the reactor following a water table rise partitioned into gas-filled voids and were released when water tables fell. In the case of N₂O, partitioning into trapped gas phases makes N₂O unavailable for enzymatic reduction, potentially enhancing N₂O fluxes under fluctuating water levels.

Denitrification is the main microbial N loss pathway on the Qinghai-Tibet Plateau above an elevation of 5000 m

Soil nitrogen (N) deficiency is the major factor contributing to low primary productivity on the Qinghai-Tibet Plateau. However, most of our understanding of N cycling is still based on human disturbed environments, and the microbial mechanisms governing N loss in low primary productivity environment remain unclear. This study explores three microbial N loss pathways in eight wetland and dryland soil profiles from the Qinghai-Tibet Plateau, at an elevation of above 5000 m with little human activity, using ¹⁵N isotopic tracing slurry technology, quantitative PCR, and high-throughput sequencing. No denitrifying anaerobic methane oxidation was detected. Anammox occurred in two of the wetland (n = 4) and dryland (n = 4) soil profiles, while denitrification widely occurred and was the dominant N loss pathway in all samples. Where denitrification and anammox co-occurred, both abundance and activity were higher in wetland than in dryland soils and higher in surface than in subsurface soils. In comparison with non-anammox sites, nitrate levels initiate anammox-related N cycling. High-throughput sequencing and network analysis of nirK, nirS, nosZ, and hzsB gene communities showed that Bradyrhizobiaceae (a family of rhizobia) may play a dominant role in N loss pathways in this region. Given the geological evolution and relatively undisturbed habitat, these findings strongly suggest that denitrification is the dominant N loss pathway in terrestrial habitats of the Qing-Tibet Plateau with minimal anthropogenic activity.

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.

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.

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.

Spatiotemporal shifts of ammonia-oxidizing archaea abundance and structure during the restoration of a multiple pond and plant-bed/ditch wetland

Ammonia-oxidizing archaea (AOA) microorganisms have been increasingly found in aquatic and terrestrial environments. These microorganisms make vital contributions to ammonia oxidation in such systems. However, their community succession characteristics in man-made wetland ecosystems have scarcely been reported. We assessed the AOA's spatiotemporal shifts in the sediments of a constructed wetland (CW) - the Shijiuyang constructed wetland (SJY-CW) - in China from the third year (2011) to the fifth year (2013) of the CW operation. The SJY-CW is composed of a pretreatment pond, a multiple plant-bed/ditch system, and a post-treatment pond. Results showed that AOA abundance in the pre- and post-treatment ponds remained invariant through 2011-2012 and decreased in 2013, while the abundance in the plant-bed/ditch system decreased gradually with wetland operation. The AOA abundance in 2013 was one order of magnitude lower than that through 2011-2012, and the AOA abundance in the plant-bed/ditch system was generally higher than that in the pre- and post-treatment ponds from 2011 to 2013. AOA diversity showed little temporal differentiation with a slightly decreasing trend for community richness index Chao1 and diversity index Shannon H' from 2011 to 2013. The AOA community was dominated by the Nitrososphaera cluster accompanied by an increasing Nitrosopumilus cluster and Nitrososphaera sister cluster within the wetland operation. Hierarchical clustering and redundancy analysis verified the horizontal shifts of AOA communities. The shifts occurred preferentially in the central plant-bed/ditch system. The operational duration of the wetland became a key factor influencing AOA abundance and community shift in SJY-CW sediments.

Biochar amendment and Calamagrostis angustifolia planting affect sources and production pathways of N2O in agricultural ditch systems

Nitrous oxide (N2O) from agricultural ditches is a non-negligible source of anthropogenic greenhouse gas (GHG) emissions, but few studies have addressed this topic in depth. On the other hand, although there are numerous reports that biochar application can affect N2O emissions from soil, the understanding of the process and source of changes is still incomplete. To examine the effect of biochar and Calamagrostis angustifolia on N2O emissions, we conducted experiments with constructed ditches where corn stalk biochar (pyrolysis temperature of 450 °C) was applied at a rate of 16.77 Mg ha-1 and C. angustifolia was planted. The sources (native sediment versus exogenous inorganic N) and production pathways (nitrification versus denitrification) of N2O emissions were discriminated using the 15N isotope tracer method. We observed that biochar application reduced the cumulative total N-N2O emissions from the native sediment by 10.8-18.7% and reduced the cumulative 15N-N2O emissions from the exogenous 15N-labelled inorganic N by 25.7-68.6%; C. angustifolia planting reduced these cumulative N2O emissions by 48.8-53.3% and 93.3-92.4%, respectively. The results showed that biochar stimulated nitrification and nitrification-derived 15N-N2O emissions, but reduced denitrification-derived 15N-N2O emissions in bare sediment microcosms; C. angustifolia effectively reduced both nitrification-derived and denitrification-derived 15N-N2O emissions. Therefore, we concluded that the effect of biochar application on N2O emissions may depend on its dominant N2O production pathway and biochar application plus C. angustifolia planting could be beneficial for the mitigation of N2O emissions in agricultural ditch systems.

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.

Impacting Microbial Communities and Absorbing Pollutants by Canna Indica and Cyperus Alternifolius in a Full-Scale Constructed Wetland System

Wetland plants that cover the wetlands play an important role in reducing pollutants. The aim of this study was to investigate the effect of two plant species on microbial communities and nitrogen-removal genes and to evaluate the contributions of absorbing pollutants by Canna indica (CI) and Cyperus alternifolius (CA) to the removal performance in both a vertical subsurface flow constructed wetland and a horizontal subsurface flow constructed wetland, which were part of a full-scale hybrid constructed wetland system. The microbial assemblages were determined using 16S rRNA high-throughput sequencing. Results showed that the presence of CI and CA positively affected microbial abundance and community in general and which was positive for the total bacteria and ammonia nitrogen removal in the CWs. The higher abundance of Nitrospirae appeared in the non-rhizosphere sediment (NRS) than that in the rhizosphere sediment (RS). More denitrification genes were found in NRS than in RS. The copy numbers of narG, nirS and nosZ genes for CA were higher than those for CI. Wetland plant species can significantly (P < 0.05) affect the distribution of microbial communities in RS. Plant selection is important to promote the development of microbial communities with a more active and diverse catabolic capability and the contribution of plant absorption to the overall removal rate of wetland system can be neglected.