Dissimilatory Nitrate Reduction To Ammonium Process Research Articles

Long-term operation and dynamic response of dissimilatory nitrate reduction to ammonium process under low-frequency infrared electromagnetic field

Dissimilatory nitrate reduction to ammonium (DNRA) received more attention for its ability to recover ammonium. This study investigated the possibility of low-frequency infrared electromagnetic field (IR-EMF) to improve DNRA. The optimal IR-EMF intensity of 0.04 μT could effectively improve DNRA activity of nonwoven fabric membrane bioreactors. In the long-term operation, the average ammonium conversion efficiency was enhanced by 117.7% and 62.5% under 0.04 μT and 0.06 μT IR-EMF, respectively. The highest nrfA-gene abundance and potential DNRA rate were obtained under 0.04 μT IR-EMF exposure. Bacteroidetes fragilis, Shewanelle oneidensis MR-1, and Thauera sp. RT1901 were selected to investigate the dynamic response of nitrogen transformation and energy metabolism to IR-EMF. The transcriptome sequencing and RT-qPCR results suggested that IR-EMF could enhance both denitrification and DNRA process, mainly by improving ATP synthesis to boost metabolic activity. This study provided an efficient method for the nitrogen recovery via DNRA process by applying IR-EMF.

Using static magnetic field to recover ammonia efficiently by DNRA process

Dissimilatory nitrate reduction to ammonium (DNRA) has garnered attention due to its ability to recover ammonia and reduce greenhouse gas emissions simultaneously. In this study, the potential of using static magnetic field (SMF) to improve DNRA process was explored from the sight of molecular biology. Functional genes, microbial community structure, and metabolism pathways were discussed. SMF of 40 mT shortened the start-up time of DNRA from 75 days to 41 days, while 80 mT SMF delayed it to 103 days. On day 80, DNRA potential rate under 40 mT SMF, reached 174 ± 11 μmol kg−1 h−1, significantly surpassing 0 mT (88 ± 6 μmol kg−1 h−1) and 80 mT SMF (52 ± 4 μmol kg−1 h−1). SMF of 40 mT also accelerated community succession and the enrichment of functional bacteria like Geobacter (from 15.71% to 32.11%). qPCR results suggested that 40 mT SMF promoted the rapid enrichment of DNRA functional gene nrfA and 80 mT SMF promoted the enrichment of nirS gene on day 40. Dynamic responses of Thauera sp. RT1901, Stutzerimonas stutzeri, Shewanella oneidensis MR-1, and Shewanella loihica PV-4 to SMF at transcriptional levels confirmed SMF could improve the nitrogen removal and electron transfer of DNRA and denitrification bacteria. Consequently, this work validated the possibility of using SMF to improve DNRA process for ammonia recovery and investigated the underlying mechanisms, which could promote the application of DNRA in full-scale.

Spatiotemporal patterns and co-occurrence patterns of dissimilatory nitrate reduction to ammonium community in sediments of the Lancang River cascade reservoirs.

Dissimilatory nitrate reduction to ammonium (DNRA) is an important nitrate reduction pathway in freshwater sediments. Many studies have focused on the DNRA process in various natural habitats. However, the joint operation of cascade reservoirs will affect the physical and chemical properties of sediments, which may change the DNRA process and bacterial community pattern in the surface sediments of cascade reservoirs. Our study was the first to investigate the spatiotemporal distribution patterns of potential DNRA rate, nrfA gene abundances, and DNRA bacterial community diversity in surface sediments of the Lancang River cascade reservoirs. The results of slurry incubation experiments combined with the 15N isotope tracer experiment ascertained that the potential rates of DNRA were 0.01-0.15 nmol-N cm-3 h-1, and qPCR results indicated that the abundance range of nrfA was 1.08 × 105-2.51 × 106 copies g-1 dry weight. High throughput sequencing of the nrfA gene revealed that the relative abundance of Anaeromyxobacter (4.52% on average), Polyangium (4.09%), Archangium (1.86%), Geobacter (1.34%), and Lacunisphaera (1.32%) were high. Pearson and RDA correlation analysis exhibited that nrfA gene abundance was positively correlated with altitude, pH, OC, and sand concentration. Anaeromyxobacter was positively correlated with reservoir age and DNRA potential rate. The deterministic environmental selection process plays a crucial role in the formation of the DNRA bacterial community. Network analysis displayed that the dominant DNRA genus was the key population of the DNRA microbial community in the sediments of Lancang River cascade reservoirs. This study reveals that the variation of DNRA bacterial activity and community structure is largely driven by the construction of cascade reservoirs, and provides a new idea for further understanding the characteristics of the DNRA community in the cascade reservoir ecosystem.

Impact of bioelectricity on DNRA process and microbial community composition within cathodic biofilms in dual-chambered bioelectrode microbial fuel cell (MFC)

The synchronous bioelectricity generation and dissimilatory nitrate reduction to ammonium (DNRA) pathway in Klebsiella variicola C1 was investigated. The presence of bioelectricity facilitated cell growth on the anodic biofilms, consequently enhancing the nitrate removal efficiency decreasing total nitrogen levels and causing a negligible accumulation of NO2– in the supernatant. Genomic analysis revealed that K. variicola C1 possessed a complete DNRA pathway and largely annotated electron shuttles. The up-regulated expression of genes narG and nirB, encoding nitrite oxidoreductase and nitrite reductase respectively, was closely associated with increased extracellular electron transfer (EET). High-throughput sequencing analysis was employed to investigate the impact of bioelectricity on microbial community composition within cathodic biofilms. Results indicated that Halomonas, Marinobacter and Prolixibacteraceae were enriched at the cathode electrodes. In conclusion, the integration of a DNRA strain with MFC facilitated the efficient removal of wastewater containing high concentrations of NO3– and enabled the environmentally friendly recovery of NH4+.

Biodegradable microplastics boost dissimilatory nitrate reduction to ammonium (DNRA) process contributing to ammonium nitrogen retention in farmland soils

Microplastics (MPs) pollution in agriculture ecosystems profoundly affects biogeochemical cycles. However, the impacts of MPs, particularly biodegradable MPs, on dissimilatory nitrate reduction to ammonium (DNRA) process remain unclear. Herein, the alterations in denitrification and DNRA pathways and underlying mechanism by different MPs exposures (including conventional nonbiodegradable MPs (polyethylene (PE) and polypropylene (PP)) and biodegradable MPs (polylactic acid (PLA) and polybutylene adipate terephthalate (PBAT)) were explored via soil microcosm experiments accompanied with 15N isotope tracer technology and 16S rRNA high-throughput sequencing. Compared with control, PE and PP increased the rates of denitrification by 117–126% but significantly decreased DNRA rates by 39.3–63.4%, whereas DNRA activities in PLA and PBAT treatments were remarkably boosted by 163–242% and dominated soil nitrate bioreduction (6.67–8.69 μg N g−1 d −1). This differentiation was attributed to the nutrient imbalance induced by MPs and distinctly selective effects of MPs on functional microbes. Microbial interaction mechanism further suggested that limited bioavailable carbon intensified the competition between heterotrophic denitrifiers (e.g., Bacillus, Sphingomonas and Microvirga) and DNRA bacteria (e.g., Luteitalea and Streptomyces) for electron donor in nonbiodegradable MPs treatments, reducing the abundances of DNRA bacteria. However, the high bioavailable C/NOx– ratio resulting from biodegradable MPs degradation released such competition and more greatly promoted the activities of DNRA bacteria than denitrifiers, facilitating ammonium retention in soils. Collectively, this study highlights the promotion of biodegradable MPs on soil ammonium recycle via DNRA process in farmland ecosystems and can shed some light on current plastic mulch-film management toward a sustainable and cleaner agricultural industry.

Microbial interaction patterns and nitrogen cycling regularities in lake sediments under different trophic conditions

Exploring how nitrogen (N) cycling microbes interact in eutrophic lake sediments and how biogenic elements influence the nitrogen cycle is crucial for understanding biogeochemical cycles and nitrogen accumulation mechanisms. In this study, sediment samples were collected from various areas of Taihu Lake with different trophic conditions in all four seasons from 2015 to 2017. Using high-throughput sequencing and molecular ecological network analysis, we investigated the microbial interaction patterns and the role of nitrogen cycling in sediments from lakes with different trophic conditions. The results showed distinct structures of sediment microbial networks between lake areas with different trophic conditions. In the more eutrophic region, network indices indicate higher transfer efficiency of energy, material, and information, more significant competition, and weaker niche differentiation of the microbial community. The sedimentary environment in the moderately eutrophic area exhibited greater potential for denitrification, nitrification, and anammox compared to the mesotrophic area, but the inhibition between N functional microbes and limitations in N removal processes were also more likely to occur. The topological structure of the networks showed that the carbon (C), sulfur (S), and iron (Fe) cycles had a strong influence on the nitrogen cycle in both lake areas. In the moderately eutrophic lake area, C- and S-cycling functional bacteria facilitated a closed cycle of the coupled N fixation–nitrification–DNRA (dissimilatory nitrate reduction to ammonium) process and reduced N removal. In the mesotrophic lake area, C- and S-cycling functional bacteria promoted both N fixation and mineralization, and Fe-cycling functional bacteria coupled with denitrifiers enhanced the nitrogen removal process of products from nitrogen fixation and mineralization. This study improved the understanding of the nitrogen cycling mechanism in lake sediments under different trophic conditions.

Effects of Fe(II) and organic carbon on nitrate reduction in surficial sediments of a large shallow freshwater lake

Nitrate-reducing ferrous [Fe(II)]-oxidizing (NRFO) has been reported from lake sediments as a natural reduction pathway. However, the effects of the contents of Fe(II) and sediment organic carbon (SOC) on the NRFO process still remain unclear. In this study, the influences of Fe(II) and organic carbon on nitrate reduction were analyzed quantitatively at two typical seasonal temperatures (25 °C representing summers and 5 °C for winters) by conducting a series of batch incubation experiments, using surficial sediments at the western zone of Lake Taihu (Eastern China). Results showed that Fe(II) greatly promoted NO3‾-N reduction by denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) processes at high-temperature (25 °C, representing summer season). As Fe (II) increased (e.g., Fe(II)/NO3‾ = 4), the promotion effect on NO3‾-N reduction was weakened, but on the other side, the DNRA process was enhanced. In comparison, the NO3‾-N reduction rate obviously decreased at low-temperature (5 °C, representing the winter season). NRFO in sediments mainly belongs to biological rather than abiotic processes. A relatively high SOC content apparently increased the rate of NO3‾-N reduction (r = 0.023–0.053 mM/d), particularly on the heterotrophic NRFO. It is interesting that the Fe(II) consistently remained active in the nitrate reduction processes no matter whether SOC was sufficient in the sediment, particularly at high-temperature. Overall, the combining effects of both Fe(II) and SOC in surficial sediments made a great contribution towards NO3‾-N reduction and N removal in a lake system. These results provide a better understanding and estimation of N transformation in sediments of the aquatic ecosystem under different environmental conditions.

Assembly mechanism and co-occurrence patterns of DNRA microbial communities and imprint of nitrate reduction in the Songhua River sediments of China's largest old industrial base

Dissimilatory nitrate reduction to ammonium (DNRA) reduces nitrate to ammonium nitrogen instead of nitrogen gas, which is an important internal linking process of nitrogen cycle. No literature has reported the assembly mechanism of DNRA microbial communities. Here, the deep assembly mechanisms and co-occurrence patterns of DNRA microbial communities and imprint of nitrate reduction in Songhua River Basin (an inland river located in northeastern China) were studied. The DNRA potential rates were detected at six sampling sites, ranging from 0.25 ± 0.23 to 4.22 ± 0.61 μmol N/L/h, accounting for 33.07% to 98.08% of the total nitrate reduction. Spearman analysis indicated that the nrfA gene abundance was significantly positively correlated with the concentration of total nitrogen (TN) in sediments (p < 0.05), suggesting that nutrient inputs may enhance the metabolic potential of DNRA bacteria. Co-occurrence network and Spearman correlation analyses showed that the DNRA rates were significantly correlated with the abundance of keystone species, but not with the dominant genera (p < 0.05). Variance partitioning analysis (VPA) revealed that sediment physicochemical properties and spatial factors explained only 21.07% and 14.51% of the DNRA community variation. Null model and neutral community model both revealed stochastic processes play a major role in shaping DNRA microbial community structure. The drift was the most important process, explaining 36.36% of the community variation, followed by homogeneous selection and homogenizing dispersal, which accounted for 27.27% and 22.73%, respectively. Additionally, Geobacteraceae played an important role in DNRA and the entire bacterial community in Songhua River. This study explored the underestimated DNRA process and the deep community assembly mechanisms, which will contribute to understanding nitrate conversion and the impact of nitrogen pollution on microbial communities of river sediments in cold regions.

Dissimilatory nitrate reduction to ammonium (DNRA) potentially facilitates the accumulation of phosphorus in lake water from sediment.

Nitrogen (N) and phosphorus (P) are crucial nutrients for eutrophication in the lacustrine ecosystem and attract the attention worldwide. However, the interaction between them need further clarification. This study aimed to assess the influence of dissimilatory nitrate reduction to ammonia (DNRA) on the cycle of P in lacustrine sediment. Different fractions of N and P in the pore water were measured using high-resolution in-situ measurement techniques, HR-Peeper and DGT, coupling with sequential extraction for solid sediment from a shallow freshwater lake. The results showed that elevated nitrate (NO3-) reduction via DNRA rather than denitrification was verified at deeper sediment layer, suggesting the generation of inorganic ammonia (NH4+) as electron donor under anaerobic episodes. High abundance of DNRA bacteria (nrfA gene) obtained using high-throughput sequencing analysis were detected at upper layer and responsible for the accumulation of NH4+ in the sediment coupling with chemolithoautotrophic metabolism. Additionally, significant desorption of ionic ferrous iron (Fe2+) and dissolved reactive phosphate (DRP) from solid phase and the enrichment in the solution was simultaneously detected. Higher concentration of solid Fe bound P (Fe-P) at deeper layer indicated the potential re-oxidation of Fe2+ as electron donor during DNRA process and sorption of DRP toward the Fe-containing minerals. However, obvious evidence of desorption proved by DGT indicated that higher NH4+ concentrations favored the reduction of Fe(III) oxy(hydr)oxides and the desorption of DRP into the pore water and diffusion toward the overlying water. Finally, noteworthy S2- release from solid sediment was speculated to stimulate the DNRA and facilitated the accumulation of NH4+ in the solution, which further induced the enrichment of DRP in water from the solid phase. Overall, DNRA potentially facilitates the accumulation of P in lake water, and the synchronous control of N and P is important for the eutrophication management and restoration of lake eutrophication.

Enhancement of dissimilatory nitrate/nitrite reduction to ammonium of Escherichia coli sp. SZQ1 by ascorbic acid: Mechanism and performance

Dissimilatory nitrate reduction to ammonium (DNRA) can be used for nitrogen recovery. However, due to the low conversion efficiency of the DNRA process of microorganisms, the process cannot be industrially applied. Ascorbic acid (ASA) can improve DNRA efficiency of Escherichia coli sp. SZQ1 (E. coli). Experimental studies suggest that 10 g L−1 ASA promoted DNRA process of E. coli at high concentrations of nitrite (10–20 mM). In the 5 g L−1 ASA system, 9.2 mM nitrite was reduced to 8.21 mM ammonium by E. coli in 120 h. Mechanistic studies reveal that ASA reduced the oxidation-reduction potential (ORP) of the system and scavenged reactive oxygen species (ROS) in the cell of E. coli. Meanwhile, ASA was utilized by E. coli as the sole carbon source and provided electrons to DNRA process through ASA metabolic pathways. This study proposes a new strategy for increasing the efficiency of DNRA.

Biochar amendment alters the partitioning of nitrate reduction by significantly enhancing DNRA in a paddy field

Using 15N tracer technique, we investigated the potential rates of denitrification, anaerobic ammonium oxidation, dissimilatory nitrate reduction to ammonium (DNRA), and their partitioning among nitrate reduction, as well as the N2O emission rates in a paddy soil receiving various biochar (0%, 0.03%, 0.1%, 0.5%, and 1.0%; w/w) and straw (0.1%) over six consecutive years. Results showed that except for the 1.0% amendment treatment, biochar significantly (P < 0.05) increased denitrification rates by 10.19‒75.96% compared with non-biochar amended treatment, and that biochar significantly (P < 0.05) increased DNRA rates by 1.63‒6.84 folds relative to non-biochar amended treatment. Consequently, biochar shifted more NO3– partitioning toward DNRA process, as suggested by the increased DNRA/(denitrification + DNRA) ratios from 1.60 to 13.18%. On the other hand, biochar significantly (P < 0.05) reduced N2O emission rates by 61.86–97.65% accompanied by a significant decrease in N2O/(N2O + N2) ratios (65.29–98.39%), indicating biochar amendment facilitated the reduction of N2O to N2. The promoting effects of biochar on DNRA rates and DNRA/(denitrification + DNRA) ratios were attributed to the increased carbon availability and the altered nitrate reducer communities. Collectively, our study suggests that biochar amendment in the paddy soil is helpful for N conservation by favoring nitrate partitioning toward DNRA process, which deepens our understanding of how biochar mediates N cycling in the paddy field.Graphical

Electron shuttle potential of biochar promotes dissimilatory nitrate reduction to ammonium in paddy soil

Enhancing dissimilatory nitrate reduction to ammonium (DNRA) is environmentally and agronomically beneficial due to DNRA improving nitrogen (N) retention in soil. However, the rate of DNRA is generally considerably lower than that of denitrification because DNRA requires more electron donors than denitrification. Biochar has been increasingly reported to act as an “electron shuttle” to facilitate electron transfer and to promote redox reactions in soil. Thus, this study aimed to investigate whether and how biochar could enhance the DNRA process in a paddy soil. The results showed that, compared with the no-biochar control, the application of rice straw biochar increased the DNRA rate from 0.2 to 0.7 mg NH4+-N kg−1 dry soil d−1. As well, biochar simultaneously, increased the relative abundance of DNRA functional microbes (nrfA-type microbes) and functional gene (nrfA) expression levels. Biochar's enhancement of DNRA was positively correlated with the biochar properties relevant to electron shuttling (e.g., specific capacitance). In contrast, the application of electron shuttle-weakened biochar (oxidized by H2O2) did not increase, or even decreased, the DNRA rate in the paddy soil. These results demonstrate that biochar can act as an electron shuttle to enhance electron availability for DNRA functional microorganisms and consequently promote the DNRA process in paddy soil. Our results indicate that amendment of paddy soil with biochar containing a high-capacity electron shuttle function is beneficial for preserving N by transforming the mobile nitrate anion into the less mobile ammonium cation in paddy soils.

Investigation of dissimilatory nitrate reduction to ammonium (DNRA) in urban river network along the Huangpu River, China: rates, abundances, and microbial communities.

Dissimilatory nitrate reduction to ammonium (DNRA) is an essential intermediate step in the nitrogen cycle, and different sediment physicochemical properties can affect the DNRA process. But the detailed research on the environmental nitrogen cycling in urban river networks based on DNRA communities and the functional gene nrfA is lacking. In this study, the flow line of the Huangpu River in Shanghai was analyzed using isotope tracer, quantitative real-time PCR, and high-throughput sequencing techniques to evaluate the role of DNRA on the stability of the river network and marine. The significant positive correlation between the rate of DNRA and sediment organic carbon was identified. At the genus level, Anaeromyxobacter is the most dominant. Notably, both heterotrophic and autotrophic DNRA species were discovered. This study added diversity to the scope of urban freshwater river network ecosystem studies by investigating the distribution of DNRA bacteria along the Huangpu River. It provided new insights into the biological nitrogen cycle of typical urban inland rivers in eastern China.

Role of organic/sulfide ratios on competition of DNRA and denitrification in a co-driven sequencing biofilm batch reactor.

Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are two competing pathways in nitrate-reducing process. In this study, a series of C/S ratios from 8:1 to 2:4 were investigated in a sequencing biofilm batch reactor (SBBR) to determine the role of reducers (sulfide and acetate) on their competition. The results showed that the proportion of DNRA increased in high electron system, either in organic-rich system or in sulfide-rich system. The highest DNRA ratio increased to 16.4% at the C/S ratio of 2:3. Excess electron donors, particularly sulfide, were favorable for DNRA in a limited nitrate environment. Moreover, a higher reductive environment could facilitate DNRA, especially, when ORP was lower than - 400mV in this system. 16S rRNA gene sequencing analysis demonstrated that Geobacter might be the important participant involved in DNRA process in organic-rich system, while Desulfomicrobium might be the dominant DNRA bacteria in sulfide-rich system. DNRA cultivation could enrich nitrogen conversion pathways in conventional denitrification systems and deepen the insight into nitrogen removal at low C/N.

Anaerobic Oxidation of Methane Coupled with Dissimilatory Nitrate Reduction to Ammonium Fuels Anaerobic Ammonium Oxidation.

Nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) is critical for mitigating methane emission and returning reactive nitrogen to the atmosphere. The genomes of n-DAMO archaea show that they have the potential to couple anaerobic oxidation of methane to dissimilatory nitrate reduction to ammonium (DNRA). However, physiological details of DNRA for n-DAMO archaea were not reported yet. This work demonstrated n-DAMO archaea coupling the anaerobic oxidation of methane to DNRA, which fueled Anammox in a methane-fed membrane biofilm reactor with nitrate as only electron acceptor. Microelectrode analysis revealed that ammonium accumulated where nitrite built up in the biofilm. Ammonium production and significant upregulation of gene expression for DNRA were detected in suspended n-DAMO culture with nitrite exposure, indicating that nitrite triggered DNRA by n-DAMO archaea. 15N-labeling batch experiments revealed that n-DAMO archaea produced ammonium from nitrate rather than from external nitrite. Localized gradients of nitrite produced by n-DAMO archaea in biofilms induced ammonium production via the DNRA process, which promoted nitrite consumption by Anammox bacteria and in turn helped n-DAMO archaea resist stress from nitrite. As biofilms predominate in various ecosystems, anaerobic oxidation of methane coupled with DNRA could be an important link between the global carbon and nitrogen cycles that should be investigated in future research.

Nitrate assimilation, dissimilatory nitrate reduction to ammonium, and denitrification coexist in Pseudomonas putida Y-9 under aerobic conditions

The specific nitrate reduction pathway in Pseudomonas putida Y-9 under aerobic conditions was studied. Strain Y-9 removed 82% of the nitrate accompanied by an accumulation of ammonium and a decrease of total nitrogen. Ammonium inhibited nitrate transformation (removal efficiency was 22.65%), illustrating that nitrate assimilation exists in strain Y-9. The detectable ammonium in the supernatant during the nitrate reduction process came from intracellular locations in strain Y-9. The nirBD that encodes nitrite reductase had an important role in strain growth and ammonium production. A 15N isotope experiment demonstrated that strain Y-9 can conduct dissimilatory nitrate reduction to ammonium (DNRA) and nirBD controls this process. This further indicated that the loss of total nitrogen is due to denitrification. All results highlighted that strain Y-9 performs simultaneous nitrate assimilation, DNRA, and denitrification under aerobic conditions, and nirBD controls the assimilation and DNRA process. Thereinto, nitrate assimilation dominates the removal of nitrate.

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+.

Performance and Influencing Factors of Dissimilatory Nitrate Reduction to Ammonium Process by the Strain Desulfovibrio sp. CMX

The process of dissimilatory nitrate reduction to ammonium (DNRA) plays an important role in the presence and conversion of ammonium ions in nature. However, the influencing factors and mechanisms of DNRA process in sulfate-reducing bacteria (SRB) have not yet been identified. The ability, influencing factors and mechanisms of DNRA was investigated in Desulfovibrio sp. CMX isolated previously in our laboratory. The results showed that the production rate of ammonium reached 85.8% and 97.3% respectively with NO3- and NO2- (10 mmol·L-1) as the sole electron donor and nitrogen source. Meanwhile, there was no by-products such as N2 and N2O. Then the effects of different extra nitrogen sources and initial concentrations of sulfate and sulfide on DNRA process were also investigated. As a result, extra nitrogen, yeast extract, could promote DNRA process by improving both growth and metabolism of the strain. In another study, sulfate promoted the nitrate reduction stage while inhibited the nitrite reduction. Overall, the DNRA process by Desulfovibrio sp. CMX was inhibited by sulfate. In addition, both the growth and two stages of the DNRA process were inhibited in the presence of sulfide. Dose effect of sulfide was observed. Inhibition was enhanced with the increase of sulfide concentration. It's noticeable that the inhibition on nitrate reduction was stronger than the inhibition on nitrite reduction with 6 mmol·L-1 of sulfide, leading to the lower reduction rate of nitrate than that of nitrite. In this condition, no accumulation of nitrite was observed in the system.

Enrichment of DNRA bacteria in a continuous culture.

Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are competing microbial nitrate-reduction processes. The occurrence of DNRA has been shown to be effected qualitatively by various parameters in the environment. A more quantitative understanding can be obtained using enrichment cultures in a laboratory reactor, yet no successful DNRA enrichment culture has been described. We showed that a stable DNRA-dominated enrichment culture can be obtained in a chemostat system. The enrichment was based on the hypothesis that nitrate limitation is the dominant factor in selecting for DNRA. First, a conventional denitrifying culture was enriched from activated sludge, with acetate and nitrate as substrates. Next, the acetate concentration in the medium was increased to obtain nitrate-limiting conditions. As a result, conversions shifted from denitrification to DNRA. In this selection of a DNRA culture, two important factors were the nitrate limitation and a relatively low dilution rate (0.026 h(-1)). The culture was a highly enriched population of Deltaproteobacteria most closely related to Geobacter lovleyi, based on 16S rRNA gene sequencing (97% similarity). We established a stable and reproducible cultivation method for the enrichment of DNRA bacteria in a continuously operated reactor system. This enrichment method allows to further investigate the DNRA process and address the factors for competition between DNRA and denitrification, or other N-conversion pathways.

Dissimilatory nitrate reduction to ammonium (DNRA) plays an important role in soil nitrogen conservation in neutral and alkaline but not acidic rice soil

In the rice soils, the alternate wetting and drying conditions and the leakage of O2 from rice roots provides a favorable environment for many N transformation processes, such as DNRA, nitrification, mineralization, and denitrification. Since nitrification is an inevitable process providing available NO3 − for DNRA and other NO3 − loss pathways, it is logical to hypothesize that a relationship exists between nitrification and DNRA. Thus, we quantified the specific gross N transformation rates occurring simultaneously to investigate the correlation among the N transformation rates in the rice soil. In this study, three rice soils with different pH values were selected. The combination of 15N tracing experiment and numerical modeling method was performed to quantify the specific gross N transformation rates (e.g., DNRA), to analyze the correlation between DNRA and the other N transformations, and to estimate the functional role of DNRA comparing to other N transformation in the rice soil. The results showed that there were two NH4 + production pathways, organic N mineralization and DNRA, in the neutral (pH 6.2) and alkaline (pH 8.2) soils, while the DNRA process was negligible in acidic soil (pH 4.7). The mineralization rate in the acidic soil (2.69 mg kg−1d−1) was significantly higher than that in neutral and alkaline soils. The DNRA rate was 0.48 and 1.09 mg kg−1d−1, which was almost equal to the mineralization rate, in the neutral (pH 6.2) and alkaline (pH 8.2) soils, offsetting efficiently the effect of low mineralization on N availability in ecosystems. The DNRA rate linearly increased with the increase in gross nitrification rate (p < 0.05). This significant coupling relationship between DNRA and nitrification favors N conservation and availability for paddy growth in the rice ecosystem. DNRA plays an important role in soil N conservation and availability in neutral and alkaline but not acidic rice soil.