The microbial community and the nirS- and nirK-encoding denitrifiers in the intertidal sediments along Laizhou Bay in China were studied using pyrosequencing and real-time quantitative PCR (qPCR), respectively. There were three primary intertidal zones: Laizhou (La), Weifang Harbor (We), and Dongying (Do). Significant differences in composition and abundances at the different taxonomic levels were observed among the three bacterial communities. The qPCR results indicated that the nirS gene abundance varied from 8.67×105 to 5.68×106copies/gwet weight (ww), whereas the nirK gene abundance varied from 1.26×105 to 1.89×106copies/gww. The canonical correlation analysis (CCA) indicated that the sand percentage was the most important factor in shaping the bacterial community followed by silt percentage, NO2−, TOC, DO, pH, and clay percentage, whereas the clay percentage, pH, NO3−, DO, NO2−, TOC, silt percentage, and sand percentage were the most important factors associated with regulating the abundance of nirS- and nirK-encoding denitrifiers.

A nitrate-dominant synthetic wastewater simulating slightly polluted water with low C/N and poor biochemical availability was treated in lab-scale vertical-flow (VF)-constructed wetlands, which had Phragmites australis planted with different types of external carbon sources: Platanus acerifolia leaf litters, P. australis litters, glucose and a blank test with no external carbon sources. A comparison of the TN removal and N₂O flux performances among the four wetland reactors indicated higher TN removal efficiencies and N₂O release fluxes in the VF wetland columns with external carbon sources, as measured by the percentage removal of TN (P. acerifolia leaf litters 82.49 %, P. australis litters 70.55 %, glucose 62.50 % and blank 46.45 %) and N₂O flux (P. acerifolia leaf litters 2275.22 μg · m⁻² · h⁻¹, P. australis litters 1920.53 μg · m⁻² · h⁻¹, glucose 1598.57 μg · m⁻² · h⁻¹and blank 1192.08 μg · m⁻² · h⁻¹). This was primarily because of an improved supply of organic carbon from the external carbon sources for heterotrophic denitrification. And, the nitrogen released from the decomposition of plant materials resulted in the N₂O release fluxes to some extent. However, employing P. acerifolia leaf litters and P. australis litters as external carbon sources caused net increases in organics of the final effluent water. Overall, the results not only demonstrated the potential of using external plant carbon sources in VF wetlands to enhance the TN removal efficiency but also showed a risk of excessive organic release and greater N₂O flux feedback to global warming. Hence, future studies are needed to optimise the quantity and method for adding external carbon sources to VF-constructed wetlands so that sufficient nitrate removal efficiency is achieved and the N₂O flux and organic pollution are minimised.

We measured C and N cycling indicators in Appalachian watersheds impacted by mountaintop removal and valley fill (MTR/VF) coal mining, and in nearby forested watersheds. These watersheds include ephemeral, intermittent, and perennial stream reaches, and the length of time since disturbance in the MTR/VF watersheds was 5 to 11 years. In forest soils compared to VF soils, both denitrification enzyme activity (DEA) and basal respiration (BR) were elevated (factor of 6 for DEA and factor of 1.8 for BR expressed on a weight basis) and bulk density was lower. Organic matter (OM) and moisture were higher in the forest soils, which likely contributed to the elevated DEA and BR levels. Evaluation of soils data from our intermittent watersheds as a chronosequence provides some evidence of soil quality (DEA, BR, and soil moisture) improvement over the course of a decade, at least in the top 5 cm. Across the hydrological permanence gradient, sediment DEA was significantly higher (factor of 1.6) and sediment OM was significantly lower in forested than in VF watersheds, whereas sediment BR did not differ between forested and VF watersheds. Dissolved organic carbon (DOC) concentrations were not different in mining-impacted and forested streams, whereas dissolved inorganic carbon (DIC) concentrations and DOC and DIC stable carbon isotopic compositions (δ¹³C) were significantly elevated in VF streams. The δ¹³C-DIC values indicate that carbonate dissolution was a dominant source of dissolved carbon in MTR/VF mining-impacted streams. The disturbance associated with MTR/VF mining significantly impacts C and N processing in soils, stream sediments, and stream water although our data suggests some improvement of soil quality during the first decade of reclamation.

The simultaneous removal of phenol and ammonium using heterotrophic nitrifying-denitrifying bacterium Serratia sp. LJ-1 was investigated. The maximum removal rates of ammonium nitrogen and phenol were 1.08 ± 0.05 and 2.14 ± 0.08 mg L⁻¹ h⁻¹, respectively. The ammonium oxidation had much higher tolerance to phenol toxicity than that of the autotrophic nitrifying bacteria. The increase in phenol concentration led to an increase in ammonium oxidation rate under the phenol concentration of 600 mg L⁻¹. The increase in ammonium concentration caused an increase in phenol biodegradation rate under the ammonium nitrogen concentration of 150 mg L⁻¹. Maximum rates of phenol biodegradation and total nitrogen removal in the treatments with nitrification metabolite (nitrate or nitrite) as the sole nitrogen source were more than 30 % lower than those of the treatment with ammonium as the sole nitrogen source. Ammonium was removed through nitrification and subsequent aerobic denitrification while phenol was biodegraded through the ortho-cleavage pathway and subsequently mineralized. Since phenol often coexists with nitrogen pollutants, these findings have significant environmental implications in terms of the simultaneous removal of these contaminants.

This study examines the effects of land use change on nitrate concentration and the abundances of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), and narG-containing denitrifiers in shallow groundwater. The results reveal a general increase of nitrate in shallow groundwater following the change of land use from paddy fields to vegetable patches. Furthermore, a significant relationship between NO₃ ⁻-N concentrations was observed both in groundwater and in soil at soil depths of 0–20, 20–40, 40–60, 60–80, and 80–100 cm. With regard to gene abundance in groundwater, the AOB amoA gene was most abundant and the AOA amoA gene copy numbers were lowest from the field with long-term paddy cultivation compared with the field under vegetable cultivation. The narG gene copy numbers were higher from the field under short-term vegetable cultivation compared with fields under long-term vegetable cultivation. The NO₃ ⁻-N concentrations in groundwater correlated positively with AOA amoA gene copy numbers, negatively with the AOB amoA gene, but with no significant relationship with the narG gene. In conclusion, land use change from paddy fields to vegetable patches increases nitrate in groundwater, which is correlated significantly with nitrate in soil and the abundance of the amoA gene, but is not related to the narG gene in groundwater. This study also suggests that the removal of groundwater nitrate pollution is not feasible through biological denitrification without additional denitrifiers and that it might even become more aggravated because of the AOA.

The purpose of this research was to thoroughly analyze the influences of environmental factors on denitrification processes in urban riparian soils. Besides, the study was also carried out to identify whether the denitrification processes in urban riparian soils could control nonpoint source nitrogen pollution in urban areas. The denitrification rates (DR) over 1 year were measured using an acetylene inhibition technique during the incubation of intact soil cores from six urban riparian sites, which could be divided into three types according to their vegetation. The soil samples were analyzed to determine the soil organic carbon (SOC), soil total nitrogen (STN), C/N ratio, extractable NO₃⁻-N and NH₄⁺-N, pH value, soil water content (SWC), and the soil nitrification potential to evaluate which of these factors determined the final outcome of denitrification. A nitrate amendment experiment further indicated that the riparian DR was responsive to added nitrate. Although the DRs were very low (0.099 ~ 33.23 ng N₂O-N g⁻¹ h⁻¹) due to the small amount of nitrogen moving into the urban riparian zone, the spatial and temporal patterns of denitrification differed significantly. The extractable NO₃⁻-N proved to be the dominant factor influencing the spatial distribution of denitrification, whereas the soil temperature was a determinant of the seasonal DR variation. The six riparian sites could also be divided into two types (a nitrate-abundant and a nitrate-stressed riparian system) according to the soil NO₃⁻-N concentration. The DR in nitrate-abundant riparian systems was significantly higher than that in the nitrate-stressed riparian systems. The DR in riparian zones that were covered with bushes and had adjacent cropland was higher than in grass-covered riparian sites. Furthermore, the riparian DR decreased with soil depth, which was mainly attributed to the concentrated nitrate in surface soils. The DR was not associated with the SOC, STN, C/N ratio, and pH. Nitrate supply and temperature finally decided the spatiotemporal distribution patterns of urban riparian denitrification. Considering both the low DR of existing riparian soils and the significance of nonpoint source nitrogen pollution, the substantial denitrification potential of urban riparian soils should be utilized to reduce nitrogen pollution using proper engineering measures that would collect the polluted urban rainfall runoff and make it flow through the riparian zones.

The biological treatment of ammonia-rich landfill leachates due to an inadequate C to N ratio requires expensive supplementation of carbon from an external carbon source. In an effort to reduce treatment costs, the objective of the study was to determine the feasibility of nitrogen removal via the nitrite pathway during landfill leachate co-treatment with municipal wastewater. Initially, the laboratory-scale sequencing batch reactor (SBR) was inoculated with nitrifying activated sludge and fed only raw municipal wastewater (RWW) during a start-up period of 9 weeks. Then, in the co-treatment period, consisting of the next 17 weeks, the system was fed a mixture of RWW and an increasing quantity of landfill leachates (from 1 to 10 % by volume). The results indicate that landfill leachate addition of up to 10 % (by volume) influenced the effluent quality, except for BOD₅. During the experiment, a positive correlation (r² = 0.908) between ammonia load in the influent and nitrite in the effluent was observed, suggesting that the second step of nitrification was partially inhibited. The partial nitrification (PN) was also confirmed by fluorescence in situ hybridisation (FISH) analysis of nitrifying bacteria. Nitrogen removal via the nitrite pathway was observed when the oxygen concentration ranged from 0.5 to 1.5 mg O₂/dm³and free ammonia (FA) ranged from 2.01 to 35.86 mg N-NH₃/dm³in the aerobic phase. Increasing ammonia load in wastewater influent was also correlated with an increasing amount of total nitrogen (TN) in the effluent, which suggested insufficient amounts of assimilable organic carbon to complete denitrification. Because nitrogen removal via the nitrite pathway is beneficial for carbon-limited and highly ammonia-loaded mixtures, obtaining PN can lead to a reduction in the external carbon source needed to support denitrification.

Ecological remediation is an important measure for the protection of lake water quality in removing nutrients, such as nitrate (NO₃⁻). In this study, four bioremediation processes (bare sediment, immobilized nitrogen cycling bacteria (INCB) added, Elodea nuttallii added, E. nuttallii-INCB assemblage) were operated at a lab to elucidate the effect of macrophyte appearance and INCB addition on NO₃⁻removal and achieve the optimal processes for biomediation.¹⁵ N-NO₃solution was added to microcosms to identify the key nitrogen transformation processes responsible for NO₃⁻removal. Results showed that nitrate removal was significantly enhanced after the addition of INCB and E. nuttallii. In the treatments with INCB added, E. nuttallii added, and INCB and E. nuttallii-INCB assemblage, nitrate removal ratio achieved 94.74, 98.76, and 99.15 %, respectively. In contrast, only 23.47 % added nitrate was removed in the control. Plant uptake and denitrification played an important role in nitrogen removal. The water quality was substantially improved by the addition of INCB and macrophyte that can accelerate denitrification and promote nitrogen assimilation of plants. The results indicated that plant uptake and microbial denitrification were key processes for nitrate removal.

This study investigated potential nitrogen fixation, net nitrification, and denitrification responses to short-term crude oil exposure that simulated oil exposure in Juncus roemerianus salt marsh sediments previously impacted following the Deepwater Horizon accident. Temperature as well as crude oil amount and type affected the nitrogen cycling rates. Total nitrogen fixation rates increased 44 and 194 % at 30 °C in 4,000 mg kg⁻¹ tar ball and 10,000 mg kg⁻¹ moderately weathered crude oil treatments, respectively; however, there was no difference from the controls at 10 and 20 °C. Net nitrification rates showed production at 20 °C and consumption at 10 and 30 °C in all oil treatments and controls. Potential denitrification rates were higher than controls in the 10 and 30 ºC treatments but responded differently to the oil type and amount. The highest rates of potential denitrification (12.7 ± 1.0 nmol N g⁻¹ wet h⁻¹) were observed in the highly weathered 4,000 mg kg⁻¹ oil treatment at 30 °C, suggesting increased rates of denitrification during the warmer summer months. These results indicate that the impacts on nitrogen cycling from a recurring oil spill could depend on the time of the year as well as the amount and type of oil contaminating the marsh. The study provides evidence for impact on nitrogen cycling in coastal marshes that are vulnerable to repeated hydrocarbon exposure.

Soils in urban areas are subjected to compaction with accelerating urbanization. The effects of anthropogenic compaction on urban soil denitrification are largely unknown. We conducted a study on an urban park soil to investigate how compaction impacts denitrification. By using¹⁵N labeling method and acetylene inhibition technique, we performed three coherent incubation experiments to quantify denitrification in compacted soil under both aerobic and anaerobic conditions. Uncompacted soil was set as the control treatment. When monitoring soil incubation without extra substrate, higher nitrous oxide (N₂O) flux and denitrification enzyme activity were observed in the compacted soil than in the uncompacted soil. In aerobic incubation with the addition of K¹⁵NO₃, N₂O production in the compacted soil reached 10.11 ng N h⁻¹ g⁻¹as compared to 0.02 ng N h⁻¹ g⁻¹in the uncompacted soil. Denitrification contributed 96 % of the emitted N₂O in the compacted soil and 36 % of the emitted N₂O in the uncompacted soil; total denitrification rate was higher in the compacted soil (up to 79.35 ng N h⁻¹ g⁻¹) than in the uncompacted soil (0.11 ng N h⁻¹ g⁻¹). Under anaerobic incubation with the addition of K¹⁵NO₃, no statistical difference in total N losses and¹⁵N-(N₂O+N₂) flux between the uncompacted soil and the compacted soil was detected. Compaction promoted soil denitrification and may impact urban N biogeochemical cycling.