Abundance Of Denitrification Genes Research Articles

Patterns of Denitrification and Methanogenesis Rates from Vernal Pools in a Temperate Forest Driven by Seasonal, Microbial Functional Gene Abundances, and Soil Chemistry

Due to their relatively small sizes, temperate forest vernal pools are less studied than other wetlands, despite being potential biogeochemical hotspots in landscapes. We investigated spatial and temporal factors driving N2O and CH4 emission rates from vernal pools in a temperate forest. We determined higher N2O (3.66 ± 0.53 × 10−6, μg N2O/m2/h) and CH4 (2.10 ± 0.7 × 10−3, μg N2O/m2/h) rates in spring relative to fall (~50% and 77% lower for N2O and CH4 rates, respectively) and winter (~70% and 94% lower for N2O and CH4 rates, respectively). Soil organic matter, nitrate content and bacterial 16S rDNA, nirS, and norB gene abundances emerged as significant drivers of N2O rates, whereas, soil pH, organic matter content and mcrA abundance were significant drivers of CH4 rates. Denitrification gene abundances were negatively correlated with N2O rates, whereas mcrA abundance correlated positively with CH4 rates. Results suggest that CH4 rates may be directly coupled to methanogen abundance, whereas N2O rates may be directly impacted by a variety of abiotic variables and indirectly coupled to the abundance of potential denitrifier assemblages. Overall, additional studies examining these dynamics over extended periods are needed to provide more insights into their control.

Investigating the role of organic carbon amendments and microbial denitrification gene abundance in nitrogen removal from experimental agricultural drainage ditches with low-grade weirs.

Low-grade weirs placed within agricultural drainage ditches in the Lower Mississippi Alluvial Valley can be used as a management practice to enhance nitrogen removal. The addition of organic carbon amendments within ditches that contain weirs could further increase nitrogen removal. Through repeated trials, changes in -N concentration between inflow and outflow were variable in the ditch without weirs, while only decreases in concentration were observed in ditches with weirs. Significant differences in -N concentrations were observed between treatments, with greater removal of -N observed in dissolved organic carbon treatments compared to control and particulate organic carbon treatments. At medium- and high-flow rates, respectively, dissolved organic carbon treatments resulted in greater -N concentration decreases of 31.6% and 27.1% compared to 19% and 11.6% in particulate organic carbon treatments and 18.6% and 17.2% in control treatments. Significant effects of weirs and sampling date on nirS, nirK, nosZ, and 16S rRNA gene abundances were observed. Observed increases in -N removal with organic carbon amendments, provides support for continued investigation on improving the efficacy of organic carbon amendments as a best management practice for -N removal in agricultural drainage ditches. PRACTITIONER POINTS: Dissolved organic carbon amendments increased nitrate-nitrogen removal. Only decreases in nitrate-nitrogen concentration were observed in ditches with weirs. Increasing flow rate did not affect nitrate-nitrogen removal. Abundance of denitrification-performing microbes likely did not affect N removal. Lack of anaerobic soil conditions and short residence time reduced nitrate-N removal.

Effect of urease and nitrification inhibitors on ammonia volatilization and abundance of N‐cycling genes in an agricultural soil

AbstractThe effect of the combined application of urease and nitrification inhibitors on ammonia volatilization and the abundance of nitrifier and denitrifier communities is largely unknown. Here, in a mesocosm experiment, ammonia volatilization was monitored in an agricultural soil treated with urea and either or both of the urease inhibitor N‐(n‐butyl) thiophosphoric triamide (NBPT) and the nitrification inhibitor 3,4‐dimethylpyrazole phosphate (DMPP), with 50% and 80% water‐filled pore space (WFPS). The effect of the treatments on the abundance of bacteria and archaea was estimated by quantitative PCR (qPCR) amplification of their respective 16S rRNA gene, that of nitrifiers using amoA genes, and that of denitrifiers by qPCR of the norB and nosZI denitrification genes. After application of urea, N losses due to NH3 volatilization accounted for 23.0% and 9.2% at 50% and 80% WFPS, respectively. NBPT reduced NH3 volatilization to 2.0% and 2.4%, whereas DMPP increased N losses by up to 36.8% and 26.0% at 50% and 80% WFPS, respectively. The combined application of NBPT and DMPP also increased NH3 emissions, albeit to a lesser extent than DMPP alone. As compared with unfertilized control soil, both at 50% and 80% WFPS, NBPT strongly affected the abundance of bacteria and archaea, but not that of nitrifiers, and decreased that of denitrifiers at 80% WFPS. Regardless of moisture conditions, treatment with DMPP increased the abundance of denitrifiers. DMPP, both in single and in combined application with NBPT, increased the abundance of nitrification and denitrification genes.

Using metagenomics to reveal landscape scale patterns of denitrifiers in a montane forest ecosystem

Denitrificationis an important process in the nitrogen cycle of many soil ecosystems, but the relationships between process rates and the genotype of denitrifying microorganisms are poorly understood. Genotyping may identify denitrifiers with less than the full complement of nitrogen-oxide reductases, which might be crucial for denitrification in nitrogen-limited environments, such as in montane forest landscapes. Therefore, a metagenomics survey was undertaken using soils from the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, USA where steep elevation, vegetation, and soil gradients provide a complex landscape matrix to assess occurrence patterns of the genes involved in denitrification. DNA was extracted from soils taken from three soil horizons, at three elevations, in two watersheds. Metagenomic analysis of reads showed that the relative abundance of denitrification genes within a community did not differ across soil depths but did vary among elevation zones, with total denitrification reads in High Hardwood > Spruce Fir > Low Hardwood. Reads from nirS were extremely rare, which suggests that complete denitrification is uncommon across this forest landscape. The gene with the largest proportion of denitrification specific reads was the quinol-oxidizing nitric oxide reductase, qnor, which reduces toxic nitric oxide to nitrous oxide. The relative enrichment of specialized denitrification genes involved in intermediate reactions may indicate that environmental factors are selecting for a partial denitrification process, rather than complete denitrification. High Hardwood soils had the highest denitrification gene abundance and the greatest potential rate of denitrification, indicating that metagenomic information was consistent with the process measurements. Although little energy is generated from complete denitrification, due to the acidic soil conditions and low nitrate availability in HBEF soils, the denitrifier community appears to compensate by producing particular denitrification genes. In particular, qnor may help the community cope with toxic nitric oxide produced via chemodenitrification, making it a public good.

Long-term nitrogen addition modifies microbial composition and functions for slow carbon cycling and increased sequestration in tropical forest soil.

Nitrogen (N) deposition is a component of global change that has considerable impact on belowground carbon (C) dynamics. Plant growth stimulation and alterations of fungal community composition and functions are the main mechanisms driving soil C gains following N deposition in N-limited temperate forests. In N-rich tropical forests, however, N deposition generally has minor effects on plant growth; consequently, C storage in soil may strongly depend on the microbial processes that drive litter and soil organic matter decomposition. Here, we investigated how microbial functions in old-growth tropical forest soil responded to 13years of N addition at four rates: 0 (Control), 50 (Low-N), 100 (Medium-N), and 150 (High-N) kgNha-1 year-1 . Soil organic carbon (SOC) content increased under High-N, corresponding to a 33% decrease in CO2 efflux, and reductions in relative abundances of bacteria as well as genes responsible for cellulose and chitin degradation. A 113% increase in N2 O emission was positively correlated with soil acidification and an increase in the relative abundances of denitrification genes (narG and norB). Soil acidification induced by N addition decreased available P concentrations, and was associated with reductions in the relative abundance of phytase. The decreased relative abundance of bacteria and key functional gene groups for C degradation were related to slower SOC decomposition, indicating the key mechanisms driving SOC accumulation in the tropical forest soil subjected to High-N addition. However, changes in microbial functional groups associated with N and P cycling led to coincidentally large increases in N2 O emissions, and exacerbated soil P deficiency. These two factors partially offset the perceived beneficial effects of N addition on SOC storage in tropical forest soils. These findings suggest a potential to incorporate microbial community and functions into Earth system models considering their effects on greenhouse gas emission, biogeochemical processes, and biodiversity of tropical ecosystems.

Effect of nitrogen fertilisation on nitrous oxide emission and the abundance of microbial nitrifiers and denitrifiers in the bulk and rhizosphere soil of Solanum lycopersicum and Phaseolus vulgaris

To determine the effect of three N-fertilisers on N2O emission and abundance of nitrification and denitrification genes in bulk and rhizosphere soil of tomato and common bean, two vegetable crops representative of main horticultural crops in South Spain. Four consecutive harvests of tomato and common bean fertilised with urea, ammonium or nitrate were carried out. The total abundance of bacteria, archaea, nitrifiers and denitrifiers was estimated by quantitative PCR. Soil physicochemical properties and N2O emission were also determined. Regardless of the plant species, the highest N2O emission was produced by the soil treated with urea, followed by ammonium and nitrate. Bacteria were more abundant than archaea in the bulk and rhizosphere soil. The abundance of the ammonia-oxidising archaea was greater than the ammonia-oxidising bacteria in the rhizosphere, but lower in the bulk soil. N-fertilisation increased the gene copy number of denitrifiers, which were more abundant in the bulk soil. N-fertilisation decreases N2O production due to increased abundance of the nosZ gene. The abundance of nitrification and denitrification genes in bulk and rhizosphere soils is dependent on the type of fertiliser. For both plant species, the ratio of the genes involved in production and reduction of N2O by bulk and rhizosphere was similar.

Seasonal and Spatial Variation in the Location and Reactivity of a Nitrate‐Contaminated Groundwater Discharge Zone in a Lakebed

AbstractGroundwater discharge delivering anthropogenic N from surrounding watersheds can impact lake nutrient budgets. However, upgradient groundwater processes and changing dynamics in N biogeochemistry at the groundwater‐lake interface are complex. In this study, seasonal water‐level variations in a groundwater flow‐through lake altered discharge patterns of a wastewater‐derived groundwater contaminant plume, thereby affecting biogeochemical processes controlling N transport. Pore water collected 15 cm under the lakebed along transects perpendicular to shore varied from oxic to anoxic with increasing nitrate concentrations (10–75 μM) and corresponding gradients in nitrite and nitrous oxide. Pore water depth profiles of nitrate concentrations and stable isotopic compositions largely reflected upgradient groundwater N sources and N cycle processes, with minor additional nitrate reduction in the near‐surface lakebed sediments. Potential denitrification rates determined in laboratory microcosms were 10–100 times higher in near‐surface sediments (0–5 cm) than in deeper sediments (5–30 cm) and were correlated with sediment carbon content and abundance of denitrification genes (nirS, nosZI, and nosZII). Potential anammox‐driven N2 production was detectable in deeper anoxic sediments. Injection of bromide and nitrite in the lake sediments showed that the highest net nitrite consumption rates were within the top 10 cm. However, short transit times owing to rapid upward pore water velocities (4–5 cm hr−1) limited removal of the contaminant nitrate transiting through the sediments. Results demonstrate that local hydrologic and biogeochemical processes at the point of discharge affect the distribution and discharge rate of N through lakebed sediments, but processes in the upgradient groundwater can be more important for affecting N speciation and concentration.

Effects of the Veterinary Antibiotic Sulfamethazine on N2O Emissions and the Associated Microbiological Mechanism in a Rice Field

Veterinary antibiotics can enter into croplands with animal excrement and can have effects on nitrification and denitrification processes in the agricultural soils. A field experiment was conducted to evaluate the effect of sulfamethazine (SMZ) on N2O emissions, nitrification, denitrification, and related functional gene abundances within a paddy field. Five treatments were used in the experiment, namely, no fertilizer and no antibiotics applied (CK), and pig manure used as basal fertilizer plus urea applied as topdressing with the addition of 0, 5, 15, and 30 mg·kg-1 SMZ (SMZ0, SMZ5, SMZ15, and SMZ30, respectively). Soil and gas samples were collected and analyzed periodically throughout the rice growing season. The results showed that the SMZ did not change the seasonal pattern of N2O emissions. During the entire observation period, there was a significant difference in N2O fluxes between the SMZ15 and SMZ0 treatment (P<0.05), but there was no significant differences in N2O fluxes between the SMZ30, SMZ5, and SMZ0 treatment (P>0.05). Medium and high concentrations (SMZ15, SMZ30) increased the cumulative emissions of N2O at the average level, and these values were 3.47 and 4.67 times higher than that of the SMZ0 treatment, respectively; the soil NO3--N content also increased. Medium and high concentrations had a significant activation effect on the gene abundances of total soil bacteria 16S rRNA, ammonia-oxidizing archaea (AOA) amoA, and ammonia-oxidizing bacteria (AOB) amoA during the nitrification process and the gene abundances of nirK, nirS, and nosZ during the denitrification process (P<0.05), while the SMZ treatment with a low concentration had a slight inhibitory effect on the abundance of each gene. The ratios of abundance copies of 16S rRNA, AOA amoA, AOB amoA, and the genes of nirK, nirS, and nosZ treated by SMZ30, SMZ15, and SMZ0 were 1.58, 1.77, 2.15, 1.38, 1.33, 1.42, and 1.24, 1.37, 1.08, 1.65, 1.11, 1.64, respectively, at the average level. The abundance ratios of the six above genes treated by SMZ5 and SMZ0 were less than one and only 0.80, 0.99, 0.92, 0.76, 0.76, and 0.77, respectively. The N2O fluxes were significantly and positively correlated with the abundances of the nirK gene (P<0.01), thus indicating that SMZ had an effect on N2O emissions by influencing the activity of denitrifying bacteria. Therefore, the pollution of farmland by veterinary antibiotics should not be ignored, and the use of antibiotics should be controlled reasonably at the source, so as to reduce the environmental and ecological risks.

Distinct Bacterial Communities in Wet and Dry Seasons During a Seasonal Water Level Fluctuation in the Largest Freshwater Lake (Poyang Lake) in China.

Water level fluctuations (WLFs) are an inherent feature of lake ecosystems and have been regarded as a pervasive pressure on lacustrine ecosystems globally due to anthropogenic activities and climate change. However, the impacts of WLFs on lake microbial communities is one of our knowledge gaps. Here, we used the high-throughput 16S rRNA gene sequencing approach to investigate the taxonomic and functional dynamics of bacterial communities in wet-season and dry-season of Poyang Lake (PYL) in China. The results showed that dry-season was enriched in total nitrogen (TN), nitrate (NO3-), ammonia (NH4+), and soluble reactive phosphorus (SRP), while wet-season was enriched in dissolved organic carbon (DOC) and total phosphorus (TP). Bacterial communities were distinct taxonomically and functionally in dry-season and wet-season and the nutrients especially P variation had a significant contribution to the seasonal variation of bacterial communities. Moreover, bacterial communities responded differently to nutrient dynamics in different seasons. DOC, NO3-, and SRP had strong influences on bacterial communities in dry-season while only TP in wet-season. Alpha-diversity, functional redundancy, taxonomic dissimilarities, and taxa niche width were higher in dry-season, while functional dissimilarities were higher in wet-season, suggesting that the bacterial communities were more taxonomically sensitive in dry-season while more functionally sensitive in wet-season. Bacterial communities were more efficient in nutrients utilization in wet-season and might have different nitrogen removal mechanisms in different seasons. Bacterial communities in wet-season had significantly higher relative abundance of denitrification genes but lower anammox genes than in dry-season. This study enriched our knowledge of the impacts of WLFs and seasonal dynamics of lake ecosystems. Given the increasingly pervasive pressure of WLFs on lake ecosystems worldwide, our findings have important implications for conservation and management of lake ecosystems.

Nitrate addition promotes the nitrogen cycling processes under the co-contaminated tetrabromobisphenol A and copper condition in river sediment

Tetrabromobisphenol A (TBBPA) and copper (Cu) are the main pollutants at e-waste recycling sites and the effects of their biotoxicity on microorganisms have drawn extensive attention. Nitrate-based bioremediation has been applied to organic pollutant-contaminated sediments since nitrate is a favorable electron acceptor for microbes. However, the effects of TBBPA and Cu on nitrogen (N)-cycling microorganisms and bioremediation in co-contaminated sediments remain unclear. Thus, our study examined the effects of TBBPA and Cu with/without nitrate addition on the TBBPA biodegradation efficiencies, microbial activities, and N functional genes. It was found the biodegradation efficiencies of TBBPA were improved by the nitrate addition from 34.7% to 59.3% and from 22.6% to 42.8% in the TBBPA and TBBPA-Cu contaminated groups, respectively. The inhibitions of the catalase activity increased with the nitrate addition because of the anaerobic respiration of the microorganisms. In addition, the potential denitrification rate exhibited an increasing trend from 6.46 to 8.23 mg-N kg−1 dry sediment day−1 during the period of 15–90 days after adding nitrate to the co-contaminated group, whereas the potential nitrification rate exhibited an opposite trend and decreased from 4.47 to 3.19 mg-N kg−1 dry sediment day−1. The denitrification gene abundances of the N-cycling genes were 107-108 orders of magnitude higher and significantly increased in the nitrate addition groups. The amoA gene abundances were lower than the denitrification gene abundances and were 105-106 orders of magnitude in the same groups. Moreover, the interaction types of the pollutants on the gene abundances were changed from synergistic to antagonistic as nitrate addition. Our study emphasized the gap of knowledge on nitrate addition affecting N-cycling microbes in the combined pollutants exposure sediments, and will be helpful for further bioremediation in different contaminated scenarios.

Metagenomic analysis reveals distinct patterns of denitrification gene abundance across soil moisture, nitrate gradients.

This study coupled a landscape-scale metagenomic survey of denitrification gene abundance in soils with in situ denitrification measurements to show how environmental factors shape distinct denitrification communities that exhibit varying denitrification activity. Across a hydrologic gradient, the distribution of total denitrification genes (nap/nar + nirK/nirS + cNor/qNor + nosZ) inferred from metagenomic read abundance exhibited no consistent patterns. However, when genes were considered independently, nirS, cNor and nosZ read abundance was positively associated with areas of higher soil moisture, higher nitrate and higher annual denitrification rates, whereas nirK and qNor read abundance was negatively associated with these factors. These results suggest that environmental conditions, in particular soil moisture and nitrate, select for distinct denitrification communities that are characterized by differential abundance of genes encoding apparently functionally redundant proteins. In contrast, taxonomic analysis did not identify notable variability in denitrifying community composition across sites. While the capacity to denitrify was ubiquitous across sites, denitrification genes with higher energetic costs, such as nirS and cNor, appear to confer a selective advantage in microbial communities experiencing more frequent soil saturation and greater nitrate inputs. This study suggests metagenomics can help identify denitrification hotspots that could be protected or enhanced to treat non-point source nitrogen pollution.

Nitrogen removal and responses of bacterial communities in activated sludge under different operational manipulations.

Operational conditions are often manipulated to improve the nitrogen removal performance of wastewater treatment, yet the impacts of operational conditions on microbial communities were still not well understood. There is a pressing need to understand the microbial mechanisms that link operation manipulation and nitrogen removal performance. In this study, high-throughput analysis of 16S rDNA and quantitative polymerase chain reaction of functional genes were used to identify the microbial response to operational manipulations. The results showed that alteration of operational parameters could change the bacterial communities at the genera level and denitrification guild gradually dominated in the activated sludge bacterial communities. Heterotrophic Hyphomicrobium and Chromatiaceae drove the kinetic of dominant genera and denitrification guild. Carbon source supplement was the most efficient strategy for improving nitrogen removal, and greatly increased the abundance of denitrifiers and denitrification genes. However, carbon source supplement inhibited expression activities of denitrification genes, as well as the proliferation of autotrophic denitrifiers, and it was supposed to be unfavorable in terms of cost over the long term. The result should bring new inspiration for improving the effect of WWTP performance through the manipulation of operational parameters.

Differences in N2O Fluxes and Denitrification Gene Abundance in the Wet and Dry Seasons Through Soil and Plant Residue Characteristics of Tropical Tree Crops

The conversion of forest to agricultural soils is a widespread activity in tropical systems, and its link to nitrous oxide (N2O) fluxes and nitrogen cycling gene abundance is relevant to understand environmental drivers that may interact with climate change. A current challenge to estimating N2O emissions from land use conversion is an incomplete understanding of crop-specific impacts on denitrifier communities and the N2O fluxes driven by differences in the above- and below-ground inputs with crop type. To address this knowledge gap in tree crops, we evaluated N2O fluxes and denitrification gene abundance and their relationships with soil and plant residue characteristics in citrus and eucalyptus plantations in the field and in soil incubations. We found that the accumulated N2O fluxes from soil were lower for the two agricultural field sites than those for their adjacent forest sites in dry and wet seasons. The N2O fluxes were higher in the wet season, and this seasonal difference persisted even when the soils collected from both seasons were incubated under the same moisture and temperature conditions in the lab for 30 days. Increased N2O fluxes in the wet season were accompanied by an increase in soil nirK and nosZ gene abundance, the dissolved organic carbon (DOC) concentration, and the total soil carbon (C) and nitrogen (N) content. In turn, the abundance of denitrifiers, as indicated by nirK, nirS and nosZ gene copy numbers, showed a low but significant positive correlation with soil bulk density. Our results suggest that soil moisture, leaf litter and crop residues influence the seasonal differences in both N2O fluxes and abundance of denitrifiers in citrus- and eucalyptus-cultivated soils, likely through effects on soil physicochemical characteristics. These findings highlight the overwhelming role of environmental drivers that can make investigating microbial drivers difficult in the field and open the possibility for a better understanding of N cycling processes in tropical soils based on paired field- and incubation-based experimentation.

Intermittent flooding of organic‐rich soil promotes the formation of denitrification hot moments and hot spots

AbstractAnthropogenic activities have altered the nitrogen cycle, necessitating management on the landscape level. Isolated time periods and areas, termed hot moments and hot spots, respectively, frequently account for a large percentage of nitrate removal in aquatic ecosystems. A series of experiments were conducted to determine the effect of hydrologic connectivity on denitrification rates, gene abundances, and nitrous oxide fluxes. Experimental areas were divided into flooded (always inundated), floodzone (intermittently inundated), and nonfloodzone (not inundated) locations in low‐organic and organic‐rich soil. Our results demonstrate that intermittent flood events enhance denitrification rates from days to weeks after flooding, depending on the inundation period. Microbial analysis demonstrated that short‐term flood events did not lead to increases in denitrifying gene abundances. Enhanced denitrification rates did not have a corresponding increase in the ratio of incomplete to complete denitrification. Incomplete to complete denitrification ratios were high in always‐inundated low‐organic sandy soil, peaking at 40%. Our results suggest that management strategies that promote hydrologic connectivity and intermittent flooding of organic‐rich floodplain soils promote the formation of denitrification hot moments and hot spots, with relatively low incomplete denitrification (&lt;3% of the total denitrification rates).

Easily mineralizable carbon in manure‐based biochar added to a soil influences N2O emissions and microbial‐N cycling genes

AbstractManure‐based biochar is an effective amendment to increase both carbon sequestration and the fertility of degraded soils, whereas the responses of N2O emissions and microbial‐N cycling genes to its application and the underlying mechanisms are poorly understood. Here, four biochars were produced under two pyrolysis temperatures (300 and 700°C) and with two organic C extraction procedures (water and acetone extraction). The resulting biochars were either with relative enrichment or depletion in easily mineralizable carbon (EMC) compared with recalcitrant C. We added biochars, with urea and sodium nitrate, to a degraded red soil and incubated the amended soils at moisture levels of 60% and 130% field capacity. All the biochars decreased nitrification gene abundance, that is, amoA. Biochars with EMC had greater stimulatory effects on the abundance of denitrification genes (nirK, nirS, and nosZ) and N2O emission, regardless of moisture level and N form, compared with biochar without EMC. Biochar‐induced microbial activity, biochar aliphatics, and alkyl groups correlated positively with N2O emission and denitrification gene abundance. The water dissolved organic C of biochar facilitated the conversion of N2O to N2, whereas the acetone extractable C fraction postponed the completion of denitrification. In conclusion, the N2O emission and denitrification gene abundance increased with decreasing biochar aromaticity and increasing EMC content. Our study emphasizes that the EMC supply in manure‐based biochars also importantly mediates soil N2O emission and microbial N cycling genes, adding to current understanding of influencing factors such as biochar pH, porosity, N availability, and redox property.

Dynamic Changes of Nitrogen-Transforming and Phosphorus-Accumulating Bacteria Along with the Formation of Cyanobacterial Blooms

In order to reveal the dynamic changes of nitrogen-transforming and phosphorus-accumulating bacteria in the process of cyanobacterial bloom formation, bacterial community structure and functional genes associated with nitrogen cycling were analyzed using high-throughput sequencing and real-time quantitative PCR (qPCR). The high-throughput sequencing results showed that the diversity of bacterial communities decreased during cyanobacterial blooms, and the lacustrine free-living and bloom-attached bacterial communities varied with increased bloom density. With increasing density of cyanobacteria, the relative abundance of Actinobacteria and Bacteroidetes decreased, whereas that of Firmicutes increased. Moreover, the growth of phosphate-accumulating organisms was increased in the phycosphere of cyanobacterial blooms, while that of nitrifiers was inhibited. Denitrifiers increased significantly under moderate blooms. The qPCR data also revealed that the abundance of nitrification and denitrification genes decreased or disappeared with the development of cyanobacterial blooms, indicating that the nitrogen transformation process might be inhibited, which showed a positive feedback of bacterial community to the development of Microcystis blooms by satisfying the needs of the nutrients budget.

Effect of Silver Nanoparticles on Denitrification and Functional Gene Abundances of Sediment in Dagu River Estuary and Northwest of Jiaozhou Bay

The influence of silver nanoparticles (AgNPs) on the denitrification performance, enzyme activity, and functional gene relative abundances of sediment was investigated based on the methods of laboratory simulation incubation in the Dagu River estuary and bay area in the northwest of Jiaozhou Bay. The different dosages of AgNPs (i.e., 0, 135, and 1350 mg·L-1 in final concentration) was added to the incubation system containing surface sediments and in-situ bottom water. During six days' incubation, the concentrations of NO3- and NO2-, NO3-, and NO2- reductases activity, and relative abundances of narG and nirS genes were measured to explore the effects of AgNPs on denitrification and its mechanism. The results showed that AgNPs significantly inhibited NO3- and NO2- reductive capacity, NO3- and NO2- reductase activity, and narG and nirS gene relative abundances, which led to aggravated accumulation of NO2-. The inhibition of NO2- reductase was significantly greater than that of NO3- reductase, and the inhibition of the nirS gene was significantly higher than that of the narG gene. The inhibition of NO3- reduction was mainly ascribed to the inhibition of functional genes, but the inhibition of NO2- reduction was mainly due to the inhibition of reductase activity. The inhibition of NO3- and NO2- reductive capacity, NO3- reductase activity, and narG and nirS gene relative abundances in the northwest of Jiaozhou Bay was significantly higher than that in the Dagu River estuary.

The Abundance of Fungi, Bacteria and Denitrification Genes during Insect Outbreaks in Scots Pine Forests

Outbreaks of defoliating insects may affect microbial populations in forests and thereby mass balances and ecosystem functioning. Here, we investigated the microbial dynamics in Scots pine (Pinus sylvestris L.) forests during outbreaks of the nun moth (Lymantria monacha L.) and the pine-tree lappet (Dendrolimus pini L.). We used real-time PCR (polymerase chain reaction) to quantify genes that characterize bacterial and fungal abundance and the denitrification processes (nirK, nirS, nosZ clades I and II) in different forest compartments and we analyzed the C and N content of pine needles, insect feces, larvae, vegetation layers, organic layers, and mineral soil horizons. The infestation of the nun moth increased the bacterial abundance on pine needles, in the vegetation layer, and in the upper organic layer, while fungal populations were increased in the vegetation layer and upper organic layer during both outbreaks. In soil, the abundance of nirK increased after insect defoliation, while the C/N ratios decreased. nosZ clades I and II showed variable responses in different soil layers and to different defoliating insects. Our results illustrate changes in the microbial populations in pine forests that were infested by defoliating insects and changes in the chemical soil properties that foster these populations, indicating a genetic potential for increased soil N2O emissions during the defoliation peak of insect outbreak events.

Plant-Microbe Interactions Drive Denitrification Rates, Dissolved Nitrogen Removal, and the Abundance of Denitrification Genes in Stormwater Control Measures.

The microbial community and function along with nitrate/nitrite (NOx) removal rates, and nitrogen (N) partitioning into "uptake", "denitrification", and "remaining" via isotope tracers, were studied in soil bioretention mesocolumns (8 unique plant species). Total denitrification gene reads per million (rpm) were positively correlated with % denitrified ( r = 0.69) but negatively correlated with total NOx removal following simulated rain events ( r = -0.79). This is likely due to plant-microbe interactions. Plant species with greater root volume, plant and microbial assimilation %, and NOx removal % had lower denitrification genes and rates. This implies that although microorganisms have access to N, advantageous functions, like denitrification, may not increase. At the conclusion of the 1.5-year experiment, the microbial community was strongly influenced by plant species within the Top zone dominated by plant roots, and the presence or absence of a saturated zone influenced the microbial community within the Bottom zone. Leptospermum continentale was an outlier from the other plants and had much lower denitrification gene rpm (average 228) compared to the other species (range: 277 to 413). The antimicrobial properties and large root volume of Leptospermum continentale likely caused this denitrification gene depression.

Microbial communities and functional genes of nitrogen cycling in an electrolysis augmented constructed wetland treating wastewater treatment plant effluent

To enhance nitrogen removal efficiency, a new electrolysis augmented constructed wetland (E-CW) was applied for nitrogen removal from waste water treatment plant (WWTP) effluent. This work demonstrated that E-CW could remove NO3− efficiently (45.5%–83.4%) under low CCOD/N ratio (average 2.29 ± 0.45) with little amount of NH4+ and NO2− generation. High throughput 16S rRNA sequence analysis revealed that Proteobacteria, Actinobacteria, Cyanobacteria, Bacteroidetes, and Verrucomicrobia were the dominant phyla in the E-CW. However, abundance of denitrifiers and denitrification genes decreased along with the operation of E-CW. Four functional gene pairs of anammox-amoA, (narG + napA)-(nirK + nirS), (narG + napA)-nosZ and qnorB-nosZ showed positive correlations with each other. Co-occurrence network analysis results indicated that functional guilds of FeOB and FeRB coupled with denitrifiers and contributed to the process of nitrogen removal in the E-CW. Overall, this work illustrated E-CW was a feasible and effective technology for enhancing nitrogen removal, and provided a theoretical basis for better design and operation of E-CW.