End Product Of Denitrification Research Articles

Denitrification and Potential Nitrous Oxide and Carbon Dioxide Production in Brownfield Wetland Soils

Brownfields, previously developed sites that are derelict, vacant, or underused, are ubiquitous in urban areas. Wetlands on brownfields often retain rain and stormwater longer than the surrounding landscape because they are low-lying; this increases the possibility for these areas to process waterborne contaminants from the urban environment. In the northeastern United States, atmospheric deposition of nitrate (NO) is high. Denitrification, a microbial process common in wetlands, is a means of removing excess NO. Nitrogen gas is the desired end product of denitrification, but incomplete denitrification results in the production of NO, a greenhouse gas. The goal of this study was to investigate the potential of brownfield wetlands to serve as sinks for inorganic nitrogen and sources of greenhouse gases. We examined limitations to denitrification and NO production in brownfield wetland soils in New Jersey. Soil C:N ratios were high (18-40) and intact core denitrification (-0.78 to 11.6 μg NO-N kg dry soil d) and N mineralization (0.11-2.97 mg N kg dry soil d) were low for all sites. However, soil NO increased during dry periods. Nitrate additions to soil slurries increased denitrification rates, whereas labile C additions did not, indicating that soil denitrifiers were nitrogen limited. Incubations indicated that the end product of denitrification was primarily NO and not N. These results indicate that brownfield wetlands can develop significant denitrification capacity, potentially causing NO limitation. They might be significant sinks for atmospheric NO but may also become a significant source of NO if NO deposition were to increase.

Estimating Rates of Denitrification Enzyme Activity in Wetland Soils with Direct Simultaneous Quantification of Nitrogen and Nitrous Oxide by Membrane Inlet Mass Spectrometry

The microbial mediated process of denitrification is a major pathway for the removal of reactive nitrogen a pollutant in natural environments and waste treatment facilities. Denitrification potential, measured as denitrification enzyme activity (DEA), was quantified in novel short-term (4 h) anaerobic assays using a more sensitive and precise technique, membrane inlet mass spectrometry (MIMS) rather than the traditional headspace electron capture gas chromatography (GC-ECD) method. Using MIMS modifications made to the instrument and sample handling allowed for the simultaneous and direct measurement of reaction products nitrous oxide (N2O), a potent greenhouse gas, and the chemically unreactive dinitrogen (N2). Rate determinations were made from the slope of a linear curve generated by plotting increasing concentrations of the reaction products with time. Strong evidence for the validity of MIMS measured DEA rates was provided by showing consistent, linear accumulations of N2O or N2 and close agreement in rates from replicate reactions. Reactions were performed using wetland soils and cultures of Pseudomonas aeruginosa and P. chloroaphis that generated denitrification end products of N2 and N2O, respectively. Under acetylene inhibition P. aeruginosa produced the N2O end product at a rate equivalent to the rate obtained in the uninhibited reaction that produced N2. No significant (p>0.05) difference was observed between MIMS or headspace with GC-ECD, determined DEA in wetland soil reactions under acetylene inhibition. Because of anoxic conditions in the reaction vessels used with MIMS, detectable rates of N2O accumulation were only observed in acetylene blocked reactions or in cultures of P. chloroaphis. This method has potential applications ranging from near realtime wastewater treatment process measurements to field studies of nitrogen cycling. The continued development and application of these types of methods are needed to improve our understanding of the mechanisms regulating denitrification and its benign, N2, and harmful, N2O, end-products.

Phylogenetic and Functional Diversity of Denitrifying Bacteria Isolated from Various Rice Paddy and Rice-Soybean Rotation Fields

Denitrifiers can produce and consume nitrous oxide (N(2)O). While little N(2)O is emitted from rice paddy soil, the same soil produces N(2)O when the land is drained and used for upland crop cultivation. In this study, we collected soils from two types of fields each at three locations in Japan; one type of field had been used for continuous cultivation of rice and the other for rotational cultivation of rice and soybean. Active denitrifiers were isolated from these soils using a functional single-cell isolation method, and their taxonomy and denitrifying properties were examined. A total of 110 denitrifiers were obtained, including those previously detected by a culture-independent analysis. Strains belonging to the genus Pseudogulbenkiania were dominant at all locations, suggesting that Pseudogulbenkiania denitrifiers are ubiquitous in various rice paddy soils. Potential denitrifying activity was similar among the strains, regardless of the differences in taxonomic position and soil of origin. However, relative amounts of N(2) in denitrification end products varied among strains isolated from different locations. Our results also showed that crop rotation had minimal impact on the functional diversity of the denitrifying strains. These results indicate that soil and other environmental factors, excluding cropping systems, could select for N(2)-producing denitrifiers.

Importance of denitrifiers lacking the genes encoding the nitrous oxide reductase for N2O emissions from soil

Analyses of the complete genomes of sequenced denitrifying bacteria revealed that approximately 1/3 have a truncated denitrification pathway, lacking the nosZ gene encoding the nitrous oxide reductase. We investigated whether the number of denitrifiers lacking the genetic ability to synthesize the nitrous oxide reductase in soils is important for the proportion of N2O emitted by denitrification. Serial dilutions of the denitrifying strain Agrobacterium tumefaciens C58 lacking the nosZ gene were inoculated into three different soils to modify the proportion of denitrifiers having the nitrous oxide reductase genes. The potential denitrification and N2O emissions increased when the size of inoculated C58 population in the soils was in the same range as the indigenous nosZ community. However, in two of the three soils, the increase in potential denitrification in inoculated microcosms compared with the noninoculated microcosms was higher than the increase in N2O emissions. This suggests that the indigenous denitrifier community was capable of acting as a sink for the N2O produced by A. tumefaciens. The relative amount of N2O emitted also increased in two soils with the number of inoculated C58 cells, establishing a direct causal link between the denitrifier community composition and potential N2O emissions by manipulating the proportion of denitrifiers having the nosZ gene. However, the number of denitrifiers which do not possess a nitrous oxide reductase might not be as important for N2O emissions in soils having a high N2O uptake capacity compared with those with lower. In conclusion, we provide a proof of principle that the inability of some denitrifiers to synthesize the nitrous oxide reductase can influence the nature of the denitrification end products, indicating that the extent of the reduction of N2O to N2 by the denitrifying community can have a genetic basis.

Nitrogen dynamics at undisturbed and burned Mediterranean shrublands of Salento Peninsula, Southern Italy

Fire is a major disturbance in shrubland ecosystems of the Mediterranean basin, with high potential to alter ecosystem nitrogen (N) stocks and N cycling. However, postfire effects on gross rates of soil N turnover (ammonification, nitrification, microbial immobilization, denitrification) have rarely been investigated. We determined gross rates of N turnover including nitrous oxide fluxes and dinitrogen emissions in the mineral soil of unburned and burned shrublands of Southern Italy 6 months after a natural fire. In soil of burned plots, both gross ammonification and gross nitrification were significantly higher than in soil of unburned plots (2.2 ± 0.3 versus 0.6 ± 0.1 mg N kg−1 sdw day−1 for ammonification and 1.1 ± 0.1 versus 0.5 ± 0.1 mg N kg−1 sdw day−1 for nitrification). Microbial immobilization, in particular of nitrate, could not compensate for the increase in inorganic N production, therefore soil nitrate concentrations were considerably higher at the burned plots. Soil microbial biomass carbon and nitrogen concentrations were significantly lower in soils of burned plots than in soils of unburned plots. Dinitrogen was the dominant end product of denitrification and emitted at higher rates from the unburned plots than from the burned plots (0.094 ± 0.003 versus 0.004 ± 0.002 mg N kg−1 sdw day−1, while there was no net nitrous oxide flux (burned plots) or slight net nitrous oxide uptake (control plots). These results show that postfire patterns of gross N turnover in soil can exhibit a significant reduction of both microbial N retention and N gas losses via denitrification.

Insights into the Effect of Soil pH on N 2 O and N 2 Emissions and Denitrifier Community Size and Activity

The objective of this study was to investigate how changes in soil pH affect the N(2)O and N(2) emissions, denitrification activity, and size of a denitrifier community. We established a field experiment, situated in a grassland area, which consisted of three treatments which were repeatedly amended with a KOH solution (alkaline soil), an H(2)SO(4) solution (acidic soil), or water (natural pH soil) over 10 months. At the site, we determined field N(2)O and N(2) emissions using the (15)N gas flux method and collected soil samples for the measurement of potential denitrification activity and quantification of the size of the denitrifying community by quantitative PCR of the narG, napA, nirS, nirK, and nosZ denitrification genes. Overall, our results indicate that soil pH is of importance in determining the nature of denitrification end products. Thus, we found that the N(2)O/(N(2)O + N(2)) ratio increased with decreasing pH due to changes in the total denitrification activity, while no changes in N(2)O production were observed. Denitrification activity and N(2)O emissions measured under laboratory conditions were correlated with N fluxes in situ and therefore reflected treatment differences in the field. The size of the denitrifying community was uncoupled from in situ N fluxes, but potential denitrification was correlated with the count of NirS denitrifiers. Significant relationships were observed between nirS, napA, and narG gene copy numbers and the N(2)O/(N(2)O + N(2)) ratio, which are difficult to explain. However, this highlights the need for further studies combining analysis of denitrifier ecology and quantification of denitrification end products for a comprehensive understanding of the regulation of N fluxes by denitrification.

New approach for measuring denitrification in the rhizosphere of vegetated marsh sediments

This study presents a new approach for measuring denitrification at depth in the marsh rhizosphere (below 5 cm). The method combines the push‐pull technique and the 15N‐isotope pairing technique, here referred to as the PPIPT. The PPIPT allows ambient denitrification rates to be measured in situ at depth in the marsh rhizosphere with minimal damage to roots and rhizomes and minimal disturbance of the ambient pore water composition. In this method, pore water is extracted from the sediment using micropiezometers without contact with the atmosphere. The porewater is amended with 15NO3− to trace denitrification, and Ar(g) is added to trace potential gas loss of denitrification end products. The “spiked” pore water is injected back into the sediment for incubation. Subsequently, samples are sequentially extracted from the sediment for analysis of 28N2, 29N2, 30N2, and Ar. Denitrification rates are calculated using the isotope paring technique and corrected for dilution and gaseous losses using the argon tracer. After several trials in the laboratory and in the field to optimize the method, the PPIPT was applied in the field to measure denitrification in the rhizosphere of various vegetation zones in two New England marshes in northeastern USA. The field applications verified the PPIPT method as a useful tool for measuring ambient rates of denitrification at depth in salt marsh rhizospheres. Generally, the rates measured were low (0–12 µmol m−2 h−1) and showed a high spatial variability in the marsh rhizosphere among different vegetation zones.

Thiocyanate decomposition under aerobic and oxygen-free conditions by the aboriginal bacterial community isolated from the waste water of a metallurgical works

A mesophilic alkalitolerant aboriginal bacterial community capable of autotrophic thiocyanate decomposition under aerobic and oxygen-free conditions was isolated from reused water of a metallurgical works. The growth of the aboriginal bacterial community was optimal at pH 9.0. Ammonium and sulfate were the end products of thiocyanate decomposition under both aerobic and oxygen-free conditions. Under oxygen-free conditions, thiocyanate decomposition occurred in the presence of nitrate. Nitrite was accumulated as an intermediate product in the course of denitrification, and was subsequently used as an electron acceptor for thiocyanate oxidation. Dinitrogen was the end product of denitrification.

Model evaluation of different mechanisms driving freeze–thaw N 2O emissions

N 2O emissions from soil contribute significantly to global warming. Pulse emissions of N 2O from soils during freeze-thawing were recently recognized as important atmospheric sources. In this modelling study we explore three different hypotheses for explaining freeze–thaw related N 2O emissions: (1) soil frost or snow cover may reduce gas diffusion and create anaerobic conditions that stimulate N 2O production via denitrification, (2) microbes that die of frost deliver easy decomposable organic carbon and nitrogen to the soil, which stimulates microbial growth and vigorous N 2O production during freeze–thaw, and (3) the enzyme nitrous oxide reductase, which is responsible for the reduction of N 2O to N 2 during denitrification, is more sensitive to low temperatures than other enzymes, so that N 2O becomes the dominating end-product of denitrification at low temperatures. These hypotheses were tested with a biogeochemical model that combines hydrology and physics calculations with a newly developed, parameter-poor biochemistry module. The model was first calibrated with field datasets on soil–atmosphere fluxes of N 2O, NO and CO 2 and soil NO 3 and NH 4 concentrations that were measured in a spruce forest in Southeast Germany in the years 1994–1997. Subsequently, additional model mechanisms were implemented that allow the model to describe the outlined mechanisms potentially driving freeze–thaw N 2O fluxes. After each implementation the model was recalibrated. We were able to mimic dimension and timing of high N 2O emissions when either one of the first two hypotheses were assumed, but found no confirmation for the third. The best model fit was achieved by combining hypothesis one and two, indicating that freeze–thaw N 2O emissions are not mono-causal.

Use of spent sulfidic caustic for autotrophic denitrification in the biological nitrogen removal processes: Lab-scale and pilot-scale experiments

Caustic is utilized in petrochemical plants for the removal of hydrogen sulfide (H 2S) from a variety of hydrocarbon streams. Because spent sulfidic caustic (SSC) harbors a high level of H 2S and high alkalinity, it was injected into the anoxic zones of the biological nitrogen removal process as the electron donor and buffering agent for sulfur-based autotrophic denitrificaton. In order to determine the optimal SSC dosage, a modified Ludzack–Ettinger (MLE) process was first conducted at laboratory scale. As the result of the lab-scale experiments, the chemical oxygen demand (COD) increment of effluent and nitrification failure were observed, because SSC harbors barely biodegradable matter, and the caustic content was high, in accordance with the requirements. Thus, during the pilot-scale experiments, a hybrid Bardenpho process was designed and the SSC was neutralized from pH 13.3 to 11.5. These strategies were successful because no COD increment of effluent was observed and the pH of the unit process was stable. The heterotrophic and autotrophic denitrification ratios at each condition were calculated and, as the end product of sulfur-based autotrophic denitrification, the sulfate concentration was monitored.

Disentangling the rhizosphere effect on nitrate reducers and denitrifiers: insight into the role of root exudates

To determine to which extent root-derived carbon contributes to the effects of plants on nitrate reducers and denitrifiers, four solutions containing different proportions of sugar, organic acids and amino acids mimicking maize root exudates were added daily to soil microcosms at a concentration of 150 microg C g(-1) of soil. Water-amended soils were used as controls. After 1 month, the size and structure of the nitrate reducer and denitrifier communities were analysed using the narG and napA, and the nirK, nirS and nosZ genes as molecular markers respectively. Addition of artificial root exudates (ARE) did not strongly affect the structure or the density of nitrate reducer and denitrifier communities whereas potential nitrate reductase and denitrification activities were stimulated by the addition of root exudates. An effect of ARE composition was also observed on N(2)O production with an N(2)O:(N(2)O + N(2)) ratio of 0.3 in microcosms amended with ARE containing 80% of sugar and of 1 in microcosms amended with ARE containing 40% of sugar. Our study indicated that ARE stimulated nitrate reduction or denitrification activity with increases in the range of those observed with the whole plant. Furthermore, we demonstrated that the composition of the ARE affected the nature of the end-product of denitrification and could thus have a putative impact on greenhouse gas emissions.

The relationship between N2O, NO, and N2 fluxes from fertilized and irrigated dryland soils of the Aral Sea Basin, Uzbekistan

Microbial respiratory reduction of nitrous oxide (N2O) to dinitrogen (N2) via denitrification plays a key role within the global N-cycle since it is the most important process for converting reactive nitrogen back into inert molecular N2. However, due to methodological constraints, we still lack a comprehensive, quantitative understanding of denitrification rates and controlling factors across various ecosystems. We investigated N2, N2O and NO emissions from irrigated cotton fields within the Aral Sera Basin using the He/O2 atmosphere gas flow soil core technique and an incubation assay. NH4NO3 fertilizer, equivalent to 75 kg ha−1 and irrigation water, adjusting the water holding capacity to 70, 100 and 130% were applied to the incubation vessels to assess its influence on gaseous N emissions. Under soil conditions as they are naturally found after concomitant irrigation and fertilization, denitrification was the dominant process and N2 the main end product of denitrification. The mean ratios of N2/N2O emissions increased with increasing soil moisture content. N2 emissions exceeded N2O emissions by a factor of 5 ± 2 at 70% soil water holding capacity (WHC) and a factor of 55 ± 27 at 130% WHC. The mean ratios of N2O/NO emissions varied between 1.5 ± 0.4 (70% WHC) and 644 ± 108 (130% WHC). The magnitude of N2 emissions for irrigated cotton was estimated to be in the range of 24 ± 9 to 175 ± 65 kg-N ha−1season−1, while emissions of NO were only of minor importance (between 0.1 to 0.7 kg-N ha−1 season−1). The findings demonstrate that for irrigated dryland soils in the Aral Sera Basin, denitrification is a major pathway of N-loss and that substantial amounts of N-fertilizer are lost as N2 to the atmosphere for irrigated dryland soils.

Dinitrogen emissions and the N 2:N 2O emission ratio of a Rendzic Leptosol as influenced by pH and forest thinning

Reduction of nitrous oxide (N 2O) to dinitrogen (N 2) by denitrification in soils is of outstanding ecological significance since it is the prevailing natural process converting reactive nitrogen back into inert molecular dinitrogen. Furthermore, the extent to which N 2O is reduced to N 2 via denitrification is a major regulating factor affecting the magnitude of N 2O emission from soils. However, due to methodological problems in the past, extremely little information is available on N 2 emission and the N 2:N 2O emission ratio for soils of terrestrial ecosystems. In this study, we simultaneously determined N 2 and N 2O emissions from intact soil cores taken from a mountainous beech forest ecosystem. The soil cores were taken from plots with distinct differences in microclimate (warm-dry versus cool-moist) and silvicultural treatment (untreated control versus heavy thinning). Due to different microclimates, the plots showed pronounced differences in pH values (range: 6.3–7.3). N 2O emission from the soil cores was generally very low (2.0 ± 0.5–6.3 ± 3.8 μg N m −2 h −1 at the warm-dry site and 7.1 ± 3.1–57.4 ± 28.5 μg N m −2 h −1 at the cool-moist site), thus confirming results from field measurements. However, N 2 emission exceeded N 2O emission by a factor of 21 ± 6–220 ± 122 at the investigated plots. This illustrates that the dominant end product of denitrification at our plots and under the given environmental conditions is N 2 rather than N 2O. N 2 emission showed a huge variability (range: 161 ± 64–1070 ± 499 μg N m −2 h −1), so that potential effects of microclimate or silvicultural treatment on N 2 emission could not be identified with certainty. However, there was a significant effect of microclimate on the magnitude of N 2O emission as well as on the mean N 2:N 2O emission ratio. N 2:N 2O emission ratios were higher and N 2O emissions were lower for soil cores taken from the plots with warm-dry microclimate as compared to soil cores taken from the cool-moist microclimate plots. We hypothesize that the increase in the N 2:N 2O emission ratio at the warm-dry site was due to higher N 2O reductase activity provoked by the higher soil pH value of this site. Overall, the results of this study show that the N 2:N 2O emission ratio is crucial for understanding the regulation of N 2O fluxes of the investigated soil and that reliable estimates of N 2 emissions are an indispensable prerequisite for accurately calculating total N gas budgets for the investigated ecosystem and very likely for many other terrestrial upland ecosystems as well.

Elevated Carbon Dioxide and Irrigation Effects on Soil Nitrogen Gas Exchange in Irrigated Sorghum

The impacts of increasing atmospheric CO2, an important greenhouse gas, on soil microbial production and consumption of other greenhouse gases such as N2O are uncertain. This study was conducted during the 1998 and 1999 summer growing seasons at the Free‐Air CO2 Enrichment (FACE) site in Maricopa, AZ. The objective was to measure N2O and denitrification emission rates in a C4 sorghum [Sorghum bicolor (L.) Moench] production system with ample and limited flood irrigation rates under FACE (seasonal mean = 579 μmol mol−1) and control (seasonal mean = 396 μmol mol−1) CO2 Plots were sampled for N2O flux using both chamber and intact incubated soil core techniques. Nitrogen gas (N2O plus N2) emissions were measured using intact incubated soil cores with C2H2 inhibition. Nitrous oxide emissions measured with chambers increased markedly after irrigation and fertilization following prolonged periods without water under both elevated and control CO2 conditions. Within 5 d of fertilization and irrigation, N2O emissions measured with chambers were <250 g N2O‐N ha−1 d−1 until subsequent irrigations. Emissions measured from cores ranged from −0.11 to >250 g N2O‐N ha−1 d−1 Seasonal cumulative N2O‐N emissions measured using chambers were <1.5 kg N ha−1 Seasonal N‐gas losses measured during 1999 were as high as 3.7 kg N ha−1, and were highest with elevated CO2 and the high irrigation treatment. During periods when significant emissions were recorded, the primary end product of denitrification was N2 rather than N2O. Water‐filled pore space (WFPS) was the most important single factor controlling N‐gas emissions, with the largest emissions (>500 g N2O‐N ha−1 d−1) coming with >55% WFPS. Neither soil NO3− nor soil organic C alone limited N gas emissions. Elevated CO2 did not result in increased N2O or N‐gas emissions with either ample or limited irrigation.

Effect of Siberian tree species on N2O production and consumption

The effect of six Siberian tree species on two stages of denitrification-N2O production and consumption-was studied. Broadleaf species (aspen and birch) proved to have lower rates of N2O consumption compared to coniferous species. The factors influencing production and consumption of N2O were also evaluated. The replacement of coniferous forests with broadleaf trees will double the N2O/N2 ratio in the denitrification end-products. Doubled N2O emission from Siberian forest soils to the atmosphere can be expected due to changes in tree species composition of forest ecosystems even without considering changes in water and temperature regimes in soil.

Nitrous oxide emissions from secondary activated sludge in nitrifying conditions of urban wastewater treatment plants: Effect of oxygenation level

In order to better understand the mechanisms of N 2O emissions from nitrifying activated sludge of urban WWTPs, sludge from the Valenton plant (Paris conurbation) are subjected to lab-scale batch experiments under various conditions of oxygenation. The results show that the highest N 2O emissions (7.1 μgN-N 2O gSS −1 h −1 in average) occur at a dissolved oxygen (DO) concentration of around 1 mgO 2 L −1. These high emissions at low oxygenation (from 0.1 to 2 mgO 2 L −1) are due to two processes: autotrophic nitrifier denitrification and heterotrophic denitrification. Nitrifier denitrification always dominates, representing from 58% to 83% of the N 2O production. This N 2O production originating from nitrifying activated sludge becomes 8 times higher when nitrite is added at a DO of 1 mgO 2 L −1; a decrease is observed both at higher and lower oxygenation. Heterotrophic denitrification represents less than 50% of the N 2O production, decreasing from 42% to 17% when oxygenation increases from 0.1 to 2 mgO 2 L −1. We show that ammonium oxidizing bacteria (AOB) can shift to nitrifier denitrification when oxygen is depleted in the environments including in the WWTPs, nitrite then plays the role of oxygen as the final electron acceptor. As opposed to what happens in nitrification, the end products of nitrifier denitrification are gaseous forms of nitrogen, where N 2O is not negligible compared to N 2. Overall, N 2O emissions represent 0.1–0.4% of oxidized NH 4 +, depending on the oxygenation level. N 2O emissions would range from 0.11 to 0.42 T N-N 2O day −1 for a tertiary treatment of the Paris wastewater effluents, consisting exclusively of activated sludge nitrification.

In Vivo Emission of Dinitrogen by Earthworms via Denitrifying Bacteria in the Gut

Earthworms emit the greenhouse gas nitrous oxide (N2O), and ingested denitrifiers in the gut appear to be the main source of this N2O. The primary goal of this study was to determine if earthworms also emit dinitrogen (N2), the end product of complete denitrification. When [15N]nitrate was injected into the gut, the earthworms Aporrectodea caliginosa and Lumbricus terrestris emitted labeled N2 (and also labeled N2O) under in vivo conditions; emission of N2 by these two earthworms was relatively linear and approximated 1.2 and 6.6 nmol N2 per h per g (fresh weight), respectively. Isolated gut contents also produced [15N]nitrate-derived N2 and N2O under anoxic conditions. N2 is formed by N2O reductase, and acetylene, an inhibitor of this enzyme, inhibited the emission of [15N]nitrate-derived N2 by living earthworms. Standard gas chromatographic analysis demonstrated that the amount of N2O emitted was relatively linear during initial incubation periods and increased in response to acetylene. The calculated rates for the native emissions of N2 (i.e., without added nitrate) by A. caliginosa and L. terrestris were 1.1 and 1.5 nmol N2 per h per g (fresh weight), respectively; these emission rates approximated that of N2O. These collective observations indicate that (i) earthworms emit N2 concomitant with the emission of N2O via the in situ activity of denitrifying bacteria in the gut and (ii) N2O is quantitatively an important denitrification-derived end product under in situ conditions.

Aerobic denitrifiers isolated from an alternating activated sludge system

One hundred and sixty-nine bacterial strains were isolated from activated sludge from a waste water treatment basin operating under alternating aerobic/anaerobic conditions. Sixteen strains from a subsample of 23 nitrogen oxide reducers were true respiratory denitrifiers, and all denitrified under both anaerobic and aerobic conditions. REP-PCR band analysis showed different patterns for all strains. One strain (strain 1) produced large amounts of N2O and was studied in detail. Nitrous oxide was the major end product of denitrification by this strain, and NO−2 was reduced more efficiently than NO−3. The aerobic denitrification was most pronounced with NO−2 as electron acceptor, and the reduction of NO−2 was not coupled to NH+4 oxidation.

Correlation of Denitrifying Capability with the Existence of nap, nir, nor and nos Genes in Diverse Strains of Soybean Bradyrhizobia

To clarify the genetic basis to the diversity of denitrifying ability in soybean bradyrhizobia, we compared the end-products of denitrification (N2, N2O and NO2 � ) with the existence of denitrifying genes (napA, nirK, norCB and nosZ) of sixty phylogenetically diverse strains of Bradyrhizobium japonicum and B. elkanii. The results indicate that the existence of denitrifying genes directly determines phenotype (end-products) in most strains of B. japonicum. The denitrifying capability and gene set were reflected by phylogenetic position based on repeated sequences (RS)-fingerprints and 16S rRNA gene sequences. However, the denitrifying genes in HRS (highly repeated sequence-possessing) strains of B. japonicum, which were identified based on RS-fingerprints as having heavy hybridization, resulted in an inconsistent correlation probably because of genomic rearrangements. The evolutionary and ecological implications of the denitrifying genes and capability in soybean bradyrhizobia are discussed.

Emissions and spatial variability of N 2O, N 2 and nitrous oxide mole fraction at the field scale, revealed with 15N isotopic techniques

The accurate measurement of nitrous oxide (N 2O) and dinitrogen (N 2) during the denitrification process in soils is a challenge which will help to estimate the contribution of soil N 2O emissions to global warming. Oxygen concentration, nitrate concentration and carbon availability are generally the main factors that control soil denitrification rate and the amount of N 2O or N 2 emitted. The aim of this paper is to present a database of the N 2O mole fraction measured at the field scale, and to test hypotheses concerning its regulation. A 15N-nitrate tracer solution was added to 36 undisturbed soil cores on a 20 m×20 m cultivated field plot. Fluxes of CO 2, N 2O and N 2 from the soil surface were monitored for 24 h. Soil moisture, bulk density, carbon, nitrogen and mineral nitrogen concentration were also measured to investigate possible spatial relationships between their variations and those of N 2O, N 2 and nitrous oxide mole fraction. Under high water content, nitrous oxide and N 2 emissions were highly variable with variation coefficients of 70–140%. N 2O emission rates were about twice as high as those of N 2, with a total denitrification rate ranging from 269 to 3843 g N ha −1 d −1. After 24 h of incubation, the values of nitrous oxide mole fraction ranged from 0.15 to 0.94 and no significant decline during incubation time was observed. Spatial variability of N 2O, N 2 and nitrous oxide mole fraction was high and no spatial dependence was observed at the scale of the experimental plot. Only tenuous relationships between gaseous nitrogen emissions and soil properties (mainly nitrate concentration and moisture content) were found. Meanwhile, a positive correlation was observed between N 2 and CO 2 emissions. This result supports the hypothesis that an increase in soil available organic carbon leads to N 2 emissions as the end product of denitrification.