Ng N2O Research Articles

Temporal enrichment of comammox Nitrospira and Ca. Nitrosocosmicus in a coastal plastisphere.

Plastic marine debris is known to harbor a unique microbiome (termed the "plastisphere") that can be important in marine biogeochemical cycles. However, the temporal dynamics in the plastisphere and their implications for marine biogeochemistry remain poorly understood. Here, we characterized the temporal dynamics of nitrifying communities in the plastisphere of plastic ropes exposed to a mangrove intertidal zone. The 39-month colonization experiment revealed that the relative abundances of Nitrospira and Candidatus Nitrosocosmicus representatives increased over time according to 16S rRNA gene amplicon sequencing analysis. The relative abundances of amoA genes in metagenomes implied that comammox Nitrospira were the dominant ammonia oxidizers in the plastisphere, and their dominance increased over time. The relative abundances of two metagenome-assembled genomes of comammox Nitrospira also increased with time and positively correlated with extracellular polymeric substances content of the plastisphere but negatively correlated with NH4+ concentration in seawater, indicating the long-term succession of these two parameters significantly influenced the ammonia-oxidizing community in the coastal plastisphere. At the end of the colonization experiment, the plastisphere exhibited high nitrification activity, leading to the release of N2O (2.52ng N2O N g-1) in a 3-day nitrification experiment. The predicted relative contribution of comammox Nitrospira to N2O production (17.9%) was higher than that of ammonia-oxidizing bacteria (4.8%) but lower than that of ammonia-oxidizing archaea (21.4%). These results provide evidence that from a long-term perspective, some coastal plastispheres will become dominated by comammox Nitrospira and thereby act as hotspots of ammonia oxidation and N2O production.

Nitrous oxide production from soybean and maize under the influence of weedicides and zero tillage conservation agriculture

Current experiment envisages evaluating N2O production from nitrification and denitrification under the influence of weedicides, cropping systems and conservation agriculture (CA). The weed control treatments were conventional hand weeding (no weedicide), pre emergence weedicide pendimethalin and post emergence weedicide imazethapyr for soybean, atrazine for maize. Experiment was laid out in randomized block design with three replicates. Soils were collected from different depths and incubated at different moisture holding capacity (MHC). N2O production from nitrification varied from 2.77 to 6.04 ng N2O g−1 soil d−1 and from denitrification varied from 0.05 to 1.34 ng N2O g−1 soil d−1. Potential nitrification rate (0.16–0.39 mM NO3 produced g−1 soil d−1) was higher than potential denitrification rate (0.45–0.93 mM NO3 reduced g−1 soil d−1). N2O production, nitrification, denitrification, and microbial gene abundance were higher in maize than soybean. Both N2O production and nitrification decreased (p < 0.05) with soil depth, while denitrification increased (p < 0.05) with soil depth. Abundance of eubacteria and ammonia oxidizing bacteria (AOB) were high (p < 0.01) at upper soil layer and declined with depth. Abundance of ammonia oxidizing archaea (AOA) increased (p < 0.05) with soil depth. Study concludes that intensive use of weedicides in CA may stimulate N2O production.

Sea animal activity controls CO2, CH4 and N2O emission hotspots on South Georgia, sub-Antarctica

Colonies of mammals and sea birds are known hotspots for biogeochemical cycles with potentially large element stocks and fast turnover in the soil. Although penguin and seal colonies are sites with potentially extremely fast biogeochemical turnover, these sites as potential sources of GHG have largely been neglected. This study aims to quantify magnitudes and environmental drivers of GHG fluxes effected by the activity of Antarctic fur seal (Arctocephalus gazella) and King penguin (Aptenodytes patagonicus) in South Georgia, sub-Antarctica. In-situ CO2 and CH4 gas flux measurements were combined with laboratory incubations of intact soil cores collected from sites along two transects away from a seal and a penguin colony. Variations in laboratory incubations were consistent with the in-situ fluxes measured in February 2017, and showed high ecosystem respiration rates at colonies (mean 44.3 μg CO2 cm−3 h−1 for seal colony, and 52.9–159 μg CO2 cm−3 h−1 for penguin colony) and a marked decrease away from these hotspots. Moderate methane production rates were found within the colonies (mean 0.1 ng CH4 cm−3 h−1 for seal colony, 44–145.5 ng CH4 cm−3 h−1 for penguin colony), while relatively high consumption rates (mean −1.8 ng CH4 cm−3 h−1) occurred outside the colonies. Incubations also included N2O production rates, which were highly variable within the colonies (1.1–293 ng N2O cm−3 h−1 for seal colony, 0.8–594.7 ng N2O cm−3 h−1 for penguin colony) and decreased markedly to near zero away from the colonies. Bacterial and pmoA communities and drivers of GHG turnover and microbe community along both transects varied according to the sea animal activity intensity gradient. This is the first study at these latitudes to quantify the overall capacity of in-situ soil methane uptake at hot spots linked to sea animal colonies versus ambient conditions. All colony sites in this study were net sources of N2O, while sites beyond colonies, which dominate the ice-free areas in South Georgia, were sinks of methane.

Effects of the nitrification inhibitor dicyandiamide (DCD) on N2Oemissions and the abundance of nitrifiers and denitrifiers in two contrasting agricultural soils

Nitrous oxide (N2O) is a trace gas and is mainly produced through the microbial-mediated nitrification and denitrification processes in agricultural soils. Dicyandiamide (DCD) is a powerful nitrification inhibitor (NI) which can mitigate N2O production from soil. However, it remains poorly understood how N2O emission and associated microbes are affected by DCD application across different agricultural soils. Here, we carried out a 21-day laboratory microcosm incubation experiment to examine the impact of ammonium sulfate and DCD applications on N2O production and the abundance of nitrifiers and denitrifiers in two contrasting agricultural soils. The abundance of the functional genes was quantified by quantitative PCR to investigate the population changes in response to ammonium sulfate and DCD addition. Higher N2O production emitted followed the application of ammonium sulfate (287.9 ng N2O–N g−1 soil) in the alluvial soil was obtained compared to that (84.9 ng N2O–N g−1 soil) in the red soil. The average N2O emission rate in the NH4 + treatment from the alluvial soil was significantly higher than that from the red soil, and DCD addition remarkably reduced N2O emission rates in both soils during the 21-day incubation. We also observed that the abundance of ammonia-oxidizing bacteria (AOB) amoA gene and the nirK gene was positively correlated with total N2O emissions in the alluvial soil, but only AOB amoA gene abundance was significantly related to the total N2O emissions in the red soil. These results suggested that DCD was more effective in inhibiting N2O production in the alluvial soil than in the red soil. DCD could effectively reduce N2O emission by suppressing the growth of microorganisms containing AOB amoA gene and the nirK gene in the alluvial soil.

Abiotic dissolution and biological uptake of nitrous oxide in Mediterranean woodland and pasture soil

Soil is generally regarded as a net emitter of nitrous oxide (N2O). However, there are numerous field studies showing net uptake of N2O from soil in different ecosystems. Consumption of N2O may be abiotic (absorption by water; adsorption by soil matrix) and biotic (microbial reduction of N2O). This study is the first using undisturbed soil cores to determine the capacity of soil to consume N2O and discuss the fate of N2O.We exposed the base of undisturbed soil cores from Mediterranean pasture and woodland soil to elevated concentrations of N2O and sulphur hexafluoride (SF6; as tracer gas). Headspace concentrations of N2O and SF6 were determined over time and consumption rates of N2O were calculated ranging from 148.8 ± 19.8 ng N2O min−1 g−1 to 163.8 ± 17.2 ng N2O min−1 g−1 in woodland soil and from 117.2 ± 36.1 ng N2O min−1 g−1 to 145.1 ± 19.4 ng N2O min−1 g−1 in pasture soil. Absorption of N2O by soil water contributed 17–49% of the total N2O consumption. The remaining N2O consumed by the cores was due to adsorbtion by the soil matrix and/or reduction by microbes.Mediterranean soil from different ecosystems with different nitrogen (N) loads has a great potential to store and consume N2O, if exposed to an N2O elevated atmosphere.

Potential of denitrification and nitrous oxide production from agricultural soil profiles (Seine Basin, France)

The denitrification process and the associated nitrous oxide (N2O) production in soils have been poorly documented, especially in terms of soil profiles; most work on denitrification has concentrated on the upper layer (first 20 cm). The objectives of this study were to examine the origin of N2O emission and the effects of in situ controlling factors on soil denitrification and N2O production, also allowing the (N2O production)/(NO3−–N reduction) ratio to be determined through (1) the position on a slope reaching a river and (2) the depth (soil horizons: 10–30 and 90–110 cm). In 2009 and 2010, slurry batch experiments combined with molecular investigations of bacterial communities were conducted in a corn field and an adjacent riparian buffer strip. Denitrification rates, ranging from 0.30 μg NO3−–N g−1 dry soil h−1 to 1.44 μg NO3−–N g−1 dry soil h−1, showed no significant variation along the slope and depth. N2O production assessed simultaneously differed considerably over the depth and ranged from 0.4 ng N2O–N g−1 dry soil h−1 in subsoils (the 90–110-cm layer) to 155.1 ng N2O–N g−1 dry soil h−1 in the topsoils (the 10–30-cm layer). In the topsoils, N2O–N production accounted for 8.5–48.0% of the total denitrified NO3−–N, but for less than 1% in the subsoils. Similarly, N2O-consuming bacterial communities from the subsoils greatly differed from those of the topsoils, as revealed by their nosZ DGGE fingerprints. High N2O-SPPR (nitrous oxide semi potential production rates) in comparison to NO3-SPDR (nitrate semi potential reduction rates) for the topsoils indicated significant potential greenhouse N2O gas production, whereas lower horizons could play a role in fully removing nitrate into inert atmospheric N2. In terms of landscape management, these results call for caution in rehabilitating or constructing buffer zones for agricultural nitrate removal.

Impact of invasive plant and environmental conditions on denitrification potential in urban riparian ecosystems

One of the most serious problems involved in riparian restoration is the proliferation of invasive plants during or after a restoration project. Although many studies have assessed ecological influences of invasive plants, life history in particular, only a few have clarified the functional consequences of such changes. In this study, we aimed to determine the influences of an invasive plant, Humulus japonicus, and environmental conditions on riparian ecosystem functions, focusing on denitrification. Soil samples from five riparian ecosystems in Korea were collected on four occasions over a one-year period, and denitrification enzyme activity (DEA) was measured using an acetylene blocking method. DEA varied between 2.5 and>7000 ng N2O g−1 soil h−1. Overall results suggest that DEA was fairly high in winter, but the influences of H. japonicus were minimal. The results suggest that water availability may be a more dominant controlling variable than the presence of H. japonicus for DEA.

Spatial variability of nitrous oxide emissions from an Australian irrigated dairy pasture

Understanding spatial variability of emissions of nitrous oxide (N2O) is essential to understanding of N2O emissions from soils to the atmosphere and in the design of statistically valid measurement programs to determine plot, farm and regional emission rates. Two afternoon, ‘snap-shot’ experiments were conducted; one in the summer and one in the autumn of 2004, to examine the statistics and soil variables affecting the spatial variability of N2O emissions at paddock scale. Small, static chambers (mini-chambers) were placed at 100 locations over an 8,100 m2 area of irrigated dairy pasture in northern Victoria, Australia. Chamber headspace was sampled for N2O and soil samples taken below each mini-chamber were analysed for soil nitrate (NO3 -), ammonium (NH4 +) and other chemical and physical properties known to affect N2O emissions. The experiments took place immediately after the sequence of grazing, urea application and irrigation. Nitrous oxide emissions and soil variables were analysed using classical statistics to investigate the effect of soil variables on N2O emissions. Geostatistics were used to investigate spatial patterns of N2O emissions and soil variables over the measurement area. Nitrous oxide emissions were extremely variable; 45–765 ng N2O–N m−2 s−1 and 20–953 ng N2O–N m−2 s−1 for the two experiments with corresponding averages of 165 and 138 ng N2O–N m−2 s−1. Nitrous oxide emissions showed spatial dependence up to 73 and 51 m for the two experiments. Nitrous oxide emissions showed significant correlation with soil nutrients in decreasing order of NO3 -, NH4 + and available-P concentrations. There was no significant correlation of N2O emissions with measured soil physical properties.

N2O emission from soil following combined application of fertiliser-N and ground weed residues

Emissions of N2O and CO2 were measured following combined applications of 15N-labelled fertiliser (100 μg N g−1; 10 atom % excess 15N) and organic olive crop weed residues (Avena sativa, Ononis viscosa, Ridolfia segetum and Olea europea; 100 μg N g−1) to a silt loam soil under controlled environment conditions. The objective was to determine the effect of varying combinations of inorganic fertiliser and plant residues on these emissions and soil mineral N dynamics. Emissions were generally increased following application of residues alone, with 23 ng N2O–N g−1 soil (2 ng N2O–N g−1 soil mg−1 biomass) and 389 μg CO2–C g−1 soil (39 μg CO2–C g−1 soil mg−1 biomass) emitted over 28 days after addition of the Ridolfia residues in the absence of fertiliser-N. N2O emissions from these residue-only treatments were strongly negatively correlated with residue lignin content (r = −0.91; P < 0.05), total carbon content (r = −0.90; P < 0.05) and (lignin + polyphenol)-to-N ratio (r = −0.70; P < 0.1). However, changes in the net input of these compounds through application of 25:75, 50:50 and 75:25 proportional mixtures of Avena and Ononis residues had no effect on emissions compared to their single (0:100 or 100:0) applications. Addition of fertiliser-N increased emissions (by up to 30 ng N2O–N g−1 28 days−1; 123%), particularly from the low residue-N treatments (Avena and Ridolfia) where a greater quantity of biomass was applied, resulting in emissions above that of the sum from the unfertilised residue and fertilised control treatments. In contrast, fertiliser application had no impact on emissions from the Olea treatment with the highest polyphenol (2%) and lignin (11%) contents due to strong immobilisation of soil N, and the 15N–N2O data indicated that residue quality had no effect on the denitrification of applied fertiliser-N. Such apparent inconsistencies mean that before the potential for manipulating N input (organic + inorganic) to lower gaseous N losses can be realised, first the nature and extent of interactions between the different N sources and any interactions with other compounds released from the residues need to be better understood.

Nitrate loss from a restored floodplain in the Lower Cosumnes River, California

Floodplain restoration has been advocated as a means to restore several ecological services associated with floodplains: water quality improvement, fish rearing habitat, wildlife habitat, flood control, and groundwater recharge. A history of agricultural encroachment on the lower Cosumnes River has resulted in extensive channelization and levee construction. In fall 1998, an experimental floodplain was established by breaching a levee in order to restore the connection between the main channel and its historic floodplain. In this study, we examined how effective this newly restored floodplain was at processing nitrate (NO3−) before reentering the main channel downstream. Two methods were used to examine nitrate loss. In December 2001, we determined denitrification potentials of the floodplain soils before seasonal flooding inundated the floodplain. Next, we conducted a series of field soil column (mesocosm) experiments from March to June 2002 to study NO3−-N loss from the overlying water in both sandy and clayey soils and at three levels of NO3−-N (ambient, +1 mg N l−1, +5 mg N l−1). In addition, we examined NO3−-N loss from mesocosms with water only to determine if loss was related primarily to soil or water column processes. Denitrification potentials were highly variable ranging from 1.6 to 769 ng N2O–N cm−3 h−1. In addition, the denitrification potential was highly correlated with the amount of bioavailable carbon indicating that carbon was a limiting factor for denitrification. Nitrate-N loss rates from mesocosms ranged from 2.9 to 21.0 μg N l−1 h−1 over all treatments and all 3 months examined. Significant loss of NO3−-N (60–93%) from the water only treatment only occurred in June when warmer temperatures and solar radiation most likely increased NO3−-N uptake by phytoplankton. When scaled up, potential NO3−-N loss from the restored floodplain represented 0.6–4.4% of the annual N load from the Lower Cosumnes River during a typical wet year and ~24% during a dry year. During dry water years, the effectiveness of the floodplain for reducing nitrate is limited by the amount of N supplied to the floodplain. Results from this study suggest that restored floodplains can be an effective NO3− sink.

Probability Distribution and Spatial Dependence of Nitrous Oxide Emission

Climate controls soil N2O emissions via its effects on soil properties such as water‐filled pore space. Changes in climate should produce changes in the probability distribution and spatial dependency of soil N2O data. Knowing the extent of the changes in the distribution of this data is important for validating model predictions. The objectives of this study were to describe the probability distributions and estimate the spatial dependency of soil N2O emission data. On a hummocky, agricultural landscape in Saskatchewan, N2O emission data and related soil variables were taken from a 128‐point transect 15 times over 2 yr. Probability distributions were compared using a Chi‐square test. The range in spatial correlation was determined using the indicator semivariogram with a nested model fit approach. The mean N2O flux ranged from 25.3 to −0.2 ng N2O–N m−2s−1Probability distributions ranged in shape from reverse J‐shape through log normal to symmetrical. The majority of distributions were statistically different from each other, showing a lack of temporal stability. Mean N2O flux and distribution shape followed an event‐based/background emission pattern. High flux events had statistically similar, reverse J‐shaped distributions. As mean N2O flux decreased to a background level distribution, shape changed to log normal and symmetrical forms. A high nugget/sill ratio characterized the majority of sampling dates, although spatial dependency was generally moderate. Flux values in the fourth quartile tended to have a spatial dependency of 15 m, probably reflecting a topographic control at a landform element scale.

Field Scale N2O Flux Measurements from Grassland Using Eddy Covariance

In order to assess nitrous oxide (N2O) emissions from typical intensively managed grassland in northern Britain fluxes were measured by eddy covariance using tuneable diode laser absorption spectroscopy from June 2002 to June 2003 for a total period of 4000 h. With micrometeorological techniques it is possible to obtain a very detailed picture of the fluxes of N2O at field scale (103–104 m2), which are valuable for extrapolation to regional scales. In this paper three of the four fertilizer applications were investigated in detail. N2O emissions did not always show a clear response. Hourly fluxes were very large immediately after the June 2002 nitrogen fertilizer application, peaking at 2.5 mg N2O–N m−2 s−1. Daily fluxes were averaging about 300 ng N2O m−2 s−1 over the 4 days following fertilizer application. The response of N2O emissions was less evident after the August fertilization, although 2 days after fertilizer application an hourly maximum flux of 554 ng N2O–N m−2 s−1 was registered. For the rest of August the flux was undetectable. The differences between fertilization events can be explained by different environmental conditions, such as soil temperature and rainfall. A fertiliser-induced N2O emission was not observed after fertilizer application in March 2003, due to lack of rainfall. The total N2O flux from June 2002 to June 2003 was 5.5 kg N2O–N ha−1y−1, which is 2.8% of the total annual N fertilizer input.

Winter fluxes of greenhouse gases from snow‐covered agricultural soil:intra‐annual and interannual variations

Despite the length of winter in cold temperate climates, few studies refer to greenhouse gas emissions from soils during the nongrowing season. In this study, N2O and CO2 fluxes from agricultural and forest soils in southeastern Quebec (Canada) were measured during winter and spring from 1994 to 1997, and the influences of climate, soil, and snow properties on the gaseous emissions were examined. N2O fluxes were far greater from the agricultural soil (2‐187 ng N2O m−2 s−1) than from the forest soil (&lt; 3 ng N2O m−2 s−1), but CO2 fluxes were equivalent for both soil systems (2‐102 μ CO2 m−2 s−1). The higher N2O concentrations in the lower soil horizons could be explained by positive temperature gradients with depth and concomitant negative gas solubility gradients. However, the higher N2O concentrations could also be explained by variations in the expression of N2O reductase with depth, which can modify the N2/N2O ratios in relation to the availability of O2. Calculated N2O‐N fluxes showed that N losses by gaseous emissions from soils during winter and spring were comparable to, or exceeded, similar reported N losses during the growing season. The highest winter fluxes observed in 1997 were interpreted to be due to favorable meteorological conditions that prevailed for denitrification through high soil water content in summer and fall of 1996. Although interannual and nterseasonal variations of fluxes are important, this study shows that wintertime losses of N2O from agricultural soil can be up to 2 to 4 times greater than emissions measured during the growing season in similar agroecosystems.

Short communication: emission of nitrous oxide (N2O) from transgenic tobacco expressing antisense NiR mRNA

The emission of N2 and N2O from intact transgenic tobacco (clone 271) expressing antisense nitrite reductase (NiR) mRNA, and wild-type plants grown aseptically, on NO3-, NO2- or NH4+ -containing medium was investigated. 15N contents of gas sampled from gas-sealed pots, in which the plants were grown on 15N-containing medium, were analyzed by gas chromato- graphy and mass spectrometry (GC-MS). No emission of N2 was detected in either of the gas samples from plant clone 271 or the wild-type grown on NO3--containing medium. N2O emission from clone 271 grown on NO3--containing medium was detected, but not from the wild-type plants. The N2O emission rate of clone 271 was 106 ng N2O mg-1 incorporated N week-1 and the N2O emission was inhibited by tungstate (a nitrate reductase inhibitor). No emission of N2O was found from clone 271 or wild-type plants grown on medium containing NH4+. Emission of N2O also was detected from clone 271 grown on NO2--containing medium and its emission rate increased with increasing NO2- levels in plants. We speculate that NO3- is reduced to NO2- and that a part of NO2- is metabolized to N2O in clone 271.

Nitrous Oxide Emissions in Sewer Systems

AbstractNitrous oxide emissions from urban wastewater were measured in a combined sewer in Bayreuth, Germany. The closed‐chamber method was used to evaluate N2O emissions. Nitrous oxide was analysed by gas chromatography with electron capture detector. Longitudinal and daily variations of N2O fluxes were determined. N2O emissions ranged from 0.11…3.8 mg m−2 h−1 depending on the level of wastewater in the sewer. Data indicate that N2O is mainly formed in the biofilm by nitrification and/or denitrification. In laboratory experiments, samples with wastewater and wastewater/biofilm mixture were incubated with and without NaCl. After two hours of incubation, N2O release was in a range of 42…429 ng N2O per millilitre wastewater or wastewater/biofilm. Incubated samples without NaCl addition showed no significant difference between wastewater and wastewater/biofilm mixture. If salt (NaCl) was added before incubation, samples with wastewater/biofilm mixture emitted 4.8 times more N2O than wastewater samples. It is assumed that high concentrations of NaCl inhibit further microbial reduction of N2O to N2.Based on the field data, a first estimate for N2O emission from sewers is 3.5 g N2O per year and person.

Nitrous Oxide and Carbon Dioxide Fluxes from a Bare Soil Using a Micrometeorological Approach

AbstractIncreasing atmospheric carbon dioxide (CO2) and nitrous oxide (N2O) levels have prompted research on management of the soil C and N pools. The impact of C and N fertilizer addition on N2O and CO2 field emissions is not clear. We determined N2O and CO2 fluxes from a 1‐ha bare soil plot using micrometeorological methods with the objective of evaluating the effect of management practices (cultivation, irrigation, fertilizer, and sucrose applications) on the relative importance of both trace gases. Research was conducted at the Elora Research Station (Typic Hapludalf) in Ontario, Canada, over 7 mo. The N2O concentration gradients were measured using a Tunable Diode Laser Trace Gas Analyzer and the CO2 gradients using an Infra‐Red Gas Analyzer. The transport coefficients were calculated using a Bowen Ratio Energy Balance and two wind profile approaches. These three approaches resulted in similar hourly fluxes. Daily N2O fluxes for nonevent periods were 12 ng m−2 s−1 in 1991, and 2 ng m−2 s−1 in the summer of 1992, while CO2 fluxes before treatments in 1991 were 0.18 mg m−2 s−1. Sucrose addition (420 kg C ha−1) resulted in the highest N2O and CO2 daily emissions measured during the experiment at 3100 ng m−2 s−1 and 0.5 mg m−2 s−1, respectively. Peak emissions of 250 ng N2O m−2 s−1 were measured after wetting of dry soil (WFP &lt; 0.4) through irrigation in 1991, and rain in 1992. Application of ammonium sulfate (100 kg N ha−1) and irrigation increased N2O emissions to 75 ng m−2 s−1, with a smaller effect caused by two subsequent irrigations on wet soil (WFP &gt; 0.6). Carbon dioxide fluxes varied between 0.01 and 0.5 mg m−2 s−1 being the predominant gas contributing to an equivalent CO2 global‐warming potential, but addition of sucrose increased the contribution of N2O to twice the contribution of CO2. The combined effect of C and N additions (e.g. manure and legume) on the N2O emissions in irrigated or high rainfall areas should be considered in the efforts of atmospheric C sequestering.

Enhanced denitrification in plots of N2-fixing faba beans compared to plots of a non-fixing legume and non-legumes

Denitrification rates were studied using the C2H2 inhibition technique in a 2-year field experiment within plots of nodulated and non-nodulated faba beans, ryegrass, and cabbage. Denitrification rates ranged from 14.40 to 0.02 ng N2O−N g−1 soil dry weight h−1. Mean denitrification increased fourfold in plots of N2−fixing Vicia faba compared to non-nodulated V. faba mutant F48, Lolium perenne, and Brassica oleracea. The results with and without C2H2 treatment indicate that in the field the major part of this enhanced denitrification led to the endproduct N2 rather than to the ozone-degrading N2O. Higher denitrification rates of plots with N2−fixing plants in September seemed to be caused by an increase in soil NO inf3 sup- of about 20 kg ha−1 found between July and August. Soil NO inf3 sup- and soil moisture explained 67% of the variation in denitrification rates of the different soil samples over the growing seasons in the 2 years. Soil moisture explained 44% of the variation for soil planted with N2−fixing plants and 62% for soil planted with non-fixing plants. Positive exponential relationships were obtained between denitrification rates and soil nitrate (r=0.71) and soil moisture (r=0.82).