Potential Denitrifying Activity Research Articles

Implications of wetland degradation for the potential denitrifying activity and bacterial populations with nirS genes as found in a succession in Qinghai-Tibet plateau, China

Alpine wetland in the Zoige Plateau has suffered from serious degradation during the last 30 years due to global climate change and anthropogenic impact. Denitrification is a key nitrogen removal process which can be performed by different microorganisms, including bacteria harboring nirS-genes. In this study, a degradation succession was used to study the effect on potential denitrification activity (PDA) and on bacterial communities harboring nirS genes. Based on the determination of the PDA, the abundance, structural diversity, and phylogenetic identity of the soil bacteria with nirS genes were further assessed by qPCR, terminal restriction fragment length polymorphism (T-RFLP), and DNA-sequencing, respectively. The results showed that soil PDA ranged from 8.78 to 52.77 ng N2O-N g−1 dry soil h−1, being lowest in sandy soil and highest in swamp soil. The abundance of nirS genes (copies g−1 soil) were also the lowest in the sandy soil while highest in the swamp soil. The average Shannon-Wiener diversity index of the nirS denitrifying bacterial structural ranged from 2.20 in the meadow soil to 3.07 in the swamp soil. Redundancy analysis (RDA) showed that the nirS denitrifying bacterial community correlated with soil water content and available phosphorus, with water content as the major factor in shaping the nirS denitrifying bacterial community. The results of this study suggest that the wetland degradation would decrease soil PDA, and abundance and structural diversity of the denitrifying bacteria with nirS genes. These findings can contribute to support a theoretical foundation for predicting the potential influences of wetland degradation on soil denitrifying bacteria in alpine wetlands.

Abundance, composition and activity of ammonia oxidizer and denitrifier communities in metal polluted rice paddies from South China.

While microbial nitrogen transformations in soils had been known to be affected by heavy metal pollution, changes in abundance and community structure of the mediating microbial populations had been not yet well characterized in polluted rice soils. Here, by using the prevailing molecular fingerprinting and enzyme activity assays and comparisons to adjacent non-polluted soils, we examined changes in the abundance and activity of ammonia oxidizing and denitrifying communities of rice paddies in two sites with different metal accumulation situation under long-term pollution from metal mining and smelter activities. Potential nitrifying activity was significantly reduced in polluted paddies in both sites while potential denitrifying activity reduced only in the soils with high Cu accumulation up to 1300 mg kg−1. Copy numbers of amoA (AOA and AOB genes) were lower in both polluted paddies, following the trend with the enzyme assays, whereas that of nirK was not significantly affected. Analysis of the DGGE profiles revealed a shift in the community structure of AOA, and to a lesser extent, differences in the community structure of AOB and denitrifier between soils from the two sites with different pollution intensity and metal composition. All of the retrieved AOB sequences belonged to the genus Nitrosospira, among which species Cluster 4 appeared more sensitive to metal pollution. In contrast, nirK genes were widely distributed among different bacterial genera that were represented differentially between the polluted and unpolluted paddies. This could suggest either a possible non-specific target of the primers conventionally used in soil study or complex interactions between soil properties and metal contents on the observed community and activity changes, and thus on the N transformation in the polluted rice soils.

POPULASI DAN AKTIVITAS DENITRIFIKASI SERTA EMISI GAS N2O PADA LAHAN PERTANIAN ORGANIK, PERTANIAN INTENSIF, DAN HUTAN

  This research investigate the population and potentials denitrification activity from three different soils, organically farmed soil, intensive farmed soil and forest soil. Our objectives were to explore spatial gradients in denitrifier populations, examine whether populations density and its potential activity was related to soil chemical properties (C and N content), and determine the potential emission of gas N2O. Results indicated biological functional differences between these three different soil ecosystems. Forest soil had the highest population density of denitrifying bacteria and also had significant potential denitrifying activities. The highest potentials denitrifying activity in the soil affected to the lowest emission of N2O gas. The lowest population and potential denitrifying activity was measured in the intensive farmed soil. Those conditions might be promoted the potentials emission of N2O.

Influence of fertilisation regimes on a nosZ‐containing denitrifying community in a rice paddy soil

Denitrification is a microbial process that has received considerable attention during the past decade since it can result in losses of added nitrogen fertilisers from agricultural soils. Paddy soil has been known to have strong denitrifying activity, but the denitrifying microorganisms responsible for fertilisers in paddy soil are not well known. The objective of this study was to explore the impacts of 17-year application of inorganic and organic fertiliser (rice straw) on the abundance and composition of a nosZ-denitrifier community in paddy soil. Soil samples were collected from CK plots (no fertiliser), N (nitrogen fertiliser), NPK (nitrogen, phosphorus and potassium fertilisers) and NPK + OM (NPK plus organic matter). The nitrous oxide reductase gene (nosZ) community composition was analysed using terminal restriction fragment length polymorphism, and the abundance was determined by quantitative PCR. Both the largest abundance of nosZ-denitrifier and the highest potential denitrifying activity (PDA) occurred in the NPK + OM treatment with about four times higher than that in the CK and two times higher than that in the N and NPK treatments (no significant difference). Denitrifying community composition differed significantly among fertilisation treatments except for the comparison between CK and N treatments. Of the measured abiotic factors, total organic carbon was significantly correlated with the observed differences in community composition and abundance (P < 0.01 by Monte Carlo permutation). This study shows that the addition of different fertilisers affects the size and composition of the nosZ-denitrifier community in paddy soil.

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.

Seasonal and spatial variation in the denitrifying activity in estuarine and lagoonal sediments

We have elucidated the seasonal and spatial variation in the potential denitrifying activity in estuarine and coastal lagoonal sediments in Lakes Shinji and Nakaumi, Japan. The denitrifying activity increased from summer through autumn and was positively correlated with the temperature of the overlying water at all sites except one, where the bottom was always more reductive than at the other sites and there was no NO3 − as a substrate for denitrification from spring to autumn. Moreover, the relationship between the denitrifying activity and the distance from the sea showed different trends in estuarine and lagoonal sediments. These spatial differences indicate that different factors regulate the denitrification in estuarine and lagoonal sediments. Denitrifying activity in estuarine sediment was regulated by the discharge of freshwater containing NO3 − or organic matter, while in the lagoonal sediments the occurrence of nitrification via the intrusion of oxic seawater into the reductive sediment appears to be a key requirement for the process of denitrification. Therefore, the denitrifying activity in the lagoonal sediment appears to be greater near the sea. Water intrusion is one of the key factors controlling denitrification in coastal marine ecosystems by affecting the supply of substrate available for denitrification.

N 2O emission in maize-crops fertilized with pig slurry, matured pig manure or ammonium nitrate in Brittany

N 2O is a potent greenhouse gas and solutions have to be sought to reduce its emission from agriculture. This work evaluates N 2O emission from maize-crop ( Zea mays) fields submitted to different organic or mineral fertilizers (pig slurry, matured pig manure or ammonium nitrate) in Brittany (France). N 2O emission was evaluated along a year in two experimental sites receiving 110 or 180 kg N ha −1 as ammonium nitrate or pig slurry and 180 or 132 kg N ha −1 as ammonium nitrate or matured pig manure at Champ Noel and Le Rheu experimental plots, respectively. N 2O emission was evaluated by interpolation method of periodic fluxes on the field scale and by simulation with NOE algorithm using measured soil characteristics such as N content and gravimetric moisture and other soil biological properties determined in a previous study (potential denitrifying activity, N 2O/[N 2O + N 2] ratio during denitrification) or drawn from literature. On the whole N 2O emissions vary between 0.3 kg N ha −1 year −1 in an unfertilized plot and 2–4 kg N ha −1 year −1 under ammonium nitrate fertilization. They were higher under N fertilizer application than without N fertilizer but no significant effect of type of N fertilizer was observed on either site. However, N 2O losses immediately after fertilizer application were higher under pig slurry and matured pig manure, while measured and predicted fluxes showed that greater N 2O losses occurred from summer to winter under ammonium nitrate application. This could be mainly explained by higher mineral N contents at Le Rheu and higher N 2O/(N 2O + N 2) ratio at both sites. The NOE model predicted higher annual N 2O emission and emission factor with ammonium nitrate at Champ Noël only and similar emissions for both treatments at Le Rheu. These results suggest that in this climate and soil context the use of pig slurry or matured pig manure did not have a stimulating impact on N 2O emissions in comparison with a plot receiving a mineral fertilization.

Acetylene blockage technique as a tool to determine denitrification potential of a biomass fixed on an organic media treating wastewater

Biofiltration on organic media is used in many wastewater applications. However, no tools are available to survey the microorganisms' activity inside the process. The acetylene blockage technique was adapted to evaluate the potential denitrifiers' activity. The objectives of this work were to adapt and verify the applicability of the method by evaluating its sensitivity and variability. A closed cell was built and installed in an open network where pure gaseous nitrogen and acetylene had been successively injected at fixed rate on filtering media samples. N2O produced by the denitrifying biomass was monitored during the test. Assays were realized in replicates, with different quantities of filtering media, to observe the sensitivity and the variability of the method. Potential denitrifying activity and quantity of biomass correlation shows that the adapted method was sensitive for 50 to 150 g (wet basis) of filtering media. Moreover, when under an acetylene atmosphere, the coefficients of variation observed for the tests varied between 10% and 32% when under anoxic conditions. This work shows that the technique as adapted could be used as a tool to survey the potential activity of denitrifiers fixed on an organic media.Key words: denitrification, biofilter, acetylene, nitrous oxide, organic media, potential activity.

Structure and activity of the denitrifying community in a maize-cropped field fertilized with composted pig manure or ammonium nitrate

One alternative to mineral fertilization is to use organic fertilizers. Our aim was to compare the impacts of 7-year applications of composted pig manure and ammonium nitrate on the structure and activity of the denitrifying community. Mineralization and organization of N, denitrification rates and N2O/N2 ratio were also investigated. Fourteen months after the last application, the potential denitrifying activity (+319%), N mineralization (+110%) and organization (+112%) were higher under pig compost than under ammonium nitrate fertilization. On the other hand, the N2O/(N2O+N2) ratio was lower (P<0.05, n=5) under organic fertilization. These effects of organic fertilization were in accordance with its higher total carbon content and microbial biomass. Fingerprints and clone library analyses showed that the structure of the denitrifying community was affected by the fertilization regime. Our results reveal that organic or mineral fertilizer applications could affect both structure and activity of the denitrifying community, with a possible influence on in situ N2O fluxes. These effects of the fertilization regime persisted for at least 14 months after the last application.

Compared effects of long-term pig slurry applications and mineral fertilization on soil denitrification and its end products (N2O, N2)

The long-term (9 years) effect of pig slurry applications vs mineral fertilization on denitrifying activity, N2O production and soil organic carbon (C) (extractable C, microbial biomass C and total organic C) was compared at three soil depths of adjacent plots. The denitrifying activities were measured on undisturbed soil cores and on sieved soil samples with acetylene method to estimate denitrification rates under field or potential conditions. Pig slurry applications had a moderate impact on the C pools. Total organic C was increased by +6.5% and microbial biomass C by ≥25%. The potential denitrifying activity on soil suspension was stimulated (×1.8, P<0.05) 12 days after the last slurry application. This stimulation was still apparent, but not significant, 10 months later and, according to both methods of denitrifying activity measurement (r 2=0.916, P<0.01 on sieved soil; r 2=0.845, P<0.001 on soil cores), was associated with an increase in microbial biomass C above a threshold of about 105 mg kg−1. The effect of pig slurry on denitrification and N2O reduction rates was detected on the surface layer (0–20 cm) only. However, no pig slurry effect could be detected on soil cores at field conditions or after NO3 − enrichments at 20°C. Although the potential denitrifying activity in sieved soil samples was stimulated, the N2O production was lower (P<0.03) in the plot fertilized with pig slurry, indicating a lower N2O/(N2O + N2) ratio of the released gases. The pig-slurry-fertilized plot also showed a higher N2O reduction activity, which is coherent with the lower N2O production in anaerobiosis.

Potential Denitrification Activity in the Landscape of a Western Alaska Drainage Basin

We examined denitrification potentials in six of the major landscape structure (riparian soils of both meandering and braided streams, peat lands, coniferous flats, alder slopes, and groundwater seeps) of the Lake Nerka catchment in southwest Alaska. We found significant potential denitrifying activity in all the soils of the main landscape patch types of the Lake Nerka catchment. The lowest potential denitrifying activity was measured in the peat lands. A highly significant relationship was found between soil organic matter and potential denitrification activity in three landforms— coniferous flats, groundwater seeps, and riparian soils of meandering streams. These three landscape structures also had the highest denitrifying potential. The finer soils of riparian zones along spawning streams, which corresponded to meandering streams, showed a significantly higher potential denitrification activity than the coarse riparian soils along nonspawning streams. These nonspawning streams corresponded to braided streams, where finer sediments were not as prevalent. Therefore, if this high potential denitrification measured in riparian soils of spawning streams is combined with large inputs of nitrate to anaerobic sites, it can result in 15 N signatures that mimic that of the marine-derived nitrogen provided by Pacific salmon in these Alaskan ecosystems.

Short-term changes in bacterial community fingerprints and potential activities in an alfisol supplemented with solid waste leachates.

We investigated the effect on soil functioning of adding leachates from municipal solid waste incinerator (MSWI) ashes to laboratory columns (bare soil) and to field experimental plots (bare soil or grass cover). Leachate of MSWI-solidified air pollution control residues (SAPCr) contained more salts but less heavy metals than did MSWI-bottom ash (BA) leachate. Leachate-supplemented soils (BA soil, SAPCr soil) and control (water added) soil (W) were analyzed after 30 days. Potential denitrifying activity (PDA) and potential N2 fixation (acetylene reduction assay, ARA) were measured in controlled conditions. PDA was significantly lower in SAPCr soil than in W soil, both in the laboratory (-45%) and in bare soil in the field (-31%). ARA values were lower in bare SAPCr soil (-54%) and in bare BA (-25%) soil. Both activities remained unaffected by leachate supplementation in soil under permanent grass cover. Automated ribosomal intergenic spacer analysis (A-RISA) fingerprints and RFLP of nifH gene pools were used to assess changes in the structure of bacterial community. Multivariate analysis of these fingerprints revealed that SAPCr leachate had a stronger effect than BA leachate on the total and N2-fixing bacterial communities. Similar results were obtained for laboratory and bare soil field plots, but leachates did not affect nifH gene pools from soil under permanent grass cover. The stronger impact of SAPCr leachate on both structure of bacterial communities and activities supports the conclusion that observed effects would result from the abundance of salts rather than from heavy metal toxicity.

Denitrification and nitrous oxide production in sediments of the Wiske, a lowland eutrophic river

Denitrification rate and nitrous oxide production were investigated in sediments of the River Wiske, a lowland, nutrient-rich calcareous tributary of the River Swale, NE England. Slurries from samples taken on two occasions at the most downstream site were used to compare potential denitrifying activity at various depths; this activity decreased by about 40% between the 0–1 and 1–2 cm layers. Minimum and maximum values for 5-cm depth cores sampled at the same site from visually different sediments differed by a factor of four. Denitrification rate in cores measured monthly during 1996 showed the lowest values in winter, a peak in late spring and a subsequent decrease through summer and autumn. Nitrous oxide production showed a similar trend and accounted for 0–100% of total nitrogen gases produced by nitrate reduction, with an average value of 42%. Denitrification rate typically ranged over one order of magnitude on passing down the river. The relative importance of various environmental factors on denitrification rate was assessed by laboratory experimental studies, using previous data for nitrate and new studies on temperature and organic carbon. Temperature showed a different effect according to whether assays were carried out with cores or slurries; denitrification rate showed an increase up to 30°C, irrespective of the season of sampling. Addition of organic substrates had no detectable effect on denitrification rate, but did lead to a significant decrease in the proportion of nitrous oxide formed as a result of nitrate reduction. Limitations in current methodologies for assaying denitrification are discussed.

Potential denitrification activity assay in soil—With or without chloramphenicol?

A common way to characterize denitrification in soil is to determine the potential denitrifying activity (PDA). Our objectives were to compare different techniques of mathematically describing experimental data obtained in the PDA assay, both with and without use of chloramphenicol (CAP), and to quantify the effect of CAP on the process. The PDA assay was carried out in the presence of acetylene in slurries of three agricultural soils containing 1 m m glucose and 1 m m KNO 3. When CAP was not used in the assay, growth related curves of N 2O-formation were obtained for all three soils. These data were used to calculate the initial rate by: (1) assuming the initial phase to be linear and using the four first data points for linear regression; and by (2) using a growth-assciated product formation equation. The good fit to the data that was obtained with the latter method suggests that PDA is a continuous process without distinct phases. Moreover, our results clearly show that denitrifying activity is inhibited by CAP even at the lowest concentration tested, 20 mg 1 −1. The inhibiting effect increased with increasing concentrations of CAP. The PDA was 17–42% lower at 1 g CAP 1 −1 compared with assays without CAP. This shows that not only synthesis of new enzymes is affected but also that the activity of already existing enzymes is decreased. Results from our study strongly suggest that single concentrations of CAP must not be used in PDA assays. An alternative strategy could be to use multiple CAP concentrations and then extrapolate to the rate at 0 g CAP 1 −1. However, we recommend assays without CAP and that data should be fitted to the growth-associated product formation equation. By using this method, values of the PDA and the growth rate of the denitrifying bacterial population are objectively obtained.

Denitrifiers and denitrifying activity in size fractions of a mollisol under permanent pasture and continuous cultivation

The variability of the denitrification process in soils was investigated through the distribution of denitrifiers and potential denitrifying activity in the various microenvironments of a mollisol sampled under permanent pasture (PP) and under continuous cultivation (CC). Five size-separates (<2, 2–20, 20–50, 50–250, and 250–2000 μm) were obtained by mild crushing in water and physical fractionation (sieving and sedimentation) of the paired soils. Soils and size fractions were analysed for organic C, total N, and microbial biomass C content and total and denitrifying bacterial counts (most probable number). The indirect enumeration of denitrifying organisms was improved using microbial biomass C data. Denitrifying enzyme activity (DEA) and denitrifying specific activity (DSA:DEA per cell) were also determined on unfractionated soils and on fractions. All the data values were higher in the unfractionated PP soil than in the unfractionated CC soil. The <2 μm fractions of both soils contained a moderate density of denitrifiers, lower than in the unfractionated soil, but with high specific activity; three times (PP soil) to five times (CC soil) as much as the value found in the unfractionated soils. The 20-2 μm fraction contained microaggregates and exhibited the highest microbial biomass C and organic N content and a density of denitrifiers greater than in the unfractionated soil, with a moderate specific denitrifying activity. Despite these contrasting characteristics, the two finest fractions of both soils had a similar potential for denitrification and provided about 90% of the total potential denitrification of each unfractionated soil. The participation of the particulate organic matter (POM) located in the > 250 μm fraction to the total potential denitrification was negligible due to the very small number of denitrifying organisms associated with this fraction. Moreover, a high DSA was found only in the POM of the soil under permanent pasture and not in the soil under continuous cultivation.

Dynamics of residual enzyme activities, denitrification potential, and physico-chemical properties in a γ-sterilized soil

The use or sterilized reinoculated soils appears to be valuable in the study of relationships between numbers and activity of microorganisms, especially in the case of denitrifiers. In this context, residual enzymatic activities and physico-chemical changes after γ-irradiation have been studied. The dynamics of disappearance of the residual activities of denitrifying enzymes, CO; release and β-glucosiduse activity were followed for 8 weeks. Evolution of some physico-chemical properties after γ-irradiation were also tested: pH, N and C status, concentrations of exchangeable cations, soil porosity. The soil treatments were wet or dry γ-sterilized and wet or dry stored after irradiation. Complete sterilization was achieved by a 25 kGy dose in all treatments. No important changes were noticed concerning the pore volume distribution. Very small variations in pH and exchangeable cation concentrations were considered negligible. On the contrary, an increase in soluble organic C. especially in the wet treatment, was attributed both to the lysis of killed cells and to the release of organic acids. particularly through the action of peroxides in wet conditions. The measurement of the enzymatic activities showed the resistance of enzymes to irradiation, leading for example to CO 2 production, to lysis of β-glucosidic bonds or to variations in the N status through deamination of nitrogenous organic compounds or progressive protcolysis of microbial cells. Nevertheless, the location of enzymes was considered as a factor controlling resistance to γ-rays and to its subsequent activity. We suggested that denitrifying enzymes were remaining active in non-proliferating cells and that the loss of potential denitrifying activity, observed for the 8 weeks period, resulted from cell lysis. It was concluded that γ-irradiation of dry soil is less damaging than that of wet soil. Nevertheless, the soil must stabilize before use and a control (uninoculated irradiated soil) must be compared in order not to attribute residual activities of the irradiated soil to the introduced microbial populations.

Survival and potential denitrifying activity of Azospirillum lipoferum and Bradyrhizobium japonicum inoculated into sterilized soil

Potential denitrifying activity and population dynamics of Azospirillum lipoferum (137C) and Bradyrhizobium japonicum (G2sp) inoculated into a γ-sterilized soil were studied for a period of 3 weeks. The denitrifying enzyme potential of soil inoculated independently with each bacterial species was strongly stimulated by the presence of a plant (Zea mays L.). Simultaneous inoculation of both bacteria also produced a higher denitrifying enzyme potential than simple inoculation. Even with double inoculation, the presence of a plant did not modify the evolution of the activity. The response of the population dynamics to these treatments followed a different pattern. The population dynamics of A. lipoferum was not affected by the presence of the plant or by the presence of B. japonicum. In contrast, the presence of both a plant and of A. lipoferum seemed to promote the growth of B. japonicum.

The Effect of Roots on Soil Denitrification

AbstractThe effects of plant roots on denitrification were studied using the acetylene inhibition method, both with short‐term anaerobic assays for potential denitrifying activity and with intact plants in soil. Experiments with anaerobic assays demonstrated that denitrification activity was greater in the rhizosphere of both pot and field grown plants. A split‐plate technique showed that potential denitrifying activity decreased rapidly in the first few millimeters away from roots. These effects were shown to be due, at least in part, to differences in available organic matter.A new method was developed for measuring short‐term denitrification rates of soils with intact plants in an aerobic atmosphere. The soil conditions in these assays were comparable to field conditions. When soil NO3‐ concentrations were high, the results confirmed prevailing opinion that denitrification rates are increased in the rhizosphere. In contrast, the planted soils denitrified at significantly lower rates than unplanted soils when NO3‐ concentrations were low. Thus, denitrification may sometimes be reduced in the rhizosphere. These experiments also indicated that increasing NO3‐ concentration increased the ratio N2O/(N2 + N2O), but that the roots had no consistent effect on this ratio.

Reduction de l'oxyde nitreux dans les sols de rizieres du senegal: Mesure de l'activite denitrifiante

A new method to estimate the rate of denitrification in soil was developed, and two different activities were measured: (1) the initial (or true) denitrifying activity of the soil under prevailing conditions; (2) the potential denitrifying activity induced by the addition of N 2O. Less than 50 g of soil, saturated with water, were incubated anaerobically at 37°C, with a known amount of N 2O, and the disappearance of N 2O was followed by gas chromatography. In 25 paddy soils of Senegal, a highly positive rank correlation was found to exist between the denitrifying activity measured by chromatographic and respirometric methods, under the same conditions of incubation with either N 2O or NO 3 − as the hydrogen acceptor. By the N 2O reduction method, it was found that there was no initial denitrifying activity in the rhizosphere of 3-week-old rice.

Denitrification in undisturbed cores from a solodized solonetz B horizon

An ‘electrolytic’ respirometer was used to study the effects of oxygen, moisture and glucose on denitrification in undisturbed cores from a solodized solonetz B horizon. Under the experimental conditions, nitrous oxide present after 144 h was a reasonable index of total denitrification. Nitrous oxide evolution was positively correlated with moisture, quantity of added glucose, and negatively correlated with per cent oxygen. With glucose present, denitrification proceeded at high oxygen and moderate moisture levels, but was most rapid at low oxygen and high moisture. At low oxygen content, denitrification increased with increasing water content. In horizons with high potential denitrifying activity, denitrification could occur under field conditions at moderate aeration and moderately high moisture contents.