Average Denitrification Rate Research Articles

Role of volume exchange ratio and non-aeration time in spillage of nitrogen in continuously fed and intermittently decanted sequencing batch reactor

The present approach addresses to remove all forms of nitrogen; including recalcitrant dissolved organic nitrogen (rDON) from finally treated effluents. This lab-scale study, suggests that the total nitrogen (TN) removal by continuous fill sequencing batch reactor (cSBR) may not be best as compared to SBR due to spillage of nutrients entered during settling and decantation period. The phenomenon of spillage supported by corresponding dispersion model (plug flow with mild dispersion, 0.081±0.041) and stoichiometric assessment based on balanced chemical equations; revealed that the spill concentration is primarily based on volume exchange ratio, non-aeration out of total cycle time and influent soluble total Kjeldahl nitrogen (TKN). Low C/N ratio acts as limiting factor for denitrification. The C/N ratio was found to be 1.25–6.75 to give TN removals from 35-85%. The average denitrification rate of 0.24gNO3−removed/gBODd in cSBR as compared to 0.198gNO3−removed/gBODd made cSBR favourable by around 18% (stoichiometrically, 14.2%) just by virtue of availability of more BOD for denitrification due its spillage. However, the spillage of TKN reverted this advantage of cSBR, rendering SBR ahead of it with respect to TN removal.

Nitrogen Removal Comparison in Porous Ceramic Media Packed-Bed Reactors by a Consecutive Nitrification and Denitrification Process

Biological nitrogen removal, using a continuous flow packed-bed reactor (CPBR) in a consecutive nitrification and denitrification process, was evaluated. An apparent decline in the nitrification efficiency coincided with the steady increase in NH 4 +-N load. Sustained nitrification efficiency was found to be higher at longer empty bed contact times (EBCTs). The relationship between the rate of alkalinity consumption and NH 4 +-N utilization ratio followed zero-order reaction kinetics. The heterotrophic denitrification rate at a carbon-tonitrogen (C/N) ratio of >4 was found to be >74%. This rate was higher by a factor of 8.5 or 8.9 for NO3 – -N/volatile solids (VS)/day or NO3 – -N/m3 ceramic media/day, respectively, relative to the rates measured at a C/N ratio of 1.1. Autotrophic denitrification efficiencies were 80–90%. It corresponds to an average denitrification rate of 0.96 kg NO 3 – -N/m 3 ceramic media/day and a relevant average denitrification rate of 0.28 g NO 3 –-N/g VS/day, were also obtained. Results presented here also constitute the usability of an innovative porous sulfur ceramic media. This enhanced the dissolution rate of elemental sulfur via a higher contact surface area.

Biological denitrification using cross-linked starch/PCL blends as solid carbon source and biofilm carrier

A novel kind of cross-linked starch/polycaprolactone (SPCL11) was prepared and used as carbon source and biofilm attachment carrier for denitrifying bacteria. The results showed that the average denitrification rate was 0.027mg NO3-N/(g·h) in batch tests. The continuous fixed-bed experiments indicated that more than 90% NO3-N was removed, the denitrification rate reached 26.86mg NO3-N/(L·h), and NO2-N concentration was below 0.16mg/L. The formation of NH4-N was observed, but usually below 1.0mg/L. Rapid biodegradation of starch on the surfaces of SPCL11 granules could cause an initial excess release of dissolved organic compound (DOC), and shortening HRT from 2h to 1h can result in sharp decrease of DOC.

Heterotrophic Denitrification of Nitrate-Contaminated Water Using Different Solid Carbon Sources

Biodegradability and COD release of polylactic acid (PLA), polycaprolactone (PCL) and polylactic acid/Poly (3- hydroxybutyrate-co-3-hydroxyvalerate) blend (PLA/PHBV) were determined in water solution with sludge as the inoculum, and the feasibility of using them as carbon sources to remove nitrate from nitrate-contaminated water was also investigated. The experimental results indicate that PLA/PHBV is degraded more rapidly and easily than PLA and PCL. PLA is difficult to be degraded in water solution, so it is unsuitable to be used as carbon source for denitrification. Average denitrification rates supported by PCL and PLA/PHBV were 6.34 and 6.63mg L-1 h-1 with 7h HRT under a batch system, respectively. Nitrite was not detected in the treated water after 9-hour operation.

Bio‐electrochemical denitrification by a novel proton‐exchange membrane electrodialysis system—a batch mode study

AbstractBACKGROUND: Contamination of nitrate in ground and surface water has become an ever‐increasing and serious environmental problem. Biological methods hold the promise of converting nitrate into harmless nitrogen. A novel denitrification system which combines proton‐exchange membrane electrodialysis with simultaneous bio‐electrochemical autotrophic denitrification has been developed. The proton‐exchange membrane was used to transfer current and to exclude oxygen or other oxidative chemicals generated in the anode reaction. The H2 generated by the cathode was utilized by autotrophic denitrifying microorganisms in the cathode cell to reduce nitrate. In this study, the transport of H+, a denitrification kinetics model and factors influencing the denitrification rate were explored in batch mode.RESULTS: The addition of 0.03 mol L−1 H2SO4 into the anode cell enhanced proton transport and maintained the pH of the cathode cell in an appropriate range for biological denitrification. The denitrification rate was affected by applied current and biomass. Under adequate current conditions, the kinetics of the denitrification process followed a zero‐order kinetics model; the average denitrification rate for unit biomass was calculated to be 9.36 mg NO3−‐N VSS g−1 h−1.CONCLUSIONS: Results indicate that the system is suitable for denitrification. Owing to its simple structure and operation, it has the potential for use as a system to reduce nitrate in water. Copyright © 2010 Society of Chemical Industry

Different responses of denitrification rates and denitrifying bacterial communities to hydrologic pulsing in created wetlands

Abstract Hydrologic pulsing, including water level drawdown and subsequent flooding, may have a considerable impact on both biogeochemical processes and microbial communities in wetlands. Since denitrifying bacteria play a key role in water quality improvement in wetlands, changes in their activities and communities with hydrologic pulsing are an important issue. We investigated the responses of in situ denitrification rates, denitrifying bacterial community structure and their quantities using nitrite reductase ( nir ) S gene under different hydrological pulsing conditions in created wetlands in central Ohio USA. Average denitrification rates, measured from 4 different sampling locations, were 302, 133, 71 and 271 μg N 2 O–N m −2 h −1 during inundated, saturated, drying and reflooding periods, respectively. In particular, the denitrification rates in shallow water level marsh areas (SM) followed by deepwater level marsh areas (DM) showed more sensitivity and magnitude of changes to hydrologic pulsing events than did non-vegetated deepwater areas. This may have been due to the high aerobic decomposition during the drying period and nutrient flushing in shallower marsh areas after the reflooding event. In contrast, the community structure and diversity of denitrifiers based on terminal-restricted fragment length polymorphism (T-RFLP) analysis showed no significant change due to hydrologic pulsing. Instead, the presence and absence of vegetation altered denitrifying bacterial community structure. The nir S gene copy number remained relatively constant with only minor increases during water level drawdown followed by a significant decrease when a sudden reflooding event occurred. These results indicate that environmental disturbances, such as hydrologic pulsing, have a major impact on the denitrification process, but less impact on the community structures of the denitrifying bacteria. In addition, there was no relationship among the denitrification rate, the community structure, and the quantity of denitrifiers, suggesting that changes in denitrification rates during hydrologic pulsing events were not caused by the changes in microbial community structure but more by physicochemical factors, such as substrate availability and hydrology.

Influence of sludge loadings and types of substrates on nutrients removal in MBRs

It has been shown that by combining the patented process for biological phosphorus removal with post-denitrification without additional carbon dosing in membrane bioreactors (MBRs) good nutrient eliminations can be achieved. Especially, reported denitrification rates are higher than expected ones. The reason for this is not understood yet but a correlation between acetate dosing in batch trials and DNR has been reported. The present study investigated the influence of COD sludge loading and acetate inflow concentration on denitrification rates in continuous MBRs. A laboratory scale MBR operated with synthetic wastewater was switched from a multi to a mono substrate (acetate) wastewater and compared to three other MBR connected to separate sewer systems. Better eliminations for COD, TN and TP for the MBR operated with acetate were expected. However, elimination efficiencies were on a good level for all plants and configurations (eliminations: 94–97% COD, 86–94% TN, 92–99% TP) and no significant increase for mono substrate could be found. Average denitrification rate and TN elimination was even a bit lower with mono substrate. For increasing COD sludge loads increasing denitrification rates could be found. However, the variation within the rates has proved that many different influences have to be considered.

Use of agro-food wastewaters for the optimisation of the denitrification process

The aim of this work was to study the feasibility of the denitrification process enhancement, in the Ciudad Real (Spain) WWTP, by dosing agro-food wastewaters generated nearby the city. The studied agro-food wastewaters were characterised by a high COD and low nutrients concentration. The denitrification rates with these wastewaters were lower than those obtained either with acetate or urban sewage, however the dose of agro-food wastewaters raised significantly the denitrification capacity in the WWTP because of the significant increase of easily biodegradable substrates in the wastewater. From the laboratory NUR batch test it was observed that the best agro-food wastewater to enhance the denitrification process was that coming from tomato processing, which presented an average denitrification rate of 1.9 mg NOx-N/(g VSS.h) and an average denitrification yield of 0.2 mg NOx-N/mg COD. The viability of the use of tomato processing wastewater was checked in a pilot plant optimised for urban sewage treatment with biological nutrient removal. The optimum dose, 5.9 mg COD/mg NOx-N, was applied and 99% of the nitrate was removed from the wastewater without influencing negatively either the COD or P effluent concentrations.

Activity and Community Composition of Denitrifying Bacteria in Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)-Using Solid-phase Denitrification Processes

Two laboratory-scale solid-phase denitrification (SPD) reactors, designated reactors A and B, for nitrogen removal were constructed by acclimating activated sludge with pellets and flakes of poly(3-hydoxybutyrate-co-3hydroxyvalerate) (PHBV) as the sole added substrate under denitrifying conditions, respectively. The average denitrification rate in both reactors was 60 mg NO3 � -N g �1 (dry wt) h �1 under steady-state conditions, whereas washed sludge taken from the reactors showed an average denitrification rate of 20 mg NO3-N g �1 (dry wt) h �1 with fresh PHBV as the sole substrate. The difference in the denitrification rate between the two might be due to the bioavailability of intermediate metabolites as the substrate for denitrification, because acetate and 3-hydroxybutyrate were detected in the reactors. Most of the predominant denitrifiers isolated quantitatively by the platecounting method using non-selective agar medium were unable to degrade PHBV and were identified as members of genera of the class Betaproteobacteria by studying 16S rRNA gene sequence information. nirS and nosZ gene clone library-based analyses of the microbial community from SPD reactor A showed that most of the nirS and nosZ clones proved to be derived from members of the family Comamonadaceae and other phylogenetic groups of the Betaproteobacteria. These results suggest that the efficiency of denitrification in the PHBV-SPD process is affected by the availability of intermediate metabolites as possible reducing-power sources as well as of the solid substrate, and that particular species of the Betaproteobacteria play the primary role in denitrification in this process.

Seasonal variability of denitrification efficiency in northern salt marshes: An example from the St. Lawrence Estuary

In coastal ecosystems, denitrification is a key process in removing excess dissolved nitrogen oxides and participating in the control of eutrophication process. Little is known about the role of salt marshes on nitrogen budgets in cold weather coastal areas. Although coastal salt marshes are important sites for organic matter degradation and nutrient regeneration, bacterial-mediated nitrogen cycling processes, such as denitrification, remain unknown in northern and sub-arctic regions, especially under winter conditions. Using labelled nitrogen (15N), denitrification rates were measured in an eastern Canadian salt marsh in August, October and December 2005. Freshly sampled undisturbed sediment cores were incubated over 8h and maintained at their sampling temperatures to evaluate the influence of low temperatures on the denitrification rate. From 2 to 12 degrees C, average denitrification rate and dissolved oxygen consumption increased from 9.6 to 25.5 micromol N2 m-2 h-1 and from 1.3 to 1.8 mmol O2 m-2 h-1, respectively, with no statistical dependence of temperature (p>0.05). Nitrification has been identified as the major nitrate source for denitrification, supplying more than 80% of the nitrate demand. Because no more than 31% of the nitrate removed by sediment is estimated to be denitrified, the presence of a major nitrate sink in sediment is suspected. Among possible nitrate consumption mechanisms, dissimilatory reduction of nitrate to ammonium, metal and organic matter oxidation processes are discussed. Providing the first measurements of denitrification rate in a St. Lawrence Estuary salt marsh, this study evidences the necessity of preserving and restoring marshes. They constitute an efficient geochemical filter against an excess of nitrate dispersion to coastal waters even under cold northern conditions.

Hydrogen limitation—a method for controlling the performance of membrane biofilm reactor for autotrophic denitrification of wastewater

Hydrogen-driven denitrification using the fiber membrane biofilm reactor (MBfR) was evaluated for consistent operation in tertiary wastewater treatment. The possibility of controlling the process rates, as well as biofilm parameters by supplying limited amounts of electron donor (hydrogen), was tested. Limiting the hydrogen supply proved to be efficient in controlling the biofilm growth and performance of the MBfR. Denitrification rates remained unchanged for both synthetic wastewater (SWW) and real municipal wastewater (MWW) effluent as well through the fluctuations in the substrate (NO3-N) concentration. The average denitrification rates were 0.50 (+/- 0.02) g NO3-N per day per m2 for SWW and 0.59 (+/- 0.04) g NO3-N per day per m2 for MWW. Biofilm density rather than thickness was the determining factor in substrate diffusion and biofilm sloughing, ultimately determining operating stability. Limited hydrogen supply assured constant volatile solids (VS) concentration in the biofilm. It was determined that VS/TS ratio higher than 0.25 assured stable biofilm operation. Decrease of VS/TS ratio below 0.25 led to shearing of the nonbiological outer layers of the biofilm. The values of chemical oxygen demand (COD), volatile suspended solids (VSS) and total suspended solids (TSS) in the final effluent were stable and well below wastewater effluent guidelines. Substitutions of bicarbonate with gaseous carbon dioxide as the carbon source did not affect denitrification rates despite lower than optimum pH conditions.

A simple sediment process description suitable for 3D-ecosystem modelling — Development and testing in the Gulf of Finland

The ecosystem of the Gulf of Finland is currently dominated by internal phosphorus loading from sediments. The internal load is highly redox sensitive, and its successful modelling on basin-wide scale requires a simplified description of the sediment process. We present here an approach in which redox-sensitive sediment processes are directly linked to the decomposition of carbon instead of the oxygen concentration in near-bottom water. Mineralisation of organic carbon is known to be the major factor controlling sediment nutrient cycling, including denitrification and Fe(III) oxide reduction, giving rise to high phosphorus fluxes from anoxic sediments. Our sediment process description requires only four main parameters, which are here identified by using in situ CO 2, dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) flux measurements carried out by Göteborg University landers. The model was tested with the aid of time series of denitrification and DIP flux rates measured in the western Gulf of Finland. Modelled near-bottom and surface nutrient concentrations were compared with monitoring data from both the eastern and western Gulf of Finland. The model simulations showed that the average net ecosystem production entering the sediment surface from the euphotic layer was 49 g C m − 2 a − 1 . This organic load induced an average denitrification rate of 2.5 g N m − 2 a − 1 and DIP flux of 0.67 g P m − 2 a − 1 , corresponding to 20,200 t P a − 1 for the whole Gulf of Finland. The model was able to describe the seasonality of denitrification and sediment DIP flux with high precision. Further, the modelled near-bottom and surface nutrient concentrations were compatible with the available data. The results indicate that, on the scales important for coastal and open sea conditions, our simple sediment process description works well. The new tool will help us to use 3D models to study the effects of external load on the production and decomposition of organic matter, and on subsequent benthic nutrient fluxes.

Nitrogen cycling processes in Lake Illawarra, an intermittently closed/open estuary in south-east Australia

Mass balance - stoichiometric budgets, such as those developed by the Land-Ocean Interactions in the Coastal Zone (LOICZ) Programme can indicate the biogeochemical functioning of an estuary, and thus help assist understanding the dominant natural processes within the system. This is the basis to developing cost-effective management strategies. In this study, nutrient budgets for Lake Illawarra, a typical intertidal coastal barrier lagoon in New South Wales, Australia, were firstly examined utilizing the LOICZ model. The LOICZ budget classified this lake as a heterotrophic ecosystem, producing carbon through net respiration. The budget results also showed the Lake to be a net denitrifying and nitrogen limited system. Benthic flux measurements of O2, TCO2, alkalinity, NH4+, NO2- +NO3- and N2 were then made at selected stations from January 2002 to January 2003 to compare the characteristics of benthic biogeochemical processes (benthic metabolism, nutrient fluxes and denitrification) for different primary producers (seagrass or microphytobenthos (MPB)) and/or sediment types (sand or mud), and thus to verify the reliability of LOICZ budget approach. The seagrass meadows exhibited significantly higher gross primary productivity than MPB in shallow sandy or deep muddy regions of the Lake. Net CO2 effluxes and O2 uptake were generally observed for all sites, indicating the Lake was an overall net-respiratory system, meaning more organic carbon was decomposed than produced. This supported the LOICZ modelling conclusion. In general, nutrient fluxes displayed typical diurnal variations, with an efflux in the dark and uptake or reduced efflux in the light. Dissolved inorganic nitrogen (DIN) net fluxes in the unvegetated sediments were directed from the sediments towards the water column and dominated by the NH4+ fluxes; seagrass beds displayed a net DIN uptake dominated by NO2-+NO3- fluxes, which may be due to the enhanced denitrification and/or assimilation activity by rooted plants. Finally, based on the measured benthic fluxes, N2 flux rates were estimated using carbon and nitrogen stoichiometry, which gave an average net denitrification rate of 0.09 mmol m-2 h-1. This was of similar magnitude to the rates measured using the N2/Ar technique (0.04 mmol m-2 h-1) or estimated using LOICZ modelling (0.03-0.05 mmol m-2 h-1).

Denitrification potential in a Louisiana wetland receiving diverted Mississippi River water

Excess nitrate in Mississippi River water entering offshore areas is reported to contribute to low oxygen (hypoxia) conditions in the Gulf of Mexico. Excessive algal growth driven by the excess nitrogen results in a decrease in dissolved oxygen in bottom water. Reintroduction of Mississippi River waters into a Louisiana coastal wetland has the potential to reduce the amount of nitrate reaching offshore waters. In this study, reduction in the concentration of added NO3 − was determined in sediment–water-columns collected from a wetland site in Breton Sound estuary receiving nutrient inputs from the Mississippi River. The capacity of a wetland to process nitrate in floodwater was determined in the laboratory. The rates of NO3 − removal (determined from change in nitrate concentration in the floodwater) averaged 97 mg N m−2 d−1 over 16 d for a 1750-mg NO3-N m−2 addition, and 170 mg N m−2 d−1 over 16 d for a 3500-mg NO3-N m−2 addition. The total N2O-N emissions from the 1750- and 3500-mg NO3-N m−2 additions were 19 and 54 mg N m−2 accounting for 1.1% and 1.5% of the applied NO3-N, respectively. Using the acetylene-inhibition technique, the average denitrification rate was determined to be 57 and 87 mg N m−2 d−1 (21 and 32 g N m−2 yr−1) during the most active denitrification period of 5 d after incubation for 1750 and 3500 mg NO3 −-N m−2 of added nitrate in floodwater, respectively. The total N evolved over 11 d as N2O + N2 was equivalent to 436 and 921 mg N m−2 (24.9% and 26.3%, respectively, of added N). Increasing the amount of NO3 − applied to the overlying water increased the rate of NO3 − loss and N2O emission significantly. The thickness of the oxidized surface sediment layer was also influenced by the NO3 − application to the floodwater with a significant linear correlation between nitrate addition and thickness of the oxidized layer (r = 0.9998, p = 0.01). This study indicates that wetlands receiving diverted Mississippi River water have the potential to process and remove NO3 − in the river water, reducing the amount of NO3 − reaching to offshore areas.

Comparison of fate of nitrogen applied to 4 different kinds of soils with particular reference to denitrification

We determined the in situ denitrification rate of four different kinds of soils during the growing season of maize by the combination of an intact core method and an acetylene inhibition technique. The soils used were Low-humic Andosols, Cumulic Andosols, Gray Lowland Soils, and Yellow Soils to which 20 Mg ha−1 of manure was applied every year. In addition, we simultaneously investigated the N balance sheet of the same maize fields using 15N-labelled fertilizer. The average denitrification rate during the growing season of maize (from May 21 to August 24, depth: 0–20 cm) was 2.0 g N ha−1 d−1 for Low-humic Andosols, 16.1 g N ha−1 d−1 for Cumulic Andosols, 20.3 g N ha−1 d−1 for Gray Lowland Soils, and 192.4 g N ha−1 d−1 for Yellow Soils, respectively. The denitrification rates in the subsurface layer (10–20 cm) were higher than those in the surface layer (0–10 cm) irrespective of the soils. Logarithm to the average denitrification rate throughout the growing season was highly correlated with the solid-phase ratio of the soils. Total amount of denitrified N from Yellow Soils during the growing season of maize was estimated to be 12% of applied N, whereas the amount from the other soils was lower than 2%. A considerable amount of nitrate remained in the subsurface layer (30–45 cm) of the Low-humic Andosols and Cumulic Andosols at harvest time. The major part of the remaining 15N-labelled fertilizer in the Gray Lowland Soils and Yellow Soils was distributed in the surface layer, which mainly occurred in an immobilized form. At harvest time, the ratio of 15N-labelled fertilizer which was unaccounted for was 19.2 ± 11.1% of the total for Low-humic Andosols, 44.4 ± 10.6% for Cumulic Andosols, 47.4 ± 9.8% for Gray Lowland Soils, 65.8 ± 12.0% for Yellow Soils, respectively. The discrepancy between the amount of denitrification and unaccounted loss indicates that leaching of nitrate was mainly responsible for the loss of N fertilizer. However, in the Yellow Soils, denitrification was considered to be a significant cause of N loss as well as leaching of nitrate.

Moving-bed biological treatment (MBBT) of municipal wastewater: denitrification

Denitrification in a full-scale installation and a pilot plant for moving-bed biological treatment (MBBT) was subject to detailed investigation. Two different types of carriers were used in conventional activated sludge reactors: foam cubes and plastic tubes (Kaldnes). Both investigated carriers showed the same behavior with regard to denitrification capacity, temperature dependency and maximum COD and nitrate turnover. In contrast to the plastic tubes (Kaldnes), the sponge cubes stored remarkable amounts of substrate. The maximum denitrification rate with acetate as a substrate was 420 gNm-3d-1 at 10 degrees C and 730 gNm-3d-1 at 20 degrees C. An average denitrification rate of 240 gNm-3d-1 (10 degrees C) was achieved with wastewater. A maximum of 37% of the COD in the influent was denitrified with a volumetric loading rate in the anoxic zone of 2.2 kgCODm-3d-1.

Geomorphic control of denitrification in large river floodplain soils

In this manuscript we investigated the relationshipsbetween the microbiological denitrification process inriver alluvial soils with structures and patterns ofthe floodplain visible at a larger scale. Wehypothesised that both topography and soil grain sizerepresent pertinent environmental factors to forecastdenitrification activity in river floodplain. Thestudy was conducted in 15 alluvial sites along a 30 kmlong stretch of the Garonne River, a seventh-orderstream of the south west of France. Sites wereselected to encompass the widest range possible ofaverage annual flood duration (0.04 to 29 days) andfrequency (return period from 0.6 to 7 years). On anannual basis, we found that average denitrificationrates did not show any significant trend along theflood frequency gradient. Although during the studythe flood frequency and duration was higher than thecalculated average, we did not find any relationshipbetween flood duration and denitrification enzymeactivity. If flood events do not last long enough tomaintain waterlogging conditions conducive to sustaindenitrification activity for long periods, theyindirectly affect the spatial distribution ofdenitrification activity through the sorting out ofsediment deposits. Indeed, we found a significantrelationship between denitrification rates in thefloodplain soils and their texture; highest rates weremeasured in fine textured soils with high silt + claycontent. Below a threshold of 65% of silt and claycontent, the floodplain soils did not present anysignificant denitrification rates. Above thatthreshold denitrification increased linearly. Theseresults demonstrate that alluvial soil texture is alandscape scale factor which has a significant effecton denitrification in floodplains.

Phenotypic Plasticity in Groundwater Denitrifiers

Bioassays were made in a gradient of NO 3 - concentrations to examine how phenotypic plasticity in denitrification rates was associated with adaptation in groundwater bacteria to aquifers with different NO 3 - concentrations. Denitrifying activity of single strains were quantified using progress curves of N 2 O production in sediment-groundwater microcosms in which the NO 3 concentrations ranged between 0.5 and 48 mg NO 3 - -N l -1 . Denitrifiers from a fertiliser contaminated aquifer with 24.1 mg NO 3 - N 1 -1 had a shorter lag phase and more rapid growth compared to strains from an uncontaminated aquifer with 2.8 mg NO 3 - -N 1 -1 . regardless of the NO 3 concentrations they were maintained in. Strains from the uncontaminated site showed a harrow plastic response with a denitrification optimum at low NO 3 - concentration (2.8 6 mg NO 3 - -N l -1 ) suggesting that denitrifiers in the uncontaminated aquifer have undergone stabilising selection to the in situ NO 3 - concentration. Strains from the contaminated site had a broader plasticity with virtually the same denitrification rate from low to high NO 3 concentrations (2.8-24 mg NO 3 - -N l -1 ). They were superior denitrifiers also at the in situ concentration of the uncontaminated aquifer. which we suggest is due to the expression of more than one copy of the reductase genes. The average denitrification rates for strains from the contaminated site were 2-3 times higher compared to rates for strains from the uncontaminated site, but the variability was larger, possibly because two of the anaerobic denitrifiers from the contaminated site also were capable of aerobic denitrification. The results presented here can be useful when in situ bioremediation of NO 3 - contaminated aquifers is considered, since denitrifiers adapted to the conditions in the contaminated aquifer maintain high rates of N 2 O production down to a NO 3 - concentration of about 3 mg NO 3 -N l -1 .

Aquifer Denitrification as Interpreted from In Situ Microcosm Experiments

AbstractDenitrification of 40 mg L−1 NO3‐N groundwater in the vicinity of Central City, NE was stimulated with ethanol in in situ microcosms that were vibrated into the aquifer sediment and, thus, filled with a relatively undisturbed, saturated sand and gravel matrix. In microcosms receiving the 1.25 C/N ratio, nitrate disappeared within 40 h; however, NO2‐N accumulated and persisted after NO3‐N was depleted. Nitrite has been documented in groundwater redoxclines and is expected to be reported more frequently with increased use of ion chromatography and dense sampling networks. Dissolved oxygen was consumed to <2.0 mg L−1 within 15 h, and stoichiometric production of HCO−3 occurred. Average denitrification rate decreased from 28.5 to 19.3 and to 16 mg N L−1 d−1 for C/N ratios of 1.25, 2.5, and 5.0 mg L−1. Kinetic N‐isotope effects resulted in the δ15N enrichment of residual nitrate as denitrification progressed, thus, causing the initial δ15N value of ∼+6‰ to increase to > +20‰. The data support measuring additional parameters to prevent misinterpretations when using δ15N values in NO−3 source identification investigations. Apparent isotope‐enrichment factors, ε value, were derived from δ15N increases vs. NO−3 and NO3 plus NO−2 reduction. They ranged from −11 to −16‰ for δ15N vs. residual NO−3 plus NO−2 and from −2.5 to −9‰ for δ15N vs. NO−3 alone. Since NO−3 could not be resolved from NO−2 in the δ15N analysis, a δ15N depleted NO2 phase was not identified. Nevertheless, isotopically light enrichment factors are consistent with the presence of NO−2.

Denitrification losses from an irrigated sandy-clay loam under a wheat-maize cropping system receiving different fertilizer treatments

Studies were conducted on denitrification in the plough layer of an irrigated sandy-clay loam under a wheat-maize cropping system receiving different fertilizer treatments. The treatments were: N-100 (urea-N at 100kgha–1year–1), N-200 (urea-N at 200kgha–1year–1), FYM-16 (farmyard manure at 16 tonnes ha–1year–1), FYM-32 (farmyard manure at 32 tonnesha–1year–1) and the control (unfertilized). Averaged across sampling dates during the wheat season, the denitrification rate as measured by the C2H2-inhibition/soil-core incubation method was highest in N-200 (83gNha–1day–1), followed by FYM-32 (60gNha–1day–1, N-100 (51gNha–1day–1), FYM-16 (47gNha–1day–1) and the control (33gNha–1 day–1). During the maize growing season, average denitrification rate was highest in FYM-32 (525gNha–1day–1), followed by FYM-16 (408gNha–1day–1), N-200 (372gNha–1day–1, N-100 (262gNha–1day–1) and the control (203gNha–1day–1). Denitrification loss integrated over the whole vegetation period was at a maximum under FYM-32 (13.9kgNha–1), followed by N-200 (11.8kgNha–1), FYM-16 (10.6kgNha–1) and N-100 (8.0kgNha–1), whereas the minimum was observed for the control (5.8kgNha–1). Under both crops, denitrification was significantly correlated with water-filled pore space and soil NO3–-N. The best multiple regression models accounted for 52% and 70% of the variability in denitrification under wheat and maize, respectively. Results indicated that denitrification is not an important N loss mechanism in this well-drained, irrigated sandy-clay loam under a wheat-maize cropping system receiving fertilizer inputs in the range of 100–200kgNha–1year–1.