Nitrifying Biomass Research Articles

Chlorinated solvents cometabolism by an enriched nitrifying bacterial consortium

The biodegradability of three of the most frequently halogenated aliphatics (trichloroethene, chloroform and 1.1.1.-trichloroethane) found in drinking water aquifers by a nitrifying enriched mixed biomass was investigated during batch tests. Within this mixed biomass, ammonia oxidisers were the effective degraders. The presence of ammonia stimulated chlorocarbon biodegradation, and the presence of chlorocarbon inhibited ammonia oxidation. This contrasted phenomenon was explained by a balance between electron supply from ammonia necessary to sustain the chlorocarbon oxidation and competitive inhibition for the ammonia monooxygenase active site between both substrates. About 0.03 to 0.2% of the electrons generated by ammonia oxidation were used for chlorocarbon degradation. Trichloroethene and chloroform oxidation induced a biomass inactivation (around 30 to 40 mg of proteins inactivated per μmol of chlorocarbn oxidised). Biomass re-activation due to exergonic ammonia catabolism was estimated to 24±6 mg of proteins reactivated per mmol of ammonia oxidised in both cases. No inactivation of re-activation was observed in the case of 1.1.1-trichloroethane.

Exceptionally high-rate nitrification in sequencing batch reactors treating high ammonia landfill leachate

The nitrogen removal capacity of a suspended culture system treating mature landfill leachate was investigated. Leachate containing high ammonium levels of 300-900 mg N/L was nitrified in a bench scale sequencing batch reactor. Leachate from four different landfills was treated over a two year period for the removal of nitrogen. In this time, a highly specific nitrifying culture was attained that delivered exceptionally high rates of ammonia removal. No sludge was wasted from the system to increase the throughput and up to 13 g/L of MLSS was obtained. Settleability of the purely nitrifying biomass was excellent with SVI less than 40 mL/g, even at the high sludge concentrations. Nitrification rates up to 246 mg N/(L h) (5.91 g N/(L d)) and specific nitrification rates of 36 mg N/(gVSS h) (880 mg N/(gVSS d)) were obtained. The loading to the system at this time allowed complete nitrification of the leachate with a hydraulic retention time of only 5 hours. Following these successful treatability studies, a full-scale plant was designed and built at one of the landfills investigated.

Interpreting the Response to Loading Changes in a Mixed‐Culture Completely Stirred Tank Reactor

Mathematical modeling and carefully controlled laboratory experiments are used to quantify the relationships among nitrifying and heterotrophic bacteria and the key constituents of effluent quality. Mathematical model results are used to interpret experimentally measured traditional parameters, such as soluble chemical oxygen demand (COD) and total biomass. Although total biomass declines with a decrease in influent COD loading, soluble effluent quality remains nearly the same because the effluent soluble COD is controlled by biomass‐associated products. Increases in ammonia‐nitrogen (NH4+–N) loading result in increases in nitrifier biomass, and the active heterotrophs also increase as a result of formation of soluble microbial products (SMP) by autotrophic nitrifiers. However, increased NH4+–N loading causes a steady increase in effluent soluble COD, indicating that the SMP production by nitrifiers is only partially used by the heterotrophs. During the nonsteady‐state period after a loading change, nitrification responds more slowly than heterotrophic processes and controls the time needed to reach the new steady state.

Ammonium removal from primary and secondary effluents using a bioregenerated ion-exchange process

A new process for ammonia removal from sewage effluents is presented. The process uses an ion exchange material, zeolite, both as the separator of NH4+ from the wastewater and also as the carrier for a nitrifying biomass. The process is carried out in a single reactor operating in two modes: an adsorption mode in which the zeolite column acts as a typical ion exchanger and a bioregeneration mode in which the bacteria attached to the zeolite oxidizes the NH4+ to NO3–. The separation between carbonaceous removal and NH4+ removal enables the exclusive selection of nitrifiers at high concentrations attached to the zeolite, thus achieving high bioregeneration rates. This paper summarizes three years of research on the process and focuses on the operation with actual secondary and primary effluents. Process operation showed: (1) no bed clogging occurred due to suspended solids accumulation; (2) residual BOD from the adsorption phase resulted in only minimal heterotroph competition and thus, no fall in the rate of bioregeneration; (3) only a small deterioration in exchange efficiency due to zeolite biofilm coverage as compared to the ion exchange efficiency of “virgin” zeolite. Results show that both secondary and primary effluents can be successfully treated by the process.

Wastewater as a source of nitrifying bacteria in river systems: the case of the River Seine downstream from Paris

The River Seine downstream from Paris receives large amounts of ammonium (about 200 μmol/l) from treated and untreated wastewater effluents. In such large river systems, due to the slow growth of nitrifying bacteria, the small size of the nitrifying population present in the water column often represents the limiting factor for nitrification of the contaminating ammonium. In this work we demonstrate that discharge of urban effluents can represent an important seeding of nitrifying bacteria, influencing the dynamics of nitrification in the river downstream. A nitrifying bacteria biomass in wastewater was deduced from H 14CO − 3 potential nitrifying activity measurements. these were found to be higher in untreated wastewater (1–200 μgC/l) and in treated effluents (0.8–30 μgC/l) than in the receiving river water (0.5–5 μgC/l). a retrospective analysis of the nitrification process in the River Seine downstream from Paris suggests that the overall ammonium oxidation rate has been continuously reduced over the past 20 years (from 1.5 to 1.0 μmol/l/h), as a result of the improvement of the treatment of Paris wastewater and the reduction of the discharge of untreated wastewater (from 14% to 0.2% of the total wastewater discharge).

Measurement of potential activity of fixed nitrifying bacteria in biological filters used in drinking water production

Nitrification during biological filtration is being used more and more in drinking water production to remove ammonia, which can be the source of several water quality problems during distribution. In this process, ammonia is converted into nitrite and then into nitrate by fixed autotrophic nitrifying bacteria. The purpose of this work was to develop a technique to estimate fixed nitrifying biomass (sum of ammonia- and nitrite-oxidizing populations). The quantification of autotrophic nitrifying biomass was determined by potential nitrifying activity measurement. The production of oxidized forms of inorganic nitrogen (nitrates and nitrites) was measured after an incubation of 2 cm3 of colonized solid support in the presence of a 5-ml nitrifier medium containing 10 mg N-NH4 L−1 for 30 min at 32°C. The production rate of oxidized nitrogen in optimal conditions was measured and converted into nitrifying biomass by using the maximum specific oxidizing activity. This technique was shown to be appropriate for conditions encountered in the biological filters used in drinking water production and sufficiently simple to be used for routine measurements. Journal of Industrial Microbiology & Biotechnology (2000) 24, 161–166.

Supply of organic matter and bacteria to aquatic ecosystems through waste water effluents

In order to study the impact on the river Seine of the waste water effluents from the city of Paris and its suburbs, a detailed characterisation was made of both raw and treated effluents from the three main treatment plants in this area which differ both in size and type of treatment. The waste water samples were subjected to analyses of the following pools of carbon: dissolved organic carbon (DOC), particulate organic carbon (POC), biodegradable fractions of DOC (BDOC) and of POC (BPOC), the biochemical oxygen demand (BOD). Inorganic and organic forms of nitrogen and phosphorous, total bacterial biomass and nitrifying bacterial biomass were also investigated in parallel. On the basis of the results of the analysis performed, a specific load per inhabitant and per day (expressed in g inh −1 d −1) for raw and the different types of treated waste water was calculated for each variable considered in this study. For raw water, the specific loads of TOC ranged between 26.4 and 28.3 g C inh −1 d −1 with particulate organic matter constituting the main part (70–76%) and the biodegradable fraction representing between 60 and 75%. Concerning micro-organisms, the average specific load of total bacteria was around 2 g C of biomass inh −1 d −1, the nitrifying biomass represented 0.3–2.5% of the total bacterial biomass. Depending on the type of treatment, the specific load of TOC in treated water ranged between 3 and 10.8 g C inh −1 d −1, it corresponded to removal percentages in the range 59–89%. Total bacterial biomass (0.05–0.33 g C inh −1 d −1) was always lower in treated than in raw water. A significant correlation was observed between BTOC (sum of BDOC and BOPC) and BOD. BPOC represented 68% of BTOC in raw water and 43% in treated water. The total biomass of bacteria constituted 8% of the BTOC.

Organic halogen removal from chlorinated humic ground water and lake water by nitrifying fluidized-bed biomass characterised by electron microscopy and molecular methods.

The dechlorinating and genotoxicity-removing activities of nitrifying fluidized-bed reactor biomass towards chlorinated organic compounds in water were shown at level below 1 ppm. The removal rates of adsorbable organic halogens were 200 micrograms Cl (g VS day)-1 for chlorinated humic ground water and 50 micrograms Cl (g VS day)-1 for chlorinated lake water when studied in batch mode. In a sequenced batch mode the removal rates [micrograms Cl (g VS day)-1] were 2000 from chlorohumus, 1400-1800 from chlorophenols in chlorinated ground water, and 430-720 from chlorohumus in chlorinated lake water. Genotoxicity was removed to a large extent (60%-80%) from the chlorinated waters upon incubation with nitrifying reactor biomass. 2,6-Di-, 2,4,6-tri and 2,3,4,6-tetrachlorophenols competed with chlorinated water organohalogens for dechlorination. The dechlorination of chlorophenols and chlorohumus required no ammonia and was not prevented by inhibitors of ammonia oxidation, nitrapyrin, parathion, sodium diethyldithiocarbamate, or allylthiourea. Electron microscopical inspection of the biomass showed the dominance of clusters of bacteria resembling known nitrifying species, Nitrosomonas, Nitrobacter, and Nitrosospira. This was supported by polymerase chain reaction amplification of the biomass DNA with four different primers, revealing the presence of 16S rDNA sequences assignable to the same species. The most intensive band obtained with the Nitroso4E primer was shown to be closely related to Nitrosomonas europaea by restriction analysis.

Ammonium removal using ion exchange and biological regeneration

A new concept for ammonium removal from secondary effluent by zeolite followed by bioregeneration has been studied. In contrast to other studies of hybrid biological-ion exchange multireactor systems, the proposed process uses the ion exchange material, zeolite, as a carrier for the nitrifying biomass. Therefore, the entire process is carried out in a single reactor. Since all the ammonium from the original effluent is concentrated in the zeolite and released gradually during regeneration, nitrification is carried out in a small volume reactor in an almost batch mode where optimal conditions for nitrification can easily be maintained. Moreover, the conversion of ammonium cations to nitrate anions allows for regenerate recycle, where the amount of chemicals added for desorption is reduced to the amount of sodium bicarbonate added as a buffer for nitrification. As a result, operational costs and production of large volumes of brine are minimized. To achieve sufficient NH 4 + concentration in the solution to allow for high rate nitrification, the cation-rich regenerant solution (or part of it) is reused from one cycle to the next. A theoretical model including ion exchange and bioregeneration modes, indicates that the total cation concentration and each cation in the recycled regenerant should reach constant values after several cycles of adsorption–regeneration and remain constant as long as the influent characteristics and operation conditions stay similar. Experiments results verified the predicted values.

Process audit and asset assessment using on-line instrumentation

The traditional approach to assessing the performance of an Activated Sludge Plant (ASP) is to conduct an expensive and often labour intensive through-plant sampling survey. The results from this kind of survey do not give an accurate picture of the plant performance in real time, and often can be ruined by inaccurate or spoilt analytical results. A new approach to Process Audits has been developed based on on-line instrumentation and remote monitoring. The key equipment required to dynamically monitor the ASP performance is the MSL Respirometer which is used to measure organic loading, required retention time and aeration system requirements. Auto-cleaning, auto-calibrating Dissolved Oxygen sensors are used in conjunction with a Respirometer to assess the aeration system efficiency (actual Kla value) and the maximum oxygen demand (OUR) which can be sustained by the plant. Ultrasonic flow meters are used to determine flow splits and actual plant retention time which is compared to the required retention time measured by the Respirometer. Mixed Liquor Suspended Solids (MLSS) profiles are monitored continuously using a self-cleaning optical sensor, which is used along with Respirometric readings to measure the effect of poor wastage regimes and ingress of inert solids. On-line ammonia, phosphate and nitrate monitors can be used to give information on the diurnal variation of nutrients, related to the loading on the plant. As well as the normal plant operation, the effect of shock loads, storm conditions and sludge liquor returns can all be assessed. All instruments are linked via a serial communications loop to a modern and telephone line. The plant is monitored remotely, making it an unobtrusive survey allowing operations to continue as normal and giving a true picture of the plant performance. This approach has been used on several UK sewage treatment works and has proven to be a cost-effective method. In one case a proposed capital spend on final settlement tanks was shown to be unnecessary; in another case the plant was shown to be unable to support a nitrifying biomass.

Estimation of nitrifying bacterial activities by measuring oxygen uptake in the presence of the metabolic inhibitors allylthiourea and azide

The effects of two metabolic inhibitors on an enriched nitrifying biomass during incubation for short periods of time were investigated by determining respirometric measurements. Allylthiourea (86 &mgr;M) and azide (24 &mgr;M) were shown to be strong, selective inhibitors of ammonia and nitrite oxidation, respectively. Consequently, a differential respirometry method for estimating nitrifying and heterotrophic bacterial activities within a mixed biomass is proposed.

Process audit and asset assessment using on-line instrumentation

The traditional approach to assessing the performance of an Activated Sludge Plant (ASP) is to conduct an expensive and often labour intensive through-plant sampling survey. The results from this kind of survey do not give an accurate picture of the plant performance in real time, and often can be ruined by inaccurate or spoilt analytical results.A new approach to Process Audits has been developed based on on-line instrumentation and remote monitoring. The key equipment required to dynamically monitor the ASP performance is the MSL Respirometer which is used to measure organic loading, required retention time and aeration system requirements.Auto-cleaning, auto-calibrating Dissolved Oxygen sensors are used in conjunction with a Respirometer to assess the aeration system efficiency (actual Kla value) and the maximum oxygen demand (OUR) which can be sustained by the plant. Ultrasonic flow meters are used to determine flow splits and actual plant retention time which is compared to the required retention time measured by the Respirometer. Mixed Liquor Suspended Solids (MLSS) profiles are monitored continuously using a self-cleaning optical sensor, which is used along with Respirometric readings to measure the effect of poor wastage regimes and ingress of inert solids. On-line ammonia, phosphate and nitrate monitors can be used to give information on the diurnal variation of nutrients, related to the loading on the plant. As well as the normal plant operation, the effect of shock loads, storm conditions and sludge liquor returns can all be assessed.All instruments are linked via a serial communications loop to a modem and telephone line. The plant is monitored remotely, making it an unobtrusive survey allowing operations to continue as normal and giving a true picture of the plant performance.This approach has been used on several UK sewage treatment works and has proven to be a cost-effective method. In one case a proposed capital spend on final settlement tanks was shown to be unnecessary; in another case the plant was shown to be unable to support a nitrifying biomass.

Influence of dissolved oxygen concentration on nitrite accumulation in a biofilm airlift suspension reactor

The biofilm airlift suspension (BAS) reactor can treat wastewater at a high volumetric loading rate combined with a low sludge loading. Two BAS reactors were operated, with an ammonium load of 5 kg N/(m3 d), in order to study the influence of biomass and oxygen concentration on the nitrification process. After start-up the nitrifying biomass in the reactors gradually increased up to 30 g VSS/L. Due to this increased biomass concentration the gas-liquid mass transfer coefficient was negatively influenced. The resulting gradual decrease in dissolved oxygen concentration (over a 2-month period) was associated with a concomitantly nitrite build-up. Short term experiments showed a similar relation between dissolved oxygen concentration (DO) and nitrite accumulation. It was possible to obtain full ammonium conversion with approximately 50% nitrate and 50% nitrite in the effluent. The facts that (i) nitrite build up occurred only when DO dropped, (ii) the nitrite formation was stable over long periods, and (iii) fully depending on DO levels in short term experiments, led to the conclusion that it was not affected by microbial adaptations but associated with intrinsic characteristics of the microbial growth system. A simple biofilm model based on the often reported difference of oxygen affinity between ammonium and nitrite oxydizers was capable of adequately describing the phenomena. Measurements of biomass density and concentration are critical for the interpretation of the results, but highly sensitive to sampling procedures. Therefore we have developed an independent method, based on the residence time of Dextran Blue, to check the experimental methods. There was a good agreement between procedures. The relation between biomass concentration, oxygen mass transfer rate and nitrification in a BAS reactor is discussed. © 1997 John Wiley & Sons, Inc.

Influence of dissolved oxygen concentration on nitrite accumulation in a biofilm airlift suspension reactor

The biofilm airlift suspension (BAS) reactor can treat wastewater at a high volumetric loading rate combined with a low sludge loading. Two BAS reactors were operated, with an ammonium load of 5 kg N/(m(3) d), in order to study the influence of biomass and oxygen concentration on the nitrification process. After start-up the nitrifying biomass in the reactors gradually increased up to 30 g VSS/L. Due to this increased biomass concentration the gas-liquid mass transfer coefficient was negatively influenced. The resulting gradual decrease in dissolved oxygen concentration (over a 2-month period) was associated with a concomitantly nitrite build-up. Short term experiments showed a similar relation between dissolved oxygen concentration (DO) and nitrite accumulation. It was possible to obtain full ammonium conversion with approximately 50% nitrate and 50% nitrite in the effluent. The facts that (i) nitrite build up occurred only when DO dropped, (ii) the nitrite formation was stable over long periods, and (iii) fully depending on DO levels in short term experiments, led to the conclusion that it was not affected by microbial adaptations but associated with intrinsic characteristics of the microbial growth system. A simple biofilm model based on the often reported difference of oxygen affinity between ammonium and nitrite oxydizers was capable of adequately describing the phenomena.Measurements of biomass density and concentration are critical for the interpretation of the results, but highly sensitive to sampling procedures. Therefore we have developed an independent method, based on the residence time of Dextran Blue, to check the experimental methods. There was a good agreement between procedures.The relation between biomass concentration, oxygen mass transfer rate and nitrification in a BAS reactor is discussed. (c) 1997 John Wiley & Sons, Inc.

Biological-ion exchange process for ammonium removal from secondary effluent

A new concept for ammonium removal from secondary effluent by zeolite followed by bio-regeneration has been studied. In contrast to other studies of hybrid biological-ion exchange multireactor systems, the proposed process uses the ion exchange material, zeolite, as the carrier for the nitrifying biomass. This enables the two mode process to be carried out in a single reactor. In the first mode (ion exchange), secondary effluent is passed through an ion exchange column where ammonium is concentrated in the zeolite. During the second mode (bioregeneration), the absorbed ammonium is released gradually and converted to nitrate by the active biomass residing on the zeolite. Nitrification is carried out batchwise and in a small volume reactor where optimal conditions can easily be maintained. Moreover, the addition of chemicals for the desorption of ammonium is minimal due to regenerant reuse during several cycles of nitrification. As a result, operational costs and production of large volumes of brine are minimized. Batch and breakthrough experiments showed that the amount of ammonium adsorbed on the chabazite is strongly affected by the presence of competing cations present in secondary effluent. A reduction of about 75% was observed when using a typical Israeli sewage ion composition. The attached biomass did not significantly effect the efficiency of the ion exchange column. Ammonium desorption experiments showed that regeneration with 10,000 mg/L Na+ is much faster than with 2440 mg/L (more than 90% ammonium recovery after 40 and 70 bed volumes, respectively). A nitrification rate of 6 g NH4-N/(L reactor *day) was obtained in a fluidized bed reactor with chabazite as the carrier. Although this rate is in the high range of reported values for biofilm reactors, desorption experiments proved that nitrification will be the process's rate limiting step, rather than the desorption rate when regenerant solutions as low as 2440 mg/L Na+ are used.

Biological-ion exchange process for ammonium removal from secondary effluent

A new concept for ammonium removal from secondary effluent by zeolite followed by bio-regeneration has been studied. In contrast to other studies of hybrid biological-ion exchange multireactor systems, the proposed process uses the ion exchange material, zeolite, as the carrier for the nitrifying biomass. This enables the two mode process to be carried out in a single reactor.In the first mode (ion exchange), secondary effluent is passed through an ion exchange column where ammonium is concentrated in the zeolite. During the second mode (bioregeneration), the absorbed ammonium is released gradually and converted to nitrate by the active biomass residing on the zeolite. Nitrification is carried out batchwise and in a small volume reactor where optimal conditions can easily be maintained. Moreover, the addition of chemicals for the desorption of ammonium is minimal due to regenerant reuse during several cycles of nitrification. As a result, operational costs and production of large volumes of brine are minimized.Batch and breakthrough experiments showed that the amount of ammonium adsorbed on the chabazite is strongly affected by the presence of competing cations present in secondary effluent. A reduction of about 75% was observed when using a typical Israeli sewage ion composition. The attached biomass did not significantly effect the efficiency of the ion exchange column.Ammonium desorption experiments showed that regeneration with 10,000 mg/L Na+ is much faster than with 2440 mg/L (more than 90% ammonium recovery after 40 and 70 bed volumes, respectively).A nitrification rate of 6 g NH4-N/(L reactor *day) was obtained in a fluidized bed reactor with chabazite as the carrier. Although this rate is in the high range of reported values for biofilm reactors, desorption experiments proved that nitrification will be the process's rate limiting step, rather than the desorption rate when regenerant solutions as low as 2440 mg/L Na+ are used.

Estimation of the active nitrifying biomass in activated sludge

An in situ nitrifier mass estimation technique was developed to estimate the nitrifier population within an activated sludge sample using dominant cultures of nitrifying organisms. This technique was applied to activated sludge samples from the Milton, Ontario Wastewater Treatment Plant and the results were compared to nitrifier population estimates calculated using the IAWPRC (now IAWQ) Activated Sludge Model No. 1 (ASM1). Several sets of nitrification model parameters were used, including values calculated in this investigation from dominant cultures of nitrifier organisms as well as the ASM1 default estimates. The trends in active nitrifier population measured by the in situ mass estimation technique (MET) were predicted by the model simulations when the parameter estimates developed in this study were used. There was good agreement and no statistical difference existed between the model estimates and the experimentally determined masses. The current ASM1 default parameters did not predict the trends in the active population as measured by MET.

Temperature effect on nitrifying bacteria activity in biofilters: activation and free ammonia inhibition

Nitrifying bacteria activity and concentrations depend on specific free ammonia concentration (ratio NH3/biomass), that is a function of temperature, pH, ammonium concentration and nitrifying biomass concentration. So, temperature is a key parameter in the nitrification process producing two opposite effects: bacteria activation and free ammonia inhibition. These phenomena are studied in an up-flow biological aerated filter (UBAF) settled by a nitrifying biofilm (measured as Volatile Attached Solids, VAS). The plug flow allows to disclosure of both effects, activation and inhibition. For Nitrosomonas bacteria only an activation effect was observed; their activity reaches a maximum at 28-29 °C. For Nitrobacter the free ammonia inhibition prevails against the activation effect for values greater than 1 mg N-NH3/mg VAS allowing nitrite accumulation of 80%; this inhibition threshold value for nitrifying biofilm is obtained measuring the specific rate of utilization of substratum per unit of biomass (μmax/Y) by activity test. The knowledge of this threshold in a biofilm process is fundamental in order to control the nitrite accumulation in nitrifying biofilm reactors.

Dynamics of population and biofilm structure in the biofilm airlift suspension reactor for carbon and nitrogen removal

The dynamics of population and biofilm structure of nitrifying and heterotrophic biomass in biofilms on small suspended particles in an airlift reactor were measured during shifts from purely nitrification to a heterotrophic medium and back to nitrification. Biofilms from a full scale reactor with predominantly heterotrophic activity were used as start material. In the first twenty days of the nitrification period ammonia was oxidized to nitrite. Hereafter the oxidation was mainly to nitrate. A conversion of nearly 5 kgN/(m3 d) was reached in fifty days. Following the change to heterotrophic medium the nitrifying biofilm served as carrier for the development of a heterotrophic biofilm layer. The nitrification capacity of the biofilms dropped to 1 kgN/(m3 d) due to oxygen diffusion limitation in the heterotrophic layer. After the switch back to nitrification the heterotrophic biofilm layer was sheared off very rapidly, while the nitrification activity increased very fast to the level at the end of the first nitrification period due to decreased diffusion limitation.

Simulation of ecological controls on nitrification

The accurate representation of nitrification in mathematical models is necessary to account for N distribution and transport through simulated ecosystems. Because of the complexity with which environmental factors interact to control nitrification, these representations should be based upon the kinetics of the limiting reactions mediated by microbial nitrifiers, rather than upon those of NO − 3 formation as is currently done. In the nitrification model proposed here, algorithms for autotrophic nitrifier activity and growth are coupled to ones for substrate production and transport under dynamic conditions of water, temperature and NH + 4 concentration. This model is used to reproduce the time course of nitrifier biomass and activity following different NH + 4amendments under different CO 2 concentrations, temperatures and water contents in a manner consistent with literature reports. Rates of NO − 3 formation arising from simulated nitrifier activity are comparable to those measured during incubations of NH + 4-amended soil under common conditions of temperature and water content. However, the rigour of comparison is limited by the detail with which soil conditions are reported, and by the temporal resolution with which NO − 3 formation is measured, from most nitrification experiments.