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The contribution of EcM fungi to forest denitrification has been over-looked, despite the effects EcM fungi have on soil nitrate and C availability, two important factors controlling N₂O emissions.  Although fungal denitrification has been proposed as a significant source of N₂O in forest soils, the ability for EcM fungi to denitrify is unknown.  Here, I test the hypotheses that EcM fungi regulate forest N₂O production both indirectly by supplying free-living microbes with C through exudation and mycelial turnover, and directly by denitrifying themselves. Soil incubations demonstrated the importance of the quality of C sources released by EcM mycelium in driving denitrification.  Comparing N₂O production from EcM and non-mycorrhizal seedlings showed the potential for the presence of EcM mycelia to increase reduction of ¹⁵N-nitrate to ¹⁵N-N₂O and ¹⁵N-N₂.  The link to EcM C was confirmed in bacterial culture: denitrification by <i>Paracoccus denitrificans </i>1222 was greatest when using C from extracts of dead mycelium of <i>Paxillus involutus </i>and extrudates produced when the fungus was in symbiosis with a host plant.  Therefore, EcM fungi indirectly increase denitrification by providing high quality C.  The potential for a direct contribution by EcM fungi to N₂O production under oxygen-limited conditions was demonstrated in pure cultures of <i>Tylospora fibrillosa </i>and <i>Paxillus involutus.  </i>These findings enabled me to develop a schematic model describing the ecological significance of the role of EcM fungi in denitrification in relation to inorganic N availability.  Overall, my work provides the first evidence that EcM fungi have the potential to play a key role in N₂O emissions.

There has been little work done to investigate the importance of denitrification in forest soils. This has been caused by difficulties associated with measurement of the denitrification process and from the assumption that nitrification, and hence also denitrification, was insignificant in acid environments. Nitrification can, however, occur even in the acid conditions found in coniferous forest soils, and is especially important after clear-felling when levels of nitrate in soil and drainage waters are often observed to increase. A potential exists, therefore, for gaseous losses of N <i>via</i> denitrification from such soils. This thesis describes the establishment of a suitable method for measurement of denitrification using the acetylene block technique. This method was used to monitor denitrification losses of N, both as N<SUB>2</SUB> and N<SUB>2</SUB>O, from a peaty-gley soil at Kershope Forest. The total loss of N from the standing forest through denitrification was estimated to be 3.2 kg ha<SUP>-1</SUP> over the year studied. Of this loss, approximately 80% was produced as H<SUB>2</SUB>O. Gaseous loss of N through denitrification represents approximately the same order of magnitude as the N lost from the site <i>via</i> leaching. An adjacent site, clear-felled four years previously, was also monitored for denitrification losses. Although this site was denitrifying at only a slightly greater rate than the standing forest, examination of past records from the site revealed that an estimated 9-40 kg N ha<SUP>-1</SUP>y<SUP>-1</SUP> had been lost in the two years immediately after felling. To assess the factors which controlled denitrification in the field, sub-samples of the soils used for denitrification measurements were analysed for water content, extractable nitrate, and available carbon. None of these factors, however, were found to correlate clearly with the denitrification rate. Further experiments, using laboratory amendments of soil cores, indicated that nitrate concentrations had the greatest effect on denitrification rates, although both the availability of carbon and the aeration status of the soil also affected the rates measured.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

Thesis (Ph. D.)--University of Rhode Island, 2005. / Typescript. Includes bibliographical references (leaves 130-159).

Chapter I presents general information about landfill leachate, characteristics of leachates in Vietnam and review of general leachates treatment situation in the country. In chapter II, a careful bibliographical study on biological processes of nitrification and denitrification is done. In chapter III, existing activated sludge models are briefly reviewed, focusing on ASM1 and ASM3. The ASM3 model then is studied in more detail with focuses on state variables, processes; kinetic and stoichiometric parameters of the model. A careful bibliographical study on sequencing batch reactor (SBR) is done in chapter IV. Chapter V presents materials and methods that will be applied in the experiments in laboratories and modelling processes of this study. In chapter VI, an SBR bench-scale is set up in the laboratory to study partial nitrification process. Chapter VII presents the experimental studies on maximum nitrification and denitrification capability, then determination of kinetic and stoichiometric parameters that will be used for calibration in the next steps. Chapter VIII presents a study on partial nitrification by applying data analysis and experimental planning method. In chapter IX (the key part of the Thesis), the modelling of the partial nitrification and denitrification in SBR is presented. It is hoped that, this study will contribute to the major issue of leachate treatment in Vietnam, especially in the North of the country where leachate characteristics and variations are the same as what was used during our experiments. Partial nitrification seems to be easily achieved in an SBR bench-scale using leachate in Nam Son landfill site. Some important characteristics of the studied leachate, are high alkalinity, high pH leading to high free ammonia concentration in the system. This free ammonia is known as a growth rate inhibitor for nitrite oxidizing bacteria, thus limiting oxidation of nitrite to nitrate and accumulating nitrite during the nitrification period. DO concentration is also known as an important influencing factor in partial nitrification in many previous studies. But in our case, its influence is just significant when the nitrification process is nearly complete: no more ammonium remains in the system, alkalinity concentration is reduced leading to a lower buffer capacity, lower pH, and then nitrite is easily oxidized to nitrate. A sufficiently high DO concentration in this case, expresses its importance in bringing about the best nitrification efficiency, while saving aeration energy. The SBR technique has demonstrated its advantages, especially the flexibility in changing the working volume, and the operating time. Modeling of partial nitrification and denitrification processes for landfill leachate treatment using the SBR technique was the main objective of this study. The simulation software - WEST® program was very useful tool to implement this task. With this program, the available model base for activated sludge model (ASM1, ASM 2, ASM 3 etc,), presented in the Peterson matrix, the variables, kinetic, stoichiometric parameters, processes can be easily modified to another activated sludge model suitable in the scope of our study. In the present case, based on the ASM3, the ASM3_2step was developed and applied, in which nitrification and denitrification are divided into two steps with nitrite as an intermediate product. The modified ASM3_2step has shown its high accuracy during calibration process. It could be use also for the other processes/techniques using activated sludge, by adding more equations and parameters. Calibration and validation were implemented for two cases: Partial nitrification and denitrification with and without carbon addition. Good results were obtained where the simulations fit well the experimental data. The kinetic and stoichiometric parameters found are very important for the other simulations, especially in process optimisation. It also demonstrates that, through process optimisation, general productivity of the SBR system can be increased. Controlling DO, changing operating time cycle mechanisms can improve the total nitrogen removal efficiency, save some aeration energy for nitrification and carbon source for denitrification. As our results are very promising, the next step could be to implement the ANAMMOX process. Key words: Partial nitrification and denitrification, ASM3_2steps, SBR, modeling.

The greenhouse gas nitrous oxide (N2O) is produced mainly by the microbial processes of nitrification and denitrification. I hypothesized that microbial community structure (composition and abundance) is linked to differences in soil N2O emissions from these two processes. Microbial community composition (type and number of nitrifier and denitrifier genotypes), abundance and N2O emission activity were determined and compared for soils from two landscapes characteristic of the North American prairie pothole region (cultivated vs. uncultivated wetlands). The landscape difference in composition of individual microbial communities was not predictive of soil N2O emissions, indicating that there is redundancy in each microbial community in relation to N2O emission activities. However, community factors influenced the pattern and distribution of N2O emission from the soils of the study site. For example, nitrification was the dominant N2O emitting process for soils of all landforms. However, neither nitrifier amoA abundance nor community composition had predictive relationships with nitrification associated N2O emissions. This lack of relationship may be a consequence of using amoA as the gene target to characterize nitrifiers. For denitrifying bacteria, there was a temporal relationship between community composition and N2O emissions. However, this may be related to the change in water-filled pore space over time. Alternatively, the presence of fungi can be linked directly to N2O emissions from water accumulating landform elements. Under hypoxic conditions, there may be two fungal pathways contributing to N2O release: fungal denitrification via P450nor and fungal heterotrophic nitrification. Results suggest that the relative importance of these two processes is linked to root exudates such as formate. It is the interaction between the seasonal fluctuations of the microbial and environmental factors that determine the level of N2O emissions from soils.

Best or beneficial management practices (BMPs) are often relied upon as a mitigation strategy for nitrate contamination throughout Canada. At a regional scale, reducing the quantity of nutrients applied to agricultural land is one BMP approach that has been implemented internationally. While these BMP strategies have been proven to successfully reduce the environmental impact of agriculture on water systems, the time interval between BMP implementation and a noticeable improvement in groundwater quality can be quite extensive. This lag time has been observed at the agriculturally impacted Thornton Well Field in Oxford County. Despite seven years of significant reductions in fertilizer application within the capture zone of this municipal well field, declining nitrate concentrations have yet to be observed in the production water wells. In order to accelerate nitrate reductions at the Thornton Well Field, an integrated approach, combining BMPs with a stimulated in situ denitrification strategy, was implemented. This research focused on the use of a cross-injection scheme to stimulate in situ denitrification within the production aquifer units, up-gradient of the Thornton Well Field. Briefly, this strategy involves injecting a carbon source and electron donor into a high flux aquifer zone using an injection and extraction system positioned perpendicular to the regional flow field. Through altering the geochemical conditions, the injections stimulate indigenous bacteria to reduce harmful nitrate to innocuous dinitrogen gas. The main objectives of this research included: characterizing the hydrogeologic and geochemical properties of the target aquifer; pilot scale testing of the proposed in situ denitrification system; and suggesting an approach for up-scaling to a full-scale treatment scheme capable of remediating the elevated nitrate concentrations at the Thornton Well Field. Core logging, electrical resistivity studies, several methods of hydraulic characterization, tracer testing, and three-dimensional groundwater modelling were used to quantify the physical properties of the target aquifer and to develop a hydrogeologic conceptual model of the site. The aquifer unit was found to be unconfined in the experiment vicinity, consisting of a complex system of six main hydrostratigraphic layers of sand and gravel featuring variable hydraulic conductivity (K) values. Despite the hydrogeologic complexity, the geochemical properties of the aquifer were relatively uniform with depth. Anion, cation, alkalinity, pH, dissolved oxygen, and nitrous oxide data all contributed to this conjecture. Of particular interest, however, were the elevated dissolved oxygen concentrations, which rivalled atmospheric saturation throughout the entire aquifer sequence. The background physical and chemical characterization identified two main challenges that would potentially influence the performance of the in situ denitrification process: stimulating uniform denitrification in the fast flowing, complex aquifer system and overcoming the elevated oxygen concentrations to achieve the necessary anaerobic conditions. Following the initial site characterization phase, several preliminary cross-injection experiments were designed and performed. These experiments featured an injection-extraction circulation cycle which spanned five metres and was operated normal to groundwater flow. Acetate was selected as the electron donor and carbon substrate. The first test involved a single acetate injection followed by an extensive period of groundwater sampling. Unfortunately, this initial test provided no indication of stimulated in situ denitrification. All anion, cation, and nitrous oxide concentration and isotope data collected during and following this injection remained within the range of background estimates. Following the first injection experiment, a subsequent test involving multiple, repetitive acetate injections was implemented to overcome the highly aerobic nature of the aquifer and support the growth and reproduction of denitrifying bacterial populations. The second injection phase included 19 individual injections that were operated at intervals of every day to every other day over a total period of 26 days. These injections successfully lowered the dissolved oxygen concentrations within the target aquifer to an average range of 0 to 4 mg/L. The least conductive layers featured the lowest oxygen concentrations, while the higher K layers maintained elevated oxygen concentrations. The nitrite, nitrate, and enriched NO3-15N and NO3-18O isotope data suggested a high degree of stimulated denitrification in the least conductive layers and a limited degree in the high-K layers. The lower-K units corresponding to multi-level well ports ML7-2, ML7-5, and ML7-6 achieved a 46 percent reduction in nitrate, while the layer represented by ML7-1 attained a 100 percent reduction in nitrate. Alternatively, due to the constant influx of dissolved oxygen and limited residence times, very little denitrification was observed in the fast flowing layers corresponding to ports ML7-3, ML7-4, and ML7-7. Overall, a percent reduction, in terms of nitrate mass crossing the 5-m wide treatment lens, of only eleven percent was calculated. These results clearly demonstrate that the K-profile had a significant impact on stimulating in situ bioremediation. Two major system challenges were observed, including an inability to successfully stimulate denitrification within the highly permeable layers and the generation of harmful nitrite at nearly all aquifer depths. Based on these significant challenges, it was concluded that additional experimentation is required before this remediation technique can be expanded to a full-scale in situ treatment scheme. The most significant recommendation requested the development and execution of a third injection phase, consisting of multiple, consecutive substrate injections designed to systematically test various pulsing intervals, injection concentrations, and electron donors. Despite the current limitations, this approach has great potential. It is believed that with additional research, the in situ stimulation of denitrification could be used to successfully reduce the elevated nitrate concentrations at the Thornton Well Field.

Best or beneficial management practices (BMPs) are often relied upon as a mitigation strategy for nitrate contamination throughout Canada. At a regional scale, reducing the quantity of nutrients applied to agricultural land is one BMP approach that has been implemented internationally. While these BMP strategies have been proven to successfully reduce the environmental impact of agriculture on water systems, the time interval between BMP implementation and a noticeable improvement in groundwater quality can be quite extensive. This lag time has been observed at the agriculturally impacted Thornton Well Field in Oxford County. Despite seven years of significant reductions in fertilizer application within the capture zone of this municipal well field, declining nitrate concentrations have yet to be observed in the production water wells. In order to accelerate nitrate reductions at the Thornton Well Field, an integrated approach, combining BMPs with a stimulated in situ denitrification strategy, was implemented. This research focused on the use of a cross-injection scheme to stimulate in situ denitrification within the production aquifer units, up-gradient of the Thornton Well Field. Briefly, this strategy involves injecting a carbon source and electron donor into a high flux aquifer zone using an injection and extraction system positioned perpendicular to the regional flow field. Through altering the geochemical conditions, the injections stimulate indigenous bacteria to reduce harmful nitrate to innocuous dinitrogen gas. The main objectives of this research included: characterizing the hydrogeologic and geochemical properties of the target aquifer; pilot scale testing of the proposed in situ denitrification system; and suggesting an approach for up-scaling to a full-scale treatment scheme capable of remediating the elevated nitrate concentrations at the Thornton Well Field. Core logging, electrical resistivity studies, several methods of hydraulic characterization, tracer testing, and three-dimensional groundwater modelling were used to quantify the physical properties of the target aquifer and to develop a hydrogeologic conceptual model of the site. The aquifer unit was found to be unconfined in the experiment vicinity, consisting of a complex system of six main hydrostratigraphic layers of sand and gravel featuring variable hydraulic conductivity (K) values. Despite the hydrogeologic complexity, the geochemical properties of the aquifer were relatively uniform with depth. Anion, cation, alkalinity, pH, dissolved oxygen, and nitrous oxide data all contributed to this conjecture. Of particular interest, however, were the elevated dissolved oxygen concentrations, which rivalled atmospheric saturation throughout the entire aquifer sequence. The background physical and chemical characterization identified two main challenges that would potentially influence the performance of the in situ denitrification process: stimulating uniform denitrification in the fast flowing, complex aquifer system and overcoming the elevated oxygen concentrations to achieve the necessary anaerobic conditions. Following the initial site characterization phase, several preliminary cross-injection experiments were designed and performed. These experiments featured an injection-extraction circulation cycle which spanned five metres and was operated normal to groundwater flow. Acetate was selected as the electron donor and carbon substrate. The first test involved a single acetate injection followed by an extensive period of groundwater sampling. Unfortunately, this initial test provided no indication of stimulated in situ denitrification. All anion, cation, and nitrous oxide concentration and isotope data collected during and following this injection remained within the range of background estimates. Following the first injection experiment, a subsequent test involving multiple, repetitive acetate injections was implemented to overcome the highly aerobic nature of the aquifer and support the growth and reproduction of denitrifying bacterial populations. The second injection phase included 19 individual injections that were operated at intervals of every day to every other day over a total period of 26 days. These injections successfully lowered the dissolved oxygen concentrations within the target aquifer to an average range of 0 to 4 mg/L. The least conductive layers featured the lowest oxygen concentrations, while the higher K layers maintained elevated oxygen concentrations. The nitrite, nitrate, and enriched NO3-15N and NO3-18O isotope data suggested a high degree of stimulated denitrification in the least conductive layers and a limited degree in the high-K layers. The lower-K units corresponding to multi-level well ports ML7-2, ML7-5, and ML7-6 achieved a 46 percent reduction in nitrate, while the layer represented by ML7-1 attained a 100 percent reduction in nitrate. Alternatively, due to the constant influx of dissolved oxygen and limited residence times, very little denitrification was observed in the fast flowing layers corresponding to ports ML7-3, ML7-4, and ML7-7. Overall, a percent reduction, in terms of nitrate mass crossing the 5-m wide treatment lens, of only eleven percent was calculated. These results clearly demonstrate that the K-profile had a significant impact on stimulating in situ bioremediation. Two major system challenges were observed, including an inability to successfully stimulate denitrification within the highly permeable layers and the generation of harmful nitrite at nearly all aquifer depths. Based on these significant challenges, it was concluded that additional experimentation is required before this remediation technique can be expanded to a full-scale in situ treatment scheme. The most significant recommendation requested the development and execution of a third injection phase, consisting of multiple, consecutive substrate injections designed to systematically test various pulsing intervals, injection concentrations, and electron donors. Despite the current limitations, this approach has great potential. It is believed that with additional research, the in situ stimulation of denitrification could be used to successfully reduce the elevated nitrate concentrations at the Thornton Well Field.