Laboratory assessment of the effect of cattle slurry pre-treatment on organic N degradation after soil application and N<Subscript>2</Subscript>O and N<Subscript>2</Subscript> emissions

Slurry separation using mechanical and chemical methods is one of the options considered to solve problems of slurry management at the farm scale. The fractions obtained with such treatments have distinct compositions, which allow different options for their utilization (composting, direct application, and fertigation). In this study, four fractions of slurry were obtained using a combined treatment system including slurry treatment with a screw press separator (solid and liquid fractions) followed by sedimentation with the addition of Polyacrylamide (PAM) (PAM-Supernatant and PAM-Sediment) to the LF. These fractions were then incorporated into arable soil under controlled laboratory conditions and the organic N degradation from each treatment was followed for 94 days. Total N emissions (N₂O + N₂) as well as the sources of the N emissions (nitrification or denitrification) were also studied during this period. Results showed that the slurry fractions (SFs) had distinct behavior relative to the whole slurry (WS), namely in terms of N degradation in soil, where N mineralization was observed only in the WS treatment whereas N immobilization occurred in the other treatments. In terms of N₂O emissions, higher losses, expressed as a percentage of the total N added, occurred from the LF treatments (liquid, PAM-Supernatant and PAM-Sediment). This work indicates that the slurry treatment by mechanical and chemical separation may be a good option for slurry management at the farm scale since it allows greater utilization of the different fractions with a small effect on N₂O emissions after SFs’ application to soil.     

Relationship Between Gross Nitrogen Cycling and Nitrous Oxide Emissionin Grass-clover Pasture

Replacement of high-input N fertilized pastures with low-input grass-legume pastures may provide a mitigation option to reduce agricultural N₂O emissions. This study examined the relationship between N-cycling rates and N₂O production and evolution from the root zone of grass-clover pastures of various ages (production year 1, 2 and 8). The experimental approach included cross-labelling pasture monoliths with ¹⁵N-enriched substrates to identify sources of N₂O, in combination with assessment of gross N mineralization and nitrification. Nitrous oxide emissions were generally low, fluctuating between 82 and 136μg N₂O–N m⁻² d⁻¹, independent of pasture age. The ¹⁵N labelling indicated that at least 50% of the N₂O was derived from the soil NH₄⁺ pool, approaching 100% in June. In the two year old pasture the NH₄⁺ pool contributions to N₂O emissions varied significantly with sampling time. Emission rates of N₂O correlated positively with soil NH₄⁺ concentrations and the NH₄⁺ supply as expressed by gross mineralization. The N₂O emissions showed a significant inverse relationship with soil NO₃⁻, but was not correlated with the supply of NO₃⁻ as expressed by gross nitrification. The ratio N₂O vs. nitrification averaged 0.05% (range 0.004 to 0.29%) and varied with sampling time showing the lowest value in wet soil conditions.     

Nitrous oxide emissions for 6 years from a gray lowland soil cultivated with onions in Hokkaido, Japan

We studied nitrous oxide (N₂O) emissions every growing season (April to October) for 6 years (19952000), in a Gray Lowland soil cultivated with onions in central Hokkaido, Japan. Emission of N₂O from the onion field ranged from 0.00 to 1.86 mgN m^–2 h^–1. The seasonal pattern of N₂O emission was the same for 6 years. The largest N₂O emissions appeared near harvesting in August to October, and not, as might be expected, just after fertilization in May. The seasonal patterns of soil nitrate (NO₃ ^–) and, ammonium (NH₄ ⁺) levels and the ratio of N₂O to NO emission indicated that the main process of N₂O production after fertilization was nitrification, and the main process of N₂O production around harvest time was denitrification. N₂O emission was strongly influenced by the drying–wetting process of the soil, as well as by the high soil water content. The annual N₂O emission during the growing season ranged from 3.5 to 15.6 kgN ha^–1. The annual nitrogen loss by N₂O emission as a percentage of fertilizer-N ranged from 1.1 to 6.4%. About 70% of the annual N₂O emission occurred near harvesting in August to October, and less than 20% occurred just after fertilization in May to July. High N₂O fluxes around the harvesting stage and a high proportion of N₂O emission to total fertilizer-N appeared to be probably a characteristic of the study area located in central Hokkaido, Japan.     

Solids and Hydraulic Retention Time Effect on N2O Emission from Moving‐Bed Membrane Bioreactors

Biological nutrient removal was operated at different solids (SRT) and hydraulic retention times (HRT) in order to assess their influence on nitrous oxide (N₂O) emission from a hybrid moving‐bed membrane bioreactor. The observed results show that the N₂O production decreased when the SRT/HRT was decreased. The maximum N₂O gaseous concentration was measured in the aerobic reactor at the end of phase I, and it decreased through phases II and III. From mass balances over the reactors of the system, the aerated (aerobic and membrane) reactors were the largest producers of N₂O, showing that the greater part of N₂O was produced during the nitrification process.     

Anthropogenic emissions of NOX, NH3 and N2O in India

Emissions of NO_x, NH₃ and N₂O from anthropogenic activities in India have been estimated based on actual field measurements as well as available default methodologies. The NO_x emissions are mainly from the transport sector and contribute about 5% of the global NO_x emission from fossil fuel. NH₃ emissions from urea seems to be highly uncertain. However, emissions of NH₃ from fertilizers and livestock are estimated to be 1175 Gg and 1433 Gg, respectively. N₂O emissions seem to be derived predominantly from fertilizer applications, resulting in the release of 199–279 Gg N₂O. Other sources of N₂O, viz. agricultural residue burning, biomass burning for energy and nitric acid production are estimated to be 3, 35–187 and 2–7 Gg, respectively.     

Fungal and bacterial contributions to codenitrification emissions of N<Subscript>2</Subscript>O and N<Subscript>2</Subscript> following urea deposition to soil

Grazed pastures contribute significantly to anthropogenic emissions of N₂O but the respective contributions of archaea, bacteria and fungi to codenitrification in such systems is unresolved. This study examined the relative contributions of bacteria and fungi to rates of denitrification and codenitrification under a simulated ruminant urine event. It was hypothesised that fungi would be primarily responsible for both codenitrification and total N₂O and N₂ emissions. The effects of bacterial (streptomycin), fungal (cycloheximide), and combined inhibitor treatments were measured in a laboratory mesocosm experiment, on soil that had received ¹⁵N labelled urea. Soil inorganic-N concentrations, N₂O and N₂ gas fluxes were measured over 51 days. On Days 42 and 51, when nitrification was actively proceeding in the positive control, the inhibitor treatments inhibited nitrification as evidenced by increased soil NH ₄ ⁺ -N concentrations and decreased soil NO ₂ ^? -N and NO ₃ ^? -N concentrations. Codenitrification was observed to contribute to total fluxes of both N₂O (≥ 33%) and N₂ (≥ 3%) in urine-amended grassland soils. Cycloheximide inhibition decreased NH ₄ ⁺ –¹⁵N enrichment and reduced N₂O fluxes while reducing the contribution of codenitrification to total N₂O fluxes by ≥ 66 and ≥ 42%, respectively. Thus, given archaea do not respond to significant urea deposition, it is proposed that fungi, not bacteria, dominated total N₂O fluxes, and the codenitrification N₂O fluxes, from a simulated urine amended pasture soil.     

Mitigation of N<Subscript>2</Subscript>O and CH<Subscript>4</Subscript> Emission from Rice and Wheat Cropping Systems Using Dicyandiamide and Hydroquinone

Agriculture contributes considerably to the emission of greenhouse gases, such as N₂O and CH₄. Here we summarize results from previous pot experiments assessing the effectiveness of urease and nitrification inhibitors reducing both N₂O and CH₄ emissions from wheat and rice cropping systems fertilized with urea (U). For the wheat cropping system, using a cambisol, we observed that the application of U with hydroquinone (HQ, a urease inhibitor), U with dicyandiamide (DCD, a nitrification inhibitor) and U with HQ plus DCD decreased the N₂O emissions by 11.4, 22.3 and 25.1%, respectively. For the rice copping system, using a luvisol, we found that the application of U with HQ, U with DCD and U with HQ plus DCD decreased N₂O emissions by 10.6, 47.0 and 62.3%, respectively, and CH₄ emissions by 30.1, 53.1 and 58.3%, respectively. In terms of total global warming potential (GWP) a reduction of 61.2% could be realized via the combined addition of HQ and DCD. The addition of wheat straw reduced the activity of HQ and DCD in the rice cropping experiments. In terms of total GWP only a reduction of 30.7% could be achieved. In general, both in upland and flooded conditions, the application of HQ and DCD alone was less effective than HQ in combination with DCD, but not significantly for U plus DCD treatment. Our observations may be further constrained, however, by practical, economic or social problems and should therefore be tested at the scale of a region (e.g. a watershed) and related to an integrated abatement of agricultural N losses.     

Nitrogen dynamics during till and no-till pasture restoration sequences in Rondônia,Brazil

The clearing of tropical rain forest in the Amazon basin has created large areas of cattle pasture that are now declining in productivity. Practices adopted by ranchers to restore productivity to degraded pastures have the potential to alter soil N availability and gaseous N losses from soils. We examined how soil inorganic N pools, net N mineralization and net nitrification rates, nitrification potential and NO and N₂O emissions from soils of a degraded pasture responded to the following restoration treatments: (1) soil tillage followed by replanting of grass and fertilization, (2) no-till application of non-selective herbicide, planting of rice, harvest followed by no-till replanting of grass and fertilization, and (3) the same no-till sequence with soybeans instead of rice. Tillage increased soil NH₄⁺ and NO₃^? pools but NH₄⁺ and NO₃^? pools remained relatively constant in the control and no-till treatments. Cumulative rates of net N mineralization and net nitrification during the first 6 months after treatment varied widely but were hightest in the tilled treatment. Emissions of NO and N₂O fluxes increased with tillage and with N fertilization. There were no clear relationships among rates of N fertilizer application, net N mineralization, net nitrification, NO, N₂O and total N oxide emissions. Our results indicate that pasture restoration sequences involving tilling and fertilizing will increase emissions of N oxides, but the magnitude of the increase is likely to differ based on timing of fertilizer application relative to the presence of plants and the magnitude of plant N demand. Emissions of N oxides appear to be decreased by the use of restoration sequences that minimize reductions in pasture grass cover.     

Assessment of CH4 and N2O fluxes in a Danish beech (Fagus sylvatica) forest and an adjacent N-fertilised barley (Hordeum vulgare) field: effects of sewage sludge amendments

Fluxes of CH₄ and N₂O were measured regularly in an agricultural field treated with 280 g m⁻² of sewage sludge. In a nearby beech forest N₂O and CH₄ fluxes were measured in a well-drained (dry) area and in a wet area adjacent to a drainage canal. We observed brief increases of both CH₄ and N₂O emissions immediately following soil applications of digested sewage sludge. Cumulated values for CH₄ emissions over the course of 328 days after sludge applications indicated a small net source in sludge treated plots (7.6 mg C m⁻²) whereas sludge-free soil constituted a small sink (-0.9 mg C m⁻²). The CH₄ emission amounted 0.01% of the sludge-C. Extrapolated to current rates of sludge applications in Danish agriculture this amounts to 0.1% of the total agricultural derived CH₄. Sludge applications did not affect cumulated fluxes of N₂O showing 312 mg N₂O–N m⁻² and 304 mg N m⁻² with and without sludge, respectively. Four months after the sludge applications a significant effect on CO₂ and NO emissions was still obvious in the field, the latter perhaps due to elevated nitrification. Nitrous oxide emission in the beech forest was about six times smaller (45 mg N m⁻²) than in the field and independent of drainage status. Methane oxidation was observed all-year round in the forest cumulating to -225 mg C m⁻² and -84 mg C m⁻² in dry and wet areas. In a model experiment with incubated soil cores, nitrogen amendment (NH₄Cl) and perturbation significantly reduced CH₄ oxidation in the forest soil, presumably as a result of increased nitrification activity. Sludge also induced net CH₄ production in the otherwise strong CH₄ oxidising forest soil. This emphasises the potential for CH₄ emissions from sewage sludge applications onto land. The study shows, however, that emissions of N₂O and CH₄ induced by sewage sludge in the field is of minor importance and that factors such as land use (agriculture versus forest) is a much stronger controller on the source/sink strengths of CH₄ and N₂O. This revised version was published online in August 2006 with corrections to the Cover Date.     

Toward Improved Coefficients for Predicting Direct N<Subscript>2</Subscript>O Emissions from Soil in Canadian Agroecosystems

Agricultural soils emit nitrous oxide (N₂O), a potent greenhouse gas. Predicting and mitigating N₂O emissions is not easy. To derive national coefficients for N₂O emissions from soil, we collated over 400 treatment evaluations (measurements) of N₂O fluxes from farming systems in various ecoregions across Canada. A simple linear coefficient for fertilizer-induced emission of N₂O in non-manured soils (1.18% of N applied) was comparable to that used by the Intergovernmental Panel on Climate Change (IPCC) (1.25% of N applied). Emissions were correlated to soil and crop management practices (manure addition, N fertilizer addition and inclusion of legumes in the rotation) as well as to annual precipitation. The effect of tillage on emissions was inconsistent, varying among experiments and even within experiments from year to year. In humid regions (e.g., Eastern Canada) no-tillage tended to enhance N₂O emissions; in arid regions (e.g., Western Prairies) no-tillage sometimes reduced emissions. The variability of N₂O fluxes shows that we cannot yet always distinguish between potential mitigation practices with small (e.g., <10%) differences in emission. Our analysis also emphasizes the need for developing consistent experimental approaches (e.g., ‘control’ treatments) and methodologies (i.e. measurement period lengths) for estimating N₂O emissions.     

Sources of nitrous oxide in soils

Research to identify sources of nitrous oxide (N₂O) in soils has indicated that most, if not all, of the N₂O evolved from soils is produced by biological processes and that little, if any, is produced by chemical processes such as chemodenitrification. Early workers assumed that denitrification was the only biological process responsible for N₂O production in soils and that essentially all of the N₂O evolved from soils was produced through reduction of nitrate by denitrifying microorganisms under anaerobic conditions. It is now well established, however, that nitrifying microorganisms contribute significantly to emissions of N₂O from soils and that most of the N₂O evolved from aerobic soils treated with ammonium or ammonium-yielding fertilizers such as urea is produced during oxidation of ammonium to nitrate by these microorganisms. Support for the conclusion that chemoautotrophic nitrifiers such as Nitrosomonas europaea contribute significantly to production of N₂O in soils treated with N fertilizers has been provided by studies showing that N₂O emissions from such soils can be greatly reduced through addition of nitrification inhibitors such as nitrapyrin, which retard oxidation of ammonium by chemoautotrophic nitrifiers but do not retard reduction of nitrate by denitrifying microorganisms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nitrous oxide emissions from agricultural soils

This paper addresses three topics related to N₂O emissions from agricultural soils. First, an assessment of the current knowledge of N₂O emissions from agricultural soils and the role of agricultural systems in the global N₂O are discussed. Secondly, a critique on the methodology presented in the OECD/OCDE (1991) program on national inventories of N₂O is presented. Finally, technical options for controlling N₂O emissions from agricultural fields are discussed.The amount of N₂O derived from nitrogen applied to agricultural soils from atmospheric deposition, mineral N fertilizer, animal wastes or biologically fixed N, is not accurately known. It is estimated that the world-wide N₂O emitteddirectly from agricultural fields as a result of the deposition of all the above nitrogen sources is 2–3 Tg N annually. This amounts to 20–30% of the total N₂O emitted annually from the earth's surface. An unknown, but probably significant, amount of N₂O is generated indirectly in on and off farm activities associated with food production and consumption.Management options to limitdirect N₂O emissions from N-fertilized soils should emphasize improving N-use efficiency. Such management options include managing irrigation frequency, timing and quantity; applying N only to meet crop demand through multiple applications during the growing season or by using controlled release fertilizers; applying sufficient N only to meet crop needs; or using nitrification inhibitors. Most of these options have not been field tested. Agricultural management practices may not appreciably affect indirect N₂O emissions.     

N2O and NO emissions from different N sources and under a range of soil water contents

Emissions of nitrous oxide (N₂O) and nitric oxide (NO) have been identified as one of the most important sources of atmospheric pollution from grasslands. Soils are major sources for the production of N₂O and NO, which are by-products or intermediate products of microbial nitrification and denitrification processes. Some studies have tried to evaluate the importance of denitrification or nitrification in the formation of N₂O or NO but there are few that have considered emissions of both gases as affected by a wide range of different factors. In this study, the importance of a number of factors (soil moisture, fertiliser type and temperature) was determined for N₂O and NO emissions. Nitrous oxide and NO evolution in time and the possibility of using the ratio NO:N₂O as an indicator for the processes involved were also explored. Dinitrogen (N₂) and ammonia (NH₃) emissions were estimated and a mass balance for N fluxes was performed. Nitrous oxide and NO were produced by nitrification and denitrification in soils fertilised with and by denitrification in soils fertilised with . Water content in the soil was the most important factor affecting N₂O and NO emissions. Our N₂O and NO data were fitted to quadratic (r=0.8) and negative exponential (r=0.7) equations, respectively. A long lag phase was observed for the N₂O emitted from soils fertilised with (denitrification), which was not observed for the soils fertilised with (nitrification) and was possibly due to a greater inhibiting effect of low temperatures on microbial activity controlling denitrification rather than on nitrification. The use of the NO:N₂O ratio as a possible indicator of denitrification or nitrification in the formation of N₂O and NO was discounted for soils fertilised with . The N mass balance indicated that about 50 kg N ha⁻¹ was immobilised by microorganisms and/or taken up by plant roots, and that most of the losses ocurred in wet soils (WFPS >60%) as N₂ and NH₃ losses (>55%).     

Gross mineralization, nitrification and N2O emission under different tillage in the North China Plain

No-tillage cropping can increase soil carbon (C) stocks and aggregation, and subsequently impact the internal nitrogen (N) cycle and gas loss. The ¹⁵N pool dilution method was used to study gross N transformations, and relative proportions of nitrous oxide (N₂O) emissions derived from denitrification versus nitrification-related processes under long-term tillage systems (no-tillage, rotary tillage and conventional tillage) in the North China Plain. In-field incubation experiments were repeated in successive growing seasons during April?CNovember in 2007. Gross mineralization rates for rotary and mouldboard plough tillage (3.6?±?0.3?C10.6?±?1.5?mg?N?kg^?1?days^?1) were significantly higher than for no-tillage (1.7?±?0.8?C6.8?±?1.1?mg?N?kg^?1?days^?1). Gross mineralization was positively correlated with soil moisture and temperature, as well as with microbial biomass N and C. However, there was no consistent tillage effect on gross nitrification, and gross nitrification was positively correlated with soil moisture, but not with gross mineralization and microbial biomass. N₂O emissions were higher in no-tillage (NT) than for conventional tillage (CT) during May?CAugust. The ¹⁵N labelling indicated that 26?C92?% of the N₂O was directly derived from the soil ammonium (NH₄ ⁺) pool. Emission rates of N₂O from both nitrification and denitrification were positively correlated with NH₄ ⁺ supply as expressed by gross mineralization, but not correlated with supply of nitrate as expressed by gross nitrification. The fraction of nitrified N emitted as N₂O was positively correlated with changes in soil moisture and varied within 0.01?C2.51???. Our results showed that the tillage management impact on gross N transformation was not consistent with N₂O emission, and more detailed information on the controls over N₂O formation needs to be sought.     

Ammonia and nitrous oxide emissions from grass and alfalfa mulches

Ammonia (NH₃) and nitrous oxide (N_-2O) emissions were measured in the field for three months from three different herbage mulches and from bare soil, used as a control. The mulches were grass with a low N-content (1.15% N in DM), grass with a high N-content (2.12% N in DM) and alfalfa with a high N-content (4.33% N in DM). NH₃ volatilization was measured using a micrometeorological technique. N_-2O emissions were measured using closed chambers. NH₃ and N_-2O emissions were found to be much higher from the N-rich mulches than from the low-N grass and bare soil, which did not differ significantly. Volatilization losses of NH₃ and N_-2O occurred mainly during the first month after applying the herbage and were highest from wet material shortly after a rain. The extent of NH₃-N losses was difficult to estimate, due to the low frequency of measurements and some problems with the denuder technique, used on the first occasions of measurements. Nevertheless, the results indicate that NH₃-N losses from herbage mulch rich in N can be substantial. Estimated losses of NH₃-N ranged from the equivalent of 17% of the applied N for alfalfa to 39% for high-N grass. These losses not only represent a reduction in the fertilizer value of the mulch, but also contribute appreciably to atmospheric pollution. The estimated loss of N_-2O-N during the measurement period amounted to 1% of the applied N in the N-rich materials, which is equivalent to at least 13 kg N_-2O-N ha^-1 lost from alfalfa and 6 kg ha^-1 lost from high-N grass. These emission values greatly exceed the 0.2 kg N_-2O-N ha^-1 released from bare soil, and thus contribute to greenhouse gas emissions.     

Catalytic decomposition of propellant N2O Over Ir/Al2O3 catalyst

N₂O, as a green propellant alternative to N₂H₄, shows potential application in satellite propulsion system. The state of Ir species and the reaction behaviors on Ir/Al₂O₃ in the oxidative environment during N₂O decomposition were identified here. Two types of Ir sites existed in this catalyst and affected the process of N₂O decomposition. The strong Ir sites facilitated the dissociative adsorption of N₂O to form N₂ and adsorbed O atoms with adsorption heat of as high as 281 kJ/mol, which promoted the desorption of adsorbed O atoms and favored the self‐sustaining decomposition of N₂O by raising the catalyst bed temperature. The other Ir sites interacted weakly with O atoms but facilitated their combination to form O₂. The Ir/Al₂O₃ catalyst then exhibited an excellent performance in initiating the decomposition of N₂O at low temperature of 200°C and good stability in 0.1 N microthruster for orbit adjustment and attitude control of satellite. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3973–3981, 2016     

Nitrogen inputs to rivers, estuaries and continental shelves and related nitrous oxide emissions in 1990 and 2050: a global model

The purpose of the current paper is to estimate future trends (up to the year 2050) in the global geographical distribution of nitrous oxide (N₂O) emissions in rivers, estuaries, and continental shelf regions due to biological processes, particularly as they are affected by anthropogenic nitrogen (N) inputs, and to compare these to 1990 emissions. The methodology used is from Seitzinger and Kroeze (1998) who estimated 1990 emissions assuming that N₂O production in these systems is related to nitrification and denitrification. Nitrification and denitrification in rivers and estuaries were related to external inputs of nitrogen to those systems. The model results indicate that between 1990 and 2050 the dissolved inorganic nitrogen (DIN) export by rivers more than doubles to 47.2 Tg N in 2050. This increase results from a growing world population, associated with increases in fertilizer use and atmospheric deposition of nitrogen oxides (NOy). By 2050, 90% of river DIN export can be considered anthropogenic. N₂O emissions from rivers, estuaries and continental shelves are calculated to amount to 4.9 (1.3 – 13.0) Tg N in 2050, of which two-thirds are from rivers. Aquatic emissions of N₂O are calculated to increase faster than DIN export rates: between 1990 and 2050, estuarine and river emissions increase by a factor of 3 and 4, respectively. Emissions from continental shelves, on the other hand, are calculated to increase by only 12.5%.     

Nitrification and denitrification derived N2O production from a grassland soil under application of DCD and Actilith F2

The relative contribution of nitrification and denitrification to N₂O production was investigated by means of soil incubations with acetylene in a mixed clover/ryegrass sown sward 5 days after application of a mineral fertiliser (calcium ammonium nitrate) or an organic one (cattle slurry) with and without the addition of the nitrification inhibitor dicyandiamide (DCD) and the commercial slurry additive Actilith-F2. At this time, maximum field N₂O emissions were taking place. N₂O production by the slurry amended soil was twice as high as that of the mineral amended one. N₂O came in a greater proportion from nitrification rather than from denitrification in the slurry treatment, while for the mineral fertilisation most N₂O came from denitrification. The addition of DCD to slurry produced a decrease in N₂O production both from nitrification and denitrification. No reduction in N₂O losses was observed from addition of DCD to the mineral fertilisation, although DCD resulted effective in reducing the nitrification rate by 53% both in the slurry and the mineral fertilisation. Actilith F2 induced a high nitrification rate and N₂O production from denitrification was reduced while that from nitrification was not. This revised version was published online in August 2006 with corrections to the Cover Date.     

生活污水不同生物脱氮过程中N_2O产量及控制

利用好氧-缺氧SBR反应器和全程曝气SBBR反应器处理生活污水,分别实现了全程、短程和同步硝化反硝化脱氮过程,研究了不同脱氮过程中N2O的产生及释放情况,同时考察了不同DO条件下同步脱氮效率及N2O产生量。结果表明,全程、短程生物脱氮过程中N2O主要产生于硝化过程,反硝化过程有利于降低系统N2O产量。全程、短程、同步硝化反硝化脱氮过程中N2O产量分别为4.67、6.48和0.35mg.L-1。硝化过程中NO2-N的积累是导致系统N2O产生的主要原因。部分AOB在限氧条件下以NH4+-N作为电子供体,NO2-N作为电子受体进行反硝化,最终产物是N2O。不同DO条件下同步硝化反硝化过程中N2O的产生表明:控制SBBR系统中DO浓度达到稳定的同步脱氮效率可使系统N2O产量最低。     

Fabrication and Characterization of In Situ Porous Si3N4‐Si2N2O‐BN Ceramic

Porous Si₃N₄‐Si₂N₂O‐BN ceramic was fabricated at 1750°C using Si₃N₄, BN, and (NH₄)₂HPO₄ as starting materials. During the sintering process, oxygen from the decomposed products of (NH₄)₂HPO₄ would bond Si and N in the liquid phase to form Si₂N₂O. The microstructure and properties of the porous ceramics were investigated. With the (NH₄)₂HPO₄ content varied from 10 to 50 vol.%, porosity of the porous Si₃N₄‐Si₂N₂O‐BN ceramic increased from 43.5% to 51%. The microstructure, mechanical, and dielectric properties was well controlled by adjusting (NH₄)₂HPO₄ contents. The present technique offers a more simple way of synthesizing porous Si₃N₄‐Si₂N₂O‐BN ceramics.