Effect of biochar on nitrous oxide emission and its potential mechanisms

Extensive use of biochar to mitigate nitrous oxide (N₂O) emission is limited by the lack of understanding on the exact mechanisms altering N₂O emission from biochar-amended soil. Biochars produced from rice straw and dairy manure at 350 and 500 °C by oxygen-limited pyrolysis were used to investigate their influence on N₂O emission. A quadratic effect of biochar levels was observed on the N₂O emissions. The potential mechanisms were explored by terminal restriction fragment length polymorphism (T-RFLP) and real-time polymerase chain reaction (qPCR). A lower relative abundance of bacteria, which included ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), was observed at 4% biochar application rate. Reduced copy numbers of the ammonia monooxygenase gene amoA and the nitrite reductase gene nirS coincided with decreased N₂O emissions. Therefore, biochar may potentially alter N₂O emission by affecting ammonia-oxidizing and denitrification bacteria, which is determined by the application rate of biochar in soil.

Implications:Biochar research has received increased interest in recent years because of the potential beneficial effects of biochar on soil properties. Recent research shows that biochar can alter the rates of nitrogen cycling in soil systems by influencing nitrification and denitrification, which are key sources of the greenhouse gas nitrous oxide (N₂O). However, there are still some controversial data. The purpose of this research was to (1) examine how applications of different dose of biochar to soil affect emission of N₂O and (2) improve the understanding of the underlying mechanisms.     

分段进水SBR短程生物脱氮过程中N2O产生及控制

利用SBR,控制曝气量为60 L/h,利用在线pH曲线控制曝气时间,成功实现了短程生物脱氮过程,并考察了不同进水方式下SBR运行性能及N2O产量。结果表明,分段进水能够有效降低短程生物脱氮过程中外加碳源投加量。在原水进水碳氮比较低时,采用递增进水量的进水方式,能够有效降低生物脱氮过程中NO-2积累量,从而降低系统N2O产量。1次进水、2次等量进水和2次递增进水方式下,生物脱氮过程中N2O产量分别为11.1、8.86和5.04 mg/L。硝化过程中NO-2-N的积累是导致系统N2O产生的主要原因。部分氨氧化菌(AOB)在限氧条件下以NH+4-N作为电子供体,NO-2-N作为电子受体进行反硝化,最终产物是N2O。     

A study on the evaluations of emission factors and uncertainty ranges for methane and nitrous oxide from combined-cycle power plant in Korea

In this research, in order to develop technology/country-specific emission factors of methane (CH₄) and nitrous oxide (N₂O), a total of 585 samples from eight gas-fired turbine combined cycle (GTCC) power plants were measured and analyzed. The research found that the emission factor for CH₄ stood at “0.82 kg/TJ”, which was an 18 % lower than the emission factor for liquefied natural gas (LNG) GTCC “1 kg/TJ” presented by Intergovernmental Panel on Climate Change (IPCC). The result was 8 % up when compared with the emission factor of Japan which stands at “0.75 kg/TJ”. The emission factor for N₂O was “0.65 kg/TJ”, which is significantly lower than “3 kg/TJ” of the emission factor for LNG GTCC presented by IPCC, but over six times higher than the default N₂O emission factor of LNG. The evaluation of uncertainty was conducted based on the estimated non-CO₂ emission factors, and the ranges of uncertainty for CH₄ and N₂O were between ?12.96 and +13.89 %, and ?11.43 and +12.86 %, respectively, which is significantly lower than uncertainties presented by IPCC. These differences proved that non-CO₂ emissions can change depending on combustion technologies; therefore, it is vital to establish country/technology-specific emission factors.     

Nitrous oxide generation in denitrifying phosphorus removal process: main causes and control measures

Despite the many benefits of denitrifying phosphorus removal process, the significant generation of nitrous oxide (N₂O), a potent greenhouse gas, remains a problem for this innovative and promising process. To better understand and more effectively control N₂O generation in denitrifying phosphorus removal process, batch experiments were carried out to investigate the main causes of N₂O generation, based on which the control measures were subsequently proposed. The results showed that N₂O generation accounted for 0.41 % of the total nitrogen removal in denitrifying phosphorus removal process, whereas, in contrast, almost no N₂O was generated in conventional denitrification process. It was further demonstrated that the weak competition of N₂O reductase for electrons and the high nitrite accumulation were the two main causes for N₂O generation, evidenced by N₂O production and reduction rates under different conditions. Accordingly, the reduction of N₂O generation was successfully achieved via two control measures: (1) the use of continuous nitrate addition reducing N₂O generation by around 91.4 % and (2) the use of propionate as the carbon source reducing N₂O generation by around 69.8 %.     

Salt marsh plants as key mediators on the level of cadmium impact on microbial denitrification

The fate of excess nitrogen in estuaries is determined by the microbial-driven nitrogen cycle, being denitrification a key process since it definitely removes fixed nitrogen as N₂. However, estuaries receive and retain metals, which may negatively affect this process efficiency. In this study, we evaluated the role of salt marsh plants in mediating cadmium (Cd) impact on microbial denitrification process. Juncus maritimus and Phragmites australis from an estuary were collected together with the sediment involving their roots, each placed in vessels and maintained in a greenhouse, exposed to natural light, with tides simulation. Similar non-vegetated sediment vessels were prepared. After 3 weeks of accommodation, nine vessels (three per plant species plus three non-vegetated) were doped with 20 mg/L Cd²⁺ saline solution, nine vessels were doped with 2 mg/L Cd²⁺ saline solution and nine vessels were left undoped. After 10 weeks, vessels were dissembled and denitrification potential was measured in sediment slurries. Results revealed that the addition of Cd did not cause an effect on the denitrification process in non-vegetated sediment but had a clear stimulation in colonized ones (39 % for P. australis and 36 % for J. maritimus). In addition, this increase on denitrification rates was followed by a decrease on N₂O emissions and on N₂O/N₂ ratios in both J. maritimus and P. australis sediments, increasing the efficiency of the N₂O step of denitrification pathway. Therefore, our results suggested that the presence of salt marsh plants functioned as key mediators on the degree of Cd impact on microbial denitrification.     

Influence of environmental factors on net N2 and N2O production in sediment of freshwater rivers

Denitrification is an important N removal process in aquatic systems but is also implicated as a potential source of global N₂O emissions. However, the key factors controlling this process as well as N₂O emissions remain unclear. In this study, we identified the main factors that regulate the production of net N₂ and N₂O in sediments collected from rivers with a large amount of sewage input in the Taihu Lake region. Net N₂ and N₂O production were strongly associated with the addition of NO₃ ^?-N and NH₄ ⁺-N. Specifically, NO₃ ^?-N controlled net N₂ production following Michaelis–Menten kinetics. The maximum rate of net N₂ production (V _max) was 116.3 μmol N₂-N m^?2 h^?1, and the apparent half-saturation concentration (k _m) was 0.65 mg N L^?1. N₂O to N₂ ratios increased from 0.18?±?0.03 to 0.68?±?0.16 with the addition of NO₃ ^?-N, suggesting that increasing NO₃ ^?-N concentrations favored the production of N₂O more than N₂. The addition of acetate enhanced net N₂ production and N₂O to N₂ ratios, but the ratios decreased by about 59.5 % when acetate concentrations increased from 50 to 100 mg C L^?1, suggesting that the increase of N₂O to N₂ ratios had more to do with the net N₂ production rate rather than acetate addition in this experiment. The addition of Cl^? did not affect the net N₂ production rates, but significantly enhanced N₂O to N₂ ratios (the ratios increased from 0.02?±?0.00 to 0.10?±?0.00), demonstrating that the high salinity effect might have a significant regional effect on N₂O production. Our results suggest that the presence of N-enriching sewage discharges appear to stimulate N removal but also increase N₂O to N₂ ratios.     

Spatial and temporal variations of nitrous oxide flux between coastal marsh and the atmosphere in the Yellow River estuary of China

To investigate the spatial and seasonal variations of nitrous oxide (N₂O) fluxes and understand the key controlling factors, we explored N₂O fluxes and environmental variables in high marsh (HM), middle marsh (MM), low marsh (LM), and mudflat (MF) in the Yellow River estuary throughout a year. Fluxes of N₂O differed significantly between sampling periods as well as between sampling positions. During all times of day and the seasons measured, N₂O fluxes ranged from ?0.0051 to 0.0805 mg N₂O m^?2 h^?1, and high N₂O emissions occurred during spring (0.0278 mg N₂O m^?2 h^?1) and winter (0.0139 mg N₂O m^?2 h^?1) while low fluxes were observed during summer (0.0065 mg N₂O m^?2 h^?1) and autumn (0.0060 mg N₂O m^?2 h^?1). The annual average N₂O flux from the intertidal zone was 0.0117 mg N₂O m^?2 h^?1, and the cumulative N₂O emission throughout a year was 113.03 mg N₂O m^?2, indicating that coastal marsh acted as N₂O source. Over all seasons, N₂O fluxes from the four marshes were significantly different (p?<?0.05), in the order of HM (0.0256?±?0.0040 mg N₂O m^?2 h^?1)?>?MF (0.0107?±?0.0027 mg N₂O m^?2 h^?1)?>?LM (0.0073?±?0.0020 mg N₂O m^?2 h^?1)?>?MM (0.0026?±?0.0011 mg N₂O m^?2 h^?1). Temporal variations of N₂O emissions were related to the vegetations (Suaeda salsa, Phragmites australis, and Tamarix chinensis) and the limited C and mineral N in soils during summer and autumn and the frequent freeze/thaw cycles in soils during spring and winter, while spatial variations were mainly affected by tidal fluctuation and plant composition at spatial scale. This study indicated the importance of seasonal N₂O contributions (particularly during non-growing season) to the estimation of local N₂O inventory, and highlighted both the large spatial variation of N₂O fluxes across the coastal marsh (CV?=?158.31 %) and the potential effect of exogenous nitrogen loading to the Yellow River estuary on N₂O emission should be considered before the annual or local N₂O inventory was evaluated accurately.     

Nitrogen transformations and balance in constructed wetlands for slightly polluted river water treatment using different macrophytes

Nitrogen removal processing in different constructed wetlands treating different kinds of wastewater often varies, and the contribution to nitrogen removal by various pathways remains unclear. In this study, the seasonal nitrogen removal and transformations as well as nitrogen balance in wetland microcosms treating slightly polluted river water was investigated. The results showed that the average total nitrogen removal rates varied in different seasons. According to the mass balance approach, plant uptake removed 8.4–34.3 % of the total nitrogen input, while sediment storage and N₂O emission contributed 20.5–34.4 % and 0.6–1.9 % of nitrogen removal, respectively. However, the percentage of other nitrogen loss such as N₂ emission due to nitrification and denitrification was estimated to be 2.0–23.5 %. The results indicated that plant uptake and sediment storage were the key factors limiting nitrogen removal besides microbial processes in surface constructed wetland for treating slightly polluted river water.     

连续流反应器污水硝化过程中的N2O释放

污水生物脱氮硝化阶段是温室气体一氧化二氮(N2O)的重要释放源。采用连续流反应器在2种进水氨氮(NH4-N,低氮反应器60 mg/L和高氮反应器180 mg/L)浓度条件下驯化硝化菌,并研究了不同初始NH4-N浓度和不同初始亚硝酸盐(NO2-N)浓度条件下所驯化硝化菌释放N2O的特征。结果表明在反应器运行过程中2个反应器释放N2O较少,均小于去除NH4-N浓度的0.01%;N2O的释放均随着初始NH4-N浓度或初始NO2-N浓度的升高而增加;不同初始NH4-N浓度条件下,低氮反应器驯化硝化菌的N2O释放率在0.51%~1.40%之间,高氮反应器驯化硝化菌在0.29%~1.27%之间;不同初始NO2-N浓度条件下,低氮反应器驯化硝化菌的N2O释放率在1.38%~3.78%之间,高氮反应器驯化硝化菌在1.16-5.81%之间。     

Nitric oxide and nitrous oxide emission from Hungarian forest soils; linked with atmospheric N-deposition

Studies of forest nitrogen (N) budgets generally measure inputs from the atmosphere in wet and dry deposition and outputs via hydrologic export. Although denitrification has been shown to be important in many wetland ecosystems, emission of N oxides from forest soils is an important, and often overlooked, component of an ecosystem N budget. During 1 year (2002–03), emissions of nitric oxide (NO) and nitrous oxide (N₂O) were measured from Sessile oak and Norway spruce forest soils in northeast Hungary. Accumulation in small static chambers followed by gas chromatography-mass spectrometry detection was used for the estimation of N₂O emission flux. Because there are rapid chemical reactions of NO and ozone, small dynamic chambers were used for in situ NO flux measurements. Average soil emissions of NO were 1.2 and 2.1 μg N m⁻² h⁻¹, and for N₂O were 15 and 20 μg N m⁻² h⁻¹, for spruce and oak soils, respectively. Due to the relatively high soil water content, and low C/N ratio in soil, denitrification processes dominate, resulting in an order of magnitude greater N₂O emission rate compared to NO. The previously determined N balance between the atmosphere and the forest ecosystem was re-calculated using these soil emission figures. The total (dry+wet) atmospheric N-deposition to the soil was 1.42 and 1.59 g N m⁻² yr⁻¹ for spruce and oak, respectively, while the soil emissions are 0.14 and 0.20 g N m⁻² yr⁻¹. Thus, about 10–13% of N compounds deposited to the soil, mostly as and , were transformed in the soil and emitted back to the atmosphere, mostly as greenhouse gas (N₂O).     

Impacts of feedlot floor condition,deposition frequency,and inhibitors on N2O and CH4 emissions from feedlot dung and urine patches

Patches of dung and urine are major contributors to the feedlot gas emissions. This study investigated the impacts of dung deposition frequency (partly reflecting animal stocking density of a feedlot), dairy feedlot floor conditions (old floor indicated with the presence of consolidated manure pad [CMP] vs. new floor with the absence of consolidated manure pad [CMPn]), and application of dicyandiamide (DCD) and hydroquinone (HQ) on nitrous oxide (N₂O) and methane (CH₄) emissions from patches in the laboratory, and the integrative impacts were expressed in terms of global warming potential (CO₂-equivalent). Dung deposition frequency, feedlot floor condition, and application of inhibitors showed inverse impacts on N₂O and CH₄ emissions from patches. Greenhouse gas (GHG) emissions from the dung, urine, and dung+urine patches on the CMP feedlot surface were approximately 7.48, 87.35, and 7.10 times those on the CMPn feedlot surface (P < 0.05). Meanwhile, GHG emissions from CMP and CMPn feedlot surfaces under high deposition frequency condition were approximately 10 and 1.7 times those under low-frequency condition. Moreover, application of HQ slightly reduced the GHG emission from urine patches, by 14.9% (P > 0.05), while applying DCD or DCD+HQ significantly reduced the GHG, by 60.3% and 65.0%, respectively (P < 0.05). Overall, it is necessary to include feedlot management such as animal stocking density and feedlot floor condition to the process of determining emission factors for feedlots. In the future, field measurements to quantitatively evaluate the relative contribution of nitrification and denitrification to the N₂O emissions of feedlot surfaces are highly required for effective N₂O control.

Implications: This study shows that feedlot CH₄ and N₂O emissions inversely respond to the dicyandiamide (DCD) application. Applying DCD significantly reduces GHG emissions of feedlot urine patches. Feedlot floor condition and stocking density strongly impact feedlot GHG emissions. Including feedlot floor condition and stocking density in the feedlot EF determining process is necessary.     

A2O工艺中N2O的产生与逸散特征简

目前污水处理过程中产生温室气体的问题已经引起普遍关注。本文通过实验室小试,研究了不同污水水质条件下A²O工艺中N₂O的产生特征,以及氧化亚氮还原酶编码基因nosZ含量对N₂O产生量的影响。结果表明,在A²O工艺中的各单元均有N₂O产生,其中厌氧池产生量最大,约占总产生量的32%~85%;A²O工艺产生的N₂O主要通过逸散进入大气,少量随二沉池出水进入到环境中。N₂O的产生量与污泥中nosZ的含量成负相关,而碳源和DO对含有nosZ基因的反硝化细菌有明显的影响,低DO环境和充足的碳源能够极大的促进其含量的提高,从而显著减少N₂O的产生量。     

Effect of influent C/N ratio on N2O emissions from anaerobic/anoxic/oxic biological nitrogen removal processes

The problem of producing strong greenhouse gas of nitrous oxide (N₂O) from biological nitrogen removal (BNR) process in wastewater treatment plants (WWTP) has elicited great concern from various sectors. In this study, three laboratory-scale wastewater treatment systems, with influent C/N ratios of 3.4, 5.4, and 7.5, were set up to study the effect of influent C/N ratio on N₂O generation in anaerobic/anoxic/oxic (A²O) process. Results showed, with the increased influent C/N ratio, N₂O generation from both nitrification and denitrification process was decreased, and the N₂O-N conversion ratio of the process was obviously reduced from 2.23 to 0.05%. Nitrification rate in oxic section was reduced, while denitrification rate in anaerobic and anoxic section was elevated and the removal efficiency of COD, NH₄ ⁺-N, TN, and TP was enhanced in different extent. As the C/N ratio increased from 3.4 to 7.5, activities of three key denitrifying enzymes of nitrate reductase, nitrite reductase, and nitrous oxide reductase were increased. Moreover, microorganism analysis indicated that the relative abundance of ammonium-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were positively correlated with N₂O generation, which was reduced from (8.42 ± 3.65) to (3.61 ± 1.66)% and (10.38 ± 4.12) to (4.67 ± 1.62)%, respectively. NosZ gene copy numbers of the A²O system were increased from (1.19 ± 0.49) × 10⁷ to (2.84 ± 0.54) × 10⁸ copies/g MLSS with the influent C/N ratio elevated from 3.4 to 7.5. Hence, appropriate influent C/N condition of A²O process could optimize the microbial community structure that simultaneously improve treatment efficiency and decrease the N₂O generation.

     

Indirect nitrous oxide emission from a nitrogen saturated spruce forest and general accuracy of the IPCC methodology

The relevance of indirect N₂O emission is a controversial topic which is subject to much uncertainty. Only a small number of studies measure the indirect N₂O emission at the interface from soil to stream. In addition, the majority of studies undertaken only cover a short-term period (<1 year). Therefore, limited information is available regarding the influence of seasonal or event effects, nor is there much information as to whether indirect N₂O emissions are reflected by N₂O in soil solutions. The present study aimed at clarifying these two questions along with the general relevance of dissolved nitrous oxide. A wetness gradient involving soil solutions of different soil types and surface waters within an N-saturated forest catchment (3.2 ha) was monitored over a period of 1 year. N₂O concentrations in soil solutions (0.09–16.6 μg N l⁻¹) were affected by events such as dry–wet cycles but did not reflect to the actual, indirect N₂O emission at the soil-stream interface. It was assumed that N₂O emission was due to N transformation processes. The N₂O concentration at the spring was three times higher than the N₂O concentrations in the soil solutions. Nevertheless, indirect N₂O emission was still subordinate (<1%) to the direct emission of N₂O. The weekly amount of indirect N₂O emissions depended only on the stream flow rate (62% of the total annual amount). For this reason it was necessary to measure indirect N₂O emission at short intervals and at the interface between soil and stream over a longer time period. Our results and the results of the reviewed studies show that the default IPCC emission factor (EF5-g=1.5%) overestimates the indirect N₂O emission from ecosystems. The emission factor should therefore be lowered to about 0.1–0.3%. In addition, the results indicate that indirect N₂O emission is an insignificant pathway in the N cycle of most ecosystems. However, final judgement will depend on long-term studies.     

Validation of DNDC for 22 long-term N2O field emission measurements

Twenty-two long-term measurements of direct N₂O emissions from soils in an intensive agricultural area were used for the validation of the process-based DNDC model (version 8.3P). Model simulations were evaluated for temporal patterns of N₂O, NH₄⁺, NO₃⁻ and water-filled pore space (WFPS) and total N₂O emissions. Several soil and crop input parameter adjustments to the model were evaluated but only the recalculation of the WFPS at wilting point and at field capacity, using pedotransfer functions, resulted in a clear improvement of the simulated variables (WFPS in all cases, N₂O in some cases). Therefore, only this adjustment was made to DNDC 8.3P. This change, however, resulted for some cases (both cropland and grassland) in retardation of nitrate leaching and to a lesser extent of NH₄⁺ to the deeper soil layers. The goodness of fit of the simulated temporal pattern of N₂O varied considerably between sites. The total simulated N₂O emissions from cropland showed a good agreement with the measurements, although there was a systematic overestimation of 7.4 kg N₂O-N ha⁻¹. Grassland soils, in contrast, gave a low agreement between total simulated and measured N₂O losses. On the basis of all measured data a regional emission factor of 3.16 with a 95% confidence interval of −0.89 to 7.21 could be calculated. DNDC simulations resulted in an emission factor of 6.49 with a 95% confidence interval of 4.04–8.93. The overall outcome of the N₂O emission measurements and DNDC simulations were compared with several empirical regression models, which may be applicable for a temperate climate system. All of the tested regression models showed reliable results up to a N₂O emission of 10 kg N₂O-N ha⁻¹. Higher emissions, however, were systematically underestimated. Though DNDC both under- and overestimated specific sites, the general agreement, over the whole range between measurements and simulations of total N₂O losses (simulations=0.82×meas.+6.2), was better than for the different regression models.     

Mitigating Global Warming Potentials of Methane and Nitrous Oxide Gases from Rice Paddies under different irrigation regimes

A field experiment was conducted in Bangladesh Agricultural University Farm to investigate the mitigating effects of soil amendments such as calcium carbide, calcium silicate, phosphogypsum, and biochar with urea fertilizer on global warming potentials (GWPs) of methane (CH₄) and nitrous oxide (N₂O) gases during rice cultivation under continuous and intermittent irrigations. Among the amendments phosphogypsum and silicate fertilizer, being potential source of electron acceptors, decreased maximum level of seasonal CH₄ flux by 25–27 % and 32–38 % in continuous and intermittent irrigations, respectively. Biochar and calcium carbide amendments, acting as nitrification inhibitors, decreased N₂O emissions by 36–40 % and 26–30 % under continuous and intermittent irrigations, respectively. The total GWP of CH₄ and N₂O gases were decreased by 7–27 % and 6–34 % with calcium carbide, phosphogypsum, and silicate fertilizer amendments under continuous and intermittent irrigations, respectively. However, biochar amendments increased overall GWP of CH₄ and N₂O gases.     

Impact of raw pig slurry and pig farming practices on physicochemical parameters and on atmospheric N2O and CH4 emissions of tropical soils,Uvéa Island (South Pacific)

Emissions of CH₄ and N₂O related to private pig farming under a tropical climate in Uvéa Island were studied in this paper. Physicochemical soil parameters such as nitrate, nitrite, ammonium, Kjeldahl nitrogen, total organic carbon, pH and moisture were measured. Gaseous soil emissions as well as physicochemical parameters were compared in two private pig farming strategies encountered on this island on two different soils (calcareous and ferralitic) in order to determine the best pig farming management: in small concrete pens or in large land pens. Ammonium levels were higher in control areas while nitrate and nitrite levels were higher in soils with pig slurry inputs, indicating that nitrification was the predominant process related to N₂O emissions. Nitrate contents in soils near concrete pens were important (≥55 μg N/g) and can thus be a threat for the groundwater. For both pig farming strategies, N₂O and CH₄ fluxes can reach high levels up to 1 mg N/m²/h and 1 mg C/m²/h, respectively. CH₄ emissions near concrete pens were very high (≥10.4 mg C/m²/h). Former land pens converted into agricultural land recover low N₂O emission rates (≤0.03 mg N/m²/h), and methane uptake dominates. N₂O emissions were related to nitrate content whereas CH₄ emissions were found to be moisture dependent. As a result relating to the physicochemical parameters as well as to the gaseous emissions, we demonstrate that pig farming in large land pens is the best strategy for sustainable family pig breeding in Uvéa Islands and therefore in similar small tropical islands.     

Influence of operating conditions on nitrous oxide formation during nitritation and nitrification

Nitrous oxide (N₂O), a strong greenhouse gas, can be produced by ammonium-oxidizing bacteria (AOB) as a by-product of ammonium oxidation and can potentially be formed in all types of nitrification processes. However, partial nitritation has been reported to cause significantly higher N₂O emissions than complete nitrification. In the study presented here, the mechanisms and factors that drive N₂O formation by AOB were investigated with respect to different operational strategies to achieve nitrite accumulation base on combined evaluation of oxygen uptake rate (OUR) and N₂O formation rate. On the one hand, N₂O formation during partial nitritation and nitrification in a continuously stirred tank reactor (CSTR) with continuous aerobic conditions was observed. On the other hand, the effect of intermittent aeration on N₂O formation during nitrification was investigated. The presence of nitrite, the extend of sludge-specific ammonium loading, low oxygen concentration, and transition from aerobic to anoxic conditions significantly increased N₂O formation in this reactor independently from each other, indicating that different formation pathways, supposedly via nitrite or hydroxylamine, were active.     

A/O工艺N_2O的产生与释放的影响因素

采用A/O工艺,在连续运行条件下,以DO、SRT和硝化液回流比(R)为影响因素,对A/O生物脱氮工艺处理模拟城市生活污水过程中N2O的释放进行了研究。实验结果表明,SRT对A/O工艺N2O释放的影响最大,其次是DO,R的影响最小。N2O转化率随着SRT的升高而降低,当SRT从10 d升高到20 d时,总N2O平均转化率从0.319%下降到0.002%。总N2O转化率随着好氧池DO的升高先降低后有所升高,当DO分别为0.6 mg O2/L、1.2 mg O2/L、2.5 mg O2/L时,反应器的总N2O平均转化率分别为0.306%、0.007%和0.013%。R对N2O释放的影响差异不明显,总N2O平均转化率在300%时最低,为0.007%。N2O释放量最低的工艺运行条件组合是SRT为20 d、DO为1.2 mg O2/L、R为300%。     

Nitrous oxide emission from an agricultural field: Comparison between measurements by flux chamber and micrometerological techniques

The soil in a drained fjord area, reclaimed for arable farming, produced N₂O mainly at 75–105 cm depth, just above the ground water level. Surface emissions of N₂O were measured from discrete small areas by closed and open-flow chamber methods, using gas chromatographic analysis and over larger areas by integrative methods: flux gradient (analysis by FTIR), conditional sampling (analysis by TDLAS), and eddy covariance (analysis by TDLAS). The mean emission of N₂O as determined by chamber procedures during a 9-day campaign was 162–202 μg N₂ONm⁻²h⁻¹ from a wheat stubble and 328–467 μg N₂ONm⁻² h⁻¹ from a carrot field. The integrative approaches gave N₂O emissions of 149–495 μg N₂ONm⁻² h⁻¹, i.e. a range similar to those determined with the chamber methods. Wind direction affected the comparison of chamber and integrative methods because of patchiness of the N₂O emission over the area. When a uniform area with a single type of vegetation had a dominant effect on the N₂O gradient at the sampling mast, the temporal variation in N₂O emission determined by the flux gradient/FTIR method and chamber methods was very similar, with differences of only 18% or less in mean N₂O emission, well below the variation encountered with the chamber methods themselves. A detailed comparison of FTIR gradient and chamber data taking into account the precise emission footprint showed good agreement. It is concluded that there was no bias between the different approaches used to measure the N₂O emission and that the precision of the measurements was determined by the spatial variability of the N₂O emission at the site and the variability inherent in the individual techniques. These results confirm that measurements of N₂O emissions from different ecosystems obtained by the different methods can be meaningfully compared.