N2O emission from cropland in China

Based on the regionalization of uplands and paddy fields in China, the crop intensity in each region and the available field measurements, N₂O emission from cropland in China in 1995 was estimated to be 398 Gg N, in which, 310 Gg N was from uplands, accounting for 78% of the total. 88 Gg N–N₂O was emitted from paddy fields with 35 Gg N emitted during the rice growing season and 53 Gg N emitted during upland crop season. N₂O emission from upland and from paddy field during upland land crop season accounted for 91% of the total emission.     

Preliminary studies on N2O emission fluxes from upland soils and paddy soils in China

In this paper, we presented the preliminary results of N₂O fluxes from Chinese upland and rice paddy fields. The mean N₂O flux from upland fields of North China is 30.6 μg N₂O-N m^-2 h^-1; the average N₂O flux from Chinese rice paddy field is 39.5 μg N₂O-N m^-2 h^-1. The effects of cropping system, water management and application of N fertilizer and organic manure on N₂O emission from rice paddy field have also been presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

NO and N2O emissions from savanna soils following the first simulated rains of the season

Data on the emissions of oxides of nitrogen from the soil during the early part of the wet season are reported for nutrient-rich and nutrient-poor sandy soils at Nylsvley, South Africa. The emissions of NO_x and N₂O following the first wetting event of the season are elevated relative to subsequent events. The observed high emission rates (76 ng N-NO m^-2 s^-1) are partially attributed to the sandiness of the soil, which permits NO to diffuse out of the soil rapidly. The pulse of high emissions following wetting is maintained for approximately 72 hours, thereafter continuing at around 20 ng NO m^-2 s^-1 while the soil remains moist. The initial pulse is suggested to be due to the accumulation of a substrate pool during the dry period, coupled with an inability of plants and microbes to use it effectively during the first few days after wetting. There were no significant differences in the peak or subsequent emission rates for either NO or N₂O between two sites of differing nitrogen mineralisation potentials. N₂O emissions averaged 8% of NO_x emissions. The enhanced emissions of NO_x which follow the first wetting after a prolonged dry period do not make a very large contribution to the annual gaseous N emission budget, but could be a significant contributor to the high tropospheric ozone levels observed over southern Africa in springtime.     

N2O and NO emissions from grassland soils after the application of cattle and swine excreta

N₂O and NO fluxes from grassland soil after the application of cattle and swine excreta were measured by a closed chamber method in the autumn and winter of 1994 to 1995. Fresh excrement and urine were spread on the grassland experimental plots and these gas fluxes were measured one or two times a week. In the autumn experiment, N₂O and NO fluxes began to increase several days after the application, the NO flux reaching a maximum after 16 days. In the winter experiment, N₂O and NO fluxes began to increase 45 days after the application and reached a maximum after 80 days. Nitrous oxide flux was influenced by soil water content, high water content leading to high N₂O flux. The ratio of NO-N/N₂O-N in the flux was in the range of 1.1 to 13.7, and negatively correlated to the soil water content. In the winter experiment, the total emission rate of NO was 0.48% and 0.45% of total nitrogen in the applied cattle and swine excreta, respectively. The total emission rate of N₂O was 0.085% and 0.098% in the applied cattle and swine excreta, respectively. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fluxes of NO and N2O from temperate forest soils: impact of forest type, N deposition and of liming on the NO and N2O emissions

Annual cycles of NO, NO₂ and N₂O emission rates from soil were determined with high temporal resolution at a spruce (control and limed plot) and beech forest site (Höglwald) in Southern Germany (Bavaria) by use of fully automated measuring systems. The fully automated measuring system used for the determination of NO and NO₂ flux rates is described in detail. In addition, NO, NO₂ and N₂O emission rates from soils of different pine forest ecosystems of Northeastern Germany (Brandenburg) were determined during 2 measuring campaigns in 1995. Mean monthly NO and N₂O emission rates (July 1994–June 1995) of the untreated spruce plot at the Höglwald site were in the range of 20–130 µg NO-N m^-2 h^-1 and 3.5–16.4 µg N₂O-N m^-2 h^-1, respectively. Generally, NO emission exceeded N₂O emission. Liming of a spruce plot resulted in a reduction of NO emission rates (monthly means: 15–140 µg NO-N m^-2 h^-1) by 25-30% as compared to the control spruce plot. On the other hand, liming of a spruce plot significantly enhanced over the entire observation period N₂O emission rates (monthly means: 6.2–22.1 µg N₂O-N m^-2 h^-1). Contrary to the spruce stand, mean monthly N₂O emission rates from soil of the beech plot (range: 7.9–102 µg N₂O-N m^-2 h^-1) were generally significantly higher than NO emission rates (range: 6.1–47.0 µg NO-N m^-2 h^-1). Results obtained from measuring campaigns in three different pine forest ecosystems revealed mean N₂O emission rates between 6.0 and 53.0 µg N₂O-N m^-2 h^-1 and mean NO emission rates between 2.6 and 31.1 µg NO-N m^-2 h^-1. The NO and N₂O flux rates reported here for the different measuring sites are high compared to other reported fluxes from temperate forests. Ratios of NO/N₂O emission rates were >> 1 for the spruce control and limed plot of the Höglwald site and << 1 for the beech plot. The pine forest ecosystems showed ratios of NO/N₂O emission rates of 0.9 ± 0.4. These results indicate a strong differentiating impact of tree species on the ratio of NO to N₂O emitted from soil.     

Using a boundary line approach to analyze N2O flux data from agricultural soils

Predicting the N₂O flux from soils is difficult because of the complex interplay of the various processes involved. In this study a boundary line approach was used to apply results from mechanistic experiments to N₂O flux data resulting from measurements on field scale in southern Germany. Boundary lines were fitted to the rim of the data points in scattergrams depicting readily obtainable soil variables against the measured N₂O flux. The boundary line approach is based on the hypothesis that this line depicts the functional dependency between the two variables. For determining these boundary lines a novel method was applied. The function best representing the relationship between the N₂O flux and soil temperature had a maximum above 23 °C and the one between the N₂O flux and the water filled pore space (WFPS, to represent water content) had a maximum at 72% WFPS. In the range of 0–20 mg N kg^-1 the relationship between N₂O flux and nitrate in the soil was best described by a linear function, whereas in the range of 0–35 mg N kg^-1 a Michaelis–Menten function was more appropriate. The boundary lines specified in this study are in agreement with existing theoretical concepts as well as experimental results obtained under controlled and field conditions as reported in the literature. Therefore, the boundary line approach can be used to improve empirical models for predicting the N₂O flux in the field.     

Diffusion analysis of N2O cycling in a fertilized soil

The behavior of nitrous oxide (N₂O) in fertilized soil was studied in terms of soil fluxes, the production rates at various depths and the turnover in soil. The diffusive losses of N₂O to the atmosphere calculated from soil N₂O profile compared favorably with the flux directly determined with a closed chamber technique. The estimate of N₂O production rates at several depths demonstrated that the sites of N₂O production was only near the soil surface. The calculated residence time of N₂O in the entire soil column studied was only 1.4 hour during active emission period and less than 1 day even in the later period having trace N₂O emission. The prolonged N₂O emission observed after the active phase was due likely to a lasting N₂O production rather than a supply from the soil N₂O reservoir. The results suggested that most N₂O in soil was emitted quite promptly to the atmosphere after its production. A minor role of soil as an N₂O reservoir is emphasized from the viewpoint of the origin of groundwater N₂O. This revised version was published online in August 2006 with corrections to the Cover Date.     

Highly Efficient Fe-silicalite Zeolite in Direct Propane Ammoxidation with N2O and O2

A novel process for the direct ammoxidation of propane over steam-activated Fe-silicalite at 723–823 K is reported. Yields of acrylonitrile (ACN) and acetonitrile (AcCN) below 5% were obtained using N₂O or O₂ as the oxidant. Co-feeding N₂O and O₂ boosts the performance of Fe-silicalite compared to the individual oxidants, leading to AcCN yields of 14% and ACN yields of 11% (propane conversions of 40% and products selectivity of 25–30%). The beneficial effect of O₂ on the propane ammoxidation with N₂O contrasts with other N₂O-mediated selective oxidations over iron-containing zeolites (e.g. hydroxylation of benzene and oxidative dehydrogenation of propane), where a small amount of O₂ in the feed dramatically reduces the selectivity to the desired product. It is shown that the productivity of ACN and especially AcCN, expressed as mol product h⁻¹ kg_cat⁻¹, is significantly higher over Fe-silicalite than over active propane ammoxidation catalysts reported in the literature. Our results open new perspectives to improve the performance of alkane ammoxidation catalysts.     

Catalytic Conversion of N2O to N2 over Metal-Based Catalysts in the Presence of Hydrocarbons and Oxygen

The catalytic conversion of N₂O to N₂ in the presence or the absence of propene and oxygen was studied. The catalysts examined in this work were synthesized impregnating metals (Rh, Ru, Pd, Co, Cu, Fe, In) on different supports (Al₂O₃, SiO₂, TiO₂, ZrO₂ and calcined hydrotalcite MgAl₂(OH)₈·H₂O). The experimental results varied both with the type of the active site and with the type of the support. Rh and Ru impregnated on -alumina exhibited the highest activity. The performance of the above most promising catalysts was studied using various hydrocarbons (CH₄, C₃H₆, C₃H₈) as reducing agents. These experimental results showed that the type of reducing agent does not affect the reaction yield. The temperature where complete conversion of N₂O to N₂ was measured was independent of the reductant type. The activity of the most active catalysts was also measured in the presence of SO₂ and H₂O in the feed. A shift of the N₂O conversion versus temperature curve to higher temperatures was observed when SO₂ and H₂O were added, separately or simultaneously, to the feed. The inhibition caused by SO₂ was attributed to the formation of sulfates and that caused by water to the competitive chemisorption of H₂O and N₂O on the same active sites.     

Effect of low-sulfur fuels upon NH3 and N2O emission during operation of commercial three-way catalytic converters

The use of low-sulfur fuel is known to improve the performance of the three-way catalytic converter (TWC). However, in this work we report how low-sulfur operation of commercial TWC also favors formation of N₂O and NH₃ as by products. We found that low-sulfur rich operation above 300 °C increases the production of NH₃, inhibiting the formation of N₂O characteristic of high-sulfur operation. During lean operation, the production of N₂O near the stoichiometric point is not significantly affected by the sulfur level. The large production of N₂O observed during light-off is not affected by SO₂ when the operation is lean, but under rich conditions N₂O is produced up to 575 °C. The increased production of NH₃ and N₂O in TWC as a result of the introduction of low-sulfur gasoline is an area that requires further analysis because of its implication upon public health in large urban settings.     

Isotopic Study of N2O Decomposition on an Ion-Exchanged Fe-Zeolite Catalyst: Mechanism of O2 Formation

N₂O decomposition on an ion-exchanged Fe-MFI catalyst has been studied using an ¹⁸O-tracer technique in order to reveal the reaction mechanism. N₂ ¹⁶O was pulsed onto an ¹⁸O₂-treated Fe-MFI catalyst at 693 K, and the O₂ molecules produced were monitored by means of mass spectrometry. The ¹⁸O fraction in the produced oxygen had almost half the value of that on the surface oxygen, and ¹⁸O¹⁸O was not detected. The result shows that O₂ formation proceeds via the Eley–Rideal mechanism (N₂ ¹⁶O + ¹⁸O(a) N₂ + ¹⁶O¹⁸O).     

The state of adsorbed oxygen species formed in the decomposition of N2O on CaO

Temperature programmed desorption, FT-IR spectra and hydrolysis of adsorbed oxygen species revealed that a considerable amount of adsorbed peroxide species were formed on CaO by decomposition of N₂O, whereas no adsorbed species were formed by molecular oxygen.     

Photocatalytic reduction of N2O on metal-supported TiO2 powder at room temperature in the presence of H2O and CH3OH vapor

Ag- and Cu-supported TiO₂ photocatalysts showed high activity for the reduction of N₂O to N₂ at room temperature in the presence of CH₃OH and H₂O vapor. The suppression by H₂O on the activity was not observed in the present photocatalyst system. The remarkable behavior of the Ag and Cu co-catalysts for TiO₂ photocatalysts agreed well with that of electro- and thermal catalyses. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of N2O emissions from soils at three temperate agricultural sites: simulations of year-round measurements by four models

Nitrous oxide (N₂O) flux simulations by four models were compared with year-round field measurements from five temperate agricultural sites in three countries. The field sites included an unfertilized, semi-arid rangeland with low N₂O fluxes in eastern Colorado, USA; two fertilizer treatments (urea and nitrate) on a fertilized grass ley cut for silage in Scotland; and two fertilized, cultivated crop fields in Germany where N₂O loss during the winter was quite high. The models used were daily trace gas versions of the CENTURY model, DNDC, ExpertN, and the NASA-Ames version of the CASA model. These models included similar components (soil physics, decomposition, plant growth, and nitrogen transformations), but in some cases used very different algorithms for these processes. All models generated similar results for the general cycling of nitrogen through the agro-ecosystems, but simulated nitrogen trace gas fluxes were quite different. In most cases the simulated N₂O fluxes were within a factor of about 2 of the observed annual fluxes, but even when models produced similar N₂O fluxes they often produced very different estimates of gaseous N loss as nitric oxide (NO), dinitrogen (N₂), and ammonia (NH₃). Accurate simulation of soil moisture appears to be a key requirement for reliable simulation of N₂O emissions. All models simulated the general pattern of low background fluxes with high fluxes following fertilization at the Scottish sites, but they could not (or were not designed to) accurately capture the observed effects of different fertilizer types on N₂O flux. None of the models were able to reliably generate large pulses of N₂O during brief winter thaws that were observed at the two German sites. All models except DNDC simulated very low N₂O fluxes for the dry site in Colorado. The US Trace Gas Network (TRAGNET) has provided a mechanism for this model and site intercomparison. Additional intercomparisons are needed with these and other models and additional data sets; these should include both tropical agro-ecosystems and new agricultural management techniques designed for sustainability.     

Possible role of nitrite/nitrate redox cycles in N2O decomposition and light-off over Fe-ZSM-5

Surface nitrite/nitrate redox cycles were proposed to explain light-off behavior that was observed during the decomposition of N₂O over Fe-ZSM-5. Further study has demonstrated that while the nitrite/nitrate model can explain the original observations as an isothermal, mechanistic phenomenon, the light-off behavior is thermal, and not a mechanistic effect. Nonetheless, a pathway involving nitrite/nitrate redox cycles appears to be more consistent with experimental observation than the simple two-step pathway involving cation redox cycles. In particular, the nitrite/nitrate pathway can explain the effect of added NO upon the reaction kinetics and the reported isotopic product composition when unlabeled N₂O reacts over an oxygen-labeled catalyst. Further, a nitrite/nitrate pathway is consistent with the steady-state kinetics as well as published thermal desorption and infrared spectroscopic results.     

An investigation into the effect of Cs promotion on the catalytic activity of NiO in the direct decomposition of N2O

A series of Cs promoted NiO catalysts have been prepared and tested for direct decomposition of N₂O. These catalysts are characterized by BET surface area, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption of N₂O (TPD-N₂O) and X-ray photo electron spectroscopy (XPS). The Cs promoted NiO catalysts exhibit higher activity for the decomposition of N₂O compared to bulk NiO. The catalyst with Cs/Ni ratio of 0.1 showed highest activity. The enhancement in catalytic activity of the Cs promoted catalysts is attributed to the change in the electronic properties of NiO. The characterization techniques suggest weakening of Ni–O bond thereby the desorption of oxygen becomes more facile during the reaction. The Cs promoted NiO catalyst is effective at low reaction temperature and also in the presence of oxygen and steam in the feed stream. IICT Communication No: 070523.     

Enhancement of C2H6 Oxidation by O2 in the Presence of N2O over Fe Ion-Exchanged BEA Zeolite Catalyst

Selective catalytic reduction (SCR) of N₂O with C₂H₆ took place effectively over Fe ion-exchanged BEA zeolite catalyst (Fe-BEA) even in the presence of excess oxygen. The mechanism in the SCR of N₂O with C₂H₆ over Fe-BEA catalyst was studied by a transient response experiment and an in situ DRIFT spectroscopy. No oxidation of C₂H₆ by O₂ took place below 350 °C (in C₂H₆/O₂). In the N₂O/C₂H₆/O₂ system, however, it was found that the reaction of C₂H₆ with O₂ was drastically enhanced by the presence of N₂O even at low temperatures (200-300 °C). Therefore, it was concluded that N₂O played an important role in the oxidation of C₂H₆ (i.e., activation of C₂H₆ at an initial step). On the basis of these findings, the mechanism in the SCR of N₂O with C₂H₆ is discussed.     

Kinetic evaluation of mechanistic models for O2 release from ZSM-5-supported [Cu2+ –O–Cu2+] ions by thermal reduction or chemical interaction with impinging N2O molecules

For a series of oxidized Cu-ZSM-5 catalysts which were characterized in the catalytic amounts of the oxygen-bridged Cu²⁺-dimers, [Cu²⁺–O–Cu²⁺], activation energies required for the reduction of the Cu²⁺-dimer species by O₂ release were determined using the temperature-programmed experiments of thermal O₂ desorption (TPD) and N₂O decomposition reaction. The activation energy for the thermal reduction of the Cu²⁺-dimers during the TPD decreased linearly with increasing molar number of the Cu²⁺-dimers available on the ZSM-5, suggesting that the energy barrier of the O₂ formation via a Langmuir-Hinshelwood (LH) mechanism increased in proportion to the distance between the two Cu²⁺-dimers in the nearest neighbor. Activation energies of thermal O₂ release were comparable to the literature-reported binding energies of the differently spaced Cu²⁺-dimers. It was also revealed that the activation energy of O₂ release during the temperature programmed N₂O decomposition reaction over an oxidized catalyst was generally low as compared to that in the TPD, and that the degree of reduction of the Cu²⁺-dimers was much greater in the N₂O decomposition reaction than in the TPD at the same temperatures. These beneficial effects N₂O decomposition on the reduction of the Cu²⁺-dimers were discussed in respect of the removal mechanism of the Cu²⁺-dimer bridged oxygen.     

Emission of nitrous oxide from soils used for agriculture

Nitrous oxide is emitted into the atmosphere as a result of biomass burning, and biological processes in soils. Biomass burning is not only an instantaneous source of nitrous oxide, but it results in a longer term enhancement of the biogenic production of this gas. Measurements of nitrous oxide emissions from soils before and after a controlled burn showed that significantly more nitrous oxide was exhaled after the burn. The current belief is that 90% of the emissions come from soils. Nitrous oxide is formed in soils during the microbiological processes nitrification and denitrification. Because nitrous oxide is a gas it can escape from soil during these transformations. Nitrous oxide production is controlled by temperature, pH, water holding capacity of the soil, irrigation practices, fertilizer rate, tillage practice, soil type, oxygen concentration, availability of carbon, vegetation, land use practices and use of chemicals. Nitrous oxide emissions from agricultural soils are increased by the addition of fertilizer nitrogen and by the growth of legumes to fix atmospheric nitrogen. A recent analysis suggests that emissions of nitrous oxide from fertilized soils are not related to the type of fertilizer nitrogen applied and emissions can be calculated from the amount of nitrogen applied. Legumes also contribute to nitrous oxide emission in a number of ways, viz. atmospheric nitrogen fixed by legumes can be nitrified and denitrified in the same way as fertilizer nitrogen, thus providing a source of nitrous oxide, and symbiotically living Rhizobia in root nodules are able to denitrify and produce nitrous oxide. Conversion of tropical forests to crop production and pasture has a significant effect on the emission of nitrous oxide. Emissions of nitrous oxide increased by about a factor of two when a forest in central Brazil was clear cut, and pasture soils in the same area produced three times as much nitrous oxide as adjacent forest soils. Studies on temperate and tropical rice fields show that less than 0.1% of the applied nitrogen is emitted as nitrous oxide if the soils are flooded for a number of days before fertilizer application. However, if mineral nitrogen is present in the soil before flooding it will serve as a source of nitrous oxide during wetting and drying cycles before permanent flooding. Thus dry seeded rice can be a source of considerable nitrous oxide. There are also indirect contributions to nitrous oxide emission through volatilization of ammonia and emission of nitric oxides into the atmosphere, and their redistribution over the landscape through wet and dry deposition. In general nitrous oxide emissions can be decreased by management practices which optimize the crop's natural ability to compete with processes whereby plant available nitrogen is lost from the soil-plant system. If these options were implemented they would also result in increased productivity and reduced inputs. This revised version was published online in August 2006 with corrections to the Cover Date.     

Room temperature decomposition of N2O in the presence of gaseous oxygen on prereduced Rh supported catalysts

The room temperature decomposition of N₂O over prereduced Rh‐based catalysts (Rh supported on ceria, zirconia and titania–alumina) is studied as a function of the oxygen content in the feed. Results indicate that Rh supported on titania–alumina shows lower degree of inhibition by gaseous oxygen on this reaction, attributed to the role of the metal particle–support interface region in the reaction. The effect of Rh loading and of the reaction temperature are consistent with the hypothesis. This revised version was published online in July 2006 with corrections to the Cover Date.