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.     

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.     

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.     

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.     

N2O and CH4 emissions from fertilized agricultural soils in southwest France

Emissions of nitrogen compounds from heavily fertilized and irrigated maize fields have been studied in the Southwest of France, over an annual cultivation cycle. Strong nitrous oxide emissions from denitrification were observed after application of nitrogen fertilizer. Flux intensity appears to be stimulated by rain or irrigation. Emission algorithms, taking into account both nitrogen input and soil water content were established on the basis of the experimental data set. They allowed us to estimate annual nitrogen loss in the form of nitrous oxide modulated by rainfall. Production of methane is observed at the level of the water table under anoxic conditions. Nevertheless, the net flux between soil and atmosphere is negative for most of the time. When methane is produced, fluxes were very low due to methane oxidation in the soil surface layer.     

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.     

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.     

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.     

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.     

Emissions of N2O from Scottish agricultural soils, as a function of fertilizer N

Potato fields and cut (ungrazed) grassland in SE Scotland gave greater annual N₂O emissions per ha (1.0–3.2 kg N₂O–N ha^-1) than spring barley or winter wheat fields (0.3–0.8 kg N₂O–N ha^-1), but in terms of emission per unit of N applied the order was potatoes > barley > grass > wheat. On the arable land, especially the potato fields, a large part of the emissions occurred after harvest.When the grassland data were combined with those for 2 years' earlier work at the same site, the mean emission over 3 years, for fertilization with ammonium nitrate, was 2.24 kg N₂O–N ha^-1 (0.62% of the N applied). Also, a very strong relationship between N₂O emission and soil nitrate content was found for the grassland, provided the water-filled pore space was > 70%. Significant relationships were also found between the emissions from potato fields and the soil mineral N content, with the added feature that the emission per unit of soil mineral N was an order of magnitude larger after harvest than before, possibly due to the effect of labile organic residues on denitrification.Generally the emissions measured were lower, as a function of the N applied, than those used as the basis for the current value adopted by IPCC, possibly because spring/early summer temperatures in SE Scotland are lower than those where the other data were obtained. The role of other factors contributing to emissions, e.g. winter freeze–thaw events and green manure inputs, are discussed, together with the possible implications of future increases in nitrogen fertilizer use in the tropics.     

Nitrous oxide emissions from agricultural fields during winter and spring thaw as affected by management practices

Highest rates of N₂O emissions from fertilized as well as natural ecosystems have often been measured at spring thaw. But, it is not clear if management practices have an effect on winter and spring thaw emissions, or if measurements conducted over several years would reveal different emission patterns depending on winter conditions. In this study, we present N₂O fluxes obtained using the flux-gradient approach over four winter and spring thaw periods, spanning from 1993 to 1996, at two locations in Ontario, Canada. Several agricultural fields (bare soil, barley, soybean, canola, grass, corn) subjected to various management practices (manure and nitrogen fertilizer addition, alfalfa ploughing, fallowing) were monitored. Nitrous oxide emissions from these fields from January to April over four years ranged between 0 and 4.8 kg N ha^-1. These thaw emissions are substantial and should be considered in the nitrous oxide budgets in regions where thaw periods occur. Our study indicates that agricultural management can play a role in mitigating these emissions. Our data show that fallowing, manure application and alfalfa incorporation in the fall lead to high spring emissions, while the presence of plants (as in the case of alfalfa or grass) can result in negligible emissions during thaw. This presents an opportunity for mitigation of N₂O emissions through the use of over-wintering cover crops.     

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%.     

N2O decomposition over [Fe]-ZSM-5 and Fe-HZSM-5 zeolites

The N₂O decomposition over an [Fe]-ZSM-5 and an Fe-HZSM-5 zeolite was studied. We found that framework incorporated iron species were much more active than Fe(III) introduced as framework charge countercations by ion exchange (TOF at 0.1 vol% N₂O:1.47 × 10^–4 at 280°C for [Fe]-ZSM-5 vs. 2.58 × 10^–4 at 468°C for Fe-HZSM-5). The higher activity of [Fe]-ZSM-5 was attributed to the uniqueness of framework iron species. Both [Fe]-ZSM-5 and Fe-HZSM-5 zeolites showed enhanced activity in the presence of excess oxygen. This is in sharp contrast to ruthenium exchanged zeolites which showed strong oxygen inhibiting effect on the rate of N₂O decomposition.     

Comparing a process-based agro-ecosystem model to the IPCC methodology for developing a national inventory of N2O emissions from arable lands in China

Nations are now obligated to assess their greenhouse gas emissions under the protocols of Article 4 of the United Nations Framework Convention on Climate Change. The IPCC has developed `spreadsheet-format' methodologies for countries to estimate national greenhouse gas emissions by economic sector. Each activity has a magnitude and emission rate and their product is summed over all included activities to generate a national total (IPCC, 1997). For N₂O emissions from cropland soils, field studies have shown that there are important factors that influence N₂O emissions at specific field sites that are not considered in the IPCC methodology. We used DNDC, a process-oriented agroecosystem model, to develop an unofficial national inventory of direct N₂O emissions from cropland in China. We assembled county-scale data on soil properties, daily weather, crop areas, N-fertilizer use, livestock populations (for manure inputs to cropland), and agricultural management for the 2500 counties in mainland China. Total 1990 cropland area was 0.95 million km². Total N-fertilizer use in China in 1990 was 16.6 Tg N. The average fertilization rate was 175 kg N ha⁻¹ cropland. One-year simulations with DNDC were run for each crop type in each county to generate estimates of direct N₂O emissions from soils. National totals were the sum of results for all crop simulations across all counties. Baseline simulations estimated that total N₂O emission from arable land in China in 1990 was 0.31 Tg N₂O-N yr⁻¹. We also ran simulations with zero N-fertilizer input; the difference between the zero-fertilizer and the baseline run is an estimate of fertilizer-induced N₂O emissions. The fertilizer-induced emission was 0.13 Tg N₂O-N yr⁻¹, about 0.8% of total N-fertilizer use (lower than the mean but within the IPCC range of 1.25±1.0%). We compared these results to our estimates of county-scale IPCC methodology emissions. Total emissions were similar but geographical patterns were quite different. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Measurement of nitrous oxide and di-nitrogen emissions from agricultural soils

Nitrous oxide can be produced during nitrification, denitrification, dissimilatory reduction of NO ₃ ^- to NH ₄ ⁺ and chemo-denitrification. Since soils are a mosaic of aerobic and anaerobic zones, it is likely that multiple processes are contributing simultaneously to N₂O production in a soil profile. The N₂O produced by all processes may mix to form one pool before being reduced to N₂ by denitrification. Reliable methods are needed for measuring the fluxes of N₂O and N₂ simultaneously from agricultural soils. The C₂H₂ inhibition and ¹⁵N gas-flux methods are suitable for use in undisturbed soils in the field. The main disadvantage of C₂H₂ is that as well as blocking N₂O reductase, it also blocks nitrification and dissimilatory reduction of NO ₃ ^- to NH ₄ ⁺ . Potentially the ¹⁵ N gas-flux method can give reliable measurements of the fluxes of N₂O and N₂ when all N transformation processes proceed naturally. The analysis of ¹⁵N in N₂ and N₂O is now fully automated by continuous-flow isotope-ratio mass spectrometry for 12-ml gas samples contained in septum-capped vials. Depending on the methodology, the limit of detection ranges from 4 to 11 g N ha^-1day^-1 for N₂ and 4 to 15 g N ha^-1day^-1 for N₂O. By measuring the ¹⁵N content and distribution of ¹⁵N atoms in the N₂O molecules, information can also be obtained to help diagnose the sources of N₂O and the processes producing it. Only a limited number of field studies have been done using the ¹⁵N gas-flux method on agricultural soils. The measured flux rates and mole fractions of N₂O have been highly variable. In rain-fed agricultural soils, soil temperature and water-filled pore space change with the weather and so are difficult to modify. Soil organic C, NO ₃ ^- and pH should be amenable to more control. The effect of organic C depends on the degree of anaerobiosis generated as a result of its metabolism. If conditions for denitrification are not limiting, split applications of organic C will produce more N₂O than a single application because of the time lag in the synthesis of N₂O reductase. Increasing the NO ₃ ^- concentration above the K _m value for NO ₃ ^- reductase, or decreasing soil pH from 7 to 5, will have little effect on denitrification rate but will increase the mole fraction of N₂O. The effect of NO ₃ ^- concentration on the mole fraction of N₂O is enhanced at low pH. Manipulating the interaction between NO ₃ ^- supply and soil pH offers the best hope for minimising N₂O and N₂ fluxes.     

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.     

Influence of soil physical properties, fertiliser type and moisture tension on N2O and NO emissions from nearly saturated Japanese upland soils

In Japan, upland soils are an important source of nitrous oxide (N₂O) and nitric oxide (NO) gas emissions. This paper reports on an investigation of the effect of soil moisture near saturation on N₂O and NO emission rates from four upland soils in Japan of contrasting texture. The aim was to relate these effects to soil physical properties. Intact cores of each soil type were incubated in the laboratory at different moisture tensions after fertilisation with NH₄-N, NO₃-N or zero N. Emissions of N₂O and NO were measured regularly over a 16–20 day period. At the end of the incubation, soil cores were analysed for physical properties. Moisture and N fertiliser significantly affected rates of emissions of both N₂O and NO with large differences between the soil types. Nitrous oxide emissions were greatest in the finer-textured soils, whereas NO emissions were greater in the coarser-textured soils. Emissions of N₂O increased at higher moisture contents in all soils, but the magnitude of increase was much greater in finer-textured soils. Nitric oxide emissions were only significant in soils fertilised with NH₄-N and were negatively correlated with soil moisture. Analysis of soil properties showed that there was a strong relationship between the magnitude of emissions and soil physical properties. The importance of soil wetness to gas emissions was mainly through its influence on soil air-filled porosity, which itself was related to gas diffusivity. From the results of this research, we can now estimate likely effects of soil texture on emissions through the influence of soil type on soil aeration and soil drainage. This is of particular value in modelling N₂O and NO emissions from soil moisture status and land use inputs.     

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.