Measurement of nitrous oxide emissions from two rice-based cropping systems in China

A field experiment was conducted to investigate the effects of winter management and N fertilization on N₂O emission from a double rice-based cropping system. A rice field was either cropped with milk vetch (plot V) or left fallow (plot F) during the winter between rice crops. The milk vetch was incorporated in situ when the plot was prepared for rice transplanting. Then the plots V and F were divided into two sub-plots, which were then fertilized with 276 kg urea-N ha^–1 (referred to as plot VN and plot FN) or not fertilized (referred to as plot VU and plot FU). N₂O emission was measured periodically during the winter season and double rice growing seasons. The average N₂O flux was 11.0 and 18.1 g N m^–2 h^–1 for plot V and plot F, respectively, during winter season. During the early rice growing period, N₂O emission from plot VN averaged 167 g N m^–2 h^–1, which was eight- to fifteen-fold higher than that from the other three treatments (17.8, 21.0 and 10.8 g N m^–2 h^–1 for plots VU, FN, and FU, respectively). During the late rice growing period, the mean N₂O flux was 14.5, 11.1, 12.1 and 9.9 g N m^–2 h^–1 for plots VN, VU, FN and FU, respectively. The annual N₂O emission rates from green manure-double rice and fallow-double rice cropping systems were 3.6 kg N ha^–1 and 1.3 kg N ha^–1, respectively, with synthetic N fertilizer, and were 0.99 kg N ha^–1 and 1.12 kg N ha^–1, respectively, without synthetic N fertilizer. Generally, both green manure N and synthetic fertilizer N contribute to N₂O emission during double rice season.     

Methane and nitrous oxide emissions from rice and maize production in diversified rice cropping systems

Nutrient Cycling in Agroecosystems - Traditional irrigated double-rice cropping systems have to cope with reduced water availability due to changes of climate and economic conditions. To quantify...     

Modelling nitrous oxide emissions from dairy-grazed pastures

Soil N₂O emissions were measured during four seasons from two highly productive grass-clover dairy pastures to assess the influences of soil moisture, temperature, availability of N (NH ₄ ⁺ and NO ₃ ^– ) and soluble C on N₂O emissions, and to use the emission data to validate and refine a simulation model (DNDC). The soils at these pasture sites (Karapoti fine sandy loam, and Tokomaru silt loam) differed in texture and drainage characteristics. Emission peaks for N₂O coincided with rainfall events and high soil moisture content. Large inherent variations in N₂O fluxes were observed throughout the year in both the ungrazed (control) and grazed pastures. Fluxes averaged 4.3 and 5.0 g N₂O/ha/day for the two ungrazed sites. The N₂O fluxes from the grazed sites were much higher than for the ungrazed sites, averaging 26.4 g N₂O/ha/day for the fine sandy loam soil, and 32.0 g N₂O/ha/day for the silt loam soil. Our results showed that excretal and fertiliser-N input, and water-filled pore space (WFPS) were the variables that most strongly regulated N₂O fluxes. The DNDC model was modified to include the effects of day length on pasture growth, and of excretal-N inputs from grazing animals; the value of the WFPS threshold was also modified. The modified model NZ-DNDC simulated effectively most of the WFPS and N₂O emission pulses and trends from both the ungrazed and grazed pastures. The modified model fairly reproduced the real variability in underlying processes regulating N₂O emissions and could be suitable for simulating N₂O emissions from a range of New Zealand grazed pastures. The NZ-DNDC estimates of total yearly emissions of N₂O from the grazed and ungrazed sites of both farms were within the uncertainty range of the measured emissions. The measured emissions changed with changes in soil moisture resulting from rainfall and were about 20% higher in the poorly drained silt loam soil than in the well-drained sandy loam soil. The model accounts for these climatic variations in rainfall, and was also able to pick up differences in emissions resulting from differences in soil texture.     

Nitrous oxide emissions from paddy soil in three rice-based cropping systems in China

Rice-flooding fallow, rice-wheat, and double rice-wheat systems were adopted in pot experiment in an annual rotation to investigate the effects of cropping system on N₂O emission from rice-based cropping systems. The annual N₂O emission from the rice-wheat and the double rice-wheat cropping systems were 4.3 kg N ha^–1 and 3.9 kg N ha^–1, respectively, higher than that from rice-flooding fallow cropping system, 1.4 kg N ha^–1. The average N₂O flux was 115 and 118 g N m^–2 h^–1 for rice season in rice-wheat system and early rice season in double rice-wheat system, respectively, 68.6 and 35.3 g N m^–2 h^–1 for the late rice season in double rice-wheat system and rice season in rice-flooding fallow, respectively, and only 3.1–5.3 g N m^–2 h^–1 for winter wheat or flooding fallow season. Temporal variations of N₂O emission during rice growing seasons differed and high N₂O emission occurred when soil conditions changed from upland crop to flooded rice.     

Combined application of organic and inorganic fertilizers mitigates ammonia and nitrous oxide emissions in a maize field

Nutrient Cycling in Agroecosystems - This long-term study used a lysimeter platform to monitor the NH3 and N2O emissions of summer maize resulting from various fertilization treatments in the...     

The effect of biological oxygen demand of cattle slurry and soil moisture on nitrous oxide emissions

The application of animal manure slurries to soils may cause high short-term emissions of nitrous oxide (N₂O). We performed studies on N₂O emissions varying the contents of NH₄-N and microbial available organic carbon (measured as biological oxygen demand, BOD) of cattle slurry. Additionally the effect of slurry BOD on N₂O emissions at different soil water contents (35, 54, 71% water filled pore space, WFPS) was studied. Slurries from an anaerobic digestion plant (digested slurry, BOD: 1.2 g O₂ l⁻¹) or untreated slurry (BOD: 6.8 g O₂ l⁻¹) were applied at 30 m³ ha⁻¹ and incubated at 20°C. The higher the WFPS the more N₂O was emitted independent from the type of slurry applied. At low and medium soil water contents, the digested slurry induced significantly lower N₂O emissions than the untreated slurry. The N₂O emissions were directly correlated with the BOD content of the slurry (R ²=0.61, P≤0.001). We also compared the effect of NH₄-N concentration and BOD on emissions from the slurries at 54% WFPS. Again the BOD had a significant influence on N₂O emissions but a reduction of NH₄-N had no effect on the amount of N₂O emitted. The microbially available organic carbon seems to determine the amount of N₂O emitted shortly after slurry application. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil carbon,soil nitrate,and soil emissions of nitrous oxide during cultivation of energy crops

Carbon (C) sequestration and soil emissions of nitrous oxide (N₂O) affect the carbon dioxide (CO₂) advantage of energy crops. A long-term study has been performed to evaluate the environmental effects of energy crop cultivation on the loamy sand soil of the drier northeast region of Germany. The experimental field, established in 1994, consisted of columns (0.25 ha each) cultivated with short rotation coppice (SRC: Salix and Populus) and columns cultivated with annual crops. The columns were subdivided into four blocks, with each receiving different fertilization treatments. The soil C content was measured annually from 1994 until 1997, and then in 2006. Soil N₂O levels were measured several times per week from 1999 to 2007. Water-filled pore space (WFPS) and soil nitrate measurements have been performed weekly since 2003. Increased C stocks were found in SRC columns, and C loss was observed in blocks with annual crops. The soil from fertilized blocks had higher levels of C than the soil from non-fertilized blocks. SRC cropping systems on dry, loamy sand soils are advantageous relative to annual cropping systems because of higher C sequestration, lower fertilized-induced N₂O emissions, and reduced background N₂O emissions in these soils. SRC cropping systems on dry, loamy sand soils have a CO₂ advantage (approximately 4 Mg CO₂ ha⁻¹ year⁻¹) relative to annual cropping systems.     

Methane and nitrous oxide emissions: an introduction

Methane and nitrous oxide are important greenhouse gases. They contribute to global warming. To a large extent, emissions of methane and nitrous oxide are connected with the intensification of food production. Therefore, feeding a growing world population and at the same time controlling these emissions is a great challenge. Important anthropogenic sources of biogenic methane are wet rice fields, cattle, animal waste, landfills and biomass burning. Important anthropogenic sources of biogenic nitrous oxide are land-use change, fertilizer production and use and manure application. The ultimate objective of the Framework Convention on Climate Change implies a stabilization of greenhouse gas concentrations in the atmosphere. As a small first step towards achieving this objective, the Convention requires the industrialized countries to bring their anthropogenic emissions of greenhouse gases by 2000 back to 1990 levels. It was also agreed that all parties would make national inventories of anthropogenic greenhouse gas emissions and programmes for control (UN, 1992).In this context, in February 1993 an international workshop was held in Amersfoort in the Netherlands to discuss methods in national emission inventories for methane and nitrous oxide, and options for control (Van Amstel, 1993). A selection of the papers presented in Amersfoort that focus on agricultural sources is published in this volume. This introductory chapter gives background information on biogenic sources and sinks of methane and nitrous oxide and options for their control. The goal of the Climate Convention is described as well as the IPCC effort to develop an internationally accepted methodology for the monitoring of greenhouse gas emissions and sinks. Finally, some preliminary results from country inventories are given. It is concluded that a common reporting framework and transparency of the inventories are important to obtain comparable results that can be used for complying with the requirements of the Climate Convention and for facilitating the international debate about appropriate response strategies.     

Increased emissions of nitric oxide and nitrous oxide following tillage of a perennial pasture

About 40% of the agricultural land in the European Union (EU) is grassland used for animal production. When grassland is tilled, organically bound carbon and nitrogen are released, providing substrates for nitrifying and denitrifying microorganisms. The aim of this study was to examine the immediate effects of tillage of a perennial grassland carried out on different dates, on the emissions of nitric oxide (NO) and nitrous oxide (N₂O), monitored intensively over a 5-day period, in a humid, dairy farming area of northern Spain. Soil was tilled 12 days and 2 days prior to fertiliser application. Tillage, time of tillage, and N fertiliser application affected NO and N₂O emissions. Tillage 12 days before the start of the flux measurements resulted in higher emissions than tillage one day before, the difference being related to differences in soil mineral N and water-filled pore space (WFPS). Emissions of NO peaked at a WFPS of 50–60%, while N₂O fluxes peaked at 70–90% WFPS. Loss of N was greater as N₂O than as NO. The total loss of N as N₂O plus NO ranged from 0.027 kg N ha^–1 in unfertilised plots to 0.56 kg N ha^–1 in the tilled and N fertilised plot. Thereafter emissions decreased rapidly to low values. The results of this study indicate that tillage of perennial grassland may release large amounts of NO and N₂O, the amounts also depending on moisture conditions and addition of N fertiliser. We suggest that in order to reduce such emissions, application of N fertiliser should not immediately follow tillage of perennial grassland, as there is an extra supply of N from mineralisation of organic matter at this time.     

Algorithms for calculating methane and nitrous oxide emissions from manure management

Biogenic emissions of methane (CH₄) and nitrous oxide (N₂O) from animal manure are stimulated by the degradation of volatile solids (VS) which serves as an energy source and a sink for atmospheric oxygen. Algorithms are presented which link carbon and nitrogen turnover in a dynamic prediction of CH₄ and N₂O emissions during handling and use of liquid manure (slurry). A sub-model for CH₄ emissions during storage relates CH₄ emissions to VS, temperature and storage time, and estimates the reduction in VS. A second sub-model estimates N₂O emissions from field-applied slurry as a function of VS, slurry N and soil water potential, but emissions are estimated using emission factors. The model indicated that daily flushing of slurry from cattle houses would reduce total annual CH₄ + N₂O emissions by 35% (CO₂ eq.), and that cooling of pig slurry in-house would reduce total annual CH₄ + N₂O emissions by 21% (CO₂ eq.). Anaerobic digestion of slurry and organic waste produces CH₄ at the expense of VS. Accordingly, the model predicted a 90% reduction of CH₄ emissions from outside stores with digested slurry, and a >50% reduction of N₂O emissions after spring application of digested as opposed to untreated slurry. The sensitivity of the model towards storage temperature and soil water potential was examined. This study indicates that simple algorithms to account for ambient climatic conditions may significantly improve the prediction of CH₄ and N₂O emissions from animal manure.     

A farm-scale basis for predicting nitrous oxide emissions from dairy farms

The IPCC methodology was used to provide farm-scale estimates of N₂O emissions from 2 dairy farms in S.W. England. Emissions were 16.5 and 15.9 kg N₂O-N per ha from Farm A and B, respectively, but a large degree of uncertainty was associated with these estimates (range 5.5–50.1 and 4.6–42.5 kg N₂O-N per ha for Farm A and B, respectively). The generalised assumptions and emission factors employed in this methodology can be refined at a farm scale, where more detailed information is available. Two alternative methodologies were therefore developed. The first was an improved IPCC approach using emission factors based on current literature and approaches and incorporating predictions of leached N from an existing UK model (NCYCLE). The second used the process-based model DNDC to provide estimates of N₂O emission from soil. Using the improved IPCC approach, total emission was 5.1 and 8.9 kg N₂O-N per ha from Farm A and B, respectively. Emission from the soil sector was decreased by 64% and 23% for Farm A and B, respectively, relative to the IPCC method. The decrease in the soil sector was largely due to a reduction in emission from grazing animals and applied animal manures. The use of NCYCLE-based estimates of nitrate leaching in the improved IPCC approach resulted in a 77% and 61% reduction in indirect emission at Farms A and B, respectively, reducing both the total emission and the proportion of the total that was due to the indirect sector. The large effect of components of the indirect sector calculations on IPCC estimates was demonstrated in a sensitivity analysis of the methodology. Data on which to base estimates of emission from indirect sources remain scarce. Preliminary measurements of indirect losses on a farm taken over 6 months confirm that indirect sources make a substantial contribution to the total emission. Estimates from the DNDC method of emission from the soil sector were larger than those of the other methods (8.2 and 7.0 kg N₂O-N per ha from Farm A and B, respectively) . Use of a dynamic model such as DNDC for the estimation of emissions at a farm scale would provide a greatly improved capability for scenario testing and hence the development of mitigation strategies. However, some calibration and development of DNDC would be required before confident estimates of emissions from all sectors could be made. This revised version was published online in August 2006 with corrections to the Cover Date.     

Controlling nitrous oxide emissions from grassland livestock production systems

There is growing awareness that grassland livestock production systems are major sources of nitrous oxide (N₂O). Controlling these emissions requires a thorough understanding of all sources and controlling factors at the farm level. This paper examines the various controlling factors and proposes farm management measures to decrease N₂O emissions from intensively managed grassland livestock farming systems. Two types of regulating mechanisms of N₂O emissions can be distinguished, i.e. environmental regulators and farm management regulators. Both types of regulators may influence the number and size of N₂O sources, and the timing of the emissions. At the field and farm scales, two clusters of environmental regulating factors have been identified, i.e. soil and climate, and three levels of management regulators, i.e. strategic, tactical and operational. Though the understanding of these controls is still incomplete, the available information suggests that there is large scope for diminishing N₂O emissions at the farm scale, using strategies that have been identified already. For example, model calculations indicate that it may be possible to decrease total N₂O emissions from intensively managed dairy farms in The Netherlands in the short term from a mean of about 19 to about 13 kg N per ha per year by more effective nutrient management, whilst maintaining productivity. There is scope for a further reduction to a level of about 6 kg N per ha per year. Advisory tools for controlling N₂O emissions have to be developed for all three management levels, i.e. strategic, tactical and operational, to be able to effectively implement emission reduction options and strategies in practice. Some strategies and best management practices to decrease N₂O emissions from grassland livestock farming systems are suggested.     

Developing a country specific method for estimating nitrous oxide emissions from agricultural soils in Canada

Accurate estimates of nitrous oxide (N₂O) emissions from agricultural soils and management factors that influence emissions are necessary to capture the impact of mitigation measures and carry out life cycle analyses aimed at identifying best practices to reduce greenhouse gas emissions. We propose improvements to a country specific method for estimating N₂O emissions from agricultural soils in Canada based on a compilation of soil N₂O flux data from recent published literature. We provide a framework for the development of empirical models that could be applied in regions where similar data and information on N₂O emissions are available. The method considers spatial elements such as soil texture, topography and climate based on a quantitative empirical relationship between synthetic N-induced soil N₂O emission factor (EF) and growing season precipitation (P) {N₂OEF?=?e^{(0.00558P?7.7)}}. Emission factors vary from less than 0.0025 kg N₂O-N kg N^?1 in semi-arid regions of Canada to greater than 0.025 kg N₂O-N kg N^?1 in humid regions. This approach differentiates soil N₂O EFs based on management factors. Specifically, empirical ratio factors are applied for sources of N of 1.0, 0.84, and 0.28 for synthetic N, animal manure N and crop residue N, respectively. Crop type ratio factors where soil N₂O EFs from applied manure- and synthetic-N on perennial crops are approximately 19% of those on annual crops. This proposed approach improves the accuracy of the dominant factors that modulate N₂O emissions from N application to soils.

     

Assessment of German nitrous oxide emissions using empirical modelling approaches

Direct nitrous oxide (N₂O) emissions from agricultural soils contribute considerably to anthropogenic GHG emissions. Albeit a key source of emissions in many countries, direct N₂O emissions are still calculated and reported to the United Nations Convention on Climate Change using default emission factors defined in the IPCC guidelines (IPCC 1996, 2006). It is known that processes controlling production and transport of N₂O are highly sensitive to environmental conditions defined by weather, soil and management. The accuracy of N₂O emission budgets and the efficiency of mitigation can be improved if those dependencies are considered with regionalized emission factors. In this study an empirical method originating from soft computing techniques based on measured data is developed and applied to quantify direct N₂O emissions from agricultural soils at field and national level in Germany between 1990 and 2005. The method is used to derive maps of emission factor distribution of direct N₂O emissions of agricultural land in Germany. Model results are compared with alternative empirical approaches from literature. Results from developing empirical models show that grassland and cropland have to be differentiated according to the key controls driving N₂O emissions. N₂O emissions of German croplands are highly influenced by climatic conditions and soil properties. The variability of N₂O fluxes on grasslands is mainly driven by the fertilizer N applied. The model comparison using measured European N₂O emissions exhibits profound discrepancies between the models used on a regional scale. The nationwide budgets derived span a narrow range of −8 to 28% relative to direct N₂O emissions quantified by the German national inventory report. The emission factor of German agriculture estimated by the developed model is 0.91% of fertilizer N applied.     

Soil-land-use-system approach to estimate nitrous oxide emissions from agricultural soils

Emissions of nitrous oxide (N₂O) from agricultural soils contribute significantly to the anthropogenic greenhouse effect. Numerous studies have been conducted during the last three decades to improve the understanding of the processes involved in the release of N₂O from agricultural soils. This enabled the creation of process based models on site and field scale. In addition, a growing number of N₂O emission data are available for different soil-land-use-systems from various climates. The integration of these data in global and national N₂O budgets leads to more improved estimations. Surprisingly, N₂O-emission calculations are rare on regional meso and macro scales. The spatial identification of areas with a high efflux of N₂O on regional meso and macro scales is essential for the implementation of N₂O emission mitigation strategies, thus leading to an increased sustainability of land use. On the basis of the ecosystem approach of Matson and Vitousek (1990), we introduce a new method to estimate regional N₂O emissions from agricultural soils on meso and macro scales. This method considers spatial environmental information from available spatial and statistical data as well as quantitative and qualitative expertise by using the tools of a geographic information system (GIS). An environmental information system (EIS) was built up for a dairy farm region in Southern Germany which includes soil, land use, topography, N₂O emission and farm management data. Using all information in the EIS, it was possible (i) to identify different spatial soil-land-use-systems, (ii) to link emission data and process knowledge to these soil-land-use-systems and (iii) to visualize spatial emission potentials. On this basis, N₂O emission potentials for each of the communities in the study region and the whole region were estimated. The estimated annual N₂O emission potential from agricultural soils for the examined dairy farm region in Southern Germany covering around 775 km² is about 3.0 kg N₂O-N ha⁻¹. This revised version was published online in August 2006 with corrections to the Cover Date.     

Root-derived nitrous oxide emissions from an Upper Midwest agricultural ecosystem

Agroecosystems are the dominant source of anthropogenic nitrous oxide (N₂O) emissions globally, yet the partitioning of nitrogen sources supporting N₂O emissions is not well understood. Fertilizer-derived N₂O emissions receive significant attention, while N₂O emissions from organic nitrogen sources, particularly belowground sources, are rarely studied. Here, in situ corn roots (Zea mays L.) were isotopically-labeled with nitrogen (N) and carbon (C) to examine effects of different long-term management systems on root-derived N₂O emissions measured during the following soybean crop in southwest Minnesota, USA. Systems differed in management intensity (tillage and fertilization), crop rotation diversity (two or four crops), and fertilizer type (inorganic or organic). The average contribution of root-derived nitrogen to cumulative N₂O–N emitted over the growing season was 8%, and was higher in 2-year (11%) than 4-year rotations (6%). The fractional loss of root-derived N as N₂O, which is an estimate of the annual emission factor for root-derived N₂O, was small (0.07–0.52%). Management intensity effects on root-derived N₂O emissions and on the root-derived fraction of N₂O emitted differed between two growing seasons as did the effects of fertilizer type on root-derived N cycling rates. Overall, rotation diversity (2 vs. 4-year rotations) exhibited the strongest management effect on root-derived N₂O emissions, suggesting that root-derived N₂O emissions could be mitigated by greater crop rotation diversity.     

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.     

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.     

Entrapment and displacement of nitrous oxide in a drained pasture soil

The objective of this work was to investigate a possible reason for the `unaccounted for ¹⁵N' fraction, of ¹⁵N mass balances, being so large in pasture systems, namely: the displacement and physical release of entrapped N₂O gas from within a soil profile. A soil core was placed inside a purpose built perspex glovebox and the internal N₂O concentration was continuously monitored. KNO₃ was applied followed by periodic applications of distilled water. After 256 h the soil core was physically broken open in an attempt to release any N₂O which may have been entrapped in the soil core. Instantaneous increases in glovebox N₂O concentrations occurred when surface applied water displaced N₂O from the base of the soil core and when the soil core was broken open (equal to 9.5% of N applied). The relative contribution these two mechanisms make will depend on the concentration of denitrification products present, the frequency and volume of irrigation/rainfall and the duration of the experiment.