Spatial and sectorial disaggregation of N2O emissions from agriculture in Belgium

In this study N₂O emissions from agriculture in Belgium have been split up per agro-pedological region and calculated per farm type. The N₂O emissions were calculated according to the `Revised 1996 IPCC guidelines for national greenhouse gas inventories'. Input data were weighed averages of the N balance of a large number of farms per agro-pedological region and per farm type. As such, the input data represent a theoretical farm in each agro-pedological region and for each distinguished farm type. In a first part, N₂O emissions were calculated for 10 agro-pedological regions in Belgium. The yearly N₂O emissions varied between 225 and 462 kg N₂O-N. The highest N₂O emissions (around 400 kg N₂O-N yr⁻¹) were found in regions with fertile soils, dominated by crop production or a combination of crop production and cattle breeding. The lowest emissions (around 250 kg N₂O-N yr⁻¹) were found in regions with extensive cattle breeding. N₂O emissions of 300 ± 15 kg N₂O-N yr⁻¹ were found in regions with less extensive cattle breeding or in regions with combinations of cattle, pig and poultry breeding. The N₂O emission per ha varied between 6 and 14 kg N₂O-N yr⁻¹. In a second part, N₂O emissions were calculated for 12 different farm types. The yearly N₂O emissions varied between 273 and 512 kg N₂O-N. The highest emissions were found on farms with crop production or a combination of crop production and cattle breeding. The lowest emissions were found on farms specialised in only one activity of animal breeding. Specialised pig farms and farms with combinations of cattle caused the greatest threat with respect to N₂O releases from agriculture. Their N₂O emission per ha was 18–40 kg N₂O-N yr⁻¹, which was significantly higher than the average N₂O release (10 kg N₂O-N yr⁻¹ ha⁻¹) for the other farm types. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nitrous oxide emissions from agricultural soils

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

The impact of grassland ploughing on CO2 and N2O emissions in the Netherlands

The contribution of ploughing permanent grassland and leys to emissions of N₂O and CO₂ is not yet well known. In this paper, the contribution of ploughing permanent grassland and leys, including grassland renovation, to CO₂ and N₂O emissions and mitigation options are explored. Land use changes in the Netherlands during 1970–2020 are used as a case study. Three grassland management operations are defined: (i) conversion of permanent grassland to arable land and leys; (ii) rotations of leys with arable crops or bulbs; and (iii) grassland renovation. The Introductory Carbon Balance Model (ICBM) is modified to calculate C and N accumulation and release. Model calibration is based on ICBM parameters, soil organic N data and C to N ratios. IPCC emission factors are used to estimate N₂O-emissions. The model is validated with data from the Rothamsted Park Grass experiments. Conversion of permanent grassland to arable land, a ley arable rotation of 3 years ley and 3 years arable crops, and a ley bulb rotation of 6 years ley and one year bulbs, result in calculated N₂O and CO₂ emissions totalling 250, 150 and 30 ton CO₂-equivalents ha^–1, respectively. Most of this comes from CO₂. Emissions are very high directly after ploughing and decrease slowly over a period of more than 50 years. N₂O emissions in 3/3 ley arable rotation and 6/1 ley bulb rotation are 2.1 and 11.0 ton CO₂-equivalents ha^–1 year^–1, respectively. From each grassland renovation, N₂O emissions amount to 1.8 to 5.5 ton CO₂-equivalents ha^–1. The calculated total annual emissions caused by ploughing in the Netherlands range from 0.5 to 0.65 Mton CO₂-equivalents year^–1. Grassland renovation in spring offers realistic opportunities to lower the N₂O emissions. Developing appropriate combinations of ley, arable crops and bulbs, will reduce the need for conversion of permanent pasture. It will also decrease the rotational losses, due to a decreased proportion of leys in rotations. Also spatial policies are effective in reducing emissions of CO₂ and N₂O. Grassland ploughing contributes significantly to N₂O and CO₂ emissions. The conclusion can be drawn that total N₂O emissions are underestimated, because emissions from grassland ploughing are not taken into account. Specific emission factors and the development of mitigation options are required to account for the emissions and to realise a reduction of emissions due to the changes in grassland ploughing.     

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.     

Process-based modelling of nitrous oxide emissions from different nitrogen sources in mown grassland

The process-based Pasture Simulation Model (PaSim 2.5) has been extended to simulate N₂O production and emission from grassland caused by nitrogen inputs from different sources. The model was used to assess the influence of management on N₂O emissions, such as the effect of shifts in the amount and timing of fertilizer application. Model performance has been tested against season-long field measurements at two different field sites. Simulation results agreed favourably with measured N₂O emission and soil air concentrations, except during an extremely wet period at one site when grass growth was very poor. The results of short-term and long-term simulation runs demonstrated the potential of the model to estimate N₂O emission factors under various conditions. During the first growing season, simulated emissions from organic fertilizers were lower than from synthetic fertilizers because more of the nitrogen was used to build up soil organic matter. The relative difference between the fertilizer types became larger with increasing application rate. The difference between fertilizer types was smaller at steady-state when higher soil organic matter content from repeated application of organic fertilizer over time led to enhanced mineralization and N₂O emissions. The dependence of simulated N₂O emissions on N input was close to linear at low, but non-linear at high fertilization rates. Emission factors calculated from the linear part of the curve suggested that the factors used in the current IPCC method underestimate the long-term effects of changes in fertilizer management. Furthermore the simulations show that N₂O emissions caused by nitrogen inputs from the decomposition of harvest losses and from biological fixation in grassland can be considerable and should not be neglected in national emission inventories. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nitrous Oxide Emissions from New Zealand Agriculture – key Sources and Mitigation Strategies

In most countries, nitrous oxide (N₂O) emissions typically contribute less than 10% of the CO₂ equivalent greenhouse gas (GHG) emissions. In New Zealand, however, this gas contributes 17% of the nation’s total GHG emissions due to the dominance of the agricultural sector. New Zealand’s target under the Kyoto Protocol is to reduce GHG emissions to 1990 levels. Currently total GHG emissions are 17% above 1990 levels. The single largest source of N₂O emission in New Zealand is animal excreta deposited during grazing (80% of agricultural N₂O emissions), while N fertilizer use currently contributes only 14% of agricultural emissions. Nitrogen fertilizer use has, however, increased 4-fold since 1990. Mitigation strategies for reducing N₂O emissions in New Zealand focus on (i) reducing the amount of N excreted to pasture, e.g. through diet manipulation; (ii) increasing the N use efficiency of excreta or fertilizer, e.g. through grazing management or use of nitrification inhibitors; or (iii) avoiding soil conditions that favour denitrification e.g. improving drainage and reducing soil compaction. Current estimates suggest that, if fully implemented, these individual measures can reduce agricultural N₂O emissions by 7–20%. The highest reduction potentials are obtained from measures that reduce the amount of excreta N, or increase the N use efficiency of excreta or fertilizer. However, New Zealand’s currently used N₂O inventory methodology will require refinement to ensure that a reduction in N₂O emissions achieved through implementation of any of these mitigation strategies can be fully accounted for. Furthermore, as many of these mitigation strategies also affect other greenhouse gas emissions or other environmental losses, it is crucial that both the economic and total environmental impacts of N₂O mitigation strategies are evaluated at a farm system’s level.     

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.     

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.     

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.     

Trends in Global Nitrous Oxide Emissions from Animal Production Systems

Wastes from animal production systems contribute as much as 30–50% to the global N₂O emissions from agriculture, but relatively little attention has been given on improving the accuracy of the estimates and on developing mitigation options. This paper discusses trends and uncertainties in global N₂O emission from animal waste and discusses possible mitigation strategies, on the basis of literature data and results of simple calculations. Total N₂O emissions from animal production systems are estimated at 1.5 Tg. Dung and urine from grazing animals deposited in pastures (41%), indirect sources (27%), animal wastes in stables and storages (19%), application of animal wastes to land (10%) and burning of dung (3%) are the five sources distinguished. Most sensitive factors are N excretion per animal head, the emission factor for grazing animals and that for indirect emissions. Total N₂O emissions are related to type and number of animals, N excretion per animal, and the management of animal wastes. Projections by FAO suggest that animal numbers will increase by 40% between 2000 and 2030. Mean N excretion per animal head will probably also increase. These trends combined suggest a strong increase in total N₂O emission from animal production systems in the near future, which is opposite to the objectives of the Kyoto Protocol. Improving N use efficiency, combined with anaerobic digestion of animal wastes for bio fuel generation are the most feasible options for mitigation, but these options seem insufficient to reverse the trend of increasing N₂O emission. In conclusion, animal production systems are a major and increasing source of N₂O in agriculture. The uncertainties in the emission estimates are large, due to the many complexities involved and the lack of accurate data, especially about N excretion and the management of animal wastes in practice. Suggestions are made how to increase the accuracy of the emission estimates and to mitigate N₂O emission from animal production systems.     

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.     

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

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

Nitrogen transformation and nitrous oxide emissions from various types of farm effluents

With land disposal of the farm effluents as an accepted practice, concerns are viewed for its effect on the nitrous oxide (N₂O) emissions. This study was undertaken to determine the effect land application of different farm effluents (treated farm dairy effluent (TFDE), untreated farm dairy effluent (UFDE), treated piggery farm effluent (TPFE) and treated meat effluent (TME)) have on N₂O emissions from soil. N₂O emissions were measured in the field using closed chamber technique. Effluents were added to the plots at a constant hydraulic loading of 25 mm with total volume of effluent applied for each plot being 50 l. Some soil properties like Soil bulk density, water filled pore space, oxygen diffusion rate (ODR), mineral nitrogen and dissolved organic carbon were measured along with the N₂O flux measurement to assess their correlation with variation observed in N₂O flux. The emissions rate was affected by the type of the effluent with TPFE emitting the highest (0.585 kg N₂O–N ha⁻¹ or 2.17% of the total added effluent-N) during autumn application and TME emitting the highest (0.286 kg N ha⁻¹ 0.84% of the total effluent-N added) during winter application. The difference in the N₂O emissions among the effluents could be attributed to the difference in their C:N ratio. The return to pre-application N₂O emissions rates within 2 weeks of autumn effluent application and 3 weeks of winter effluent application indicates that the effect of effluent application on flux is short lived. Correlation studies indicate that N₂O flux was affected by some of the above mentioned soil properties.     

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

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

Greenhouse gas emissions from simulated beef and dairy livestock systems in the United States

Computer spreadsheets were developed to evaluate greenhouse gas (GHG) emissions from U.S. beef and dairy livestock systems from nine locations. Of the beef systems the cow-calf herd emitted the most and feedlot cattle the least methane (CH₄) and nitrous oxide (N₂O) per unit product. Carbon dioxide (CO₂) emissions per unit product were the least for the cow-calf and greatest for the feedlot scenarios. In the dairy systems approximately one-half of the total GHG CO₂ equivalents were from CH₄ and one-third from N₂O. Mitigation strategies, such as intensive grazing, reduced GHG emissions by approximately 10%. This revised version was published online in August 2006 with corrections to the Cover Date.     

N2O and NO emission from agricultural fields and soils under natural vegetation: summarizing available measurement data and modeling of global annual emissions

The number of published N₂O and NO emissions measurements is increasing steadily, providing additional information about driving factors of these emissions and allowing an improvement of statistical N-emission models. We summarized information from 1008 N₂O and 189 NO emission measurements for agricultural fields, and 207 N₂O and 210 NO measurements for soils under natural vegetation. The factors that significantly influence agricultural N₂O emissions were N application rate, crop type, fertilizer type, soil organic C content, soil pH and texture, and those for NO emissions include N application rate, soil N content and climate. Compared to an earlier analysis the 20% increase in the number of N₂O measurements for agriculture did not yield more insight or reduced uncertainty, because the representation of environmental and management conditions in agro-ecosystems did not improve, while for NO emissions the additional measurements in agricultural systems did yield a considerable improvement. N₂O emissions from soils under natural vegetation are significantly influenced by vegetation type, soil organic C content, soil pH, bulk density and drainage, while vegetation type and soil C content are major factors for NO emissions. Statistical models of these factors were used to calculate global annual emissions from fertilized cropland (3.3 Tg N₂O-N and 1.4 Tg NO-N) and grassland (0.8 Tg N₂O-N and 0.4 Tg NO-N). Global emissions were not calculated for soils under natural vegetation due to lack of data for many vegetation types.     

Impact of different forms of N fertilizer on N2O emissions from intensive grassland

Nitrous oxide (N₂O) emissions were measured over two years from an intensively managed grassland site in the UK. Emissions from ammonium nitrate (AN) and urea (UR) were compared to those from urea modified by various inhibitors (a nitrification inhibitor, UR(N), a urease inhibitor, UR(U), and both inhibitors together, SU), as well as a controlled release urea (CR). N₂O fluxes varied through time and between treatments. The differences between the treatments were not consistent throughout the year. After the spring and early summer fertilizer applications, fluxes from AN plots were greater than fluxes from UR plots, e.g. the cumulative fluxes for one month after N application in June 1999 were 5.2 ± 1.1 kg N₂O-N ha^–1 from the AN plots, compared to 1.4 ± 1.0 kg N₂O-N ha^–1 from the UR plots. However, after the late summer application, there was no difference between the two treatments, e.g. cumulative fluxes for the month following N application in August 2000 were 3.3 ± 0.7 kg N₂O-N ha^–1 from the AN plots and 2.9 ± 1.1 kg N₂O-N ha^–1 from the UR plots. After all N applications, fluxes from the UR(N) plots were much less than those from either the AN or the UR plots, e.g. 0.2 ± 0.1 kg N₂O-N ha^–1 in June 1999 and 1.1 ± 0.3 kg N₂O-N ha^–1 in August 2000. Combining the results of this experiment with earlier work showed that there was a greater N₂O emission response to rainfall around the time of fertilizer application in the AN plots than in the UR plots. It was concluded that there is scope for reducing N₂O emissions from N-fertilized grassland by applying UR instead of AN to wet soils in cool conditions, e.g. when grass growth begins in spring. Applying UR with a nitrification inhibitor could cut emissions further.     

Fertilizers, agronomy and N2O

N₂O is emitted from agricultural soils due to microbial transformation of N from fertilizers, manures and soil N reserves. N₂O also derives from N lost from agriculture to other ecosystems: as NH₃ or through NO ₃ ^- leaching. Increased efficiency in crop N uptake and reduction of N losses should in principle diminish the amount of N₂O from agricultural sources. Precision in crop nutrient management is developing rapidly and should increase this efficiency. It should be possible to design guidelines on good agricultural practices for low N₂O emissions in special situations, e.g. irrigated agriculture, and for special operations, e.g. deep placement of fertilizers and manures. However, current information is insufficient for such guidelines. Slow-release fertilizers and fertilizers with inhibitors of soil enzymatic processes show promise as products which give reduced N₂O emissions, but they are expensive and have had little market penetration. Benefits and possible problems with their use needs further clarification.     

Use of the 15?N gas flux method to measure the source and level of N2O and N2 emissions from grazed grassland

Understanding the contribution of nitrification and denitrification to production of nitrous oxide (N₂O), a potent greenhouse gas, is important in devising effective mitigation strategies to reduce emissions. In this study the ¹⁵N gas flux method was used to investigate N₂O and N₂ emissions following an application of ¹⁵N labelled ammonium nitrate (0.71?mol?N?m^?2) to intensive grassland swards (grazed at 2.74 or 2.05 livestock units ha^?1 year^?1) at a site in Southern Ireland. The ¹⁵N labelled fertiliser (NO₃ moiety ¹⁵N labelled at 60 at. %) was applied to designated soil areas in the field, enclosed by static chambers, in June 2009, September 2009 and March 2010. Fluxes of N₂O and N₂ were determined over 12?days on each occasion. N₂O and N₂ emissions were significantly (P?<?0.001) lower in March 2010 than in June or September 2009. There was little difference between the two swards grazed at different stocking rates on N₂O or N₂ emissions. Mean cumulative N₂O emissions over 103?h were 212.9, 279.5 and 62.06?mg?m^?2 for June 2009, September 2009 and March 2010, respectively. Mean cumulative N₂ emissions for the three time periods were 818.8, 893.8 and 87?mg?m^?2, respectively. The N₂O mole fraction averaged 0.21 and 0.23 in June 2009 and September 2009, respectively, but increased to 0.41 in March 2010 which may have been due to changes in denitrifier community composition or due to N₂O reductase being sensitive to low soil temperatures. The results point to denitrification of nitrate as the major source of N₂O at this site which may have implications for choice of fertiliser in moist temperate climates.     

Nitrous oxide emissions from a fertile grassland in Western Norway following the application of inorganic and organic fertilizers

In Norway, 65 % of the agricultural land is under grassland for feeding ruminants. The objective of the present study was to quantify N₂O emissions from grassland on a fertile sandy loam in Western Norway, and to estimate the response of seasonal N₂O emissions to added inorganic N, cattle slurry (CS) N and clover N. Ammonium nitrate (AN) and CS were applied manually at annual rates of 0, 100, 150, 200 and 250 kg AN-N ha^?1, 80 kg CS-N ha^?1 or as a combination of 200 kg AN-N ha^?1 and 80 kg CS-N ha^?1. Background N₂O emissions were five times higher in summer season 2009 than in 2010, but the relative amount of N₂O derived from AN was constant in both periods, amounting to 0.11 % of applied N. CS had no measurable impact on N₂O emissions in 2009, but 0.15 % of CS-N was emitted as N₂O during summer 2010. In the warm year of 2009, which included a drought period, 1–24 % of the N₂O emissions were attributed to the effect of clover depending on fertilization. Clover had no effect on N₂O fluxes in the cool and moist year 2010. Our results suggest that N₂O emissions in fertile Norwegian grasslands are to a great extent controlled by inter-annual variations in background emissions and variable contribution of biologically fixed N and CS-N.