Changes in patterns of fertilizer nitrogen use in Asia and its consequences for N2O emissions from agricultural systems

In most soils, formation and emissions of N₂O to the atmosphere are enhanced by an increase in available mineral nitrogen (N) through increased rates of nitrification and denitrification. Therefore, addition of N, whether in the form of organic or inorganic compounds eventually leads to enhanced N₂O emissions. Global N₂O emissions from agricultural systems have previously been related primarily to fertilizer N input from synthetic sources. Little attention has been paid to N input from other N sources or to the N₂O produced from N that has moved through agricultural systems. In a new methodology used to estimate N₂O emissions on the country or regional scale, that is briefly described in this paper, the anthropogenic N input data used include synthetic fertilizer, animal waste (feces and urine) used as fertilizer, N derived from enhanced biological N-fixation through N₂ fixing crops and crop residue returned to the field. Using FAO database information which includes data on synthetic fertilizer consumption, live animal production and crop production and estimates of N input from recycling of animal and crop N, estimates of total N into Asian agricultural systems and resulting N₂O emissions are described over the time period 1961 through 1994.During this time the quantity and relative amounts of different types of materials applied to agricultural soils in Asia as nitrogen (N) fertilizer have changed dramatically. In 1961, using the earliest entry from the FAO database, of the approximately 15.7 Tg of fertilizer N applied to agricultural fields 2.1 Tg N (13.5% of total N applied) was from synthetic sources, approximately 6.9 Tg N from animal wastes, 1.7 Tg N from biological N-fixation, and another 5 Tg N from reutilization of crop residue. In 1994, 40.2 Tg from synthetic fertilizer N (57.8% of total), 14.2 Tg from animal wastes, 2.5 Tg from biological N-fixation and 12.6 Tg from crop residue totalling 69.5 Tg N were utilized within agricultural soils in all Asian countries.The increases in N utilization have increased the emission of nitrous oxide from agricultural systems. Estimated N₂O from agricultural systems in Asia increased from about 0.8 Tg N₂O-N in 1961 to about 2.1 in 1994. The period of time when increases in N input and resulting N₂O emissions were greatest was during 1970–1990.This evaluation of N input into Asian agricultural systems and the resulting N₂O emissions demonstrates the large change in global agriculture that has occurred in recent decades. Because of the increased need for food production increases in N input are likely. Although the rate of increase of N input and N₂O emissions during the 1990s appears to have declined, we ask if this slowed rate of increase is a general long term trend or if global food production pressures will tend to accelerate N input demand and resulting N₂O emissions as we move into the 21st century.     

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.     

Closing the global N2O budget: nitrous oxide emissions through the agricultural nitrogen cycle

In 1995 a working group was assembled at the request of OECD/IPCC/IEA to revise the methodology for N₂O from agriculture for the National Greenhouse Gas Inventories Methodology. The basics of the methodology developed to calculate annual country level nitrous oxide (N₂O) emissions from agricultural soils is presented herein. Three sources of N₂O are distinguished in the new methodology: (i) direct emissions from agricultural soils, (ii) emissions from animal production, and (iii) N₂O emissions indirectly induced by agricultural activities. The methodology is a simple approach which requires only input data that are available from FAO databases. The methodology attempts to relate N₂O emissions to the agricultural nitrogen (N) cycle and to systems into which N is transported once it leaves agricultural systems. These estimates are made with the realization that increased utilization of crop nutrients, including N, will be required to meet rapidly growing needs for food and fiber production in our immediate future. Anthropogenic N input into agricultural systems include N from synthetic fertilizer, animal wastes, increased biological N-fixation, cultivation of mineral and organic soils through enhanced organic matter mineralization, and mineralization of crop residue returned to the field. Nitrous oxide may be emitted directly to the atmosphere in agricultural fields, animal confinements or pastoral systems or be transported from agricultural systems into ground and surface waters through surface runoff. Nitrate leaching and runoff and food consumption by humans and introduction into sewage systems transport the N ultimately into surface water (rivers and oceans) where additional N₂O is produced. Ammonia and oxides of N (NO_x) are also emitted from agricultural systems and may be transported off-site and serve to fertilize other systems which leads to enhanced production of N₂O. Eventually, all N that moves through the soil system will be either terminally sequestered in buried sediments or denitrified in aquatic systems. We estimated global N₂O–N emissions for the year 1989, using midpoint emission factors from our methodology and the FAO data for 1989. Direct emissions from agricultural soils totaled 2.1 Tg N, direct emissions from animal production totaled 2.1 Tg N and indirect emissions resulting from agricultural N input into the atmosphere and aquatic systems totaled 2.1 Tg N₂O–N for an annual total of 6.3 Tg N₂O–N. The N₂O input to the atmosphere from agricultural production as a whole has apparently been previously underestimated. These new estimates suggest that the missing N₂O sources discussed in earlier IPCC reports is likely a biogenic (agricultural) one.     

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.     

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.     

Rates and controls of nitrous oxide and nitric oxide emissions following conversion of forest to pasture in Rondônia

Tropical soils are important sources of nitrous oxide (N₂O) and nitric oxide (NO) emissions from the Earths terrestrial ecosystems. Clearing of tropical rainforest for pasture has the potential to alter N₂O and NO emissions from soils by altering moisture, nitrogen supply or other factors that control N oxide production. In this review we report annual rates of N₂O and NO emissions from forest and pastures of different ages in the western Brazilian Amazon state of Rondônia and examine how forest clearing alters the major controls of N oxide production. Forests had annual N₂O emissions of 1.7 to 4.3 kg N ha^-1 y^-1 and annual NO emissions of 1.4 kg N ha^-1 y^-1. Young pastures of 1–3 years old had higher N₂O emissions than the original forest (3.1–5.1 kg N ha^-1 y^-1) but older pastures of 6 years or more had lower emissions (0.1 to 0.4 kg N ha^-1 y^-1). Both soil moisture and indices of soil N cycling were relatively poor predictors of N₂O, NO and combined N₂O + NO emissions. In forest, high N₂O emissions occurred at soil moistures above 30 water-filled pore space, while NO emissions occurred at all measured soil moistures (18–43). In pastures, low N availability led to low N₂O and NO emissions across the entire range of soil moistures. Based on these patterns and results of field fertilization experiments, we concluded that: (1) nitrification was the source of NO from forest soils, (2) denitrification was not a major source of N₂O production from forest soils or was not limited by NO- supply, (3) denitrification was a major source of N₂O production from pasture soils but only when NO₃^- was available, and (4) nitrification was not a major source of 3 NO production in pasture soils. Pulse wettings after prolonged dry periods increased N₂O and NO₃^- emissions for only short periods and not enough to appreciably affect annual emission rates. We project that Basin-wide, the effect of clearing for pasture in the future will be a small reduction in total N₂O emissions if the extensive pastures of the Amazon continue to be managed in a way similar to current practices. In the future, both N₂Oand NO fluxes could increase if uses of pastures change to include greater use of N fertilizers or N-fixing crops. Predicting the consequences of these changes for N oxide production will require an understanding of how the processes of nitrification and denitrification interact with soil type and regional moisture regimes to control N₂O and NO production from these new anthropogenic N sources.     

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.     

Effects of fertiliser type and the presence or absence of plants on nitrous oxide emissions from irrigated soils

Nitrous oxide (N₂O) emissions and denitrification losses from an irrigated sandy loam soil amended with composted municipal solid waste (MSW), sheep manure (SM), surface applied pig slurry (SPS), incorporated pig slurry (IPS) or urea (U) were studied under Mediterranean conditions. We quantified emissions, in both the presence and absence of maize and N₂O production, via denitrification and nitrification pathways using varying concentrations of acetylene. Discounting the N₂O lost in the Control, the percentages of N₂O lost in relation to the total N applied were greater for urea (1.80%) than for MSW (0.50%), SM (0.46%), SPS (1.02%) or IPS (1.27%). In general, plots treated with organic fertilisers emitted higher amounts of N₂O when under maize than bare soil plots. On the other hand, greater denitrification losses were also recorded for plots in the absence of plants (between 9.7 and 29.3 kg N₂O-N ha⁻¹) than for areas with plants (between 7.1 and 24.1 kg N₂O-N ha⁻¹). The proportion of N₂O produced via denitrification was greater from fertiliser treatments than for the controls and also greater without plants (between 66 and 91 % of the N₂O emitted) than with plants (between 48 and 81%).     

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.     

Nitrogen Cycling in an Irrigated Wheat System in Sonora, Mexico: Measurements and Modeling

An improved version of an ecosystem nitrogen cycling model (NLOSS) is described, tested, and used to analyze nitrogen cycling in the Yaqui Valley, Sonora, Mexico. In addition to previously described modules in NLOSS that simulate soil water and solute fluxes, soil evaporation, soil energy balance, and denitrification, modules were added to estimate crop growth, soil carbon cycling, urea hydrolysis, and nitrification. We first tested the model against season-long measurements of soil NO₃⁻, NO₂⁻, and NH₄⁺ aqueous concentrations; NO and N₂O soil effluxes; and crop biomass accumulation in three fertilizer treatments. We used NLOSS to test the sensitivity of wheat production, NO₃⁻ losses, and trace-gas emissions to fertilizer application rate. With the␣model, we compared the typical farmer’s fertilization of 250 kg N ha⁻¹ with five other fertilization scenarios, ranging from 110 to 220 kg N ha⁻¹. The typical farmer’s practice produced higher wheat yield than the lower fertilization treatments. However, the increase in yield per increase in kg N applied decreased with increasing fertilizer addition as a result of higher leaching losses, higher residual N, and higher trace-gas emissions. In addition, with respect to the lowest fertilization treatment, the highest fertilization treatment resulted in an 11% decrease, a 10% increase, and a 157% increase in N₂, N₂O, and NO emissions, respectively, and a 41% increase in leached NO₃⁻ + NO₂⁻. These results demonstrate that a small decrease in fertilizer application rate can increase N-use efficiency for wheat growth, while reducing leaching losses and emissions of harmful trace gas fluxes.     

Nitrous oxide emissions from an irrigated soil as affected by fertilizer and straw management

Nitrous oxide (N₂O) emission from farmland is a concern for both environmental quality and agricultural productivity. Field experiments were conducted in 1996–1997 to assess soil N₂O emissions as affected by timing of N fertilizer application and straw/tillage practices for crop production under irrigation in southern Alberta. The crops were soft wheat (Triticum aestivumL.) in 1996 and canola (Brassica napusL.) in 1997. Nitrous oxide flux from soil was measured using a vented chamber technique and calculated from the increase in concentration with time. Nitrous oxide fluxes for all treatments varied greatly during the year, with the greatest fluxes occurring in association with freeze-thaw events during March and April. Emissions were greater when N fertilizer (100 kg N ha⁻¹) was applied in the fall compared to spring application. Straw removal at harvest in the fall increased N₂O emissions when N fertilizer was applied in the fall, but decreased emissions when no fertilizer was applied. Fall plowing also increased N₂O emissions compared to spring plowing or direct seeding. The study showed that N₂O emissions may be minimized by applying N fertilizer in spring, retaining straw, and incorporating it in spring. The estimates of regional N₂O emissions based on a fixed proportion of applied N may be tenuous since N₂O emission varied widely depending on straw and fertilizer management practices. This revised version was published online in August 2006 with corrections to the Cover Date.     

Changes in nitrogen budgets and nitrogen use efficiency in the agroecosystems of the Changjiang River basin between 1980 and 2000

To investigate both the temporal and spatial changes in the nitrogen use efficiency (NUE) of agroecosystems in the different agricultural regions of the Changjiang (Yangtze) River basin, we constructed a nitrogen (N) budget by using a database of county-level agricultural statistics that was collected every 10 years from 1980 to 2000. Based on the mass balance model, we defined the NUE of agroecosystems as the proportion of all N inputs that are exported via the harvested crop biomass. According to our estimates, the mean total N inputs increased from 8.68 Tg N in 1980 to 13.4 Tg N in 1990 and to 19.8 Tg N in 2000 due to regional human activities. The proportion of anthropogenic new reactive N to the total inputs increased from 42% in 1980 to 68% in 2000 while the proportion of recycled N decreased. N from synthetic fertilizers was the largest contributor to the basin and dramatically increased to 12.23 Tg N in 2000, corresponding to a fivefold increase over that in 1980. While the amount of N from atmospheric deposition, biological N fixation, and recycled N varied slightly between 1980 and 2000, the proportion of N in harvested crops to the total N inputs decreased. Furthermore, the proportion of N lost by denitrification, volatilization, and riverine N transport, and that stored in soil increased between 1980 and 2000 as a result of intensified agricultural activities. It was found that the change pattern of the NUE differs both temporally and spatially. In the Sichuan basin and the plains in the middle and lower reaches that comprise the main agricultural regions of the Changjiang River basin, the NUE increased between 1980 and 1990; however, it dramatically decreased in almost the entire area between 1990 and 2000. On the other hand, in the mountainous and hilly regions of the lower Jinshajiang and Wujiang watersheds, the NUE decreased between 1980 and 1990 but increased between 1990 and 2000. As a result, the total amount of N transported to the surface waters from the agroecosystem reached 4.32 Tg N in 2000, showing a 2.4-fold increase over that in 1980. The export of riverine N increased, and the areas that exported large amounts of riverine N expanded widely from the Changjiang lower plain to the Changjiang middle plain and the surrounding areas between 1980 and 2000. It was apparent that the high rates of N fertilizer application were the most important factor that led to the dramatic decrease in NUE between 1990 and 2000.     

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.     

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.     

Quantification of N-losses as NH3, NO, and N2O and N2 from fertilized maize fields in southwestern France

Emissions of nitrogen compounds (NO, NH₃, N₂O and N₂) from heavily fertilized (280 kg(N) ha^-1) and irrigated maize fields were studied over an annual cultivation cycle in southwestern France. NO and N₂O emissions were measured by chamber techniques throughout the year. During fertilization and maize growth periods, chamber measurements were intensified and complemented by flux-gradient micrometeorological measurements of NO_x and NH₃. The two methods used, Bowen ratio and a simplified aerodynamical techniques, agree quite well and quantify NO_x and NH₃ flux variations during the period of intense emission which followed fertilizer application. Over a yearly cycle, nitrogen loss in the form of NH₃, NO and N₂O were calculated using micrometeorological flux measurements and emission algorithms calibrated with field data (chambers). The soil denitrification potential represented by the ratio N₂O/(N₂O+N₂) was measured in the laboratory to calculate potential total gaseous nitrogen loss. Taking into account all uncertainties, the total N loss into the atmosphere represents 30 to 110 kg(N) ha^-1 with about less than 1% as NH₃, 40% as NO, 14% as N₂O and 46% as N₂. This is in agreement with the agronomic nitrogen budget based on the N fertilizer input and soil furniture and, on the N-output by crops and crop residues, which displays a net imbalance of 50 to 100 kg(N) ha^-1.     

An assessment of N loss from agricultural fields to the environment in China

Using the 1997 IPCC Guidelines for National Greenhouse Gas Inventory Methodology, and statistical data from the China Agricultural Yearbook, we estimated that the direct N₂O emission from agricultural fields in China in 1990 was 0.282 Tg N. Based on micro-meteorological field measurement of NH₃ volatilization from agricultural fields in different regions and under different cropping systems, the total NH₃ volatilization from agricultural fields in China in 1990 was calculated to be 1.80 Tg N, which accounted for 11% of the applied synthetic fertilizer N. Ammonia volatilization from agricultural soil was related to the cropping system and the form of N fertilizer. Ammonia volatilization from paddy fields was higher than that from uplands, and NH₄HCO₃ had a higher potential of NH₃ volatilization than urea. N loss through leaching from uplands in north China accounted for 0.5–4.2% of the applied synthetic fertilizer N. In south China, the leaching of applied N and soil N from paddy fields ranged from 6.75 to 27.0 kg N ha^-1 yr^-1, while N runoff was between 2.45 and 19.0 kg N ha^-1 yr^-1.     

Impact of mineral N fertilizer application rates on N2O emissions from arable soils under winter wheat

Nitrogen fertilizers are a major source of nitrous oxide (N₂O) emissions from arable soils. The relationship between nitrogen application rates and N₂O emissions was evaluated during the growth period of winter wheat (~140 days) at six field sites in north-western Germany. Nitrogen was applied as calcium–ammonium–nitrate, with application rates ranging between 0 and 400 kg N ha^?1. One trial was conducted in 2010, three trials in 2011 and two trials in 2012. Additionally, post-harvest N₂O emissions were evaluated at two field sites during autumn and winter (2012–2013). The emission factors (during the growth period) varied between 0.10 and 0.37 %. Annual N₂O emissions ranged between 0.46 and 0.53 % and were consistently lower across all sites and years than to the IPCC Tier 1 default value (1.0 %). Across all sites and years, the relationship between N₂O and N application rate was best described by linear regression even if nitrogen amounts applied were higher than the nitrogen uptake of the crop. Additionally, annual N₂O emissions per unit of harvested wheat grain were calculated for two field sites to assess the environmental impact of wheat grain production. Yield-scaled N₂O emissions followed a hyperbolic function with a minimum of 177 and 191 g N₂O–N t grain yield^?1 at application rates of 127 and 150 kg N ha^?1, followed by an increase at higher N application rates. This relationship indicates that wheat crop fertilization does not necessarily harm the environment through N₂O emissions compared to zero fertilization. Thus, improving nitrogen use efficiency may be the best management practice for mitigating yield-scaled N₂O emissions.     

Sources of nitrous oxide in soils

Research to identify sources of nitrous oxide (N₂O) in soils has indicated that most, if not all, of the N₂O evolved from soils is produced by biological processes and that little, if any, is produced by chemical processes such as chemodenitrification. Early workers assumed that denitrification was the only biological process responsible for N₂O production in soils and that essentially all of the N₂O evolved from soils was produced through reduction of nitrate by denitrifying microorganisms under anaerobic conditions. It is now well established, however, that nitrifying microorganisms contribute significantly to emissions of N₂O from soils and that most of the N₂O evolved from aerobic soils treated with ammonium or ammonium-yielding fertilizers such as urea is produced during oxidation of ammonium to nitrate by these microorganisms. Support for the conclusion that chemoautotrophic nitrifiers such as Nitrosomonas europaea contribute significantly to production of N₂O in soils treated with N fertilizers has been provided by studies showing that N₂O emissions from such soils can be greatly reduced through addition of nitrification inhibitors such as nitrapyrin, which retard oxidation of ammonium by chemoautotrophic nitrifiers but do not retard reduction of nitrate by denitrifying microorganisms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Gross mineralization, nitrification and N2O emission under different tillage in the North China Plain

No-tillage cropping can increase soil carbon (C) stocks and aggregation, and subsequently impact the internal nitrogen (N) cycle and gas loss. The ¹⁵N pool dilution method was used to study gross N transformations, and relative proportions of nitrous oxide (N₂O) emissions derived from denitrification versus nitrification-related processes under long-term tillage systems (no-tillage, rotary tillage and conventional tillage) in the North China Plain. In-field incubation experiments were repeated in successive growing seasons during April?CNovember in 2007. Gross mineralization rates for rotary and mouldboard plough tillage (3.6?±?0.3?C10.6?±?1.5?mg?N?kg^?1?days^?1) were significantly higher than for no-tillage (1.7?±?0.8?C6.8?±?1.1?mg?N?kg^?1?days^?1). Gross mineralization was positively correlated with soil moisture and temperature, as well as with microbial biomass N and C. However, there was no consistent tillage effect on gross nitrification, and gross nitrification was positively correlated with soil moisture, but not with gross mineralization and microbial biomass. N₂O emissions were higher in no-tillage (NT) than for conventional tillage (CT) during May?CAugust. The ¹⁵N labelling indicated that 26?C92?% of the N₂O was directly derived from the soil ammonium (NH₄ ⁺) pool. Emission rates of N₂O from both nitrification and denitrification were positively correlated with NH₄ ⁺ supply as expressed by gross mineralization, but not correlated with supply of nitrate as expressed by gross nitrification. The fraction of nitrified N emitted as N₂O was positively correlated with changes in soil moisture and varied within 0.01?C2.51???. Our results showed that the tillage management impact on gross N transformation was not consistent with N₂O emission, and more detailed information on the controls over N₂O formation needs to be sought.     

Measurement and modeling of nitrous and nitric oxide emissions from a tea field in subtropical central China

Tea fields represent an important source of nitrous oxide (N₂O) and nitric oxide (NO) emissions due to high nitrogen (N) fertilizer applications and very low soil pH. To investigate the temporal characteristics of N₂O and NO emissions, daily emissions were measured over 2½ years period using static closed-chamber/gas chromatograph and chemiluminescent measurement system in a tea field of subtropical central China. Our results revealed that N₂O and NO fluxes showed similar temporal trends, which were generally driven by temporal variations in soil temperature and soil moisture content and were also affected by fertilization events. The measured average annual N₂O and NO emissions were 10.9 and 3.3 kg N ha^?1 year^?1, respectively, highlighting the high N₂O and NO emissions from tea fields. To improve our understanding of N-cycling processes in tea ecosystems, we developed a new nitrogenous gas emission module for the water and nitrogen management model (WNMM, V2) that simulated daily N₂O and NO fluxes, in which the NO was simulated as being emitted from both nitrification and nitrite chemical decomposition. The results demonstrated that the WNMM captured the general temporal dynamics of N₂O (NSE = 0.40; R² = 0.52, RMSE = 0.03 kg N ha^?1 day^?1, P < 0.001) and NO (NSE = 0.41; R² = 0.44, RMSE = 0.01 kg N ha^?1 day^?1, P < 0.001) emissions. According to the simulation, denitrification was identified as the dominant process contributing 76.5% of the total N₂O emissions, while nitrification and nitrite chemical decomposition accounted for 52.3 and 47.7% of the total NO emissions, respectively.