Soil nitrous oxide and methane fluxes are low from a bioenergy crop (canola) grown in a semi-arid climate

Understanding nitrous oxide (N₂O) and methane (CH₄) fluxes from agricultural soils in semi‐arid climates is necessary to fully assess greenhouse gas emissions from bioenergy cropping systems, and to improve our knowledge of global terrestrial gaseous exchange. Canola is grown globally as a feedstock for biodiesel production, however, resulting soil greenhouse gas fluxes are rarely reported for semi‐arid climates. We measured soil N₂O and CH₄ fluxes from a rain‐fed canola crop in a semi‐arid region of south‐western Australia for 1 year on a subdaily basis. The site included N fertilized (75 kg N ha^?1 yr^?1) and nonfertilized plots. Daily N₂O fluxes were low (?1.5 to 4.7 g N₂O‐N ha^?1 day^?1) and culminated in an annual loss of 128 g N₂O‐N ha^?1 (standard error, 12 g N₂O‐N ha^?1) from N fertilized soil and 80 g N₂O‐N ha^?1 (standard error, 11 g N₂O‐N ha^?1) from nonfertilized soil. Daily CH₄ fluxes were also low (?10.3 to 11.9 g CH₄‐C ha^?1 day^?1), and did not differ with treatments, with an average annual net emission of 6.7 g CH₄–C ha^?1 (standard error, 20 g CH₄–C ha^?1). Greatest daily N₂O fluxes occurred when the soil was fallow, and following a series of summer rainfall events. Summer rainfall increased soil water contents and available N, and occurred when soil temperatures were >25 °C, and when there was no active plant growth to compete with soil microorganisms for mineralized N; conditions known to promote N₂O production. The proportion of N fertilizer emitted as N₂O, after correction for emissions from the no N fertilizer treatment, was 0.06%; 17 times lower than IPCC default value for the application of synthetic N fertilizers to land (1.0%). Soil greenhouse gas fluxes from bioenergy crop production in semi‐arid regions are likely to have less influence on the net global warming potential of biofuel production than in temperate climates.     

Soil‐derived trace gas fluxes from different energy crops – results from a field experiment in Southwest Germany

Willow coppice, energy maize and Miscanthus were evaluated regarding their soil‐derived trace gas emission potential involving a nonfertilized and a crop‐adapted slow‐release nitrogen (N) fertilizer scheme. The N application rate was 80 kg N ha^?1 yr^?1 for the perennial crops and 240 kg N ha^?1 yr^?1 for the annual maize. A replicated field experiment was conducted with 1‐year measurements of soil fluxes of CH₄, CO₂ and N₂O in weekly intervals using static chambers. The measurements revealed a clear seasonal trend in soil CO₂ emissions, with highest emissions being found for the N‐fertilized Miscanthus plots (annual mean: 50 mg C m^?² h^?1). Significant differences between the cropping systems were found in soil N₂O emissions due to their dependency on amount and timing of N fertilization. N‐fertilized maize plots had highest N₂O emissions by far, which accumulated to 3.6 kg N₂O ha^?1 yr^?1. The contribution of CH₄ fluxes to the total soil greenhouse gas subsumption was very small compared with N₂O and CO₂. CH₄ fluxes were mostly negative indicating that the investigated soils mainly acted as weak sinks for atmospheric CH₄. To identify the system providing the best ratio of yield to soil N₂O emissions, a subsumption relative to biomass yields was calculated. N‐fertilized maize caused the highest soil N₂O emissions relative to dry matter yields. Moreover, unfertilized maize had higher relative soil N₂O emissions than unfertilized Miscanthus and willow. These results favour perennial crops for bioenergy production, as they are able to provide high yields with low N₂O emissions in the field.     

Afforestation does not necessarily reduce nitrous oxide emissions from managed boreal peat soils

Pristine peatlands have generally low nitrous oxide (N₂O) emissions but drainage and management practices enhance the microbial processes and associated N₂O emissions. It is assumed that leaving peat soils from intensive management, such as agriculture, will decrease their N₂O emissions. In this paper we report how the annual N₂O emission rates will change when agricultural peat soil is either left abandoned or afforested and also N₂O emissions from afforested peat extraction sites. In addition, we evaluated a biogeochemical model (DNDC) with a view to explaining GHG emissions from peat soils under different land uses. The abandoned agricultural peat soils had lower mean annual N₂O emissions (5.5?±?5.4?kg?N?ha^?1) than the peat soils in active agricultural use in Finland. Surprisingly, N₂O emissions from afforested organic agricultural soils (12.8?±?9.4?kg?N?ha^?1) were similar to those from organic agricultural soils in active use. These emissions were much higher than those from the forests on nutrient rich peat soils. Abandoned and afforested peat extraction sites emitted more N₂O, (2.4?±?2.1?kg?N?ha^?1), than the areas under active peat extraction (0.7?±?0.5?kg?N?ha^?1). Emissions outside the growing season contributed significantly, 40% on an average, to the annual emissions. The DNDC model overestimated N₂O emission rates during the growing season and indicated no emissions during winter. The differences in the N₂O emission rates were not associated with the age of the land use change, vegetation characteristics, peat depth or peat bulk density. The highest N₂O emissions occurred when the soil C:N ratio was below 20 with a significant variability within the measured C:N range (13–27). Low soil pH, high nitrate availability and water table depth (50–70?cm) were also associated with high N₂O emissions. Mineral soil has been added to most of the soils studied here to improve the fertility and this may have an impact on the N₂O emissions. We infer from the multi-site dataset presented in this paper that afforestation is not necessarily an efficient way to reduce N₂O emissions from drained boreal organic fields.     

Climate- and crop-responsive emission factors significantly alter estimates of current and future nitrous oxide emissions from fertilizer use

The current Intergovernmental Panel on Climate Change (IPCC) default methodology (tier 1) for calculating nitrous oxide (N₂O) emissions from nitrogen applied to agricultural soils takes no account of either crop type or climatic conditions. As a result, the methodology omits factors that are crucial in determining current emissions, and has no mechanism to assess the potential impact of future climate and land‐use change. Scotland is used as a case study to illustrate the development of a new methodology, which retains the simple structure of the IPCC tier 1 methodology, but incorporates crop‐ and climate‐dependent emission factors (EFs). It also includes a factor to account for the effect of soil compaction because of trampling by grazing animals. These factors are based on recent field studies in Scotland and elsewhere in the UK. Under current conditions, the new methodology produces significantly higher estimates of annual N₂O emissions than the IPCC default methodology, almost entirely because of the increased contribution of grazed pasture. Total emissions from applied fertilizer and N deposited by grazing animals are estimated at 10 662 t N₂O‐N yr^?1 using the newly derived EFs, as opposed to 6 796 t N₂O‐N yr^?1 using the IPCC default EFs. On a spatial basis, emission levels are closer to those calculated using field observations and detailed soil modelling than to estimates made using the IPCC default methodology. This can be illustrated by parts of the western Ayrshire basin, which have previously been calculated to emit 8–9 kg N₂O‐N ha^?1 yr^?1 and are estimated here as 6.25–8.75 kg N₂O‐N ha^?1 yr^?1, while the IPCC default methodology gives a maximum emission level of only 3.75 kg N₂O‐N ha^?1 yr^?1 for the whole area. The new methodology is also applied in conjunction with scenarios for future climate‐ and land‐use patterns, to assess how these emissions may change in the future. The results suggest that by 2080, Scottish N₂O emissions may increase by up to 14%, depending on the climate scenario, if fertilizer and land management practices remain unchanged. Reductions in agricultural land use, however, have the potential to mitigate these increases and, depending on the replacement land use, may even reduce emissions to below current levels.     

Greenhouse gas fluxes from an Australian subtropical cropland under long‐term contrasting management regimes

The long‐term effects of conservation management practices on greenhouse gas fluxes from tropical/subtropical croplands remain to be uncertain. Using both manual and automatic sampling chambers, we measured N₂O and CH₄ fluxes at a long‐term experimental site (1968–present) in Queensland, Australia from 2006 to 2009. Annual net greenhouse gas fluxes (NGGF) were calculated from the 3‐year mean N₂O and CH₄ fluxes and the long‐term soil organic carbon changes. N₂O emissions exhibited clear daily, seasonal and interannual variations, highlighting the importance of whole‐year measurement over multiple years for obtaining temporally representative annual emissions. Averaged over 3 years, annual N₂O emissions from the unfertilized and fertilized soils (90 kg N ha^?1 yr^?1 as urea) amounted to 138 and 902 g N ha^?1, respectively. The average annual N₂O emissions from the fertilized soil were 388 g N ha^?1 lower under no‐till (NT) than under conventional tillage (CT) and 259 g N ha^?1 higher under stubble retention (SR) than under stubble burning (SB). Annual N₂O emissions from the unfertilized soil were similar between the contrasting tillage and stubble management practices. The average emission factors of fertilizer N were 0.91%, 1.20%, 0.52% and 0.77% for the CT‐SB, CT‐SR, NT‐SB and NT‐SR treatments, respectively. Annual CH₄ fluxes from the soil were very small (?200–300 g CH₄ ha^?1 yr^?1) with no significant difference between treatments. The NGGF were 277–350 kg CO₂‐e ha^?1 yr^?1 for the unfertilized treatments and 401–710 kg CO₂‐e ha^?1 yr^?1 for the fertilized treatments. Among the fertilized treatments, N₂O emissions accounted for 52–97% of NGGF and NT‐SR resulted in the lowest NGGF (401 kg CO₂‐e ha^?1 yr^?1 or 140 kg CO₂‐e t^?1 grain). Therefore, NT‐SR with improved N fertilizer management practices was considered the most promising management regime for simultaneously achieving maximal yield and minimal NGGF.     

Soil nitrous oxide emissions from a typical semiarid temperate steppe in inner Mongolia: effects of mineral nitrogen fertilizer levels and forms

Nitrous oxide (N₂O) emissions can be significantly affected by the amounts and forms of nitrogen (N) available in soils, but the effect is highly dependent on local climate and soil conditions in specific ecosystem. To improve our understanding of the response of N₂O emissions to different N sources of fertilizer in a typical semiarid temperate steppe in Inner Mongolia, a 2-year field experiment was conducted to investigate the effects of high, medium and low N fertilizer levels (HN: 200 kg N?ha^-1y^-1, MN: 100 kg N ha^-1y^-1, and LN: 50 kg N ha^-1y^-1) respectively and N fertilizer forms (CAN: calcium ammonium nitrate, AS: ammonium sulphate and NS: sodium nitrate) on N₂O emissions using static closed chamber method. Our data showed that peak N₂O fluxes induced by N treatments were concentrated in short periods (2 to 3 weeks) after fertilization in summer and in soil thawing periods in early spring; there were similarly low N₂O fluxes from all treatments in the remaining seasons of the year. The three N levels increased annual N₂O emissions significantly (P?<?0.05) in the order of MN > HN > LN compared with the CK (control) treatment in year 1; in year 2, the elevation of annual N₂O emissions was significant (P?<?0.05) by HN and MN treatments but was insignificant by LN treatments (P?>?0.05). The three N forms also had strong effects on N₂O emissions. Significantly (P?<?0.05) higher annual N₂O emissions were observed in the soils of CAN and AS fertilizer treatments than in the soils of NS fertilizer treatments in both measured years, but the difference between CAN and AS was not significant (P?>?0.05). Annual N₂O emission factors (EF) ranged from 0.060 to 0.298% for different N fertilizer treatments in the two observed years, with an overall EF value of 0.125%. The EF values were by far less than the mean default EF proposed by the Intergovernmental Panel on Climate Change (IPCC).     

Soil-atmosphere exchange of nitrous oxide and methane in New Zealand terrestrial ecosystems and their mitigation options: a review

The two non-CO₂ greenhouse gases (GHGs) nitrous oxide (N₂O) and methane (CH₄) comprise 54.8% of total New Zealand emissions. Nitrous oxide is mainly generated from mineral N originating from animal dung and urine, applied fertiliser N, biologically fixed N₂, and mineralisation of soil organic N. Even though about 96% of the anthropogenic CH₄ emitted in New Zealand is from ruminant animals (methanogenesis), methane uptake by aerobic soils (methanotrophy) can significantly contribute to the removal of CH₄ from the atmpsphere, as the global estimates confirm. Both the net uptake of CH₄ by soils and N₂O emissions from soils are strongly influenced by changes in land use and land management. Quantitative information on the fluxes of these two non-CO₂ GHGs is required for a range of land-use and land-management ecosystems to determine their contribution to the national emissions inventory, and for assessing the potential of mitigation options. Here we report soil N₂O fluxes and CH₄ uptake for a range of land-use and land-management systems collated from published and unpublished New Zealand studies. Nitrous oxide emissions are highest in dairy-grazed pastures (10–12 kg N₂O–N ha^?1 year^{? 1}), intermediate in sheep-grazed pastures, (4–6 kg N₂O–N ha^?1 year^?1), and lowest in forest, shrubland and ungrazed pasture soils (1–2 kg N₂O–N ha^?1 year^?1). N deposited in the form of animal urine and dung, and N applied as fertiliser, are the principal sources of N₂O production. Generally, N₂O emissions from grazed pasture soils are high when the soil water-filled pore-space is above field capacity, and net CH₄ uptake is low or absent. Although nitrification inhibitors have shown some promise in reducing N₂O emissions from grazed pasture systems, their efficacy as an integral part of farm management has yet to be tested. Methane uptake was highest for a New Zealand Beech forest soil (10–11 kg CH₄ ha^?1 year^?1), intermediate in some pine forest soils (4–6 kg CH₄ ha^?1 year^?1), and lowest in most pasture (<1 kg CH₄ ha^?1 year^?1) and cropped soils (1.5 kg CH₄ ha^?1 year^?1). Afforestation /reforestation of pastures results in increases in soil CH₄ uptake, largely as a result of increases in soil aeration status and changes in the population and activities of methanotrophs. Soil CH₄ uptake is also seasonally dependent, being about two to three times higher in a dry summer and autumn than in a wet winter. There are no practical ways yet available to reduce CH₄ emissions from agricultural systems. The mitigation options to reduce gaseous emissions are discussed and future research needs identified.     

Are the benefits of yield responses to nitrogen fertilizer application in the bioenergy crop Miscanthus × giganteus offset by increased soil emissions of nitrous oxide?

A field trial was carried out on a 15 year old Miscanthus stand, subject to nitrogen fertilizer treatments of 0, 63 and 125 kg‐N ha^?1, measuring N₂O emissions, as well as annual crop yield over a full year. N₂O emission intensity (N₂O emissions calculated as a function of above‐ground biomass) was significantly affected by fertilizer application, with values of 52.2 and 59.4 g N₂O‐N t^?1 observed at 63 and 125 kg‐N ha^?1, respectively, compared to 31.3 g N₂O‐N t^?1 in the zero fertilizer control. A life cycle analyses approach was applied to calculate the increase in yield required to offset N₂O emissions from Miscanthus through fossil fuel substitution in the fuel chain. For the conditions observed during the field trial yield increases of 0.33 and 0.39 t ha^?1 were found to be required to offset N₂O emissions from the 63 kg‐N ha^?1 treatment, when replacing peat and coal, respectively, while increases of 0.71 and 0.83 t ha^?1 were required for the 125 kg‐N ha^?1 treatment, for each fuel. These values are considerably less than the mean above‐ground biomass yield increases observed here of 1.57 and 2.79 t ha^?1 at fertilization rates 63 and 125 kg‐N ha^?1 respectively. Extending this analysis to include a range of fertilizer application rates and N₂O emission factors found increases in yield necessary to offset soil N₂O emissions ranging from 0.26 to 2.54 t ha^?1. These relatively low yield increase requirements indicate that where nitrogen fertilizer application improves yield, the benefits of such a response will not be offset by soil N₂O emissions.     

A meta‐analysis of fertilizer‐induced soil NO and combined NO+N2O emissions

Soils are among the important sources of atmospheric nitric oxide (NO) and nitrous oxide (N₂O), acting as a critical role in atmospheric chemistry. Updated data derived from 114 peer‐reviewed publications with 520 field measurements were synthesized using meta‐analysis procedure to examine the N fertilizer‐induced soil NO and the combined NO+N₂O emissions across global soils. Besides factors identified in earlier reviews, additional factors responsible for NO fluxes were fertilizer type, soil C/N ratio, crop residue incorporation, tillage, atmospheric carbon dioxide concentration, drought and biomass burning. When averaged across all measurements, soil NO‐N fluxes were estimated to be 4.06 kg ha^?1 yr^?1, with the greatest (9.75 kg ha^?1 yr^?1) in vegetable croplands and the lowest (0.11 kg ha^?1 yr^?1) in rice paddies. Soil NO emissions were more enhanced by synthetic N fertilizer (+38%), relative to organic (+20%) or mixed N (+18%) sources. Compared with synthetic N fertilizer alone, synthetic N fertilizer combined with nitrification inhibitors substantially reduced soil NO emissions by 81%. The global mean direct emission factors of N fertilizer for NO (EF_NO) and combined NO+N₂O (EF_c) were estimated to be 1.16% and 2.58%, with 95% confidence intervals of 0.71–1.61% and 1.81–3.35%, respectively. Forests had the greatest EF_NO (2.39%). Within the croplands, the EF_NO (1.71%) and EF_c (4.13%) were the greatest in vegetable cropping fields. Among different chemical N fertilizer varieties, ammonium nitrate had the greatest EF_NO (2.93%) and EF_c (5.97%). Some options such as organic instead of synthetic N fertilizer, decreasing N fertilizer input rate, nitrification inhibitor and low irrigation frequency could be adopted to mitigate soil NO emissions. More field measurements over multiyears are highly needed to minimize the estimate uncertainties and mitigate soil NO emissions, particularly in forests and vegetable croplands.     

Nitrous oxide fluxes from corn fields: on-farm assessment of the amount and timing of nitrogen fertilizer

Nitrogen fertilization is considered as an important source of atmospheric N₂O emission. A seven site‐year on‐farm field experiment was conducted at Ottawa and Guelph, ON and Saint‐Valentin, QC, Canada to characterize the affect of the amount and timing of N fertilizer on N₂O emission in corn (Zea mays L.) production. Using the static chamber method, gas samples were collected for 28‐days after preplant and 28‐days after sidedress fertilization at the seven site‐year, resulting in 14 monitoring periods. For both methods of fertilization, peak N₂O flux and cumulative emission increased with the amount of N applied, with rates ranging from 30 to 900 μg N m^?2 h^?1. Depending on N amount and time of application, cumulative emission varied from 0.05 to 2.42 kg N ha^?1, equivalent to 0.03% to 1.45% of the N fertilizer applied. Differences in N₂O emission peaks among fertilizer treatments were clearly separated in 13 out of 14 monitoring periods. Total N₂O emissions may have been underestimated compared with annual monitoring in 10 out of the 49 cases because the monitoring period ended before N₂O efflux returned to the baseline level. The flux of N₂O was negligible when soil mineral N in the 0–15 cm layer was < 20 mg N kg^?1. While rainfall stimulated emission, soil temperature > 15 °C was likely the driving force responsible for the higher levels of N₂O found for sidedress than preplant application methods. However, caution must be taken when interpreting these later results as preplant fertilization may have continuously stimulated N₂O emissions after the 28‐days monitoring period, especially in situations where N₂O effluxes have not fallen back to their baseline levels. Increasing fertilizer rates from 90 to 150 kg N ha^?1 resulted in slight increases in yields, but doubled cumulative N₂O emissions.     

Nitrous oxide emissions from agricultural fields: Assessment, measurement and mitigation

In this paper we discuss three topics concerning N₂O emissions from agricultural systems. First, we present an appraisal of N₂O emissions from agricultural soils (Assessment). Secondly, we discuss some recent efforts to improve N₂O flux estimates in agricultural fields (Measurement), and finally, we relate recent studies which use nitrification inhibitors to decrease N₂O emissions from N-fertilized fields (Mitigation).To assess the global emission of N₂O from agricultural soils, the total flux should represent N₂O from all possible sources; native soil N, N from recent atmospheric deposition, past years fertilization, N from crop residues, N₂O from subsurface aquifers below the study area, and current N fertilization. Of these N sources only synthetic fertilizer and animal manures and the area of fields cropped with legumes have sufficient global data to estimate their input for N₂O production. The assessment of direct and indirect N₂O emissions we present was made by multiplying the amount of fertilizer N applied to agricultural lands by 2% and the area of land cropped to legumes by 4 kg N₂O-N ha^-1. No regard to method of N application, type of N, crop, climate or soil was given in these calculations, because the data are not available to include these variables in large scale assessments. Improved assessments should include these variables and should be used to drive process models for field, area, region and global scales.Several N₂O flux measurement techniques have been used in recent field studies which utilize small and ultralarge chambers and micrometeorological along with new analytical techniques to measure N₂O fluxes. These studies reveal that it is not the measurement technique that is providing much of the uncertainty in N₂O flux values found in the literature but rather the diverse combinations of physical and biological factors which control gas fluxes. A careful comparison of published literature narrows the range of observed fluxes as noted in the section on assessment. An array of careful field studies which compare a series of crops, fertilizer sources, and management techniques in controlled parallel experiments throughout the calendar year are needed to improve flux estimates and decrease uncertainty in prediction capability.There are a variety of management techniques which should conserve N and decrease the amount of N application needed to grow crops and to limit N₂O emissions. Using nitrification inhibitors is an option for decreasing fertilizer N use and additionally directly mitigating N₂O emissions. Case studies are presented which demonstrate the potential for using nitrification inhibitors to limit N₂O emissions from agricultural soils. Inhibitors may be selected for climatic conditions and type of cropping system as well as the type of nitrogen (solid mineral N, mineral N in solution, or organic waste materials) and applied with the fertilizers.     

Net annual global warming potential and greenhouse gas intensity in Chinese double rice‐cropping systems: a 3‐year field measurement in long‐term fertilizer experiments

The impact of agricultural management on global warming potential (GWP) and greenhouse gas intensity (GHGI) is not well documented. A long‐term fertilizer experiment in Chinese double rice‐cropping systems initiated in 1990 was used in this study to gain an insight into a complete greenhouse gas accounting of GWP and GHGI. The six fertilizer treatments included inorganic fertilizer [nitrogen and phosphorus fertilizer (NP), nitrogen and potassium fertilizer (NK), and balanced inorganic fertilizer (NPK)], combined inorganic/organic fertilizers at full and reduced rate (FOM and ROM), and no fertilizer application as a control. Methane (CH₄) and nitrous oxide (N₂O) fluxes were measured using static chamber method from November 2006 through October 2009, and the net ecosystem carbon balance was estimated by the changes in topsoil (0–20 cm) organic carbon (SOC) density over the 10‐year period 1999–2009. Long‐term fertilizer application significantly increased grain yields, except for no difference between the NK and control plots. Annual topsoil SOC sequestration rate was estimated to be 0.96 t C ha^?1 yr^?1 for the control and 1.01–1.43 t C ha^?1 yr^?1 for the fertilizer plots. Long‐term inorganic fertilizer application tended to increase CH₄ emissions during the flooded rice season and significantly increased N₂O emissions from drained soils during the nonrice season. Annual mean CH₄ emissions ranged from 621 kg CH₄ ha^?1 for the control to 1175 kg CH₄ ha^?1 for the FOM plots, 63–83% of which derived from the late‐rice season. Annual N₂O emission averaged 1.15–4.11 kg N₂O–N ha^?1 in the double rice‐cropping systems. Compared with the control, inorganic fertilizer application slightly increased the net annual GWPs, while they were remarkably increased by combined inorganic/organic fertilizer application. The GHGI was lowest for the NP and NPK plots and highest for the FOM and ROM plots. The results of this study suggest that agricultural economic viability and GHGs mitigation can be simultaneously achieved by balanced fertilizer application.     

Direct nitrous oxide emissions from oilseed rape cropping – a meta‐analysis

Oilseed rape is one of the leading feedstocks for biofuel production in Europe. The climate change mitigation effect of rape methyl ester (RME) is particularly challenged by the greenhouse gas (GHG) emissions during crop production, mainly as nitrous oxide (N₂O) from soils. Oilseed rape requires high nitrogen fertilization and crop residues are rich in nitrogen, both potentially causing enhanced N₂O emissions. However, GHG emissions of oilseed rape production are often estimated using emission factors that account for crop‐type specifics only with respect to crop residues. This meta‐analysis therefore aimed to assess annual N₂O emissions from winter oilseed rape, to compare them to those of cereals and to explore the underlying reasons for differences. For the identification of the most important factors, linear mixed effects models were fitted with 43 N₂O emission data points deriving from 12 different field sites. N₂O emissions increased exponentially with N‐fertilization rates, but interyear and site‐specific variability were high and climate variables or soil parameters did not improve the prediction model. Annual N₂O emissions from winter oilseed rape were 22% higher than those from winter cereals fertilized at the same rate. At a common fertilization rate of 200 kg N ha^?1 yr^?1, the mean fraction of fertilizer N that was lost as N₂O‐N was 1.27% for oilseed rape compared to 1.04% for cereals. The risk of high yield‐scaled N₂O emissions increased after a critical N surplus of about 80 kg N ha^?1 yr^?1. The difference in N₂O emissions between oilseed rape and cereal cultivation was especially high after harvest due to the high N contents in oilseed rape's crop residues. However, annual N₂O emissions of winter oilseed rape were still lower than predicted by the Stehfest and Bouwman model. Hence, the assignment of oilseed rape to the crop‐type classes of cereals or other crops should be reconsidered.     

Predicting N2O emissions from agricultural land through related soil parameters

An empirical model of nitrous oxide emission from agricultural soils has been developed. It is based on the relationship between N₂O and three soil parameters – soil mineral N (ammonium plus nitrate) content in the topsoil, soil water‐filled pore space and soil temperature – determined in a study on a fertilized grassland in 1992 and 1993. The model gave a satisfactory prediction of seasonal fluxes in other seasons when fluxes were much higher, and also from other grassland sites and from cereal and oilseed rape crops, over a wide flux range (< 1 to > 20 kg N₂O‐N ha^?1 y^?1). However, the model underestimated emissions from potato and broccoli crops; possible reasons for this are discussed. This modelling approach, based as it is on well‐established and widely used soil measurements, has the potential to provide flux estimates from a much wider range of agricultural sites than would be possible by direct measurement of N₂O emissions.     

Nitrous oxide fluxes from a grain–legume crop (narrow‐leafed lupin) grown in a semiarid climate

Understanding nitrous oxide (N₂O) fluxes from grain–legume crops in semiarid and arid regions is necessary if we are to improve our knowledge of global terrestrial N₂O losses resulting from biological N₂ fixation. N₂O fluxes were measured from a rain‐fed soil, cropped to a grain–legume in a semiarid region of southwestern Australia for 1 year on a subdaily basis. The site included plots planted to narrow‐leafed lupin (Lupinus angustifolius; ‘lupin’) and plots left bare (no lupin). Fluxes were measured using soil chambers connected to a fully automated system that measured N₂O by gas chromatography. Daily N₂O fluxes were low (?0.5 to 24 g N₂O‐N ha^?1 day^?1) and not different between treatments, culminating in an annual loss of 127 g N₂O‐N ha^?1. Greatest daily N₂O fluxes occurred from both treatments in the postharvest period, and following a series of summer and autumn rainfall events. At this time of the year, soil conditions were conducive to soil microbial N₂O production: elevated soil water contents, increased inorganic nitrogen (N) and dissolved organic carbon concentrations, and soil temperatures generally > 25 °C; furthermore, there was no active plant growth to compete for mineralized N. N₂O emissions from the decomposition of legume crop residue were low, and approximately half that predicted using the currently recommended IPCC methodology. Furthermore, the contribution of the biological N₂ fixation process to N₂O emissions appeared negligible in the present study, supporting its omission as a source of N₂O from the IPCC methodology for preparing national greenhouse gas inventories.     

Nonlinear nitrous oxide (N2O) response to nitrogen fertilizer in on‐farm corn crops of the US Midwest

Row‐crop agriculture is a major source of nitrous oxide (N₂O) globally, and results from recent field experiments suggest that significant decreases in N₂O emissions may be possible by decreasing nitrogen (N) fertilizer inputs without affecting economic return from grain yield. We tested this hypothesis on five commercially farmed fields in Michigan, USA planted with corn in 2007 and 2008. Six rates of N fertilizer (0–225 kg N ha^?1) were broadcast and incorporated before planting, as per local practice. Across all sites and years, increases in N₂O flux were best described by a nonlinear, exponentially increasing response to increasing N rate. N₂O emission factors per unit of N applied ranged from 0.6% to 1.5% and increased with increasing N application across all sites and years, especially at N rates above those required for maximum crop yield. At the two N fertilizer rates above those recommended for maximum economic return (135 kg N ha^?1), average N₂O fluxes were 43% (18 g N₂O–N ha^?1 day^?1) and 115% (26 g N₂O–N ha^?1 day^?1) higher than were fluxes at the recommended rate, respectively. The maximum return to nitrogen rate of 154 kg N ha^?1 yielded an average 8.3 Mg grain ha^?1. Our study shows the potential to lower agricultural N₂O fluxes within a range of N fertilization that does not affect economic return from grain yield.     

Nonlinear response of N2O flux to incremental fertilizer addition in a continuous maize (Zea mays L.) cropping system

The relationship between nitrous oxide (N₂O) flux and N availability in agricultural ecosystems is usually assumed to be linear, with the same proportion of nitrogen lost as N₂O regardless of input level. We conducted a 3‐year, high‐resolution N fertilizer response study in southwest Michigan USA to test the hypothesis that N₂O fluxes increase mainly in response to N additions that exceed crop N needs. We added urea ammonium nitrate or granular urea at nine levels (0–292 kg N ha^?1) to four replicate plots of continuous maize. We measured N₂O fluxes and available soil N biweekly following fertilization and grain yields at the end of the growing season. From 2001 to 2003 N₂O fluxes were moderately low (ca. 20 g N₂O‐N ha^?1 day^?1) at levels of N addition to 101 kg N ha^?1, where grain yields were maximized, after which fluxes more than doubled (to >50 g N₂O‐N ha^?1 day^?1). This threshold N₂O response to N fertilization suggests that agricultural N₂O fluxes could be reduced with no or little yield penalty by reducing N fertilizer inputs to levels that just satisfy crop needs.     

Woody biochar's greenhouse gas mitigation potential across fertilized and unfertilized agricultural soils and soil moisture regimes

Biochar has been widely researched as an important technology for climate smart agriculture, yet work is still necessary to identify the magnitude of potential greenhouse gas (GHG) mitigation and mechanisms involved. This study measured slow‐pyrolysis wood‐derived biochar's impact on GHG efflux, mineral N dynamics, and soil organic C in a series of two incubations across fertilized and unfertilized agricultural soils and soil moisture regimes. This research explored the magnitude of biochar's full GHG mitigation potential and drivers of such impacts. Results of this incubation indicate slow‐pyrolysis wood‐derived biochar has potential to provide annual emission reductions of 0.58–1.72 Mg CO₂‐eq ha^?1 at a 25 Mg ha^?1 biochar application rate. The greatest GHG mitigation potential was from C sequestration and nitrous oxide (N₂O) reduction in mineral N fertilized soils, with minimal impacts on N₂O emissions in unfertilized soils, carbon dioxide (CO₂) emissions, and methane (CH₄) uptake. Analysis of mineral N dynamics in the bulk soil and on biochar isolates indicated that neither biochar impacts on net mineralization and nitrification nor retention of ammonium () on biochar isolates could explain biochar's N₂O reduction. Instead, biochar amendments exhibited consistent N₂O emission reductions relative to the N₂O emission in the control soil regardless of soil type and fertilization. Results across a soil moisture gradient suggest that woody biochar may aerate soils shifting redox conditions and subsequent N₂O production. Understanding the magnitude of biochar's GHG reduction potential and the mechanisms driving these effects can help inform biochar modeling efforts, explain field results and identify agricultural applications that maximize biochar's full GHG mitigation potential.     

Intensive measurement of nitrous oxide emissions from a corn–soybean–wheat rotation under two contrasting management systems over 5 years

No‐tillage (NT), a practice that has been shown to increase carbon sequestration in soils, has resulted in contradictory effects on nitrous oxide (N₂O) emissions. Moreover, it is not clear how mitigation practices for N₂O emission reduction, such as applying nitrogen (N) fertilizer according to soil N reserves and matching the time of application to crop uptake, interact with NT practices. N₂O fluxes from two management systems [conventional (CP), and best management practices: NT + reduced fertilizer (BMP)] applied to a corn (Zea mays L.), soybean (Glycine max L.), winter‐wheat (Triticum aestivum L.) rotation in Ontario, Canada, were measured from January 2000 to April 2005, using a micrometeorological method. The superimposition of interannual variability of weather and management resulted in mean monthly N₂O fluxes ranging from − 1.9 to 61.3 g N ha⁻¹ day⁻¹. Mean annual N₂O emissions over the 5‐year period decreased significantly by 0.79 from 2.19 kg N ha⁻¹ for CP to 1.41 kg N ha⁻¹ for BMP. Growing season (May–October) N₂O emissions were reduced on average by 0.16 kg N ha⁻¹ (20% of total reduction), and this decrease only occurred in the corn year of the rotation. Nongrowing season (November–April) emissions, comprised between 30% and 90% of the annual emissions, mostly due to increased N₂O fluxes during soil thawing. These emissions were well correlated (r²= 0.90) to the accumulated degree‐hours below 0 °C at 5 cm depth, a measure of duration and intensity of soil freezing. Soil management in BMP (NT) significantly reduced N₂O emissions during thaw (80% of total reduction) by reducing soil freezing due to the insulating effects of the larger snow cover plus corn and wheat residue during winter. In conclusion, significant reductions in net greenhouse gas emissions can be obtained when NT is combined with a strategy that matches N application rate and timing to crop needs.     

Infield greenhouse gas emissions from sugarcane soils in Brazil: effects from synthetic and organic fertilizer application and crop trash accumulation

Bioethanol from sugarcane is becoming an increasingly important alternative energy source worldwide as it is considered to be both economically and environmentally sustainable. Besides being produced from a tropical perennial grass with high photosynthetic efficiency, sugarcane ethanol is commonly associated with low N fertilizer use because sugarcane from Brazil, the world's largest sugarcane producer, has a low N demand. In recent years, several models have predicted that the use of sugarcane ethanol in replacement to fossil fuel could lead to high greenhouse gas (GHG) emission savings. However, empirical data that can be used to validate model predictions and estimates from indirect methodologies are scarce, especially with regard to emissions associated with different fertilization methods and agricultural management practices commonly used in sugarcane agriculture in Brazil. In this study, we provide in situ data on emissions of three GHG (CO₂, N₂O, and CH₄) from sugarcane soils in Brazil and assess how they vary with fertilization methods and management practices. We measured emissions during the two main phases of the sugarcane crop cycle (plant and ratoon cane), which include different fertilization methods and field conditions. Our results show that N₂O and CO₂ emissions in plant cane varied significantly depending on the fertilization method and that waste products from ethanol production used as organic fertilizers with mineral fertilizer, as it is the common practice in Brazil, increase emission rates significantly. Cumulatively, the highest emissions were observed for ratoon cane treated with vinasse (liquid waste from ethanol production) especially as the amount of crop trash on the soil surface increased. Emissions of CO₂ and N₂O were 6.9 kg ha^?1 yr^?1 and 7.5 kg ha^?1 yr^?1, respectively, totaling about 3000 kg in CO₂ equivalent ha^?1 yr^?1.