Integrated management practices significantly affect N2O emissions and wheat–maize production at field scale in the North China Plain

In the North China Plain, a field experiment was conducted to measure nitrous oxide (N₂O) and methane (CH₄) fluxes from a typical winter wheat–summer maize rotation system under five integrated agricultural management practices: conventional regime [excessive nitrogen (N) fertilization, flood irrigation, and rotary tillage before wheat sowing; CON], recommended regime 1 (balanced N fertilization, decreased irrigation, and deep plowing before wheat sowing; REC-1), recommended regime 2 (balanced N fertilization, decreased irrigation, and no tillage; REC-2), recommended regime 3 (controlled release N fertilizer, decreased irrigation, and no tillage; REC-3), and no N fertilizer (CK). Field measurements indicated that pulse emissions after N fertilization and irrigation contributed 19–49 % of annual N₂O emissions. In contrast to CON (2.21 kg N₂O-N ha^?1 year^?1), the other treatments resulted in significant declines in cumulative N₂O emissions, which ranged from 0.96 to 1.76 kg N₂O-N ha^?1 year^?1, indicating that the recommended practices (e.g., balanced N fertilization, controlled release N fertilizer, and decreased irrigation) offered substantial benefits for both sustaining grain yield and reducing N₂O emissions. Emission factors of N fertilizer were 0.21, 0.22, 0.23, and 0.37 % under CON, REC-1, REC-3, and REC-2, respectively. Emissions of N₂O during the freeze–thaw cycle period and the winter freezing period accounted for 9.7 and 5.1 % of the annual N₂O budget, respectively. Thus, we recommend that the monitoring frequency should be increased during the freeze–thaw cycle period to obtain a proper estimate of total emissions. Annual CH₄ fluxes from the soil were low (?1.54 to ?1.12 kg CH₄-C ha^?1 year^?1), and N fertilizer application had no obvious effects on CH₄ uptake. Values of global warming potential were predominantly determined by N₂O emissions, which were 411 kg CO₂-eq ha^?1 year^?1 in the CK and 694–982 kg CO₂-eq ha^?1 year^?1 in the N fertilization regimes. When comprehensively considering grain yield, global warming potential intensity values in REC-1, REC-2, and REC-3 were significantly lower than in CON. Meanwhile, grain yield increased slightly under REC-1 and REC-3 compared to CON. Generally, REC-1 and REC-3 are recommended as promising management regimes to attain the dual objectives of sustaining grain yield and reducing greenhouse gas emissions in the North China Plain.     

Nitrous oxide (N2O) emissions in response to increasing fertilizer addition in maize (Zea mays L.) agriculture in western Kenya

National and regional efforts are underway to increase fertilizer use in sub-Saharan Africa, where attaining food security is a perennial challenge and mean fertilizer use in many countries is <10 % of nationally recommended rates. Increases in nitrogen (N) inputs will likely cause increased emissions of the greenhouse gas nitrous oxide (N₂O). We established experimental plots with different rates of N applied to maize (Zea mays) in a field with a history of nutrient additions in western Kenya and measured N₂O fluxes. Fertilizer was applied by hand at 0, 50, 75, 100, and 200 kg N ha^?1 in a split application on March 22 and April 20, 2010. Gas sampling was conducted daily during the week following applications, and was otherwise collected weekly or biweekly until June 29, 2010. Cumulative fluxes were highest from the 200 kg N ha^?1 treatment, with emissions of 810 g N₂O–N ha^?1; fluxes from other treatments ranged from 620 to 710 g N₂O–N ha^?1, but with no significant differences among treatments. Emissions of N₂O during the 99-day measurement period represented <0.1 % of added fertilizer N for all treatments. Though limited to a single year, these results provide further evidence that African agricultural systems may have N₂O emission factors substantially lower than the global mean.     

Nitrous oxide emissions and herbage accumulation in smooth bromegrass pastures with nitrogen fertilizer and ruminant urine application

Agricultural soils contribute significantly to nitrous oxide (N₂O) emissions, but little data is available on N₂O emissions from smooth bromegrass (Bromus inermis Leyss.) pastures. This study evaluated soil N₂O emissions and herbage accumulation from smooth bromegrass pasture in eastern Nebraska, USA. Nitrous oxide emissions were measured biweekly from March to October in 2011 and 2012 using vented static chambers on smooth bromegrass plots treated with a factorial combination of five urea nitrogen (N) fertilizer rates (0, 45, 90, 135, and 180 kg ha^?1) and two ruminant urine treatments (distilled water and urine). Urine input strongly affected daily and cumulative N₂O emissions, but responses to N fertilizer rate depended on growing season rainfall. In 2011, when rainfall was normal, cumulative N₂O emissions increased exponentially with N fertilizer rate. In 2012, drought reduced daily and cumulative N₂O emission responses to N fertilizer rate. Herbage accumulation ranged from 4.46 Mg ha^?1 in unfertilized pasture with distilled water to 16.01 Mg ha^?1 in pasture with 180 kg N ha^?1 and urine in 2011. In 2012, plots treated with urine had 2.2 times more herbage accumulation than plots treated with distilled water but showed no response to N fertilizer rate. Total applied N lost as N₂O ranged from 0–0.6 to 0.5–1.7 % across N fertilizer rates in distilled water and urine treatments, respectively, and thus, support the Intergovernmental Panel on Climate Change default direct emission factors of 1.0 % for N fertilizer additions and 2.0 % for urine excreted by cattle on pasture.     

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.     

Soil N2O and CO2 emissions from cotton in Australia under varying irrigation management

Irrigation is known to stimulate soil microbial carbon and nitrogen turnover and potentially the emissions of nitrous oxide (N₂O) and carbon dioxide (CO₂). We conducted a study to evaluate the effect of three different irrigation intensities on soil N₂O and CO₂ fluxes and to determine if irrigation management can be used to mitigate N₂O emissions from irrigated cotton on black vertisols in South-Eastern Queensland, Australia. Fluxes were measured over the entire 2009/2010 cotton growing season with a fully automated chamber system that measured emissions on a sub-daily basis. Irrigation intensity had a significant effect on CO₂ emission. More frequent irrigation stimulated soil respiration and seasonal CO₂ fluxes ranged from 2.7 to 4.1 Mg-C ha^?1 for the treatments with the lowest and highest irrigation frequency, respectively. N₂O emission happened episodic with highest emissions when heavy rainfall or irrigation coincided with elevated soil mineral N levels and seasonal emissions ranged from 0.80 to 1.07 kg N₂O-N ha^?1 for the different treatments. Emission factors (EF = proportion of N fertilizer emitted as N₂O) over the cotton cropping season, uncorrected for background emissions, ranged from 0.40 to 0.53 % of total N applied for the different treatments. There was no significant effect of the different irrigation treatments on soil N₂O fluxes because highest emission happened in all treatments following heavy rainfall caused by a series of summer thunderstorms which overrode the effect of the irrigation treatment. However, higher irrigation intensity increased the cotton yield and therefore reduced the N₂O intensity (N₂O emission per lint yield) of this cropping system. Our data suggest that there is only limited scope to reduce absolute N₂O emissions by different irrigation intensities in irrigated cotton systems with summer dominated rainfall. However, the significant impact of the irrigation treatments on the N₂O intensity clearly shows that irrigation can easily be used to optimize the N₂O intensity of such a system.     

Nitrous oxide emissions as influenced by legume cover crops and nitrogen fertilization

In this study, we measured nitrous oxide (N₂O) fluxes from plots of fall-planted hairy vetch (HV, Vicia villosa) and spring-planted broadleaf vetch (BLV, Vicia narbonensis) grown as nitrogen (N) sources for following summer forage crabgrass (Digitaria sanguinalis). Comparisons also included 60 kg ha^?1 inorganic N fertilizer for crabgrass at planting (60-N) and a control without N fertilizer. Each treatment had six replicated plots across the slope. Fluxes were measured with closed chamber systems during the period between spring growth of cover crops and first-cut of crabgrass in mid-July. HV had strong stand and aboveground biomass had 185?±?50 kg N ha^?1 (mean?±?standard error, n?=?6) at termination. However, BLV did not establish well and aboveground biomass had only 35?±?15 kg N ha^?1. Ratio vegetation index of crabgrass measured as proxy of biomass growth was highest in HV treatment. However, total aboveground biomass of crabgrass was statistically similar to 60-N plots. Fluxes of N₂O were low prior to termination of cover crops but were as high as 8.2 kg N₂O ha^?1 day^?1 from HV plots after termination. The fluxes were enhanced by large rainfall events recorded after biomass incorporation. Rainfall enhanced N₂O fluxes were also observed in other treatments, but their magnitudes were much smaller. The high N₂O fluxes from HV plots contributed to emissions of 30.3?±?12.4 kg N₂O ha^?1 within 30 days of biomass incorporation. Emissions were only 2.0?±?0.7, 3.4?±?1.3 and 1.0?±?0.4 kg N₂O ha^?1 from BLV, 60-N and control plots, respectively.     

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.     

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 and methane emissions from cultivated seasonal wetland (dambo) soils with inorganic,organic and integrated nutrient management

In many smallholder farming areas southern Africa, the cultivation of seasonal wetlands (dambos) represent an important adaptation to climate change. Frequent droughts and poor performance of rain-fed crops in upland fields have resulted in mounting pressure to cultivate dambos where both organic and inorganic amendments are used to sustain crop yields. Dambo cultivation potentially increases greenhouse gas (GHG) emissions. The objective of the study was to quantify the effects of applying different rates of inorganic nitrogen (N) fertilisers (60, 120, 240 kg N ha^?1) as NH₄NO₃, organic manures (5,000, 10,000 and 15,000 kg ha^?1) and a combination of both sources (integrated management) on GHG emissions in cultivated dambos planted to rape (Brassica napus). Nitrous oxide (N₂O) emissions in plots with organic manures ranged from 218 to 894 µg m^?2 h^?1, while for inorganic N and integrated nutrient management, emissions ranged from 555 to 5,186 µg m^?2 h^?1 and 356–2,702 µg m^?2 h^?1 respectively. Cropped and fertilised dambos were weak sources of methane (CH₄), with emissions ranging from ?0.02 to 0.9 mg m^?2 h^?1, while manures and integrated management increased carbon dioxide (CO₂) emissions. However, crop yields were better under integrated nutrient management. The use of inorganic fertilisers resulted in higher N₂O emission per kg yield obtained (6–14 g N₂O kg^?1 yield), compared to 0.7–4.5 g N₂O kg^?1 yield and 1.6–4.6 g N₂O kg^?1 yield for organic manures and integrated nutrient management respectively. This suggests that the use of organic and integrated nutrient management has the potential to increase yield and reduce yield scaled N₂O emissions.     

Seasonal N<Subscript>2</Subscript>O emissions respond differently to environmental and microbial factors after fertilization in wheat–maize agroecosystem

Biogeochemical processes regulating cropland soil nitrous oxide (N₂O) emissions are complex, and the controlling factors need to be better understood, especially for seasonal variation after fertilization. Seasonal patterns of N₂O emissions and abundances of archaeal ammonia monooxygenase (amoA), bacterial amoA, nitrate reductase (narG), nitrite reductase (nirS/nirK), and nitrous oxide reductase (nosZ) genes in long-term fertilized wheat–maize soils have been studied to understand the roles of microbes in N₂O emissions. The results showed that fertilization greatly stimulated N₂O emission with higher values in pig manure-treated soil (OM, 2.88 kg N ha^?1 year^?1) than in straw-returned (CRNPK, 0.79 kg N ha^?1 year^?1) and mineral fertilizer-treated (NPK, 0.90 kg N ha^?1 year^?1) soils. Most (52.2–88.9%) cumulative N₂O emissions occurred within 3 weeks after fertilization. Meanwhile, N₂O emissions within 3 weeks after fertilization showed a positive correlation with narG gene copy number and a negative correlation with soil NO₃^? contents. The abundances of narG and nosZ genes had larger direct effects (1.06) than ammonium oxidizers (0.42) on N₂O emissions according to partial least squares path modeling. Stepwise multiple regression also showed that log narG was a predictor variable for N₂O emissions. This study suggested that denitrification was the major process responsible for N₂O emissions within 3 weeks after fertilization. During the remaining period of crop growth, insufficient N substrate and low temperature became the primary limiting factors for N₂O emission according to the results of the regression models.     

Season and location–specific nitrous oxide emissions in an almond orchard in California

Almonds are an important commodity in California and account for around 15% of the state’s fertilizer nitrogen (N) consumption. Motivated by strong correlations typically observed between fertilizer N inputs and emissions of the potent greenhouse gas and ozone depleting molecule nitrous oxide (N₂O), this study aimed to characterize spatial and temporal patterns in N₂O emissions in an almond orchard under typical agronomic management. N₂O fluxes were measured for a total of 2.5 years, including 3 growing seasons and 2 dormant seasons. Measurements targeted two functional locations, defined as tree rows and tractor rows. In conjunction with the flux measurements, we determined driving variables including soil ammonium (NH₄ ⁺) and nitrate (NO₃ ^?), dissolved organic carbon (DOC), soil water-filled pore space (WFPS), soil pH, air temperature and precipitation. Cumulative annual N₂O emissions were low (0.65 ± 0.07 and 0.53 ± 0.19 kg N₂O–N ha^?1 year^?1 in year 1 and 2, respectively), likely due to the coarse soil texture and microject sprinkler irrigation and fertigation system. Emission factors (EF), conservatively calculated as the ratio of N₂O emitted to fertilizer N applied, were 0.25 ± 0.03% and 0.19 ± 0.07% for year 1 and 2, respectively, which is below the IPCC EF range of 0.3–3%. Correlation analyses between N₂O and driving variables suggested that overall N₂O production was limited by microbial activity and nitrification was likely the major source process, but specific drivers of N₂O emissions varied between seasons and functional locations.     

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.     

Recovery of groundwater N2O at the soil surface and its contribution to total N2O emissions

Production and accumulation of the major greenhouse gas nitrous oxide (N₂O) in surface groundwater might contribute to N₂O emissions to the atmosphere. We report on a ¹⁵N tracer study conducted in the Fuhrberger Feld aquifer in northern Germany. A K¹⁵NO₃ tracer solution (60 atom%) was applied to the surface groundwater on an 8 m² measuring plot using 45 injection points in order to stimulate production of ¹⁵N₂O by denitrification and to detect its contribution to emissions at the soil surface. Samples from the surface groundwater, from the unsaturated zone and at the soil surface were collected in regular intervals over a 72-days period. Total N₂O fluxes at the soil surface were low and in a range between ?7.6 and 29.1 μg N₂O-N m^?2 h^?1. ¹⁵N enrichment of N₂O decreased considerably upwards in the profile. In the surface groundwater, we found a ¹⁵N enrichment of N₂O between 13 and 42 atom%. In contrast, ¹⁵N enrichment of N₂O in flux chambers at the soil surface was very low, but a detectable ¹⁵N enrichment was found at all sampling events. Fluxes of groundwater-derived ¹⁵N-N₂O were very low and ranged between 0.0002 and 0.0018 kg N₂O-N ha^?1 year^?1, indicating that indirect N₂O emissions from the surface groundwater of the Fuhrberger Feld aquifer occurring via upward diffusion are hardly significant. Due to these observations we concluded that N₂O dynamics at the soil–atmosphere interface is predominantly governed by topsoil parameters. However, highest ¹⁵N enrichments of N₂O throughout the profile were obtained in the course of a rapid drawdown of the groundwater table. We assume that such fluctuations may enhance diffusive N₂O fluxes from the surface groundwater to the atmosphere for a short time.     

Nitrous oxide fluxes in a Brazilian clayey oxisol after 24 years of integrated crop-livestock management

Integrated crop-livestock systems have been recently adopted in several agricultural regions of Brazil. Studies involving the effect of adopting integrated systems on greenhouse gas mitigation are essential for choosing sustainable agricultural systems. In this study, the emissions of nitrous oxide in a crop-livestock system (4-year crop/pasture rotation) compared with two continuous crop (CC) areas under conventional and no-tillage management were investigated. The treatments consisted of continuous cropping under no-tillage (CC-NT), continuous cropping with annual heavy disc harrow (CC-CT), an integrated crop-livestock system under no-tillage (CLS-NT) and native Cerrado as a reference. Considering the cumulative N₂O emissions in a year, the CC-CT emitted 2.55 kg N-N₂O ha^?1, higher than the Cerrado, which emitted 0.55 kg N-N₂O ha^?1. All the agricultural systems emitted more N₂O than the Cerrado, however, the two conservation systems CC-NT and CLS-NT had lower emissions than the CC-CT, and were responsible for 1.90 and 1.52 kg N-N₂O ha^?1, respectively. In the agroecosystems, the highest N₂O fluxes were observed after fertilization and rainfall events. In the CC systems, N₂O emissions were greater than in the integrated system during the sorghum/off-season period, but in the CC-CT emissions were greater than in CC-NT. During the soybean cycle no differences in emissions were observed between both CC systems, which surpassed that in CLS-NT that was occupied by Brachiaria pasture. The annual cumulative N₂O emissions in CLS-NT were close to that observed in the Cerrado indicating this system to be an agricultural practice with potential to mitigate N₂O emissions.     

The Effect of Future Climate Perturbations on N<Subscript>2</Subscript>O Emissions from a Fertilized Humid Grassland

N₂O emissions from a fertilized humid grassland near Cork, Ireland were continuously measured during 2003 using an eddy covariance system. For most of the year emissions were close to zero and 60% of the emissions occurred in eight major events of 2–20 days’ duration. Two hundred and seven kg ha⁻¹ of synthetic N and 130 kg ha⁻¹ organic N were applied over the year and the total measured annual N₂O emission was 11.6 kg N ha⁻¹. The flux data were used to test the prediction of N₂O emissions by the DNDC (DeNitrification – DeComposition) model. The model predicted total emissions of 15.4 kg N ha⁻¹, 32 % more than the observed emissions. On this basis the model was further used to simulate (a) background (non-anthropogenic) N₂O emissions and (b) the effect on N₂O emissions of future climate perturbations based on the Hadley Center model output of the IS92a scenario for Ireland. DNDC predicts 1.7 kg N ha⁻¹ year⁻¹ of background N₂O emissions, accounting for 15% of the observed emissions. Climate shifts will increase total annual modeled N₂O emissions from 15.4 kg N ha⁻¹ to 22.4 kg N ha⁻¹ if current levels of N applications are maintained, or to 21.2 kg N ha⁻¹ if synthetic N applications are reduced to 170 kg N ha⁻¹ to comply with recent EU water quality legislation. Thus the projected increase in N₂O emissions due to climate change is far larger than the decrease expected from reduced fertilizer applications.     

Soil organic matter,greenhouse gases and net global warming potential of irrigated conventional,reduced-tillage and organic cropping systems

Reducing tillage intensity and diversifying crop rotations may improve the sustainability of irrigated cropping systems in semi-arid regions. The objective of this study was to compare the greenhouse gas (GHG) emissions, soil organic matter, and net global warming potential (net GWP) of a sugar beet (Beta vulgaris L.)-corn (Zea mays L,) rotation under conventional (CT) and reduced-tillage (RT) and a corn-dry bean (Phaseolus vulgaris L.) rotation under organic (OR) management during the third and fourth years of 4-year crop rotations. The gas and soil samples were collected during April 2011–March 2013, and were analyzed for carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) emissions, water-filled pore space (WFPS), soil nitrate (NO₃ ^?–N) and ammonium (NH₄ ⁺–N) concentrations, soil organic carbon (SOC) and total nitrogen (TN), and net global warming potential (net GWP). Soils under RT had 26% lower CO₂ emissions compared to 10.2 kg C ha^?1 day^?1 and 43% lower N₂O emissions compared to 17.5 g N ha^?1 day^?1 in CT during cropping season 2011, and no difference in CO₂ and N₂O emissions during cropping season 2012. The OR emitted 31% less N₂O, but 74% more CO₂ than CT during crop season 2011. The RT had 34% higher SOC content than CT (17.9 Mg ha^?1) while OR was comparable with CT. Net GWP was negative for RT and OR and positive for CT. The RT and OR can increase SOC sequestration, mitigate GWP and thereby support in the development of sustainable cropping systems in semiarid agroecosystems.     

Mitigation potential of greenhouse gases under different scenarios of optimal synthetic nitrogen application rate for grain crops in China

Proper management of synthetic nitrogen (N) fertilizer can reduce direct N₂O emission from soil and indirect CO₂ emission from production and transportation of synthetic N. In the late 1990s, the average application rates of synthetic N were 212, 207 and 207 kg ha^?1, respectively, for rice, wheat, and maize in China’s croplands. But research suggests that the optimal synthetic N application rates for the main grain crops in China should be in the range of 110–150 kg ha^?1. Excessive application of synthetic N has undoubtedly resulted in massive emission of greenhouse gases. Therefore, optimizing N application rates for grain crops in China has a great potential for mitigating the emission of greenhouse gases. Nevertheless, this mitigation potential (MP) has not yet been well quantified. This study aimed at estimating the MP of N₂O and CO₂ emissions associated with synthetic N production and transportation in China based on the provincial level statistical data. Our research indicates that the total consumption of synthetic N on grain crops in China can be reduced by 5.0–8.4 Tg yr^?1 (28–47 % of the total consumption) if the synthetic N application rate is controlled at 110–150 kg ha^?1. The estimated total MP of greenhouse gases, including direct N₂O emission from croplands and indirect CO₂ emission from production and transportation of synthetic N, ranges from 41.7 to 70.1 Tg CO₂_eq. yr^?1. It was concluded that reducing synthetic N application rate for grain crops in China to a reasonable level of 110–150 kg ha^?1 can greatly reduce the emission of greenhouse gases, especially in the major grain-crop production provinces such as Shandong, Henan, Jiangsu, Hebei, Anhui and Liaoning.     

Nitrogen leaching and indirect nitrous oxide emissions from fertilized croplands in Zimbabwe

Agricultural efforts to end hunger in Africa are hampered by low fertilizer-use-efficiency exposing applied nutrients to losses. This constitutes economic losses and environmental concerns related to leaching and greenhouse gas emissions. The effects of NH₄NO₃ (0, 60 and 120?kg?N?ha^?1) on N uptake, N-leaching and indirect N₂O emissions were studied during three maize (Zea mays L.) cropping seasons on clay (Chromic luvisol) and sandy loam (Haplic lixisol) soils in Zimbabwe. Leaching was measured using lysimeters, while indirect N₂O emissions were calculated from leached N using the emission factor methodology. Results showed accelerated N-leaching (3?C26?kg?ha^?1?season^?1) and N-uptake (10?C92?kg?ha^?1) with N input. Leached N in groundwater had potential to produce emission increments of 0?C94?g N₂O-N?ha^?1?season^?1 on clay soil, and 5?C133?g N₂O-N?ha^?1?season^?1 on sandy loam soil following the application of NH₄NO₃. In view of this short-term response intensive cropping using relatively high N rate may be more appropriate for maize in areas whose soils and climatic conditions are similar to those investigated in this study, compared with using lower N rates or no N over relatively larger areas to attain a targeted food security level.     

Effects of the herbicides butachlor and bensulfuron-methyl on N2O emissions from a dry-seeded rice field

Independent field and laboratory incubation experiments were conducted to investigate the effects of two commonly used herbicides butachlor and bensulfuron-methyl on N₂O emissions from a dry-seeded rice field. Three treatments were applied in field experiments: a fertilized control without herbicide, fertilized plots amended with butachlor equivalent to 2.55 L ha^?1 of 60 % by weight active ingredient and fertilized plots amended with bensulfuron-methyl equivalent to 300 g ha^?1 of 10 % by weight active ingredient. Herbicides were applied twice in the rice growing season according to local farming practices. The same treatments were used in laboratory incubation experiments, i.e., a fertilized control without herbicide and fertilized soil amended with the herbicide butachlor or bensulfuron-methyl. The soil moisture was adjusted to 0.55 g g^?1 in the lab incubation experiments based on the average water content determined in the dry-seeded rice field. The field and laboratory simulation experiments all showed that the butachlor applications led to significantly increased N₂O emissions (p < 0.05), whereas bensulfuron-methyl had no effect on N₂O emissions (p > 0.05). Butachlor enhanced the N₂O emissions by up to 177.5 % over the entire rice growing season. Moreover, butachlor and bensulfuron-methyl treatment led to a marginal stimulation of the soil respiration rates. A further investigation in the field experiments suggested that the butachlor-enhanced N₂O emissions resulted from increased soil ammonium nitrogen and nitrate nitrogen contents and the more abundance of ammonia-oxidizing and denitrifying bacteria in the late stage after the herbicide application. The bensulfuron-methyl treatment had no influence on N₂O emissions during the rice growing season, which was attributed to the low soil nitrate nitrogen contents during this period.     

Reducing nitrous oxide emissions and optimizing nitrogen-use efficiency in dryland crop rotations with different nitrogen rates

Recent interests in improving agricultural production while minimizing environmental footprints emphasized the need for research on management strategies that reduce nitrous oxide (N₂O) emissions and increase nitrogen-use efficiency (NUE) of cropping systems. This study aimed to evaluate N₂O emissions, annualized crop grain yield, emission factor, and yield-scaled- and NUE-scaled N₂O emissions under continuous spring wheat (Triticum aestivum L.) (CW) and spring wheat–pea (Pisum sativum L.) (WP) rotations with four N fertilization rates (0, 50, 100, and 150 kg N ha^?1). The N₂O fluxes peaked immediately after N fertilization, intense precipitation, and snowmelt, which accounted for 75–85% of the total annual flux. Cumulative N₂O flux usually increased with increased N fertilization rate in all crop rotations and years. Annualized crop yield and NUE were greater in WP than CW for 0 kg N ha^?1 in all years, but the trend reversed with 100 kg N ha^?1 in 2013 and 2015. Crop yield maximized at 100 kg N ha^?1, but NUE declined linearly with increased N fertilization rate in all crop rotations and years. As N fertilization rate increased, N fertilizer-scaled N₂O flux decreased, but NUE-scaled N₂O flux increased non-linearly in all years, regardless of crop rotations. The yield-scaled N₂O flux decreased from 0 to 50 kg N ha^?1 and then increased with increased N fertilization rate. Because of non-significant difference of N₂O fluxes between 50 and 100 kg N ha^?1, but increased crop yield, N₂O emissions can be minimized while dryland crop yields and NUE can be optimized with 100 kg N ha^?1, regardless of crop rotations.