Average N2O Emissions Research Articles

N2O Generation, Key Influencing Factors, and Emission Reduction Strategies of A2O Process in Municipal Wastewater Treatment Plant

To achieve non-carbon dioxide greenhouse gas emission reduction and control in municipal wastewater treatment plants （WWTPs）, this study conducted one-year long-term monitoring of nitrous oxide （N2O） in the anaerobic-anoxic-aerobic （A2O） process of a large-scale municipal wastewater treatment plant in Beijing. The experimental results showed that the anaerobic and anoxic zones of the A2O process could effectively remove dissolved N2O contained in the return sludge, while the aerobic zone was the main area for N2O generation and emission, and its generation pathway may have been dominated by ammonia oxidizing bacteria （AOB） denitrification. A significant difference was observed between winter and summer N2O production, and the difference in the average N2O release flux was up to 7.6 times, and the average monthly N2O emission in winter was 32.75 kg, which was significantly higher than that in summer （6.06 kg）. The accumulation of nitrite （NO2--N） and the concentration of dissolved oxygen （DO） had a significant impact on N2O production. Therefore, to achieve N2O reduction in the A2O process, the concentration of NO2--N in the aerobic zone should be controlled below 0.40 mg·L-1 in winter and 0.10 mg·L-1 in summer, while the DO concentration should be maintained above 1.2 mg·L-1.

One–third substitution of nitrogen with cow manure or biochar greatly reduced N2O emission and carbon footprint in saline–alkali soils

The rapid expansion of farmland and long−term excessive nitrogen (N) application have caused huge environmental risks in fragile ecosystems facing global warming. Partially substituting N fertilizer with organic fertilizers offers an alternative field management strategy to alleviate the pressure of the ecological environment. To this end, the influence of one−third substitution of N fertilizer with cow manure or biochar field experiment was conducted under maize in Tarim River Basin since 2019. Five treatments with three replications were applied: CK (Fallow); no fertilization (0 N); conventional N fertilizer (N; N: 300 kg N ha−1, organic fertilizer: 0 kg N ha−1); one−third substitution of N with biochar (NB; N: 200 kg N ha−1, Biochar: 100 kg N ha−1) and one−third substitution of N with cow manure (NM; N: 200 kg N ha−1, Cow manure: 100 kg N ha−1) under maize season in saline−alkali soils. The greenhouse gas (GHG) emissions, net ecosystem carbon budget (NECB), soil organic carbon (SOC), maize yield, carbon footprint (CF), and yield carbon footprint (CFy) were analyzed from 2021 to 2022. The results showed that NB treatment decreased the average cumulative CO2 emissions by 21 %, while NM treatment showed no difference compared to N treatment. NB and NM treatments reduced the average cumulative N2O emissions (−61 %, −49 %), CF (−68 %, −10 %), CFy (−66 %, −19 %) and increased maize yield (+3 %, +2 %), SOC storage (+43 %, +6 %), NECB (+80 %, +24 %), and agronomic N use efficiency (ANUE) (+5 %, +3 %), compared to N treatment. NB treatment had the lowest emission factors (EF) (0.19 %) and the highest sustainability index (1.58) compared to NM treatment (0.26 %, 0.61) and N treatment (0.53 %, 0.84). To sum up, substituting one−third of N fertilizer with biochar or manure in saline−alkali soils was proved to be a multi−benefit strategy to increase yields and reduce GHG emissions and CF.

Significant diurnal variations in nitrous oxide (N2O) emissions from two contrasting habitats in a large eutrophic lake (Lake Taihu, China)

Algae and macrophytes in lake ecosystems regulate nitrous oxide (N2O) emissions from eutrophic lakes. However, knowledge of diurnal N2O emission patterns from different habitats remains limited. To understand the diurnal patterns and driving mechanisms of N2O emissions from contrasting habitats, continuous in situ observations (72 h) of N2O fluxes from an algae-dominated zone (ADZ) and reed-dominated zone (RDZ) in Lake Taihu were conducted using the Floating Chamber method. The results showed average N2O emission fluxes of 0.15 ± 0.06 and 0.02 ± 0.04 μmol m−2 h−1 in the ADZ and RDZ in autumn, respectively. The significantly higher (p < 0.05) N2O fluxes in the ADZ were mainly attributed to differences in nitrogen (N) levels. The results also showed significant diurnal differences (p < 0.05) in the N2O emission fluxes within the ADZ and RDZ, and daytime fluxes were significantly higher (p < 0.05) than nighttime fluxes. The statistical results indicated that N2O emissions from the ADZ were mainly driven by diurnal variations in N loading and the dissolved oxygen (DO) concentration, and those from the RDZ were more influenced by DO, redox potential, and pH. Finally, we determined the proper time for routine monitoring of N2O flux in the two habitats. Our results highlight the importance of considering diverse habitats and diurnal variations when estimating N2O budgets at a whole-lake scale.

N2O Emission Factors from Wastewater Treatment Plants Based on Literature Statistics and Model Fitting

The emission of nitrous oxide （N2O） during wastewater treatment cannot be ignored. The analysis of statistical data from literature based on 126 empirical studies revealed that the geographical factors of wastewater treatment plants （WWTPs） had a significant impact on N2O emission factors. However, the N2O emission factors of WWTPs in all regions of the world were generally lower than the Intergovernmental Panel on Climate Change （IPCC） recommended values. In China, the N2O emission factors （in N2O-N/Ninfluent） of WWTPs were approximately 0.000 35-0.065 20 kg·kg-1. Meanwhile, the N2O emission factors of different wastewater treatment processes were also significantly different, especially since the sequencing batch reactor （SBR） process had higher emissions. The use of uniform default emission factors for accounting was prone to overestimate N2O emissions, and it is recommended that countries conduct actual monitoring or modeling studies to develop categorical emission factors suitable for local conditions. In addition, the N2O emission factor based on total nitrogen （TN） removal was weakly negatively correlated with TN removal in 126 empirical data, which was more in line with bioprocessing stoichiometry and could provide an accurate accounting method for N2O. To this end, a digital twin model was developed to dynamically simulate a case anaerobic-anoxic-aerobic （AAO） WWTP to comprehensively quantify the dynamic emission behavior of N2O, which demonstrated that N2O emissions had significant seasonal and daily variability and were only equivalent to 11% of the calculated value of the emission factor based on the IPCC recommendation. Comparing the scatter linear fitting and categorical mean exponential fitting methods, it was found that the latter could more accurately reflect the negative correlation between the N2O emission factors and the TN removal rate, and an exponential regression equation between the average N2O emission factor based on the amount of TN removed and the TN removal rate was further developed to predict the N2O emission. The dynamic simulation and categorical index fitting methods provided in this study are important references for the accurate accounting of N2O emissions in similar WWTPs and provide help for understanding and responding to the N2O emission problems.

Nitrous oxide emissions and microbial communities variation in low dissolved oxygen and low carbon-to-nitrogen ratio anoxic-oxic wastewater treatment plant.

Dissolved oxygen (DO) levels and carbon-to-nitrogen (C/N) ratio affect nitrous oxide (N2O) emissions by influencing the physiological and ecological dynamics of nitrifying and denitrifying microbial communities in activated sludge systems. For example, Nitrosomonas is a common N2O producing nitrifying bacteria in wastewater treatment plants (WWTPs), and DO conditions can affect the N2O production capacity. Previous studies have reported N2O emission characteristics under adequate DO and C/N conditions in A/O WWTPs. However, in actual operation, owing to economic and managerial factors, some WWTPs have a long-term state of low DO levels in oxic tanks and low influent C/N. Research on N2O emission characteristics in low DO-limited and low C/N ratio WWTPs is limited. This study investigated N2O emissions and the corresponding shifts in microorganisms within an anoxic-oxic (A/O) WWTP over 9-month. Quantitative PCR was used to assess the abundance of ten functional genes related to nitrification and denitrification processes, and high-throughput sequencing of the 16S rRNA gene was employed to determine the composition change of microorganisms. The findings revealed that 1) the average N2O emission factor was 1.07% in the studied WWTP; 2) the DO-limited oxic tank primarily contributed to N2O; 3) NO2-, TOC, and C/N ratios were key factors for dissolved N2O in the aerobic tank; and 4) Nitrosomonas and Terrimonas exhibited a robust correlation with N2O emissions. This research provides data references for estimating N2O emission factors and developing N2O reduction policies in WWTPs with DO-limited and low C/N ratios.

Estimation of Potential Nitrous Oxide Emissions from Landfills in the United States: 2010–2020

Nitrous oxide (N2O), a major greenhouse gas, has the potential to be emitted from waste landfills. Previous studies have demonstrated the propensity of landfilling facilities to emit significant quantities of N2O, a fact underscored by the IPCC Guidelines, which emphasize the importance of researching this phenomenon. However, due to the absence of established international guidelines for quantifying N2O emissions from landfills, many countries, including the United States, have excluded N2O from greenhouse gas inventories. Therefore, this study aims to estimate N2O emissions from landfills in the United States, a country with a significant landfill waste volume. In this study, N2O emissions from U.S. landfills over an 11-year period (2010–2020) are estimated by using the emission estimation formula provided in CDM AM0083 and emission factors from the 2006 IPCC Guidelines. Additionally, emissions were calculated spatially for each state and individual landfill facility. As a result, the impact of integrating N2O emissions from landfills into the national greenhouse gas inventory was assessed. The average annual landfill N2O emission in the United States over the 11-year period was estimated to be 3,214,693 ton-CO2-equivalent/year, with an overall decreasing trend. In 2020, Indiana, Michigan, and Oregon exhibited high landfill N2O emissions per capita, while the Virgin Islands, Connecticut, and Massachusetts demonstrated lower emissions. When incorporated into the U.S. greenhouse gas inventory, landfill N2O emissions represent 10.41% of the total sector N2O emissions. Although N2O emissions are declining alongside reduced waste landfilling in the United States, the quantity remains significant and should be factored into greenhouse gas inventory calculations and emission scenarios for the next CMIP6. Further research investigating N2O emission coefficients across different regions and waste types is necessary. Ultimately, this study aims to support the United Nations (UN)’s Sustainable Development Goals (SDGs), particularly SDG 11 (Sustainable Cities and Communities), 12 (Responsible Consumption and Production), and 13 (Climate Action), by enhancing the tools for accurate greenhouse gas inventory and promoting sustainable waste management.

Enhanced efficiency nitrogen fertilizers (EENFs) can reduce nitrous oxide emissions and maintain high grain yields in a rain-fed spring maize cropping system

ContextEnhanced efficiency nitrogen fertilizers (EENFs) have provided opportunities for the simultaneous mitigation of nitrous oxide (N2O) emissions and increases in crop productions. However, the practice of combining EENFs with optimal fertilization under conditions of misaligned water and fertilizer inputs in rain-fed spring maize systems remains inadequately understood. ObjectiveThis study aims to determine the factors controlling N2O emissions under the soil climatic conditions in rain-fed plastic mulching cropping systems treated with different nitrogen (N) fertilization rates and EENFs, and to determine the most effective N fertilization measure to reduce N2O emissions while maintaining crop yields. MethodsA 3-year field experiment was conducted in a spring maize system located in the semiarid rain-fed region of the Loess Plateau, applying two types of EENFs: a nitrification inhibitor dicyandiamide (DCD) and a slow-release fertilizer (SRF). It comprised five fertilization treatments: unfertilized (CK), conventional N rate (CON, 200 kg N ha−1), optimal N rate (OPT, 160 kg N ha−1), OPT with the addition of DCD (OPT+DCD), and OPT using SRF (OPT-SRF). ResultsThe average annual cumulative N2O emissions over the 3-year experimental period ranged from 0.74 to 1.87 kg N2O-N ha–1. Fertilizer N contributed 26−60% to annual N2O emissions. The highest N2O fluxes (50–161 μg N2O-N m–2 h–1) typically occurred within the initial 10 days following fertilization, likely induced by strong nitrification processes, constituting 12–19% of the annual N2O emissions during this brief period. In comparison to the CON treatment, the OPT, OPT+DCD, and OPT-SRF treatments resulted in significant reductions in annual N2O emissions of 24%, 46%, and 34%, respectively, without causing any significant decrease in maize grain yields. The application of DCD led to increased ammonium (NH4+-N) concentrations while reducing nitrate (NO3–-N). Conversely, using SRF resulted in a decrease in both NH4+-N and NO3–-N concentrations. ConclusionsUsing EENFs at the ideal fertilization rate significantly curtailed yearly N2O emissions without adversely affecting maize yields in a rain-fed plastic mulching spring maize system prevalent in the Loess Plateau. Most N2O emissions occurred post-fertilization, with rainfall events further influencing these emissions. ImplicationsOur findings recommend the incorporation of nitrification inhibitors, such as DCD, as the most effective fertilizer measure for reducing N2O emissions in the rain-fed region of the Loess Plateau.

Effects of Long-Term Controlled-Release Urea on Soil Greenhouse Gas Emissions in an Open-Field Lettuce System.

Controlled-release urea (CRU) fertilizers are widely used in agricultural production to reduce conventional nitrogen (N) fertilization-induced agricultural greenhouse gas emissions (GHGs) and improve N use efficiency (NUE). However, the long-term effects of different CRU fertilizers on GHGs and crop yields in vegetable fields remain relatively unexplored. This study investigated the variations in GHG emissions at four growth stages of lettuce in the spring and autumn seasons based on a five-year field experiment in the North China Plain. Four treatments were setup: CK (without N application), U (conventional urea-N application), ON (20% reduction in urea-N application), CRU (20% reduction in polyurethane-coated urea without topdressing), and DCRU (20% reduction in polyurethane-coated urea containing dicyandiamide [DCD] without topdressing). The results show that N application treatments significantly increased the GHG emissions and the lettuce yield and net yield, and DCRU exhibited the lowest N2O and CO2 emissions, the highest lettuce yield and net yield, and the highest lettuce N content of the N application treatments. When compared to U, the N2O emission peak under CRU and DCRU treatments was notably decreased and delayed, and their average N2O emission fluxes were significantly reduced by 10.20-20.72% and 17.51-29.35%, respectively, leading to a significant reduction in mean cumulative N2O emissions during the 2017-2021 period. When compared to U, the CO2 fluxes of DCRU significantly decreased by 8.0-16.54% in the seedling period, and mean cumulative CO2 emission decreased by 9.28%. Moreover, compared to U, the global warming potential (GWP) and greenhouse gas intensity (GHGI) of the DCRU treatment was significantly alleviated by 9.02-17.13% and 16.68-20.36%, respectively. Compared to U, the N content of lettuce under DCRU was significantly increased by 6.48-17.25%, and the lettuce net yield was also significantly increased by 5.41-7.71%. These observations indicated that the simple and efficient N management strategy to strike a balance between enhancing lettuce yields and reduce GHG emissions in open-field lettuce fields could be obtained by applying controlled-release urea containing DCD without topdressing.

Regulatory potential of soil available carbon, nitrogen, and functional genes on N2O emissions in two upland plantation systems

Dynamic nitrification and denitrification processes are affected by changes in soil redox conditions, and they play a vital role in regulating soil N2O emissions in rice-based cultivation. It is imperative to understand the influences of different upland crop planting systems on soil N2O emissions. In this study, we focused on two representative rotation systems in Central China: rapeseed–rice (RR) and wheat–rice (WR). We examined the biotic and abiotic processes underlying the impacts of these upland plantings on soil N2O emissions. The results revealed that during the rapeseed-cultivated seasons in the RR rotation system, the average N2O emissions were 1.24±0.20 and 0.81±0.11 kg N ha−1 for the first and second seasons, respectively. These values were comparable to the N2O emissions observed during the first and second wheat-cultivated seasons in the WR rotation system (0.98±0.25 and 0.70±0.04 kg N ha−1, respectively). This suggests that upland cultivation has minimal impacts on soil N2O emissions in the two rotation systems. Strong positive correlations were found between N2O fluxes and soil ammonium (NH4+), nitrate (NO3−), microbial biomass nitrogen (MBN), and the ratio of soil dissolved organic carbon (DOC) to NO3− in both RR and WR rotation systems. Moreover, the presence of the AOA-amoA and nirK genes were positively associated with soil N2O fluxes in the RR and WR systems, respectively. This implies that these genes may have different potential roles in facilitating microbial N2O production in various upland plantation models. By using a structural equation model, we found that soil moisture, mineral N, MBN, and the AOA-amoA gene accounted for over 50% of the effects on N2O emissions in the RR rotation system. In the WR rotation system, soil moisture, mineral N, MBN, and the AOA-amoA and nirK genes had a combined impact of over 70% on N2O emissions. These findings demonstrate the interactive effects of functional genes and soil factors, including soil physical characteristics, available carbon and nitrogen, and their ratio, on soil N2O emissions during upland cultivation seasons under rice-upland rotations.

Relative importance between nitrification and denitrification to N2O from a global perspective

AbstractNitrous oxide (N2O) is a potent greenhouse gas, and its mitigation is a pressing task in the coming decade. However, it remains unclear which specific process between concurrent nitrification and denitrification dominates worldwide N2O emission. We snagged an opportunity to ascertain whence the N2O came and which were the controlling factors on the basis of 1315 soil N2O observations from 74 peer‐reviewed articles. The average N2O emission derived from nitrification (N2On) was higher than that from denitrification (N2Od) worldwide. The ratios of nitrification‐derived N2O to denitrification‐derived N2O, hereof N2On:N2Od, exhibited large variations across terrestrial ecosystems. Although soil carbon and nitrogen content, pH, moisture, and clay content accounted for a part of the geographical variations in the N2On:N2Od ratio, ammonia‐oxidizing microorganisms (AOM):denitrifier ratio was the pivotal driver for the N2On:N2Od ratios, since the AOM:denitrfier ratio accounted for 53.7% of geographical variations in N2On:N2Od ratios. Compared with natural ecosystems, soil pH exerted a more remarkable role to dictate the N2On:N2Od ratio in croplands. This study emphasizes the vital role of functional soil microorganisms in geographical variations of N2On:N2Od ratio and lays the foundation for the incorporation of soil AOM:denitrfier ratio into models to better predict N2On:N2Od ratio. Identifying soil N2O derivation will provide a global potential benchmark for N2O mitigation by manipulating the nitrification or denitrification.

Long-term application of controlled-release urea reduced ammonia volatilization, raising the risk of N2O emissions and improved summer maize yield

ContextWith the extensive promotion of controlled-release urea (CRU), its N-release pattern has been shown to match the crop’s N-demand period and improve crop yield by reducing N losses. Nevertheless, the impacts of CRU on summer maize yield, especially on greenhouse gas emissions, have rarely been reported during its long-term use. ObjectiveIt is necessary through long-term positioning field experiment with CRU application to investigate the regulatory mechanisms of maize yield formation, nitrogen use efficiency (NUE), ammonia volatilization and greenhouse gas emissions. MethodsThe long-term fertilizer positioning experiment for ten years, four treatments were performed: polymeric-coated controlled-release urea (CRF, 42% N, 3-months controlled-release period), sulfur-coated controlled-release urea (SCF, 35% N, 3-months controlled-release period), common urea (CCF, 46% N) and no nitrogen fertilizer (CK). ResultsResults showed that the two coated controlled-release fertilizers significantly increased NUE and dry matter accumulation in summer maize, and ultimately improved grain yield. Compared to CCF, CRF and SCF treatments increased post-silking dry matter accumulation by 16.3% and 13.3%; NUE by 42.3% and 28.9%. The 2-year mean grain yield of CRF and SCF treatment increased 9.0% and 12.4% compared to CCF treatment. CRF treatment significantly increased the NO3--N content in 0–60 cm soil layer; while CRF and SCF treatments showed a significantly higher NH4+-N concentration in the 0–20 cm soil layer than CCF treatment. Compared to CCF treatment, the CRF and SCF treatments significantly reduced peak ammonia volatilization, and the average cumulative emissions of ammonia volatilization of two years were reduced by 80.8% and 53.2%, respectively. CRF treatment had lower peak of N2O emissions but longer N2O emission duration than other N application treatments. The two-year average cumulative N2O emissions from CRF treatment increased by 49.6%, and SCF treatment increased by 61.1% compared to CCF treatment. Compared to CK treatment, total cumulative CH4 uptake was 36.1% lower in CRF treatment, 60.9% in SCF and 78.9% in CCF treatment. The higher cumulative N2O emissions from CRF and SCF treatment resulted in a significantly higher global warming potential (GWP) than CCF treatment. The two-year mean increase in greenhouse gas intensity (GHGI) was 36.2% for CRF treatment and 44.7% for SCF treatment compared to CCF treatment. ConclusionsLong-term application of coated controlled-release fertilizers could achieve high yields and nitrogen use efficiency. However, there is a risk of increased N2O emissions, causing elevated GHGI. SignificanceIn the future, it is necessary to strengthen the research on the comprehensive effects of long-term reduced application of CRU on crop yield and greenhouse gas emissions, in order to promote the wide application of environmentally friendly CRU in the field.

Environmental parameters and management as factors affecting greenhouse gas emissions from clay soil

ABSTRACT Greenhouse gas emissions (GHG) drive climate change, with agricultural land significantly contributing, influenced by soil properties. While extensive research exists on environmental and management impacts on GHG emissions across various soils and climates, understanding key factors influencing GHG emissions from clay soil in temperate climates is limited. This study aims to investigate the combination of environmental and management factors reducing N2O, CO2, and CH4 emissions from clay soil in temperate climates. Recognising the potential of reduced tillage and legume-based crop rotations in mitigating GHG emissions, we investigate their impact on soil emissions. The conventionally managed field with spring barley, field beans, winter wheat, and winter rapeseed rotation demonstrates the lowest average N2O emission (3.7 g N2O ha−1 d− 1), while the field with winter crops in a reduced tillage rotation shows the highest N2O emission (8.5 g N2O ha−1 d− 1). A rotation with winter crops, beans twice, and barley, under conventional management, demonstrates the highest CO2 emission (140.2 kg CO2 ha−1 d− 1), while the lowest CO2 emission is observed in a rotation with winter crops, beans, and barley under reduced tillage management (100.8 kg CO2 ha−1 d− 1). CH4 assimilation ranges from 3.1 to 5.4 CH4 g ha−1 d−1 across all rotation and tillage combinations. However, ANCOVA results indicate that the volumes of GHG emissions are significantly influenced by the interaction of environmental and management factors, where precipitation is the most significant factor in the interaction with other environmental factors, soil tillage, and crop residues for N2O and CO2 emissions, while CH4 emissions are influenced by the interaction of air temperature with other environmental factors, soil tillage, and crop residues. This underscores the need to consider both management and relevant environmental factors when evaluating the impact of practices on GHG emissions from clay soil in temperate climates.

Contrasting effects of intensified dry-season drought and extended dry-season length on soil greenhouse gas emissions in a subtropical forest

Over two-thirds of the Earth's land surface is subjected to seasonal precipitation changes along with climate warming, including the subtropical forests that represent one of the Earth's most important carbon sink and source. However, few experiments have been conducted to understand the response of soil greenhouse gas (GHGs) emissions from these forests to seasonal changes in precipitation. Herein, we conducted a field experiment in a subtropical forest of southern China including two precipitation seasonality treatments: an intensified dry-season (Oct–Mar) drought and wetter wet-season (Jun–Sep) treatment (ID) and an extended dry-season (Apr–May) length and wetter wet-season treatment (ED); for both ID and ED, the annual precipitation amount was kept the same as under ambient control (AC). Compared to AC, the decreased annual CO2 emissions for ID were mainly due to decreased WFPS in Oct–Mar of 2013–2014 and increased WFPS during Jun–Sep of 2013; the increased annual CH4 uptake for ID was predominantly attributed to decreased WFPS in Oct–Mar of 2013–2014; the decreased annual N2O emissions for ID were mainly due to decreased WFPS in Oct–Mar of 2013; the increased annual N2O emissions for ID in 2014 were mainly attributed to increased WFPS in Jun–Sep (p < 0.05). Relative to AC, the increased annual CO2 and N2O emissions from ED were predominantly attributed to decreased WFPS in Apr–May and increased WFPS in Jun–Sep during 2013–2014, respectively (p < 0.05). The average annual CO2-equivalent CH4 and N2O emissions increased under ED but decreased under ID compared to AC (p < 0.05). Although our two precipitation manipulation scenarios simulated seasonal drought impacts without changing annual precipitation amount, ED and ID had distinct impacts on soil GHGs emissions, which have important implications for modeling the subtropical forests GHG emissions and managing the forests to mitigate climate change.

Simulated nitrous oxide emissions from multiple agroecosystems in the U.S. Corn Belt using the modified SWAT-C model

Agriculture is a major source of nitrous oxide (N2O) emissions into the atmosphere. However, assessing the impacts of agricultural conservation practices, land use change, and climate adaptation measures on N2O emissions at a large scale is a challenge for process-based model applications. Here, we integrated six N2O emission algorithms for the nitrification processes and seven N2O emission algorithms for the denitrification process into the Soil and Water Assessment Tool-Carbon (SWAT-C). We evaluated the different combinations of methods in simulating N2O emissions under corn (Zea mays L.) production systems with various conservation practices, including fertilization, tillage, and crop rotation (represented by 14 experimental treatments and 83 treatment-years) at five experimental sites across the U.S. Midwest. The SWAT-C model exhibited wide variability in simulating daily average N2O emissions across treatment-years with different method configurations, as indicated by the ranges of R2, NSE, and BIAS (0.04–0.68, −1.78–0.60, and −0.94–0.001, respectively). Our results indicate that the denitrification process has a stronger impact on N2O emissions than the nitrification process. The best performing N2O emission algorithms are those rooted in the CENTURY model, which considers soil pH and respiration effects that were overlooked by other algorithms. The optimal N2O emission algorithm explained about 63% of the variability of annual average N2O emissions, with NSE and BIAS of 0.60 and −0.033, respectively. The model can reasonably represent the impacts of agricultural conservation practices on N2O emissions. We anticipate that the improved SWAT-C model, with its flexible configurations and robust modeling and assessment capabilities, will provide a valuable tool for studying and managing N2O emissions from agroecosystems.

Higher N2O emissions from organic compared to synthetic N fertilisers on sandy soils in a cool temperate climate

The atmospheric increase in N2O is mainly derived from N fertilisation in agriculture, and improved emission estimates are needed for effective mitigation. This study presents first estimates of country-specific N2O emissions from synthetic and liquid organic fertilisers in Denmark. Representative crop rotations were established in four locations across Denmark to provide a realistic context for the estimation of N2O emissions, i.e., a dairy farm rotation in Western Denmark, dairy and pig farm rotations in Southwestern Denmark, and an arable rotation in Eastern Denmark. The four sites were light-textured and typical for Northern Europe, whereas rainfall varied considerably among sites and years. A randomised block design was used, and all crops were represented in triplicate each year with monitoring of N2O emissions between April 2020 and March 2022. Spring barley was part of all rotations, and here three synthetic fertilisers (NS, NPK and urea ammonium nitrate) and eight organic fertilisers (three cattle slurries, three pig slurries and two digestates) were applied in 1 m2 plots at either two or four sites in order to compare N2O emissions from the same N fertiliser materials under contrasting site conditions. Identical methodologies for management, fertiliser application, and N2O measurement and flux calculation, were used at all sites to ensure comparability. Manually operated chambers were used for N2O flux measurements. The continuous monitoring indicated a strong seasonal pattern across all four sites with the main part of N2O emissions during spring. The side-by-side comparison of several N sources at four sites in two years showed for synthetic fertilisers an average N2O emission factor for the spring period of 0.15% (95% C.I. −0.17 to 0.37%, n = 16), and for liquid organic fertilisers (pig and cattly slurries, and digestates) an average of 1.02% (95% C.I. 0.75 – 1.30%, n = 44). The higher N2O emissions from organic fertilisers, which was significant at each of the four sites, is in opposition to new N2O emission factors recently proposed in a refinement of the IPCC methodology for national inventories. The conflicting results are discussed with reference to region-specific site conditions and fertiliser types, and in particular the predominance of soils with a low clay content, and of liquid manure management, may explain the deviations from global estimates. A comparison of annual and spring N2O emissions indicated a difference in the order of 0.1 – 0.2% of the N input (n = 8), and the feasibility of estimating N2O emission factors based on emissions during the growing season only is discussed.

Data-driven modeling on the global annual soil nitrous oxide emissions: Spatial pattern and attributes

Previous assessments generated divergent estimates of global terrestrial soil nitrous oxide (N2O) emission and its spatial distributions, which did not match the observed data well. The objectives of this study were to generate a global map of terrestrial soil N2O emissions based on field observations (n = 5549) and quantify the contribution of different variables for predicting the global variation of N2O emissions. We provided spatially explicit maps of annual soil N2O emission rates across forest, grassland and cropland using the random forest approach. The global mean soil N2O emission rate in our data-driven model was 0.059 ± 0.006 g N m−2 year−1, which was lower than the estimates from previous model ensembles. Soil N2O emissions were higher in the northern than southern hemisphere. The average annual soil N2O emission rate of cropland (0.094 ± 0.009 g N m−2 year−1) was higher than that of forest (0.039 ± 0.004 g N m−2 year−1) and grassland (0.045 ± 0.007 g N m−2 year−1). In addition, we found that soil nitrogen substrates dominated the changes in soil N2O emissions and the relative importance of nitrate, ammonium, and fertilizer in predicting soil N2O emissions was greater than that of mean annual temperature and precipitation. Our data-driven model results implied that previous process-based model may overestimate the global soil N2O emission rates due to limited validation data and incomplete assumptions on related-mechanisms. This study highlights the importance of global field observations in N2O emission estimation, which can provide an independent dataset to constrain previous process-based models for better prediction.

Analysis of Greenhouse Gas Emissions Characteristics and Emissions Reduction Measures of Animal Husbandry in Inner Mongolia

Global warming has had a profound impact on human life, with animal husbandry being a significant contributor to greenhouse gas emissions and playing a crucial role in the global greenhouse gas budget. Inner Mongolia is a major contributor to these emissions, making it vital to study the link between greenhouse gas emissions and animal husbandry in this region for the purpose of reducing emissions. In this study, the emissions of greenhouse gases (CH4, N2O, and CO2) from livestock and poultry breeding from 2010 to 2020 and the emissions of each city from 2020 were estimated, the emissions characteristics were analysed, and the low carbon emissions reduction technical measures were proposed. The results show that (1) the overall greenhouse gas emissions from 2010 to 2020 in Inner Mongolia showed a fluctuating trend; the main emissions sources were gastrointestinal fermentation and faecal management. The annual average CH4 emissions were 994,400 ta−1, and the annual average N2O emissions were 35,100 ta−1. (2) In 2020, the total emissions of each league city were 38.05 million t equivalent of CO2, and the emissions gradually decreased from east to west, with a significant emissions reduction potential. Based on these findings, this study also proposed technical measures for reducing carbon emissions, offering theoretical support to drive the industrial transformation and upgrading of the livestock industry, and promoting green economic development in Inner Mongolia as part of its carbon peaking and neutrality goals.

Denitrification contributes to N2O emission in paddy soils.

Denitrification is vital to nitrogen removal and N2O release in ecosystems; in this regard, paddy soils exhibit strong denitrifying ability. However, the underlying mechanism of N2O emission from denitrification in paddy soils is yet to be elucidated. In this study, the potential N2O emission rate, enzymatic activity for N2O production and reduction, gene abundance, and community composition during denitrification were investigated using the 15N isotope tracer technique combined with slurry incubation, enzymatic activity detection, quantitative polymerase chain reaction (qPCR), and metagenomic sequencing. Results of incubation experiments showed that the average potential N2O emission rates were 0.51 ± 0.20 μmol⋅N⋅kg-1⋅h-1, which constituted 2.16 ± 0.85% of the denitrification end-products. The enzymatic activity for N2O production was 2.77-8.94 times than that for N2O reduction, indicating an imbalance between N2O production and reduction. The gene abundance ratio of nir to nosZ from qPCR results further supported the imbalance. Results of metagenomic analysis showed that, although Proteobacteria was the common phylum for denitrification genes, other dominant community compositions varied for different denitrification genes. Gammaproteobacteria and other phyla containing the norB gene without nosZ genes, including Actinobacteria, Planctomycetes, Desulfobacterota, Cyanobacteria, Acidobacteria, Bacteroidetes, and Myxococcus, may contribute to N2O emission from paddy soils. Our results suggest that denitrification is highly modular, with different microbial communities collaborating to complete the denitrification process, thus resulting in an emission estimation of 13.67 ± 5.44 g N2O⋅m-2⋅yr-1 in surface paddy soils.

Effects of ant nesting on seasonal dynamics of soil N2O emission in a secondary tropical forest.

We assessed the seasonal dynamics of N2O emission in ant nests soils in secondary tropical Millettia leptobotrya forest of Xishuangbanna by using the static chamber-gas chromatography method, and determined the lin-kages between ant-mediated changes in soil properties (e.g., carbon pool, nitrogen pool, and temperature and humidity) and N2O emission. The results showed that ant nesting significantly affected soil N2O emission. The ave-rage soil N2O emission (0.67 mg·m-2·h-1) in ant nests was 40.2% higher than that in the control (0.48 mg·m-2·h-1). N2O emission in ant nests and the control showed substantial seasonal variation, with higher rate in June (0.90 and 0.83 mg·m-2·h-1, respectively) than that in March (0.38 and 0.19 mg·m-2·h-1, respectively). Ant nesting significantly increased the values (7.1%-74.1%) of moisture, temperature, organic carbon, total nitrogen, hydrolytic nitrogen, ammonium nitrogen, nitrate nitrogen, and microbial biomass carbon, but decreased pH (9.9%) compared with the control. Results of structural equation model showed that soil N2O emission was promoted by soil C and N pool, temperature, and humidity, but was inhibited by soil pH. The explained extents of soil nitrogen pool, carbon pool, temperature and humidity, and pH for N2O emission changes were 37.2%, 27.7%, 22.9% and 9.4%, respectively. Therefore, ant nesting regulated N2O emission dynamics by changing nitrification and denitrification substrates (e.g., nitrate and ammoniacal nitrogen), carbon pool, and micro-habitat (temperature and moisture) of soil in the secondary tropical forest.

Sewage sludge application stimulated soil N2O emissions with a low heavy metal pollution risk in Eucalyptus plantations

Sewage sludge (SS) has been extensively used as an alternative fertilizer in forest plantations, which are beneficial in supplying timbers and mitigating climate change. However, whether the extra nitrogen (N) applied by SS would enhance the soil nitrous oxide (N2O) emission, an important greenhouse gas, in forest plantations have not been well understood. The objective of this study is to evaluate the ecological effects of SS application on soils, by investigating the soil N2O emission and the toxicity of the potentially toxic elements (PTEs) in soil. A field fertilization experiment was conducted in Eucalyptus plantations with four fertilization rates (0 kg m−2, 1.5 kg m−2, 3.0 kg m−2, and 4.5 kg m−2). The soil N2O emissions were monitored at a soil depth of 0–10 cm using static chamber method, soil chemical properties, and PTEs were determined at soil depths of 0–10 cm, 10–20 cm, and 20–40 cm. The average soil N2O emission rate was 8.1 μg N2O–N h−1 m−2 in plots without SS application (control). The application of SS significantly increased the soil N2O emissions by 7–10 times as to control. The increased N2O emissions were positively related to the soil total phosphorus and nitrogen and negatively correlated with copper and zinc, which increased with the SS application. However, the potential ecological risk index (Ei) and the comprehensive potential ecological risk index (RI) of PTEs were lower than 40 and 150 respectively, which indicating a low toxicity of PTEs to soil health. After seven months of SS application, the priming effects of SS on soil N2O emissions gradually diminished. These findings suggest that the application of SS may increase N2O emissions at the initial stages of application (<7 months) and may have a low PTEs pollution risk, even at a high SS addition rate (4.5 kg m−2).