N2O Emission Factors Research Articles

Identification of key water parameters and microbiological compositions triggering intensive N2O emissions during landfill leachate treatment process

Landfill leachate treatment processes tend to emit more N2O compared to domestic wastewater treatment. This discrepancy may be ascribed to leachate water characteristics such as high refractory COD, ammonium (NH4+) content, and salinity. In this work, the leachate influent was varied to examine the N2O emission scenarios. NH4+-N, COD, and Cl− concentrations ranged between 1000–2500, 1000–10,000, and 500–3000 mg L−1, respectively. Simultaneously, we attempted to combine statistical analysis with high-throughput sequencing to understand the microbial mechanism with regards to N2O emission. Results show that the strong N2O emissions occur in the nitrifying tank due to the intensive aeration. The system receiving the lowest COD shows the maximum N2O emission factor of 42.7% of the removed nitrogen. Both redundancy analysis and a structural equation model verify that insufficient degradable organics are the key water parameter triggering intensive N2O emission within the designed influent limits. Furthermore, two essential but non-abundant functional bacteria, Flavobacterium (acting as a denitrifier) and Nitrosomonas (acting as a nitrifier), are identified as the core functional species that dramatically influence N2O emissions. An increase in influent COD promotes the proliferation of Flavobacterium and inhibits Nitrosomonas, which in turn reduce N2O release. Meanwhile, two keystone species of Castellaniella and Saprospiraceae unclassified are recognized. They may supply a suitable niche and integrity of the microbial community for N-cycle functional bacteria. These findings reveal the essential role of non-abundant species in microbial community, and expand the current understanding of microbial interactions underlying N2O dynamics in leachate treatment systems.

Functional N-cycle genes in soil and N2O emissions in tropical grass-maize intercropping systems

There is evidence that forage grasses such as Megathyrsus and Urochloa can suppress nitrification, with direct or indirect consequences on soil inorganic N dynamics and nitrous oxide (N2O) emissions. However, the influence of soil chemical properties on the dynamics of functional N-genes and losses of N in maize (Zea mays L.) intercropped with forage grasses under N fertilization is poorly understood. In this study, soil samples and N2O emissions were analyzed from a field experiment in which maize (fertilized or not with ammonium-based fertilizer) was intercropped with Guinea grass (M. maximus cv. Tanzânia), palisade grass (U. brizantha cv. Marandu), and ruzigrass (U. ruziziensis cv. Comum). Soil N-cycle microorganisms [16S rRNA of bacteria and archaea, nifH (gene encoding N2-fixing bacteria), ammonia-oxidizing bacteria (AOB) and archaea (AOA), nirS (encoding nitrite reductase), and nosZ (encoding nitrous oxide reductase)] were influenced by forage grass, N fertilization, and sampling time, but no evidence of biological nitrification inhibition was found. Palisade grass was associated with a higher abundance of nifH (7.0 × 105 gene copies g−1 soil, on average) in the absence of N compared with the other grasses (4.3 × 105 gene copies g−1 soil, on average). Nitrogen fertilization increased the abundance of AOB but not AOA. Furthermore, N2O flux was influenced by AOB, water-filled pore space, and N fertilization, whereas the cumulative N2O emission and fertilizer-induced emission factor (0.36%, on average) were not affected by the grasses. In conclusion, this study reveals the strong dominance of AOB under ammonium supply, potentially stimulating N2O emissions in maize-forage grass intercropping systems.

Roots and other residues from leys with or without red clover: Quality and effects on N2O emission factors in a partly frozen soil following autumn ploughing

Revised IPCC guidelines assume that a constant share of N in decomposing crop residues is directly emitted as N2O (emission factor: EFN2O), and calculate the amount of nitrogen (N) in non-removable residues of temporary grasslands proportionally to the average annual herbage yield. However, EFN2O depends on the intrinsic quality of the residues and their interactions with environmental conditions. Only a few field studies on N2O emissions from grassland renewal are available, and none have simultaneously quantified the N amount and quality of non-removable residues (roots and stubble).To gain insight into the effect of non-removable residue quality on EFN2O, we studied the amount and quality of roots and stubble and their effect on EFN2O following the ploughing of three-year-old swards.The measured amount of N in non-removable residues was approximately 20, 25, and 31 kg N per 1 Mg average annual dry matter yield in grass, red clover-grass, and red clover, and 70–83% of it was below ground. However, the EFN2O of non-removable residues measured over 252 days was lower (0.24%, SE = 14% for grass and red clover-grass) than the IPCC default value (0.6%, CV: 50%) for wet regions, although within the uncertainty margin, and was significantly lower than the EFN2O of incorporated herbage, which was related to differences in EFCO2. We advocate for more specific studies that separate the effects of belowground and aboveground residues (AGR), considering the possibility of simplifying the accounting of N2O emissions from belowground residues while improving that of non-removable AGR from temporary grasslands and other green crops.We observed the accumulation of N2O in the frozen soil under snow, which was released during diurnal percolation of meltwater. N2O emissions from frozen soil accounted for 30% or more of the total emissions.

Conservation Agriculture With Optimum Fertilizer Nitrogen Rate Reduces GWP for Rice Cultivation in Floodplain Soils

Wetland rice cultivation contributes significantly to global warming potential (GWP), an effect which is largely attributed to emissions of methane (CH4). Emerging technologies for wetland rice production such as conservation agriculture (CA) may mitigate greenhouse gas (GHG) emissions, but the effects are not well defined. Investigations were carried out in an irrigated rice (Boro rice) field in the fifth crop after conversion of conventional tillage (CT) to strip tillage (ST). Two crop residue levels (low versus high, LR versus HR) and three nitrogen (N) application rates (N1 = 108, N2 = 144, and N3 = 180 kg N ha−1) were laid out in a split-plot experiment with three replicates. Yield-scaled GHG emissions and GWP were estimated to evaluate the impacts of CA on mitigating CH4 and N2O emissions in the rice paddy field. There was a 55% higher N2O emission in ST with HR coupled with N3 than that in CT with LR coupled with N1. The N2O emission factors ranged from 0.43 to 0.75% in ST and 0.45 to 0.59% in CT, irrespective of the residue level and N rate. By contrast, CH4 emissions were significantly lower in CA than in the conventional practices (CT plus LR). The ST with LR in N2 reduced the GWP by 39% over the GWP in CT with HR in N1 and 16% over the conventional practices. Based on our investigation of the combination of tillage, residue, and N rate treatments, the adoption of CA with high and low residue levels reduced the GWP by 10 and 16%, respectively, because of lower CH4 and N2O emissions than the current management practices. The relatively high N2O emission factors suggest that mitigation of this GHG in wetland rice systems needs greater attention.

Low water level drives high nitrous oxide emissions from treatment wetland

Wetlands that are restored for carbon sequestration or created for water treatment are an important sources of greenhouse gases, especially methane. The emission of nitrous oxide (N2O) from these systems is often considered negligible due to the inundation and anerobic conditions that support complete denitrification. We used closed chamber method to analyze N2O fluxes over a long-term period across heterogeneous wetland ecosystem constructed for treating nitrate-rich agricultural runoff. Our results showed that the water depth and temperature were most important factors affecting high N2O emissions. The shallow areas where water depth was less than 9 cm created N2O hot spots that emitted 48.8% of the total wetlands annual emission while only covering 6% of the total area. The annual emission from shallow-water hot spots with dense helophytic vegetation was 4.85 ± 0.5 g N2O–N m−2 y−1 while it was only 0.37 ± 0.01 g N2O–N m−2 y−1 in deeper zones. While the water depth was the main factor for high N2O emissions, the temperatures increased the magnitude of the flux and therefore summer droughts and water drawdown created even larger hot spots. These results also suggest that IPCC benchmarks could underestimate N2O emission from shallow waterbodies. Thus, it is important that the shallow zones and water level drawdown in the created or restored wetlands is avoided to minimize the N2O flux.

Emission factors for Vietnamese beef cattle manure sun-drying and the effects of drying on manure microbial community.

Livestock manure and its management are significant sources of greenhouse gas (GHG). In most Southeast Asian countries, the current GHG emissions are estimated by the Intergovernmental Panel on Climate Change (IPCC) Tier 1 approach using default emission factors. Sun-drying is the dominant manure treatment in Vietnam, and in this study, we measured GHG emissions during manure drying using a chamber-based approach. Results show the emission factors for CH4 and N2O were 0.295 ± 0.078 g kg-1 volatile solids (VS) and 0.132 ± 0.136 g N2O-N kg-1 Ninitial, respectively. We monitored the total bacterial/archaeal community using 16S rRNA gene amplicon sequencing and measured the abundance of functional genes required for methanogenesis (mcrA), nitrification (amoA) and denitrification (nirK, nirS and nosZ) processes. Methane emission occurred only at the beginning of the drying process (days 1 to 3). The results of amplicon sequencing indicated that the relative abundance of methanogens also decreased during this period. Although some nitrification activity was detected, there was no significant N2O emission. These findings well describe the manure management system in south Vietnam and the GHG emission from this manure category, paving the way for higher Tier estimations using country-specific values.

Appropriate N fertilizer addition mitigates N2O emissions from forage crop fields

Forage crops are widely cultivated as livestock feed to relieve grazing pressure in agro-pastoral regions with arid climates. However, gaseous losses of soil nitrogen (N) following N fertilizer application have been considerable in response to the pursuit of increased crop yield. A two-year experiment was carried out in a typical saline field under a temperate continental arid climate to investigate the effect of N application rate on N2O emissions from barley (Hordeum vulgare L.), corngrass (Zea mays × Zea Mexicana), rye (Secale cereale L.), and sorghum-sudangrass hybrid (Sorghum bicolor × Sorghum sudanense). The dynamics of N2O emissions, hay yield, and crude protein (CP) yield were measured under four N application rates (0, 150, 200, and 250 kg ha−1) in 2016 and 2017. An N2O emission peak was observed for all crop species five days after each N application. Cumulative N2O fluxes in the growing season ranged from 0.66 to 2.40 kg ha−1 and responded exponentially to N application rate. Emission factors of N2O showed a linear increase with N application rate for all crop species, but the linear slopes significantly differed between barley or rye and corngrass and sorghum-sudangrass hybrid. The hay and CP yields of all forage grasses significantly increased with the increase of N application rate from 0 to 200 kg ha−1. Barley and rye with lower hay and CP yields showed higher N2O emission intensities. The increased level of N2O emission intensity was higher from 200 to 250 kg ha−1 than from 150 to 200 kg ha−1. At N application rates of 200 and 250 kg ha−1, CP yield had a significantly negative correlation with cumulative N2O emission and explained 50.5% and 62.9% of the variation, respectively. In conclusion, ~200 kg ha−1 is the optimal N rate for forage crops to minimize N2O emission while maintaining yield in continental arid regions.

Would transitioning from conventional to organic oat grains production reduce environmental impacts? A LCA case study in North-East Canada

Oat is mainly produced in northern latitude countries such as Canada, which is world's second largest producer. Hence it is necessary to study the environmental impacts of alternative farming systems to conventional oat production in order to improve its environmental performance. A life cycle assessment (LCA) was conducted to compare the environmental impacts and potential improvement opportunities of conventional and organic oat grain production using three different functional units (FU); per mass (t−1 of grains), surface area (ha−1) and gross sales income (1000 CAD−1). Most data were collected from a major farm located in Quebec, Canada. Our analysis shows that organic production is globally preferable based on the area and monetary FUs, whereas conventional production has lower impacts for 11 midpoint impact categories per ton of grain produced. In addition, organic production has higher damages on human health and ecosystem quality, both per ton of grain and per hectare. The relevance of each FU is discussed in the article. The hotspots of conventional oat production are the production and use of synthetic fertilizers, while those of organic oat production are the use of organic fertilizers and manure transportation. A sensitivity analysis showed that the choice of the N2O emission factor and phosphorus emission model considerably affects the magnitude of the impacts on climate change and freshwater eutrophication. Also, the implementation of green manure in the rotation system and the reduction of manure transport distances would significantly affect the results. In conclusion, our LCA case study does not support the idea that converting conventional oat grain production to organic agriculture would systematically decrease environmental impacts in eastern Canada. The environmental benefits largely depend on specific farmer practices and regional context.

Distinctive Microbial Processes and Controlling Factors Related to Indirect N2O Emission from Agricultural and Urban Rivers in Taihu Watershed.

Inland rivers are hotspots of anthropogenic indirect nitrous oxide (N2O) emissions, but the underlying microbial processes remain poorly understood. This study measured N2O fluxes from agricultural and urban rivers in Taihu watershed and investigated the microbial processes driving N2O production and consumption. The N2O fluxes were significantly higher in agricultural rivers (140.1 ± 89.1 μmol m-2 d-1) than in urban rivers (25.1 ± 27.0 μmol m-2 d-1) (p < 0.001). All wind-based models significantly underestimated N2O flux in urban rivers (p < 0.05) when using the Intergovernmental Panel on Climate Change method because they underestimated the N2O emission factor (EF5r). Wind speed and nitrate were the key factors affecting N2O fluxes in agricultural and urban rivers, respectively. NirK-type denitrifiers produced N2O in urban river water, while nirS-type denitrifiers consumed N2O in the sediments of all rivers. Co-occurrence network analysis indicated organics from Microcystis served as electron donors for denitrifiers (dominated by Flavobacterium) in water, while direct interspecies electron transfer between Thiobacillus and methanogens and between Dechloromonas and sulfate-reducing bacteria enhanced N2O reduction in sediments. This study advances our knowledge on the distinctive microbial processes that determine N2O emissions in inland rivers and illustrates the need to revise EF5r for N2O estimation in urban rivers.

Low nitrous oxide concentration and spatial microbial community transition across an urban river affected by treated sewage

Urban rivers receive used water derived from anthropogenic activities and are a crucial source of the potent greenhouse gas nitrous oxide (N2O). However, considerable uncertainties still exist regarding the variation and mechanisms of N2O production in response to the discharge of treated sewage from municipal wastewater treatment plants (WWTPs). This study investigated N2O concentrations and microbial processes responsible for nitrogen conversion upstream and downstream of WWTPs along the Tama River flowing through Tokyo, Japan. We evaluated the effect of treated sewage on dissolved N2O concentrations and inherent N2O consumption activities in the river sediments. In summer and winter, the mean dissolved N2O concentrations were 0.67 µg-N L−1 and 0.82 µg-N L−1, respectively. Although the dissolved N2O was supersaturated (mean 288.7% in summer, mean 240.7% in winter) in the river, the N2O emission factors (EF5r, 0.013%–0.025%) were significantly lower than those in other urban rivers and the Intergovernmental Panel on Climate Change default value (0.25%). The nitrate (NO3−) concentration in the Tama River increased downstream of the WWTPs discharge sites, and it was the main nitrogen constituent. An increasing trend of NO3− concentration was observed from upstream to downstream, along with an increase in the N2O consumption potential of the river sediment. A multiple regression model showed that NO3− is the crucial factor influencing N2O saturation. The diversity in the upstream microbial communities was greater than that in the downstream ones, indicating the involvement of treated sewage discharge in shaping the microbial communities. Functional gene quantification for N2O production and consumption suggested that nirK-type denitrifiers likely contributed to N2O production. Structural equation models (SEMs) revealed that treated sewage discharged from WWTPs increased the NO3− loading from upstream to downstream in the river, inducing changes in the microbial communities and enhancing the N2O consumption activities. Collectively, aerobic conditions limited denitrification and in turn facilitated nitrification, leading to low N2O emissions even despite high NO3− loadings in the Tama River. Our findings unravel an overestimation of the N2O emission potential in an urban oxygen-rich river affected by treated sewage discharge.

Diffusive Fluxes and Controls of N2O from Coastal Rivers in Tianjin City

Rivers are an important emission source of greenhouse gases. To explore the spatial characteristics and influencing factors of N2O emission from the coastal rivers in Tianjin City, six rivers into the Bohai Sea from different land-use types were selected, and the N2O concentrations, saturation, and diffusive fluxes were measured using the headspace-gas chromatography method. The N2O concentration was in supersaturation, and the rivers were the source of atmospheric N2O. The average concentration, saturation, and diffusive fluxes of N2O were (23.85±15.20) nmol·L-1, (309.71±197.38)%, and (27.04±16.46) μmol·(m2·d)-1, with the ranges of 12.70-115.69 nmol·L-1, 164%-1502%, and 9.17-244.79 μmol·(m2·d)-1, respectively. The N2O concentrations and diffusive fluxes of the rivers presented great spatial heterogeneity, with the sewage river (Huangdipai River)>urban river (Haihe River main stream, Jiyun River)>suburban river (Duliujian River, Yongding Xinhe River)>agricultural river (Chaobai Xinhe River). The N2O concentration and diffusion fluxes were significantly correlated with salinity, nutrients, and carbon sources. NO3--N and TP contributed greatly to the diffusive flux differences. N2O production and emission greatly related to the nitrogen cycle process in the Tianjin River, and different forms of nitrogen variously contributed to N2O diffusive fluxes. The salinity gradient had the opposite effect on the N2O emission in urban rivers and drainage rivers. The N2O diffusive fluxes of the sewage river in Tianjin were significantly higher than that of other river types. In the future, due to the development of urbanization and the expansion of urban land, more management measures should focus on the hotspots such as the downstream of wastewater treatment plants of sewage rivers, the estuaries of urban rivers, and the residential gathering areas of suburban rivers to reduce N2O emission.

ЭМИССИЯ ПАРНИКОВЫХ ГАЗОВ НА ТОРФОРАЗРАБОТКАХ В ЦЕНТРЕ ЕВРОПЕЙСКОЙ РОССИИ

The carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes measured at a peat extraction site in the Moscow region in different seasons of 2005-2015 are considered. The average daily (taking into account intradaily dynamics) and average annual fluxes (emission factors) of CO2, CH4, and N2O from the interditch spacing (including microscale elevations and depressions) and drainage channels were estimated. The values of CO2 and N2O fluxes from the main interditch surface were equal to 3.3 t C-CO2/(ha year) and 1.1 kg N-N2O/(ha year), respectively; the value of the CH4 flux from the channel was 3200 kg CH4/(ha year). The resulting estimates are consistent with the default emission factors proposed for the “peatland under extraction” land category by the 2013 Supplement to the IPCC 2006 Guidelines for National Greenhouse Gas Inventories: Wetlands. The results can be used for preparing the national report of the Russian Federation on the emission and absorption of greenhouse gases not controlled by the Montreal Protocol

Within-field spatial variations in subsoil bulk density related to crop yield and potential CO2 and N2O emissions

Subsoil compaction is an increasing problem in modern agriculture, but is not easily recognized in practice, also because of possible within-field spatial variations. This paper addresses the question of how within-field spatial variations in soil bulk density and other soil characteristics relate to within-field spatial variations in crop yield and potential CO2 and N2O emissions from soil. Four fields (5 to 20 ha each) were selected at the suggestion of crop farmers, and sampled using a random soil sampling design (100 samples per field). Undisturbed soil samples were taken at depth of 5–10, 30–35, and 50–55 cm and soil bulk density and potential CO2 and N2O emissions measured under controlled conditions. At each sampling point, also top soil (0–20 cm) samples were taken for determination of pH, texture, SOM, and (micro)nutrients, and soil penetration resistance measurements and visual assessments of soil structure were made. Wheat yields were recorded with harvesters equipped with GPS and yield recorders.Mean soil bulk density in the sub-soil (30–35 cm) ranged between fields from 1.36 ± 0.08 to 1.60 ± 0.11 g cm−3. Mean wheat yields ranged between fields and years from 7.6 ± 0.6 and 11.3 ± 2.4 Mg ha−1. Semi-variogram analyses showed that crop yields and soil properties were mostly spatially dependent; nugget-to-sill ratios were < 25% with ranges of 137 to 773 m. The ratio of CO2 emissions to N2O emissions was negatively related to soil bulk density, especially following N application.In conclusion, within-field spatial variations in subsoil bulk density were successfully related to spatial variations in crop yield and potential CO2 and N2O emissions. The ratio of CO2 emissions to N2O emissions had a much greater response to spatial variations in soil bulk density than wheat yield. Our study suggests that N2O emission factors may depend on (sub)soil bulk density.

Assessing the spatial and temporal variability of greenhouse gas emissions from different configurations of on-site wastewater treatment system using discrete and continuous gas flux measurement

Abstract. Global emissions linked to wastewater treatment are estimated to account for up to 1.5 % of total greenhouse gas (GHG) emissions globally. However, few studies have measured GHG emissions from domestic on-site treatment systems (DWWTSs) directly. In this study, two DWWTSs were monitored for 446 d and &gt; 42 000 gas flux measurements were conducted using both discrete spot measurements and continuous flux chamber deployments. The observed GHG fluxes from biological activity in the soil and water phase were found to be highly spatially and temporally variable and correlated to environmental factors, water usage patterns and system design. In total, the results show that a septic tank discharging effluent into a well-designed soil treatment unit is estimated to emit a net 9.99 kg-CO2eq.cap-1yr-1, with approximately 63 %, 27 % and 10 % of the total CO2-equivalent net emissions in the form of CO2, CH4 and N2O, respectively. Emissions from the septic tank surface contributed over 50 % of total emissions and tended to be strongly underestimated by one-off discrete measurements, especially when episodic ebullitive events are to be considered. Fluxes from the soil treatment unit (STU) stemmed from both the soil surface and the vent system. Soil fluxes were mostly influenced by temperature but peaked regularly under conditions of rapidly changing soil water content. Vent fluxes were mostly governed by effluent, quality and a low number of high-emission events were responsible for the majority of total observed vent emissions. Owing to the strong overall spatial and temporal heterogeneity of observed fluxes from DWWTSs across all modules, future studies should focus on continuous deployments of a number of flux chambers over discrete measurements to accurately assess GHG emissions from on-site systems. This study also provided insights into managing GHG emissions from DWWTSs by different system configuration design, as well as indicating that the current IPCC emission factors for CH4 and N2O significantly overestimate emissions for on-site wastewater treatment systems.

Estimating natural nitrous oxide emissions from the Qinghai–Tibetan Plateau using a process-based model: Historical spatiotemporal patterns and future trends

Fluctuations in natural nitrous oxide emissions (N2O) are not fully understood, especially on the Qinghai–Tibetan Plateau. In order to characterize the differences in historical N2O variations among different regions of the Qinghai-Tibetan Plateau and investigate how a warmer and wetter climate change would affect the N2O fluxes over the Qinghai-Tibetan Plateau, a process-based model (TRIPLEX-GHG) has been validated using 14 sites with annual N2O emission fluxes, and is used to estimate the spatiotemporal patterns of natural N2O fluxes, and predict its future trend for the Qinghai-Tibetan Plateau (the temperature increasing by 1.5 °C and the precipitation increasing by 10%). Considering the model performance evaluation indicators (R2=0.83, rRMSE=44.6%, PBIAS=7.18%), the performance of the model for simulating annual N2O fluxes is “moderate”. The estimated average total natural N2O emissions of the Qinghai-Tibetan Plateau from 1970 to 2017 were 0.313 Tg N yr−1, ranging from 0.305 Tg N yr−1 (1983) to 0.325 Tg N yr−1 (2006). The estimated total natural N2O emissions of the Qinghai-Tibetan Plateau slightly increased from 1970 to 2017, with a mean increase of 0.0002 Tg N yr−1. Between the 1970s and 2010s, most increases in N2O flux were observed in the northern and southeastern regions of the Qinghai-Tibetan Plateau. The area in which the rate of N2O released was greater than 15 µg N m−2 h−1 was mainly found in plateau temperate regions. Regions with relatively high temperatures released more N2O, which indicates that temperature is a dominant factor for N2O emissions in high-altitude regions. Natural N2O emissions of the Qinghai-Tibetan Plateau will increase up to 0.335 Tg N yr−1 (by approximately 6.13%) by 2050 if the climate continues to become warmer and wetter.

Increasing grain yield, nitrogen use efficiency of summer maize and reducing greenhouse gas emissions by applying urea ammonium nitrate solution

AbstractThe contradiction between the high grain yield of summer maize (Zea mays L.) and greenhouse gas (GHG) emissions in the field can be resolved by selecting effective nitrogen (N) sources and fertilization methods. In this study, two fertilization methods were used: (a) urea ammonium nitrate solution (UAN) and urea (210 kg ha–1 nitrogen applied) were applied by farmers' traditional fertilization method; and (b) fertigation of four UAN application rates: CK (no N applied), AF1 (126 kg N ha–1), AF2 (168 kg N ha–1), AF3 (210 kg N ha–1). The results showed that both hybrids (DH518 and DH605) applied UAN under the traditional fertilization method increased grain yield by 2.4 and 1.8% and enhanced nitrogen use efficiency (NUE) by 25.1 and 9.9%, respectively, compared with urea. While under fertigation treatment, the grain yield increased by 7.6 and 6.6%, and the NUE increased by 23.2 and 12.4%. Grain yield increased with the increase of UAN application, and the NUE showed a trend of rising first and then falling under fertigation treatment. Both the cumulative N2O emissions and N2O emission factor showed an increasing trend as well. An application rate of UAN at 168 kg N ha–1 (AF2) resulted in a NUE that was 49% higher than urea. Compared with urea, the global warming potential (GWP) and greenhouse gas intensity (GHGI) of AF2 were reduced by 29.5% and 31.2%, respectively. These indicated that optimizing the application rate of UAN through fertigation would achieve better summer maize yield and environmental benefits.

Nitrous oxide emissions from subtropical estuaries: Insights for environmental controls and implications

Estuaries are expected to contribute large nitrous oxide (N2O) emissions, however the environmental controls and implications of N2O emissions have not been well understood. Here we investigated water N2O concentrations, fluxes and sources in wet and dry seasons for 2019–2020 in five subtropical estuaries spanning hydrologic characteristics and nitrogen concentrations gradient. Water dissolved N2O concentrations and fluxes were in a range of 15.8–84.9 nmol L–1 and 0.66–22.2 µg m–2 h–1, respectively. These studied estuaries were oversaturated in N2O, with the saturations of 118–615%. Water dissolved N2O concentrations, saturations and fluxes increased significantly as nitrogen concentrations increase, whereas they did not differ significantly between the wet and dry seasons. Water N2O emissions, however, were also lower in the estuaries characterized by large discharge and water flow. N2O saturations and fluxes were determined directly by water nitrogen and oxygen concentrations and more indirectly by water temperature and velocity. The δ15N-N2O and site preference-N2O varied respectively from 2.86 to 11.31‰ and from 1.58 to 11.72‰, which overlapped the values between nitrification and denitrification. Nitrification and denitrification were responsible for 18.7–38.1% and 61.9–81.3% of N2O emissions, respectively. Indirect N2O emission factors were 0.08–0.14% and decreased with increasing total nitrogen concentrations. It is estimated that water N2O emissions in CO2 equiv could offset approximately 4.9% of average CO2 sink of China estuaries. Therefore, these results suggest that nitrogen concentrations and hydrologic characteristics together modify N2O emissions and that estuaries may be the important contributors to N2O emissions.

Sugarcane residue and N-fertilization effects on soil GHG emissions in south-central, Brazil

Ethanol derived from sugarcane (Saccharum officinarum) could replace a substantial amount of fossil fuel in Brazil, provided the greenhouse gas (GHG) budget is balanced. Green harvest systems have significantly increased the potential amount of sugarcane residue (straw) available for harvest and bioenergy production, but several questions about how those practices will affect field GHG fluxes. In this sense, we quantified GHG emissions associated with four straw harvest rates (12 [no removal]; 6 [moderate removal]; 3 [high removal] and 0 Mg ha−1 [total removal] of dry matter of straw left on the soil), with and without 100 L ha−1 vinasse + N-fertilizer [80 kg ha−1 (NH4)2SO4]. GHG fluxes were measured for 60 days using static chambers. Our results showed that total straw removal reduced CO2 and N2O emissions by 15% and 25%, respectively. After vinasse and fertilizer additions, CO2 emissions were 2.5-fold higher, and N2O were 5-fold higher, regardless of straw rates. In additional, synergic effect of vinasse + fertilizer and straw removal decreased 60% CH4 uptakes. Direct N2O emission factor was estimated at 0.32%. Net GHG emissions induced by vinasse + fertilizer applied in the sugarcane plantation were 220 g m−2 CO2e (or 60 g m−2 C-equivalent), disregarding straw harvest rate. Therefore, we conclude that maintaining at least 6 Mg ha-1 of sugarcane straw in the field will result in a ‘win-win situation’ by mitigating GHG emissions, sustaining soil, and still saving part of the raw-material for bioenergy production.

Fertilizer-induced nitrous oxide emissions from global orchards and its estimate of China

The fruit has become the third-largest agricultural planting industry after cereals and vegetables in China. Fertilization regimes (e.g., application rate and method) in fruit orchards typically differ from cereal croplands, which would incur a pronounced difference in fertilizer-induced nitrous oxide (N2O) emissions between them. However, fertilizer-induced direct N2O emissions from orchard fields remain poorly understood. We conducted a field experiment in a peach orchard and a global meta-analysis of N2O emissions from fruit orchards. The emission factor (EF) of fertilizer N for N2O averaged 0.81%, with a background N2O emission of 3.4 kg N ha–1 yr-1 in our field study. A global meta-analysis suggested that the linear regression model was the best to fit N2O emissions by fertilizer N input for most fruit types compared to the nonlinear models. When averaging all global data, the linear model projected the EF of N2O from orchards to be 0.84%, with the background emission of 1.96 kg N ha–1. The estimate of direct N2O derived from the orchard-specific nonlinear model was substantially lower than those from the nonlinear model with global cropland measurements. The fertilizer-induced direct N2O emission from Chinese orchards during the 2000s was estimated to be 32–49 Gg N yr–1, equivalent to about 14% of total direct N2O emissions from Chinese uplands. Therefore, orchard cultivation constitutes a hotspot of N2O emissions in the agricultural sector, and priority should be given to emissions reduction to achieve the transition to climate-smart agriculture.

Model-based identification of the dominant N2O emission pathway in a full-scale activated sludge system

Activated sludge models (ASMs), extended with an N2O emission module, are powerful tools to describe the operation of full-scale wastewater treatment plants (WWTPs). Specifically, such models can investigate the most contributive N2O production pathways and guide towards N2O and carbon footprint (CF) mitigation measures. A common practice is to develop and validate models using data from a single WWTP. In this study, a successfully validated model in one plant (Slupsk/Poland) was extrapolated to another full-scale WWTP (Viikinmäki/Finland). For this purpose, the previously developed ASM No. 2d with the N2O module was used. Moreover, the results of calibration and validation of that model were compared with those obtained on the basis of the ASM No. 3 with an N2O module. A novel, rigorous calibration protocol, based on the system engineering approach, was implemented to minimize the number of adjusted parameters without compromising the accuracy of model predictions. The validated model accurately predicted the behavior of the system in terms of the liquid N2O production in the bioreactor and gaseous N2O emissions. Model-based identification of N2O production pathways revealed the key role of heterotrophs duo to their high abundance in the microbial community. The N2O emission factor (EF) at the studied plant was found between 0.9 and 0.94% of the influent TN-load for the validation and calibration period, respectively. Based on the model predictions, it was estimated that the aerobic zones contributed to over 93% of the N2O emitted to the atmosphere, while the remaining portion (7%) resulted from the N2O liquid-gas transfer in the non-aerated zones. The difference between the predicted N2O EF and the empirical EF calculation would lead to almost 1000 tonnes of CO2 equivalent reduction of the annual CF of the plant, which highlights the importance of model applications in CF studies.