Higher N2O Emissions Research Articles

Soil microbial communities as potential regulators of N2O sources in highly acidic soils

• Soil pH was a key driver of N2O emission and sources in acidic soils. • N2O emission was significantly positively associated with the ratio of ITS to 16S. • N2O was significantly correlated with bacterial and fungal community composition. • Fungi contributed to N2O in highly acidic tea plantations and vegetable fields. Acidic soil is a main source of global nitrous oxide (N2O) emissions. However, the mechanism behind the high N2O emissions from acidic soils remains a knowledge gap. The objective of this microcosm incubation study was to pin-point the microbial mechanisms involved in N2O production processes in acidic soils. For that purpose, the isotopic signatures and microbial community structure and composition of four soil samples were examined. The results showed that highly acid soils (pH = 3.51) emitted 89 times more N2O than alkaline soils (pH = 7.95) under the same nitrogen (N) inputs. Fungal denitrification caused high N2O emissions in acidic soils. ITS to 16S abundance ratio was positively correlated with cumulative N2O emissions from the tested soils. The highly acid soils (pH < 4.5) showed greater fungal nirK gene abundance and lower abundance of AOA-amoA, AOB-amoA, nirK, nosZ I and nosZ II genes. The unclassified Aspergillaceae fungi (63.65%) dominated the highly acidic soils and was the most strongly correlated genus with N2O emissions. These findings highlight that soil microbial community structures, denitrifying fungi in particular, shaped by low pH (pH < 4.5) lead to high N2O emissions from acidic soils.

Arbuscular mycorrhizal fungi mitigate earthworm-induced N2O emissions from upland soil in a rice-rotated wheat farming system

Earthworms are important for soil processes in arable cropping systems, and affect the development of arbuscular mycorrhizal fungi (AMF) which may combinely play a crucial role in soil nitrogen (N) cycling. However, the information is limited about their interaction on N2O emissions. A two-factorial microcosm field experiment was established to identify the interactive effects of earthworms and AMF on N2O emissions from a rice-rotated wheat field and explore its underlying mechanisms about nitrification and denitrification processes. The results showed that earthworm addition (+E) increased cumulative N2O emissions during the whole growth stage by 236.7 % compared with no earthworms (−E). Cumulative N2O emissions during the jointing and booting stages contributed to 58.7 % of that during the whole growth stage. Higher N2O emissions under +E were attributed to changes in soil organic carbon (SOC) and NH4+ concentration. SOC was positively correlated to gene abundances associated with N2O reduction (e.g., nosZ), and earthworm activity enhanced SOC mineralization and reduced SOC concentration, thus increasing N2O emissions. A negative correlation between NH4+ and N2O emissions, and lower NH4+ under +E indicated that earthworms enhanced nitrification intensity using NH4+ as substrates. Interestingly, AMF normal (+A) significantly decreased cumulative N2O emissions by 37.3 % compared with mycorrhizal suppression (−A) and the interaction with earthworms can further decreased N2O emissions by 37.0 % compared with -E-A (P < 0.05). This can be explained by higher NH4+, lower NO3− concentrations, and lower abundance of nitrification-associated genes (e.g., AOA and AOB) at the booting stage under +A, suggesting that AMF reduced N2O emissions mainly through lowering nitrification intensity. The results highlight that earthworms increase N2O emissions while AMF can mitigate earthworm-induced N2O emissions, and enhance our understanding on the soil N cycle in terms of N2O production in the presence of AMF and earthworms.

Numerical study of NH3/CH4 MILD combustion with conjugate heat transfer model in a down-fired lab-scale furnace

Ammonia (NH3) is a promising carbon-free fuel but suffers from low flame stability and high NO emission. Moderate or intense low-oxygen dilution (MILD) combustion has been indicated to be a feasible technology for resolving these drawbacks of NH3 during its combustion. However, the effect of NH3 substitution on combustion and NO emission characteristics as well as heat transfer behaviors under MILD combustion is not well-understood. This paper reports a numerical study of NH3/CH4 combustion in a laboratory-scale furnace by using conjugate heat transfer (CHT) model, which eliminates the commonly-used fixed-temperature or fixed-heat flux assumptions for furnace walls. Main emphasis is laid on the comparison of NO emission, temperature distribution, flame structure and furnace heat transfer behaviors between traditional swirling combustion and MILD combustion under various NH3 blending ratio (XNH3) conditions. The results show that NO emission reaches to the maximum at XNH3 of 50% regardless of combustion mode., while MILD combustion generates a lower NO emission and conversion ratio than swirling combustion in the whole range of XNH3 (0% - 100%). Numerical simulation without using CHT model produces negative heat transfer in certain furnace regions, indicating the necessity of adopting CHT model on simulating heat transfer process in combustion furnaces. With the CHT model, the heat flux on the furnace wall is found to follow the same trend between the two different combustion modes, but the heat utilization efficiency of MILD combustion is slightly lower mainly due to the increased exhausted gas temperature, which is resulted from the postponed reaction zone. With the replacement of CH4 by NH3, the heat utilization efficiency gradually decreases under both two combustion modes as a result of lower peak temperature and less soot radiation.

Nitrous Oxide Fluxes in Permafrost Peatlands Remain Negligible After Wildfire and Thermokarst Disturbance

AbstractThe greenhouse gas (GHG) balance of boreal peatlands in permafrost regions will be affected by climate change through disturbances such as permafrost thaw and wildfire. Although the future GHG balance of boreal peatlands including ponds is dominated by the exchange of both carbon dioxide (CO2) and methane (CH4), disturbance impacts on fluxes of the potent GHG nitrous oxide (N2O) could contribute to shifts in the net radiative balance. Here, we measured monthly (April to October) fluxes of N2O, CH4, and CO2 from three sites located across the sporadic and discontinuous permafrost zones of western Canada. Undisturbed permafrost peat plateaus acted as N2O sinks (−0.025 mg N2O m−2 d−1), but N2O uptake was lower from burned plateaus (−0.003 mg N2O m−2 d−1) and higher following permafrost thaw in the thermokarst bogs (−0.054 mg N2O m−2 d−1). The thermokarst bogs had below‐ambient N2O soil gas concentrations, suggesting that denitrification consumed atmospheric N2O during reduction to dinitrogen. Atmospheric uptake of N2O in peat plateaus and thermokarst bogs increased with soil temperature and soil moisture, suggesting sensitivity of N2O consumption to further climate change. Four of five peatland ponds acted as N2O sinks (−0.018 mg N2O m−2 d−1), with no influence of thermokarst expansion. One pond with high nitrate concentrations had high N2O emissions (0.30 mg N2O m−2 d−1). Overall, our study suggests that the future net radiative balance of boreal peatlands will be dominated by impacts of wildfire and permafrost thaw on CH4 and CO2 fluxes, while the influence from N2O is minor.

Soil characteristics controlling nitrous oxide emissions of tropical peatlands

Emission of nitrous oxide (N2O) from peatlands contributes very significantly to the world global warming, although the factors controlling N2O emissions from peatlands are not yet clear. This study aimed to determine peat characteristics controlling N2O emissions in peatlands. N2O emissions and several soil characteristics (pH, electrical conductivity, height of water table, water-filled pore space, decomposition degree of peats, bulk density, organic carbon, total nitrogen, and concentrations of ammonium and nitrate) were observed in peatlands with different crops and land-uses: lettuce, spring-onion, albizia, shrubs-peat, and burned-peats. Correlation-regression analyses were employed to quantify peat properties influencing the rates of N2O emission from peatlands. The results of the study showed that N2O emissions varied based on the type of crops/land-uses, in which peats used for crop cultivation had higher N2O emissions than others. The results of the analysis also revealed that N2O emissions of peat were controlled by variables related to water contents and the contents of NH4 + and NO3 −. In peats with relatively similar NH4 + and NO3 − contents, N2O emissions were determined by variables related to water contents (height of water table and volumetric water contents). The result of this study shows the need for water and peat fertility management for the mitigation of N2O emission from peatlands. Thus, managing the availability of nitrogen by using biological fertilizers to reduce the amounts of inorganic nitrogen fertilizers is required to reduce N2O emissions without decreasing crop yields in peatlands.

Hot moment of N2O emissions in seasonally frozen peatlands.

Since the start of the Anthropocene, northern seasonally frozen peatlands have been warming at a rate of 0.6 °C per decade, twice that of the Earth's average rate, thereby triggering increased nitrogen mineralization with subsequent potentially large losses of nitrous oxide (N2O) to the atmosphere. Here we provide evidence that seasonally frozen peatlands are important N2O emission sources in the Northern Hemisphere and the thawing periods are the hot moment of annual N2O emissions. The flux during the hot moment of thawing in spring was 1.20 ± 0.82 mg N2O m-2 d-1, significantly higher than that during the other periods (freezing, -0.12 ± 0.02 mg N2O m-2 d-1; frozen, 0.04 ± 0.04 mg N2O m-2 d-1; thawed, 0.09 ± 0.01 mg N2O m-2 d-1) or observed for other ecosystems at the same latitude in previous studies. The observed emission flux is even higher than those of tropical forests, the World's largest natural terrestrial N2O source. Furthermore, based on soil incubation with 15N and 18O isotope tracing and differential inhibitors, heterotrophic bacterial and fungal denitrification was revealed as the main source of N2O in peatland profiles (0-200 cm). Metagenomic, metatranscriptomic, and qPCR assays further revealed that seasonally frozen peatlands have high N2O emission potential, but thawing significantly stimulates expression of genes encoding N2O-producing protein complexes (hydroxylamine dehydrogenase (hao) and nitric oxide reductase (nor)), resulting in high N2O emissions during spring. This hot moment converts seasonally frozen peatlands into an important N2O emission source when it is otherwise a sink. Extrapolation of our data to all northern peatland areas reveals that the hot moment emissions could amount to approximately 0.17 Tg of N2O yr-1. However, these N2O emissions are still not routinely included in Earth system models and global IPCC assessments.

Organic fertilizer substitutions maintain maize yield and mitigate ammonia emissions but increase nitrous oxide emissions.

Organic fertilizer can improve soil structure and enhance the nutrient content in soil and is beneficial to sustainable agricultural development. However, the influence of organic fertilizer substitutions on NH3 and N2O emissions from farmland is unclear. Thus, we set up an organic substitution field experiment in Northeast China. The experiment included six treatments: single application of chemical fertilizers (NPK: 250kg N ha-1); NO10, 10% reduction in chemical nitrogen fertilizers (225kg N ha-1) + chicken manure (25kg N ha-1); NO20, 20% reduction in chemical nitrogen fertilizers (200kg N ha-1) + chicken manure (50kg N ha-1); NO30, 30% reduction in chemical nitrogen fertilizers (175kg N ha-1) + chicken manure (75kg N ha-1); NO40, 40% reduction in chemical nitrogen fertilizers (150kg N ha-1) + chicken manure (100kg N ha-1); and no-nitrogen fertilizer (CK). This experiment investigated the effects of partial substitution of chemical nitrogen fertilizer with organic fertilizer on NH3 and N2O emissions and nitrogen use efficiency in a maize field. The results showed that, compared with chemical N, organic fertilizer mitigated NH3 volatilization but promoted the soil N2O total emissions during the whole growth stage. NH3 cumulative volatilization decreased with the increase in the substitution rate of organic fertilizer. Compared with the NPK treatment, the cumulative volatilization of NH3 in the NO30 and NO40 treatments decreased by 15.24 and 17.92%, respectively. The NO40 treatment had the highest N2O emission in the whole growth stage, and the N2O emission of the NO40 treatment increased by 10.72% compared to the NPK treatment. Moreover, the yield, partial factor productivity (PFP), nitrogen harvest index (NHI), and apparent nitrogen recovery efficiency (NRE) of NO30 treatment were the highest of all treatments, and the yields, PFP, plant N accumulation, grain N accumulation, and the cumulative emissions of NH3 and N2O were similar to N20 treatment. In conclusion, nitrogen fertilizer use efficiency was enhanced, decreasing environmental pollution from livestock under organic fertilizer substitution conditions. We suggested 20% or 30% substitution rates of organic fertilizer were proper.

Comparison and Evaluation of GHG Emissions during Simulated Thermophilic Composting of Different Municipal and Agricultural Feedstocks

Composting is widely used to recycle a variety of different organic wastes. In this study, dairy manure, chicken litter, biosolids, yard trimmings and food waste were selected as representative municipal and agricultural feedstocks and composted in simulated thermophilic composting reactors to compare and evaluate the GHG emissions. The results showed that the highest cumulative emissions of CO2, CH4 and N2O were observed during yard trimmings composting (659.14 g CO2 kg-1 DM), food waste composting (3308.85 mg CH4 kg-1 DM) and chicken litter composting (1203.92 mg N2O kg-1 DM), respectively. The majority of the carbon was lost in the form of CO2. The highest carbon loss by CO2 and CH4 emissions and the highest nitrogen loss by N2O emission occurred in dairy manure (41.41%), food waste (0.55%) and chicken litter composting (3.13%), respectively. The total GHG emission equivalent was highest during food waste composting (365.28 kg CO2-eq ton-1 DM) which generated the highest CH4 emission and second highest N2O emissions, followed by chicken litter composting (341.27 kg CO2-eq ton-1 DM), which had the highest N2O emissions. The results indicated that accounting for GHG emissions from composting processes when it is being considered as a sustainable waste management practice was of great importance.

Targeting yield and reducing nitrous oxide emission by use of single and double inhibitor treated urea during winter wheat season in Northern Germany

Nitrous oxide (N2O) is a powerful greenhouse gas and has an adverse effect on stratospheric ozone. Field application of synthetic N fertilizers is the largest source of global N2O emission and different N forms (nitrate vs. ammoniacal N) may play a significant role. In addition, the use of nitrification inhibitor (NI) is considered as a reliable way to mitigate agricultural N2O emission, whereas this effect is still debated for urease inhibitors (UI). However, the efficacy of NI or UI is still variable among different inhibitor products and environmental conditions. This study was conducted to test the efficacy of N form (calcium ammonium nitrate CAN vs. urea) and the almost unstudied UI, N-(2-nitrophenyl) phosphoric triamide (2-NPT), as well as an NI, mixture of dicyandiamide and 1H-1,2,4-triazol (DCD/TZ) and the combination of both inhibitors on N2O emission and crop yield. The measurements were carried out in winter wheat growth season in the subsequent years of 2012–2013 in the North of Germany. No difference in cumulated N2O emissions were observed between urea and CAN. The results confirmed the positive effect of NI (DCD/TZ) on reducing N2O emission. Compared with untreated urea, NI addition caused ∼75 % reduction of fertilizer derived N2O emissions within the vegetation period. The combination of UI and NI did not result in a further reduction of relative or yield-scaled N2O emission, although it resulted in higher grain yield and nitrogen recovery. Addition of UI showed no consistent effect on N2O emission compared to untreated urea, however in year 2013 a significant reduction of fertilizer derived emissions by ∼50 % was observed. Higher yields were observed for CAN fertilization compared to urea, though not significant. For both treatments including UI the yield effects, in particular N use efficiency, were stronger than for untreated urea and urea solely treated with NI. Therefore, the combined treatment with UI and NI was the most advantageous fertilizer solution for concomitantly achieving high yield, high nitrogen utilization efficiency and N2O emission reduction.

Solubility of soil phosphorus in extended waterlogged conditions: An incubation study

Understanding how extended excess soil moisture exacerbated by extreme weather events affects changes in iron (Fe) chemistry is crucial for assessing environmental risk associated with soil phosphorus (P) in high P soils. The objective of our study was to assess the effects of three soil moisture regimes (field capacity, water saturation, and waterlogging), two Fe3+ nitrate level (Fe3+ nitrate addition and no Fe3+ nitrate addition), and the duration of incubation (0, 3, 7, 14, 21, 28, 35, 49, 63, 90, and 120 days) on the (i) reduction of ferric (Fe3+) to ferrous (Fe2+) iron, (ii) solubility of soil P, and (iii) soil microbial biomass and greenhouse gas emissions. Surface soils (0–20 cm) were collected from a maize silage field located in the Fraser Valley (British Columbia, Canada). Decreased redox potential (Eh) of 155 mV in waterlogged soils coincided with the reduction of Fe3+ to Fe2+ of about 1190 mg kg−1 and an increase in soil pH of 0.8 unit compared to field capacity regime at 120 days after pre-incubation (P < 0.001). The increase of pH is due to the microbially-mediated reduction of metal cations which consumes H+ cations. Water-extractable P (Pw) concentrations increased with increasing soil moisture regimes from 1.47 to 2.27, and 2.58 mg kg−1 under field capacity, water saturation, and waterlogged regime respectively. Mehlich-3 extractable P concentrations significantly decreased from 196 to 184 and 172 mg kg−1 under water saturation, field capacity, and waterlogged regime respectively. Concomitant to Pw concentrations, microbial biomass carbon and nitrogen as well as DOC, CO2 and N2O emissions increased with increasing soil moisture regimes. The Fe3+ nitrate addition had an inhibitory effect on Fe reduction, Pw concentration at the first 35 days, and DOC but a stimulating effect on N2O emission. A high N2O emission at the first 63 days, CO2 emission after 35 days, and a non-remarkable concentration of Fe2+ at the first 63 days with Fe3+ nitrate addition under waterlogged soil suggests that NO3− is more preferable than Fe3+ as an electron acceptor. Our results showed that soils maintained under extended anoxic conditions could increase the soluble and available P and subsequent risk of P transport to surface and drainage waters, whereas Fe3+ nitrate addition could minimize or delay this effect.

Mitigation of nitrous oxide emissions in grazing systems through nitrification inhibitors: a meta-analysis

Grasslands are the largest contributor of nitrous oxide (N2O) emissions in the agriculture sector due to livestock excreta and nitrogen fertilizers applied to the soil. Nitrification inhibitors (NIs) added to N input have reduced N2O emissions, but can show a range of efficiencies depending on climate, soil, and management conditions. A meta-analysis study was conducted to investigate the factors that influence the efficiency of NIs added to fertilizer and excreta in reducing N2O emissions, focused on grazing systems. Data from peer-reviewed studies comprising 2164 N2O emission factors (EFs) of N inputs with and without NIs addition were compared. The N2O EFs varied according to N source (0.0001–8.25%). Overall, NIs reduced the N2O EF from N addition by 56.6% (51.1–61.5%), with no difference between NI types (Dicyandiamide—DCD; 3,4-Dimethylpyrazole phosphate—DMPP; and Nitrapyrin) or N source (urine, dung, slurry, and fertilizer). The NIs were more efficient in situations of high N2O emissions compared with low; the reduction was 66.0% when EF > 1.5% of N applied compared with 51.9% when EF ≤ 0.5%. DCD was more efficient when applied at rates > 10 kg ha−1. NIs were less efficient in urine with lower N content (≤ 7 g kg−1). NI efficiency was negatively correlated with soil bulk density, and positively correlated with soil moisture and temperature. Better understanding and management of NIs can optimize N2O mitigation in grazing systems, e.g., by mapping N2O risk and applying NI at variable rate, contributing to improved livestock sustainability.

The effect of upland crop planting on field N2O emission from rice-growing seasons: A case study comparing rice-wheat and rice-rapeseed rotations

Paddy fields, especially under flooded rice-upland crop rotation modes, undergo tremendous changes in soil redox conditions. The effects of different upland crop plantations on field N2O emissions from the rice-planting period and the underlying mechanisms are scarce. In this study, two types of flooded rice-upland cropping rotation modes, namely, rice-rapeseed rotation (RR) and rice-wheat rotation (RW), were selected to investigate the effects of different upland crop plantations on field N2O emissions during the rice-growing period, in central China. To confirm the effect of upland crop plantations on soil variables and associated functional genes, conventional rice planting (RR-CP and RW-CP) with a part of the non-rice area (RR-NP and RW-NP) during the rice season was implemented. Results revealed that seasonal N2O emissions from RR-NP and RW-NP treatments were 1.83 ± 0.18 and 1.43 ± 0.25, and 1.41 ± 0.09 and 0.88 ± 0.08 kg N ha−1, respectively, during two consecutive rice-plantation seasons, which were significantly higher than those from RR-CP treatment (1.38 ± 0.16 and 0.91 ± 0.15 kg N ha−1) and RW-CP treatment (0.95 ± 0.08 and 0.50 ± 0.10 kg N ha−1). Higher N2O emissions from RR rotation than from RW rotation, regardless of rice planting, indicated that upland cultivation affected soil N2O efflux. For rice planting treatments, strong positive relationships between N2O fluxes and soil-dissolved organic carbon (DOC), ammonium (NH4+), nitrate (NO3-), and functional genes (nirS, nirK, and nosZ genes) were observed, implying that soil available C and N, and related functional genes are key regulatory factors controlling N2O emissions. Compared with the RW-CP treatment, the higher abundance of the nirK gene and lower nosZ/(nirS + nirK) ratio from the RR-CP treatment may facilitate greater N2O production, while reducing conversion of N2O to N2, resulting in increased N2O emissions. Furthermore, structural equation model (SEM) showed that field flood depth (FD), soil available C and N, and the abundance of nirS gene, together, displayed more than 70 % impact effect on N2O emission for RR-CP treatment, while, FD, soil available N, and nirS and nirK genes, together, displayed more than 60 % impact effect on N2O emission for RW-CP treatment. Our results suggest that suitable upland crop rice cultivation could be beneficial for mitigating soil N2O emissions.

Influence of the Pilot Flame on the Morphology and Exhaust Emissions of NH3-CH4-Air Swirl Flames Using a Reduced-Scale Burner at Atmospheric Pressure

This work presents an experimental study on the influence of the pilot flame characteristics on the flame morphology and exhaust emissions of a turbulent swirling flame. A reduced-scale burner, inspired by that fitted in the AE-T100 micro gas turbine, was employed as the experimental platform to evaluate methane (CH4) and an ammonia-methane fuel blend with an ammonia (NH3) volume fraction of 0.7. The power ratio (PR) between the pilot flame and the main flame and the fuel composition of the pilot flame was investigated. The pilot power ratio was varied from 0 to 20% for both fuel compositions tested. The NH3 volume fraction in the pilot flame ranged from pure CH4 to pure NH3 through various NH3–CH4 blends. Flame images and exhaust emissions, namely CO2, CO, NO, and N2O were recorded. It was found that increasing the pilot power ratio produces more stable flames and influences most of the exhaust emissions measured. The CO2 concentration in the exhaust gases was roughly constant for CH4-air or NH3–CH4–air flames. In addition, a CO2 concentration reduction of about 45% was achieved for XNH3 = 0.70 compared with pure CH4, while still producing stable flames as long as PR ≥ 5%. The pilot power ratio was found to have a higher relative impact on NO emissions for CH4 than for NH3–CH4, with measured exhaust NO percentage increments of about 276% and 11%, respectively. The N2O concentration was constant for all pilot power ratios for CH4 but it decreased when the pilot power ratio increased for NH3–CH4. The pilot fuel composition highly affected the NO and N2O emissions. Pure CH4 pilot flames and higher power ratios produced higher NO emissions. Conversely, the NO concentration was roughly constant for pure NH3 pilot flames, regardless of the pilot power ratio. Qualitative OH-PLIF images were recorded to further investigate these trends. Results showed that the pilot power ratio and the pilot fuel composition modified the flame morphology and the OH concentration, which both influence NO emissions.

Transient nitrite accumulation explains the variation of N2O emissions to N fertilization in upland agricultural soils

Nitrogen (N) fertilization in agricultural soils is the largest anthropogenic source of nitrous oxide (N2O) emissions, but the underlying mechanism of the varied response of N2O emissions to N fertilizer is unclear. We investigated the response of N2O emissions to ammonium-based fertilizers and nitrification inhibitors in upland agricultural soils across northern China. Large variations in N2O emissions in different upland soils in response to N fertilization were found, whereas the variations were well correlated with soil nitrite (NO2−) dynamics (P < 0.01). A higher pH soil is generally correlated with a higher potential nitrification and denitrification rate, a higher AOB/AOA gene abundance ratio, higher NO2− content, and higher N2O emission. The efficacy of 3,4-dimethylpyrazole phosphate (DMPP) for inhibiting net nitrification increased with increasing soil pH. Further investigation showed that rapid ammonia oxidation drives oxygen (O2) depletion and NO2− accumulation in the soil matrix, which stimulates N2O emission and inhibits N2 emission under anoxic conditions. Collectively, our results showed that alkaline soils tend to have rapid ammonia oxidation potential, leading to transient NO2− accumulation and O2 depletion in the soil matrix after large ammonium addition. This might trigger significant N2O emissions from denitrification and DNRA, which explains the spatial heterogeneity of N2O emissions in response to N fertilization in various upland soils.

Temperature-Related N2O Emission and Emission Potential of Freshwater Sediment

Nitrous oxide (N2O) is a major radiative forcing and stratospheric ozone-depleting gas. Among natural sources, freshwater ecosystems are significant contributors to N2O. Although temperature is a key factor determining the N2O emissions, the respective effects of temperature on emitted and dissolved N2O in the water column of freshwater ecosystems remain unclear. In this study, 48 h incubation experiments were performed at three different temperatures; 15 °C, 25 °C, and 35 °C. For each sample, N2O emission, dissolved N2O in the overlying water and denitrification rates were measured, and N2O-related functional genes were quantified at regular intervals. The highest N2O emission was observed at an incubation of 35 °C, which was 1.5 to 2.1 factors higher than samples incubated at 25 °C and 15 °C. However, the highest level of dissolved N2O and estimated exchange flux of N2O were both observed at 25 °C and were both approximately 2 factors higher than those at 35 °C and 15 °C. The denitrification rates increased significantly during the incubation period, and samples at 25 °C and 35 °C exhibited much greater rates than those at 15 °C, which is in agreement with the N2O emission of the three incubation temperatures. The NO3− decreased in relation to the increase of N2O emissions, which confirms the dominant role of denitrification in N2O generation. Indeed, the nirK type denitrifier, which constitutes part of the denitrification process, dominated the nirS type involved in N2O generation, and the nosZ II type N2O reducer was more abundant than the nosZ I type. The results of the current study indicate that higher temperatures (35 °C) result in higher N2O emissions, but incubation at moderate temperatures (25 °C) causes higher levels of dissolved N2O, which represent a potential source of N2O emissions from freshwater ecosystems.

Characteristics of soil N2O emission and N2O-producing microbial communities in paddy fields under elevated CO2 concentrations

The effects of elevated carbon dioxide (CO2) concentration (e[CO2]) on nitrous oxide (N2O) emissions from paddy fields and the microbial processes involved in N2O emissions have recently received much attention. Ammonia-oxidizing microorganisms and denitrifying bacteria dominate the production of N2O in paddy soils. To better understand the dynamics of N2O production under e[CO2], a field experiment was conducted after five years of CO2 fumigation based on three treatments: CK (ambient atmospheric CO2), T1 (CK + increase of 40 ppm per year until 200 ppm), and T2 (CK + 200 ppm). N2O fluxes, soil physicochemical properties, and N2O production potential were quantified during the rice-growth period. The functional gene abundance and community composition of ammonia-oxidizing archaea (AOA) and bacteria (AOB) were analyzed using quantitative polymerase chain reaction (qPCR) and those of ammonia-denitrifying bacteria (nirS- and nirK-type) were analyzed using Illumina MiSeq sequencing. N2O emissions decreased by 173% and 41% under the two e[CO2] treatments during grain filling and milk ripening, respectively (P < 0.05). N2O emissions increased by 279% and 172% in the T2 treatment compared with T1 during the tillering and milk-ripening stages, respectively (P < 0.05). Furthermore, the N2O production potential was significantly higher in the CK treatment than in T1 and T2 during the elongation stage. The N2O production potential and abundance of AOA amoA genes in T1 treatment were significantly lower than those in CK treatment during the high N2O emission phase caused by mid-season drainage (P < 0.05). Although nirK- and nirS-type denitrifying bacteria community structure and diversity did not respond significantly (P > 0.05) to e[CO2], the abundance of nirK-type denitrifying bacteria significantly affected the N2O flux (P < 0.05). Linear regression analysis showed that the N2O production potential, AOA amoA gene abundance, and nirK gene abundance explained 47.2% of the variation in N2O emissions. In addition, soil nitrogen (N) significantly affected the nirK- and nirS-type denitrifier communities. Overall, our results revealed that e[CO2] suppressed N2O emissions, which was closely associated with the abundance of AOA amoA and nirK genes (P < 0.05).

Technological avenues and market mechanisms to accelerate methane and nitrous oxide emissions reductions.

Strategies targeting methane (CH4) and nitrous oxide (N2O) emissions are critical to meeting global climate targets. Existing literature estimates the emissions of these gases from specific sectors, but this knowledge must be synthesized to prioritize and incentivize CH4 and N2O mitigation. Accordingly, we review emissions sources and mitigation strategies in all key sectors (fuel extraction and combustion, landfilling, agriculture, wastewater treatment, and chemical industry) and the role of carbon markets in reducing emissions. The most accessible reduction opportunities are in the hydrocarbon extraction and waste sectors, where half (>3 Gt-CO2e/year) of the emissions in these sectors could be mitigated at no net cost. In total, 60% of CH4 emissions can be mitigated at less than $50/t-CO2. Expanding the scope of carbon markets to include these emissions could provide cost-effective decarbonization through 2050. We provide recommendations for carbon markets to improve emissions reductions and set prices to appropriately incentivize mitigation.

Analysis of low emission characteristics of NH3/H2/air mixtures under low temperature combustion conditions

The complex dynamics of NH3/H2/air mixtures have been numerically investigated in an idealized reactor environment of fast fuel/air mixing represented by perfectly stirred reactors (PSR). The complete steady state “S-curve” solutions are obtained with eigenvalue analysis for quantifying the stable/unstable characteristics of different branches. With four different chemical mechanisms, stable cool flame branches are observed for NH3/H2/air mixtures. The reactions involving species NH2, H2NO and NNH are found important for both fuel consumption and NOx formation on the cool flame branch. The parametric study further shows that with less than 5 % hydrogen addition in ammonia (in volume), stable cool flames exist over a wide range of inlet temperature, equivalence ratio and pressure. For the states in the cool flame branch, although hydrogen is almost consumed completely, only portion of the ammonia is consumed resulting in low NOx emission due to the reduction effects of remaining ammonia and high N2O emission resulting from low temperature. Inspired by the observed stable cool flame characteristics, a multi-staged low-temperature model system is formulated for low NOx emission of NH3/H2/air combustion, in which small amount of hydrogen is sequentially injected to homogeneous reactors of lean NH3/H2/air mixture. With the representative high inlet temperature of 850 K, pressure of 20 atm, fuel lean of ϕ=0.7 and critical fuel efficiency of 99.7 %, results with the 28-species Li reduced mechanism show that less than 75 ppm of NOx and 98 ppm of N2O can be achieved with two-staged homogenous combustion operating at low temperature of 1300 K and 2 % H2 addition in each stage. With three-staged homogenous combustion, less than 70 ppm of NOx and 10 ppm of N2O can be achieved at 1300 K. NOx emission is lower than those of direct NH3/H2/air combustion without aftertreatment, demonstrating the opportunity of utilizing low temperature combustion to achieve high fuel conversion, low NOx and N2O formation for lean NH3/H2/air mixtures.

Greenhouse gas emissions from Silphium perfoliatum and silage maize cropping on Stagnosols

AbstractBackgroundThe sustainability of bioenergy is strongly affected by direct field‐derived greenhouse gas (GHG) emissions and indirect emissions form land‐use change. Marginal land in low mountain ranges is suitable for feedstock production due to small impact on indirect land‐use change. However, these sites are vulnerable to high N2O emissions because of their fine soil texture and hydrology.AimsThe perennial cup plant (Silphium perfoliatum L.) might outperform silage maize (Zea mays L.) on cold, wet low mountain ranges sites regarding yield and ecosystem services. The aim of this study was to assess whether the cultivation of cup plant also provides GHG mitigation potential compared to the cultivation of maize.MethodsA t‐year field experiment was conducted in a low mountain range region in western Germany to compare area and yield‐scaled GHG emissions from cup plant and maize fields. GHG emissions were quantified using the closed chamber method.ResultsCup plant fields emitted an average of 3.6 ± 4.3 kg N2O‐N ha–1 year–1 (–85%) less than maize fields. This corresponded to 74.0 ± 94.1 g CO2‐eq kWh–1 (–78%) less emissions per produced electrical power. However, cup plant had a significantly lower productivity per hectare (–34%) and per unit of applied nitrogen (–32%) than maize.ConclusionCup plant as a feedstock reduces direct field‐derived GHG emissions compared to maize but, due to lower yields cup plant, likely increases emissions associated with land‐use changes. Therefore, the increased sustainability of bioenergy from biogas by replacing maize with cup plant is heavily dependent on the performance of maize at these sites and on the ecosystem services of cup plant in addition to GHG savings.

Similar strong impact of N fertilizer form and soil erosion state on N2O emissions from croplands

Soil erosion affects 20% of croplands worldwide. However, understanding the effect of soil erosion on N2O emissions, which is one of the most potent greenhouse gases, is still limited. This limitation is likely because the small-scale differences in soil properties and fertility induced by erosion (i.e. ranges of erosion states) have barely been considered in studies quantifying N2O emissions from croplands. There are, however, indications that the erosion state itself strongly impacts N2O emission, similar to the N fertilizer form. Therefore, our investigations aimed to further explore these indications.We measured N2O fluxes for three years and at five sites within an erosion affected field experiment. N2O emissions were quantified using a manual chamber system. Three sites were established on a summit position (Albic Luvisol; non-eroded) but differed in N fertilizer forms (organic biogas fermented residues, calcium ammonium nitrate and a mixture of both fertilizers). Two additional sites were established on an extremely eroded soil (Calcaric Regosol) and wet depositional soil in a depression (Endogleyic Colluvic Regosol) to measure the effect of soil erosion states on N2O emissions. Both additional sites were fertilized with calcium ammonium nitrate only.In case of the non-eroded soil (summit), organic fertilization resulted in the highest cumulative N2O emission (6.2 ± 0.21 kg N2O-N/ha y−1) compared to mixed (5.5 ± 0.18 kg N2O-N/ha y−1) and mineral (1.9 ± 0.17 kg N2O-N/ha y−1) fertilization. These high emissions were probably caused by soluble C and N substrates from organic fertilizer, resulting in microbial activities favoring high N2O emissions. Regarding the erosion status, we observed the highest N2O emissions in the depositional soil (2.8 ± 0.21 kg N2O-N/ha y−1), followed by the non-eroded (1.9 ± 0.17 kg N2O-N/ha y−1) and the extremely eroded soil (0.6 ± 0.03 kg N2O-N/ha y−1). These differences in N2O emissions were mainly due to the site-specific, erosion induced differences in soil properties such as soil texture, soil organic C and total N contents and stocks, water-filled pore space and soil pH. These results indicate that soil erosion state may indeed be of similar importance, as N fertilizer form, for the magnitude of N2O emissions from croplands.