Reduce N2O Emissions Research Articles

A comprehensive assessment of greenhouse gas emissions research in the Cerrado region, Brazil

The increasing demand for food, fiber, and (bio)energy boosted by population growth has accentuated agricultural expansion, increasing global greenhouse gas (GHG) emissions. This scenario is valid in Brazil, where agriculture accounts for the largest part of the nation’s GHG emissions, primarily associated with the expansion of agriculture over areas of native vegetation, especially in the Cerrado region. However, despite the contribution of this sector to GHG emissions, there is a limited understanding of how different systems affect these emissions, as well as the current state of the art on this topic. Therefore, we performed a comprehensive literature review to synthesize the information about GHG emissions in the region, including cropping systems where GHG was measured, methodological procedures, and the main results achieved. Our review shows that the subject of “GHG” has been poorly investigated, with a huge discrepancy compared to other related topics such as soil organic matter. Most studies (16 % of 236) only mentioned GHG-related terms but did not measure them. These studies were conducted mainly in the south-central part of the region and were mostly limited to short-term experiments (< 5 years) or monitoring periods (< 1 year), using manual static chambers. The analysis of the available GHG data indicated that converting Cerrado into agriculture increases N2O emissions by ∼ 0.45 kg ha−1 year−1 while decreasing CH4 influx by ∼ 3 kg ha−1 year−1. Despite that, no-tillage combined with cover crops effectively reduces N2O emissions (∼-0.3 kg ha−1 year−1). Our findings reveal a significant gap in monitoring GHG fluxes in the Cerrado region, particularly in the northern part where Brazil’s new agricultural frontier, the Matopiba region, is located. Efforts should prioritize generating comprehensive GHG data for Cerrado agriculture by employing more robust monitoring protocols. This would help guide producers, researchers, and policymakers to enhance agricultural management practices toward greater sustainability.

Optimized agricultural management reduces global cropland nitrogen losses to air and water.

Nitrogen (N) losses from croplands substantially contribute to global N pollution. Assessing the reduction in N losses through improved N management practices is complex due to varying site conditions, such as land use, climate, soil properties and local farming methods. In this Article, we conducted a meta-analysis to evaluate the effects of improved practices on N loss reduction, analysing data from 1,065 studies with 6,753 pairs of observations comparing standard and optimized practices. Without considering site-specific conditions, optimized management practices can reduce N2O emissions by 3-39%, NH3 emissions by 15-68%, N run-off by 21-37% and N leaching by 19-52%. After considering local conditions and current practices, average reductions on a global scale were 31% for N2O, 23% for NH3, 18% for N run-off and 17% for N leaching. The effectiveness of N loss reduction was mainly influenced by optimized management practices and, to a lesser extent, site conditions. The results of this study underscore the importance of implementing optimized, site-specific management to effectively reduce N losses from global croplands.

Impact of Drying–Wetting Cycles on Nitrification Inhibitors (DMPP and DMPSA) in a Greenhouse Experiment with Two Contrasting Mediterranean Soils

Studies of the impact of nitrification inhibitors (NIs), specifically DMPP and DMPSA, on N2O emissions during “hot moments” have produced conflicting results regarding their effectiveness after rewetting. This study aimed to clarify the effectiveness of NIs in reducing N2O emissions by assessing residual DMP concentration and its influence on ammonia-oxidizing bacteria (AOB) in two pot experiments using calcareous (Soil C, Calcic Haploxerept) and acidic soils (Soil A, Dystric Xerochrepts). Fertilizer treatments included urea (U), DMPP, and DMPSA. The experiments were divided into Phase I (water application to dry period, 44 days) and Phase II (rewetting from days 101 to 121). In both phases for Soil C, total N2O emissions were reduced by 88% and 90% for DMPP and DMPSA, respectively, compared with U alone. While in Phase I, the efficacy of NIs was linked to the regulation of AOB populations, in Phase II this group was not affected by NIs, suggesting that nitrification may not be the predominant process after rewetting. In Soil A, higher concentrations of DMP from DMPP were maintained compared to Soil C at the end of each phase. Despite this, NIs had no significant effect due to low nitrification rates and limited amoA gene abundance, indicating unfavorable conditions for nitrifiers. The study highlights the need to optimize NIs to reduce N2O emissions and improve nitrogen efficiency, while understanding their interactions with the soil. This knowledge is necessary in order to design fertilization strategies that improve the sustainability of agriculture under climate change.

Optimizing residue return with soil moisture and nutrient stoichiometry reduced greenhouse gas fluxes in Alfisols

Optimum soil moisture and high crop residue return (RR) can increase the active pool of soil organic carbon and nitrogen, thus modulating the magnitude of greenhouse gas (GHG) fluxes. To determine the effect of soil moisture on the threshold level of RR for the wheat production system, we analyzed the relationship between GHG fluxes and RR at four levels, namely 0, 5, 10, and 15 Mg ha−1 (R0, R5, R10, and R15) under two soil moisture content (80% FC and 100% FC) and three levels of nutrient management (NS0: no nutrient; NS1, NS2= 3x NS1). Nutrient input (N and P) in NS1 balanced the residue C/nutrient stoichiometry to achieve 30% stabilization of the residue C input in RR (R5). All RR treatments (cf. R0) were found to significantly reduce N2O emission in moderate soil moisture content (80% FC) by 22–56% across nutrient management due to enhanced soil C mineralization, microbial biomass carbon, and N immobilization. However, averaged across nutrient management, a linear increase in N2O emission was observed with increasing RR under 100% FC soil moisture. A significant decrease in CH4 emission by ca. 46% in most RR treatments was observed in 100% FC compared with the R0. The N2O emission was negatively correlated (p = &amp;lt;0.001) with nutrient stoichiometry. Partial least square (PLS) regression indicated that GHG emissions were more responsive (values &amp;gt; 0.8) to management variables (RR rate, nitrogen (N) input rate, soil moisture, and nutrient stoichiometry of C: N) and post-incubation soil properties (SMBC and NO3-N) in Alfisols. This study demonstrated that the mechanisms responsible for RR effects on soil N2O, CH4 fluxes, and carbon mineralization depend on soil moisture and nutrient management, shifting the nutrient stoichiometry of residue C: N: P.

Spatial benefit difference of fertilizer reduction and substitution policy based on grid scale: A case study of the Heihe River Basin

Agriculture green development faces the challenge of balancing the application of both organic and chemical fertilizers. Formulating effective policies to harmonize these applications is essential for ensuring food security and environmental sustainability. However, there is a lack of studies investigating the economic and environmental benefits of policy implementation at the grid scale, and insufficient research has been conducted on the subsequent impacts these policies have on diverse crops. Therefore, this paper develops the SIMPLE-G-Heihe model to simulate the impacts of green agricultural policies on multiple crops planting in arid regions. This model addresses limitations found in the SIMPLE-G model, which supported a single crop and lacked organic fertilizer substitution simulation. The results show that: (1) Merely increasing subsidies for organic fertilizers is insufficient to significantly enhance yields, and improving the efficiency of fertilizer utilization should also be emphasized. (2) Combining improvements in fertilizer technology with subsidies for organic fertilizers can concurrently support economic advancement and environmental mitigation. Research indicates that integrating a 3% organic fertilizer subsidy with an 8% improvement in chemical fertilizer technology increases vegetable output value by 0.27% and reduces N2O emissions by 1.09%. (3) The upstream areas of the Heihe River Basin show a significant reduction in fertilizer use. Fertilizer input is mainly reduced for vegetables in the upstream areas, while for wheat and maize, the reduction is primarily in the midstream and upstream areas. The findings discuss the spatial heterogeneity and flexibility of policy impacts, providing valuable insights for the implementation of green agriculture strategies in China.

Soil greenhouse gas emissions under enhanced efficiency and urea nitrogen fertilizer from Australian irrigated aerobic rice production

AbstractAerobic rice production offers a promising solution to improve water use efficiency and reduce methane (CH4) emissions by minimizing water inundation. However, alternate water‐saving methods for rice cultivation can lead to “trade‐off” emissions of nitrous oxide (N2O). A field experiment was conducted over one season measuring soil‐derived greenhouse gas emissions in irrigated aerobic rice (Oryza sativa L.) under different N fertilizer management at a rate of 220 kg N ha−1, including a nil treatment (“control”); slow release (180 days) polymer‐coated urea (“N180”); banded urea applied upfront (“urea”); and three applications of broadcast urea (“urea‐split”). The N180 treatment reduced soil N2O emissions compared with urea (p &lt; 0.001), with mean cumulative N2O emissions of 4.36 ± 1.07 kg N ha−1 and 27.9 ± 5.70 kg N ha−1, respectively. Soil N2O fluxes were high, reaching up to 1916 and 2900 µg N m2 h−1 after urea application and irrigation/rain events, and were similar to other irrigated crops grown on heavy textured soils. Fertilizer N management had no effect on soil CH4 emissions, which were negligible across all treatments ranging from 1.28 to 2.75 kg C ha−1 over the growing season. Cumulative soil carbon dioxide emissions ranged from 1936 to 3071 kg C ha−1 and were greatest in N180. This case study provides the first evidence in Australia that enhanced efficiency nitrogen fertilizer can substantially reduce N2O emissions from soils in an aerobic rice system. Our findings reinforce the CH4 mitigation potential of water saving rice approaches and demonstrate the need to consider N fertilizer management to control N2O emissions.

Forecasting nitrous oxide emissions from a full-scale wastewater treatment plant using LSTM-based deep learning models

Nitrous oxide (N2O) emissions from wastewater treatment plants (WWTPs) exhibit significant seasonal variability, making accurate predictions with conventional biokinetic models difficult due to complex and poorly understood biochemical processes. This study addresses these challenges by exploring data-driven alternatives, using long short-term memory (LSTM) based encoder-decoder models as basis. The models were developed for future integration into a model predictive control framework, aiming to reduce N2O emissions by forecasting these over varying prediction horizons. The models were trained on 12 months and tested on 3 months of data from a full-scale WWTP in Amsterdam West, the Netherlands. The dataset encompasses seasonal peaks in N2O emissions typical for winter and spring months. The best performing model, featuring a 256-256 LSTM architecture, achieved the highest accuracy with test R2 values up to 0.98 across prediction horizons spanning 0.5 to 6.0 hours ahead. Feature importance analysis identified past N2O emissions, influent flowrate, NH4+, NOx, and dissolved oxygen (DO) in the aerobic tank as most significant inputs. The observed decreasing influence of historical N2O emissions over extended prediction horizons highlights the importance and significance of process variables for the model's performance. The model's ability to accurately forecast short-term N2O emissions and capture immediate trends highlights its potential for operational use in controlling emissions in WWTPs. Further research incorporating diverse datasets and biochemical process inputs related to microbial activities in the N2O production pathways could improve the model's accuracy for longer forecasting horizons. These findings advocate for hybridising deep learning models with biokinetic and mechanistic insights to enhance prediction accuracy and interpretability.

Modifying functional groups of straw-based hydrogel provide soil N2O mitigation potential by complete denitrification

Farmed soil is considered to contribute more than 60% anthropogenic N2O emission which plays critical role in global warming potential. Straw-based hydrogels with abundant functional groups are commonly used in environmental and agronomic applications primarily as adsorbents. Those functional groups could mediate electron transfer in the process of adsorbates and might also regulate N2O emission. However, it was still unclear whether and how these functional groups affect the process. To address this knowledge gap, we employed spectral analysis and a robotized incubation system to determine mechanisms of different functional groups in influencing soil N2O. The results indicated that the straw-based hydrogel was capable of significantly reducing the emission of N2O in denitrification, and the electrochemical properties of the hydrogel had promoting effect of the microbial genetic potential for N2O reduction. Specifically, carboxymethyl (R-COO) and primary amine groups (R-NH2) promoted complete denitrification through electron supply. Furthermore, R-NH2 had a significant negative correlation with N2O emissions. To further verify the role of specific structures and functional groups in denitrification, we conducted an experiment using pure lignin as a template. This experiment resulted in an increase in R-NH2 content to 13.2% and a decrease in N2O emissions by 49.1% compared with straw hydrogel. These results prove the feasibility of straw-based hydrogel as a redox system to accelerate complete denitrification, and the electron-donating functional groups were significantly related to the reduction rate of N2O. Finally, based on these mechanisms, functional group targeted grafting technology was proposed to improve its ability to mitigate N2O emission reduction.

Unprecedented N2O production by nitrate-ammonifying Geobacteraceae with distinctive N2O isotopocule signatures.

Dissimilatory nitrate reduction to ammonium (DNRA), driven by nitrate-ammonifying bacteria, is an increasingly appreciated nitrogen-cycling pathway in terrestrial ecosystems. This process reportedly generates nitrous oxide (N2O), a strong greenhouse gas with ozone-depleting effects. However, it remains poorly understood how N2O is produced by environmental nitrate-ammonifiers and how to identify DNRA-derived N2O. In this study, we characterize two novel enzymatic pathways responsible for N2O production in Geobacteraceae strains, which are predominant nitrate-ammonifying bacteria in paddy soils. The first pathway involves a membrane-bound nitrate reductase (Nar) and a hybrid cluster protein complex (Hcp-Hcr) that catalyzes the conversion of NO2- to NO and subsequently to N2O. The second pathway is observed in Nar-deficient bacteria, where the nitrite reductase (NrfA) generates NO, which is then reduced to N2O by Hcp-Hcr. These enzyme combinations are prevalent across the domain Bacteria. Moreover, we observe distinctive isotopocule signatures of DNRA-derived N2O from other established N2O production pathways, especially through the highest 15N-site preference (SP) values (43.0‰-49.9‰) reported so far, indicating a robust means for N2O source partitioning. Our findings demonstrate two novel N2O production pathways in DNRA that can be isotopically distinguished from other pathways.IMPORTANCEStimulation of DNRA is a promising strategy to improve fertilizer efficiency and reduce N2O emission in agriculture soils. This process converts water-leachable NO3- and NO2- into soil-adsorbable NH4+, thereby alleviating nitrogen loss and N2O emission resulting from denitrification. However, several studies have noted that DNRA can also be a source of N2O, contributing to global warming. This contribution is often masked by other N2O generation processes, leading to a limited understanding of DNRA as an N2O source. Our study reveals two widespread yet overlooked N2O production pathways in Geobacteraceae, the predominant DNRA bacteria in paddy soils, along with their distinctive isotopocule signatures. These findings offer novel insights into the role of the DNRA bacteria in N2O production and underscore the significance of N2O isotopocule signatures in microbial N2O source tracking.

Improved nitrogen fertilizer management reduces nitrous oxide emissions in a northern Prairie cropland.

Arable croplands are a significant source of nitrous oxide (N2O) emissions, largely due to nitrogen (N) fertilizer applications to support crop production. Nevertheless, there is limited research on the N2O dynamics from canola-wheat rotations in the semi-arid northern Prairies, an important agricultural region. Here, we present micrometeorological N2O fluxes measured from January 2021 to April 2024 in Saskatchewan, Canada, to evaluate the impact of N fertilizer management on the year-round N2O emissions from a canola-wheat rotation. A combination of two 4R (Right Source, Right Rate, Right Time, Right Place) N management practices - a reduced N rate and an enhanced efficiency N fertilizer source - was compared to common fertilizer management practices for the region. Two periods at high risk for N2O flux events were identified, after N fertilizer applications and the following spring thaw, with the magnitude of emissions varying over the multi-year period. As for cumulative emissions, the growing season (GS) N2O emissions were 50 % of annual emissions, presenting an opportunity to mitigate N2O emissions through improved N fertilizer management. Indeed, the improved 4R N management reduced N2O emissions by 57 % over the entire study period without impacting yields. The reduction in GS N2O emissions resulted from the 4R N management lowering mean N2O flux at times of high WFPS (>50 %). The non-growing season (NGS) N2O accounted for 11-67 % of annual emissions. Fall soil nitrate levels were a strong explanatory variable of NGS emissions (r2 = 0.69, r2 = 0.39), but the rate of change and magnitude of NGS emissions depended on thawing conditions - lower for drier thaws, higher for wetter thaws. Ultimately, better N fertilizer management reduces cumulative N2O emissions from cropping systems when practiced for several years.

Study of activity stratification-controlled NH3–H2 combustion in a large-bore gas engine utilizing split-channel supercharge and fuel-air mixing technology

In this study, the split-channel supercharge and fuel-air mixing technology (SCS-FAM) is firstly proposed for large-bore ammonia (NH3)-hydrogen (H2) engine, which involves the separation of fuel and air intake ports to mitigate the risk of H2 backfire in intake system. We explored the activity stratification under different intake schemes and H2 blend ratios through simulation, as well as how it affects the NH3–H2 combined combustion characteristics including in-cylinder flow turbulence, temperature, concentration, and key intermediates distribution. The results show that in Scheme 2 when air entered the high swirl ratio intake 1 and fuel entered the low swirl ratio intake 2, a rich mixture was formed near the spark plug, accompanied by localized NH3 thermal reforming phenomena, which improved the activity distribution of H2. Additionally, the strong swirl promoted combustion in the squish zone and significantly shortened the post-combustion period. Scheme 2 achieved a higher indicated thermal efficiency (ITE) than Scheme 1 with a small amount of H2 addition, with the highest ITE of 42.1% obtained at 15 vol% H2 blend ratio. Notably, the addition of H2 promoted NH3 consumption and the generation of active radicals, resulting in reduced N2O emissions during the expansion stroke, but causing an increase in NO emissions.

Nitrous oxide emissions and yields from potato production systems as influenced by nitrogen fertilization and irrigation: A meta‐analysis

AbstractPotato (Solanum tuberosom) is a globally significant crop in relation to the scale of its consumption, being the third most consumed worldwide. The overall sustainability of global agriculture is increasingly of concern, specifically in relation to increasing anthropogenic emissions of the greenhouse gas nitrous oxide (N2O) emissions from nitrogen fertilizer additions to croplands and its contribution to climate change. Against this backdrop, a meta‐analysis of 119 experimental comparisons from 18 studies—spanning 19 study sites in 10 countries—was employed to investigate the impact of irrigation, cumulative water input, N fertilizer application rate, soil pH, and soil texture on cumulative N2O fluxes and tuber yield in potato production. Compared to non‐fertilized controls, N2O emissions from fertilized potato production decreased by 34% when irrigation provided 61%–90% of total water input (corresponding to averages of 321–473 mm). Likewise, N2O emissions increased by 53% with 200–475 mm seasonal water input and by 37% with N fertilization rates of 101–200 kg N fertilizer ha−1. Furthermore, soil pH between 7.1 and 7.5 reduced emissions by 6%, while medium‐textured soils showed an increase of 2%. Conversely, tuber yields from fertilized potato production were comparatively maximized under 31%–60% of water input as irrigation (7%) and 751–1025 mm cumulative seasonal water input (28%). Alongside 201–300 kg N fertilizer ha−1 (97%), soil pH of 7.1–7.5 (48%), and in coarse‐textured soils (49%). Overall, these findings underscore the importance of considering irrigation and N fertilization options specifically in optimizing potato production for reduced N2O emissions and enhanced tuber yield.

Combination of nitrogen and organic fertilizers reduce N2O emissions while increasing winter wheat grain yields and quality in China

Wheat grain yields, quality, and nitrous oxide (N2O) emissions were controlled through the type and application rate of nitrogen (N) fertilizer. Here, we investigated the optimal management of N fertilization by examining the combined effects of organic and N fertilizers on wheat yields, quality, and N2O emissions. Field trials under six treatments were located on campus farms at Anhui Science and Technology University, including farmer’s common practice (270 kg N ha-1, N270), 2/3 reduction in N270 (90 kg N ha-1, N90), organic fertilizer with equal N270 (OF270), 2/3 reduction in OF270 (OF90), 4/5 reduction OF270 + 1/5 reduction N270 (20% OF270 + 80% N270), and 4/5 reduction OF90 + 1/5 reduction N90 (20% OF90 + 80% N90) were applied to winter wheat. The plots were arranged in a randomized complete block experimental design. The N2O emissions were quantified under different fertilization measures in the peak wheat growing season during sowing, jointing, and grain filling stages, respectively. Compared with N270 and N90 treatments, N2O emissions were significantly decreased by 18.6% and 27.2%, respectively, under 20% OF270 + 80% N270% and 20% OF90 + 80% N90 (p &amp;lt; 0.05). Further, N2O emissions in N270 were increased by 50.8% relative to N90. Wheat yields increased significantly under 20% OF90 + 80% N90 by 27.6% (N270) and 16.4% (N90), and were considerably enhanced under 20% OF270 + 80% N270 by 40.6% (N270) and 12.7% (N90) in contrast to OF270 and N270 (p &amp;lt; 0.05). Compared with N90, the content of wet gluten, protein and starch under 20% OF90 + 80% N90 treatment significantly increased by 7.7%, 13.8% and 7.9%, and enhanced by 7.6%, 4.8%, 8.0% relative to OF90, respectively (p &amp;lt; 0.05). The starch content increased significantly by 2.0%, whereas the settlement value decreased considerably by 2.9% under 20% OF270 + 80% N270 (p &amp;lt; 0.05), and there was no notable difference in the wet gluten and protein contents (p &amp;gt; 0.05). Our findings indicated that organic fertilizer mixed with N fertilizer can effectively reduce N2O emissions, increase both the grain yields and quality in wheat field compared with N fertilizer alone.

Deep oxidation of NO via catalytic ozonation

ABSTRACT In addition to selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) which are available to remove NOx from flue gas, oxidation method is receiving more and more attention because this method makes it possible to remove NOx and SO2 from flue gas simultaneously by wet scrubbing. O3 as a strong oxidant has a high oxidation capacity and it can oxide NO to N2O5 which has a higher water solubility compared with NO2. However, it needs a long reaction time and the escape of unreacted ozone may cause secondary pollution. In this study, FeMnCo/Al2O3 catalyst and FeMnCe/Al2O3 catalyst were prepared and applied to enhance the deep oxidation of NOx with ozone and reduce O3 slip. Effects of various operating parameters such as O3/NO ratio, gas residence time and operating temperature were evaluated and the results demonstrate that N2O5 began to generate when O3/NO ratio was higher than 1.0 and increased with increasing O3/NO ratio. Little residual O3 was formed in the presence of catalyst while 350 ppm O3 was measured at the outlet gas when O3/NO ratio was controlled at 2.0. The N2O5 conversion efficiency increased with increasing gas residence time, operating temperature and the highest N2O5 conversion efficiency was achieved at 100°C. Furthermore, the conversion efficiency remained around 90% during 20 h operation over FeMnCe/Al2O3 catalyst with an O3/NO ratio of 1.73, a gas residence time of 1.2 s, and a temperature of 100°C. On the other hand, the N2O5 conversion efficiency remained around 80% during 3 h operation over FeMnCo/Al2O3 catalyst. Overall, FeMnCe/Al2O3 catalyst reveals good potential for deep oxidation of NO by O3 and can be further developed as a viable catalyst for reducing NO emission from industries.

Application of straw-derived biochar: a sustainable approach to improve soil quality and crop yield and reduce N2O emissions in paddy soil.

The burning of agricultural straw is a pressing environmental issue, and identifying effective strategies for the rational utilization of straw resources is decisive for achieving sustainable development. Owing to its high carbon content and exceptional stability, straw biochar produced via pyrolysis has emerged as a key focus in multidisciplinary research. However, the efficacy of biochar in mitigating nitrous oxide (N2O) emissions from paddy soils is not consistent. A 2-year field experiment was conducted and investigated the impact of biochar derived from two feedstocks (rice straw and wheat straw, pyrolyzed at 450°C) on N2O emissions, global warming potential (GWP), greenhouse gas intensity (GHGI), nitrogen use efficiency (NUE), crop yield, and soil quality. The static chamber technique was used for collecting N2O gas samples, and concentrations were analyzed through gas chromatography methods. The treatment combinations included BR0 (control), BR1 (NPK at the recommended dose, 120:60:40kgha-1), BR2 (wheat straw biochar, 5 t ha-1), and BR3 (rice straw biochar, 5 t ha-1). The results exhibited that cumulative N2O emissions from BR2 and BR3 treatments decreased by 10.55% and 13.75% respectively, compared to BR1. Lower GWP and GHGI were observed under both biochar treatments compared with BR1. The highest rice grain yield (3.48Mgha-1) and NUE (76.72%) were recorded from BR3, which also exhibitedthe lowest yield-scaled N2O emission. We observed positive correlations between soil nitrate, ammonia and water-filled pore spaces, while NUE showed negative correlations with N2O emissions. Significant (p < 0.05) improvements in soil quality were also detected in both the biochar treated plots, indicated by increased soil pH, water holding capacity, porosity, and nutrient contents. Overall, the results suggest that applying biochar at a rate of 5 t ha-1 in paddy soil is a viable nutrient management strategy with the potential to reduce reliance on inorganic fertilizers, mitigate N2O emissions, and contribute to sustainable food production.

The divergent role of straw return in soil O2 dynamics elucidates its confounding effect on soil N2O emission

The divergent effects of straw returns on nitrous oxide (N2O) emissions from soil require elucidation of the underlying mechanisms and factors that explain the inconsistency in in-situ conditions. We conducted a field experiment based on a long-term trial under different regimes of nitrogen (N) fertilization and straw management, complemented by laboratory incubation experiments involving visualized O2 dynamics imaging. In the field trial, we performed hourly basis high-time-resolution measurements of soil matrix oxygen (O2), N2O concentrations and fluxes during N2O “hot moment” events. We found that straw return increased cumulative N2O emissions by 32.7% under conventional high N input (Ncon), but showed no effect on N2O emission under optimized N input (Nopt). In situ O2 content and further microcosm experiments with visualized O2 spatiotemporal distribution suggested that long-term straw return increases porosity and soil O2 content, which reduced N2O emission under low N substrate conditions by improving soil pore structure and aeration during “hot moment” events. By contrast, straw return increased N2O emission via creating short-term O2 depletion zone and triggering denitrification in anoxic microsites when excess N substrate was available. Although straw return showed inconsistent effects on N2O emission under different N application rates, it consistently decreased N2O concentration in the soil matrix during the "hot moment" events, suggesting that straw return increases the transport of the produced N2O in soil matrix to the soil surface. Our study underscores the multifaceted role of straw return in soil O2 dynamics, i.e., stimulating O2 consumption in a short-term microscale of soil, but increasing soil porosity in a long-term mesoscale of soil. This explains the confounding effects of straw management on the production and transportation of soil N2O in situ and emphasizes the importance of optimized N fertilization for reducing the “hot moment” N2O emissions when straw is incorporated.

Impact of freeze-thaw cycles and influent C/N ratios on N2O emissions in subsurface wastewater infiltration systems

Subsurface wastewater infiltration system (SWIS) is important source of N2O emission, biological denitrification process relies on organic carbon source as electron donor, enabling denitrifying reductase (NAR, NIR, NOR, and N2OR) to reduce nitrate nitrogen to N2. However, freeze-thaw cycles cause changes in soil structure through physical and biological processes that increase organic matter availability and microbial activity, thereby accelerating upper (aerobic) organic matter degradation. This leads to carbon deficiency in the lower (anaerobic) layer, which reduces denitrification efficiency increasing N2O production and emissions. This study investigated effects of influent C/N ratios on N2O emissions from SWIS under freeze-thaw stress, and changes in denitrifying reductase and microbial communities. Results showed that as influent C/N ratio increased from 6:1–10:1, average TN removal of SWIS decreased from 89.59 % to 84.95 %, while N2O emission rate decreased from 0.106 mg·m−2·h−1 to 0.0333 mg·m−2·h−1, a reduction of 68.61 %. Meanwhile, N2OR activity increased by 29.95 % with C/N ratio increasing from 6 to 10, showing a significant reduction of N2O. High-throughput sequencing results revealed that Proteobacteria and Firmicutes showed a linear relationship with influent C/N. As C/N increased from 6 to 8 and 10, Proteobacteria relative abundance increased from 22.24 % ± 1.79–32.51 % ± 2.51 % and 39.59 % ± 3.45 %, while Firmicutes relative abundance decreased from 31.73 % ± 7.87–3.34 % ± 0.27 % and 2.19 % ± 0.07 %. These results indicated that sufficient supply of organic carbon provided abundant electron donors for denitrifying reductases in freeze-thaw-affected SWIS, promoting influent NO3--N reduction and decreasing N2O release rates. Conversely, under low C/N ratio conditions, electron acceptance capacity of N2OR was limited and N2OR activity was inhibited, resulting in N2O accumulation and emission. SynopsisThis paper shows how increasing the organic carbon supply in SWIS can reduce N2O emissions and improve nitrogen removal under freeze-thaw stress.

Coarse bubble mixing in anoxic zone greatly stimulates nitrous oxide emissions from biological nitrogen removal process

The biological nitrogen removal process in wastewater treatment inevitably produces nitrous oxide (N2O), a potent greenhouse gas. Coarse bubble mixing is widely employed in wastewater treatment processes to mix anoxic tanks; however, its impacts on N2O emissions are rarely reported. This study investigates the effects of coarse bubble mixing on N2O emissions in a pilot-scale mainstream nitrite shunt reactor over a 50-day steady-state period. Online and offline N2O monitoring campaigns show that coarse bubble mixing in the anoxic zones significantly elevates N2O emissions, yielding an extremely high emission factor of 15.5 ± 3.5 %. Intensive sampling and isotopic analyses suggest that the elevated emissions are primarily due to the inhibition of the N2O denitrification process by oxygen in the anoxic phase introduced by coarse bubbling. Substituting coarse bubble mixing with submersible pump mixing resulted in a substantial reduction of N2O emissions, decreasing the emission factor by an order of magnitude to 1.2 ± 0.8 %. The findings reveal that a previously overlooked factor, coarse bubble mixing, can significantly stimulate N2O emissions. The use of coarse bubble mixing in anoxic tanks of biological nitrogen removal warrants caution.

Soil nitrous oxide emissions from wheat-based rotations with different types of pulse crops

Production of agricultural crops with a low greenhouse gas (GHG) footprint is essential to mitigate the adverse effects of climate change. The inclusion of pulse crops in cereal-based rotations can enhance environmental quality by providing biologically fixed N and thereby reducing the amount of synthetic N fertilizer required for the crop rotation. The inclusion of pulse crop has the potential to reduce N2O emissions from the agricultural system in both the legume phase and the subsequent wheat phase of the rotation. However, long-term studies are necessary to thoroughly investigate N2O emissions from rotations with pulse crops, particularly in the semiarid region where pulse crops are frequently grown. In the present study, we evaluated cumulative N2O emissions and emission intensity during the rotation cycle. The assessment was conducted over 4 years, during two complete 2-yr cycles of an established rotation (years 9–12), under the climatological conditions of 2018–2021. Four rotations including wheat-wheat, pea-wheat, lentil-wheat, and chickpea-wheat were selected from a trial in Swift Current, Saskatchewan (semiarid prairies/Brown Chernozem). Our experiment was subjected to below normal precipitation, with interannual variations in climate and the last 2 years (2020–21) were drier than the first two years (2018–2019). Under such climate, PW and LW demonstrated to be environmentally sustainable, always exporting the highest N in grains (133 kg N ha−1 averaged across PW and LW and cycles) and consistently achieving the lowest N2O intensity (2.8 g N2O-N per kg exported N averaged across PW and LW and cycles). Continuous wheat presented inconsistent results, with a significant reduction in exported N from years 9–10 to 11–12 (the driest cycle). Because WW also promoted the highest cumulative N2O emissions, N2O intensity over the 2-yr was always the highest for WW. The CW consistently promoted the lowest N exports and was not resilient to dry soil conditions, with 23% lower exported N in years 11–12 than in years 9–10. Hence including pulse crops with pea or lentil in the rotation reduced N2O emissions and enhanced wheat yield resiliency.

Effects of aged biochar additions at different addition ratios on soil greenhouse gas emissions

Biochar addition is effective in reducing soil greenhouse gas (GHG) emissions, but it's essential to evaluate whether aged biochar retains this capability as its properties change over time. However, research comparing the effects of fresh and aged biochar on soil GHG emissions is limited. Moreover, exploring the priming effect of biochar on native soil organic carbon (SOC) mineralization is crucial for revealing the effect mechanism on soil CO2 emission. However, research investigating the priming effects of aged biochar is limited. In this study, the effects of aged biochar addition on soil physicochemical properties, GHG emissions, and global warming potential (GWP) were examined through an incubation experiment with three treatments: (1) soil only (CK), (2) 1 % aged maize straw biochar addition (HBC1) and (3) 4 % aged maize straw biochar addition (HBC4), and then their effects were compared with those of fresh biochar from our previous research. 13C tracer technology was used to assess the priming effect of aged biochar on native SOC mineralization. Results showed that aged biochar improved soil physicochemical properties. Compared to CK, HBC1 and HBC4 reduced CO2 emissions by 28.02 % and 20.15 %, respectively, and reduced N2O emissions by 61.54 % and 66.39 %. HBC4 significantly increased CH4 emission, whereas HBC1 reduced it. HBC1 and HBC4 reduced GWP by 29.01 % and 21.41 %, respectively. Overall, aged biochar demonstrated a greater reduction effect compared to fresh biochar at the 1 % addition ratio. The CO2 reduction is attributed to the negative priming effect of aged biochar on native SOC mineralization. The reduction in N2O emissions is attributed to aged biochar promoting microbial nitrogen fixation and reducing the ratio of denitrification to nitrification. The variation in CH4 emissions reflects differing dominant factors influencing CH4 emission across varying addition ratios. In conclusion, 1 % aged biochar addition demonstrates a more favorable long-term effect on mitigating GHG emissions.