Direct N2O Emissions Research Articles

Bio-straw resource recycling systems: Agricultural productivity and green development

Bio-straw is one of the valuable and renewable organic resources. China produces most of the world's bio-straw. Straw recycling is closely associated with global carbon and nitrogen cycles, agricultural productivity, and green development. We systematically integrated straw recycling systems from 1979 to 2019 into three major cereal crops to identify patterns of crop productivity and greenhouse gas (GHG) emissions in a life cycle assessment, and to explore GHG mitigation potential using scenario analyses. Straw recycling partially alleviated the negative potassium surplus caused by straw removal and increased wheat, maize, and rice yields by 7.8%, 5.2%, and 5.0%, respectively. We developed a new empirical model for nitrous oxide (N2O) and methane (CH4) emissions from straw recycling systems in regional crops. Straw recycling promoted direct N2O emissions from upland crops, especially maize. Nonetheless, straw recycling reduced net GHG emissions by 33.1% in wheat and 12.1% in maize due to high rates of soil carbon sequestration relative to inorganic fertilizer management alone. In rice, straw recycling depressed N2O emissions but increased net GHG emissions by 59.8% because CH4 emissions were 2.06 times greater than those under fertilizer management. When combined with optimal fertilizer inputs based on zero nutrient surplus, 100% straw recycling further reduced net GHG emissions by 28.0% in wheat and 19.5% in maize, whereas 60% straw recycling produced better results in rice (20.6% emissions reduction), in spite of increasing fertilizer-induced emissions. Enhancing the quantity and quality of straw recycling, based on crop-specific recycling targets, should contribute to the dual targets of achieving higher crop yields and green development.

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.

Trade-offs between grain yields and ecological efficiencies in a wheat-maize cropping system using optimized tillage and fertilization management on the North China Plain.

Optimized fertilizer and tillage management can be an effective strategy for high ecological efficiencies as well as crop yields. The objective of this study was to assess the impact of diverse management practices on carbon footprint, and ecosystem services in a wheat-maize cropping system. An in situ field experiment field was conducted from 2018 to 2020 on the North China Plain, and six treatments were established: deep tillage (DT), shallow tillage (ST), no tillage (NT), deep tillage + adding organic fertilizer (DTF), shallow tillage + adding organic fertilizer (STF), and no tillage + adding organic fertilizer (NTF). The results showed that adding organic fertilizer and the deeper tillage depth caused higher direct CO2 and N2O emission fluxes. DTF treatment significantly increased carbon footprint either per-unit area (CFa) or per-unit net income (CFe). Compared with DT treatment, STF treatment had higher CFa but lower CFe by increasing net income through boosted crop yields. Besides, the highest ecosystem service values (ESV) were present in STF treatment during both 2years (42,017.13 CNY ha-1 and 43,352.03 CNY ha-1). In conclusion, STF treatment was an optimal management practice to trade-off grain yields and ecological efficiencies in a wheat-maize cropping system. Furthermore, this study highlights that adding organic fertilizer could be an efficient option toward sustainable farmland utilization with high soil carbon sequestration capacity and high ESV.

Modelling soil emissions and precision agriculture in fertilization life cycle assessment - A case study of wheat production in Austria

The environmental assessment of using optical crop sensors for variable rate nitrogen application (VRNA) has been limited by the lack of a robust method to quantify site- and technology-specific impacts. This study aimed to (1) present a comparative life cycle assessment (LCA) of a conventional winter wheat production system with and without using a crop sensor for VRNA applied to an Austrian case study. Special emphasis was placed on simulating site-specific field emissions with the DeNitrification-DeComposition (DNDC) biogeochemical soil model; (2) assess the environmental impacts of only the fertilization process; and (3) compare soil emissions simulated by the DNDC with soil emissions coming from a benchmark ecoinvent wheat production process. Three nitrogen fertilization schemes – one conventional and two VRNA – were modeled. Two functional units were used – 1 ha of cultivated winter wheat and 1 kg of winter wheat produced. The system boundary includes tillage, seeding, plant protection, nitrogen fertilization, and harvesting processes. Information communication technologies (ICT) – manufacturing of the sensor, internet and computer manufacturing and usage – were also included within the boundary. Local and global environmental impacts attributed to nitrogen emissions due to fertilization were evaluated in this LCA, including climate change (CC), fine particulate matter formation (FPMF), freshwater eutrophication (FE), freshwater ecotoxicity (FET), terrestrial acidification (TA), marine eutrophication (ME), and human noncarcinogenic toxicity (HTnc). The CC of the fertilization process was 1,662.8 kg CO2 eq./ha with conventional nitrogen application versus 1,518.8 kg CO2 eq./ha as the lower of the two VRNA results, an 8.6% reduction due to less fertilizer applied. Fertilization was found to be responsible for more than 80% of the total emissions that impact CC, 55% of the FET, 44% of the HTnc, 96% of the FE and 96% of the TA. The largest greenhouse gas (GHG) emitters were soil N emissions as simulated by the DNDC, followed by the fertilizer manufacturing process in all of the impacts, except for FET and HTnc, where fertilizer production was the highest contributor. ICT components contributed less than 1% to all of the impacts assessed. The amount of applied N fertilizer has a greater influence on NH3 and NO3 indirect soil emissions than on direct N2O emissions. This study demonstrates that using optical crop sensors for VRNA could have a limited but positive environmental impact and highlights the importance of applying site-specific soil models to estimate field emissions.

Challenges in carbon footprint evaluations of state-of-the-art municipal wastewater resource recovery facilities

Wastewater treatment is an important source of direct and indirect greenhouse gas (GHG) emissions, which some wastewater operators report and account for CO2-eq impacts through carbon footprint evaluations. We investigated the challenges with GHG emissions’ accounting of three state-of-the-art energy-efficient wastewater resource recovery facilities (WRRFs) and reviewed their CO2 accounting reports. Our study aimed to highlight the major contributors and factors to estimate emissions, including direct N2O and CH4 emissions and propose recommendations for public reporting of CO2 accounting of WRRFs. We categorised emissions as direct (scope 1), background (scope 2), downstream and avoided emissions (scope 3A and 3B) and evaluated how a change in emission factor may affect how close the WRRFs are to reaching CO2 neutrality. The results show that electricity consumption and direct emissions constitute between 20 and 70% of actual CO2-eq emissions and therefore need careful consideration.All three plants have increasingly offset scope 2 emissions over 2014–2019, resulting in a total reduction of approximately 3211 tons CO2-eq, corresponding to 72% of their needed cuts by 2030 set by the Danish government.No standard factors are used across the plants to estimate emissions. We propose some general recommendations that wastewater operators can apply to correctly report and account for CO2-eq emissions. We also recommend that operators move their long-term focus from CO2 neutrality to CO2-eq reduction and make an effort to measure and quantify scope 1 direct emissions properly. A tax on N2O emissions should be introduced in future policies.

Tillage strategies optimize SOC distribution to reduce carbon footprint

Tillage methods and nitrogen (N) application are critical for soil organic carbon (SOC) sequestration and crop production. However, both tillage and N are the main contributors to the carbon footprint (CF) in agricultural production. A 6-year-long field experiment was conducted under a winter wheat-summer maize cropping system in Northern China to test how tillage methods (RT, annual rotary tillage; DT, annual deep tillage; and TT, RT applied annually with a DT interval of two years) and N rates (300 kg ha–1, N300; 225 kg ha–1, N225; 165 kg ha–1, N165) affect SOC sequestration, greenhouse gas (GHG) emissions, and annual grain yield. And, the CF was used to evaluate ecological sustainability. RT preferentially sequestrated SOC in the 0–10 cm soil layer. In the 10–30 cm soil layers, 2.87–3.82 and 1.85–2.53 Mg ha–1 greater SOC were respectively observed under DT and TT than RT, which was conducive to maximizing annual grain yield with less N application (N225) relative to traditional farming practice (RT-N300). Both increasing N rate and deep tillage resulted in obvious increases in the total GHG emissions. N fertilizer production and transportation were the greatest contributors, accounting for 40.4–47.0% of the total GHG emissions, followed by direct N2O and CH4 emissions (23.5–30.5%). TT-N225 significantly reduced CF (CF including SOC sequestration was 0.49 Mg CO2 eq ha–1 year–1 lower than DT-N225, and was 1.87 Mg CO2 eq ha–1 year–1 lower than RT-N300) and maintained high crop productivity while creating appropriate soil conditions and thus may be a much cleaner agricultural strategy in Northern China. And, the strategy of reducing N application in agricultural production by improving soil properties of the plow layer in this study can also be referred to in other ecological regions.

Direct N2O emissions from global tea plantations and mitigation potential by climate-smart practices

Estimating N2O emissions from the agricultural sector and developing effective reduction strategies are essential to achieving the Paris Agreement 2 °C target. Based on 3705 observations from 435 articles, we demonstrated that the response of N2O emissions was more sensitive to N inputs on acidic soils than alkaline soils and that climatic factors influence this difference. Total global N2O emissions from tea plantations in the 2010s were estimated to be 46.5 Gg N yr–1 using an exponential model developed herein. Tea plantations are a significant contributor to N2O emissions from the agricultural sector in several countries. The intensity of yield-scale GHG emissions from tea was significantly higher than in other upland cereals. Applying climate-smart practices in Chinese tea plantations could reduce emissions equivalent to one-third of the global total. We conclude that accurate identification of N2O emission hotspots and implementation of targeted measures are essential to achieving global temperature control targets.

Mapping direct N2O emissions from peri-urban agriculture: The case of the Metropolitan Area of Barcelona

Mapping direct N2O emissions from peri-urban agriculture: The case of the Metropolitan Area of Barcelona

Nitrous oxide emissions from five fertilizer treatments during one year – High-frequency measurements on a Swedish Cambisol

Nitrous oxide (N2O) is a strong greenhouse gas, and the emissions from managed soils are increasing. Emissions of N2O are highly variable in time and space, and there are potential triggers for emission peaks both in crop season and no-crop season. The aim of this study was to compare how fertilizer treatments, differing in rate and source of nitrogen (N), influence direct N2O emissions from soil, in crop season as well as in no-crop season, with the use of automated, high-frequency chamber measurements. Emissions were measured from cereal production on a Swedish clay-rich soil fertilized with biogas digestate, pig slurry and two levels of mineral N, as well as from control plots receiving no fertilizer N. The results showed that N2O emissions per unit area were low in all treatments, compared to other studies. Emissions from the treatment with mineral fertilizers at recommended rates were similar to the emissions from the control (0.65 and 0.48 kg N2O-N ha−1 yr−1, respectively). One-year cumulative emissions from a mineral N input rate 50 % higher than recommended were about three times higher than the control. Emissions of N2O from the pig slurry and biogas digestate treatments per unit area were of the same magnitude as from the high mineral N treatment. While the emissions from the high mineral N treatment were associated with elevated concentrations of nitrate in the drainage water, the high emissions from the organic fertilizer treatments were probably a result of large input of ammonium and degradable organic matter both in the year studied and in the preceding year. Most (approximately 75 %) of the N2O emissions occurred between harvest in autumn and sowing in spring, mainly in periods of freeze-thaw cycles. The relative differences between treatments were roughly the same during crop season and no-crop season. This study concludes that it is possible to combine high yields with very low N2O emissions - even on a clay soil in a semi-humid climate - when using mineral fertilizers at recommended rates.

Full straw incorporation into a calcareous soil increased N2O emission despite more N2O being reduced to N2 in the winter crop season

Crop straw application in combination with fertilizer nitrogen (N) dose reduction is recommended to improve crop yields and carbon sequestration in soil. This practice may also promote soil nitrous oxide (N2O) emission, thus partially counterbalancing the expected benefits. However, the full straw return effect on the reduction of soil N2O to dinitrogen (N2) is not yet well known, owing to the methodological difficulties in quantifying soil N2 fluxes against the high atmospheric N2 concentration. This study was carried out in a long-term experimental field of a calcareous soil cultivated with summer maize and winter wheat in rotation. For the three field treatments since 2006, i.e., optimal fertilizer N with and without full straw return, and the control (without N and straw application), we conducted in- or ex-situ observations on field N2O fluxes, soil N2 emissions, crop yields, soil organic carbon (SOC) contents and soil/environmental factors. During a rotation cycle, we found that soil ammonium and nitrite concentrations, moisture and temperature jointly determined the N2O emissions, while nitrate concentration, dissolvable organic carbon to nitrate-N ratio and moisture jointly dominated the N2 emissions. Compared to straw removal, the straw return promoted reduction of more soil N2O to N2 in the wheat season than in the maize season, showing increases in the N2 emissions by 57% versus − 10% (P = 0.06). Nevertheless, it significantly enhanced the N2O emissions measured in situ by approximately 137% and 21% in the wheat and maize seasons, respectively (P < 0.05). It also significantly increased the annual crop yields by 20% on average (P < 0.05). Meanwhile it tended to enhance the SOC stock (0−20 cm) by 15‰ yr−1 (P = 0.14), showing a trend of more intensive carbon sequestration than the direct soil N2O emission (−948 versus 378 kg CO2e ha−1 yr−1 on average). We conclude that the long-term full straw return helps to sequester more carbon that offsetting the enhanced N2O emissions from the calcareous soil in the rotation system as it promotes reduction of more N2O to N2 in the wheat season.

Significant Global Yield-Gap Closing Is Possible Without Increasing the Intensity of Environmentally Harmful Nitrogen Losses

Substantial increases in cereal yields are necessary if a growing global demand for food is to be met without further conversion of natural to agricultural land. However, since in many regions yields are limited by soil nutrient availability, this will increase the requirement for fertilizer inputs, specifically of nitrogen (N). Here we focus on maize cultivation, and investigate the trade-off between yield increases and environmentally harmful N-losses in the form of nitrous oxide (N2O) emissions and nitrate (NO3-) leaching. We model the evolution of N-losses as yield gaps—the difference between actual and potential yields—are closed. To do this we use the process-based, biogeochemical model LandscapeDNDC to perform global simulations on a 0.5° grid, and evaluate the response of yields and environmental N-losses to changes in N-inputs. Our simulations find current production (circa 2015) of 954 Tg (5.1 Mg/ha), direct and indirect N2O emissions of 416 Gg-N (2.2 kg-N/ha or 0.44 kg-N/Mg) andNO3-leaching of 5.9 Tg-N (31.5 kg-N/ha or 6.2 kg-N/Mg). We demonstrate that, under an “optimal” strategy for closing yield gaps, maize yields could be increased by 20–25% with concomitant stable or even slightly decreased yield-scaled N-losses. However, further yield increases would come at an ever accelerating cost in environmentally harmful N losses. This acceleration occurs when yields exceed ~70–80% of their potential.

A Decreasing Trend of Nitrous Oxide Emissions From California Cropland From 2000 to 2015

Mitigation of greenhouse gas emissions from agriculture requires an understanding of spatial‐temporal dynamics of nitrous oxide (N2O) emissions. Process‐based models can quantify N2O emissions from agricultural soils but have rarely been applied to regions with highly diverse agriculture. In this study, a process‐based biogeochemical model, DeNitrification‐DeComposition (DNDC), was applied to quantify spatial‐temporal dynamics of direct N2O emissions from California cropland employing a wide range of cropping systems. DNDC simulated direct N2O emissions from nitrogen (N) inputs through applications of synthetic fertilizers and crop residues during 2000–2015 by linking the model with a spatial‐temporal differentiated database containing data on weather, crop areas, soil properties, and management. Simulated direct N2O emissions ranged from 3,830 to 7,875 tonnes N2O‐N yr−1, representing 0.73%–1.21% of the N inputs. N2O emission rates were higher for hay and field crops and lower for orchard and vineyard. State cropland total N2O emissions showed a decreasing trend primarily driven by reductions of cropland area and N inputs, the trend toward growing more orchard, and changes in irrigation. Annual direct N2O emissions declined by 47% from 2000 to 2015. Simulations showed N2O emission variations could be explained not only by cropland area and N fertilizer inputs but also climate, soil properties, and management besides N fertilization. The detailed spatial‐temporal emission dynamics and driving factors provide knowledge toward effective N2O mitigation and highlight the importance of coupling process‐based models with high‐resolution data for characterizing the spatial‐temporal variability of N2O emissions in regions with diverse croplands.

Effects of biochar combined with nitrification/urease inhibitors on soil active nitrogen emissions from subtropical paddy soils

We examined the effects of biochar and urease inhibitors/nitrification inhibitors on nitrification process, ammonia and N2O emission in subtropical soil, and determined the best combination of biochar with nitrification and urease inhibitors. This work could provide a theoretical basis for the mitigation of the negative environmental risk caused by reactive nitrogen gas in the application of nitrogen fertilizer. A indoor aerobic culture test was conducted with seven treatments [urea+biochar (NB), urea+nitrification inhibitor (N+NI), urea+urease inhibitor (N+UI), urea+nitrification inhibitor+urease inhibitor (N+NIUI), urea+nitrification inhibitor+biochar (NB+NI), urea+urease inhibitor+biochar (NB+UI), urea+nitrification inhibitor+urease inhibitor+biochar (NB+NIUI)] and urea (N) as the control. The dynamics of soil inorganic nitrogen content, N2O emission and the volatility of ammonia volatilization were observed under combined application of biochar with urease inhibitor (NBPT)/nitrification inhibitor (DMPP). The results showed that:1)Compared to the control (5.11 mg N·kg-1·d-1) during the incubation period, NB treatment significantly increased therate constant of nitrification by 33.9%, and N+NI treatment significantly reduced the nitrification rate constant by 22.9%. NB treatment significantly increased the abundance of ammonia oxidizing bacteria (AOB) by 56.0%. 2) Compared with N treatment, N+NI and NB+NI treatments signi-ficantly enhanced the cumulative emission of NH3 by 49%. The N+UI treatment reduced the cumulative loss of NH3. The inhibition effect of NB+UI treatment was more significant. 3) The emission rate of N2O was highest in the first 10 days after fertilization. The N2O emission under NB treatment was the earliest, and that of N treatment was the highest (5.87 μg·kg-1·h-1). The combined application of DMPP and NBPT performed the best in reducing soil N2O emission. We estimated global warming potential (GWP) of the direct N2O and indirect N2O (NH3) emissions. Compared with N treatments, N+NI and NB+NI treatments increased the GWP by 34.8% and 40.9%, respectively. While the NB and NB+UI treatments significantly reduced the GWP by 45.9% and 60.5%, the combination of biochar and urease inhibitor had the best effect on reduction of GWP of soil active nitrogen emissions.

Long-term trends of direct nitrous oxide emission from fuel combustion in South Asia

An increasing concentration of nitrous oxide (N2O) in the global atmosphere can perturb the ecological balance, affecting the climate and human life. South Asia, one of the world’s most populous regions, is a hotspot for N2O emission. Although agriculture traditionally dominated the region, economic activities are rapidly shifting towards industry and energy services. These activites may become the largest emitters of N2O in future. Yet, few attempts have been made to estimate long-term direct N2O emission from fuel combustion for the different energy-consuming sectors in the South Asian region. Therefore, the present study developed a comprehensive sectoral N2O emission inventory for South Asian countries for the time period of 1990–2017, with projections till 2041. It revealed that the average N2O emission from fuel combustion in the South Asia region is about 40.96 Gg yr−1 with a possible uncertainty of ±12 Gg yr−1, showing an increase of more than 100% from 1990 to 2017. Although India is the major contributor, with an average of 34 Gg yr−1 of N2O emissions, in terms of growth, small countries like Bhutan and Maldives are dominating other South Asian countries. Sector-wise, the residential sector contributed a maximum emission of 14.52 Gg yr−1 of N2O but this is projected to reduce by more than 50% by 2041. This is because of the successful promotion of cleaner fuels like liquefied petroleum gas over more polluting fuelwood. Power generation contributed 9.43 Gg yr−1of N2O emissions, exhibiting a maximum growth of 395%, followed by road transport (289%) and industry (231%). Future N2O emissions from transport, power and industry are projected to rise by 2.8, 3.3, and 23.9 times their 2017 estimates, respectively, due to the incapability of current policies to combat rising fossil fuel consumption. Mitigation options, such as replacing diesel and compressed natural gas vehicles with electricity-driven vehicles, can decelerate N2O emissions to 45% by 2041 for road transport. A 41% reduction is possible by displacing coal with renewables in the power and industry sectors. Overall, the South Asian contribution to global N2O emissions has enlarged from 2.7% in 1990 to 5.7% in 2007–2016, meaning there is an urgent need for N2O emission mitigation in the region.

Interactive effects of straw management, tillage, and a cover crop on nitrous oxide emissions and nitrate leaching from a sandy loam soil

Minimum tillage, residue recycling and the use of cover crops are key elements of conservation agriculture that play important roles in soil carbon (C) and nitrogen (N) dynamics. This study determined the long-term effects of tillage practice (conventional ploughing vs. direct seeding), straw management (retained vs. removed), and the presence of a cover crop (CC; fodder radish in this study) on nitrous oxide (N2O) emissions, nitrate (NO3−) leaching, and soil mineral N dynamics between October 2019 and June 2020. In the factorial experiment with eight treatment combinations, cumulative N2O emissions ranged from 0.04 to 0.8 kg N ha−1, whereas NO3− leaching varied between 4 and 28 kg N ha−1. The study did not find effects of straw retention on NO3− leaching or N2O emissions. No-till reduced N2O emissions by on average 46% compared to ploughing. Fodder radish reduced NO3− leaching by 80–84%, and there was little N2O emission in the presence of the cover crop; however, after termination in spring there was a flush of N2O, cumulative N2O-N averaged 0.1 and 0.5 kg N ha−1 without and with a cover crop. With information about long-term soil C retention from straw and fodder radish, an overall greenhouse (GHG) balance was calculated for each system. Without straw retention after harvest, there was always a positive net GHG emission, and the indirect N2O emission from NO3− leaching was similar to, or greater than direct N2O emissions. However, in the presence of fodder radish, the direct N2O emissions after termination were much more important than indirect emissions, and negated the C input from fodder radish. Direct seeding, straw retention and the use of a cover crop showed positive effects on N retention and/or GHG balance and could substantially improve the carbon footprint of agroecosystems on sandy soil in a wet temperate climate.

Mapping direct N2O emissions from peri-urban agriculture: The case of the Metropolitan Area of Barcelona

Geographically explicit datasets reflecting local management of crops are needed to help improve direct nitrous oxide (N2O) emission inventories. Yet, the lack of geographically explicit datasets of relevant factors influencing the emissions make it difficult to estimate them in such way. Particularly, for local peri-urban agriculture, spatially explicit datasets of crop type, fertilizer use, irrigation, and emission factors (EFs) are hard to find, yet necessary for evaluating and promoting urban self-sufficiency, resilience, and circularity. We spatially distribute these factors for the peri-urban agriculture in the Metropolitan Area of Barcelona (AMB) and create N2O emissions maps using crop-specific EFs as well as Tier 1 IPCC EFs for comparison. Further, the role of the soil types is qualitatively assessed. When compared to Tier 1 IPCC EFs, we find 15% more emissions (i.e. 7718 kg N2O-N year−1) than those estimated with the crop-specific EFs (i.e. 6533 kg N2O-N year−1) for the entire AMB. Emissions for most rainfed crop areas like cereals (e.g. oat and barley) and non-citric fruits (e.g. cherries and peaches), which cover 24% and 13% of AMB's peri-urban agricultural area respectively, are higher with Tier 1 EF. Conversely, crop-specific EFs estimate higher emissions for irrigated horticultural crops (e.g. tomato, artichoke) which cover 33% of AMB's peri-urban agricultural area and make up 70% of the total N2O emissions (4588 kg N2O-N year−1 using crop-specific EFs). Mapping the emissions helps evaluate spatial variability of key factors such as fertilizer use and irrigation of crops but carry uncertainties due to downscaling regional data to represent urban level data gaps. It also highlighted core emitting areas. Further the usefulness of the outputs on mitigation, sustainability and circularity studies are briefly discussed.

Century-long changes and drivers of soil nitrous oxide (N2 O) emissions across the contiguous United States.

The atmospheric concentration of nitrous oxide (N2O) has increased by 23% since the pre‐industrial era, which substantially destructed the stratospheric ozone layer and changed the global climate. However, it remains uncertain about the reasons behind the increase and the spatiotemporal patterns of soil N2O emissions, a primary biogenic source. Here, we used an integrative land ecosystem model, Dynamic Land Ecosystem Model (DLEM), to quantify direct (i.e., emitted from local soil) and indirect (i.e., emissions related to local practices but occurring elsewhere) N2O emissions in the contiguous United States during 1900–2019. Newly developed geospatial data of land‐use history and crop‐specific agricultural management practices were used to force DLEM at a spatial resolution of 5 arc‐min by 5 arc‐min. The model simulation indicates that the U.S. soil N2O emissions totaled 0.97 ± 0.06 Tg N year−1 during the 2010s, with 94% and 6% from direct and indirect emissions, respectively. Hot spots of soil N2O emission are found in the US Corn Belt and Rice Belt. We find a threefold increase in total soil N2O emission in the United States since 1900, 74% of which is from agricultural soil emissions, increasing by 12 times from 0.04 Tg N year−1 in the 1900s to 0.51 Tg N year−1 in the 2010s. More than 90% of soil N2O emission increase in agricultural soils is attributed to human land‐use change and agricultural management practices, while increases in N deposition and climate warming are the dominant drivers for N2O emission increase from natural soils. Across the cropped acres, corn production stands out with a large amount of fertilizer consumption and high‐emission factors, responsible for nearly two‐thirds of direct agricultural soil N2O emission increase since 1900. Our study suggests a large N2O mitigation potential in cropland and the importance of exploring crop‐specific mitigation strategies and prioritizing management alternatives for targeted crop types.

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.

A two-year measurement of methane and nitrous oxide emissions from freshwater aquaculture ponds: Affected by aquaculture species, stocking and water management

Aquaculture ponds are of increasing worldwide concerns as critical sources of atmospheric methane (CH4) and nitrous oxide (N2O), but little is known about these gases emissions as affected by aquaculture species, stocking and water management in aquaculture ponds. Here, a two-year study was carried out to quantify CH4 and N2O emissions from freshwater crab and fish aquaculture ponds in subtropical China. We further explored how the microbial functional genes [CH4: mcrA and pmoA; N2O: archaeal and bacterial amoA (AOA + AOB), nirS, nirK, nosZ] may drive CH4 and N2O release in the crab aquaculture pond typically undergoing flooding-to-drainage alteration. Over the two-year period, annual CH4 and N2O fluxes averaged 0.95 mg m−2 h−1 and 20.94 μg m−2 h−1 in the fish aquaculture, and 0.78 mg m−2 h−1and 28.48 μg m−2 h−1 in the crab aquaculture, respectively. The direct N2O emission factors were estimated to be 0.77% and 0.36% of the total N input by feed or 1.59 g N2O-N kg−1 and 1.06 g N2O-N kg−1 aquaculture yield in the crab and fish ponds, respectively. Among three functional stocking areas, CH4 and N2O emissions were consistently the highest at the feeding area (FA) in the both aquaculture ponds, followed by at the undisturbed area (UA) and aerated area (AA). The shift in sediment soil moisture from waterlogging to drainage conditions significantly increased the abundance of AOB relative to AOA and pmoA, decreased those of denitrifying functional genes (nirS, nirK, nosZ) and mcrA, while did not alter the functional group ratio of nirS + nirK relative to nosZ. Our results highlight that a better understanding of CH4 and N2O emissions from aquaculture ponds requires taking into consideration of data sourced from more diverse aquaculture systems with different management patterns. In addition, a deep analysis of the microbial processes that drive CH4 and N2O production and consumption from aquaculture ponds remains to be addressed in future studies.