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Agricultural ditches emit disproportionate amounts of nitrous oxide (N 2 O), but their contributions to regional or global N 2 O emissions remain unclear due to limited data. The Intergovernmental Panel on Climate Change (IPCC) recommends using emission factors (EFs) to estimate indirect N 2 O emission, but the EF for ditches (EF 5g ) is categorized as groundwater, which potentially introduces a significant bias. This study conducted a regional-scale campaign in the North China Plain, one of the world's most intensive agricultural regions, and calculated the EF 5g values from agricultural ditches by the concentration method (N 2 O-N/NO 3 - -N). The results found that the regional-scale mean EF 5g value (0.0028) was less than half of the IPCC default value (0.006), illustrating that the current IPCC methodology significantly overestimates N 2 O emissions from agricultural ditches. Despite the relatively small EF 5g values, agricultural ditches exhibited a high mean N 2 O concentration (3.36 μg L -1 ) and a large regional emission (1.14 ± 0.86 Gg N 2 O-N yr -1 ), which is equal to 3.8 ± 2.9% of direct N 2 O emission from the croplands in the North China Plain. Since ditches are ubiquitous in agricultural regions and are likely to expand under climate change, refining EF 5g is crucial to accurately assess their contribution to global N 2 O budgets.

The fertilization regimes of combining manure with synthetic fertilizer are benefits for crop yields and soil fertility in cropping systems as compared to sole synthetic fertilization, but the responses of nitrous oxide (N 2 O) emissions to these practices are inconsistent in the literatures. We hypothesized that it is caused by different proportions of nitrogen (N) applied as manure and various soil properties. Here, we conducted a microcosm experiment, and measured the N 2 O emissions from control (no N) and five manure substitution treatments (supplied 100 mg N kg -1 using the combination of urea with manure) with a range of proportions of N applied as manure (0, 25%, 50%, 75%, and 100%) in three different soil types (fluvo-aquic soil, black soil, and latosol) under aerobic condition. The stimulated effect on N 2 O emissions was more pronounced after manure application in an alkaline soil with high nitrification rate, due to relatively rapid soil DOC depletion and N mineralization of manure. N 2 O emissions from partial substitution of urea with manure were significantly higher than manure-only addition under high soil pH due to abundant labile C from manure. However, there was no difference between manure substitution treatments under acid soils. Nitrification inhibitor substantially decreased N 2 O emissions with increasing soil pH, but it was less effective in mitigating N 2 O emissions with larger proportion of manure. This is likely due to the slow nitrification under low soil pH, and denitrification derived N 2 O increased with increasing manure application rate. Collectively, our study shows that the application of manure substitution to alkaline soils requires careful consideration, which might have rapid nitrification potential and hence trigger significant N 2 O emissions. The knowledge gained in this work will help the decision-makers in optimizing a sound N fertilization regime interacted with soil properties for sustainable crop production and N 2 O mitigation., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Denitrifying nitrous oxide (N 2 O) emissions in agroecosystems result from variations in microbial composition and soil properties. However, the microbial mechanisms of differential N 2 O emissions in agricultural soils are less understood. In this study, microcosm experiments using two main types of Chinese cropland soil were conducted with different supplements of nitrate and glucose to simulate the varying nitrogen and carbon conditions. The results show that N 2 O accumulation in black soil (BF) was significantly higher than that in fluvo-aquic soil (FF) independent of nitrogen and carbon. The abundance of most denitrifying genes was significantly higher in FF, but the ratios of genes responsible for N 2 O production (nirS and nirK) to the gene responsible for N 2 O reduction (nosZ) did not significantly differ between the two soils. However, the soils showed obvious discrepancies in denitrifying bacterial communities, with a higher abundance of N 2 O-generating bacteria in BF and a higher abundance of N 2 O-reducing bacteria in FF. High accumulation of N 2 O was verified by the bacterial isolates of Rhodanobacter predominated in BF due to a lack of N 2 O reduction capacity. The dominance of Castellaniella and others in FF led to a rapid reduction in N 2 O and thus less N 2 O accumulation, as demonstrated when the corresponding isolate was inoculated into the studied soils. Therefore, the different phenotypes of N 2 O metabolism of the distinct denitrifiers predominantly colonized the two soils, causing differing N 2 O accumulation. This knowledge would help to develop a strategy for mitigating N 2 O emissions in agricultural soils by regulating the phenotypes of N 2 O metabolism., (© 2022. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)

Hot moments of nitrous oxide (N 2 O) emissions induced by interactions between weather and management make a major contribution to annual N 2 O budgets in agricultural soils. The causes of N 2 O production during hot moments are not well understood under field conditions, but emerging evidence suggests that short-term fluctuations in soil oxygen (O 2 ) concentration can be critically important. We conducted high time-resolution field observations of O 2 and N 2 O concentrations during hot moments in a dryland agricultural soil in Northern China. Three typical management and weather events, including irrigation (Irr.), fertilization coupled with irrigation (Fer.+Irr.) or with extreme precipitation (Fer.+Pre.), were observed. Soil O 2 and N 2 O concentrations were measured hourly for 24 h immediately following events and measured daily for at least one week before and after the events. Soil moisture, temperature, and mineral N were simultaneously measured. Soil O 2 concentrations decreased rapidly within 4 h following irrigation in both the Irr. and Fer.+Irr. events. In the Fer.+Pre. event, soil O 2 depletion did not occur immediately following fertilization but began following subsequent continuous rainfall. The soil O 2 concentration dropped to as low as 0.2% (with the highest soil N 2 O concentration of up to 180 ppmv) following the Fer.+Pre. event, but only fell to 11.7% and 13.6% after the Fer.+Irr. and Irr. events, which were associated with soil N 2 O concentrations of 27 ppmv and 3 ppmv, respectively. During the hot moments of all three events, soil N 2 O concentrations were negatively correlated with soil O 2 concentrations (r = -0.5, P < 0.01), showing a quadratic increase as soil O 2 concentrations declined. Our results provide new understanding of the rapid short response of N 2 O production to O 2 dynamics driven by changes in soil environmental factors during hot moments. Such understanding helps improve soil management to avoid transitory O 2 depletion and reduce the risk of N 2 O production., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Oxygen (O 2 ) plays a critical and yet poorly understood role in regulating nitrous oxide (N 2 O) production in well-structured agricultural soils. We investigated the effects of in situ O 2 dynamics on N 2 O production in a typical intensively managed Chinese cropping system under a range of environmental conditions (temperature, moisture, ammonium, nitrate, dissolved organic carbon, and so forth). Climate and management (fertilization, irrigation, precipitation, and temperature), and their interactions significantly affected soil O 2 and N 2 O concentrations ( P < 0.05). Soil O 2 concentration was the most significant factor correlating with soil N 2 O concentration ( r = -0.71) when compared with temperature, water-filled pore space, and ammonium concentration ( r = 0.30, 0.25, and 0.26, respectively). Soil N 2 O concentration increased exponentially with decreasing soil O 2 concentrations. The exponential model of N treatments and fertilization with irrigation/precipitation events predicted 74-90% and 58% of the variance in soil N 2 O concentrations, respectively. Our results highlight that the soil O 2 status is the proximal, direct, and the most decisive environmental trigger for N 2 O production, outweighing the effects of other factors and could be a key variable integrating the aggregated effects of various complex interacting variables. This study offers new opportunities for developing more sensitive approaches to predicting and through appropriate management interventions mitigating N 2 O emissions from agricultural soils.

The properties of agricultural soils in various regions of the world are variable and can have a significant but poorly understood impact on soil nitrogen (N) transformations and nitrous oxide (N 2 O) emissions. For this reason, we undertook a study of gross N transformations and related N 2 O emissions in contrasting agricultural soils from China and the UK. Seven Chinese and three UK agricultural soils were collected for study using a 15 N tracing approach. The soil pH ranged from 5.4 to 8.7, with three acidic soils collected from Jinjing, Lishu and Boghall; one neutral soil collected from Changshu, and the other six alkaline soils collected from Quzhou, Zhangye, Changwu, Jinzhong, Boxworth and Stetchworth. Our results showed that the main N transformation processes were oxidation of ammonium (NH 4 + ) to nitrate (NO 3 - ) (O NH4 ), and mineralization of organic N to NH 4 + . The gross autotrophic nitrification rates calculated in the three acidic soils were between 0.25 and 4.15 mg N kg -1  d -1 , which were significantly lower (p < 0.05) than those in the remaining neutral and alkaline soils ranging from 6.94 to 14.43 mg N kg -1  d -1 . Generally, soil pH was positively correlated (p < 0.001) with gross autotrophic nitrification rate and cumulative N 2 O emissions, indicating that soil pH was an important factor regulating autotrophic nitrification and N 2 O emissions. There was also a significant positive correlation between the gross autotrophic nitrification rate and cumulative N 2 O emissions, highlighting the importance of this process for producing N 2 O emissions in these agricultural soils under aerobic conditions. Gross NH 4 + immobilization rates were very low in most soils except for the Jinjing soil with the lowest pH. In conclusion, the gross autotrophic nitrification rates and related N 2 O emissions were controlled by soil pH irrespectively of the soil's origin in these agricultural soils., (Copyright © 2019 Elsevier B.V. All rights reserved.)

The IPCC assume a linear relationship between nitrogen (N) application rate and nitrous oxide (N 2 O) emissions in inventory reporting, however, a growing number of studies show a nonlinear relationship under specific soil-climatic conditions. In the North China plain, a global hotspot of N 2 O emissions, covering a land as large as Germany, the correlation between N rate and N 2 O emissions remains unclear. We have therefore specifically investigated the N 2 O response to N applications by conducting field experiments with five N rates, and high-frequency measurements of N 2 O emissions across contrasting climatic years. Our results showed that cumulative and yield-scaled N 2 O emissions both increased exponentially as N applications were raised above the optimum rate in maize ( Zea mays L.). In wheat ( Triticum aestivum L.) there was a corresponding quadratic increase in N 2 O emissions with the magnitude of the response in 2012-2013 distinctly larger than that in 2013-2014 owing to the effects of extreme snowfall. Existing empirical models (including the IPCC approach) of the N 2 O response to N rate have overestimated N 2 O emissions in the North China plain, even at high N rates. Our study therefore provides a new and robust analysis of the effects of fertilizer rate and climatic conditions on N 2 O emissions.

Nitrous oxide, one of the powerful long-lived greenhouse gases, is emitted mainly through biological processes, especially from fertilized soil. It is critical to partition the contribution of different pathways to N 2 O emissions and the relevant characteristics of microbial communities to identify the key N 2 O processes. An microcosm was conducted to partition the N 2 O emissions from different pathways, and the changes in soil mineral nitrogen and various nitrifiers (amoA bacteria and amoA archaea) and denitrifiers (nirS, nirK, and nosZ) were also determined using qPCR and high-throughput sequencing methods. Different gas inhibitor combinations (i.e., 0.06% acetylene, pure oxygen, 0.06% acetylene in pure oxygen, and pure helium) were used to partition the N 2 O pathways. A 5% oxygen treatment, with and without acetylene, was also included so that the N 2 O emissions could be measured under lower oxygen partial pressure. Results showed that ammonia-oxidation (AO) and successive nitrifier denitrification (NiD) were the main pathways contributing to N 2 O emissions at the earlier period after ammonium sulfate application with the cumulative N 2 O emissions accounting for 30.9% and 59.2% of the total N 2 O emissions, respectively. The higher NiD N 2 O contributions occurred when the soil nitrite concentration appeared higher, especially under the lower oxygen conditions. Higher N 2 O emissions from AO and NiD were associated with the compositional proportion of some dominant AOB species. Denitrification contributed more N 2 O (63.6%-69.3%) in the later period during incubation, coinciding with the following characteristics for denitrifiers: a) lower nosZ/(nirS + nirK) ratio, b) more diversity in nirS, and c) different proportions of some dominant species in nirK. Our results demonstrated that higher AO and successive NiD were the main N 2 O emission pathways, suggesting that controlling the ammonium content and weakening the AO are critical in decreasing N 2 O emissions., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

The linkage between N 2 O emissions and the abundance of nitrifier and denitrifier genes is unclear in the intensively managed calcareous fluvo-aquic soils of the North China Plain. We investigated the abundance of bacterial amoA for nitrification and narG, nirS, nirK, and nosZ for denitrification by in situ soil sampling to determine how the abundance of these genes changes instantly during N fertilization events and is related to high N 2 O emission peaks. We also investigated how long-term incorporated straw and/or manure affect(s) the abundance of these genes based on a seven-year field experiment. The overall results demonstrate that the long-term application of urea-based fertilizer and/or manure significantly enhanced the number of bacterial amoA gene copies leading to high N 2 O emission peaks after N fertilizer applications. These peaks contributed greatly to the annual N 2 O emissions in the crop rotation. A significant correlation between annual N 2 O emissions and narG, nirS, and nirK gene numbers indicates that the abundance of these genes is related to N 2 O emission under conditions for denitrification, thus partly contributing to the annual N 2 O emissions. These findings will help to draw up appropriate measures for mitigation of N 2 O emissions in this 'hotspot' region.

The impacts of different crop rotation systems with their corresponding management practices on grain yield, greenhouse gas emissions, and fertilizer nitrogen (N) and irrigation water use efficiencies are not well documented. This holds especially for the North China Plain which provides the staple food for hundreds of millions of people and where groundwater resources are polluted with nitrate and depleted through irrigation. Here, we report on fertilizer N and irrigation water use, grain yields, and nitrous oxide (N2O) and methane (CH4) emissions of conventional and optimized winter wheat-summer maize double-cropping systems, and of three alternative cropping systems, namely a winter wheat-summer maize (or soybean)-spring maize system, with three harvests in two years; and a single spring maize system with one crop per year. The results of this two-year study show that the optimized double-cropping system led to a significant increase in grain yields and a significant decrease in fertilizer N use and net greenhouse gas intensity, but the net greenhouse gas N2O emissions plus CH4 uptake and the use of irrigation water did not decrease relative to the conventional system. Compared to the conventional system the net greenhouse gas emissions, net greenhouse gas intensity and use of fertilizer N and irrigation water decreased in the three alternative cropping systems, but at the cost of grain yields except in the winter wheat-summer maize-spring maize system. Net uptake of CH4 by the soil was little affected by cropping system. Average N2O emission factors were only 0.17% for winter wheat and 0.53% for maize. In conclusion, the winter wheat-summer maize-spring maize system has considerable potential to decrease water and N use and decrease N2O emissions while maintaining high grain yields and sustainable use of groundwater., (Crown Copyright © 2013. Published by Elsevier B.V. All rights reserved.)

An automated system for continuous measurement of N₂O fluxes on an hourly basis was employed to study N₂O emissions in an intensively managed low carbon calcareous soil under sub-humid temperate monsoon conditions. N₂O emissions occurred mainly within two weeks of application of NH₄(+) based fertilizer and total N₂O emissions in wheat (average 0.35 or 0.21 kg N ha⁻¹ season⁻¹) and maize (average 1.47 or 0.49 kg N ha⁻¹ season⁻¹) under conventional and optimum N fertilization (300 and 50-122 kg N ha⁻¹, respectively) were lower than previously reported from low frequency measurements. Results from closed static chamber showed that N₂O was produced mainly from nitrification of NH₄(+)-based fertilizer, with little denitrification occurring due to limited readily oxidizable carbon and low soil moisture despite consistently high soil nitrate-N concentrations. Significant reductions in N₂O emissions can be achieved by optimizing fertilizer N rates, using nitrification inhibitors, or changing from NH₄(+)- to NO₃(-)-based fertilizers., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Arable soil continues to be the dominant anthropogenic source of nitrous oxide (N2O) emissions owing to application of nitrogen (N) fertilizers and manures across the world. Using laboratory and in situ studies to elucidate the key factors controlling soil N2O emissions remains challenging due to the potential importance of multiple complex processes. We examined soil surface N2O fluxes in an arable soil, combined with in situ high‐frequency measurements of soil matrix oxygen (O2) and N2O concentrations, in situ 15N labeling, and N2O 15N site preference (SP). The in situ O2 concentration and further microcosm visualized spatiotemporal distribution of O2 both suggested that O2 dynamics were the proximal determining factor to matrix N2O concentration and fluxes due to quick O2 depletion after N fertilization. Further SP analysis and in situ 15N labeling experiment revealed that the main source for N2O emissions was bacterial denitrification during the hot‐wet summer with lower soil O2 concentration, while nitrification or fungal denitrification contributed about 50.0% to total emissions during the cold‐dry winter with higher soil O2 concentration. The robust positive correlation between O2 concentration and SP values underpinned that the O2 dynamics were the key factor to differentiate the composite processes of N2O production in in situ structured soil. Our findings deciphered the complexity of N2O production processes in real field conditions, and suggest that O2 dynamics rather than stimulation of functional gene abundances play a key role in controlling soil N2O production processes in undisturbed structure soils. Our results help to develop targeted N2O mitigation measures and to improve process models for constraining global N2O budget. [ABSTRACT FROM AUTHOR]

Soil nitrous oxide (N2O) is an important greenhouse gas contributing to climate change. Many processes produce N2O in soil and the production rate of each process is affected by climatic-edaphic factors, making the soil-to-atmosphere N2O flux extremely dynamic. Experimental approaches, including natural and enriched isotopic methods, have been developed to separate and quantify the N2O production from different processes. However, these methods are often costly and tedious, hampering their wide application. This study aimed to develop a mechanistic model quantifying the soil N2O production from nitrifier nitrification (NN), nitrifier denitrification (ND), and heterotrophic denitrification (HD), which are considered as the most important biological processes, and to investigate how climatic-edaphic factors affect N2O production from individual process as well as total N2O production. The developed model demonstrated its robustness and capability by reliably reproducing the N2O production from NN, ND, and HD in different types of soils under various moisture contents or oxygen concentrations. The model simulations unraveled how environmental conditions and soil properties control the total N2O production rate by variably regulating individual processes. Therefore, the mechanistic model can potentially elucidate the large spatiotemporal variances of in-situ soil N2O flux and improve the assessment of soil N2O emission at regional and global scales. [ABSTRACT FROM AUTHOR]

Purpose: The current study aimed to (1) investigate the responses of N gases (NO, N2O, and N2) and mineral N (NH4+, NO2−, and NO3−) to the different O2 availability; (2) better understand the importance of O2 availability on N2O production pathways; and (3) provide evidence for making N2O mitigation measures in this agricultural soil. Materials and methods: An incubation experiment with a fluvo-aquic soil was conducted using a robotized incubation system, with two added nitrogen sources (NH4+, NO2−) under two initial O2 concentrations (21% and 0% v/v). Gas samples (3.5 ml) were collected every 8 h during the incubation, and soil samples were destructed collected at the beginning and the end of the incubation. Results and discussion: Under initial condition of 21% O2, NH4+ combined with NO2− increased N2O emission especially at high NH4+ concentration (150 mg N kg−1). It is possibly because strong nitrification with high NH4+ addition caused a low O2 availability (< 6.2% v/v) which might have stimulated nitrifier denitrification or denitrification with N2O emission, and under initial condition of 0% O2, denitrification and anammox were the dominant processes, and N loss via gas depended on NO2− and NO3− concentration in the soil. Anammox might occur when NH4+ and NO2− were present, and this process should not be overlooked under anaerobic condition at the studied soil. Conclusions: O2 controlled the soil microbiome processes in the fluvo-aquic soil under current conditions. Under aerobic condition, nitrification and nitrification-related process (nitrifier denitrification or denitrification) were the main N2O sources, and the nitrogenous gas kinetics depended on the rates of exogenous N and N types; while under anaerobic condition, denitrification contributed the most N2O and N2 production, which closely related with NO2− and NO3− concentration in the soil. [ABSTRACT FROM AUTHOR]

Purpose: The work aimed to (1) better understand how C rate and type affecting N2O emissions when combined application with different N forms in a strong ammonia oxidation soil, (2) further explore the tradeoff and mechanism of C availability to N transformations, and (3) provide evidence for making targeted N2O mitigation measures in these kinds of agricultural soils. Materials and methods: The soil was collected from a typical farmer's field in Quzhou, Hebei province, China, classified as calcareous Cambisols (FAO classification system) and characterized with high pH (7.72), low organic C (9.1 g kg−1), and a strong ammonia oxidation potential. Two microcosm incubations were conducted in laboratory under 20 °C and 70% WFPS (water-filled pore space), and the soils were amended with different glucose rates (0, 0.5, 1.0, 2.0, 4.0 mg C kg−1) in Exp I, and with different C sources (glucose, pectin, starch, cellulose, lignin, straw) in Exp II, meanwhile with NH4+ or NO3−-based fertilizer addition. 480 g (equivalent to oven-dried weight) of pre-incubated soil was placed in a glass jar (1165 mL volume) and adjusted to a field bulk density of 1.34 g cm−3. All the glass jars were placed randomly and incubated for 15 days under aerobic condition. Results and discussion: Alone application of NH4+ released more N2O (peak at 16.27 μg N kg−1) than alone NO3− application. However, combined application of glucose with NO3− could emit more N2O than when combined with NH4+, and the highest N2O peak value was found in 0.5 g kg−1of glucose. When pectin is combined with NH4+, resulted in higher accumulated N2O emissions than other C sources, while applying straw and cellulose had negligible effects on N2O emissions but enhanced CO2 emissions under the incubation conditions. Glucose and pectin were identified as two labile C sources, significantly enhanced N2O emissions. As expected, C addition could stimulate mineral N immobilization. Conclusions: Combined application of C and N enhanced N2O and CO2 emissions albeit to different extents. A higher accumulated N2O emission was found when C/N ratio was lower (12.5). The stimulating effect of glucose and pectin as labile C sources with N addition on N2O production should be considered, especially in soils with high pH, low organic C, and a strong ammonia oxidation potential. Therefore, it may have the potential to mitigate N2O emissions from the field when avoiding applying both N and C especially rich in glucose and pectin together. [ABSTRACT FROM AUTHOR]

Many studies have shown the efficiency of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) in suppressing nitrification and nitrous oxide (N2O) emissions. However, the effect of DMPP on soil gross nitrogen transformations and the mechanism of its inhibitory effects on N2O production pathways remain unknown. A 15N tracing experiment was conducted to investigate the effect of DMPP on gross N transformation rates and pathways of N2O production in two typical Chinese and UK agricultural soils. The soils differed in organic carbon (C) and clay content but otherwise had similar properties. The results showed that the application of DMPP decreased the gross autotrophic nitrification rate (p < 0.05) by 21.6% in the Chinese soil and 9.4% in the UK soil. The lower inhibitory efficiency of DMPP in the UK soil was likely to have been due to high rates of adsorption by soil organic C and clay. The total gross rate of mineralization was lower in the presence of DMPP in both soils, likely because there was a regulatory feedback when ammonium concentrations were high. DMPP also significantly reduced cumulative N2O emissions (p < 0.05) in both soils (by between 15.8 and 68.4%), which might be attributed to the dual inhibitory effect of the DMPP on autotrophic nitrification rate and the proportion of N2O produced by autotrophic nitrification processes. This finding will help to predict the sites where DMPP is likely to be most effective and allow the user to target DMPP application to soils with particular properties. [ABSTRACT FROM AUTHOR]

A 2-year field study was conducted to measure nitrous oxide (N2O) and methane (CH4) in a rain-fed spring maize cropland in the Loess Plateau, P. R. China, and to determine the effects of optimized nitrogen (N) fertilization practices on urea-derived N2O emission factor (EF), grain yield, net greenhouse gas (NGHG) emission, and net greenhouse gas intensity (NGHGI). Five treatments were considered, including control (CK), conventional N fertilization (Con), optimal N fertilization (Opt), optimal N fertilization plus nitrification inhibitor (Opt + DCD), and optimal N fertilization with slow release urea (Opt + SR). Soil acted as a small sink for atmospheric CH4. Nitrogen fertilization and heavy rainfall events (>40 mm) were the main factors controlling N2O emissions. The annual mean EF ranged from 0.12 to 0.55%. Compared to conventional N fertilizer, nitrification inhibitor decreased the annual cumulative N2O, NGHG, and NGHGI emissions by 45, 52, and 48%, respectively, without decreasing grain yield. In conclusion, nitrification inhibitor addition was the most effective practice to reduce N2O emissions in the rain-fed regions of Loess Plateau. [ABSTRACT FROM AUTHOR]

Nitrogen (N) fertilization in agricultural soils is the largest anthropogenic source of nitrous oxide (N 2 O) emissions, but the underlying mechanism of the varied response of N 2 O emissions to N fertilizer is unclear. We investigated the response of N 2 O emissions to ammonium-based fertilizers and nitrification inhibitors in upland agricultural soils across northern China. Large variations in N 2 O emissions in different upland soils in response to N fertilization were found, whereas the variations were well correlated with soil nitrite (NO 2 −) 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 NO 2 − content, and higher N 2 O 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 (O 2) depletion and NO 2 − accumulation in the soil matrix, which stimulates N 2 O emission and inhibits N 2 emission under anoxic conditions. Collectively, our results showed that alkaline soils tend to have rapid ammonia oxidation potential, leading to transient NO 2 − accumulation and O 2 depletion in the soil matrix after large ammonium addition. This might trigger significant N 2 O emissions from denitrification and DNRA, which explains the spatial heterogeneity of N 2 O emissions in response to N fertilization in various upland soils. [Display omitted] • NO 2 − dynamic explained the varied response of N 2 O emissions to N fertilizer. • Alkaline soil inclined to NO 2 − accumulation and O 2 depletion after NH 4 + addition. • The accumulated NO 2 − triggered significant N 2 O emission under anoxic condition via DNRA. • Soil pH plays a vital role in regulating AOA and AOB communities and N 2 O emission. [ABSTRACT FROM AUTHOR]

A 56-day aerobic incubation experiment was performed with 15-nitrogen (N) tracer techniques after application of wheat straw to investigate nitrate-N (NO3-N) immobilization in a typical intensively managed calcareous Fluvaquent soil. The dynamics of concentration and isotopic abundance of soil N pools and nitrous oxide (N2O) emission were determined. As the amount of straw increased, the concentration and isotopic abundance of total soil organic N and newly formed labeled particulate organic matter (POM-N) increased while NO3-N decreased. When15NO3-N was applied combined with a large amount of straw at 5000 mg carbon (C) kg−1only 1.1 ± 0.4 mg kg−1NO3-N remained on day 56. The soil microbial biomass N (SMBN) concentration and newly formed labeled SMBN increased significantly (P < 0.05) with increasing amount of straw. Total N2O-N emissions were at levels of only micrograms kg−1soil. The results indicate that application of straw can promote the immobilization of excessive nitrate with little emission of N2O. [ABSTRACT FROM PUBLISHER]

Optimizing crop rotations is one of the proposed sustainable management strategies for increasing carbon sequestration. The main aim of this study was to evaluate the DeNitrification-DeComposition (DNDC) model for estimating soil parameters (temperature, moisture and exchangeable NO3− and NH4+), crop yield and nitrous oxide (N2O) emissions for long-term multi-cropping systems in Hebei, China. The model was validated using five years of data of soil parameters, crop yields and N2O emissions. The DNDC model effectively simulated daily soil temperature, cumulative soil nitrogen and crop yields of all crops. It predicted the trends of observed daily N2O emissions and their cumulative values well but overestimated the magnitude of some peaks. However, the model underestimated daily water filled pore space, especially in dry seasons, and had difficulties in correctly estimating daily exchangeable NO3− and NH4+. Both observed and simulated cumulative N2O results showed that optimized and alternative cropping systems used less nitrogen fertiliser, increased grain yield and decreased N2O emissions compared to the conventional cropping system. Our study shows that although the DNDC model (v. 9.5) is not perfect in estimating daily N2O emissions for these long-term multi-cropping systems, it could still be an effective tool for predicting cumulative emissions. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N 2 O) is a potent greenhouse gas and ozone-depleting chemical. Although nitrogen (N) fertilizer rate is the best single predictor of N 2 O emissions, the spatial heterogeneity of the emission factor is striking and poorly understood. Nitrite (NO 2 −) dynamics have been reported to be correlated with N 2 O production in many soils; however, NO 2 − is often overlooked owing to its rare accumulation, and the underlying mechanisms of NO 2 − accumulation and the relationship with spatiotemporal heterogeneity of N 2 O emissions in response to N addition remains unclear. Here, we collected upland agricultural soils that cover the main upland agricultural production regions of China and investigated the soil-specific effects on N 2 O emissions, changes in microbial abundances and community structures of nitrifiers, and co-occurrence networks for ammonium (NH 4 +)-based fertilizer addition. Our results showed that high NO 2 − accumulation and high N 2 O emissions were generally correlated with low nxrA /AOB gene ratios (R2 = 0.90 and 0.86, respectively, P < 0.01). Further network analyses revealed that NH 4 + addition increased the network links of the nitrification functional communities by 23% and enhanced the interrelationship between AOB and nitrite oxidizers. By synthesizing our data with previous studies in the literature, we found that soil N 2 O emission rate and nxrA /AOB gene ratio exhibit a threshold behavior worldwide, suggesting that disproportionality of NH 3 and NO 2 − oxidations rates are involved in the heterogeneity of fertilizer-induced N 2 O emissions in agricultural soils. These findings enhance our understanding of spatiotemporal heterogeneity in NO 2 − accumulation and N 2 O emissions within upland agricultural soils in view of nitrification functional communities. • The responses of the nxrA /AOB gene ratio to ammonium addition explain the variation of cumulative N 2 O emission. • The nonlinear negative relationship between nxrA /AOB with NO 2 − and N 2 O emission is prevalent in agricultural soils worldwide. • The AOB- nxrA network is crucial for elucidating the microbial mechanism of NO 2 − and N 2 O accumulation while ammonium addition. [ABSTRACT FROM AUTHOR]

The divergent effects of straw returns on nitrous oxide (N 2 O) 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 O 2 dynamics imaging. In the field trial, we performed hourly basis high-time-resolution measurements of soil matrix oxygen (O 2), N 2 O concentrations and fluxes during N 2 O "hot moment" events. We found that straw return increased cumulative N 2 O emissions by 32.7% under conventional high N input (N con), but showed no effect on N 2 O emission under optimized N input (N opt). In situ O 2 content and further microcosm experiments with visualized O 2 spatiotemporal distribution suggested that long-term straw return increases porosity and soil O 2 content, which reduced N 2 O emission under low N substrate conditions by improving soil pore structure and aeration during "hot moment" events. By contrast, straw return increased N 2 O emission via creating short-term O 2 depletion zone and triggering denitrification in anoxic microsites when excess N substrate was available. Although straw return showed inconsistent effects on N 2 O emission under different N application rates, it consistently decreased N 2 O concentration in the soil matrix during the "hot moment" events, suggesting that straw return increases the transport of the produced N 2 O in soil matrix to the soil surface. Our study underscores the multifaceted role of straw return in soil O 2 dynamics, i.e., stimulating O 2 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 N 2 O in situ and emphasizes the importance of optimized N fertilization for reducing the "hot moment" N 2 O emissions when straw is incorporated. [Display omitted] • Long-term straw return increased soil porosity and matrix O 2 concentration. • Straw return increased short-term O 2 consumption and O 2 diffusion. • The divergent role of straw return on O 2 explains its effect on N 2 O production and emission. [ABSTRACT FROM AUTHOR]

Oxygen (O 2) is a key factor driving the expression of N-cycle-related functional genes and regulating nitrogenous gas production in the soil. However, how and to what extent the associated gene transcription and corresponding gas kinetics interact with the transition of soil O 2 status caused by N fertilisation, remains poorly understood. In this context, we conducted a robotized incubation experiment using a He/O 2 atmosphere with different amounts of ammonium-based fertiliser (0 (control), 60 (AS 60) and 200 mg N kg−1 (NH 4) 2 SO 4 (AS 200)) applied to an agricultural soil with a strong nitrification potential under three different initial O 2 levels (oxic 21%, sub-oxic 3%, and anoxic 0%) over 14 days. Through repeated measurements of N 2 O, NO, and N 2 concentrations and kinetics of mineral N following the decline of O 2 concentrations, we found that higher ammonium addition (200 vs. 60 mg N kg−1) caused faster O 2 consumption in the headspace and induced up to 238 times higher net accumulation of N 2 O under initially oxic headspace condition. We speculate that this was due to: 1) the rapid transition of O 2 status from oxic to anoxic due to vigorous ammonia oxidation; and 2) the increased N 2 O/(N 2 O + NO + N 2) ratio of denitrification with higher N addition. The amoA and nosZ gene transcript numbers changed significantly in response to ammonium addition and decreasing O 2 concentration, whereas high-throughput sequencing revealed a significant structural alteration of the soil microbiota along with the transition of O 2 status. Our results highlight that the vigorousness of oxygen depletion in the soil matrix driven by rapid ammonia oxidation is the proximal factor that regulates gas kinetics in high nitrification-potential soil when O 2 diffusion is limited. This implies that practices which reduce hotspots of ammonia oxidation have the potential to mitigate N 2 O emissions from nitrifier denitrification and denitrification in agricultural soil. • High ammonium N addition induced an exponential increase in N₂O emission. • The exponential increase was due to O 2 depletion caused by rapid ammonia oxidation. • High NH 4 +- N addition increased the N 2 O/(N 2 O + NO + N 2) product ratio. • Nitrifier denitrification played a major role in N 2 O production in this soil. [ABSTRACT FROM AUTHOR]

Enhanced efficiency nitrogen fertilizers (EENFs) have provided opportunities for the simultaneous mitigation of nitrous oxide (N 2 O) emissions and increases in crop productions. However, the practice of combining EENFs with optimal fertilization under conditions of misaligned water and fertilizer inputs in rain-fed spring maize systems remains inadequately understood. This study aims to determine the factors controlling N 2 O emissions under the soil climatic conditions in rain-fed plastic mulching cropping systems treated with different nitrogen (N) fertilization rates and EENFs, and to determine the most effective N fertilization measure to reduce N 2 O emissions while maintaining crop yields. A 3-year field experiment was conducted in a spring maize system located in the semiarid rain-fed region of the Loess Plateau, applying two types of EENFs: a nitrification inhibitor dicyandiamide (DCD) and a slow-release fertilizer (SRF). It comprised five fertilization treatments: unfertilized (CK), conventional N rate (CON, 200 kg N ha−1), optimal N rate (OPT, 160 kg N ha−1), OPT with the addition of DCD (OPT+DCD), and OPT using SRF (OPT-SRF). The average annual cumulative N 2 O emissions over the 3-year experimental period ranged from 0.74 to 1.87 kg N 2 O-N ha–1. Fertilizer N contributed 26−60% to annual N 2 O emissions. The highest N 2 O fluxes (50–161 μg N 2 O-N m–2 h–1) typically occurred within the initial 10 days following fertilization, likely induced by strong nitrification processes, constituting 12–19% of the annual N 2 O emissions during this brief period. In comparison to the CON treatment, the OPT, OPT+DCD, and OPT-SRF treatments resulted in significant reductions in annual N 2 O emissions of 24%, 46%, and 34%, respectively, without causing any significant decrease in maize grain yields. The application of DCD led to increased ammonium (NH 4 +-N) concentrations while reducing nitrate (NO 3 –-N). Conversely, using SRF resulted in a decrease in both NH 4 +-N and NO 3 –-N concentrations. Using EENFs at the ideal fertilization rate significantly curtailed yearly N 2 O emissions without adversely affecting maize yields in a rain-fed plastic mulching spring maize system prevalent in the Loess Plateau. Most N 2 O emissions occurred post-fertilization, with rainfall events further influencing these emissions. Our findings recommend the incorporation of nitrification inhibitors, such as DCD, as the most effective fertilizer measure for reducing N 2 O emissions in the rain-fed region of the Loess Plateau. [ABSTRACT FROM AUTHOR]

Digestate is considered as an option for recycling resources and a part of the substitution for chemical fertilizers to reduce environmental impacts. However, its application may lead to significant nitrous oxide (N 2 O) emissions because of its high concentration of ammonium and degradable carbon. The research objectives are to evaluate how N 2 O emissions respond to digestate as compared to urea application and whether this depends on soil properties and moisture. Either digestate or urea (100 mg N kg−1) was applied with and without a nitrification inhibitor of 3,4-dimethylpyrazole phosphate (DMPP) to three soil types (fluvo-aquic soil, black soil, and latosol) under three different soil moisture conditions (45, 65, and 85% water-filled pore space (WFPS)) through microcosm incubations. Results showed that digestate- and urea-induced N 2 O emissions increased exponentially with soil moisture in the three studied soils, and the magnitude of the increase was much greater in the alkaline fluvo-aquic soil, coinciding with high net nitrification rate and transient nitrite accumulation. Compared with urea-amended soils, digestate led to significantly higher peaks in N 2 O and carbon dioxide (CO 2) emissions, which might be due to stimulated rapid oxygen consumption and mineralized N supply. Digestate-induced N 2 O emissions were all more than one time higher than those induced by urea at the three moisture levels in the three studied soils, except at 85% WFPS in the fluvo-aquic soil. DMPP was more effective at mitigating N 2 O emissions (inhibitory efficacy: 73%–99%) in wetter digestate-fertilized soils. Overall, our study shows the contrasting effect of digestate to urea on N 2 O emissions under different soil properties and moisture levels. This is of particular value for determining the optimum of applying digestate under varying soil moisture conditions to minimize stimulated N 2 O emissions in specific soil properties. [Display omitted] • N 2 O emission ratios of digestate to urea were all more than 1 except at 85% WFPS in alkaline soil. • High soil moisture triggered an exponential increase in N 2 O emissions from digestate amendment. • The greater increase in alkaline soil was due to high net nitrification rate and transient nitrite accumulation. • Nitrification inhibitor significantly reduced digestate-induced N 2 O emissions under high soil moisture conditions. [ABSTRACT FROM AUTHOR]

Mitigating greenhouse gas emissions and ensuring crop productivity simultaneously is a major challenge that demands the balance of the amounts of nitrogen (N) and straw (S) applied to agricultural soil. This study seeks to determine whether higher grain yields and lower nitrous oxide (N 2 O) emissions could be realized concomitantly by optimizing synthetic N rates and straw returning. The effect of fertilizer N rates and straw application on the inter-annual yield-scaled N 2 O emission variations was measured over a three-year period (2011–2014) in a maize-wheat cropping system on the North China Plain (NCP). Yield-scaled N 2 O emissions were expressed as g N 2 O-N per Mg grain. Six treatments with three synthetic N levels (zero N [N 0 ], optimized N [N opt ] and conventional N [N con ]) and two straw management practices (straw removal [i.e., N 0 , N opt and N con ] and straw return [i.e., N 0 + S, N opt + S and N con + S]) were used. Optimized N (N opt , N opt + S) refers to the use of approximately 50% of the fertilizer N that is used in conventional farming practices (N con , N con + S), with no significant decrease in grain yields ( P > 0.05). Optimized N reduced cumulative N 2 O emissions by 18–37%, which in turn significantly decreased yield-scaled N 2 O emissions by 38–42% ( P < 0.05). The effects of fertilizer N rates and grain yields on cumulative N 2 O emissions are described by linear and exponential models, respectively. Straw return had a positive effect on mean yield-scaled N 2 O emissions both in the maize season and annually. Yield-scaled N 2 O emissions are constructive considering the trade-off between grain yield and N 2 O emission mitigation from intensive crop production. Optimized fertilizer N rate combined with straw return reduced yield-scaled N 2 O emissions significantly in maize-wheat rotations. [ABSTRACT FROM AUTHOR]

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 (N 2 O) emission, thus partially counterbalancing the expected benefits. However, the full straw return effect on the reduction of soil N 2 O to dinitrogen (N 2) is not yet well known, owing to the methodological difficulties in quantifying soil N 2 fluxes against the high atmospheric N 2 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 N 2 O fluxes, soil N 2 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 N 2 O emissions, while nitrate concentration, dissolvable organic carbon to nitrate-N ratio and moisture jointly dominated the N 2 emissions. Compared to straw removal, the straw return promoted reduction of more soil N 2 O to N 2 in the wheat season than in the maize season, showing increases in the N 2 emissions by 57% versus − 10% (P = 0.06). Nevertheless, it significantly enhanced the N 2 O 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 N 2 O emission (−948 versus 378 kg CO 2 e ha−1 yr−1 on average). We conclude that the long-term full straw return helps to sequester more carbon that offsetting the enhanced N 2 O emissions from the calcareous soil in the rotation system as it promotes reduction of more N 2 O to N 2 in the wheat season. • Straw return increased crop yield and N2O emission in a wheat-maize rotation system. • Straw return promoted more carbon sequestration than N2O emission in 100-year CO2e. • Straw facilitated reduction of more N2O to N2 in wheat season than in maize season. • N2O and N2 emissions in wheat season were dominated by denitrifiers denitrification. • Measured soil factors well explained variances in fluxes and flux ratios of N-gases. [ABSTRACT FROM AUTHOR]

Soil dinitrogen (N 2) emissions are a key nitrogen loss pathway of terrestrial ecosystems. However, the quantification of field N 2 emissions from terrestrial ecosystems remains challenging, as sensitive field methods for measuring N 2 fluxes are lacking. Here, we report a new approach to quantify field N 2 emissions by (i) parameterizing the molar ratio of nitrous oxide (N 2 O) to N 2 O plus N 2 emissions (R N 2 O) in the laboratory and (ii) measuring field N 2 O emissions and soil factors. Soil samples were taken from a maize field and incubated in the laboratory under simulated field conditions. Soil N 2 and N 2 O emissions were determined using the gas-flow-soil-core method. The measurements revealed that the R N 2 O values were significantly higher (0.06–0.67) following urea fertilization and soil rewetting compared to those periods with no fertilization (0.03–0.08) (P < 0.01). A multivariate, nonlinear parameterization of R N 2 O against four easily measured soil factors (ammonia and nitrate concentrations, temperature, and moisture) (n = 20, r 2 = 0.92, P < 0.001) was developed. The seasonal N 2 emissions at the field scale were calculated by combining the laboratory-measured R N 2 O with the field-measured N 2 O emissions and the soil factors. Based on this approach, the cumulative emissions of N 2 and N 2 +N 2 O for the maize season were 7.2 ± 2.8 and 9.6 ± 2.1 (standard error) kg N ha−1, respectively. Using a fixed R N 2 O , i.e., disregarding the temporal and spatial variability of R N 2 O , resulted in approximately 50%–70% lower estimates. Our study shows that a combination of field N 2 O and soil factors measurements and laboratory parameterization of R N 2 O allows field N 2 emissions from croplands to be constrained. With additional measurements, including other soil properties, the development of a generalized parameterization of R N 2 O may become feasible. This approach would allow for a better understanding of gaseous N losses from agricultural ecosystems. Image 1 • A method was developed to quantify dynamical field N 2 fluxes from upland soils. • This method combined N 2 O/(N 2 O + N 2) ratios with field-measured N 2 O fluxes. • N 2 O/(N 2 O + N 2) ratio was parameterized as a function of multiple soil factors. • The obtained multivariate function was validated by independent literature data. [ABSTRACT FROM AUTHOR]