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Nitrogen (N) fertilizer application to agricultural soils results in substantial emissions of nitric oxide (NO), a key substance in tropospheric chemistry involved in climate forcing and air pollution. However, the estimates of global cropland NO emissions remain uncertain due to a lack of information on direct NO emission factors (EF d s) of applied N for various cropping systems at seasonal or annual scales. Here we quantified the crop-specific seasonal and annual-scale NO EF d s through synthesizing 1094 measurements from 125 field-based studies worldwide. The global mean crop-specific seasonal EF d was 0.53%, with the highest for vegetables (0.75%). Among cereal crops, the EF d of maize (0.45%) or wheat (0.47%) was about three times higher than for rice (0.12%). At annual scale, the mean EF d across all cropping systems was 0.58%, with tea plantations having the highest (1.54%). For other cropping systems, the annual-scale EF d s ranged from 0.02% to 1.07%. Besides crop type, also soil organic carbon, total N, and pH as well as N fertilizer type were the main factors explaining the variations of NO EF d s. Based on obtained specific EF d s for each crop type, we estimated that NO emissions due to the use of synthetic fertilizers from global croplands are about 0.42-0.62 Tg N year -1 . Our budgets are relatively lower if compared to estimates derived by the use of IPCC defaults for NO emissions (0.72-1.66 Tg N year -1 ) or reported elsewhere (0.67-1.04 Tg N year -1 ). In our estimates, cash crops (vegetable, tea and orchard), which cover only 9% of the world cropland area, contributed about 31% to total NO emissions from global fertilized croplands. Overall, our meta-analysis provides improved crop-specific NO EF d s reflecting current stage of knowledge. The work also highlights the relative importance of cash crop production as sources for atmospheric NO, that is, agricultural systems on which mitigation efforts may focus., (© 2022 John Wiley & Sons Ltd.)

To cope with challenges of food security and water scarcity in rice production, water-saving ground cover rice production systems (GCRPSs) are increasingly adopted in China and globally. Reduced soil moisture as well as increased soil aeration and temperature under GCRPSs may promote soil N transformations, and in turn give rise to environmental challenges. These include emissions of the potent greenhouse gas nitrous oxide (N 2 O) and atmospheric pollutant nitric oxide (NO). Using conventional flooding rice cultivation as a reference, a three-year field experiment was conducted to investigate the performances of GCRPSs under inorganic (urea) or integrated nutrient management (a combination of synthetic and organic fertilizers), with regards to soil N 2 O and NO emissions as well as grain yields. N 2 O and NO emissions in GCRPSs exhibited high seasonal and interannual variations along with changes in soil inorganic N content and rainfall. When urea alone was applied, the average N 2 O and NO emissions from GCRPSs were 4.11 and 0.14 kg N ha -1 , respectively. These emissions were significantly higher than those of conventional rice cultivation, with 1.47 and 0.052 kg N ha -1 for N 2 O and NO, respectively. When integrated nutrient management was performed for GCRPSs, N 2 O and NO emissions were reduced by approximately 77% and 50%, respectively, i.e., the emission magnitude comparable with N-trace gas losses from conventional rice cultivation. Moreover, GCRPSs with integrated nutrient management resulted in optimal grain yields, and thus, the yield-scaled N 2 O + NO emissions were the lowest compared to other treatments. Averaged over 3 years, the direct emission factors of N 2 O and NO for GCRPSs with urea alone were 2.58% and 0.064%, respectively. Those for GCRPSs with integrated nutrient management were 0.48% and 0.016%, respectively. The results of this study suggest that GCRPS with integrated nutrient management is an eco-friendly strategy for optimizing crop yields while mitigating N 2 O and NO emissions., (Copyright © 2018 Elsevier B.V. All rights reserved.)

High nitrogen (N) inputs in Chinese vegetable and cereal productions played key roles in increasing crop yields. However, emissions of the potent greenhouse gas nitrous oxide (N 2 O) and atmospheric pollutant nitric oxide (NO) increased too. For lowering the environmental costs of crop production, it is essential to optimize N strategies to maintain high crop productivity, while reducing the associated N losses. We performed a 2 year-round field study regarding the effect of different combinations of poultry manure and chemical N fertilizers on crop yields, N use efficiency (NUE) and N 2 O and NO fluxes from a Welsh onion-winter wheat system in the North China Plain. Annual N 2 O and NO emissions averaged 1.14-3.82 kg N ha -1 yr -1 (or 5.54-13.06 g N kg -1 N uptake) and 0.57-1.87 kg N ha -1 yr -1 (or 2.78-6.38 g N kg -1 N uptake) over all treatments, respectively. Both N 2 O and NO emissions increased linearly with increasing total N inputs, and the mean annual direct emission factors (EF d ) were 0.39% for N 2 O and 0.19% for NO. Interestingly, the EF d for chemical N fertilizers (N 2 O: 0.42-0.48%; NO: 0.07-0.11%) was significantly lower than for manure N (N 2 O: 1.35%; NO: 0.76%). Besides, a negative power relationship between yield-scaled N 2 O, NO or N 2 O + NO emissions and NUE was observed, suggesting that improving NUE in crop production is crucial for increasing crop yields while decreasing nitrogenous gas release. Compared to the current farmers' fertilization rate, alternative practices with reduced chemical N fertilizers increased NUE and decreased annual N 2 O + NO emissions substantially, while crop yields remained unaffected. As a result, annual yield-scaled N 2 O + NO emissions were reduced by > 20%. Our study shows that a reduction of current application rates of chemical N fertilizers by 30-50% does not affect crop productivity, while at the same time N 2 O and NO emissions would be reduced significantly., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Straw return in combination with synthetic N fertilizer is considered to be beneficial to soil fertility and crop yield. Such practice, however, can considerably modify soil microbial activity and relative C and N availability, both of which are known to regulate soil nitrous oxide (N 2 O) and nitric oxide (NO) emissions. Minimizing these emissions per unit of crop yield is a prerequisite to minimize the environmental footprint of agricultural production and thus, a policy objective. In our study, we quantified N 2 O and NO emissions and determined fertilizer-N use efficiencies (NUE) and crop yields of two double-cropping (summer maize/Welsh onion-winter wheat) systems with and without straw incorporation in the North China Plain. Relative to the fertilized treatment without straw amendments, straw incorporation showed a significant inhibitory effect on annual N 2 O emissions from the maize-wheat system (-31%), but no significant effect was observed for the Welsh onion-wheat system. However, straw return significantly reduced annual NO emissions by >30% for both systems. Meanwhile, straw return in both systems significantly increased the NUE and crop yields by 34-47% and 7-16%, respectively, as compared to the treatment without straw additions. Across the double-cropping systems, annual direct emission factors of N 2 O, NO and N 2 O+NO were 0.37-0.57%, 0.08-0.78% and 0.57-1.36%, respectively. Furthermore, a negative relationship between direct emission factors of N 2 O+NO and crop NUE was observed, highlighting the importance of optimizing NUE for reducing environmental risks of a cropping system. When expressing emissions on a yield basis, straw return significantly reduced annual yield-scaled N 2 O+NO emissions by 15-42% for both systems. Overall, our results show that the combined application of crop straw and synthetic N fertilizer is a promising N management strategy for maximizing crop yields while mitigating N-trace gas emissions., (Copyright © 2017 Elsevier B.V. All rights reserved.)

The gas-flow-soil-core (GFSC) technique allows to directly measure emission rates of denitrification gases of incubated soil cores. However, the technique was still suffering some drawbacks such as inadequate accuracy due to asynchronous detection of dinitrogen (N2) and other gases and low measurement frequency. Furthermore, its application was limited due to intensive manual operation. To overcome these drawbacks, we updated the GFSC system as described by Wang et al. (2011) by (a) using both a chemiluminescent detector and a gas chromatograph detector to measure nitric oxide (NO), (b) synchronizing the measurements of N2, NO, nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4), and (c) fully automating the sampling/analysis of all the gases. These technical modifications significantly reduced labor demands by at least a factor of two, increased the measurement frequency from 3 to 6 times per day and resulted in remarkable improvements in measurement accuracy (with detection limits of 0.5, 0.01, 0.05, 2.3 and 0.2μgN or Ch(-1)kg(-1)ds, or 17, 0.3, 1.8, 82, and 6μgN or Cm(-2)h(-1), for N2, N2O, NO, CO2, and CH4, respectively). In some circumstances, the modified system measured significantly more N2 and CO2 and less N2O and NO because of the enhanced measurement frequency. The modified system distinguished the differences in emissions of the denitrification gases and CO2 due to a 20% change in initial carbon supplies. It also remarkably recovered approximately 90% of consumed nitrate during incubation. These performances validate the technical improvement, and indicate that the improved GFSC system may provide a powerful research tool for obtaining deeper insights into the processes of soil carbon and nitrogen transformation during denitrification., (Copyright © 2013. Published by Elsevier Ltd.)

Nitrogen (N) fertilizer application to agricultural soils results in substantial emissions of nitric oxide (NO), a key substance in tropospheric chemistry involved in climate forcing and air pollution. However, the estimates of global cropland NO emissions remain uncertain due to a lack of information on direct NO emission factors (EFds) of applied N for various cropping systems at seasonal or annual scales. Here we quantified the crop-specific seasonal and annual-scale NO EFds through synthesizing 1094 measurements from 125 field-based studies worldwide. The global mean crop-specific seasonal EFd was 0.53%, with the highest for vegetables (0.75%). Among cereal crops, the EFd of maize (0.45%) or wheat (0.47%) was about three times higher than for rice (0.12%). At annual scale, the mean EFd across all cropping systems was 0.58%, with tea plantations having the highest (1.54%). For other cropping systems, the annual-scale EFds ranged from 0.02% to 1.07%. Besides crop type, also soil organic carbon, total N, and pH as well as N fertilizer type were the main factors explaining the variations of NO EFds. Based on obtained specific EFds for each crop type, we estimated that NO emissions due to the use of synthetic fertilizers from global croplands are about 0.42-0.62 Tg N year-1. Our budgets are relatively lower if compared to estimates derived by the use of IPCC defaults for NO emissions (0.72-1.66 Tg N year-1) or reported elsewhere (0.67-1.04 Tg N year-1). In our estimates, cash crops (vegetable, tea and orchard), which cover only 9% of the world cropland area, contributed about 31% to total NO emissions from global fertilized croplands. Overall, our meta-analysis provides improved crop-specific NO EFds reflecting current stage of knowledge. The work also highlights the relative importance of cash crop production as sources for atmospheric NO, that is, agricultural systems on which mitigation efforts may focus. [ABSTRACT FROM AUTHOR]

Abstract: Fire is a major factor controlling global carbon (C) and nitrogen (N) cycling. While direct C and N losses caused by combustion have been comparably well established, important knowledge gaps remain on postfire N losses. Here, we quantified both direct C and N combustion losses as well as postfire gaseous losses (N2O, NO and N2) and N leaching after a high‐intensity experimental fire in an old shrubland in central Spain. Combustion losses of C and N were 9.4 Mg C/ha and 129 kg N/ha, respectively, representing 66% and 58% of initial aboveground vegetation and litter stocks. Moreover, fire strongly increased soil mineral N concentrations by several magnitudes to a maximum of 44 kg N/ha 2 months after the fire, with N largely originating from dead soil microbes. Postfire soil emissions increased from 5.4 to 10.1 kg N ha−1 year−1 for N2, from 1.1 to 1.9 kg N ha−1 year−1 for NO and from 0.05 to 0.2 kg N ha−1 year−1 for N2O. Maximal leaching losses occurred 2 months after peak soil mineral N concentrations, but remained with 0.1 kg N ha−1 year−1 of minor importance for the postfire N mass balance. 15N stable isotope labelling revealed that 33% of the mineral N produced by fire was incorporated in stable soil N pools, while the remainder was lost. Overall, our work reveals significant postfire N losses dominated by emissions of N2 that need to be considered when assessing fire effects on ecosystem N cycling and mass balance. We propose indirect N gas emissions factors for the first postfire year, equalling to 7.7% (N2‐N), 2.7% (NO‐N) and 5.0% (N2O‐N) of the direct fire combustion losses of the respective N gas species. [ABSTRACT FROM AUTHOR]

High nitrogen (N) inputs in Chinese vegetable and cereal productions played key roles in increasing crop yields. However, emissions of the potent greenhouse gas nitrous oxide (N 2 O) and atmospheric pollutant nitric oxide (NO) increased too. For lowering the environmental costs of crop production, it is essential to optimize N strategies to maintain high crop productivity, while reducing the associated N losses. We performed a 2 year-round field study regarding the effect of different combinations of poultry manure and chemical N fertilizers on crop yields, N use efficiency (NUE) and N 2 O and NO fluxes from a Welsh onion-winter wheat system in the North China Plain. Annual N 2 O and NO emissions averaged 1.14–3.82 kg N ha −1 yr −1 (or 5.54–13.06 g N kg −1 N uptake) and 0.57–1.87 kg N ha −1 yr −1 (or 2.78–6.38 g N kg −1 N uptake) over all treatments, respectively. Both N 2 O and NO emissions increased linearly with increasing total N inputs, and the mean annual direct emission factors (EF d ) were 0.39% for N 2 O and 0.19% for NO. Interestingly, the EF d for chemical N fertilizers (N 2 O: 0.42–0.48%; NO: 0.07–0.11%) was significantly lower than for manure N (N 2 O: 1.35%; NO: 0.76%). Besides, a negative power relationship between yield-scaled N 2 O, NO or N 2 O + NO emissions and NUE was observed, suggesting that improving NUE in crop production is crucial for increasing crop yields while decreasing nitrogenous gas release. Compared to the current farmers’ fertilization rate, alternative practices with reduced chemical N fertilizers increased NUE and decreased annual N 2 O + NO emissions substantially, while crop yields remained unaffected. As a result, annual yield-scaled N 2 O + NO emissions were reduced by > 20%. Our study shows that a reduction of current application rates of chemical N fertilizers by 30–50% does not affect crop productivity, while at the same time N 2 O and NO emissions would be reduced significantly. [ABSTRACT FROM AUTHOR]

Background and aims: The direct measurement of denitrification dynamics and its product fractions is important for parameterizing process-oriented model(s) for nitrogen cycling in various soils. The aims of this study are to a) directly measure the denitrification potential and the fractions of nitrogenous gases as products of the process in laboratory, b) investigate the effects of the nitrate (NO) concentration on emissions of denitrification gases, and c) test the hypothesis that denitrification can be a major pathway of nitrous oxide (NO) and nitric oxide (NO) production in calcic cambisols under conditions of simultaneously sufficient supplies of carbon and nitrogen substrates and anaerobiosis as to be found to occur commonly in agricultural lands. Methods: Using the helium atmosphere (with or without oxygen) gas-flow-soil-core technique in laboratory, we directly measured the denitrification potential of a silt clay calcic cambisol and the production of nitrogen gas (N), NO and NO during denitrification under the conditions of seven levels of NO concentrations (ranging from 10 to 250 mg N kg dry soil) and an almost constant initial dissolved organic carbon concentration (300 mg C kg dry soil). Results: Almost all the soil NO was consumed during anaerobic incubation, with 80-88 % of the consumed NO recovered by measuring nitrogenous gases. The results showed that the increases in initial NO concentrations significantly enhanced the denitrification potential and the emissions of N and NO as products of this process. Despite the wide range of initial NO concentrations, the ratios of N, NO and NO products to denitrification potential showed much narrower ranges of 51-78 % for N, 14-36 % for NO and 5-22 % for NO. Conclusions: These results well support the above hypothesis and provide some parameters for simulating effects of variable soil NO concentrations on denitrification process as needed for biogeochemical models. [ABSTRACT FROM AUTHOR]

Aims: Temperate grassland is one of the major global biome types and is widely used as rangeland. Typically, cold winters are followed by a transition period with soil thawing that may last from days to weeks. Pulse NO emissions during freeze-thaw events have been observed in a range of temperate ecosystem types and may contribute significantly to annual NO emissions. It was shown recently that spring thaw pulse NO emissions dominated annual NO emissions in a steppe region of Inner Mongolia. Even though biogeochemical models are increasingly used for up scaling of NO emissions from terrestrial ecosystems, they still need to be further developed to be capable of both simulating pulse NO emission during spring thaw and accounting for the impact of grazing on soil NO emissions in general. Methods: In this study, we modified an existing biogeochemical model, Mobile-DNDC, to allow an improved simulation of plant production, snow height, and soil moisture for steppe in Inner Mongolia exposed to different grazing intensities. The newly introduced routines relate maximum snow height to end-of-season biomass (ESSB), to account for decreased plant productivity due to grazing and consider the increase of resistance (impedance) of soil ice on the soil hydraulic conductivity. Results: The implementation of the impedance concept, which means the consideration of decreased hydraulic conductivity in frozen soil, resulted in an improved simulation of soil water content and decreased simulated oxygen content in the top soil during freeze-thaw periods. Increased soil moisture and associated oxygen limitation stimulated NO emission by enhanced denitrification. Based on observations in the field, maximum snow height was limited by ESSB, protecting snow against erosion by wind. Since grazing reduced biomass and thereby snow cover, water availability during spring thaw was smaller at grazed sites as compared to ungrazed sites. In agreement with field observations, lower water content and anaerobiosis resulted in decreased NO emissions during spring thaw. Conclusions: The introduction of the impedance concept into Mobile-DNDC is a major step forward in simulating pulse NO emissions from soils during spring-thaw. [ABSTRACT FROM AUTHOR]

Plant-microbe interactions are crucial regulators of belowground nitrogen cycling in terrestrial ecosystems. However, such interactions have mostly been excluded from experimental setups for the investigation of gross inorganic N fluxes and N partitioning to plants and microorganisms. Ungulate grazing is likely to feed back on soil N fluxes, and hence it is of special importance to simultaneously investigate grazing effects on both plant and microbial N fluxes in intact plant-soil systems, where plant-microbe interactions persist during the experimental incubation. Based on the homogenous NH labelling of intact plant-soil monoliths we investigated how various stocking rates (0, 2.35, 4.8 and 7.85 sheep ha grazing season) in steppe of Inner Mongolia feedback on gross rates of N mineralization and short-term inorganic N partitioning between plant, microbial and soil N pools. Our results showed that the effect of grazing on gross N mineralization was non-uniform. At low stocking rate gross N mineralization tended to decrease but increased with higher grazing pressure. Hence, there was no significant correlation between stocking rate and gross N mineralization across the investigated grazing intensities. Grazing decreased N recovery both in plant and microbial N pools but strongly promoted NO accumulation in the soil and thus negatively affected potential ecosystem N retention. This appeared to be closely related to the grazing-induced decline in easily degradable soil C availability at increasing stocking rate. [ABSTRACT FROM AUTHOR]

In a combined field and laboratory study in the southwest of Burkina Faso, we quantified soil-atmosphere N2O and NO exchange. N2O emissions were measured during two field campaigns throughout the growing seasons 2005 and 2006 at five different experimental sites, that is, a natural savanna site and four agricultural sites planted with sorghum ( n = 2), cotton and peanut. The agricultural fields were not irrigated and not fertilized. Although N2O exchange mostly fluctuated between −2 and 8 μg N2O–N m−2 h−1, peak N2O emissions of 10–35 μg N2O–N m−2 h−1 during the second half of June 2005, and up to 150 μg N2O–N m−2 h−1 at the onset of the rainy season 2006, were observed at the native savanna site, whereas the effect of the first rain event on N2O emissions at the crop sites was low or even not detectable. Additionally, a fertilizer experiment was conducted at a sorghum field that was divided into three plots receiving different amounts of N fertilizer (plot A: 140 kg N ha−1; plot B: 52.5 kg N ha−1; plot C: control). During the first 3 weeks after fertilization, only a minor increase in N2O emissions at the two fertilized plots was detected. After 24 days, however, N2O emission rates increased exponentially at plot A up to a mean of 80 μg N2O–N m−2 h−1, whereas daily mean values at plot B reached only 19 μg N2O–N m−2 h−1, whereas N2O flux rates at plot C remained unchanged. The calculated annual N2O emission of the nature reserve site amounted to 0.52 kg N2O–N ha−1 a−1 in 2005 and to 0.67 kg N2O–N ha−1 a−1 in 2006, whereas the calculated average annual N2O release of the crop sites was only 0.19 kg N2O–N ha−1 a−1 and 0.20 kg N2O–N ha−1 a−1 in 2005 and 2006, respectively. In a laboratory study, potential N2O and NO formation under different soil moisture regimes were determined. Single wetting of dry soil to medium soil water content with subsequent drying caused the highest increase in N2O and NO emissions with maximum fluxes occurring 1 day after wetting. The stimulating effect lasted for 3–4 days. A weaker stimulation of N2O and NO fluxes was detected during daily wetting of soil to medium water content, whereas no significant stimulating effect of single or daily wetting to high soil water content (>67% WHCmax) was observed. This study demonstrates that the impact of land-use change in West African savanna on N trace gas emissions is smaller—with the caveat that there could have been potentially higher N2O and NO emissions during the initial conversion—than the effect of timing and distribution of rainfall and of the likely increase in nitrogen fertilization in the future. [ABSTRACT FROM AUTHOR]

Gross rates of N mineralization and nitrification, and soil–atmosphere fluxes of N2O, NO and NO2 were measured at differently grazed and ungrazed steppe grassland sites in the Xilin river catchment, Inner Mongolia, P. R. China, during the 2004 and 2005 growing season. The experimental sites were a plot ungrazed since 1979 (UG79), a plot ungrazed since 1999 (UG99), a plot moderately grazed in winter (WG), and an overgrazed plot (OG), all in close vicinity to each other. Gross rates of N mineralization and nitrification determined at in situ soil moisture and soil temperature conditions were in a range of 0.5–4.1 mg N kg−1 soil dry weight day−1. In 2005, gross N turnover rates were significantly higher at the UG79 plot than at the UG99 plot, which in turn had significantly higher gross N turnover rates than the WG and OG plots. The WG and the OG plot were not significantly different in gross ammonification and in gross nitrification rates. Site differences in SOC content, bulk density and texture could explain only less than 15% of the observed site differences in gross N turnover rates. N2O and NO x flux rates were very low during both growing seasons. No significant differences in N trace gas fluxes were found between plots. Mean values of N2O fluxes varied between 0.39 and 1.60 μg N2O-N m−2 h−1, equivalent to 0.03–0.14 kg N2O-N ha−1 y−1, and were considerably lower than previously reported for the same region. NO x flux rates ranged between 0.16 and 0.48 μg NO x -N m−2 h−1, equivalent to 0.01–0.04 kg NO x -N ha−1 y−1, respectively. N2O fluxes were significantly correlated with soil temperature and soil moisture. The correlations, however, explained only less than 20% of the flux variance. [ABSTRACT FROM AUTHOR]

The isotopic composition (ic) of soil nitrogen (N) and, more recently, the intramolecular distribution of 15N in the N2O molecule (site preference, SP) are powerful instruments to identify dominant N turnover processes, and to attribute N2O emissions to their source processes. Despite the process information contained in the ic of N species and the associated potential for model validation, the implementation of isotopes in ecosystem models has lagged behind. To foster the validation of ecosystem models based on the ic of N species, we developed the stable isotope model for nutrient cycles (SIMONE). SIMONE uses fluxes between ecosystem N pools (soil organic N, mineral N, plants, microbes) calculated by biogeochemical models, and literature isotope effects for these processes to calculate the ic of N species. Here, we present the concept of SIMONE, apply it to simulations of the biogeochemical model LandscapeDNDC, and assess the capability of 15N‐N2O and, to our knowledge for the first time, SP, to constrain simulated N fluxes by LandscapeDNDC. LandscapeDNDC successfully simulated N2O emission, soil nitrate, and ammonium, as well as soil environmental conditions of an intensively managed grassland site in Switzerland. Accordingly, the dynamics of 15N‐N2O and SP of soil N2O fluxes as simulated by SIMONE agreed well with measurements, though 15N‐N2O was on average underestimated and SP overestimated (root‐mean‐square error [RMSE] of 8.4‰ and 7.3‰, respectively). Although 15N‐N2O could not constrain the N cycling process descriptions of LandscapeDNDC, the overestimation of SP indicated an overestimation of simulated nitrification rates by 10–59% at low water content, suggesting the revision of the corresponding model parameterization. Our findings show that N isotope modeling in combination with only recently available high‐ frequency measurements of the N2O ic are promising tools to identify and address weaknesses in N cycling of ecosystem models. This will finally contribute to augmenting the development of model‐based strategies for mitigating N pollution. [ABSTRACT FROM AUTHOR]

• Topsoil SOC saturation (or steady-state) is reached after 12–13 years of straw return. • Soil N 2 O and NO show high interannual variability, largely driven by changes in rainfall. • Increases in SOC stocks are associated with increases in N 2 O emissions and crop yields. • Long-term straw return shows no stimulating effect on yield-scaled N 2 O and NO fluxes. Straw return to agricultural soils is considered as a key strategy to improve soil organic C (SOC) sequestration and crop production. However, changes in SOC can affect soil N-turnover and associated N 2 O and NO fluxes, as soil biogeochemical C- and N-cycles are strongly coupled. Understanding how and to what extent N-trace gas emissions per unit crop yield respond to changes in SOC following straw return, is critical for balancing food security and climate change mitigation. Here we report on an experiment in a Chinese wheat-maize system designed to understand the effects of long-term (2005–2020) different straw management options (straw return, burning or removal) on SOC stocks and yields, completed by N 2 O and NO flux measurements over the period 2016–2020. Our results showed that the increase in SOC stocks following straw return tended to level off over time, indicating that SOC saturation is reached after 12–13 years under straw return. Soil N-trace gas emissions showed high inter- and intra-annual variability, which were largely driven by changes in rainfall, soil hydrothermal conditions or soil C- and N-availability. Compared to straw burning or removal, long-term straw return increased annual N 2 O emissions by 9 %-49 %, to 2.72–2.98 kg N ha−1 yr−1, while NO emissions were decreased by 12 %-28 %, to 0.83–0.90 kg N ha−1 yr−1. The annual N 2 O emissions roughly offset the total C sink (SOC sequestration and CH 4 uptake) by 40 %-46 %, but will override the climate benefits of total C sink as the SOC pool becomes saturated. However, because yields were higher in straw return treatments, yield-scaled annual N 2 O emissions remained unchanged. Our unique dataset allows for the first time a better understanding of the dynamics between SOC sequestration, yields and N-trace gas emissions, with results calling for action to address the risk of increasing N 2 O emissions following long-term SOC increases induced by straw return. [Display omitted] [ABSTRACT FROM AUTHOR]

In China, most farmers intensively producing vegetables in greenhouses use frequent flooding irrigation and apply excessive amounts of nitrogen (N) fertilizers. This results in low resource use efficiency and high environmental N losses, partly in the form of the potent greenhouse gas nitrous oxide (N 2 O) or of the atmospheric pollutant nitric oxide (NO). For ensuring high vegetable productivity at lower environmental costs, it is crucial to develop efficient irrigation and fertilization strategies. Here we report on year-round measurements of N 2 O and NO fluxes as well as crop yields for a greenhouse cucumber cultivation, comparing common farmers' practice with two alternative managements, i.e., drip fertigation (which reduced irrigation water amount to half of that used for flood irrigation) and reduction of chemical N application by 1/3. Soils managed by farmers' practice were an important source of atmospheric N-oxides, specifically with regard to N 2 O. Annual emissions were 60.5 and 1.39 kg N ha−1 yr−1, respectively, for N 2 O and NO, which were 2.40% and 0.05% of the applied N, respectively. Compared to the farmers' practice, N 2 O emission factors under drip fertigation were decreased by 41%, while NO emission factors nearly doubled. Applying lower rates of N application reduced N 2 O and NO emission factors by 43% and 40%, respectively. Differences in the N-oxides emission strength were primarily ascribed to changes in soil microbial N turnover processes, i.e., higher moisture conditions in soils managed by farmers' practice favored denitrification in a greater extent compared with soils fertilized by drip fertigation. We also showed that drip fertigation and reducing fertilization rates by 1/3 did not compromise vegetable yields, so that lower yield-scaled N 2 O + NO emissions were calculated. Therefore, the replacement of farmers' practice by drip fertigation or by reducing the amount of N applied would help to secure productivity, while reducing the climate and air quality footprint of greenhouse vegetable production. Image 1 • The farmers' practice showed higher N 2 O emission factors than the IPCC default value. • Drip fertigation greatly reduced N 2 O emissions in comparison to the farmers' practice. • In comparison to the farmers' practice, drip fertigaton stimulated annual NO fluxes. • Reducing N rates by 1/3 decreased annual N 2 O and NO EFs by 43% and 40%, respectively. • Drip fertigation or reducing N rates by 1/3 did not compromise vegetable yields. [ABSTRACT FROM AUTHOR]

• Annual non-CO 2 greenhouse gas and NO fluxes were measured in alpine meadow and forest. • CH 4 uptake in alpine sites was generally lower than other Chinese grasslands and forests. • Alpine forest had high soil C content and N availabiltiy and thus large N-oxide fluxes. • Low CH 4 uptake in the meadow was ascribed to high soil moisture and low gas diffusion. • Alpine forest showed higher net non-CO 2 greenhouse gas emission than the alpine meadow. The importance of the world's largest plateau (i.e., the Tibetan Plateau) as a source/sink for atmospheric greenhouse gases (GHGs) still remains uncertain as field observations covering entire years and contrasting ecosystems are scarce. Here we report on year-round measurements of methane (CH 4), nitrous oxide (N 2 O) and nitric oxide (NO) fluxes from two representative ecosystems (cattle-grazed meadow and natural forest, >3300 m a.s.l) on the eastern Tibetan Plateau. In our study, the alpine soils generally functioned as sinks for atmospheric CH 4. The pronounced seasonality of CH 4 uptake, with higher uptake in the growing season and lower uptake in the non-growing season, was mainly regulated by the combined changes in soil moisture and temperature. Soil CH 4 uptake was lower at the meadow site due to high soil moisture and grazing-induced soil compaction reducing gas diffusivity and hampering CH 4 uptake. For the alpine meadow pulses of N-oxide fluxes were observed during the freeze-thaw period, while freeze-thaw related pulses did not occur at the forest site, where the seasonality of N-oxide fluxes was primarily regulated by changes in soil temperature (Q 10 = 3.0). Annual cumulative fluxes for the meadow and forest ecosystems ranged between −1.15 and −2.24 kg CH 4 -C ha−1 yr−1, 0.13-0.65 kg N 2 O-N ha−1 yr−1 and 0.022-0.047 kg NO-N ha−1 yr−1. Non-growing season fluxes contributed 34–45% and 21–51% to the annual budgets of CH 4 and N-oxides, respectively. Multiple regression analyses showed that soil N-oxide fluxes were significantly positively correlated to concentrations of soil dissolved organic C (DOC) and inorganic N, with DOC and inorganic N concentrations being higher at the forest site. Our study also shows that alpine forest is a stronger source for non-CO 2 GHGs relative to alpine meadow (higher by 195 kg CO 2 -eq ha−1 yr−1). This difference in GHG fluxes significantly offsets the net climate benefits due to increased C-sequestration associated with afforestation/reforestation. [ABSTRACT FROM AUTHOR]

Increasing demand of commercial markets for tea products is driving the conversion from rice paddies to tea plantations in subtropical China. So far, however, little is known on how this land-use change, along with the age of plantation establishment, will affect the fluxes of greenhouse gases (GHG) methane (CH 4 ) and nitrous oxide (N 2 O) and the air-pollutant nitric oxide (NO). Thus, we measured CH 4 , N 2 O and NO fluxes over an entire year from two tea plantations (one- and five-year-old plantations) and a native rice-fallow system under two system-specific nitrogen fertilizer options. On an annual scale, comparable or higher GHG emissions were measured for tea plantations as compared to the rice-fallow system. Besides the pollution swapping from CH 4 being the dominating GHG in rice paddies to N 2 O in tea plantations, annual NO emissions increased significantly. Annual direct emission factors of N 2 O and NO in tea plantations were 2.47–5.80% and 2.00–3.99%, respectively, significantly higher than in the rice-fallow system (N 2 O: 1.05–2.05%; NO: 0.033–0.051%). Differences in N fertilizer inputs and soil environmental conditions (e.g., soil water regime, pH and organic carbon) due to contrasting managements of these systems and their interactions are clearly driving the stimulation of N 2 O and NO emissions and contributing to the significant CH 4 reductions. Furthermore, we observed that increasing tea stand age, particularly under organic fertilization, further stimulated N 2 O and NO emissions that varied significantly intra-annually. Nevertheless, the higher N 2 O and NO emissions of tea plantations and the increasing emission strength with stand age deserve further research attention and consideration for future land-use conversions. [ABSTRACT FROM AUTHOR]

NO fluxes from soils are a significant source for tropospheric NO x , though global and regional estimates of the soil source strength are constrained by the paucity of measurements. In a continuous 18 month effort (2012–2014) soil NO fluxes from an intensively managed arable site in the black soil region of the Southern Ukraine (Odessa region) were measured using an automated dynamic chamber system. Measurements revealed three periods of peak NO emissions (fertigation, re-wetting of soils, and to a lower extend during winter), with a pulse emission peak during soil re-wetting in summer of 88.4 μg N m −2 h −1 . The mean annual NO flux was 5.1 ± 8.9 μg N m −2 h −1 and total annual NO emissions were 0.44 ± 0.78 kg N ha −1 yr −1 . The fertilizer induced emission factor for NO was 0.63% under beetroot. The combined effect of soil temperature, soil moisture and soil DIN (NH 4 + and NO 3 − ) concentrations were identified as drivers of the temporal and spatial variability of soil NO fluxes. This work shows that long-term measurements are needed for estimating annual fluxes and the importance of soils as a source for tropospheric NO x as the contribution of different seasons and crop growing periods to the annual budget differed markedly. [ABSTRACT FROM AUTHOR]

The DNDC, LandscapeDNDC and IAP-N-GAS models have been designed to simulate the carbon and nitrogen processes of terrestrial ecosystems. Until now, a comparison of these models using simultaneous observations has not been reported, although such a comparison is essential for further model development and application. This study aimed to evaluate the performance of the models, delineate the strengths and limitations of each model for simulating soil nitrous oxide (N 2 O) and nitric oxide (NO) emissions, and explore short-comings of these models that may require reconsideration. We conducted comparisons among the models using simultaneous observations of both gases and relevant variables from the winter wheat–summer maize rotation system at three field sites with calcareous soils. Simulations of N 2 O and NO emissions by the three models agreed well with annual observations, but not with daily observations. All models failed to correctly simulate soil moisture, which could explain some of the incorrect daily fluxes of N 2 O and NO, especially for intensive fluxes during the growing season. Multi-model ensembles are promising approaches to better simulate daily gas emissions. IAP-N-GAS underestimated the priming effect of straw incorporation on N 2 O and NO emissions, but better results were obtained with DNDC95 and LandscapeDNDC. LandscapeDNDC and IAP-N-GAS need to improve the simulation of irrigation water allocation and residue decomposition processes, respectively, and together to distinguish different irrigation methods as DNDC95 does. All three models overestimated the emissions of the nitrogenous gases for high nitrogen fertilizer (>430 kg N ha −1 yr −1 ) addition treatments, and therefore, future research should focus more on the simulation of the limitation of soil dissolvable organic carbon on denitrification in calcareous soils. [ABSTRACT FROM AUTHOR]

NO emissions from soils and ecosystems are of outstanding importance for atmospheric chemistry. Here we review the current knowledge on processes involved in the formation and consumption of NO in soils, the importance of NO for the physiological functioning of different organisms, and for inter- and intra-species signaling and competition, e.g. in the rooting zone between microbes and plants. We also show that prokaryotes and eukaryotes are able to produce NO by multiple pathways and that unspecific enzymo-oxidative mechanisms of NO production are likely to occur in soils. Nitric oxide production in soils is not only linked to NO production by nitrifying and denitrifying microorganisms, but also linked to extracellular enzymes from a wide range of microorganisms. Further investigations are needed to clarify molecular mechanisms of NO production and consumption, its controlling factors, and the significance of NO as a regulator for microbial, animal and plant processes. Such process understanding is required to elucidate the importance of soils as sources (and sinks) for atmospheric NO. [ABSTRACT FROM AUTHOR]

Highlights: [•] A technical system is improved to measure soil N2, N2O, NO, CO2 and CH4 emissions. [•] Two methods are applied for measuring NO concentrations of different levels. [•] Synchronization and automation enable more accurate quantification of the gases. [Copyright &y& Elsevier]

Abstract: We investigated nitric oxide (NO) fluxes at a summer and a winter sheepfold in the Baiyinxile livestock farm, near Xilinhot, Inner Mongolia, which are a typical feature of the regional husbandry. Using a manual static opaque chamber/chemiluminescence measuring system, we intermittently observed fluxes in the summer sheepfold between May 28th and September 26th 2005 and in both winter and summer sheepfolds between March 8th and October 18th 2006. During these periods, mean NO emissions (±S.E., in terms of mass of nitrogen) were 124.0±28.7, 134.6±23.3 (summer sheepfold) and 214.4±79.6μgNO–Nm−2 h−1 (winter sheepfold), respectively, and thus, three magnitudes higher than observed steppe NO emissions in the same region. The NO fluxes were not significantly different between the 2 years, but in summer they were much higher than in spring (p <0.05). Temperature and moisture of the faeces layer significantly regulated the NO fluxes (p <0.01). The direct NO emission factor (EF) for faeces and urine excreted in the sheepfolds was 0.7gNO–Nemitted kg−1 Nexcreted, which was almost 37 times lower than a recently reported N2O EF. We estimated the total NO emission from the sheepfolds of the Baiyinxile livestock farm to be 1.82±0.43tonsNO–Nyear−1, which accounts to approximately 12.3% of the total NO emission from this steppe region. With the rapid increase of livestock numbers, sheepfold NO emissions may further increase and contribute to high N deposition in confined areas around sheepfolds. [Copyright &y& Elsevier]

Abstract: Field operations of tillage and residue incorporation could have potentially important influences on N-trace gas fluxes, though poorly quantified. Here we studied the effects of straw incorporation in the preceding rice season and no-tillage prior to wheat sowing on nitric oxide (NO) and nitrous oxide (N2O) emissions during the non-rice period of a typical rice-wheat rotation in the Yangtze River Delta. Compared to conventional management practice (no straw incorporation along with rotary harrowing tillage to 10 cm before wheat sowing), straw incorporation alone decreased cumulative N2O emissions over the entire non-rice period by 32% (1.53 vs. 2.24 kg N ha-1, P < 0.05) but did not affect NO emissions (0.88 vs. 0.87 kg N ha−1). In contrast, no-tillage alone increased N2O emissions by 75% (P < 0.05) while reducing NO emissions by 48% (P < 0.01). Combination of no-tillage and straw incorporation led to no change in N2O emissions but a reduction in NO emissions compared to the conventional management regime. The direct N2O emission factors (EFds) of applied nitrogen fertilizers during the non-rice season ranged from 0.29% to 1.35% with a coefficient of variation (CV) as large as 68% among the investigated management regimes. The EFds for NO ranged from 0.13% to 0.32% with a CV of 50%. Adoption of these new EFds will allow us to account for management effects on N-trace gas emissions when calculating emission inventories. Nevertheless, it is noteworthy that the uncertainty remains high, since the effects of soil properties such as texture or pH on management practices are not yet well defined. [Copyright &y& Elsevier]