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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.)

Urban turfgrass ecosystems are expected to increase at unprecedented rates in upcoming decades, due to the increasing population density and urban sprawl worldwide. However, so far urban turfgrasses are among the least understood of all terrestrial ecosystems concerning their impact on biogeochemical N cycling and associated nitrous oxide (N 2 O) and nitric oxide (NO) fluxes. In this study, we aimed to characterize and quantify annual N 2 O and NO fluxes from urban turfgrasses dominated by either C4, warm-season species or C3, cool-season and shade-enduring species, based on year-round field measurements in Beijing, China. Our results showed that soil N 2 O and NO fluxes varied substantially within the studied year, characterizing by higher emissions during the growing season and lower fluxes during the non-growing season. The regression model fitted by soil temperature and soil water content explained approximately 50%-70% and 31%-38% of the variance in N 2 O and NO fluxes, respectively. Annual cumulative emissions for all urban turfgrasses ranged from 0.75 to 1.27 kg N ha -1 yr -1 for N 2 O and from 0.30 to 0.46 kg N ha -1 yr -1 for NO, both are generally higher than those of Chinese natural grasslands. Non-growing season fluxes contributed 17%-37% and 23%-30% to the annual budgets of N 2 O and NO, respectively. Our results also showed that compared to the cool-season turfgrass, annual N 2 O and NO emissions were greatly reduced by the warm-season turfgrass, with the high root system limiting the availability of inorganic N substrates to soil microbial processes of nitrification and denitrification. This study indicates the importance of enhanced N retention of urban turfgrasses through the management of effective species for alleviating the potential environmental impacts of these rapidly expanding ecosystems., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

A portion of alpine meadows has been and will continue to be cultivated due to the concurrent increasing demands for animal- and crop-oriented foods and global warming. However, it remains unclear how these long-term changes in land use will affect nitric oxide (NO) emission. At a field site with a calcareous soil on the Qinghai-Tibetan Plateau, the authors measured the year-round NO fluxes and related variables in a typically winter-grazed natural alpine meadow (NAM) and its adjacent forage oat field (FOF). The results showed that long-term plow tillage, fertilization and growing forage oats significantly yielded ca. 2.7 times more (p < 0.01) NO emissions from the FOF than the NAM (conservatively 208 vs. 56 g N/(ha·year) on average). The spring freeze-thaw period and non-growing season accounted for 17%-35% of the annual emissions, respectively. The Q 10 of surface soil temperature (T s ) was 8.9 in the NAM (vs. 3.8 in the FOF), indicating increases of 24%-93% in NO emissions per 1-3 °C increase. However, the warming-induced increases could be smaller than those due to land use change and management practices. The T s and concentrations of ammonium, nitrate and water-extractable organic carbon jointly explained 69% of the variance in daily NO fluxes from both fields during the annual period (p < 0.001). This result indicates that temporally and/or spatially distributed NO fluxes from landscapes with calcareous soils across native alpine meadows and/or fields cultivated with forage oats can be predicted by simultaneous observations of these four soil variables., (Copyright © 2018. Published by Elsevier B.V.)

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.)

As intensive vegetable cultivation is rapidly expanding in China and elsewhere worldwide, its environmental consequences on nitrous oxide (N(2)O) and nitric oxide (NO) emissions deserve attention. We measured N(2)O and NO fluxes simultaneously for a full year from Chinese subtropical vegetable fields. Clearly, both N(2)O and NO emissions varied greatly in different vegetable crop seasons within a year, highlighting the importance of whole-year measurement for achieving temporally accurate annual direct emission factors. A revised "hole-in-the-pipe" model well described quantitative relationships between N(2)O plus NO fluxes and soil-specific conditions. Annual background N(2)O and NO emissions were 0.73-5.0 and 0.26-0.56 kg N ha(-1) yr(-1), respectively, for the vegetable cultivations. The farmers' fertilization practice increased N(2)O and NO emissions. Annual direct emission factors for greenhouse and conventional vegetable fields, respectively, were 1.1% and 1.9% for N(2)O, and 0.36% and 0.32% for NO, indicating there is a need to consider a differentiation of emission factors for managed vegetable cultivations., (Copyright © 2014 Elsevier Ltd. 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.)

A method for measuring NO and NO2 exchanges between cultivated lands and the atmosphere in a FACE (free-air CO2 enrichment) study is described. With this method, gas is sampled with a technique of static-opaque-chamber and the fluxes of NO and NO2 exchanges are determined by analyzing the NO and NO2 concentrations with a chemiluminescent NOx analyzer. Application of this method in the FACE study of a rice-wheat ecosystem has indicated that reliable data on the exchange fluxes could be obtained. Over the non-waterlogged period of a rice-wheat rotation, net emission of NO from the fields was observed, while net uptake of NO2 occurred. The daily net emission of NO did not correlated with the soil temperature, but negatively depended upon soil moisture (R2 = 0.82, P < 0.001). A significant seasonal variation in the net uptake of NO2 was observed, which was regulated by wheat growth status. The daily uptake of NO2 depended upon both soil temperature and soil moisture. The dependence for each could be described with a parabola function (for soil temperature: R2 = 0.74, P < 0.001; for soil moisture: R2 = 0.69, P < 0.001). An elevation of atmospheric CO2 by 200 +/- 40 mumol.mol-1 mitigated the net emission of NO by 19% (t-test P = 0.096) and might be possible to reduce the net uptake of NO2 by 10% (t-test P = 0.26), which was likely due to the stimulated wheat growth.

To meet increasing demands, tea plantations are rapidly expanding in China. Although the emissions of nitrous oxide (N2O) and nitric oxide (NO) from tea plantations may be substantially influenced by soil pH reduction and intensive nitrogen fertilization, process model-based studies on this issue are still rare. In this study, the process-oriented biogeochemical model, Catchment Nutrient Management Model – DeNitrification-DeComposition (CNMM-DNDC), was modified by adding tea-growth-related processes that may induce a soil pH reduction. Using a dataset for intensively managed tea plantations at a subtropical site, the performances of the original and modified models for simulating the emissions of both gases subject to different fertilization alternatives and stand ages were evaluated. Compared with the observations in the early stage of a tea plantation, the original and modified models showed comparable performances for simulating the daily gas fluxes (with a Nash–Sutcliffe index (NSI) of 0.10 versus 0.18 for N2O and 0.32 versus 0.33 for NO), annual emissions (with an NSI of 0.81 versus 0.94 for N2O and 0.92 versus 0.94 for NO) and annual direct emission factors (EF d s). For the modified model, the observations and simulations demonstrated that the short-term replacement of urea with oil cake stimulated N2O emissions by ∼62 % and ∼36 % and mitigated NO emissions by ∼25 % and ∼14 %, respectively. The model simulations resulted in a positive dependence of EF d s of either gas on nitrogen doses, implicating the importance of model-based quantification of this key parameter for inventory purposes. In addition, the modified model with pH-related scientific processes showed overall inhibitory effects on the gases' emissions in the middle to late stages during a full tea plant lifetime. In conclusion, the modified CNMM-DNDC exhibits the potential for quantifying N2O and NO emissions from tea plantations under various conditions. Nevertheless, wider validation is still required for simulation of long-term soil pH variations and emissions of both gases from tea plantations. [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]

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]

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]

We conducted multi-year observations of nitric oxide (NO) fluxes from typical vegetable fields in the Yangtze River delta, which is located in southeastern China. Flux measurements were performed manually twice per week at intervals of 2¿3 days, in both fertilized and unfertilized fields, over an investigation period of 1448 days (September 2004¿August 2008). In total, twelve vegetable-growing periods and a short fallow period were investigated. On average, the NO fluxes from the fertilized plots were 21 times higher than fluxes from the unfertilized plots (p < 0.001). Peak NO emissions usually occurred soon after the addition of nitrogenous fertilizer. Peak emissions took place during about 15% of the whole investigation time, but contributed to approximately 89% of the total NO release. The annual background NO emissions (from fields without nitrogen amendment) were observed at 0.290 ± 0.019 (standard deviation of 3 observations) kg N ha¿1. The total amounts of NO emitted during the individual vegetable-growing periods correlated positively and exponentially with the products of seasonal mean soil temperatures and nitrogen addition rates (R 2 = 0.87, p < 0.001). The mean direct NO emission factor (EFd, the loss rate of fertilizer nitrogen via NO emissions) for the four-year period was determined to be 0.51% ± 0.11% (standard error of 3 observations). The EFds of individual vegetable-growing seasons ranged from 0.05% to 1.24%, varying linearly and positively with the products of seasonal mean soil temperatures and nitrogen addition rates (R 2 = 0.58, p < 0.01). The observed interaction of soil temperature and nitrogen addition on NO emission in seasonal totals and EFds occurred in soils with moisture contents ranging from 55% to 100% water-filled pore space (mean: 79%; standard deviation: 9%). The results of this study indicate that when other conditions remain relatively stable, the direct emission factor, a key parameter for compiling an inventory of NO emissions from vegetable fields, may vary with not only soil temperature but also nitrogen addition. [Copyright &y& Elsevier]

Urban turfgrass ecosystems are expected to increase at unprecedented rates in upcoming decades, due to the increasing population density and urban sprawl worldwide. However, so far urban turfgrasses are among the least understood of all terrestrial ecosystems concerning their impact on biogeochemical N cycling and associated nitrous oxide (N 2 O) and nitric oxide (NO) fluxes. In this study, we aimed to characterize and quantify annual N 2 O and NO fluxes from urban turfgrasses dominated by either C4, warm-season species or C3, cool-season and shade-enduring species, based on year-round field measurements in Beijing, China. Our results showed that soil N 2 O and NO fluxes varied substantially within the studied year, characterizing by higher emissions during the growing season and lower fluxes during the non-growing season. The regression model fitted by soil temperature and soil water content explained approximately 50%–70% and 31%–38% of the variance in N 2 O and NO fluxes, respectively. Annual cumulative emissions for all urban turfgrasses ranged from 0.75 to 1.27 kg N ha−1 yr−1 for N 2 O and from 0.30 to 0.46 kg N ha−1 yr−1 for NO, both are generally higher than those of Chinese natural grasslands. Non-growing season fluxes contributed 17%–37% and 23%–30% to the annual budgets of N 2 O and NO, respectively. Our results also showed that compared to the cool-season turfgrass, annual N 2 O and NO emissions were greatly reduced by the warm-season turfgrass, with the high root system limiting the availability of inorganic N substrates to soil microbial processes of nitrification and denitrification. This study indicates the importance of enhanced N retention of urban turfgrasses through the management of effective species for alleviating the potential environmental impacts of these rapidly expanding ecosystems. [Display omitted] • Annual N 2 O and NO fluxes were characterized across various urban turfgrass types. • The magnitude of N 2 O and NO fluxes is greatly modulated by urban turfgrass species. • Changes in soil hydrothermal conditions determine the variance of N 2 O and NO fluxes. • Urban turfgrass systems are nonnegligible sources in N 2 O and NO budget estimation. [ABSTRACT FROM AUTHOR]

Abstract Measurements of nitric oxide (NO) emission from a typical Chinese rice–wheat rotation are continuously made during the non-waterlogged period by using an automatic system based on static chamber techniques. A positive correlation exists between NO emission and soil moisture content when surface soil temperature is > 20 °C. The diurnal variability in NO emission is characterized with day-peak, night-peak and irregular patterns, which are in close association with wheat growth. The diurnal NO emission under the day-peak pattern is correlated with the simultaneously observed surface soil temperature, whereas that under the night-peak and irregular pattern is dependent on surface soil temperature at 7 ± 2 and 3 ± 2 h before NO observation, respectively. The effect of soil temperature on NO emission is well described by F = α·e β·T , where F is NO flux, T soil temperature, and α and b empirical coefficients. The parameter Q 10 ; that is, the change in NO emission per 10 °C soil temperature, is correlated with the rates of fertilizer-N application. An approach orientated from the Arrhenius equation, &formm101;&formm102;, is developed in order to predict diurnal NO emission, where T K is the daily average soil temperature, ΔT K the deviation of soil temperature from the daily average, E a the apparent activation energy, R the gas constant, t a given time within the one-day cycle, t delayed time for appearance of the diurnal NO emission peak, and k an empirical coefficient. Based on the results, the authors recommend that intermittent measurement of NO emission from a similar ecosystem would be best taken around 17:00. The molar ratio of NO/N2 O over the non-waterlogged period is > 1 when soil moisture content was less than the field capacity, suggesting that NO emission was mainly derived from nitrification under this condition. [ABSTRACT FROM AUTHOR]

Abstract Nitric oxide emissions from a typical rice–wheat rotation system in southeastern China were continuously measured with an automatic system in 1996–1997. The seasonal pattern of the NO emissions was characterized for the non-waterlogged period of a rotation cycle. Nitric oxide emissions during the period from March through June were 3.9–6.3 folds for the fertilized plots and 1.6 folds for the unfertilized plot larger than those from November through December. Nitric oxide emissions were not detectable during the winter period from January through February. Amendment of synthetic fertilizer N significantly enhanced the NO emission by a factor of 6.5, but the enhancement was significantly mitigated by 25% through substituting ca. 16% of the synthetic fertilizer N with organic N from fermented crop residues or by 21% through deep tillage. The NO–N emission factor, defined as the amount of NO–N released per unit of synthetic fertilizer N input, was determined to be 0.025 kg NO–N kg-1 of N applied for the non-waterlogged period, which was reduced by 32% through substituting part of the synthetic N fertilizer with fermented crop residues or by 24% through deep tillage. In addition, the NO emission factor, defined as the amount of NO–N emitted from unit unfertilized area per day, was observed to be ca. 3.8 g N ha-1 d-1 . Approximately 0.55 Tg N yr-1 was likely released as NO from Chinese cultivated lands. [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]

Abstract: Intensive nitrogen (N) fertilizer application in the vegetable fields of China has commonly occurred during recent decades and may substantially increase both nitrous oxide (N2O) and nitric oxide (NO) emissions. However, the quantification of N2O and NO emissions from vegetable fields has been rare due to both the lack of long-term field measurements and reliable methods for extrapolating these measurements. Using a unique dataset from a four-year measurement study of an intensively managed conventional vegetable field in southeastern China, we tested a process-based biogeochemical model, denitrification-decomposition (DNDC), for its applicability for quantifying the impact of fertilizer management practices on emissions of N trace gases from vegetable production. The results from the model validation indicate that the simulations of vegetable yields and seasonal cumulative N2O and NO emissions are consistent with the observations. In addition, DNDC can generally capture the measured temporal pattern of daily N2O and NO fluxes. The modeled impacts of fertilization alternatives can be summarized as follows: (a) both the type and application rate of N fertilizers play important roles in regulating N2O and NO emissions as well as vegetable growth; (b) reducing the N application rate to 75% of the conventional amount decreased N2O and NO emissions by 31% and showed little impact on vegetable biomasses, suggesting that reducing the N dose to a reasonable level would be advisable for both the mitigation of N gases emissions and the maintenance of vegetable production; and (c) replacing synthetic fertilizer under the conventional management practices with organic manure may significantly stimulate N2O emission by 62% while decreasing vegetable yields. The results from this modeling study may provide useful information for the ongoing debate regarding the optimization of fertilizer use strategies in China. This study also demonstrates the potential of utilizing process-based models, such as DNDC, to quantify and mitigate N2O and NO emissions from intensive vegetable production through interpreting, integrating, and extrapolating field observations. [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]

• 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]

• An EC system with a QCL-based closed-path analyzer was used to measure NO fluxes. • Detection limit for half-hourly fluxes was 5.6 μg n m −2 h −1 (95% confidence interval). • The system was qualified for obtaining daily fluxes during low emission period. • The QCL-based analyzer is an effective option for measuring turbulent NO fluxes. Croplands are important sources of atmospheric nitric oxide (NO). However, high-frequency measurements of NO fluxes over croplands using the eddy covariance (EC) technique are still scarce, mainly due to instrumental limitation. In this study, a closed-path NO analyzer based on a quantum cascade laser (QCL) absorption spectrometer was employed for EC flux measurements over a subtropical vegetable field during a two-month summer period with the lowest NO emission intensity of the year. The purpose was to investigate the detection limit of the EC system based on this NO analyzer and evaluate its applicability for measuring the turbulent fluxes of NO under field conditions. The performance of the analyzer was stable, showing an average precision (0.1 s) of 0.338 nmol mol−1 and a corresponding flux detection limit of 5.6 μg N m −2 h −1 at the 95% confidence interval. The measured turbulent NO fluxes ranged from −7.1 to 61.4 μg N m −2 h −1 (median: 3.5 μg N m −2 h −1), with a relative random error of 386% before field ploughing and 76% thereafter. The systematic errors due to the high-frequency loss and the use of lag times of carbon dioxide for NO flux calculation were estimated at 12% and 3%, respectively. During the measurement period, 37% of the observed half-hourly fluxes were larger than the detection limit; the magnitude of these fluxes is comparable with that measured by the static chambers. Nevertheless, this EC system could be still qualified for measuring turbulent NO fluxes over common croplands if the flux averaged at daily or longer timescales are of interest, because either flux detection limit or random error would decrease by an order of n , wherein n is the number of half-hourly fluxes being taken for averages. This study shows that the closed-path dual-QCL analyzer could be an effective option for EC measurements of turbulent NO fluxes with the advantages of (i) stable performance, (ii) high precision and fast response, and (iii) feasible instrumental maintenance for long-term field measurements. However, the observed turbulent fluxes still underestimated the soil NO emissions likely due to chemical reaction loss of NO below the sensor height. Further studies are necessary to address this systematic error. [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]

Abstract: Nitrogen amendment followed by flooding irrigation is a general management practice for a wheat–maize rotation in the North China Plain, which may favor nitrification and denitrification. Consequently, high emissions of nitrous oxide (N2O) and nitric oxide (NO) are hypothesized to occur. To test this hypothesis, we performed year-round field measurements of N2O and NO fluxes from irrigated wheat–maize fields on a calcareous soil applied with all crop residues using a static, opaque chamber measuring system. To interpret the field data, laboratory experiments using intact soil cores with added carbon (glucose) and nitrogen (nitrate, ammonium) substrates were performed. Our field measurements showed that pulse emissions after fertilization and irrigation/rainfall contributed to 73% and 88% of the annual N2O and NO emissions, respectively. Soil moisture and mineral nitrogen contents significantly affected the emissions of both gases. Annual emissions from fields fertilized at the conventional rate (600 kg N ha−1 yr−1) totaled 4.0 ± 0.2 and 3.0 ± 0.2 kg N ha−1 yr−1 for N2O and NO, respectively, while those from unfertilized fields were much lower (0.5 ± 0.02 kg N ha−1 yr−1 and 0.4 ± 0.05 kg N ha−1 yr−1, respectively). Direct emission factors (EFds) of N2O and NO for the fertilizer nitrogen were estimated to be 0.59 ± 0.04% and 0.44 ± 0.04%, respectively. By summarizing the results of our study and others, we recommended specific EFds (N2O: 0.54 ± 0.09%; NO: 0.45 ± 0.04%) for estimating emissions from irrigated croplands on calcareous soils with organic carbon ranging from 5 to 16 g kg−1. Nitrification dominated the processes driving the emissions of both gases following fertilization. It was evident that insufficient available carbon limited microbial denitrification and thus N2O emission. This implicates that efforts to enhance carbon sink in calcareous soils likely increase their N2O emissions. [Copyright &y& Elsevier]

Abstract: One-year winter wheat–summer maize rotation is the most popular double cropping system in north-central China, and this highly productive system is an important source of nitrous oxide (N2O) and nitric oxide (NO) emissions due to the high fertilizer N and irrigation water inputs. To sustain the high crop production and mitigate the detrimental impacts of N2O and NO emissions, improved management practices are extensively applied. The aim of this study is therefore to evaluate the effects of an improved management practice of irrigation, fertilization and crop straw on grain yield and N2O and NO emissions for a wheat–maize rotation field in northern China. Using automated and manual chamber measuring systems, we monitored N2O and NO fluxes for the conventional (CT, 2007–2008), improved (IT, 2007–2008), straw-amended (WS, 2008–2009), straw-not-amended (NS, 2008–2009), and no N-fertilizer treatments (WS–NN, 2008–2009), respectively, for one rotation-year. The grain yields were determined for CT and IT for three rotation-years (2005–2008) and for WS, NS and WS–NN for one rotation-year (2008–2009). The improved management of irrigation and fertilization reduced the annual N fertilization rate and irrigation amount by 17% and 30%, respectively; increased the maize yield by 7–14%; and significantly decreased the N2O and NO emissions by 7% (p <0.05) and 29% (p <0.01), respectively. The incorporation of wheat straw increased the cumulative N2O and NO emissions in the following maize season by 58% (p <0.01) and 13%, respectively, whereas the effects of maize straw application were not remarkable. The N2O and NO emission factors of applied N were 2.32±2.32% and 0.42±1.69% for wheat straw and 0.67±0.23% and 0.54±0.15% for chemical N-fertilizers, respectively. Compared to conventional management practices using high application rates of irrigation water and chemical N-fertilizer as well as the field burning of crop straw, the improved management strategy presented here has obvious environmentally positive effects on grain yield and mitigation of N2O and NO emissions. [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]