[Nitric Oxide Emissions from Chinese Upland Cropping Systems and Mitigation Strategies: A Meta-analysis].

Agroecosystems are a significant source of nitric oxide (NO), a potent atmospheric pollutant. It has been well documented that the NO emissions from upland cropping systems and their emission factors are large relative to those from paddy fields. However, a clear understanding of their uncertainty and regulating factors is still lacking. To date, various field experiments have been conducted to investigate NO emissions and mitigation, providing an opportunity for a Meta-analysis. The aims of this study were to 1 investigate the uncertainty and regulating factors of NO emissions and emission factors from maize-winter wheat rotations, non-waterlogging period in rice-winter wheat rotations, vegetable fields, tea plantations, and fruit orchards across China by extracting data from peer-reviewed publications, and 2 quantify the mitigation potential of management practices, such as reducing nitrogen fertilizer input, organic substitution with chemical fertilizers, and application of enhanced-efficiency nitrogen fertilizers or biochar by performing a pairwise Meta-analysis. A total of 49 references (published from 2006 to 2021) were collected. The results showed that annual NO emissions from the maize-winter wheat rotations, tea plantations, and fruit orchards averaged 1.44, 7.45, and 0.92 kg·hm-2, respectively, with significant differences among the three cropping systems (P<0.05). The seasonal NO emissions from the non-waterlogging period in rice-winter wheat rotations and vegetable fields within a single growth period averaged 2.13 kg·hm-2 and 2.09 kg·hm-2, respectively. The NO emissions positively related to nitrogen inputs in the maize-winter wheat rotations, non-waterlogging period in rice-winter wheat rotations, and tea plantations (P<0.01) but not in the vegetable fields and fruit orchards. The emission factors averaged 0.31%, 0.71%, 0.96%, 1.74%, and 0.13% in the maize-winter wheat rotations, non-waterlogging period in rice-winter wheat rotations, vegetable fields, tea plantations, and fruit orchards, respectively, with significant differences among the cropping systems (P<0.01), except between the maize-winter wheat rotations and non-waterlogging period in rice-winter wheat rotations or vegetable fields (P>0.05). Considering the substantial differences in emission factors among the cropping systems, a specific emission factor for each system should be applied when estimating an agricultural NO budget at a regional or national scale. Reducing nitrogen input only mitigated NO emissions (by 36%) at a reducing nitrogen ratio above 25% but did not impact emission factors. An optimal reducing nitrogen ratio has to be further evaluated without crop productivity penalties. Organic substitution in soils with organic carbon content<15 g·kg-1 or pH<7 and application of enhanced-efficiency fertilizers in the maize-winter wheat rotation simultaneously mitigated NO emissions (by -46%- -38%) and emission factors (by -62%- -45%). By contrast, biochar amendment had no significant effects on either NO emissions or emission factors. These findings highlight a possibility of choosing an effective NO mitigation strategy under specific field conditions.

[Effects of Stalk Incorporation on Soil Carbon Sequestration, Nitrous Oxide Emissions, and Global Warming Potential of a Winter Wheat-Summer Maize Field in Guanzhong Plain].

The net greenhouse gas emissions from upland soils, as indicated by global warming potential (GWP), mainly depend on the soil carbon sequestration and nitrous oxide (N2O) emissions. The annual changes in surface (0-20 cm) soil organic carbon (SOC) content from 2010 to 2017 and the N2O emissions from 2014 to 2017 were measured within a long-term fertilization experiment. The objective was to quantify the effect of stalk incorporation on the soil carbon sequestration, annual N2O emissions, and GWP of a winter wheat-summer maize field in the Guanzhong Plain. The field experiment included three treatments:conventional fertilization (CF), conventional fertilization plus maize stalks (CFS), and an unfertilized control (CK). The CF and CFS treatments received the same amount of urea per year, with nitrogen (N) input at 165 kg·hm-2 and 188 kg·hm-2 in the winter wheat season and summer maize season, respectively. The CF treatment retained the stubbles (about 10 cm above ground) when harvesting the winter wheat and summer maize crops. The CFS treatment retained the same wheat stubbles and all maize stalks (containing approximately 40 kg·hm-2 of N). The CK treatment was unfertilized throughout the year, with the stubble management the same as that in the CF treatment. The results showed that the CK treatment displayed few changes in SOC content and low N2O emissions, with GWP varying from 0.04 to 0.11 t·(hm2·a)-1. The SOC contents in the CF and CFS treatments increased linearly with the fertilization years (P<0.001), and their SOC sequestration rates were 0.69 t·(hm2·a)-1 and 0.97 t·(hm2·a)-1, respectively. The N2O emissions from the CF and CFS treatments varied from 1.65 to 5.36 kg·(hm2·a)-1 and from 3.08 to 7.73 kg·(hm2·a)-1, respectively. The annual N2O emissions from the CFS treatment were 43%-94% higher than those from the CF treatment, whereas the difference was only significant between 2015 and 2016 (P<0.05). The GWP of the CF and CFS treatments varied from -1.95 to -0.28 t·(hm2·a)-1 and from -2.59 to -0.35 t·(hm2·a)-1, respectively. The cumulative GWP of the CFS treatment was 42% lower than that of the CF treatment between 2014 and 2017. In summary, the studied winter wheat-summer maize field acted as a sink of greenhouse gases under the conventional fertilization regime. The stalk incorporation further favored greenhouse gas mitigation despite the trade-offs between SOC sequestration and N2O emissions.

[Effect of Long-term Dairy Manure Amendment on N2O and NO Emissions from Summer Maize-Winter Wheat Cropping Systems].

Annual nitrous oxide (N2O) and nitric oxide (NO) emissions were measured within a 27 year fertilization experiment in Guanzhong Plain. Gas samples were collected using static chambers from June 2017 to June 2018. The primary objectives of this study were to quantify the variations in N2O and NO emissions and evaluate the effect of manure amendment on gas losses. Three treatments were set up in the field using a completely random block design. The control treatment (CK) remained unfertilized throughout the year. The synthetic fertilizers (NPK) and NPK plus dairy manure (NPKM) treatments received an annual nitrogen (N) input at a rate of 353 kg·hm-2. In the summer maize season, the NPK and NPKM treatments received urea as a N source at 188 kg·hm-2. In the winter wheat season, the NPK treatments received urea at 165 kg·hm-2. The NPKM treatment received the same amount of N as the NPK treatment but with 30% from urea and 70% from dairy manure. The results showed that N2O and NO emissions from the CK treatment were consistently low during the experimental period. Large emission peaks were captured in the NPK and NPKM treatments, mostly responding to fertilizer application and irrigation. The largest N2O and NO peaks were up to 103.0 g·(hm2·d)-1 and 71.0 g·(hm2·d)-1, respectively, and both occurred in the NPKM treatment during the summer maize season. The NO/N2O ratio was negatively related to soil water-filled pore space (P<0.01) at soil temperatures above 20℃ for the NPK and NPKM treatments, indicating the regulatory effect of soil temperature and water content on gas fluxes. Annual N2O emissions from the CK, NPK, and NPKM treatments were 0.21 kg·hm-2, 2.32 kg·hm-2, and 2.15 kg·hm-2, respectively, with a non-significant difference between the NPK and NPKM treatments (P=0.74). Annual NO emissions from the CK, NPK, and NPKM treatments were 0.23 kg·hm-2, 0.80 kg·hm-2, and 1.46 kg·hm-2, respectively, with a significant difference between the NPK and NPKM treatments (P<0.05). We concluded that long-term dairy manure amendment did not influence N2O emissions but increased NO emissions.

[Effect of Long-term Organic Amendments on Nitric Oxide Emissions from the Summer Maize-Winter Wheat Cropping System in Guanzhong Plain].

Agricultural soil is a significant source of nitric oxide (NO). The primary aim of this study was to quantify the effect of long-term organic amendments on NO emissions from the summer maize-winter wheat cropping system in Guanzhong Plain. NO fluxes were regularly measured by the static chamber method for one year (June 2016 to June 2017). Field experiments included four fertilizer treatments that commenced in 1990. The control (CK, 0 kg·hm-2) treatment was unfertilized throughout the years. The fertilized treatments were synthetic fertilizer (NPK, 165 kg·hm-2), synthetic fertilizer plus maize stalk (NPKS, (165+40) kg·hm-2), and synthetic fertilizer plus dairy manure (NPKM, (50+115) kg·hm-2) during the winter wheat season. They were fertilized with synthetic fertilizer (188 kg·hm-2) during the summer maize season. The results showed small NO emission [<12.2 g·(hm2·d)-1] from the CK treatment within the experimental period. Large NO fluxes [up to 112.0 g·(hm2·d)-1 in NPK treatment] were captured following sowing and fertilization during the summer maize season and following fertilization during the winter wheat season for all fertilized treatments. Annual NO emissions and direct emission factors ranged from 0.13 to 0.57 kg·hm-2 and from 0.04% to 0.12%, respectively. Annual NO emissions from the NPKS and NPKM treatments were 17.6% lower and 68.0% (P<0.05) larger than those from the NPK treatment, respectively. Seasonal NO emissions from the NPKS and NPKM treatments were 41.1%-60.0% (P<0.05) lower than those from the NPK treatment during the winter wheat season, indicating that organic amendments reduced NO emissions. Seasonal NO emissions from the NPKS and NPKM treatments were 25.2%-292.1% (P<0.05) larger than that from the NPK treatment during the summer maize season, mostly due to the positive effect of soil organic matter content on NO emissions.

[Effects of Long-term Organic Amendments on Soil N2 O Emissions from Winter Wheat-maize Cropping Systems in the Guanzhong Plain].

The primary aim of this study was to quantify the effects of long-term organic amendments on soil nitrous oxide (N2O) emissions. Using static chamber-gas chromatograph technique, we measured N2O fluxes from winter wheat-maize rotation system and related environmental factors in the Guanzhong Plain for one year (October 2014 to October 2015). Field experiments were based on the "Chinese National Loess Fertility and Fertilizer Effects Long-term Monitoring Experiment". Four treatments were control (CK, 0 kg·hm-2), NPK (NPK, 353 kg·hm-2), NPK combined with maize straw[NPKS, (353+40) kg·hm-2] and cattle waste[NPKM, (238+115) kg·hm-2]. During the experimental period, N2O fluxes from CK treatment were small[<2.9 g·(hm2·d)-1]; while emissions from fertilized treatments peaked after fertilization[up to 113.4 g·(hm2·d)-1 for NPKS] and irrigation[up to 495.0 g·(hm2·d)-1 for NPKM] during winter wheat and maize seasons, respectively. N2O flux was significantly correlated to soil water-filled pore space for all treatments (r>0.28,P<0.05). Annual N2O emissions were (0.1±0.0), (2.6±0.1), (3.4±0.7) and (2.9±0.3) kg·hm-2 for CK, NPK, NPKS and NPKM, respectively. The fertilized treatments released higher N2O emissions than CK treatment (P<0.05), indicating that fertilization stimulated N2O emissions. However, the differences in N2O emissions were not significant among the fertilized treatments (P=0.06), suggesting that organic amendments did not increase N2O emissions obviously. The direct emission factors were 0.72%, 0.83% and 0.80% for NPK, NPKS and NPKM, respectively, all of which were lower than the IPCC default of 1%. The yield-scaled N2O emission for NPKM was the lowest among the fertilized treatments.

[Characteristics of nitrous oxide flux in spring wheat field ecosystem in Sanjiang Plain of Northeast China].

With static-closed-chamber and GC (Agilent 4890), the nitrous oxide (N2O) flux in spring wheat field ecosystem in the Sanjiang Plain of Northeast China was continuously observed in situ from May 2004 to October 2006. The results showed that the N2O flux presented obvious seasonal and inter-annual variation, which was mainly related with precipitation and field water management. The diurnal variation of N2O flux had correlations with the air temperature and the temperature at 5 cm underground. Wheat field ecosystem was the strong sources of atmosphere N2O during plant growth season. The N2O flux decreased obviously in fallowing period, was weaker in freezing period, and increased slowly when soil thawing. The average N2O flux in wheat growth season was 0.190 mg x m(-2) x h(-1), the flux was 0.077 mg x m(-2) x h(-1) from after-reaping to before-freezing period, and 0.017 mg x m(-2) x h(-1) in freezing and thawing period.