| Nitric oxide（NO）is a highly active trace gas in the atmosphere,which plays an important role in tropospheric chemistry and photochemistry.NO not only affects the concentration of oxidizing agents（such as hydroxyl free radicals and ozone）in the troposphere,but also promotes the formation of atmospheric particulate matter and acid deposition.It is one of the most important gaseous pollutants affecting human health and ecosystem productivity.Nitric oxide is mainly derived from fossil fuels and biomass combustion,and agricultural soil is the main source of NO emission in rural areas.Excessive fertilization is an important reason for the increase of NO emission.A large number of studies have shown that straw returning and cow dung application can alleviate some of the environmental problems caused by excessive fertilizer application（such as soil degradation,nitrate leaching,ammonia volatilization,etc.）,but their effects on NO emission are highly spatiotemporal.Since climate,soil and farmland management measures all have significant effects on NO emissions,in-situ observation of NO emissions from farmland under specific conditions is a basic method to quantitatively study NO emissions and its influencing factors.At present,most of the in-situ observation experiments adopt the artificial static box method,which is trapped by the technical problem of low sampling frequency,leading to the difference between the field observation results and the actual situation.However,the model simulation can well compensate for this defect.In recent years,the establishment and development of mathematical models has gradually become an important means to study NO emissions.Therefore,in situ observation and model simulation were used in this study.On the one hand,annual observation was conducted on NO emission and its main regulating factors in winter wheat-summer maize fields in Guanzhong Plain,and the effects of long-term straw returning and cow dung application on crop yield and NO emission were evaluated.On the other hand,a boundary line model based on a few parameters was established,and the accuracy of the simulation results of NO emission flux by this model was verified.The main purpose is to provide basic data support for the accurate assessment of regional NO emission and its emission reduction.The annual observation of NO emission flux from winter wheat to summer corn fields in Guanzhong Plain was conducted from October 2018 to October 2019,and the observation method was artificial static box method.The field trials were based on long-term localized fertilization trials（started in 1990）and included total fertilizer（NPK）,fertilizer plus straw（NPKS）,fertilizer plus dairy manure（NPKM）,and no fertilizer（CK）treatments.Based on the observation results of the previous period（October 2016 to October 2018）and the boundary analysis method,a boundary model with soil temperature,void water content and nitrogen substrate content as input parameters was constructed,and the simulation results of the model were tested with the NO emission flux of the current observation year.The main results of field observation experiment are as follows:（1）There was a significant seasonal variation of NO emission under fertilization[-0.21～33.8 g·（hm2·d）-1)],and the cumulative NO emission in summer maize season was significantly higher than that in winter wheat season（p<0.05）;Fertilization is the main cause of NO pulse emission.（2）The emission coefficients of NPK,NPKS and NPKM were 0.08%,0.08%and0.13%,respectively,which were all lower than the average emission coefficients of 0.7%in the global fertilized farmland.The annual cumulative emissions of the three fertilization treatments were 0.49,0.68 and 0.65 kg·hm-2,respectively.Compared with the application of chemical fertilizer,straw returning and cow dung application increased NO emissions by32.7%and 38.8%,respectively（p<0.05）.（3）The NO fluxes of the four treatments were significantly correlated with soil temperature（p<0.05）;Multiple regression analysis showed that NO flux was related to soil WFPS and nitrogen substrate concentration.（4）NPKS and NPKM increased the total crop yield by 17.1%and 17.5%,respectively.The model simulation results are as follows:（1）The boundary line model can well simulate the variation of NO flux under three fertilization treatments in Guanzhong Plain,and the variation of NO flux between simulated values and field observations is basically consistent.There was a significant correlation between the simulated values and the observed values in the field（p<0.001）.（2）The simulated value of the boundary line model for the annual total amount of NO under fertilization treatment was higher,and the NPK,NPKS and NPKM were overestimated by 32%,29%and 24%,respectively.The main conclusions are as follows:（1）Compared with chemical fertilizer application alone,organic materials returning to the field could increase the crop yield of wheat and maize seasons in Guanzhong Plain,but at the same time,it also increased the total annual NO emission in the field.The emission coefficients of the three treatments are low,which are all less than 0.7%of the emission coefficients of the global fertilizing fields.（2）The boundary line model can well simulate the dynamic change of NO and the total emission under fertilization in Guanzhong Plain.However,additional environmental factors are needed to construct a more widely applicable and accurate boundary model. |
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