Effects Of Long-term Dairy Manure Application On Nitric Oxide Emissions From Cropland: A DNDC Modelling Study

Nitric oxide（NO）is an active gas in the atmosphere.The continuous increase of anthropogenic emissions will have an important impact on the formation of ozone,photochemical smog,smog and acid rain,thus bringing serious environmental problems.Due to excessive application of nitrogen（N）fertilizer in agricultural production,farmland has become an important anthropogenic source of NO emission.The ecological environment in Guanzhong Plain is fragile.Therefore,manure returning is widely used in this area to slow down soil degradation,improve soil fertility and food yield,and affect soil NO emission by regulating the balance of carbon（C）and nitrogen（N）substrates and microbial and enzyme activities in soil.The results of many studies show that the research conclusions on the impact of manure returning to the field on NO emission are inconsistent,and previous studies mainly focus on the short-term impact of manure replacing chemical fertilizer,lacking a clear understanding of the long-term impact of manure replacing chemical fertilizer.Therefore,it is necessary to quantitatively study the long-term effects of manure instead of chemical fertilizer on NO emission.In this study,two field treatments are set up at the National Soil Quality Observation and Experimental Station in Yangling District,Guanzhong Plain,which are total chemical fertilizer（NPK）and chemical fertilizer+dairy manure（70%NPKM）,and two treatments（40%NPKM and 100%M）are added when DNDC model is used for long-term simulation,and the nitrogen application rate of each treatment is the same.The DNDC model is calibrated and validated based on field measurements for 3 years.The long-term changes of soil nitric oxide（NO）emission and crop yield under different fertilization practices under different future climate scenarios（SSP245,SSP370 and SSP585）are quantitatively simulated,and the long-term effects of different fertilization practices on soil nitric oxide（NO）emission and crop yield are evaluated,and potential emission reduction strategies are explored.The model simulation results show that:（1）The DNDC model can effectively simulate the annual and seasonal changes of soil temperature,moisture,crop yield and NO emission in different treatments,and accurately reflect the effects of different climatic conditions and farmland management measures such as fertilization and irrigation on crop yield and NO emission.The evaluation parameters of crop yield in different fertilization treatments,MBE=-125.20～157.17kg·hm^-2,n RMSE=1.60%～ 4.13%,R²=0.84～0.98,The evaluation parameters of NO emission,MBE=-0.03～0.08kg·hm^-2,n RMSE=7.27%～10.83%,R²=0.96～0.98.However,DNDC model is poor in simulating the diurnal variation of soil NO₃^--N,NH₄⁺-N content and NO emission flux.（2）Sensitivity analysis of seven driving factors（annual average temperature,annual precipitation,soil clay ratio,soil SOC content,nitrogen application rate,irrigation rate and tillage depth）are carried out.The results show that crop yield under different fertilization measures is most sensitive to annual precipitation;Soil NO emission under different fertilization measures is most sensitive to nitrogen application rate.Under different driving factors,the absolute values of sensitivity parameters of 70%NPKM treatment is lower than those of NPK treatment,which indicates that returning dairy manure to the field can effectively maintain the soil environment.（3）NO emission and crop yield under different fertilization treatments are affected by both air temperature and precipitation.The increasing rates of NO emission,crop yield and yield-scale NO emission per unit are in the range of 12.8～64.0%,10.8～21.5%,-0.8～38.7%,respectively,and follow the rule of 100%M>70%NPKM>40%NPKM>NPK.Under equal nitrogen treatment,compared with NPK treatment,continuous application of dairy manure returning to field for 110 years could significantly increase the yield of corn and wheat（P<0.05）.The higher the proportion of dairy manure substitution,the shorter the time required for wheat to reach the best yield.The standard deviation of crop yield under different fertilization treatments show that returning dairy manure to field is beneficial to improve the stability of maize and wheat.（4）Under equal nitrogen treatment,compared with single application of chemical fertilizer,short-term（about 16 years）continuous application of dairy manure returning to field can reduce NO emission.From the scale of 110 years,the continuous application of dairy manure returning measures will increase NO emission and offset the NO emission reduction effect of the previous treatment.Therefore,in the long run,returning dairy manure to farmland is not only ineffective in reducing NO gas emission in Guanzhong area,but also significantly increases NO emission.In a word,DNDC model can be used to simulate the dynamic changes of crop yield and soil NO under the measures of returning dairy manure to field in Guanzhong area.Long-term dairy manure returning to field can effectively improve and ensure crop yield and production stability,but it will increase NO emission from farmland soil at the same time.The long-term simulation results of DNDC model show that in order to ensure a certain crop yield level and reduce NO emission due to fertilization in the summer maize-winter wheat rotation model in Guanzhong area,full chemical fertilizer treatment is the best fertilization scheme in this area.