Comprehensive Quantification Of Global Cropland Nitrous Oxide And Ammonia Emissions Based On DNDC Model

Chemical nitrogen（N）fertilizers are intensively used in agricultural activities to maintain optimum yield.However,high N fertilizer inputs and low N use efficiency lead to more N loss from cropping system into water bodies through runoff and into atmosphere by gas emissions.Agricultural soil is the main source of nitrous oxide（N₂O）and ammonia（NH₃）emissions.As one of the important long-lived greenhouse gases,N₂O is the major substance destroying stratospheric ozone.As an essential alkaline gas,NH₃ can react with acidic substances to form ammonium salts,which contribute to fine and specific pollution,lead to regional haze,and degrade human health.However,the global magnitude and spatial and temporal patterns of cropland N₂O and NH₃ emissions still remain unclear.Mitigation measures also are not discussed comprehensively.Here,we estimate global cropland N₂O and NH₃emissions based on DNDC model,elucidate temporal trends and spatial patterns,project future N₂O and NH₃ emission and discuss potential mitigation strategies.The main research conclusions are as follows:（1）The magnitude of cropland N₂O and NH₃ emissions are clarified.Global cropland N₂O emissions are estimated to be 3.6 Tg N yr^-1.Chemical N fertilizers,crop residues plus N fixation and N deposition account to 48%,27%and 25%of total cropland N₂O emissions,respectively.Although N deposition only account for 6%of soil N inputs,emission factors of N deposition are 7 times than that of N fertilizer in croplands through a more direct way for nitrification and denitrification.Farmland NH₃ emissions are estimated to be 26 Tg N yr^-1,accounting for about 26%of total soil N losses.Chemical N fertilizers are the largest contributor to cropland NH₃ emissions,explaining about 88%of total cropland NH₃ emissions(22.5 Tg N yr^-1).（2）The global cropland N₂O and NH₃ emissions have regional and time-varying characteristics,including several major hotspots:Eastern China,India,Thailand,Eastern United States and Western Europe.N₂O and NH₃ emissions in Eastern China are much higher than other hotspots,and are approximately 1.6 times and 3.3 times that of the Eastern United States,respectively.The largest growth rate of N₂O and NH₃ emissions occur in Eastern China(3.5%yr^-1 and 2.3%yr^-1),followed by India(3.6%yr^-1 and 2.1%yr^-1)and Eastern United States(2%yr^-1 and 1.9%yr^-1)during1996-2013.（3）Predict future N₂O and NH₃ emissions and discuss mitigation measures.Cropland N₂O emissions will increase at 0.9%yr^-1 and 1.4%yr^-1 from 2010 to 2100under RCP4.5 and RCP8.5 scenarios,respectively.At the end of the 21st century,N₂O emissions will reach the maximum(7.6 and 11.8 Tg N yr^-1 under RCP4.5 and RCP8.5,respectively).Cropland NH₃ emissions will increase by 10%compared with 2010under the no N fertilizer increase scenario.Meanwhile,considering that control policy may be launched in the future in countries such as Western Europe,the United States and China,if N fertilizer use changes by-0.5%yr^-1（under current technology and agricultural management levels）,NH₃ emissions will decrease by 18-71%.Finally,the potential mitigation practices of N₂O and NH₃ were discussed from the aspects of fertilization amount,N fertilizer types,and agricultural management measures.