Reconstruction Of Historical Nitrogen Fertilizer Data In China And Assessment Of Nitrous Oxide (N₂O) Emissions From Agro-ecosystem

To maintain high productivity and meet the demand for food in China,chemical nitrogen（N）fertilizers have been widely used in agricultural production activities.However,excessive use of N fertilizer has become a serious problem.Due to high N input and low utilization efficiency,N enters water bodies through runoff or lost to the atmosphere in the form of gas,causing significant negative impacts on aquatic and atmospheric environments.The agricultural ecosystem is one of the major sources of nitrous oxide（N₂O）emissions-a long-lived greenhouse gas with strong ozone-depleting capacity.Currently,there is a lack of long-term,spatial explicit N fertilizer application data,leading to large uncertainties in N₂O assessments in China’s agricultural ecosystem.This study reconstructed the N fertilizer application rates for each major crops planted in China from 1952 to 2019,based on various sources of N fertilizer statistics.Based on the reconstructed N fertilizer data,we estimated the N₂O emissions in Chinese cropland using the DNDC（De Nitrification-De Composition Model）model.Furthermore,the simulated,crop-specific N₂O in 2019 were further examined.The N fertilizer management strategies were also discussed to help improve environmental protection and policy making in China.The study arrives at the following conclusions:（1）The total amount of N fertilizer used in China has increased by about 52 times from0.06Tg in 1952 to 31.15Tg in 2014,with the average fertilization rate rising from 0.05g N m^-2yr^-1 in the corresponding year to 18.83g N m^-2 yr^-1,an increase of approximately 377 times.The N fertilizer was most intensively used in the traditional agricultural regions,such as northern China Plain and Sichuan Plain.These astonishing results indicate a tremendous transformation in China’s agricultural ecosystem over the past 60 years.（2）Based on the simulation results from DNDC,it was found that the N₂O emissions has been increasing from 1952 to 2019 in China’s cropland ecosystem.The emission hotspots include the Huadong Plain,Chengdu Plain,Lianghu region,Northeast Plain,and East China.In 2019,the N₂O emissions from mono-crops such as maize,wheat,and rice accounted for 58.8%of the total cropland ecosystem emissions.Among these crops,maize was the highest contributor of N₂O emissions,reaching 0.0863Tg,which accounted for 27.5%of the total emissions from the cropland ecosystem in 2019.