Optimizing Nitrogen Management And Quantitative Analysis Of Their Environmental Effect

Agricultural practices have significant effects on crop productivity,nitrogen leaching as well as on greenhouse gas?GHG?emissions.High nitrogen input leads to environmental problems,such as greenhouse gas?GHG?emissions and groundwater nitrite pollution.There is an urgent need to re-assess the management strategies to improve the eco-efficiency of the cropping systems,that is,to maintain the same productivity with less GHG emissions and other environmental costs.In this study,we chose Huantai County as the study area,and winter wheat-summer maize rotation system as research object.Based on the previous work in our research group,the database of basic physiochemical properties and water and nitrogen management of Huantai County was improved through collecting historical information,questionnaire survey and GIS software.On the basis of this database,we used APSIM model,after calibration and validation,to simulate crop growth,water and nitrogen use efficiency,nitrogen leaching,and greenhouse gas emissions under different water and nitrogen management in 60 years at County Scale,and quantitatively analysed the simulated results to obtain the improved management.In addition,we also chose Gongzhuling Couty?Jilin Province?as research site.Long-term dynamics of yield and soil GHG emissions under differen management scenarios in a maize mono-cropping system of northeast China were estimated using a farming system model?APSIM?.Combining with life cycle assessment and questionnaire survey among farmers,we estimated life-cycel GHG emissions and GHG intensity.The main conclusions are as follows:1.Huantai County?1?After the directly calibration of parameters relating to the change soil organic carbon based on the previous research,we conducted validity test on APSIM?Agricultural Production Systems sIMulator?using the experimental data at Huantai of Shandong Province in North China,and the results showed that the calibrated model can well simulated aboveground biomass?Coefficient of determination R²>0.79?,yield?R²>0.76?,nitrogen in aboveground biomass?R²>0.88?and grain?R²?0.79?,N₂O emissions?R²=0.85?,and soil water dynamic of 0-20 cm?R²=0.76?and soil nitrogen dynamics of 0-200 cm?R²>0.29?.Although there existed potential problems in term of daily N₂O emissions simulation by APSIM,it can well reflect the effect of different N?N?application on the total emissions in a growing season and a year.?2?Under conventional irrigation with total irrigation amount of 420 mm and 7 times of irrigation,reducing nitrogen input by 15%is feasible to maintain crop yield,while reducing N input by 30%will incur obverse yield reduction.Automatic irrigation based on soil moisture could increase crop yield via improve the water use efficiency and partial factor productivity of N.under the scenario of reducing nitrogen input by 30%with automatic irrigation,the total irrigation amount is 330 mm with 6 times of irrigation,while the yield is only decreased by 4%comparing with that of conventional water and nitrogen management,which indicated that the nitrogen and irrigation water input could be further reduced.?3?Under conventional water and nitrogen management,the average N leaching and N₂O emissions are?110 and?5.17 kg N/ha,respectively.When multiplying by the cropland area,the total N leaching and N₂O emissions are?4149 and?194 Mg N in Huantai County,respectively.The nitrogen input in the northeast and southwest part of Huantai County is higher and incurred higher N leaching and N₂O emissions than other parts.Under the same nitrogen input,automatic irrigation could reduce N leaching,while increase N₂O emissions.?4?Under current technology,reducing nitrogen input by 15%is feasible to decrease N leaching by 39%??2076 Mg N?and greenhouse gas emissions by 17%??54Gg CO₂-eq?while still maintaining crop yield in Huantai County.However,with the development of soil electronic sensor and irrigation equipment,if the automatic irrigation could be applied,and combining with optimized N input,the N leaching and greenhouse gas emissions could be further reduced.2.Gongzhuling site?1?The life-cycle GHG emissions?GHGL?and GHG intensity?GHG emissions per unit yield?of current farmers' practice in the studied system were about 4.37 Mg CO₂-eq ha^-1 yr^-1 and 0.49 Mg CO₂-eq Mg-1,respectively,and emissions from machinery input?including production,transportation,repair and maintenance?and soil organic carbon?SOC?change accounted for 14.6%of GHGL,while emissions from nitrogen fertilizer input?production and transportation?and direct N₂O emissions accounted for the majority??60%of GHGL?.?2?Scenario analysis suggested that current farmers' practice in terms of N application and residue management are nearly optimal for crop production but not for climate change mitigation.?3?Improving management by reducing N input by 25 kg N ha^-1 and returning the burned or abandoned straw to field can not only secure crop yield and maintain SOC balance,but also can reduce GHGL and GHG intensity by 561 kg CO₂-eq ha^-1 yr-¹ and 0.06 Mg CO₂-eq Mg^-1,respectively.However,it is not feasible to incorporate the straw used as energy source into soil,which could incur substantial fossil fuel CO₂ emissions resulting from the substitution of coal for straw.These results highlight opportunities to further optimize management strategies?especially for the nitrogen and straw management?to reduce GHG emissions while maintaining high yield.