Effects Of Applying Organic Fertilizers On N₂O Emissions And Nitrogen Fertilizer Use Efficiency From Farmland: Meta-analysis And Long-term Located Fertilization Experiment

Agroecosystems are a major source of nitrous oxide（N₂O）,a potent greenhouse gas.In recent years,a large amount of nitrogen（N）fertilizers has been applied to agroecosystems for promoting crop yield,while the N fertilizer use efficiency（NUE）declines to 30%.The surplus of N in soil has led to a significant increase in N₂O emissions.The N₂O mitigation is a great challenge for a sustainable agriculture.The combined application of organic and chemical fertilizers is one measure of conservation agriculture,which can reduce the input rate of chemical fertilizers and improve soil fertility.Due to differences in crop types,climatic conditions,soil properties,and management practices,the impacts of organic amendments on N₂O emissions and NUE displayed large heterogeneity in space and time.In this thesis,the effects of organic amendments on N₂O emissions and NUE and their major regulating factors were quantified scientific data for achieving a sustainable agriculture and“carbon neutral”on the basis of meta-analysis and field experiment.Peer-reviewed publications were collected from databases of CNKI,Google Scholar and Web of Science using keywords as“nitrous oxide”,“corn”,“maize”,“wheat”,“wheat”,“rice”and“rice”.Finally,105 references were retained,with 628 valid data recording the N₂O from Chinese cereal cropland since 2006.The dataset was used to evaluate how the effects of organic amendments on N₂O emissions and NUE responding to different climates（climate type,annual average temperature,annual average rainfall）,soil properties（soil organic carbon content,total N content,soil texture,p H）,and management measures（crop types,organic fertilizer types,irrigation methods）.Since the studies involved in the Meta-analysis are mostly short-term fertilization experiments of 1 to 3 years,the effects of long-term application of organic fertilizers on N₂O emissions and NUE still need to be explored.Therefore,a long-term positioning fertilization experiment in the Guanzhong Plain was selected to carry out field observations.The crops were winter wheat-summer maize cropping systems.Field experiment included four treatments:unfertilized control（CK）,full chemical fertilizer（NPK）,maize stalks chemical fertilizer（NPKS）,dairy manure plus chemical fertilizers（NPKM）.The N₂O emissions and crop yields were measured from October 2018 to October 2019.The field experiments were to clarify the seasonal variation of N₂O emissions and their regulating factors,and to assess the impacts of maize stalks and dairy manure on N₂O emissions and NUE.The results of Meta-analysis show that:（1）The N₂O emission,emission factors,yield-scaled N₂O emissions,and crop yield did not relate to the N application rate;while the NUE related negatively to the N application rate（r²=0.13,P<0.01）.（2）The N₂O emissions did not related to NUE,while the yield-scaled N₂O emissions related negatively to NUE（r²=0.23,P<0.01）.（3）Origanic amendments reduced the yield-scaled N₂O emissions and increased crop yields and NUE under the climatic conditions with an average annual temperature of 15～20°C and an average annual rainfall of 750～1000 mm.Organic amendments reduced the yield-scaled N₂O emissions and increased NUE in alkaline soils（p H>7.3）and soils with total N content ranging from 0.5 to 2 g·kg^-1 and soil organic carbon content below 10 g·kg^-1.（4）The application of animal wastes significantly reduced the yield-scaled N₂O emissions and increased crop yields.Straw returning did not display significant effects on the emission factors,yield-scaled N₂O emissions,crop yields and NUE.（5）Organic amendments significantly reduced the yield-scaled N₂O emissions and increased crop yields and NUE in wheat and maize field.Organic amendments did not impact the emission factors,yield-scaled N₂O emissions,crop yields and NUE in rice fields.（6）Organic amendments reduced the yield-scaled N₂O emissions and increase crop yields under split fertilization regime（basic fertilizer and topdressing）.Organic amendments reduced yield-scaled N₂O emissions and increased crop yields and NUE under One-time regime.Organic amendments reduced the yield-scaled N₂O emissions under flood irrigation.The results of the long-term positioning fertilization experiment show that:（1）The N₂O emissions displayed large seasonal variations.The emission peaks[the highest peak 163 g·（hm²·d）^-1]were mostly related to fertilization.Soil moisture and ammonium nitrogen content were the dominant regulating factors for the N₂O emissions in each treatment.（2）The annual N₂O emissions treated by CK,NPK,NPKS and NPKM are 1.05,2.61,2.93,and 2.75 kg·hm^-2,respectively.（3）The yield-scaled N₂O emissions ranged from 0.16 to 0.19 g·kg^-1 across the treatments.Relative to NPK treatment,the NPKM treatment displayed lower yield-scaled N₂O emissions.（4）The crop yields and NUE ranged from 13.0 to 17.6 t·hm^-2,and from 20.8%to 29.4%,respectively,across the treatments.Application of maize stalks and dairy manure increased crop yields and NUE.（5）There yield-scaled N₂O emissions did not relate to NUE in winter wheat season while the relationship was significantly in the summer maize season（r²=71;P<0.01）.In summary,organic amendments reduced the yield-scaled N₂O emissions and increased NUE under the climatic with an average temperature of 15～20℃and an average annual rainfall of 750～1000 mm,and in alkaline soils（p H>7.3）and soils with total N content ranging from 0.5 to 2 g·kg^-1 and soil organic carbon content below 10 g·kg^-1.The application of animal wastes significantly reduced the yield-scaled N₂O emissions and increased crop yields under one-time fertilization regime in wheat and maize field.