Characteristics Of Greenhouse Gas Emissions And Nitrogen Losses Under Long-term Nitrogen Fertilization And Straw Incorporation In The North China Plain

North China Plain(NCP)has experienced long-term excessive utilization of synthetic nitrogen(N)fertilizer,which results in high risks of greenhouse gas(GHG)emissions and nitrogen losses.Straw incorporation was strongly recommended by researchers and government agencies in NCP,which was believed to have significant effects on the mitigation of GHG emissions and on the control of N losses.Our study established several treatments including different N fertilization and straw management practices,and used meta-analysis,long-term field study and in-situ 15N tracer to evaluate the effects of long-term N fertilization and straw incorporation on GHG emissions,N fates and N losses in wheat-maize cropping system of NCP.The main findings are as follows:(1)Meta-analysis showed that the mean values of N2O emissions in wheat season and maize season of NCP were 0.68 and 2.10 kgN2NO-N ha-1 season-1,respectively.Precipitation,temperature,soil pH and texture had no significant impacts on annual GHG emissions(P>0.05),because of limited variation of these factors in the NCP.The N2O emissions increased exponentially with mineral fertilizer N application rate(P<0.05),with y = 0.239e0.0058x for wheat season and y = 0.365e0.0071x for maize season(P<0.05).Mixed application of organic manure with reduced mineral fertilizer N could reduce annual N2O emissions by 16%relative to mineral N application alone(P<0.05)while maintaining a high crop yield.Straw incorporation significantly increased annual N2O emissions,by 26%relative to straw removal(P<0.05).Compared with conventional tillage,no-tillage significantly reduced N2O emissions by-30%in the wheat season,whereas it increased those emissions by～10%in the maize season(P<0.05).(2)Field study from 2013 to 2016 revealed that,practice of no-tillage and straw incorporation under conventional N fertilization rate(345 kg N ha-1 season-1)would induce high risk of GHG emissions;optimized N fertilization rate(225 kg N ha-season-1)could significantly reduce N2O emission by 31%(P<0.05),while maintained high crop yield,which could be recognized as the most effective measure for N2O mitigation;optimized N fertilization combined with rotary tillage and straw incorporation was the most promising integrated practice for GHG mitigation in the NCP.Compared with initial soil N level of 2009,soil N stock(0-2 m)increased apparently with the rate of 8-419 kg N ha-1 yr-1 after 7-year N fertilization;optimized N fertilization could reduce N losses by the rate of 35 kg N ha-1 yr-1,compared with conventional N fertilization;long-term straw incorporation achieved more N accumulation(34-112 kg N ha-1 yr-1)in 0-2 m soil than straw removal.(3)15N tracer study exhibited that,in long-term high N-fertilization soils,most of the fertilizer-derived N2O emissions were lost in three weeks following fertilization events.In wheat season,fertilizer-derived and non-fertilizer-derived N2O emissions both accounted for about 50%of the total emission,while in maize season,fertilizer-derived and non-fertilizer-derived N2O emissions accounted for about 70%and 30%of the total emission,respectively.Application of N fertilizer had a significant priming effect on the native soil N pool(P<0.05),which may increase the risk of N2O loss from N accumulated in the soil.In maize season,compared to sole N fertilization,N fertilization+straw incorporation would enhance the turnover of native soil N and/or straw N,which in turn increased non-fertilizer-derived N2O emission.(4)Fate of 15N fertilizer showed that,in long-term intensively fertilized soils of NCP,the percentages of fertilizer N recovered in grain,straw,1 m soil,N2O emission and NH3 volatilization were 30%,3%,37%,0.11%and 2%,respectively,in wheat season,and 20%,16%,37%,0.24%and 6%,respectively,in maize season.In the NCP,compared with straw removal,straw incorporation could result in fertilizer-N immobilization by 27-43 kg N ha-1 season-1 and could achieve a reduction of fertilizer-N losses by 13%-42%.Our study highlights that,in long-term intensively managed soils,although straw incorporation has a tendency for enhancing direct GHG emissions,long-term application of straw incorporation integrated with optimized N fertilization and rotary tillage has great potential for GHG mitigation in the NCP.Moreover,both long-term experiment and 15N tracer study showed that straw incorporation had great benefits on the reduction of N losses and the increment of soil N stock,which was worth to be implemented in the NCP.