Simulation Research On Greenhouse Gas Emission Of Major Rice-based Cropping Systems In Hubei Province

Global warming which caused by greenhouse gases（GHGs）has received widespread attention,and the resulting environmental problems have gradually affected human life,and the reduction of GHG emissions is very important.Rice fields are considered to be an important source of GHG emissions.Hubei Province plays a pivotal role in food production in our country,with a large area of paddy fields and rich and diverse planting systems.Studying the characteristics of GHG emissions and emission reduction potential of different rice-based cropping systems in Hubei Province is of great significance to the sustainable development of agriculture.This study used the DNDC model to simulate the GHG emissions of rice-wheat system（RW）,rice-rape system（RR）,double rice system（DR）,rice-fallow system（RF）and rice crayfish coculture system（RC）in Hubei Province,and took Qianjiang City as an example to explore the influencing factors of GHG emissions under different rice-based cropping systems.Combining the DNDC model and remote sensing technology,it more accurately reflected the current status of GHG emissions from rice fields in Qianjiang City.In addition,it has been proved that factors such as the amount of straw returning to the field,the nitrogen application rate and the time of drying the field in the management measures can affect the GHG emissions.The main findings are as follows:（1）The actual measurement results showed that there were some differences in seasonal GHG emissions among different rice-based cropping systems,and the annual global warming potential（GWP）was mainly from CH₄ emissions in rice season.The proportion of CH₄ in annual GWP of RC,RF,RW,RR and DR was 95.23%,94.30%,94.88%,70.00%and 93.17%,respectively,while the proportion of CH₄ emission of rice season in annual CH4 emission in each rice-based cropping system was 74.77%,82.17%,98.55%,95.74%and 100.00%,respectively.The contribution of N₂O to annual GWP was small,and it was closely related to winter crop type and field management.The DNDC model was calibrated and verified with the measurement results.Various evaluation indicators indicated that the DNDC model was suitable for the simulation of GHG emissions in the main rice-based cropping system in Hubei Province.（2）The sensitivity of various types of parameters to GHG emissions was different under different rice-based cropping systems.Among the soil parameters,the bulk density clay content,SOC and p H value have the most significant effects on GHG emissions.Among the meteorological parameters,except for the RC,the annual average temperature and atmospheric CO₂ concentration are more sensitive parameters,and the water drop in the RW and RR are also more sensitive parameters.Among the management parameters,the amount of nitrogen application,the time of drying the field and the amount of straw returning to the field have the greatest impact on the GHG emissions of each model.（3）The annual CH₄ emissions and the annual GWP per unit area of Hubei Province from 2006 to 2015 were as follows:RF<RR<RF<DR,while the overall performance of the annual N₂O emissions per unit area were RF<DR<RW<RR.The annual GWP per unit area of Shennongjia Forest Region,Suizhou City,Xiangyang City,Yichang City,Shiyan City and Enshi Tujia and Miao Autonomous Prefecture were at low levels,so they are more suitable to develop rice-based cropping systems in terms of agricultural emission reduction.（4）The area of RW and RR showed a fluctuating upward trend,while the area of DR and RF showed a slow downward trend in Hubei Province from 2006 to 2015.The simulation results show that the GHG emission trends of each system were consistent with the area change trends.The CH₄ emissions of the main rice-based cropping systems in Hubei Province from 2006 to 2015 were 5.60×10⁵ t·yr^-1,the N₂O emissions were5.29×10³ t·yr^-1,and the GWP was 2.06×10⁷ t CO₂-eq·yr^-1.The main rice-based cropping systems contribute to the GWP as RR（34.35%）>RW（31.62%）>DR（24.39%）>RF（9.64%）.The regional GHG emissions were highest in Jingzhou City,Huanggang City,Xiaogan City,Xiangyang City and Jingmen City,while Shennongjia Forest District,Ezhou City,Shiyan City and Qianjiang City were the lowest.（5）The RC has the effect of emission reduction,and each systems can achieve the greenhouse gas emission reduction target by appropriately reducing the amount of straw returning and nitrogen application and increasing the time of field drying.Combined with remote sensing technology,a 1 km×1 km area simulation of Qianjiang City in 2019 was carried out,and the overall emission status of Qianjiang City was more accurately visualized.In 2019,the area of RC extracted by remote sensing in Qianjiang City was58395.64 hm²,and the area of other systems was between 10930.95 hm²-25793.52 hm².The CH₄ and N₂O emissions of RC were 394.50 kg·hm^-2 and 1.43 kg·hm^-2,both lower than Other systems.RC had the effect of carbon sequestration and emission reduction.After switching to other systems,Qianjiang’s CH₄ emissions will increase by2.31%-11.25%,N₂O emissions will increase by 11.49%-67.09%,and carbon fixation will be reduced by 9.95%-22.81%.The amount of straw returned to the field had the most significant impact on CH₄ emissions.Compared with the current half amount of straw returned to the field,the non-return of straw will reduce the CH₄emissions by13.21%-46.82%.Nitrogen application rate has the greatest impact on N₂O emissions.Compared with the current conventional nitrogen application level,a 20%reduction in nitrogen will reduce N₂O emissions by 5.35%-17.91%.The effect of the time of drying the field on CH₄ and N₂O emissions is opposite.Compared with the current light drying the field,severe drying the field will increase CH₄ emissions by 17.27%-23.36%,and increase N₂O emissions by 6.03%-10.30%.Increasing the time of drying the field will reduce the comprehensive GWP.