The Characteristics Of Greenhouse Gas Emissions From Rice Paddy Fields In Jianghan Plain

Mitigation of greenhouse gas emissions is a hot topic in the world.Global warming caused by increase in greenhouse gas concentrations results in a series of environmental problems.Paddy field is an impotant source of greenhouse gas methane（CH₄）and nitrous oxide（N₂O）.Annual planting area of rice is about 12 million hm² in Hubei Province.So,it is impotant for studying characteristics of greenhouse gas emission from paddy field in Hubei province to mitigate greenhouse gas emissions of China’s rice fields.At present,DeNitrification-DeComposition（DNDC）model,one of the best models,has been widely used to study greenhouse gas emissions in China.In this study,we assessed greenhouse gase emissions from paddy fields of Jianghan plain that is the typical rice production area of Hubei province using the DNDC model.The field experiment located in Qianjiang city was conducted to observe CH₄ and N₂O annual emissions from paddy fields.The model was verified through comparing the observed data from fields with the simulated data.The climate,soil,crop,agricultural management databases of DNDC was established based on the results of DNDC model testing at site,then greenhouse gas emission characteristics from paddy fields were simulated under different cropping systems,straw returning level,tillage depths and sowing dates in Jiang plain region using the model.The results show that CH₄ fluxes from paddy fields ranged from-2.80 mg·m^-2·h^-1to 55.64 mg·m^-2·h^-1,and N₂O fluxes were 0.0043 mg·m^-2·h^-11.90 mg·m^-2·h^-1 in Jianghan Plain.The results of field experiment conducted in Qianjiang city show that the DNDC model well simulated greenhouse gas emissions from three paddy cropping systems that included middle rice-wheat（RW）,middle rice-rapeseed（RR）and medium rice-fallow cropping systems（RF）.According to the simulation of DNDC model,the CH₄ emissions was estimated to be 0.29 Tg C·yr^-1 and the N₂O emission was 0.0092TgN.yr^-1 from paddy field in Jianghan plain.Under different cropping systems,global warming potential（GWP）followed the order of RW>RR>RF,suggesting that RR system can not only reduce the greenhouse gas emissions but also increase the economic benefits.Under different straw returning levels,CH₄ and N₂O emissions,soil organic C（SOC）,and grain yield increased with increasing the amounts of straw returning.100%straw returning treatment resulted in the highest SOC and grain yields among different straw returning levels.Under different tillage depth treatments,the CH₄fluxes from paddy fields followed the order of tilling at 20cm depth>tilling at 10cm depth>no-tillage,while N₂O fluxes followed the order of no-tillage>tilling at 10cm depth>tilling at 20cm depth.The GWP of tilling at 10cm depth was the lowest.Under different sowing dates,sowing 7 days in advance resulted in the highest CH₄ and N₂O emissions,and sowing 7 days in delay showed the lowest the emissions.In conclusion,100%straws returned into the soils+tilling soil at 10 cm depth+delaying seeding under rice-rapeseed cropping systems can guarantee rice yield,increase soil SOC,and reduce GWP of paddy fields.