Carbon Footprints And Carbon Sequestration Capacity Of Different Rice Cropping Systems In The Middle Yangtze River Region

Increased global greenhouse gas?GHG?emissions have resulted in global warming and raised public concern.Agriculture production systems play an important role in GHG emissions,and paddy fields are the main source of CH₄ emissions and important carbon?C?sequestration site.Therefore,optimizing rice planting system may be an effective strategy for reducing greenhouse gas emissions and improving carbon sequestration capacity of the paddy agroecosystems.It is of great significance for China to develop low-carbon agriculture with characterics of agricultural energy conservation and emission reduction.Using the life cycle assessment?LCA?,in which a system boundary was defined as the duration from rape seeding to rice harvest,a field experiment was established during 2017-2019 in Modern Agriculture Demonstration Center of Wuxue,Hubei,to evaluate the carbon footprint per unit area?CF_a?and per kg yield?CF_y?,the carbon sequestration capacity per unit area?NEP_a?and per kg yield?NEP_y?of six rice-based cropping systems in the middle Yangtze river region,and to identify the sustainable rice cropping system for balancing low carbon footprint,high carbon sequestration capacity,and high yield.The six rice cropping systems included fallow-early rice-late rice?FEL?,rape-early rice-late rice?REL?,fallow-ratoon rice?FRa R?,rape-ratoon rice?RRa R?,fallow-middle rice?FMR?,and rape-middle rice?RMR?.In current study,static opaque chamber-gas chromatograph method was employed to measure GHG?CO₂,CH₄,and N₂O?emissions continuously,and the C contents of the harvested grains,roots,straws,and leaves were analyzed separately using elemental analyzer,the soil organic carbon?SOC?concentration was determined by the potassium dichromate?K₂Cr₂O₇?heating method.The main results are as follows:?1?The rape yield had no significant difference among the three rape-rice croppingsystems?REL,RRa R and RMR?,and the total of rice yields decreased in the order of double rice cropping system>ratoon rice cropping system>middle rice cropping system;FEL?16.16?>FRa R?15.41?>RMR?13.22?>FMR?10.81?;Compared to FEL,FRa R and FMR under fallow-rice system,the annual yields in the REL,RRa R and RMR under rape-rice system increased by 20.0%,7.1%and 22.3%,respectively;Therefore,REL and RRa R systems consisting of three crops growth seasons are conducive to increase in grain yield.?2?Compared with the FEL,FRa R and FMR systems under fallow-rice croppingsystems,the annual indirect GHG emissions of the REL,RRa R and RMR systems under rape-rice cropping systems increased by 36.7%,50.4%and 73.1%,respectively;the annual indirect GHG emissions of middle rice cropping system were significantly lower than that of ratoon rice cropping systems,followed by double rice cropping systems.The emissions from the diesel consumption were the main components of the annual indirect GHG emissions,followed by fertilizer.?3?The annual CH₄ emissions were mainly occurred in the rice growing period,in which CH₄ emissions were the largest contributor of annual total GHG emissions among the six rice cropping systems;the annual N₂O emissions had little contribution to annual total GHG emissions,and N₂O emissions were mainly occurred in the rape growing season.Therefore,reducing CH₄ emissions in the rice growing period is the key to reducing GHG emissions in rice cropping systems.Compared to FEL,FRa R and FMR under fallow-rice systems,annual CH₄ emissions in the REL,RRa R and RMR under rape-rice systems accordingly reduced by 29.5%,28.8%and 29.4%,respectively;annual CH₄ emissions of middle rice cropping system were significantly lower than that of ratoon rice,followed by double rice cropping system.These data indicate that RMR and RRa R systems show better emission reduction effect among the six rice cropping systems.?4?The ranking of SOC sequestration(kg CO₂-eq ha^-1 yr^-1)among six croppingsystems was FRa R?3495.1?>RRa R?2740.4?>FMR?2655?>RMR?2118.1?>FEL?2046.7?>REL?611.7?.The increase in the SOC sequestration in the topsoil?0-20 cm?from 2017 to 2019 for six rice cropping systems indicated improving SOC storage could offset part of GHG emissions and had a positive effect on carbon footprint.?5?The annul CF_aof the six rice cropping systems were 21696-36647 kg CO₂ eq ha^-1FMR?2.18?>REL?1.76?>RMR?1.65?>FRa R?1.6?>RRa R?1.45?,considering SOC sequestration.The rape-rice cropping systems?REL,RRa R and RMR?had lower carbon footprint,compared to the fallow-rice cropping systems?FEL,FRa R and FMR?,respectively.The RRa R system had the lowest CF_y values among the six cropping systems,which was mainly caused by the low CH₄ emissions from paddy fields in RRa R system.CH₄ emissions occurred in the rice growing peroid were the main contributor of the carbon footprint,followed by fertilizers and diesel consumption from agricultural inputs,while N₂O emissions constituted only a minority of the carbon footprint.?6?The NEP_a of the six cropping systems?FEL,REL,FRa R,RRa R,FMR and RMR?were 6.43,8.21,6.56,9.65,0.64 and 3.75 t C ha^-1,respectively,and the NEP_y of FRa R,REL,FEL,and RMR were 0.40,0.42,0.43,0.57,0.06 and 0.28 kg C kg^-1,respectively,ranking as RRa R>FRa R>REL>FEL>RMR>FMR.The rape-rice cropping systems including REL,RRa R and RMR had the significant higher carbon sequestration capacity,compared to the fallow-rice cropping systems including FEL,FRa R and FMR,respectively.Similarly,the RRa R system had the both highest NEP_a and NEP_y among the six cropping systems.In conclusion,the rape-rice cropping systems were beneficial to the increase in the annual yields and the reduction in the CH₄ emissions from paddy fields,thus reducing the annual carbon footprints and improving the annual carbon sequestration capacity.Due to the characteristics of"one crop and two harvests",the ratoon rice cropping system had relatively low agricultural inputs and CH₄ emissions,meanwhile its biomass C fixation was the highest.The RRa R system sustainably achieved high annual yield,mitigated GHG emissions,resulting in the reduced carbon footprints and improved carbon sequestration capacity of the paddy agroecosystems.Therefore,the RRa R system is an appropriate cropping system for the energy conservation,emission reduction,carbon sequestration and sustainable development of the agriculture in the middle Yangtze River region in China.