Research On The Effect Of Crop Diversification On Farmland Greenhouse Gas Emissions In Oasis Irrigation Area

Food security and greenhouse gas emission（GHG）reduction are two major issues of sustainable agriculture.How to achieve food security with lower GHG emissions has been a serious challenge in agricultural production.Diversified cropping systems based on intercropping has typical features like higher efficiency and lower emissions.Nevertheless,increasing crop diversity on the same plot annually lack of the theoretical and practical bases,referring to achievement of lowering GHG emissions without compromising yield of crops.Afield experiment was conducted at Hexi oasis irrigation area in 2018-2020,consisted with five cropping systems included sole cropping spring wheat and maize（W,M,one crop a year）,spring wheat/maize intercropping and multiple cropping green manure after wheat harvest（W//M,W-G,two crops a year）,multiple cropping green manure in wheat/maize intercropping after wheat harvest（W-G//M,three crops a year）.The objective of this study was to reveal a basic mechanism on regulation effect of crop diversity levels on farmland productivity and soil GHG emissions.The aim was to provide a theoretical base for higher efficient diversified cropping systems in oasis irrigation areas.The main results are as follows:（1）Increasing crop diversity significantly reduced soil GHG emissions during the growth period of crops.Intercropping system was more conducive to soil GHG reduction than multiple cropping system.Soil CO₂ emissions of the W-G//M treatments were 9.1%lower than that of the W-G treatment,but there was no significant difference with wheat/maize intercropping（W//M）and sole cropping maize（M）.The W-G//M and W//M treatments of soil CO₂ emissions was increased by 16.5%and 9.8%,compared with the W treatment,respectively.Soil CO₂ emissions of the W//M treatment were 14.2%lower than that of the W-G treatment.The total N₂O emission of soil was affected by cropping systems,and the W-G//M treatment of soil N₂O emissions was decreased by 20.3%compared with W-G,16.3%and 16.6%compared with M and W treatments,respectively.The total of N₂O emissions of the W//M treatment was decreased by 29.1%,compared with the W-G treatment.The warming potential（GWP）of WG//M was decreased by14.3%in comparison to the W-G treatment.There was no significant difference with the WG//M and W//M treatments.The GWP of WG//M was reduced by 12.1%compared with M treatment.At the same time,the W//M treatment is 16.4%lower than that of the W-G treatment.（2）Crop diversification affected soil GHG emissions by regulating soil organic matter（SOM）,total nitrogen（TN）and soil C/N ratio at different periods.The correlation between soil CO₂emissions and the SOC and TN was mainly performed before wheat harvest.The correlation between soil N₂O emissions and SOM and TN was mainly performed after wheat harvest.Before wheat harvest,the W-G//M treatment reduced SOM by 18.7%and 14.9%,and TN reduced by 9.4%and 12.0%compared with M and W-G treatments,respectively.The SOM of the W-G//M treatment was increased by 7.8%and 12.4%compared with the W//M and W treatments,respectively.After wheat harvest,the W-G//M treatment reduced TN of soil by 6.1%and 7.6%compared with M and W-G treatments,which is beneficial to soil N₂O emissions reduction.Compared with the W treatment,the TN with the W//M treatment was increased by 9.4%after wheat harvest.For all cropping systems,the soil C/N ratio is between 15.8-19.8.Decreasing the soil C/N ratio is contributed to reducing soil GHG emissions in this range,and crop diversification could reduce the soil C/N ratio,which was beneficial to soil GHG emissions reduction（3）Increasing crop diversity could decrease soil microbial biomass and soil enzyme activity,which was contributed to soil GHG reduction in the cropping systems with green manure.Compared with the W-G treatment,the soil microbial biomass carbon（SMBC）and soil microbial biomass nitrogen（SMBN）of the W-G//M treatment were decreased by 7.2%and 11.6%,respectively.The soil urease（SUA）and catalase activities（SCA）were decreased by 14.4%and10.9%,respectively.Compared with the W//M treatment,the SMBC and SMBN of the W-G//M treatment were increased by 11.1%and 12.3%,the SUA and SCA was increased by 17.2%and36.2%,respectively.Crop diversification reduces the SMBC during the growth period of crops,thereby reducing soil CO₂ emissions.And crop diversification reduces the SMBN at green manure return after 15 days,which was contributed to soil N₂O emissions reduction.Meanwhile,crop diversification significantly reduces the SUA and SCA at green manure return 15 days,thereby reducing the soil CO₂ and N₂O emissions at this period.（4）The interaction between various influencing factors determined soil GHG emissions.Increasing crop diversity by intercropping reduced TN by reducing SOM,and significantly reduces soil CO₂ emissions.The soil C/N ratio plays an important role in regulation soil N₂O emissions.The interaction between soil C/N ratio and TN determined soil N₂O emissions,an in the TN decrease the C/N ratio,which was beneficial to soil N₂O reduction.Crop diversification decreased the SMBC leads to the reduction of SMBN,which was conducive to soil CO₂ emissions reduction.The results of canonical correlation analysis further confirmed the results of correlation analysis and overall path analysis.The results showed that the SMBN and GWP were ranked the first and the TN and soil N₂O emission fluxes ranked second among the influencing factors and emission indicators,respectively.There was a negative correlation between soil N₂O emissions and the TN.（5）Increasing crop diversity significantly increased crop yields,reduced GHG emission intensity(GHG_EI)and promoted soil carbon sequestration.It contributed to improving the sustainability of cropping systems.Intercropping system had better GHG emissions reduction effect and sustainability than multiple cropping system.Compared with the M treatment,the grain yield of W-G//M and W//M treatments increased by 13.0%and 9.3%,the energy yield increased by 52.5%and 30.1%,and the energy yield of W-G//M increased by 16.0%compared with the W//M treatment,respectively.Compared with the M treatment,the GHG emission intensity based on grain yield(GHG _EIg)of the W-G//M and W//M treatments were reduced by 19.9%and 18.7%,and the greenhouse gas emission intensity based on the energy yield(GHG _EIe)was reduced by35.8%and 31.1%,respectively.Although there was no significant difference between the GHG _EIgof the W-G//M and W//M treatments,but the GHG _EIeof W-G//M treatment was 13.4%lower than that of the W//M treatment.In addition,the W-G//M treatment has the highest evaluation index（0.97）.The W-G//M treatment significantly increased evaluation index by 9.0%and 27.6%compared to the W//M and M treatments.In conclusion,increasing crop diversity by integrating multiple cropping green manure into wheat/maize intercropping contributed to increasing yield and mitigating soil GHG emissions of cropping systems.Thus,this practice can be considered as the sustainable cropping system in arid irrigation areas.