Effects Of Green Manure On Greenhouse Gas Emissions And Relevant Soil Microbial Community In Double Rice Cropping System

Green manure crops play an important role in reducing chemical fertilizer applications while enhancing soil fertility.However,its effects on rice paddy greenhouse gas?GHG?emissions and its underlying mechanism is poorly understood.Based on two long-term green manure-based experiments,we analyzed the impacts of green manure on?i?rice yield,?ii?GHG emissions,?iii?methanogenic and methanotrophic communities;and the integrated impacts of milkvetch and straw on?iv?rice yield,?v?GHG emissions and?vi?bacterial community structure.The first experiment,located in Qiyang and started in 1982,consisted of four treatments:ryegrass-rice-rice?Ry?,milkvetch rice-rice?Mv?,rapeseed-rice-rice?Rp?and fallow-rice-rice?Fa?.The second experiment,which is located in Jinxian and started in 1981,consisted of four treatments:100%NPK,reduced NPK+milkvetch?NPK+Mv?,reduced NPK+milkvetch+straw?NPK+Mv+S?and no fertilizer?CK?.Incorporation of winter-planted green manure had more impacts on yield and CH₄ emissions in the early rice season than in the late rice season.Compared to Fa,Mv,Rp and Ry significantly increased annual rice yield by 19.91%,14.45%,and 10.65%,respectively,over both years.Meanwhile,Ry and Rp significantly increased annual CH₄ emissions by 116.73%and 52.45%,respectively,while Mv reduced CH₄ emissions by 16.33%though not significant.The CH₄ emissions showed significant positive correlations with the C/N ratio of the incorporated green manure?r=0.88?.Although there was no significant difference in annual cumulative N₂O emissions among the treatments,Ry and Rp increased the annual area-scaled GHG emissions by 66.70%and 31.51%,respectively while in Mv,it was 11.06%lower in comparison with Fa.Similarly,annual yield-scaled GHG emissions in Ry and Rp were 70.27%and 30.96%,higher,respectively,while Mv was 13.14%lower,compared to Fa.Significant impacts of green manure application were found on soil microbial community related to methane emission.CH₄ emissions links to soil physicochemical and microbial properties showed that the cumulative CH₄ emissions significantly correlated with dissolved organic carbon?DOC??r=0.71?and the abundance of mcrA gene?r=0.71?,but not pmoA.The relative abundance of acetoclastic methanogens of the family Methanosarcinaceae,and hydrogenotrophic methanogens of the orders Methanocellales and Methanobacteriales were significantly lower in Mv,in comparison with Ry,and significantly correlated with cumulative CH₄ emissions?r=0.92,0.85,0.95,respectively?.There were significant effects of green manure and crop straw application on GHG emissions and soil microbial community.On combining milkvetch and straw mulch,NPK+Mv+S significantly reduced cumulative CH₄ emissions,compared to 100%NPK;however,it was not significantly different from NPK+Mv?P>0.05?.DOC and available phosphorus?AP?significantly correlated with the annual cumulative CH₄ emissions?r=0.64,0.68,respectively,P<0.05?.The presence of Methylobacter and higher abundance of acetate-reducing bacteria,Geobacter,support the lower CH₄ emission observed in NPK+Mv+S.On the other hand,no significant difference existed in N₂O emissions in both the early and the late rice seasons.Compared to 100%NPK,NPK+Mv+S significantly increased annual rice yield by7.8%.Our results suggest that Mv improves rice yield while mitigating CH₄ emissions by regulating methanogenic communities.Also,the combination of milkvetch and straw mulch benefits yield and lowers CH₄ emissions.Therefore,milkvetch application,with or without straw,is recommendable for yield improvements and lower CH₄ emissions in the double rice cropping system.