Effects Of Different Water And Nitrogen Fertilizer Management Practices On Rice Growth And Greenhouse Gas Emissions In The Double Rice Cropping System

To seek the effective water-nitrogen（N）management practices that can increase rice yield and reduce paddy greenhouse gas（GHG）emissions,a field experiment was conducted in the double rice cropping system:（1）U+CI:urea with conventional traditional irrigation,as the control;（2）U+SWD:urea with shallow water depth with alternate wetting-drying irrigation;（3）CRU+SWD:polymer-coated controlled release urea with SWD;（4）NU+HQ+SWD:nitrapyrin crystal urea with hydroquinone and SWD;（5）EM+U+SWD:urea with EM and SWD.The results are as follows:（1）Compared with U+CI,U+SWD improved the leaf SPAD value,leaf area index（LAI）and biomass at the grain filling stage during late rice season,but had no significant effect during early rice season.The spike rate can be increased by U+SWD.Compared with U+SWD,CRU+SWD and NU+HQ+SWD increased plant height,leaf SPAD value,LAI,and biomass during early and late rice seasons,and further inhibited the number of ineffective tillers.The effect of EM+U+SWD on the rice growth was positive during early rice season,but was not obvious during late rice season.（2）Compared with U+CI,U+SWD reduced the seed setting rate of early rice,resulting in the slight yield reduction by 3.5%-6.7%,but the difference was not significant.The grain number per panicle was significantly increased,led to improving the late rice yield by 7.6%-8.5%.Compared with U+SWD,CRU+SWD and NU+HQ+SWD increased the grain number per panicle of early rice and late rice,and effective panicle number per unit area of late rice,giving rise to increasing yield during both two seasons.EM+U+SWD had no significant effect on late rice yield but increased early rice yield by 2.0%-7.6%,compared with U+SWD.（3）Different water-N management had the obvious impacts on CH₄ emissions in paddy fields.Compared with U+CI,U+SWD significantly reduced CH₄ emissions by 12.6%-44.9%during rice growing seasons.Compared with U+SWD,CRU+SWD,NU+HQ+SWD,and EM+U+SWD can further reduce CH₄ emissions in the cropping season.Water-N management in the former period had a follow-up effect on the subsequent CH₄ emissions.The implementation of SWD in early rice season can reduce CH₄ emission of land preparation period before late rice season.The application of CRU and NU+HQ were favorable for the absorption of atmospheric CH₄ in winter fallow period.（4）Changes in irrigation mode caused the trade-off in CH₄ and N₂O emissions.Compared with U+CI,U+SWD significantly increased N₂O emissions during early and late rice growing seasons.Compared to U+SWD,N₂O emissions from rice growing seasons under CRU+SWD,NU+HQ+SWD,and EM+U+SWD were decreased,with the average reductions of 14.5%,27.2%and 12.6%,respectively.However,the application of CRU and NU+HQ increased N₂O emissions during late-season land preparation and fallow periods.（5）Compared with U+SWD and U+CI,SOC sequestration rates from CRU+SWD and NU+HQ+SWD were larger,but that from EM+U+SWD was slightly decreased.SWD can reduce net total GHG(T_GHG)emissions,and the reduction magnitude depended on the type of N fertilizer.Compared with U+CI,U+SWD significantly reduced net T_GHG emissions by 19.8%.Compared with U+SWD,CRU,NU+HQ,and EM+U combined with SWD can further reduce the net T_GHG emissions by 6.8%-37.5%,and achieve the lower emission intensity.（6）Compared with U+CI,U+SWD increased 60.8-83.5%irrigation water use efficiency and 29.6%-57.8%total water use efficiency.N use efficiency of early rice under U+SWD was slightly lower than U+SWD,but the difference was not significant.Yet,U+SWD significantly increased the N use efficiency of late rice.Compared with U+SWD,CRU+SWD,NU+HQ+SWD and EM+U+SWD can further increase water use efficiency and N use efficiency.In summary,SWD,or SWD combined with the application of CRU,NU+HQ and EM+U are the effective technology that can simultaneously mitigate GHG emissions and achieve high grain yield in the double rice cropping system.