Effects Of Ridge Irrigation And Alternative Nitrogen Application On CH₄ And N₂O Emission From Double-cropping Rice Field And Its Mechanism

At present,there are many researches about the effects of irrigation or fertilization on greenhouse gas emissions from paddy fields.However,the mechanisms of rice water and nitrogen utilization and greenhouse gas emission reduction from paddy fields with ridge irrigation in different ridge widths and nitrogen（N）fertilizer subsitution under ridge irrigation are still unclear.In this study,the double-cropping rice fields in the south of the Tropic of Cancer,Guangxi,were selected as the research objects.Combining with the local climate characteristics,the field and pot experiments were carried out to investigate the greenhouse gas emission reduction effect and mechanism under ridge irrigation and N fertilizer subsitution,so as to enhance the efficient utilization of water and N and achieve the effective measures.The main findings are as follows:1.Effects of ridge irrigation on water-saving and yield-increasing of rice under different N levels.A pot experiment was conducted to study soil water dynamics,the water consumption at different growth stages and yield of double-cropping rice under three irrigation methods and three nitrogen levels.The results showed that under three nitrogen levels,compared with conventional irrigation（CI）,ridge irrigation（RI）and"thin-wet-dry"irrigation（TI）improved rice yield by 6.21%-8.02%and 5.53%-7.39%,and increased thousand-grain weight,grain number,ear number,ear length and seed setting rate significantly.The rice yield in RI mode was higher than that in TI mode.Compared with TI mode,RI mode reduced the water consumption by 4.97%-6.10%.Under the same N level,RI and TI modes had higher N content in rice straw than CI modes.At the tillering,booting,heading and maturing stages,the soil NH₄⁺-N and NO₃^--N contents in the RI and TI modes were higher than those in the CI mode.Compared with the CI mode,both RI and TI modes increased the microbial biomass carbon（MBC）and nitrogen（MBN）and enzyme activities in soils.Appropriate increase in N fertilizer was beneficial for increasing rice yield.Under three irrigation methods,compared with the N1 level,the N2 and N3 levels increased the rice yield by 53.78%-66.77%,and yield in N2 level was 59.09%～66.77%higher than that in N1 level.The 1000-grain weight,ear number,ear number,ear length,and seed setting rate in N2 and N3 levels were also significantly higher than those of N1 level.Compared with the N1 level,increasing the N fertilizer（N2,N3）significantly increased the water consumption of rice.Compared with the N1 level,the N2 and N3 levels increased the N,P,and K content of rice straw and grains.Compared with the N1 level,the N2 and N3 levels increased soil NH₄⁺-N and NO₃^--N contents at the tillering,booting,and heading stages.Compared with N1 level,N2 and N3 levels increased MBC,MBN and enzyme activities in soils at different growth stages.2.Effect and mechanism of ridge irrigation with different ridge widths on CH₄ and N₂O emissions from double-cropping rice fields.Taking CI mode as a control,field experiments were conducted to investigate the effects of RI mode with different ridge widths on rice yield,CH₄ and N₂O emissions from paddy fields,global warming potential（GWP）and greenhouse gas emission intensity（GHGI）and soil indicators,and then the relationships between CH₄ and N₂O fluxes in rice fields and soil indicators were analyzed.The results showed that compared with the CI mode,all RI modes increased the late-season rice yield by 8.16%-17.6%,and the late-season rice yield in RI80 mode was significantly higher than that of RI100 mode by 8.83%.Compared with the CI mode,all RI modes increased the effective panicle number and thousand-grain weight of early and late-season rice significantly.Compared with the CI mode,the RI mode significantly reduced the irrigation amount in the double-cropping rice by 29.8%-47.0%,and the RI80 mode had the lowest irrigation amount in the double-cropping rice.Compared to the CI mode,the RI mode reduced the soil NH₄⁺-N content slightly,but increased the soil NO₃^--N content.Compared with the CI mode,the RI mode increased the soil MBC and MBN significantly.Under different irrigation methods,the activities of soil catalase,urease,invertase,and nitrate reductase reached the maximum at the booting stage,the soil phosphatase activity reached the maximum at the milk maturing stage,and the soil nitrite reductase reached the maximum at the tillering stage.During the whole growth period of rice,the RI mode had higher enzyme activity in soils than the CI mode.Compared with the CI mode,all RI modes reduced CH₄ emissions by 49.4%-70.3%from late-season rice fields during the whole growth period,and the RI60 and RI80 modes reduced CH₄ emissions by 41.6%and 45.6%from early-season rice fields.However,the RI mode only increased the N₂O flux from the double-cropping rice fields slightly.The CH₄ flux from paddy field was positively correlated with soil microbial biomass N,ammonium nitrogen content,temperature and Eh value,but negatively correlated with urease activity,catalase activity and p H value.There was a significant positive correlation between the N₂O flux from paddy field and the soil temperature and Eh value,and a significant negative correlation between the N₂O flux from paddy field and the soil p H value.Thus soil temperature,Eh value and p H value significantly affected the CH₄ and N₂O fluxes from paddy field.Compared with the CI mode,all RI modes reduced the global warming potential（GWP）by 47.9%-61.4%and greenhouse gas emission intensity（GHGI）by 53.7%-74.8%from late-season rice fields,and the RI60 and RI80 modes respectively decreased the GHGI by 44.6%and 52.5%in the early-season rice fields.Thus the RI80 mode increased the yield of double-cropping rice and reduced the CH₄ emissions,GWP and GHGI in paddy fields.3.Effect and mechanism of alternative nitrogen（N）application on CH₄ and N₂O emissions from double-cropping rice fields under ridge irrigation.Field experiments were conducted to study double-cropping rice yield,CH₄ and N₂O emissions and soil indicators under two irrigation methods（CI and RI）and four nitrogen treatments.GWP and GHGI under different treatments and the correlations between CH₄ and N₂O fluxes and soil indexes were analyzed.The results showed that under two irrigation methods,compared with only urea application（F0）,the three alternative N application treatments（F1-green manure instead of 30%urea,F2-pig manure instead of 30%urea,and F3-controlled release urea instead of 30%urea）increased double cropping rice yield.In CI mode,compared to F0,F1,F2 and F3 increased the yields of early-and late-season rice by 20.2%,23.2%and 14.6%,and 16.9%,19.8%and11.7%,respectively.In RI mode,the rice yield in F1,F2 and F3 was22.2%,23.0%and 12.4%,and 17.0%,20.7%and 12.6%higher than that in F0.Under the three alternative N application treatments,the yields of double-cropping rice in the RI mode were higher than those in the CI mode by 8.5%,9.4%,8.0%and 5.9%,and 9.7%,9.7%,10.6%and 10.9%.Compared with the CI mode,the RI mode significantly reduced the irrigation amount in the double-cropping rice.Under the same irrigation method,compared with F0,F1,F2 and F3 treatments increased the irrigation amount in the double-cropping rice.RI mode and three alternative N application treatments significantly improved the irrigation water use efficiency of double-cropping rice.Compared with CI,RI mode improved soil NH₄⁺-N,NO₃^--N,MBC,MBN and enzyme activities during the growth period of early-season rice.In RI mode,the three alternative N application treatments increased soil NH₄⁺-N,NO₃^--N,MBC,MBN and enzyme activity compared with only urea application.Under the two irrigation methods,the replacement of 30%urea with green manure reduced CH₄ emissions from rice fields,while the replacement of 30%urea with pig manure and controlled release urea promoted CH₄ emissions from rice fields.The three alternative N application treatments also decreased N₂O emissions from rice fields.There was a significant positive correlation between CH₄ flux in paddy field and soil acid phosphatase activity,while there was a significant negative correlation between CH₄ flux in paddy field and soil invertase activity,temperature and Eh value.N₂O flux from rice fields had a significant positive correlation with soil urease activity,invertase activity,temperature and soil Eh value,while it had a significant negative correlation with soil microbial biomass carbon and acid phosphatase activity.Therefore,soil invertase and acid phosphatase activities,temperature and Eh value significantly influenced the CH₄and N₂O fluxes from rice fields.Under different irrigation method,compared with F0,F1 reduced GWP and GHGI in early-season rice fields by23.1%-31.3%and 39.0%-41.2%,while F2 increased GWP and GHGI in early-season rice fields by 53.9%-111.6%and 23.8%-59.6%,respectively.Under different N treatment,the GWP and GHGI in the RI field were7.8%-34.0%and 12.8%-34.8%lower than those of the CI field,respectively.In conclusion,the ridge irrigation with a ridge width of 80 cm was the optimal RI mode to increase rice yield and reduced CH₄and N₂O from rice fields.Replacing 30%urea with green manure under ridge irrigation was a suitable alternative N application method for increasing rice yield and decreasing GWP and GHGI.Soil temperature,Eh value and p H value significantly affected the CH₄ and N₂O fluxes from paddy field under ridge irrigation with different ridge widths,and soil invertase and acid phosphatase activities,temperature and Eh value significantly influenced the CH₄and N₂O fluxes from rice fields under different irrigation modes and alternative nitrogen application methods.