Effects Of Different Elevated CO₂ Concentrations On CH₄ Emission And Its Mechanism In Paddy Fields

The concentration of carbon dioxide（CO₂）in the atmosphere continues to rise due to human activities,increasing from 280 ppm before the western industrial revolution to 410.5ppm in 2019.In the past 10 years,atmospheric CO₂ concentration has been increasing at an average annual rate of 2.37 ppm.Under the scenario of moderate stabilization of greenhouse gas（GHGs）emissions,the atmospheric CO₂ concentrations are expected to reach 538–670 ppm by 2100.Methane（CH₄）contributes 16%to global warming,second only to CO₂;its global warming potential（GWP）over a 100–year scale is 28–34 times that of CO₂.CH₄ emissions from paddy fields are approximately 31–112 Tg yr^–1,accounting for 5–19%of the total global CH₄ emissions.CH₄ in paddy fields is produced by methanogens through decomposing the methanogenic substrates under strict anaerobic conditions.At present,most researchers believe that elevated carbon dioxide concentration（e[CO₂]）（usually 200 ppm above ambient）promotes CH₄ emissions in paddy fields.However,there are large differences in the atmospheric CO₂concentration under different carbon emission scenarios in the future.The atmospheric CO₂concentration will gradually increase rather than abruptly increase and maintain a constant high level.Therefore,the effects of gradual and abrupt increase of CO₂ concentration on CH₄emission in paddy fields may be different.In addition,it is generally believed that the e[CO₂]cannot directly change soil methanogenic community,but can indirectly affect soil methanogens through affecting plant growth,soil physicochemical properties,and soil enzyme activity,thus affecting the CH₄ emission in paddy fields.However,it is still difficult to simply explain how e[CO₂]affects CH₄ emissions in paddy fields due to the complexity of rhizosphere processes.Under the climate change scenario,it is of great significance for agriculture to adapt to climate change and carbon neutralization to clarify the spatio–temporal variation of CH₄ flux in rice fields,accurately estimate the CH₄ emission per unit rice yield,and explore the mechanism affecting the CH₄ emission in rice fields.Based on the open–top chamber（OTCs）CO₂ concentration automatic regulation platform established in 2016,field experiments were conducted in the growing season of rice（Oryza sativa L.）from 2018 to 2020 to explore the effects and mechanism of different e[CO₂]on CH₄emission in paddy fields,The experimental treatments included three CO₂ concentrations:control（CK）:ambient atmospheric CO₂concentration;gradual increase of CO₂ concentration（C₁）:CK+120 ppm in 2018,+160 ppm in 2019,and+200 ppm in 2020;abrupt increase of CO₂ concentration（C₂）:direct increase of CO₂ concentration by 200 ppm.The rice variety was Nanjing 9108.In 2018,the water management method was continuous flooding（CF）,while in2019 and 2020,the water management method was flooding–midseason drainage–reflooding（FDFM）.The CH₄ fluxes were measured using a transparent static chamber–laser greenhouse gas analyzer technique.The methane production potential（MPP）was analyzed using the slurry incubation.The abundance of methanogens was detected using Quantitative Real–time PCR（q PCR）.Additionally,the community composition and structure of methanogens were also compared with Illumina Mi Seq sequencing.The main results of this study are as follows:（1）The effect of e[CO₂]on CH₄ emission in paddy fields may be a long-term effect,and CH₄ emission in paddy fields can be significantly increased only when CO₂concentration continues to rise for 5 years.In the 2018 and 2019 rice growing seasons,compared with CK treatment,C₁ and C₂ treatments had no significant effect on the cumulative amount of CH₄ emission（CAC）.In contrast,in the 2020 rice growing season,C₁ and C₂treatments significantly increased CAC.Interestingly,e[CO₂]significantly increased CAC/yield in 2018–2020 rice growing seasons,and there was a significant proportional relationship between CAC/yield and different CO₂ concentration levels in the 2018 and 2019rice growing seasons.In the 2020 rice season,the CAC/yield of C₂ treatment was significantly increased by 12.6%compared with that of C₁ treatment.（2）Rice plants have a certain buffer capacity to the short-term（less than 4 years）increase of CO₂ concentration,and e[CO₂]（+200 ppm）for 5 years can significantly increase the shoot biomass of rice and thus improve soil DOC concentration.In the 2018and 2019 rice seasons,C₁ and C₂ treatments had no significant effect on shoot biomass of rice.In the 2020 rice season,the shoot biomass of C₂ treatment was significantly increased by 25.3%compared with that of CK treatment.Meanwhile,the shoot biomass of C₂ treatment was significantly increased by 12.2%compared with that of C₁ treatment.Similarly,e[CO₂]had no significant effect on soil dissolved organic carbon（DOC）concentration in the 2018 and 2019rice seasons,but significantly increased the seasonal average soil DOC concentration in the2020 rice season.Regression analysis further showed that there was a significant positive linear correlation between soil DOC concentration and shoot biomass of rice.In addition,e[CO₂]significantly increased the seasonal average soil invertase activity,but had no significant effects on rice yield,soil ammonium nitrogen（NH₄⁺–N）and nitrate nitrogen（NO₃^––N）concentrations,and soil p H in three rice seasons.It is worth noting that e[CO₂]by 120 ppm and 160 ppm did not significantly affect the activities of urease and catalase,but e[CO₂]by 200 ppm significantly enhanced the seasonal average activities of urease and catalase in 2018 and 2020.Meanwhile,in the 2020 rice season,the seasonal average soil urease activity under C₂ treatment was significantly increased by 3.0%compared with C₁ treatment.（3）The short-term（less than 4 years）changes of organic matter input and environmental factors caused by e[CO₂]may not be enough to cause significant changes in the structure and diversity of methanogenic communities.In general,the effects of e[CO₂]on soil methanogenic abundance in the 2018 and 2019 rice growing seasons were not significant.In 2020,compared with CK treatment,C₁ treatment significantly increased soil methanogenic abundance at the tillering and milk–ripening stages,while C₂ treatment significantly increased soil methanogenic abundance at each growth stage.Similarly,in the 2018 and 2019 rice growing seasons,compared with CK treatment,C₁ and C₂ treatments had certain positive effects on soil MPP from tillering to milk–ripening stages,but these positive effects were not all significant.In 2020 rice growing season,C₁ and C₂ treatments significantly increased soil MPP compared with CK treatment.Notably,in the 2020 rice season,compared with C₁treatment,the methanogenic abundance and MPP of C₂ treatment at the tillering stage were significantly increased by 59.3%and 24.1%,respectively;the methanogenic abundance of C₂treatment at the elongation stage was significantly increased by 72.2%;the MPP of C₂ treatment at the grain–filling stage was significantly increased by 24.4%.In addition,compared with CK treatment,C₂ treatment significantly increased the relative abundance of Methanoregula in2018 rice season.In 2020 rice season,C₁ and C₂ treatments significantly increased the relative abundance of Methanospirillum compared with CK treatment.However,although e[CO₂]significantly changed the structure of methanogenic communities at the elongation stage in2018 rice season,there was no significant difference in the structure and diversity of methanogenic communities under different CO₂ treatments in three rice seasons.（4）E[CO₂]affected soil physiochemical properties and enzyme activities by affecting the shoot biomass of rice,which indirectly changed the growth environment of methanogenic community and further affected CH₄ emission in paddy fields.In the 2018-2020 rice seasons,there was a significant linear relationship between CH₄ emission and shoot biomass and yield of rice.Meanwhile,the linear model based on soil methane production potential,methanogenic abundance,and DOC concentration could explain 67.5%–81.6%variation of CAC in paddy fields.In addition,stepwise regression analysis further showed that the linear models based on soil invertase,DOC,and NH₄⁺–N could explain 51.0%and 59.3%variation of MPP in 2018 and 2019 rice growing seasons,respectively,while the linear models based on soil DOC,NO₃^––N,and p H could explain 72.2%variation of MPP in 2020 rice growing season.Meanwhile,there was a significant linear relationship between methanogenic abundance and soil DOC concentration in paddy soil during three rice growing seasons.In conclusion,although e[CO₂]significantly changed the relative abundance of individual methanogens,in general,the e[CO₂]did not significantly change the structure and diversity of methanogenic communities.The effect of e[CO₂]on paddy ecosystem was reflected in the long–term effect.E[CO₂]for 5 years could increase the shoot biomass of rice,improve soil DOC concentration,and thus improve the activity and abundance of methanogens,and ultimately increase CH₄ emissions in paddy fields.Notably,e[CO₂]significantly increased CH₄emissions per unit rice yield in all three rice seasons.In addition,in the 2020 rice season,compared with C₁ treatment,the CAC/yield,shoot biomass,and urease activity of C₂ treatment were significantly increased by 12.6%,12.2%,and 3.0%,respectively;meanwhile,the methanogenic activity and abundance of C₂ treatment at the tillering stage were significantly increased by 24.1%and 59.3%,respectively.Therefore,it is inappropriate to ignore the fact that CO₂ concentration increases gradually and only examine the effects of high e[CO₂]on farmland ecosystems.