Effects Of Hydrological Changes And Nutrient Addition On Greenhouse Gas Fluxes In The Water-level-fluctuating Zone Of The Three Gorges Reservoir And Its Mechanism

Global climate change is one of the most severe environmental problems facing global society today.It not only affects the Earth’s ecosystem but also endangers human living environments.CO₂,CH₄,and N₂O are the three main long-lived greenhouse gases in the atmosphere and play an important role in global climate change.Worldwide hydroengineering creates novel riparian ecosystems with unique hydrological conditions.However,with changes in global precipitation patterns and due to human activities and large amounts of livestock manure leading to large amounts of nitrogen and phosphorus inputs,the greenhouse gas emission mechanism in the novel riparian ecosystems remains unclear.In this study,2019was a normal rainfall year while 2020 experienced extreme rainfall.Hydrological changes and nitrogen and phosphorus enrichment were simulated.We combined extreme rainfall weather and field experiments to comprehensively investigate the impacts of hydrological changes and nitrogen and phosphorus additions on greenhouse gas emission fluxes in a riparian zone of reservoir and explore the underlying mechanisms.The main research results are as follows:（1）In both 2019（normal rainfall）and 2020（extreme rainfall）,hydrological changes significantly affected soil and vegetation characteristics.In 2019,long-term flooding increased soil p H and soil water content in the 0-10cm soil layer,but decreased soil temperature,total organic carbon,total carbon content,and carbon-to-nitrogen ratio.Periodic flooding also increased soil p H in the 0-10cm soil layer,but decreased carbon-to-nitrogen ratio.Additionally,long-term flooding increased soil p H and soil water content in the 10-20cm soil layer,but decreased soil total nitrogen and total carbon content.Periodic flooding increased soil water content and nitrate nitrogen content in the 10-20cm soil layer.Furthermore,both long-term and periodic flooding increased root nitrogen content and vegetation diversity（H）.In 2020,long-term flooding increased soil p H in the 0-10cm soil layer and decreased dissolved organic carbon concentration in the 10-20cm soil layer,while periodic flooding increased soil p H and NO₃^--N content in the 10-20cm soil layer.Additionally,long-term flooding decreased root carbon-to-nitrogen ratio,while periodic flooding increased root carbon and nitrogen content.Compared to hydrological changes,the effects of nitrogen and phosphorus addition on soil and vegetation characteristics were limited in both years.The only effect was that nitrogen addition increased root nitrogen content in2020.Compared to normal rainfall,extreme rainfall significantly increased soil total carbon content at both depths under long-term flooding conditions and total organic carbon content in the 10-20cm soil layer,and increased vegetation diversity（H）under periodic flooding conditions.Additionally,under simultaneous phosphorus and nitrogen-phosphorus addition,extreme rainfall did not significantly increase soil water content,but increased total organic carbon content in the 10-20cm soil layer under phosphorus addition.（2）Changes in hydrology and nitrogen and phosphorus additions significantly affected greenhouse gas fluxes under normal rainfall conditions in 2019.Long-term flooding significantly reduced CO₂emissions by 24%,but increased CH₄emissions by about 4 times.Compared to long-term flooding,periodic flooding significantly increased N₂O emissions by about 3 times.In addition,nitrogen addition increased CO₂emissions by 37%.Compared to2019,hydrology changes and nitrogen and phosphorus additions had no significant impact on greenhouse gas fluxes under extreme rainfall conditions in 2020.Overall,extreme rainfall reduced CO₂fluxes by 76%,but increased CH₄and N₂O fluxes by 2 times.It is worth noting that the impact of extreme rainfall on CH₄fluxes was not observed under phosphorus addition and nitrogen and phosphorus simultaneous addition treatments,and the impact on N₂O fluxes was only observed under long-term flooding and phosphorus addition treatments.（3）There is a significant relationship between greenhouse gas fluxes and soil and vegetation characteristics.According to stepwise regression analysis,soil characteristics（p H,soil moisture,dissolved organic carbon,and carbon-to-nitrogen ratio）and vegetation characteristics（aboveground biomass,belowground biomass,root carbon and nitrogen content）explained 75%of CO₂flux variation.Among them,p H and ammoniacal nitrogen content were important driving factors for CH₄and N₂O fluxes,respectively.According to structural equation modeling analysis,extreme rainfall and hydrological changes affect CO₂fluxes by changing soil and vegetation characteristics.For CH₄,extreme rainfall affects CH₄fluxes by changing soil and vegetation characteristics,while hydrological changes affect CH₄fluxes only by changing soil characteristics.For N₂O,extreme rainfall and hydrological changes affect N₂O fluxes by changing soil characteristics.Compared to hydrological changes,the total effect of extreme rainfall on CO₂,CH₄,and N₂O fluxes is about 3 times,1 times,and5 times higher,respectively.（4）Extreme rainfall modulated the response of global warming potential（GWP）to flood frequency and nutrient addition.In a normal rainfall year,long-term flooding significantly reduced GWP by 31%,while nitrogen addition and nitrogen and phosphorus simultaneous addition significantly increased GWP by 39%and 32%,respectively.These effects were not observed in the year of extreme rainfall.Overall,extreme rainfall reduced GWP by 60%and shifted the dominant greenhouse gas from CO₂to CH₄.In summary,continuous flooding reduced CO₂emissions but increased CH₄emissions due to the increase in soil moisture.N addition promoted CO₂emissions because it increased nutrients for plant and soil respiration.These phenomena were not observed in the year of extreme rainfall,which is attributed to the overriding effect of extreme rainfall on soil properties and vegetation characteristics.In addition,P addition weakened the impact of extreme rainfall on CH₄fluxes because it reduced soil moisture,while P addition and continuous flooding promoted the impact of extreme rainfall on N₂O fluxes because it increased carbon availability.Overall,extreme rainfall significantly reduced greenhouse gas emissions and exceeded the impacts of hydrological changes and nitrogen and phosphorus additions on greenhouse gas emissions.