Mechanism Of Drought Period And Nitrogen Deposition On N₂O And CH₄ Flux In Leymus Chinensis Grassland

Greenhouse gas emissions are an important cause of global warming,and nitrous oxide（N₂O）and methane（CH₄）are trace greenhouse gases with strong warming effects,with 298 and 25 times the warming potential of CO₂,respectively.Grasslands,as the second largest ecosystem in the world,emit 2.5 Tg of N₂O per year and have an important position in the global N₂O emissions from terrestrial ecosystems.There are also many studies showing that grassland ecosystems are important sinks for atmospheric CH₄.In recent years,increased human activities have led to increased atmospheric nitrogen deposition and frequent droughts,and nitrogen deposition and droughts have become two important factors in global climate change,especially drought during growing season,which seriously affect the structure and function of grassland ecosystems.Songnen grassland is located at the eastern end of the Eurasian steppe belt,and is an important base for the development of grassland animal husbandry in China.Nitrogen deposition can affect grassland N₂O and CH₄fluxes directly or indirectly by affecting soil available nitrogen content and plant growth.Changes in rainfall patterns,such as drought,can change soil microbial population and activity by directly affecting soil moisture content,which in turn can lead to changes in N₂O and CH₄fluxes.However,most studies focused on the effects of nitrogen deposition and drought on N₂O and CH₄fluxes,and less studies researching effects of drought periods and its interaction with nitrogen deposition.Studying the characteristics of N₂O and CH₄fluxes and their responses to nitrogen deposition and drought periods is important to further understand the relationship between grassland ecosystems and global climate change.Therefore,in this study,N₂O and CH₄fluxes and related soil physicochemical properties and microbial and plant indicators were measured in a mesocosm experiment from June 2021 to September 2022 to investigate the mediating mechanisms of the responses in N₂O and CH₄fluxes to nitrogen deposition and drought periods and their interactions in Songnen grassland.The results showed that:（1）Leymus chinensis grassland was a source of atmospheric N₂O emissions,and N₂O fluxes were significantly and positively correlated with air temperature and soil temperature,with the growing season being the main period of N₂O emissions,accounting for about more than 85%of annual emissions.Both nitrogen deposition and drought periods had significant effects on N₂O emissions.Among them,nitrogen deposition significantly contributed to N₂O emissions,mainly due to the fact that nitrogen deposition increased the available nitrogen content in the soil and increased the substrate for N₂O production.Drought periods affected N₂O emissions mainly by altering nitrification,with 14.72%,35.50%,and 4.02%increase in persistent drought,early drought,and later drought,respectively,compared to ambient precipitation treatments,with early drought having the greatest effect on grassland N₂O emissions and later drought having no significant effect on grassland N₂O emissions.In addition,nitrogen deposition influenced the response of N₂O fluxes to drought periods to some extent.Specifically,nitrogen deposition reduced the promotion of N₂O emissions during drought periods.（2）Leymus chinensis grassland is a sink for atmospheric CH₄,and grassland CH₄uptake was significantly correlated with soil water content,soil available nitrogen content,and soil soluble organic carbon content.The results showed that the main factors affecting CH₄uptake in grassland were the number and activity of methane-oxidizing bacteria,and methanogenic bacteria played a minimal role in influencing grassland CH₄emissions in this study.Nitrogen deposition significantly increased CH₄uptake by affecting the soil nitrogen pool and increasing the soil available nitrogen content,which in turn affected the activity and number of methanogenic oxidizing bacteria.Drought periods affected methane-oxidizing bacteria activity by reducing soil water content,which reduced CH₄uptake.In summary,nitrogen deposition increased grassland N₂O emissions and promoted grassland CH₄uptake.The effects of different drought period treatments on N₂O emissions from grassland were as follows:continuous drought at and early drought treatments significantly increased N₂O emissions from grassland,while later drought treatments had no significant effects on N₂O emissions from grassland.The effects of different drought period treatments on grassland CH₄uptake were as follows:continuous drought,early drought treatment and later drought significantly reduced grassland CH₄uptake.Changes in nitrate and ammonium nitrogen content due to nitrogen deposition had the most significant effect on N₂O emissions,and soil temperature had the most significant effect on CH₄uptake.Therefore,progressively increasing nitrogen deposition in the future may have an impact on the source-sink relationship between soil and atmospheric N₂O and CH₄in Songnen grassland.This study is scientifically important for predicting the response of N₂O and CH₄fluxes to nitrogen deposition and to periods of drought in the context of global climate change,as well as explaining the mechanisms of N₂O emissions and CH₄uptake in Leymus chinensis grasslands,providing a scientific basis for suppressing N₂O emissions and promoting CH₄uptake,and providing data to support the assessment of global greenhouse gas flux.