Characteristics And Driving Mechanism Of Soil Reactive Nitrogen Emissions From The Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau is the"third pole"in the world,the"sensitive area"in global climate change research,and one of the most important ecological regions in the world.The high altitude determines its unique characteristics of strong atmospheric oxidation,and has significantly different soil microbial community structure and nitrogen cycle process.The soil nitrogen cycle process can not only emit nitric oxide（NO）and nitrogen oxide（NO_x）,but also gaseous nitrite（HONO）,and then participate in the cycle of atmospheric hydrogen and oxygen free radicals（OH·）,ozone（O₃）and volatile organic compounds（VOCs）,directly or indirectly affecting the atmospheric oxidation and air quality.The highest amount of HONO emitted from soil can account for 60%of the atmospheric HONO concentration,which is the frontier of the cross study of soil nitrogen cycle and atmospheric chemistry.However,there have been no relevant reports on soil HONO emissions in the Qinghai-Tibet Plateau region,and the potential pathways and driving mechanisms of soil HONO emissions in this region are still unclear.Based on this,this study measured the emission fluxes of HONO,NO,and NO_x from soil in the Qinghai-Tibet Plateau through field sampling and indoor dynamic box systems;The annual emission flux of soil active nitrogen gas in Tibet was estimated using the empirical"dry-wet alternation"model,and the emission characteristics and spatial distribution pattern of soil active nitrogen gas in the region were revealed;Through the determination of soil physical and chemical properties,microbial community and correlation analysis,the key biological or abiotic factors affecting the emission of HONO,NO and NO_x in the soil of the Qinghai-Tibet Plateau were analyzed;Simulate the NO_x concentration in the atmosphere in the field,explore the impact of different concentrations of NO/NO_x+O₃ fumigation on the emission of soil active nitrogen gas,and clarify the driving mechanism of soil active nitrogen gas emission.The main conclusions are as follows:（1）The peak range of HONO,NO and NO_x emissions from the soil in the Qinghai-Tibet Plateau is 2.52～235.15 ng N m^-2 s^-1,1.18～268.41 ng N m^-2 s^-1,9.66～282.13 ng N m^-2 s^-1,and The The integrated emission range of a single dry and wet alternative process is 0.01～1.77 mg N m^-2,0～3.73 mg N m^-2,and 0～3.66 mg N m^-2,respectively.Among them,the peak value of HONO emission from cropland soil is the highest(42.10±26.24 ng N m^-2 s^-1),and the peak value of wetland emissions is the lowest(2.95±0.18 ng N m^-2 s^-1);The peak emission of NO and NO_xin bare soil is the highest(94.15±72.11 ng N m^-2 s^-1;97.13±69.03 ng N m^-2 s^-1),and the peak emission of NO and NO_x in wetland is the lowest(3.15±0.76 ng N m^-2 s^-1;11.19±0.69 ng N m^-2s^-1).（2）The soil HONO emission in the Qinghai-Tibet Plateau is significantly affected by nitrate nitrogen（NO₃^--N）concentration and the diversity of archaea community（Simpson index）;NO and NO_x emissions were significantly affected by ammonium nitrogen（NH₄⁺-N）,NO₃^--N concentration,total nitrogen（TN）,total carbon（TC）content,amo A,nap A,nir K,nos Z and other nitrification and denitrification function gene abundance;NO_x emission is also significantly affected by the lower soil pH and the evenness of archaea community（Shanoneven index）.（3）Based on the empirical"dry-wet alternation"model,the annual emission fluxes of HONO,NO and NO_x from soils in Tibet are 7.01 Gg N yr^-1,11.6 Gg N yr^-1 and 20.3 Gg N yr^-1 of which the bare soil contributes the most to the total regional emissions（43%、46%and 67%）.The spatial distribution of annual emissions of soil active nitrogen shows that there are more in the southeast of Tibet and less in the northwest of Tibet.There are four HONO emission hotspots,namely,Changdu region,Shannan region,Shigatse and Lhasa region.（4）Fumigation with high concentration of NO/NO_x+O₃ promoted the emission of HONO,NO,and NO_x from forest soil,with a maximum emission peak at 10 ppb NO;Inhibiting the emission of HONO,NO,and NO_x from grassland soil;The impact on HONO,NO,and NO_x emissions from farmland soil is weak,with a minimum emission peak at a concentration of 10 ppb NO_x+30 ppb O₃.The change in total emissions is consistent with the peak emissions.After fumigation with different NO and NO_x+O₃ concentrations,changes in soil pH,NO₃^--N,and NO₂^--N concentrations are potential factors driving soil HONO,NO,and NO_x emissions.The change of pH strongly affects the emission of HONO;Changes in the concentration of NO₃^--N may be a leading potential factor in promoting the emission of HONO,NO,and NO_x.In addition,changes in the concentration of NH₄⁺-N affect the emission of NO.Overall,there are significant differences in HONO,NO,and NO_x emissions from different ecological types of soils in the Qinghai-Tibet Plateau,which are affected by soil physical and chemical properties,nitrogen cycle function gene abundance,and microbial diversity.The annual emission fluxes of soil active nitrogen gases in Tibet are 7.01 Gg N yr^-1（HONO）,11.62 Gg N yr^-1（NO）,and 20.31 Gg N yr^-1（NO_x）,respectively.Bare land and grassland are the main emission sources.Forest land,grassland,and farmland soils have different responses to high concentrations of NO/NO_x+O₃ fumigation,while soil pH and mineral nitrogen are potential driving factors.This study indicates that soil active nitrogen gas emissions,especially from bare land and grassland,may enhance atmospheric oxidation and exacerbate air pollution in the Qinghai-Tibet Plateau.