Study On Effects Of Nitrogen Deposition,Grazing And Extreme Precipitution On N₂O Emissions In A Leymus Chinensis Meadow

Soil is a significant source of atmospheric nitrous oxide（N₂O）.Soil N₂O emissions at global scale are affected by the intensification of global climate change and land use intensity,including nitrogen deposition,extreme precipitation and grazing.The contribution of N₂O fluxes from grassland ecosystems to global cannot be underestimated,as grasslands cover about 1/4 of the global land area.The production and consumption of N₂O are mainly regulated by biochemical processes（nitrification and denitrification processes,etc.）.Although,there are differences in the intensity and pathways of action,extreme precipitation,nitrogen deposition and grazing can alter the rates of nitrification and denitrification processes,thus affecting the rate and direction of soil N₂O exchange with the atmosphere.Currently,researchers have focused on the effects of single factor,such as climate change and human disturbance factors on grassland ecosystems,but the interaction of multiple influences is relatively rare.However,extreme precipitation,nitrogen deposition and grazing often occur simultaneously in the management and maintenance of grasslands,so it is important to understand the response of N₂O fluxes in grassland ecosystems to joint changes in multiple environmental factors and their potential regulatory mechanisms to properly assess the characteristics of global greenhouse gas emissions under climate change and increased human disturbance.This study was conducted in a natural Leymus chinensis meadow in the central part of Songneng meadow,China.Experimental platform including treatments of simulated N addition(urea,10 g N m^-2 yr^-1)and grazing（moderate,cattle）which was established to carry out a study on the effects of multi-factor interactions of N addition and grazing on soil N₂O fluxes,at the same time,to explore the long-term and short-term effects of simulated accidental extreme precipitation events on soil N₂O fluxes in the context of N addition and grazing.We explain the responses of soil N₂O fluxes to co-occurrence in the future of changes in precipitation patterns,increased nitrogen deposition and moderate grazing,identify key regulatory factors,and explore the influence pathways and underlying mechanisms,by comparing the differences in soil N₂O fluxes under different treatments and analyzing their associations with plant,soil physicochemical properties,soil nutrient and soil microbial content in L.chinensis meadow.The following main experimental results and conclusions were obtained through a large controlled experiment which simulating nitrogen deposition and grazing in the field and a microcosm experiment simulating extreme precipitation(60 mm d^-1),nitrogen deposition and grazing.（1）The annual cumulative of N₂O fluxes was 1.31±0.015 kg ha^-1 in L.chinensis meadow,which is the source of N₂O emissions.There was a stronger dependence of N₂O fluxes on temperature and precipitation in this study area,as N₂O fluxes peaked in July and the lowest emissions in January.The growing season dominates the cumulative N₂O fluxes throughout the year,accounting for about 90%,because both high temperature and precipitation area are concentrated in the growing season in the studied area,meanwhile,the dry and cold winter was not conducive to production of N₂O.The nitrification was the main source of N₂O fluxes,because,the water-filled pore space（WFPS）was less than 40%in this study area,which favored to the nitrification process.N deposition significantly simulated N₂O emissions compared to the control;grazing increased N₂O fluxes by only 13.7%,which did not reach significance level;N addition and grazing interacted on N₂O fluxes significantly.There were some key controlling factors affecting N₂O fluxes,such as above-ground biomass,soil bulk density,soil temperature,WFPS,ammonium nitrogen,soil potential net nitrogen mineralization rate and soil potential net nitrification rate.Results of the structural equation model indicated that N addition and grazing affected the nitrification process by altering N₂O fluxes mainly through increasing the ammonium N content in the soil.It emphasized that there were the coupling effects of grazing and nitrogen deposition on N₂O fluxes.In the context of future global change,though,there has the risk of increasing greenhouse gas emissions from moderate grazing,that reminds human to pay attention to the ecological benefits of grasslands as well as their economic benefits.（2）The extreme precipitation legacy effect,defined as the impact of historical precipitation on ecosystem structure and function,has been recognized as an important driver in shaping the temporal variability of N₂O fluxes in soil.There has rarely been quantitatively assessed on the legacy effects of extreme precipitation on N₂O fluxes in grassland ecosystems.In this study,we continuously monitored meteorological,plant,soil environment,soil microbial parameters and N₂O fluxes in L.chinensis meadow from April 6-July 18,2017 and April 6-July 18,2018,and conducted the contrastive analysis between before and after the extreme precipitation to assess the interannual legacy effects of extreme precipitation.The results showed that extreme precipitation had a significant legacy effect on above-and below-ground biomass and N₂O fluxes.Because increased soil moisture in the previous year stimulated bud pool of plants,which lead to the distinct growth advantage of plant in the second growing season.Plant growth consumed large amount soil N,which caused the microbial activity was limited by N-reactive substrates then inhibited N cycling rates.Nitrogen deposition significantly suppressed the negative legacy effect of extreme precipitation on N₂O fluxes as increased soil inorganic N,which relieved competition between plant and microbial for N substrates.Conversely,grazing contributed to the negative legacy effect of extreme precipitation on N₂O fluxes destructively,mainly attributed to animal foraging promoting compensatory growth of plant,increased microbial limitation of nitrogen substrate.In the context of future global change,episodic extreme precipitation may slow down greenhouse gas emissions from grasslands while favoring biomass accumulation.Excitingly,extreme precipitation can mitigate the contribution of nitrogen deposition to N₂O fluxes.Although moderate grazing has coupling effects with extreme precipitation to reduce greenhouse gas emissions from grasslands,physical damage to grasses and soils from animal trampling on grasslands with high water content cannot be avoided.Therefore,grazing behavior should be carefully managed to maintain sustainable development of grasslands after the occurrence of extreme precipitation.（3）Episodic extreme precipitation is characterized by sudden occurrence,short duration and large variability.Changes in the frequency of extreme weather events may have serious impacts on ecosystem structure and function,etc.,and may change the reaction rates of biogeochemical processes,and may even change the direction of biogeochemical process,whose effects on greenhouse gases in grassland ecosystems are poorly understood.In this study,we investigated the short-term effects of extreme precipitation on soil N₂O fluxes under different treatments by artificial watering to simulate extreme precipitation,adding urea to simulate nitrogen deposition and mowing to simulate grazing using microcosm experiments.We found that extreme precipitation significantly increased soil N₂O fluxes.During the drainage period after flooding,WFPS,inorganic nitrogen and soluble organic carbon had strong combined effects on N₂O fluxes,and when they reached certain level,in which nitrification and co-denitrification processes jointly dominate N₂O production.Nitrogen addition promoted production of N₂O by extreme precipitation,mainly because N addition provides substrates for nitrification and denitrification processes.Grazing mitigated the increase in N₂O emissions due to extreme precipitation,and grazing reduces the litter,resulting in reduction of available N in the soil,which inhibits N cycling processes through substrates.Therefore,reasonable grazing will mitigate the negative effects of climate extremes.Comprehensively,the productivity of L.chinensis meadow will be benefited more from increased nitrogen deposition and the legacy effects of extreme precipitation under global climate change.Reasonable grazing can mitigate the loss of diversity due to nitrogen deposition,but there is a risk of enhancement in N₂O emissions.In addition,there are adverse effects of extreme precipitation on N₂O fluxes on both short-and long-term scales that may lead to increased greenhouse gas emissions from L.chinensis meadow.In the future,global precipitation and nitrogen deposition continue to increase,proper grazing management of grasslands can mitigate the adverse effects of extreme weather events and increased nitrogen deposition on grassland ecosystems.This study provides scientific basis for understanding the characteristics and influencing factors of greenhouse gas emissions from grassland ecosystems in response to global climate change and anthropogenic disturbances.Moreover,this study provides data to support the assessment of global greenhouse gas flux changes,which is important for the evaluation of regional climate change.