Net Ecosystem CO₂ Exchange Of Meadoe Steppe In Hulunbuir,Inner Mongolia Under The Addition Of Nitrogen And Phosphorus Nutrients

Net ecosystem CO₂ exchange（NEE）is an important component of terrestrial ecosystem carbon cycle,which represents the capacity of terrestrial ecosystem to absorb atmospheric CO₂ and determines the carbon source and sink capacity of the ecosystem.The increasing global atmospheric nitrogen deposition had increased the availability of nitrogen and phosphorus in terrestrial ecosystems,affecting vegetation,soil and carbon cycle in global ecosystems.Carbon,nitrogen and phosphorus cycles in terrestrial ecosystems are strongly coupled.Therefore,it is of great significance to study how nitrogen and phosphorus addition affect net ecosystem CO₂ exchange in grassland for studying carbon balance and carbon source/sink function of grassland ecosystem.In order to explore the effect of nitrogen and phosphorus on NEE of meadow grassland in Inner Mongolia,a field experiment of nitrogen and phosphorus addition was conducted on meadow grassland in Erguna City,Hulunbuir City,Inner Mongolia.The diurnal and seasonal dynamics of NEE and the key processes affecting its change were measured and analyzed.The main results were as follows:（1）During the growing season,NEE of Hulunbuir meadow steppe ecosystem has obvious diurnal dynamic characteristics;and the NEE shows obvious seasonal changes with the change of rainfall.Carbon source is the key parameter affecting the drought on the ecosystem of Hulunbuir meadow grassland in the growing season of 2018,and the function of carbon source/sink was not changed by the addition of nitrogen and phosphorus.（2）Nitrogen addition greatly inhibited NEE.The reason is that its effect on ecosystem respiration（ER）is greater than that on total primary productivity （GPP）.Nitrogen addition promotes autotrophic respiration in plants（AR）. Nitrogen addition promoted plant root respiration（RR）and inhibited heterotrophic respiration（HR）,resulting in no significant effect on soil respiration（SR）.（3）Phosphorus addition had no significant effect on RR,but significantly promoted microbial heterotrophic respiration,so it had a significant effect on SR.At the same time,phosphorus addition significantly inhibited AR and GPP of autotrophic respiration,thus inhibited ER.But it had no significant effect on NEE.It was also found that the inhibitory effects of phosphorus addition on ER,GPP and AR depended on the availability of nitrogen.（4）Nitrogen and phosphorus together inhibited NEE and promoted ER,but had no significant effect on other carbon cycle processes.（5）NEE had no obvious correlation with above-ground biomass（AGB）, below-ground biomass（BGB）and total biomass.At the same time,ER and GPP also had no obvious correlation with BGB.In contrast,ER had significantly positive correlation with AGB and total biomass,while GPP had significant negative correlation with them.