Simulated Carbon Expenditure Potential Of Three Grassland Types In Inner Mongolia Based On DNDC Model

As an important part of the Eurasian steppe,Inner Mongolia grassland is a natural experimental grassland to study the soil carbon and nitrogen characteristics in the background of global climate change.Nitrogen deposition may cause changes in the vegetation community composition in the region and then affect the carbon cycle.At present,although the characteristics of carbon budget in Inner Mongolia under the change of nitrogen settlement have been studied,the carbon balance of grassland ecosystem under various nitrogen addition gradient needs to be deeply studied,it is important to clarify the effect of nitrogen addition on the grassland ecosystem.In this study,three kinds of grassland（meadow grassland,typical grassland and desert grassland）in Inner Mongolia grassland were used,seven nitrogen added gradient CK(0 g N·m^-2·yr^-1),N1(5 g N·m^-2·yr^-1),N2(10 g N·m^-2·yr^-1),N3(15 g N·m^-2·yr^-1),N4(20g N·m^-2·yr^-1),N5(25 g N·m^-2·yr^-1),N6(30 g N·m^-2·yr^-1)were set,based on the simulation of DNDC model,the effect of nitrogen addition on the three grassland types,It aims to provide basic data on strategic knowledge reserves to mitigate greenhouse gas emissions and ecosystem management,At the same time,it also provides scientific basis and basic data for the accurate analysis and prediction of grassland carbon source sink changes and carbon budget under the background of nitrogen settlement.The main conclusions are as follows:（1）The proportion of different nitrogen addition gradients on the aboveground biological carbon reserves of meadow grassland is greater than that of typical grassland and desert grassland,he saturation threshold of aboveground carbon reservoirs of meadow grassland may be 15 g N·m^-2·yr^-1.The promoting effect of nitrogen addition on the typical grassland underground biological carbon reservoir reserves,soil organic carbon reserves and total carbon reserves is greater than that of meadow grassland and desert grassland.The aboveground biological carbon reservoir reserves of the three grassland ecosystems are meadow grassland>desert grassland>typical grassland.Underground biological carbon bank reserves、soil organic carbon reserves and total carbon reserves are meadow grassland>typical grassland>desert grassland.Besides,compared with the control,the underground biocarbon bank reserves of the typical grassland were significantly increased under N3treatment（P<0.05）,none of the rest showed significant changes（P>0.05）,while soil organic carbon reserves increased only under the N4 gradient of typical grassland（P<0.05）.And it is worth noting that the total carbon reserves all peaked under the N4gradient.（2）There is a significant positive correlation between aboveground and underground biological carbon bank reserves and soil organic carbon reserves under different nitrogen addition gradients（P<0.01）,underground biomass carbon reserves and total carbon reserves,soil p H and average annual temperature have significant negative effects（P<0.01）,while average annual precipitation,soil organic carbon and soil quick-acting nitrogen have significant positive effects on the three（P<0.01）.The aboveground biomass carbon reserves were significantly negatively associated with average annual temperature and soil p H（P<0.01）,and showed a significant positive correlation with soil organic carbon and average annual precipitation（P<0.01）.（3）The"point point mode"based on the DNDC model determines a set of localization parameters to simulate the carbon cycle characteristics of meadow grassland,typical grassland and desert grassland ecosystem in Inner Mongolia,selected parameters for sensitivity analysis before using soil temperature,soil water content,aboveground biomass carbon and soil organic carbon indicators corrected and verified the high simulation accuracy,the model can largely reflect the real situation of the ecosystems of these three grasslands.DNDC simulations showed that the peak of community ecosystem carbon flux（NEE,ER,GEP）under different nitrogen application conditions appeared in June and July,and the lowest values appeared in May and August,but nitrogen application The seasonal trend was not changed,and the concentration of the control showed a unimodal curve.The carbon flux change characteristic of typical grassland and desert grassland ecosystem is one month behind that meadow grassland,and the net ecosystem carbon exchange amount of meadow grassland is higher than that of typical grassland and desert grassland,and the carbon fixation capacity of grassland is stronger under humid climate conditions is stronger.（4）Different levels of nitrogen addition promotes the carbon flux in the ecosystem,and has a relatively strong excitation effect on the typical grassland and desert grassland.Compare gross ecosystem productivity（GEP）,ecosystem respiration（ER）,and net ecosystem carbon exchange（NEE）showed that the Inner Mongolia grassland ecosystem reached large carbon flux at the nitrogen addition concentration of 15-20 g N·m^-2·yr^-1.According to the analysis of the DNDC model,The nitrogen concentration gradients of NEE of the three grassland ecosystems were15 g N·m^-2·yr^-1,5 g N·m^-2·yr^-1and 20 g N·m^-2·yr^-1.Correlation analysis showed that soil moisture,soil temperature,rainfall and temperature all had significant positive correlation on ER and GEP（P<0.01）and significant negative effects on NEE（P<0.01）and significant positive correlation between ER and GEP（P<0.01）.