Methane Fluxes And Their Responses To Climate Change In Different Grassland Ecosystems In Inner Mongolia

Methane（CH₄）is the second largest greenhouse gas after carbon dioxide（CO₂）,and its source and sink intensity has been widely concerned.Grassland is the second largest ecosystem in the world.Many studies have shown that it is an important sink of atmospheric CH₄.Inner Mongolia grassland is a typical representative of the temperate grassland in the north.The natural grassland area accounts for 1/5 of the national grassland area,and the sown pasture construction area ranks first in the country.It is of great significance for us to further understand the relationship between grassland ecosystem and global climate change to study the characteristics of soil CH₄ flux and its response to climate change in different grassland types in Inner Mongolia.This study adopted the methods of literature analysis and model simulation.Firstly,the database of CH₄ flux and influencing factors of different types of grassland in Inner Mongolia was established using the data in the literature and the existing measured data of the research group.We estimated the CH₄ uptake of different types of grassland,and explored the impact of environmental factors and grazing intensity on CH₄ uptake of different types of grassland.Then,the DNDC（De Nitrification-De Composition）model was validated by means of the CH₄ flux and the impact factors measured at the typical sites of natural and sown pasture.The past and future climate change scenarios were set,and the validated DNDC model was used to simulate and analyze the responses of CH₄ fluxes of different grassland types to climate change.The main results were as follows:（1）The grassland in Inner Mongolia was a sink of atmospheric CH₄,and its annual mean uptake rate was about 21.4～27.5 g C ha^-1 d^-1.The order of CH₄ uptake in different types of natural grassland in growing season was desert steppe>typical steppe>meadow steppe.（2）Temperature had a significant effect on CH₄ flux in typical steppe,desert steppe and sown pasture（p<0.05）.Precipitation,soil organic carbon and soil p H had significant effects on CH₄ uptake of the three types of natural grassland（p<0.05）.Soil NH₄⁺significantly affected CH₄ uptake in typical steppe and desert steppe（p<0.05）.Soil NO₃^-significantly affected CH₄ uptake in meadow steppe and typical steppe（p<0.05）.There was a significant negative correlation between CH₄ uptake and soil bulk density in sown pasture and typical steppe（p<0.05）.（3）CH₄ uptake was significantly inhibited by heavy grazing（HG）in grassland（typical steppe,desert steppe and sown pasture）（p<0.05）,while light grazing（LG）and moderate grazing（MG）had no significant effect on CH₄ uptake.（4）Through parameter sensitivity analysis of DNDC model,it was found that CH₄ flux of natural grassland and sown pasture was highly sensitive to temperature,precipitation,soil organic carbon combined soil bulk density and field water capacity.DNDC model can accurately simulate CH₄ flux,soil temperature and soil moisture on typical observing sites of natural grassland and sown pasture.The error（RMSE）between simulated results and measured results was lower than 5,and the fitting R²between simulated values and measured values was more than 0.6.（5）According to the meteorological data from 1961 to 1990 and the future climate change pattern provided by IPCC,we set the past and future climate change scenarios,and used the DNDC model to simulate the CH₄flux of natural grassland and sown pasture.It could be seen that the CH₄uptake of natural grassland and sown pasture increased with the temperature in the past 30 years,and the response of CH₄ uptake of natural grassland and sown pasture to precipitation was different.Under the future climate scenario,the CH₄ uptake of grassland ecosystem would gradually increase.Moreover,the CH₄ uptake of sown pasture was better than that of natural grassland.In the future,the construction of sown pasture could be encouraged,so as to further enhance the intensity of grassland carbon sink of methane.