Abstract: On the basis of the principle of which CO₂ is absorbed by the solution of NaOH, the release amount of CO₂ from soils under different vegetation was observed round the clock in Changyanbao of Xi'an, and study the release amount of CO₂ and its change law as well as the factors affecting the release of CO₂.The observed material shows that the release amount of CO₂ in 24 hours from March to May per square metre is between 900 and 4500 mg from grassland, and is between 1000 and 5200 mg from forest land. The release amount of CO₂ from soils under different vegetation is increased from March to May, which indicates that the release amount of CO₂ is much when temperature is high. The release amount from different soils changes from low to high and then to low from morning to the next morning. By comparing the curves of air temperature change with the curves of the release amount curves of CO₂, it is known that the time stage of much release amount of CO₂ is corresponding to that of low temperature, and the time stage of less release amount of CO₂ is corresponding to that of high temperature. The reason why the phenomenon occurs is that the change of soil temperature is about 4～6 hours later than change of the air temperature. This shows that the change of the release amount of CO₂ from soils is determined by soil temperature. The release amount is much when the soil temperature is high and is less when the soil temperature is low. There is the difference of the release amount of CO₂ from the soils under different vegetation. The release amount of CO₂ from forest land is more than that from grassland, and the latter is more than that from exposed land. Vegetation play a part in affecting the release amount of CO₂.The vegetation develops well and produce much organic substance, which is favourable to the living of microorganism and producing of much CO₂.The release amount from forest land and grassland in night takes about 70% in day and takes about 30%. The release amount of CO₂ in exposed land in night is slightly greater than that in day.

Key words: Southern suburbs of Xi'an, CO2release from soils, release law, environment