| This study focuses on such areas as the southern and eastern part of Xi'an and near Chang'an county. The topsoil layer is usually composed of Malan loess. Because its composition is homogeneous and its construction is loose, it's the ideal layer to be studied for C02 emission. The landforms of southern and eastern part of Xi'an and Chang'an county are characterized respectively by flat second terrace of Wei river, Bailu tableland and Shaoling tableland. In the research areas, natural vegetation is hardly seen, but crops, artificial vegetation and secondary vegetation. This region has warm-temperate and continental monsoon climate with 4 distinct seasons.The method used in this study is alkali aqua absorption. The observation, which lasted nearly 2 years from September 1999 to June 2001, shows that the amount of C02 emission doesn't have the positive correlation with the change of temperature, which is different from other studies on other areas, that is to say the emission amount in the daytime is lower than that at night.The research shows that the amount of C02 emission from Soil in Xi'an decreases, and then rises and again decreases throughout the whole day from 8:OOAM to 8:OOAM next day. The release of C02 in the soil tends to keep pace with the temperature change, however the change of the former is later than that of the latter. Usually, from 11:00 to 14:00, the least is released from the soil, and then the amount of C02release increases rapidly and reaches its climax around the time period from 19:00 to 23:00. About 6:00 in the morning, the amount declines apparently.The amount of C02release from soil in Xi'an changes all the year round, with the highest amount in the summer, next autumn, and the lowest in the winter. Usually, the amount of C02release per day is found to be the highest in July, which is over 8 g/m2-d, next in August, and the lowest in December, January and February, which is less than 0. 5 g/m2-d.Temperature is the most important factor affecting the C02 release. In the term of seasonal change, temperature plays an apparently more important part. By statistic analysis, the amount of C02 release shows extremely obvious positive correlation with temperature. With the globebecoming warm, the amount of C02 emission will increase.The release of C02 in the soil is sensitive to the texture and structure of soil. More C02 is released from loose layers than solid ones. By putting certain number of observation sites, soil can be evaluated by the amount of C02 release.The amount of C02 emission varied under the different conditions of vegetation. Generally speaking, there is a higher amount of C02 emission on the bare ground, but a lower amount on the woodland and grassland. The amount of C02 emission would rise after the farmland is deserted. It's found that the C02release from the bare ground changes frequently, while it is more stable if the ground is covered by vegetation.The soil C02 is primarily produced by microorganism. According to the change of C02release, it's estimated that microorganism is more active at night than in the daytime.As far as the soil formed on the loess, C02come not only from the shallow layer about 1m underground, but also below 1m.The experiment shows vegetation plays an important role in slowing down the release of soil carbon and protecting the environment. The protection of vegetation and enhancement of ecosystem is one of the effective measures to curb C02 in the atmosphere. Therefore, the restoration and construction of the forest, and enlargement of Carbon sinks on the continent is an important aspect in the environment construction from now on.
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