Abstract: The soil samples were collected from 0-10 cm topsoil in the semi-fixed sand land and fixed sand land under the shrubberies and arbor forests regenerated for 23-54 years in the MU US sandy land in North Shaanxi Province. Each soil sample was separated as the light and heavy fractions by density grouping method,and the evolution and accumulation rates of organic carbon and total nitrogen,C/N and the characteristics of contribution proportion for sequestrating carbon and nitrogen in these two soil fractions were analyzed. The results indicated that the organic carbon contents in soil light fraction under the shrubberies and arbor forests regenerated for 23-54 years increased by 14.0-40.6 times and 8.8-19.2 times respectively,which were significantly higher than those in the corresponding heavy fractions (3.2-7.7 times and 3.5-8.1 times). In the same period of vegetation regeneration,the total nitrogen contents in soil light fraction under the arbor forests and shrubberies were increased by 14.5-40.9 times and 11.8-29.1 times respectively,which were also significantly higher than those of the corresponding heavy fractions (4.6-8.5 times and 4.4-12.6 times). These results indicated that the soil organic carbon and total nitrogen in light fraction were more sensitive to the desertification reversion than that in heavy fraction. Increase of the contents of organic carbon and total nitrogen in the light fraction and heavy fraction of soil also made the growth rates of organic carbon density of light fraction under the forests and shrubberies reach to 0.57 mg·hm^-2·a^-1 and 0.26 mg·hm^-2·a^-1 respectively. The growth rates of the organic carbon density in heavy fraction under the forests and shrubberies were only 0.18 mg·hm^-2·a^-1 and 0.20 mg·hm^-2·a^-1 respectively. Moreover,the growth rates of the total nitrogen density in light fraction reached to 0.03 mg·hm^-2·a^-1 and 0.02 mg·hm^-2·a^-,and they in heavy fraction reached to 0.02 mg·hm^-2·a^-1 and 0.04 mg·hm^-2·a^-1,respectively. According to these growth rates of carbon and nitrogen fractions,the carbon of light fraction under the forests and shrubberies could contribute 75.9% and 59.4% of the total organic carbon increment respectively during the 54-year restoration. The nitrogen in heavy fraction could contribute 44.6% and 63.9% of total nitrogen increment respectively. In addition,the C/N ratios of soil heavy fraction under the two woodlands regenerated for 54 years were significantly reduced by 11.4% and 38.5% respectively compared with those in semi-fixed sand land. However,there was no significant change of C/N in the light fraction under the forests,and the C/N in the light fraction under the shrubberies regenerated for 23-54 years was decreased by 21.7%-31.0%. As a result the quality of soil carbon pool was improved significantly. Therefore,the soil of restoration under the sand-fixing forests in North Shaanxi affected significantly the sequestration of carbon and nitrogen. Moreover,arbor forests has better carbon sequestration ability and shrubberies has better nitrogen sequestration effect.

Key words: sand-fixing forest, restoration process, soil carbon and nitrogen, carbon sequestration, Mu Us sandy land