Active soil organic carbon, derived from the decomposition of litter, root exudates, soil organic matter hydrolyzation, soil microbe per se and its metabolite is the active fraction of soil organic matter. Despite of the lower percent of the active soil organic carbon to the total soil organic matter, it was a more sensitive indicator of the soil effects caused by management and environmental changes. Therefore, active soil organic carbon was a critical component in maintaining the balance of soil organic carbon and soil fertility. The production, turnover and decomposition of fine roots are major process in the carbon and nutrient dynamics. The input of nutrients and organic matter to the soil by fine roots contributes to the soil fertility and carbon sequestration. Most previous studies focused on the soil organic carbon in plantations converted from agricultural lands, however, little information was available on the active soil organic carbon during the process decomposition process of fine root (grass root). The carbon dynamic of fine roots (grass roots) decomposition and soil microbial biomass carbon (SMBC),water soluble organic carbon (WSOC),readily oxidizable carbon (ROC) and total organic carbon (TOC) in land use conversion from agricultural lands into forest lands were evaluated by the intact core methods in the present study. 560 intact soil cores were sampled from the plots of four models viz. Betula luminifera-Hemarthria compressa mixed pattern, Betula luminifera plantation, Hemarthria compressa grassland and Cryptomeria fortunei plantation for one year. After being placed in nylon bags, these intact soil cores were put back to the soil and collected again after 30, 90, 180, 270 and 365 d. Soil microbial biomass carbon (SMBC), water soluble organic carbon (WSOC),readily oxidizable carbon (ROC),total organic carbon (TOC) were then measured for each sample. The results showed that fine roots (grass roots) released net carbon during the decomposition in four models. Carbon loss of decomposing roots was fit by a single exponential decay model. SMBC, WSOC, ROC and TOC content were higher in Betula luminifera-Hemarthria compressa mixed pattern(P＜0.05) than other three models. The contribution ratio of SMBC to TOC were 1.2%-3.3%、0.7%-1.5%、0.8%-2.2%、0.5%-0.8%, respectively across the four models. ROC/TOC in Betula luminifera-Hemarthria compressa mixed pattern was also highest among the four plantations. Moreover, ROC content was positively correlated with TOC content among the four models (P＜0.05). The results indicated that soil organic carbon in Betula luminifera-Hemarthria compressa mixed pattern was much easier to be transformed compared to the other three plantations, and the ROC content in the four models depended on the content of TOC at sites with similar environmental conditions.