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The dynamics of soil organic carbon (SOC) under different land uses can be beneficial for accurately assessing carbon sequestration to deal with global climate change. The aim of this study was to quantify the SOC content in various fractions under different land uses. Soil samples were collected from the top 0.1 m and 0.1-0.3 m of cropland, grassland, and forest in Huachi County of Gansu Province, China. Significant differences in physical fractions were found in the top 0.1-m layer, with cropland having the highest proportion of micro-aggregates (< 250 mu m), forest having the highest proportion of small macro-aggregates (250-2000 mu m), and grassland tending to form large macro-aggregates (> 2000 mu m). SOC values were 6.9, 11.3, and 10.3 g kg(-1) in the top 0.1-m layer for cropland, grassland, and forest, respectively. The difference in d13C between the light and heavy fraction in small macro-aggregates was smaller than that in both large macro-aggregates and micro-aggregates. These results indicated that small macro-aggregates conserved SOC relative to micro-aggregates and large macro-aggregates. The differences in d13C between the light and heavy fraction in all aggregate size fractions of grassland were much larger than in forest, indicating that there was higher microbial decomposition in grasslands. Overall, our results suggested that the accumulation of SOC in grassland was derived from the abundant carbon input, but the protection of SOC from small macro-aggregates was important to forest soil.