Aims Soil aggregate is the main habitat for decomposition and transformation of soil organic carbon (SOC) and is important to regulate SOC sequestration. The mechanisms of the stability of SOC fractions may vary among different aggregate sizes. The aims of this study were to explore the characteristics of SOC "fractionation" in soil aggregates, and to reveal the mechanisms of carbon (C) sequestration in soil aggregates of mollisols after 31-year changes in vegetation cover. Methods A long-term field experiment with different vegetation cover (grassland, farmland and bareland) was established in National Observation Station of Hailun Agro-ecosystem System. Soil aggregate fractionation, the density and humus fractionation within different aggregate sizes were further carried out. Important findings The results showed that after 31 years of land cover change, the surface SOC and total nitrogen (TN) contents in grassland with higher C inputs increased significantly with time, while the SOC and TN contents decreased significantly in bareland, but with no statistical significance in farmland. The 2-0.25 mm (include 2 mm, the same below) aggregates was the excellent fraction for SOC sequestration under all three land cover. The stability of soil aggregate was in the order of: grassland > farmland > bareland. The mass proportion of

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