It is unclear whether the soil carbon sequestration capacity of plantation forests can reach the level of natural forests,especially under the declining area of natural forests and increasing the area of plantation forests on a global scale.Taking planted and natural Pinus tabuliformis forests in the Ziwuling Forest Region of Loess Plateau as the research object,the distribution characteristics and correlations of soil aggregate composition,soil organic carbon (SOC),different degrees of labile SOC and Glomalin-related soil protein (GRSP) in 0—20 cm soil layer of different density plantations and natural forests were compared and analyzed.The results showed that:(1) The total SOC content and the content of its composition:very-labile,less-labile,and non-labile SOC in the whole soil of the plantations were significantly lower than those in the natural forests.The total SOC and non-labile SOC contents increased with increasing stand density,while the less-labile SOC content was the opposite.(2) The total SOC and its component contents of each particle size aggregates in the plantations were significantly lower than those in the natural forests.With increasing stand density,the weight percentage of macroaggregates,total SOC content and non-labile SOC content of macroaggregates of plantation forests increased,but were significantly lower than those of natural forests.(3) The SOC content of the plantations increased with the increase of stand density,and its peak value was 17.95 g/kg,which was 65.5% of the SOC content of the natural forests.Among them,the number of macroaggregates and their SOC content was significantly reduced,which was the main reason for the difference.The percentage of labile SOC was negatively correlated with the increase of total SOC content,GRSP increased with the increase of stand density but was lower than that of natural forest,and the total SOC content was increased by increasing the number of macroaggregates.Increasing the plantation stand density can improve its soil carbon sequestration capacity by increasing surface soil biomass,the number of soil macroaggregates,and the GRSP content,and reducing the percentage of labile SOC,but significantly reduces the plant diversity of the plantations.