Abstract: In this study,in order to assess the impacts of the first cycle of Larix olgensis afforestation on soil organic carbon,the authors used Dunhua City in northeast China as a case region to investigate the soil carbon in seven Larix olgensis stands during different afforestation periods,with an adjacent 250-year-old natural forest as the control for the possible long term soil carbon storing in northeast China. Mineral soil was sampled and the carbon contained in forest biomass and forest floors was measured to explore the qualitative as well as quantitative changes of soil carbon.Results showed that soil carbon decreased to a minimum level of 75.87 Mg/hm² in the first 12 years and then gradually recovered.In the 21st year after afforestation,the soil organic carbon density of forests rose to a level as high as that in the former cropland,ie 84.28 Mg/hm².Thereafter,the soil carbon started to accumulate.Within the short time span of 33 years,the amounts of carbon lost or gained by soil were much smaller than those accumulated in forest biomass.The soil carbon accounts for 63.4% of the ecosystem carbon stock in the 250-year-old afforested stand,suggesting that nutrient-rich afforestation soils may serve as the potential sinks for carbon in long term.Particulate organic matter(POM) was a sensitive index to reflect the soil quality.In the 0-30 cm soil layer,the percentages of coarse POM for the L.olgensis and natural forests were 2.7 and 3.4 times as high as that for the cropland.The proportion of particulate organic carbon in the total soil carbon increased,which indicates an improvement of the soil quality after afforestation.