Precise classification of soil organic fractions is the basis for studying soil organic carbon(C)and nitrogen(N)dynamics.However,relevant information about the secondary poplar-birch forest(SPBF)is scarce,although it is one of the main natural forests in Northeast China.In this study,we discuss the potential of soil C sequestration and soil fertility in this forest.For this purpose,soils were sampled from layers at depths of 0-10,10-20,and 20-30 cm in a typical SPBF in Changbai Mountain.Collected soils were divided into five fractions according to their physicochemical stabilities,which included sand and stable aggregates soil fraction(SA),acid insoluble soil fraction(AI),easily oxidized soil fraction(EO),particle soil fraction(P),and soluble soil fraction(S).Mass fraction(percentage of each component in the intact soil),concentration of organic C and N,allocation proportions of organic C and N,C/N ratio,and five infrared functional groups in all fractions were analyzed.Results showed that nearly 90%of all soil fractions was classified as AI(66.21%)and SA(22.11%),which resulted in the highest amount of soil C sequestration(about 2/3 of the entire soil C).This amount was occluded in stable fractions of soil with a 2- 9 fold higher C/N ratio than the active fractions of P and EO.The 4-80 fold higher N concentration in the active fractions of P and EO resulted in higher allocation proportion of N in these two fractions(33.1% and 26.0%, respectively).Close correlation between organic C and N was generally found among fractions or within each of the fractions,with the exception of those in the P and S fractions owing to their extremely low proportion in the total intact soil.This difference in storage pattern of organic C and N among different fractions favors the long-term storage of C in soil with rapid N supply for plant growth.Together with the changes in organic C and N in different soil fractions,different infrared functional groups were observed in different soil fractions.The relative contents of most of infrared functional groups in the AI fraction were the lowest,while the relative contents of most of infrared functional groups in the P and S fractions were higher than those in other groups.Significant positive correlations among each couple of organic C,N,and most of the infrared functional groups in the same fraction were determined, indicating that the infrared functional groups can probably maintain the functions of soil C and N.Otherwise, significant correlations between the infrared functional groups and C/N ratio indicated the relative contents of infrared functional groups of the soil fractions,revealing various chemical activities of different soil fractions.Our findings indicate the importance of clarifying the mechanism of soil C sequestration and N availability in soils in SPBF.These data provide a basis for understanding changes in soil C and N dynamics in the SPBF and their possible performance in response to global warming process.