Objective  Exploring the stability of soil structure and the changes of organic carbon-nitrogen content after the conversion of secondary forest into Pinus koraiensis plantation, and analyzing whether the mixing of different broadleaved tree species with P. koraiensis could alleviate the decline of soil fertility caused by the pure P. koraiensis plantation, so as to provide a basis for the selection of mixed tree species and the targeted improvement of forest soil quality.

  Method  Taking Juglans mandshurica-P. koraiensis plantation, Fraxinus mandshurica-P. koraiensis plantation, Phellodendron amurensis-P. koraiensis plantation and the pure P. koraiensis plantation built in Maoershan Forest Farm of Northeast Forestry University as the research object, and taking the secondary forest with main tree species of J. mandshurica and F. mandshurica as the reference, the soil aggregate classification was carried out by the combination of dry screening and wet screening, and the aggregate distribution, carbon-nitrogen content of each particle size were measured. Changes on soil aggregate stability and carbon-nitrogen distribution after the transformation of the secondary forest into P. koraiensis plantation were analyzed by calculating the parameters on stability of soil structure and the contribution rates of organic carbon-nitrogen in the aggregates with different particle sizes.

  Result  After the secondary forest being transformed into P. koraiensis plantation, the mass fraction of > 2 mm particle size aggregates decreased, among which J. mandshurica-P. koraiensis plantation decreased by 17.94%, the mass fraction of < 0.053 mm particle size aggregates increased, and the pure P. koraiensis plantation increased by 45.78%. MWD and GMD of the soil decreased, and the stability of aggregates decreased. The contents of organic carbon and total nitrogen of aggregates with different particle sizes decreased in varying degrees, and the decline degree of J. mandshurica-P. koraiensis plantation was the lowest; the contribution rates of organic carbon and nitrogen in soil aggregates of the secondary forest and three mixed P. koraiensis plantations were mostly large aggregates, while the contribution rates of organic carbon and nitrogen in soil aggregates of the pure P. koraiensis plantation were mainly micro aggregates in 10−20 cm and 20−30 cm soil layers.

  Conclusion  After secondary forest being transformed into P. koraiensis plantations, the soil aggregate stability, carbon and nitrogen content decrease in varying degrees. From the analysis of soil aggregate distribution, stability and organic carbon-nitrogen content, J. mandshurica, F. mandshurica and P. amurense are the suitable mixed tree species, which could improve the nutritional status of P. koraiensis plantation.