Abstract: Clarifying the characteristics of soil organic carbon and its fractions and enzyme activities across different forest types can provide the reference for ecological restoration and soil quality improvement. In this study, we investigated the distribution patterns of soil organic carbon, its fractions, and enzyme activities as well as their relationships in the 0-40 cm soil layer in five different types of plantations (Populus alba var. pyramidalis stand, Robinia pseudoacacia stand, Platycladus orientalis stand, a mixed stand of Populus alba var. pyramidalis and Robinia pseudoacacia, and a mixed stand of Platycladus orientalis and Robinia pseudoacacia) on the southern and northern mountains of Lanzhou City. Among the five stand types, Platycladus orientalis stand had the highest average soil organic carbon and light and heavy fraction organic carbon, followed by R. pseudoacacia stand. The highest average soil organic carbon and light and heavy fraction organic carbon were recorded in May. Soil organic carbon and light and heavy fraction organic carbon at 0-20 cm layer in Platycladus orientalis stand were higher than those in R. pseudoacacia stand. At 20-40 cm layer, they showed fluctuating changes among different forest stand types. The highest average soil invertase, amylase, and cellobiase activities were found in the Populus alba var. pyramidalis and R. pseudoacacia mixed stand. Soil enzyme activities of each stand were highest in June. The activities of soil enzymes in the 0-20 cm soil layer showed Populus alba var. pyramidalis and Robinia pseudoacacia mixed stand > Platycladus orientalis and R. pseudoacacia mixed stand. In the 20-40 cm soil layer, the activities of the three soil enzymes were pure forest > mixed forest. There were significant differences in soil enzyme activities among different forest types. The contents of soil organic carbon and its fractions and soil enzyme activities of different forest stand types all showed a significant downward trend with increasing soil depth (P<0.05). Soil organic carbon and its fractions were extremely significantly correlated with soil invertase activity (P<0.01). Soil organic carbon and its fractions as well as soil enzyme activities were affected by forest stand types, with the highest levels in the Platycladus orientalis stand and Populus alba var. pyramidalis and R. pseudoacacia mixed stand, respectively, indicating that the establishment of coniferous forests and mixed forests is of great significance for ecological restoration.

Key words: forest type, artificial forest, organic carbon, light and heavy-fraction organic carbon, soil enzyme activity