Objective  Our project objective is to explore the spatial distribution characteristics of soil organic carbon and its influencing factors of the main vegetation community types formed after returning farmland to forest land (grassland) in the loess region of northern Shaanxi Province, northwestern China and to evaluate soil carbon sink management and ecological benefits of planted forests in the future, in order to provide reference basis and accumulate basic data for related research on forest soil carbon in northern China.

  Method  Taking the Pinus tabuliformis forest, Hippophae rhamnoides forest, grassland and mixed forest of Pinus tabuliformis and Hippophae rhamnoides in Dajigou Forest Park of Wuqi County as the research objects, the typical sample plots were selected, and the single factor analysis of variance and gray correlation method were used to explore the different types of vegetation in 0−100 cm soil organic carbon vertical change and its main influencing factors.

  Result   (1) The soil organic carbon content and storage in the study area had obvious surface aggregation phenomenon, and decreased with increasing soil depth. (2) The average content of soil organic carbon under different vegetation types was shown as Hippophae rhamnoides forest (7.03 g/kg) > low-slope Pinus tabuliformis-Hippophae rhamnoides mixed forest (5.34 g/kg) > grassland (5.16 g/kg) > high-slope Pinus tabuliformis-Hippophae rhamnoides mixed forest (3.87 g/kg) > Pinus tabulacformis forest (3 g/kg). Significant differences in average soil organic carbon content were found between Hippophae rhamnoides forest and Pinus tabuliformis forest, high-slope Pinus tabuliformis-Hippophae rhamnoides mixed forest (P < 0.05). (3) Soil organic carbon reserve of different vegetation types ranged from 41.11 to 74.76 t/ha. (4) The C/N value of soil profiles of different vegetation ranged from 16.41 to 39.11, and the average C/N values varied from big to small as Hippophae rhamnoides forest (34.68) > low-slope Pinus tabuliformis-Hippophae rhamnoides mixed forest (25.88) > grassland (25.82) > Pinus tabuliformis forest (23.08) > High-slope Pinus tabuliformis-Hippophae rhamnoides mixed forest (22.71). (5) The correlations between soil physical and chemical factors and organic carbon content of different vegetation types are all above medium correlation, and closely related to organic carbon content.

  Conclusion  In the future construction of carbon sink forests in the study area, the influencing factors of soil organic carbon should be fully considered, and preferentially select dominant tree species such as Hippophae rhamnoides forest.