Objective  Farmland soil organic carbon (C) and nitrogen（N）density are important indicators to measure soil fertility and soil quality. To explore the current status and differences of organic C and N accumulation in farmland ecosystems in alpine regions.

  Method  Soil samples were collected in layers from the southern slope of the middle section of Qilian Mountains, and the spatial distribution of organic C, total N contents and vertical distribution characteristics of their density along the altitude was measured indoors.

  Result  ① The soil organic C density of the 0-50 cm soil layer in the study area reached a maximum of 10.18 kg m⁻² at an altitude of 2800 m. The total N density reached a maximum of 1.86 kg m⁻² at an altitude of 3000 m, and the soil organic C density increased with the altitude. The increase showed a “U”-shaped curve, and the total N density showed a unimodal curve with the increase of altitude. ② On the 0-50 cm soil profile, soil organic C density and total N densities decreased with the increase of soil depth at altitudes ≤ 3000 m, and increased with soil depth at altitudes of 3100 m. ③ Altitude has a significant positive correlation with soil total N content (P < 0.01), and at the same time, it has a significant positive correlation with total N density (P < 0.01). Soil depth has a significant negative correlation with organic C content (P < 0.01), but there is a very significant positive correlation with soil organic C density and total N density (P < 0.01).

  Conclusion  Altitude and soil layer are the key factors affecting the distribution of soil organic C and total N in farmland on the southern slope of Qilian Mountains.