Soil organic carbon as functions of slope aspects and soil depths in a semiarid alpine region of Northwest China

- Effects of slope aspects and depths on SOC in a semiarid alpine region were studied.
- SOC density ranged from 9.73 to 35.21 kg m− 2 at 0-60 cm at the hill scale.
- SOC density on the north aspects was about 3.2, 2.9 and 1.9 times larger than the south, southwest and west aspects.
- SOC could be expressed as functions of soil depths and slope aspects.

Soils in alpine regions associated with complex topography are characterized by large variability in the spatial distribution of soil organic carbon (SOC). However, the patterns and topographic controls on SOC at the hill scale in semiarid alpine regions are not well understood. In this study, the effects of slope aspects and depths on SOC were quantified based on field investigations in a mainly undisturbed region of the Qilian Mountains in northwestern China. Soil samples were collected at 0-10, 10-20, 20-40 and 40-60 cm on south-, southwest-, west-, and north-facing slopes of three hills. Results showed that the SOC density at 0-60 cm varied from 9.73 to 35.21 kg m− 2, and increased from the south- to north-facing slopes. The average SOC density on the north-facing slopes was about 3.2, 2.9 and 1.9 times larger than on the south-, southwest- and west-facing slopes. Both the general linear model and mixed linear model suggested that, at the hill scale, the slope aspects and soil depths explained the main variations of SOC concentration in our study. The profile pedotransfer function method indicated that the SOC varied predictably with soil depths and aspects, and the prediction functions well predicted the SOC data from literature. Our results highlight the importance of slope aspect as an indicator of the SOC, and demonstrate that the transformed aspect is a good continuous variable in predicting the SOC in the semiarid alpine region.