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

This research achievement is published on CATENA 

Keywords: Semiarid alpine region; Soil organic carbon; Slope aspect; Soil depth; Profile pedotransfer function 

Fig. Location of study area and distribution of sampling plots on hillslopes. (a) Tridimensional surface map of study area, (b) images of Hills A, B and C, (c) 3 m contour map of Hill A indicating the distribution of soil profiles. The sampling method on Hills B and C is the same as Hill A. The SFS, SWFS, WFS, and NFS represent south-, southwest-, west-, and north-facing slope, respectively. 

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