Variation in soil organic carbon by slope aspect in the middle of the Qilian Mountains in the upper Heihe River Basin, China

Topography has an influence on both abiotic (e.g. soil) and biotic (e.g. vegetation) factors influencing soil organic carbon (SOC). In this study, three slope aspects, south-facing, semi-sunny (south-west), and semi-shady (north-west) of mountainous grassland in the middle of the Qilian Mountains were sampled to explore the variation in SOC caused by topography. Soil properties were analyzed at different depths (0–10, 10–20, 20–40, and 40–60cm). The results showed that except soil total organic phosphate, SOC, soil water content, soil bulk density, daily soil temperature, biomass, vegetation cover, and species richness on the semi-shady slope aspect were significantly different to those on two other slope aspects. The SOC, vegetation cover, and biomass significantly increased, from the south-facing to semi-shady slope aspect, and their values on the semi-shady slope aspect were around two times greater than on the south-facing slope aspect. The plant community composition also varied by slope aspect, with Agropyron cristatum and Stipa grandis dominant on the south-facing slope aspect, and Agropyron cristatum and Carex aridula dominant on the semi-sunny slope aspect, and Kobresia humilis and Carex crebra dominant on the semi-shady slope aspect. Among all measured abiotic and biotic factors, daily soil temperature, soil bulk density and total soil organic nitrogen could be used to predict SOC, explaining 69.0% of the variation in the data (p<0.001). On all slope aspects, SOC was greatest in the top 20cm of soil, representing around 60.0% of the total SOC to 60cm depth. The results indicated that the distribution of SOC was regulated by slope aspect through its influence on vegetation and soil properties in this region. Therefore, SOC protection measures should be aspect specific.