Chemical composition of soil organic carbon and aggregate stability along an elevation gradient in Helan Mountains, northwest China

•Soil organic carbon content (SOC) increased along elevation gradient.•Low elevation promoted accumulation microaggregates while mid and high elevations promoted accumulation macroaggregates.•Soil aggregate structure was more stable at mid-high elevations.•SOC at mid elevation had higher degrees of aromaticity and hydrophobicity.•Soil labile carbon pool was significantly correlated to the stability of aggregate. Exploring the composition of soil organic carbon (SOC) and aggregate stability over a range of elevations is essential to evaluate the stability of SOC and the conversion of carbon source or sink of mountain ecosystems. However, information of those changes in mountain ecosystems in arid and semi-arid regions is still scarce to date. In this study, we estimated the response of SOC stability along elevation gradient (1,380–2,438 m) by combining physical protection of aggregates with chemical protection of SOC using a redundancy analysis method in the Helan Mountains, northwest China. Results showed that the dominated aggregates of Helan Mountains changed from microaggregates to macroaggregates with increasing elevation, and the mean weight diameter (MWD) in mid and high elevations (2,139–2,248 m) was significantly higher than that in low elevations (1,380–1,650 m). Solid-state 13C NMR spectra showed that O-alkyl C was the main chemical composition of SOC in the different elevations. Regression analysis indicated that with the increase of elevation, the proportion of aromatic C and aromaticity gradually increased, peaked at the mid elevation and subsequently decreased. These findings indicated that the soil structure and chemical composition of SOC are more stable in the mid elevation. Macroaggregates were positively correlated with O-alkyl C and aromatic C, while silt and clay were positively correlated with alkyl C, indicative of a lower decomposition rate of macroaggregates. The RDA analysis confirmed that aggregate stability were significant associated with SOC content and chemical composition such as O-alkyl C, indicating that the soil labile carbon pool was most related to affect aggregate stability. Our findings suggest that the Helan Mountains had a better soil structure and a more stable SOC chemical composition at mid elevation ranges. The soil structure at low elevations is sensitive to environment change; therefore, a reduction in anthropogenic disturbance is suggested for improved sustainability of the ecosystem.