Tunnels construction are widely used to facilitate large transport infrastructure projects, which may lead to a significant impact on soil organic carbon (SOC) dynamics and SOC chemical stability through depleting groundwater. However, the mechanisms of the effects tunnel construction on the chemical stability of SOC remain unexplored, which has become one of the most important but the least studied field. Therefore, this study aimed to investigate how tunnel construction impacts the chemical stability of SOC in shrublands along three altitude gradients (3240, 3420 and 3600 m above sea level) in the Eastern Tibetan Plateau. The results showed that tunnel construction did not significantly change SOC and its fractions regardless of altitude levels. However, tunnel construction increased O-alkyl carbon (C) by 10-15%, and decreased alkyl C (8-13%) and aromatic C (8-12%). These changes reduced SOC disintegration (alkyl C/O-alkyl C) and increased aliphaticity (aliphatic C/aromatic C) along three altitudes, causing a decrease in the chemical stability of SOC. This phenomenon was attributed to the reduced microbial activity caused by the decrease in soil water content after tunnel construction and an increase in active functional groups due to the increase of fine root biomass revealed by structural equation model. Our finding suggests that tunnel construction had no impact on SOC content, but the reduction in chemical stability of SOC may have a profound impact on long-term SOC sequestration. These findings offered new insights in predicting long-term SOC dynamics following giant construction engineering.

Process-based erosion models are efficient tools that can be used to predict where and when erosion occurs. On unpaved roads that have been recognized as important sediment sources, soil loss along road segments should be precisely predicted. This study was performed using the hillslope version of the Water Erosion Prediction Project (WEPP) to estimate soil loss from 20 typical road segments in the red soil region of South China. Terrestrial laser scanning (TLS)-measured soil losses were used to validate the model simulations. The results showed that the WEPP model could reasonably predict the total soil loss in relatively short (less than 100 m) and gentle (slope gradient lower than 10%) road segments. In contrast, the WEPP-simulated soil loss was underestimated for long or steep road segments. Detailed outputs along roads revealed that most of the peak soil loss rates could not be adequately calculated. The linear critical shear stress and the sediment equilibrium theory in the WEPP model for soil detachment simulation might be responsible for the underestimation. Additionally, the lack of upslope flow and the curved road tortuosity were found to be connected to the relatively low efficiency of the model outputs. Nevertheless, the WEPP simulation could accurately fit the trend of soil loss variation along road segments despite underestimation. Furthermore, the simulated results could provide a reliable prediction of the maximum soil loss positions. Therefore, the WEPP model could be adopted to evaluate the erosion risk of unpaved roads in the red soil region of South China.