Effects of the construction of the lower Yarlung Tsangpo River tunnel project on the stability of soil organic carbon

Tunnels have been widely constructed in plateau mountains. However, the effects of tunnel construction on soil organic carbon stability have not been extensively studied. Soil organic carbon stability is an important indicator to evaluate the response of soil ecosystems to environmental changes. To detect the resistance of soil organic carbon (SOC) to tunnel construction in the lower Yarlung Tsangpo River tunnel, we compared the response of SOC stability to tunnel construction in the engineering impact (ED) and undisturbed (CK) areas by combining labile organic carbon (LOC), soil agglomerates, and soil enzymes. The results showed no significant differences among CK 255.31 and 91.19 mg·kg⁻¹ and ED 291.40 and 110.28 mg·kg⁻¹ in terms of SOC and LOC contents (P > 0.05), respectively. The proportion of > 0.25 mm aggregates in all soil fractions was over 80%. With the decrease of aggregate size, the content of organic carbon in aggregate showed a decreasing trend, but no significant difference between the ED and CK areas was observed. This study showed that the tunnel construction did not change litterfall, indicating that tunnel construction did not affect the input and output of SOC. Moreover, there were no significant differences in soil enzymes, microbial biomass carbon, and soil agglomerate composition between ED and CK, indicating that the tunnel construction had no significant effect on the microbiological regulation mechanism of SOC stability and the physical protection mechanism of soil agglomerates. In conclusion, our study indicated that the tunnel construction in the lower Yarlung Tsangpo River did not have a significant effect on the stability of soil organic carbon. These findings can aid in evaluating the impact of tunnel construction on the environment and providing an important basis for ecological protection in the lower Yarlung Tsangpo River.