Reduced contribution of microbial necromass carbon to soil organic carbon following tunnel construction in the eastern Qinghai-Tibet Plateau

Microbial necromass carbon (MNC) is one of the primary sources of stable soil organic carbon (SOC), playing a crucial role in global carbon cycling. Despite its significance, the impact of tunnel construction, a commonly adopted human activity in railway development, on MNC and its underlying mechanisms remains underexplored. To address this gap, a field survey was conducted in shrubland ecosystems on the eastern edge of the Qinghai-Tibet Plateau. Our study aimed to investigate the influence of tunnel construction on MNC and its contribution to SOC along an altitudinal gradient (3240 m, 3420 m, and 3600 m) and at two soil depths (0–10 cm and 10–30 cm). Results indicated that tunnel construction did not exert a significant impact on SOC and MNC. However, it led to a substantial decrease in the contribution of MNC to SOC (15.6%–19.9% in tunnel-affected areas) compared to the control areas (19.04%–30.97%). This outcome was likely attributed to the decreased bacterial necromass carbon (BNC) and the increased input of underground plant-derived carbon. MNC at 10–30 cm was significantly lower than that at 0–10 cm, and it exhibited a notable decrease at 3420 m compared to both 3240 m and 3600 m. These observations were closely related to reduced carbon input and microbial necromass stability. Principal component structural equation modeling revealed that soil microbial activities were the most important for MNC and SOC accumulation. Consequently, tunnel construction influenced SOC formation by altering the relative contributions from plant and microbial sources. This study provides valuable evidence for assessing the impacts of engineering construction on SOC and its sources in the context of climate change. Moreover, it offers scientific support for ecosystem conservation and sustainable development in the Qinghai-Tibet Plateau region during engineering construction.