Microbial Co-Occurrence Network Robustness, Not Diversity, Is a Key Predictor of Soil Organic Carbon in High-Altitude Mountain Forests

Altitude-driven environmental changes influence the persistence of soil organic carbon (SOC) via microbial metabolic pathways. However, the degree to which the network robustness of microbial communities directly predicts the persistence of organic carbon in alpine mountain forests remains unclear. This study focused on the Qilian Sabina przewalskii forest, situated along an altitude gradient of 2900–3400 m in the Qilian Mountains, systematically exploring the organization of soil microbial communities, the co-occurrence networks’ robustness, and their predictive capacity for organic carbon storage. The results indicate that altitude, as a critical driving factor, not only alters the physicochemical properties, microbial composition, and diversity of the soil but also significantly impacts its complexity and network robustness. The complexity and robustness of the microbial network are highest in the mid-altitude region (3100–3200 m), which is conducive to the development of robust microbial networks. Both bacterial α diversity and network robustness exhibit positive correlations with SOC, whereas fungal diversity shows a negative correlation with SOC. Furthermore, statistical modeling revealed that indices of microbial co-occurrence network robustness were stronger predictors of SOC storage than classical alpha-diversity indices. The structural equation model reveals that microbial biomass nitrogen (MBN) serves as a key mediating factor linking microbial diversity and SOC. Soil characteristics emerge as the primary direct driving factor, whereas the robustness of microbial networks exerts a significant yet minor direct and mediating influence. This study underscores that the robustness of microbial networks, rather than their diversity, is a critical predictor of soil organic carbon in high-altitude mountain forests. It offers a novel theoretical framework for understanding the mechanisms of the carbon cycle in mountain forest ecosystems in the context of climate warming.