The accumulation capacity of microbial residues in the rhizosphere increased along an elevation gradient

Root activities fuel higher microbial growth and metabolic activity, which may greatly promote the formation efficiency of microbial residues and their contribution to soil organic carbon (SOC) sequestration in the rhizosphere. However, the contribution of microbial residues to SOC accumulation in the rhizosphere across elevation, as well as the associated microbial metabolic traits, remains poorly understood. Here, we investigated the microbial residue C (MRC) concentrations and their contribution to SOC (i.e., microbial C pump (MCP) efficacy) in the rhizosphere and bulk soil along an elevation gradient on the Tibetan Plateau. We further assessed the accumulation capacity of microbial residues in the rhizosphere (RMCP efficacy) by calculating the ratio of the MCP efficacy between the rhizosphere and bulk soil, and by analyzing the associated microbial metabolic traits. The results showed that the MCP efficacy in the rhizosphere was significantly higher than in bulk soil across all elevations, suggesting that the rhizosphere exerts a considerable capacity to accumulate microbial residues. Moreover, the RMCP efficacy increased with elevation, which was accompanied by an enhancement in microbial metabolic efficiency (i.e., increased carbon use efficiency and reduced enzyme investment), implying that microbial trait-based strategies shift from a resource acquisition strategy to a high growth yield strategy with elevation. Collectively, our findings underscore the importance of trade-offs in microbial trait-based strategies for regulating rhizosphere SOC formation and associated soil C source/sink functions in forests facing changing environments.