Physicochemical Protection Outweighs Microbial Carbon Use Efficiency in Regulating Soil Organic Carbon Across an Aridity Gradient

Microbial carbon use efficiency (CUE) has been proposed as a critical determinant of soil organic carbon (SOC) dynamics, yet its relative importance compared with plant carbon inputs, edaphic conditions, microbial attributes, and physicochemical protection remains poorly understood. Here, we conducted large-scale soil samplings across a 3000-km aridity gradient on the Tibetan Plateau to evaluate the contribution of microbial CUE to SOC variation. Microbial CUE increased with increasing aridity, whereas SOC declined, resulting in a weak negative relationship between CUE and SOC along the aridity gradient. This increase in CUE was primarily driven by reduced microbial respiration, while microbial growth remained largely unchanged. By contrast, correlation analyses revealed strong positive relationships between SOC and physicochemical protection, plant diversity and carbon inputs, edaphic properties, and microbial attributes. Random forest and structural equation models further showed that SOC variation was more strongly associated with indicators of physicochemical protection, including mineral-associated organic carbon, clay content, calcium-bound carbon, and microbial residue carbon than with microbial CUE. Together, our findings challenge the view that microbial CUE alone governs SOC accumulation and highlight the importance of stabilization processes in regulating SOC variations across the aridity gradient.