Rhizosphere priming effect can accelerate or decelerate the decomposition of soil organic matter. Using a natural abundance ¹³C tracer method allowing partitioning of native soil organic carbon (SOC) decomposition and plant rhizosphere respiration, we studied the effects of eight tree species on the strength of the rhizosphere priming. All tree species enhanced the rate of SOC decomposition, by 82% on average. Mean diameter of first-order roots and root exudate-derived respiration were positively correlated with the RPE, together explaining a large part of the observed variation in the RPE (R² = 0.72), whereas root branching density was negatively associated with the RPE. Path analyses further suggested that mean diameter of first-order roots was the main driver of the RPE owing to its positive direct effect on the RPE and its indirect effects via root exudate-derived respiration and root branching density. These results demonstrate that the magnitude of the RPE is regulated by complementary aspects of root morphology, architecture and physiology, implying that comprehensive approaches are needed to reveal the multiple mechanisms driving plant effects on the RPE.