Root functional traits determine the magnitude of the rhizosphere priming effect among eight tree species

Living roots and their rhizodeposits can accelerate or decelerate the decomposition of soil organic matter which refers to the rhizosphere priming effect (RPE). However, whereas plant traits are thought to be key factors controlling the RPE, little is known about how root traits representative of plant biomass allocation, morphology, architecture, or physiology influence the magnitude of the RPE. Using a natural abundance C-13 tracer method allowing partitioning of native soil organic carbon (SOC) decomposition and plant rhizosphere respiration, we studied here the effects of eight C-3 tree species featuring contrasting functional traits on C-4 soil respiration over a 204-day period in a microcosm experiment. All tree species enhanced the rate of SOC decomposition, by 82% on average, but the strength of the rhizosphere priming significantly differed among species. Mean diameter of first-order roots and root exudate-derived respiration were positively correlated with the RPE, together explaining a large part of observed variation in the RPE (R-2 = 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. Our study demonstrates 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. Overall, our results emphasize the relevance of integrating root traits in biogeochemical cycling models to improve model performance for predicting soil C dynamics.