IntroductionThe rhizosphere priming effect (RPE) accelerates soil organic matter decomposition and acts as a pivotal feedback mechanism in the global carbon cycle. However, the regulatory role of distinct plant functional traits remains poorly understood. This study aimed to examine how nitrogen-phosphorus (N-P) fertilization levels and specific plant traits interactively shape the magnitude and direction of RPE.MethodsWe conducted a pot experiment comparing Citrus reticulata Blanco (Mandarin orange, a C3 shrub) and Medicago sativa L. (alfalfa, a C3 herbaceous legume) across an N-P fertilization gradient, including low N-P fertilization (P4N10: 4 g P m-2 + 10 g N m-2), high N-P fertilization (P8N20: 8 g P m-2 + 20 g N m-2), and corresponding unplanted controls. Plant biomass distribution, root chemical traits such as root nitrogen concentration, soil pH, extracellular enzyme activities, and RPE were measured at Days 45 and 90. ANOVA and regression analyses were used to identify the main drivers of RPE.ResultsBoth species induced predominantly positive RPEs across sampling periods and fertilization gradients, ranging from −1.5% to +127%, but the underlying drivers shifted with nutrient availability. Under low fertilization, leaf biomass emerged as the best predictor of interspecific variation, explaining 41% of the variance and supporting a “source-strength” mechanism by Day 45. Conversely, under high fertilization, root nitrogen concentration became the dominant driver, explaining 44% of the variance by Day 90 and indicating a shift toward a substrate-quality control mechanism. While fertilization enhanced leaf photosynthetic traits and specific rhizosphere respiration, it concurrently acidified the soil and suppressed extracellular enzyme activities involved in nitrogen mining.DiscussionThe net RPE was determined by the balance between stimulation of plant carbon inputs and direct inhibition of microbial decomposition processes. Overall, our results demonstrate that the plasticity of plant biomass allocation and root chemistry governs RPE variation. Incorporating these trait-specific responses is essential for accurately predicting soil carbon dynamics under changing nutrient regimes.