The plant economics spectrum is increasingly recognized as a major determinant of plant species effects on terrestrial ecosystem functioning related to carbon cycling. However, the role of plant economic strategies in the effects of living root activity on soil organic carbon (SOC) dynamics through rhizodeposition remains unexplored, despite SOC being the largest terrestrial carbon pool. Using a continuous 13C‐labelling method allowing partitioning of plant and soil sources to carbon fluxes and pools, we studied here the linkages between plant economic strategies and SOC cycling processes in a ‘common garden’ greenhouse experiment. It includes a panel of 12 grassland species selected along a gradient of economic traits and belonging to three functionnal groups (C3 grasses, forbs and legumes). All species induced an acceleration of native SOC mineralization but this rhizosphere priming effect (RPE) substantially differed across species and varied eleven‐fold by the end of the experiment (from +26% to +295% relative to unplanted soil). Interspecific variation in RPE was primarily linked to plant photosynthetic activity associated to species economic strategies of light and CO2 resource acquisition and processing. Fast‐growing acquisitive species, such as legumes, featured large RPE, in relation with their high canopy photosynthesis coupled to high leaf photosynthetic capacity and large net primary productivity allocated above‐ground. This large RPE was further associated with high root metabolic activity, rhizodeposition and soil microbial activity. In contrast, fine‐root growth and economic traits related to soil resource foraging ability were poor predictors of RPE. The formation of new root‐derived SOC varied nine‐fold across species and was similarly positively related to the net primary productivity allocated above‐ground. Fast‐growing acquisitive species with a high photosynthetic activity induced a disproportionately large RPE relative to SOC formation. Synthesis. Overall, our study demonstrates that rhizodeposition is a major mechanism through which plant economic strategies of grassland species control soil carbon dynamics. Acquisitive versus conservative species were associated with high versus low rates of photosynthesis and rhizodeposition, in turn leading to fast versus slow SOC turnover. This emphasizes the importance of considering rhizosphere processes for understanding plant species effects on soil biogeochemistry.

Understanding how plant species influence soil nutrient cycling is a major theme in terrestrial ecosystem ecology. However, the prevailing paradigm has mostly focused on litter decomposition, while rhizosphere effects on soil organic matter (SOM) decomposition have attracted little attention. Using a dual 13 C/ 15 N labeling approach in a 'common garden' glasshouse experiment, we investigated how the economic strategies of 12 grassland plant species (graminoids, forbs and legumes) drive soil nitrogen (N) cycling via rhizosphere processes, and how this in turn affects plant N acquisition and growth. Acquisitive species with higher photosynthesis, carbon rhizodeposition and N uptake than conservative species induced a stronger acceleration of soil N cycling through rhizosphere priming of SOM decomposition. This allowed them to take up larger amounts of N and allocate it above ground to promote photosynthesis, thereby sustaining their faster growth. The N 2-fixation ability of legumes enhanced rhizosphere priming by promoting photosynthesis and rhizodeposition. Our study demonstrates that the economic strategies of plant species regulate a plant-soil carbon-nitrogen feedback operating through the rhizosphere. These findings provide novel mechanistic insights into how plant species with contrasting economic strategies sustain their nutrition and growth through regulating the cycling of nutrients by soil microbes in their rhizosphere.