Forty years of research on the rhizosphere priming effect (RPE) has demonstrated the potentially large increase (up to a factor 3) of soil organic matter mineralization induced by plant roots, but failed to directly quantify its contribution to the carbon (C) balance. Combining continuous CO2 flux measurements with RPE measurements has thus far been technically challenging. Here, we present an experimental platform of 40 mesocosms (volume = 88L; surface = 0.049 m2), including a 13C-labeled CO2 air-production system with a maximum capacity of 4 m3min−1 and customizable labeling intensity. For this study,13C depleted fossil C was used as source of labeled CO2 and the experiment was run for 250 days. Continuous net CO2 exchange measurements allowed us to estimate net ecosystem productivity, gross primary production and ecosystem respiration of the studied plant-soil systems. The RPE was regularly (bi-monthly to monthly) quantified by measuring the accumulation and isotopic composition of CO2 in dark chambers placed over the mesocosms. Our results show a good re- lationship between night plant-soil respiration (from continuous CO2 exchange measurements) and dark plant- soil respiration (from CO2 accumulation in dark chambers). This result suggests that our estimates of RPE and plant-soil fluxes based on the different methods are comparable. Preliminary results obtained in spring with grasses cultivated under ambient or elevated CO2 indicate that the RPE represents 1.22 ± 0.16% of gross primary production and 4.64 ± 1.12% of ecosystem respiration. The RPE estimates may have an uncertainty linked to the possible deviation in delta 13C between C sources (soil or plant) and released CO2 from these sources. We performed a sensitivity analysis on how the variation in intensity of isotopic labeling (difference in delta 13C between plant and soil) affects the uncertainty of RPE estimates considering 1‰ delta 13C deviation. Estimation of the RPE with an uncertainty lower than 10% of the estimated value requires a labeling intensity higher than 60 ‰. The developed platform will help to scale up the study of the RPE control on C cycling to the ecosystem level.