Soil CO2 flux measurement is a key method that can be used to monitor the hazards in an active volcanic area. In order to determine accurately the variations of the CO2 soil emission we propose an approach based on the radiocarbon (14C) deficiency recorded in the plants grown in and around the Solfatara (Naples, Italy). We twice sampled selected poaceae plants in 17 defined sites around the Solfatara volcano. 14C measurements by liquid scintillation counting (LSC) were achieved on the grass samples. The 14C deficiency determined in the sampled plants, compared to the atmosphere 14C activity, ranged from 6.6 to 51.6%. We then compared the proportion of magmatic CO2 inferred to the instantaneous measurements of CO2 fluxes from soil performed by the accumulation chamber CO2 degassing measurement at the moment of the sampling at each site. The results show a clear correlation (r=0.88) between soil CO2 fluxes and 14C activity. The determination of the plants 14C deficiency provides an estimate of the CO2 rate within a few square meters, integrating CO2 soil degassing variations and meteorological incidences over a few months. It can therefore become an efficient bio-sensor and can be used as a proxy to cartography of the soil CO2 and to determine its variations through time.