Direct and indirect effects of temperature, drought and elevated CO2 on grassland ecosystems have been well established when considering one or two abiotic factors, but rarely when considering the interactions of the three factors. For example, the positive effects of elevated CO2 and temperature on growth and plant C balance may be counterbalanced by drought effects, depending on the thresholds. For the below-ground compartment, experiments on grassland ecosystems have shown that elevated atmospheric CO2 can affect root dynamics in terms of production, life span, and mortality and decomposition rates. Then it is expected that C cycling is fast and C accumulation is small in soil of grassland subjected to elevated CO2. Here, we report results of dynamics and demography of roots and soil respiration of perennial grassland monoliths exposed to four different treatments: Control (C), Temperature (T, +3°C in comparison with control), Temperature x Drought (TD, +3°C and 20% lower summer precipitations than in control) and Temperature x Drought x CO2, according to A2 scenario (IPCC, 2001) (TDCO2, +3°C, -20% summer precipitations and CO2 200ppm above control). Root dynamics was measured by minirhizotron method and soil respiration was measured by a closed chamber technique. Results obtained for the three first years showed a significant increase of root growth (+33%) and new roots mean diameter (+6%) in the TDCO2 compared to control. Under TD and TDCO2 soil respiration was 35% higher than in control. These first results confirm higher root activity under A2 scenario (TDCO2).