Under the scenario of global warming, the response of carbon (C) fluxes of arid and semi-arid ecosystems, is still not well understood. A field warming experiment using open top chambers (OTCs) was conducted in a shrub-grass patagonian steppe to evaluate the effects on bare soil respiration (Rsoil), and ecosystem respiration (Reco), gross primary productivity (GPP) and net C exchange (NEE) during the growing season. Air (Tair) and soil (Tsoil) temperature, and soil available phosphorus changed significantly while there were no changes in soil moisture, soil organic carbon, total soil nitrogen and root biomass, after one-year of treatment. Inside OTCs Rsoil increased by 61%, with larger changes found during the daytime than during nighttime and tended to increase the temperature sensitivity (Q10) from 1.49 to 1.58. Enhanced daytime NEE was observed inside OTCs mainly in the shrub Senecio filaginoides patches where C sequestration increased by 37% and GPP by 35% without effects on Reco. In the grass Poa ligularis patches changes in C fluxes were not significant, but lower decrease in GPP than in Reco tended to increase C uptake inside OTCs. This study reveals that future climate scenarios will lead to an enhanced C sequestration of vegetated patches but also to a higher bare soil CO2 emission which may turn this ecosystem in a substantial C source if bare soil fraction increases due to global changes.