Despite the advance in our understanding of the carbon exchange between terrestrial ecosystems and the atmo-sphere, semiarid ecosystems have been poorly investigated and little is known about their role in the global carbon balance. We used eddy covariance measurements to determine the exchange of CO 2 between a semiarid steppe and the atmosphere over 3 years. The vegetation is a perennial grassland of Stipa tenacissima L. located in the SE of Spain. We examined diurnal, seasonal and interannual variations in the net ecosystem carbon balance (NECB) in relation to biophysical variables. Cumulative NECB was a net source of 65.7, 143.6 and 92.1 g C m À2 yr À1 for the 3 years stud-ied, respectively. We separated the year into two distinctive periods: dry period and growing season. The ecosystem was a net source of CO 2 to the atmosphere, particularly during the dry period when large CO 2 positive fluxes of up to 15 lmol m À2 s À1 were observed in concomitance with large wind speeds. Over the growing season, the ecosystem was a slight sink or neutral with maximum rates of À2.3 lmol m À2 s À1 . Rainfall events caused large fluxes of CO 2 to the atmosphere and determined the length of the growing season. In this season, photosynthetic photon flux density controlled day-time NECB just below 1000 lmol m À2 s À1 . The analyses of the diurnal and seasonal data and preli-minary geological and gas-geochemical evaluations, including C isotopic analyses, suggest that the CO 2 released was not only biogenic but most likely included a component of geothermal origin, presumably related to deep fluids occurring in the area. These results highlight the importance of considering geological carbon sources, as well as the need to carefully interpret the results of eddy covariance partitioning techniques when applied in geologically active areas potentially affected by CO 2 -rich geofluid circulation.