Wind as a main driver of the net ecosystem carbon balance of a semiarid Mediterranean steppe in the South East of Spain ANA REY*, LUCA BELELLI-MARCHESINI † , ANA WERE*, PENELOPE SERRANO-ORTIZ*, GIUSEPPE ETIOPE ‡ § , DARIO PAPALE † , FRANCISCO DOMINGO* andEMILIANO PEGORARO* – *Department of Desertification and Geoecology, Experimental Station of Arid Zones (EEZA), High Spanish Scientific Council (CSIC), Almerı ´a, Spain, †Department for Innovation in Biological, Agro-Food and Forest Systems (DIBAF), University of La Tuscia, Viterbo, Italy, ‡Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2, Rome, Italy, §Faculty of Environmental Science and Engineering, Babes-Bolyai University, Cluj-Napoca, Romania Abstract 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. Keywords: alpha grass, carbon sequestration, ecosystem respiration, eddy covariance, geogas, geothermal activity, grasslands, net ecosystem carbon balance Received 20 May 2011 and accepted 25 July 2011 Introduction The carbon balance of a large array of ecosystems across biomes has been intensively studied since the establishment of the international network of eddy flux sites FLUXNET (with more than 400 towers worldwide, http://www.fluxnet.ornl.gov/fluxnet/index.cfm). As a result, our understanding of ecosystem functioning across a large range of climatic zones has improved considerably over the last decade (Baldocchi, 2008) and our confidence in the estimates of the biosphere carbon sink is much greater, amounting to 3 Gt C yr À1 (Le Que ´re ´ et al., 2009). However, arid and semiarid ecosys- tems have received much less attention in global terres- trial carbon estimates, and much less is known about their carbon balance and future changes in response to climate change (Schimel, 2010). Given their large exten- sion, indeed they constitute the largest biome in the world (Schimel, 2010), it is clear that a reliable estimate of the global carbon balance requires the inclusion of ecosystems located in semiarid and arid regions. So far, few long-term estimates of their current carbon seques- tration potential have been made (e.g. Hunt et al., 2004; Hastings et al., 2005; Luo et al., 2007; Rotenberg & ¶Deceased. Correspondence: A. Rey, Museo Nacional de Ciencias Naturales CSIC, Serrano 115 E-28006, Madrid, Spain, tel. + 34 917 822 101, fax + 34 915 640 800, e-mail: arey@mncn.csic.es © 2011 Blackwell Publishing Ltd 539 Global Change Biology (2012) 18, 539–554, doi: 10.1111/j.1365-2486.2011.02534.x