Recent research highlights the important role of (semi-)arid ecosystems in the global carbon (C) cycle. However, detailed process based investigations are still necessary in order to fully understand how drylands behave and to determine the main factors currently affecting their C balance with the aim of predicting how climate change will affect their structure and functions. Here, we explore the potential biological and non-biological processes that may compose net CO2 exchange in a semiarid grassland in southeast Spain by means of eddy covariance measurements registered over six hydrological years (2009–2015). Results point out the great importance of subterranean ventilation, an advective transport process causing net CO2 release, especially during drought periods and under high-turbulence conditions. Accordingly, extreme CO2 release, far exceeding that found in the literature, was measured over the whole study period (2009–2015) averaging 230 g C m−2 year−1; this occurred mostly during the dry season and was very unlikely to correspond to concurrent biological activity and variations of in situ organic C pools. Underground CO2 concentrations corroborate this finding. In this regard, the potential origins of the released CO2 could be geological degassing and/or subterranean translocation of CO2 in both gaseous and aqueous phases. However, future research is needed in order to understand how CO2 transport and production processes interact and modulate drylands’ terrestrial C balance. Overall, the present study exposes how subterranean ventilation and hydrogeochemistry can complicate the interpretation of the terrestrial C cycle.