CO2 exchange components of a temperate semi-desert sand grassland ecosystem in Hungary were measured 21 times in 2000-2001 using a closed IRGA system. Stand CO2 uptake and release, soil respiration rate (Rs), and micrometeorological values were determined with two types of closed system chambers to investigate the daily courses of gas exchange. The maximum CO2 uptake and release were -3.240 and 1.903 μmol m-2 s-1, respectively, indicating a relatively low carbon sequestration potential. The maximum and the minimum Rs were 1.470 and 0.226 μmol(CO2) m-2 s-1, respectively. Water shortage was probably more effective in decreasing photosynthetic rates than Rs, indicating water supply as the primary driving variable for the sink-source relations in this ecosystem type. and J. Balogh ... [et al.].