Soil respiration of terrestrial ecosystems, a major component in the global carbon cycle is affected by elevated atmospheric CO<sub>2</sub> concentrations. However, seasonal differences of feedback effects of elevated CO<sub>2</sub> have rarely been studied. At the Giessen Free-Air CO<sub>2</sub> Enrichment (GiFACE) site, the effects of +20% above ambient CO<sub>2</sub> concentration (corresponds to conditions reached 2035–2045) have been investigated since 1998 in a temperate grassland ecosystem. We defined five distinct annual periods, with respect to management practices and phenological cycles. For a period of three years (2008–2010), weekly measurements of soil respiration were carried out with a survey chamber on vegetation-free subplots. The results revealed a pronounced and repeated increase of soil respiration during late autumn and winter dormancy. Increased CO<sub>2</sub> losses during the autumn period (September–October) were 15.7% higher and during the winter period (November–March) were 17.4% higher compared to respiration from control plots. <br><br> However, during spring time and summer, which are characterized by strong above- and below-ground plant growth, no significant change in soil respiration was observed at the FACE site under elevated CO<sub>2</sub>. This suggests (i) that soil respiration measurements, carried out only during the vegetative growth period under elevated CO<sub>2</sub> may underestimate the true soil-respiratory CO<sub>2</sub> loss (i.e. overestimate the C sequestered) and (ii) that additional C assimilated by plants during the growing period and transferred below-ground will quickly be lost via enhanced heterotrophic respiration outside the main vegetation period.