<p>Large amounts of total organic carbon are temporarily stored in soils, which makes soil respiration one of the major sources of terrestrial CO<sub>2</sub> fluxes within the global carbon cycle. More than half of global soil organic carbon (SOC) is stored in subsoils (below 30 cm), which represent a significant C pool. Although several studies and models have investigated soil respiration, little is known about the quantitative contribution of subsoils to total soil respiration or about the sources of CO<sub>2</sub> production in subsoils. In a two-year field study in a European beech forest in northern Germany, vertical CO<sub>2</sub> concentration profiles were continuously measured at three locations and CO<sub>2</sub> production quantified in the topsoil and the subsoil. To determine the contribution of fresh litter-derived C to CO<sub>2</sub> production in the three soil profiles, an isotopic labelling experiment using <sup>13</sup>C-enriched leaf litter was performed. Additionally, radiocarbon measurements of CO<sub>2</sub> in the soil atmosphere were used to obtain information about the age of the C source in CO<sub>2</sub> production. At the study site, it was found that 90 % of total soil respiration was produced in the first 30 cm of the soil profile where 53 % of the SOC stock is stored. Freshly labelled litter inputs in the form of dissolved organic matter were only a minor source for CO<sub>2</sub> production below a depth of 10 cm. In the first two months after litter application, fresh litter-derived C contributed on average 1 % at 10 cm depth and 0.1 % at 150 cm depth to CO<sub>2</sub> in the soil profile. Thereafter, its contribution was less than 0.3 % and 0.05 % at 10 cm and 150 cm depths respectively. Furthermore CO<sub>2</sub> in the soil profile had the same modern radiocarbon signature at all depths, indicating that CO<sub>2</sub> in the subsoil originated from young C sources, despite a radiocarbon age bulk SOC in the subsoil. This suggests that fresh C inputs in subsoils in the form of roots and root exudates are rapidly respired and that other subsoil SOC seems to be relatively stable. The field labelling experiment also revealed a downward diffusion of 13CO<sub>2</sub> in the soil profile against the total CO<sub>2</sub> gradient. This isotopic dependency should be taken into account when using labelled <sup>13</sup>CO<sub>2</sub> and <sup>14</sup>C isotope data as an age proxy for CO<sub>2</sub> sources in the soil.</p>