Although considerable research has been conducted on the importance of recent litter compared with older soil organic matter as sources of dissolved organic carbon (DOC) in forest soils, a more thorough evaluation of this mechanism is necessary. We studied water-extractable organic carbon (WEOC) in a soil profile under a cool-temperate beech forest by analysing the isotopic composition (13C and 14C) of WEOC and its fractions after separation on a DAX-8 resin. With depth, WEOC became more enriched in 13C, which reflects the increasing proportion of the hydrophilic, isotopically heavier fraction. The 14C content in WEOC and its fractions decreased with depth, paralleling the 14C trend in soil organic matter (SOM). These results indicate a dynamic equilibrium of WEOC and soil organic carbon. The dominant process maintaining the WEOC pool in the mineral soil appears to be the microbial release of water-soluble compounds from the SOM, which alters in time-scales of decades to centuries.