Soil organic carbon decomposition from recently added and older sources estimated by δ13C values of CO2 and organic matter

► The δ13C signature of soil CO2 reflects that of organics decomposed. ► The admixture of atmospheric CO2 to soil CO2 was removed by Miller/Tans models. ► The contribution of younger and older C to CO2 and to SOM was compared. ► This allows to estimate microbial availability of younger relative to older C sources. ► Younger C (<2 years) is seven times more available for microorganisms than older C.