C and N in soil organic matter density fractions under elevated atmospheric CO2: Turnover vs. stabilization

► Free POM possessed the shortest MRTC & MRTN vs. oPOM & Mineral-associated OM. ► Increase of fPOM-C/N & MRTC showed recalcitrancy of plant inputs under elevated CO2. ► No difference in MRT of oPOM 1.6 & oPOM 2.0 was due to fast turnover of aggregates. ► Constant MRTC in Min-OM did not reveal the stabilization of SOM under elevated CO2.