Soil organic matter (SOM) is a critical player in the global carbon cycle and a common paleoclimate archive, yet the mechanisms governing its evolution, particularly in subsoil (>30 cm) systems, remain less well understood. To better characterize subsoil SOM dynamics, we investigated the isotopic compositions of SOM in fine (<2 mu m) and coarser (>2 mu m) grain-size fractions of the top 2 m of a soil profile developing in eolian sediments on the Chinese Loess Plateau. We found that in the subsoil, SOM in the fine fraction, characterized by lower C/N and higher delta C-13, exhibits consistently younger radiocarbon ages than those in the coarser fraction. Such offsets suggest that organo-mineral interactions facilitate fresh carbon input to the fine fraction rather than preventing SOM decomposition. We then developed a mass-balance model to interpret the radiocarbon data. The results suggest continual carbon input from the above-ground biosphere to the 2-m deep subsoil. The active subsoil organic carbon cycling can lead to significant variations in delta C-13(SOM) under an environment that experienced substantial vegetation changes, resulting in time-averaged signals when using delta C-13(SOM) for paleoenvironmental reconstructions. Our study underscores the dynamic nature of the subsoil carbon pool, emphasizing its potentially underestimated role in the contemporary carbon cycle.