Particle‐Size‐Specific Radiocarbon Constraints Imply an Active Subsoil Organic Carbon Pool

Abstract 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 μm) and coarser (>2 μm) 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 δ 13 C, 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 δ 13 C SOM under an environment that experienced substantial vegetation changes, resulting in time‐averaged signals when using δ 13 C 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.