Declines in organic matter persistence with increased soil carbon.

The persistence of soil organic matter (SOM) is critical for predicting carbon-climate feedbacks, yet how increasing soil organic carbon (SOC) relates to long-term SOM persistence across environmental gradients remains unclear. Soil radiocarbon, SOM physicochemical composition, and microbial carbon use efficiency (CUE) are analyzed along a precipitation-driven gradient integrated with global datasets. Here we show, as SOC content increases, its persistence as reflected by radiocarbon signatures declines at both the transect and global scales. This pattern indicates that higher SOC soils are increasingly dominated by younger, faster-cycling carbon rather than older, stabilized pools. Plant carbon inputs strongly predict SOM persistence and are associated with younger, less persistent SOC. While microbial CUE increases with SOC, higher CUE does not necessarily enhance long-term stabilization. Our findings suggest that SOC content alone provides limited insight into long-term soil carbon persistence and highlight the importance of explicitly representing plant carbon inputs and microbially mediated persistence in Earth system models.