Decoupling of microbial carbon use efficiency from soil carbon storage in boreal forests

Microbial carbon use efficiency (CUE) is a cornerstone metric for predicting soil organic carbon (SOC) storage globally. However, its predictive power in vulnerable frozen boreal forests, where physical preservation can override biological processing, remains a critical unknown. Here, we investigated the CUE-SOC relationship across the climatically sensitive permafrost transitional zone in the Greater Khingan Mountains. Our results revealed a stark dichotomy that challenges the universal applicability of this microbial efficiency–SOC paradigm. In the warmer, non-permafrost soils, microbial CUE was the primary positive driver of SOC accumulation, consistent with global patterns. Conversely, this relationship completely vanished in adjacent permafrost soils, in which SOC accumulation was decoupled from microbial efficiency and was instead overwhelmingly controlled by high retention of plant carbon residues (e.g., NDVI, particulate organic matter) and their physical cryo-preservation. This fundamental decoupling of microbial processing from soil carbon storage demonstrated that the biogeochemical rules governing SOC in much of the world do not apply in these frozen landscapes. Our findings provide critical mechanistic evidence that ecosystem carbon model must shift priority toward controls on plant inputs and physical cryo-preservation over microbial CUE to accurately forecast the fate of the vast and vulnerable northern carbon stocks in a future climate.