Microbial carbon use efficiency links litter quality to soil organic carbon stabilization

Although litter quality is widely assumed to regulate soil organic carbon (SOC) formation through decomposition, it remains unclear whether decomposition itself or microbial carbon allocation determines the fate of plant-derived carbon. Using six 13C-labelled litter substrates from dominant plant functional groups in an alpine meadow ecosystem, we traced litter-derived carbon into microbial respiration, biomass and necromass during a long-term incubation. Litter quality strongly influenced microbial carbon use efficiency (CUE′) and microbial-derived carbon accumulation but had little effect on native SOC mineralization. Fast-decomposing forb litter did not generate the greatest microbial necromass accumulation; instead, legume litter produced the highest microbial CUE′, necromass accumulation and carbon accumulation efficiency. Microbial CUE′ emerged as the primary determinant of microbial carbon accumulation, while fungal necromass drove variation in total microbial necromass. These findings reveal a decoupling between litter decomposition and carbon stabilization and demonstrate that microbial CUE′, rather than decomposition rate, determines whether plant-derived carbon is retained as persistent microbial-derived carbon or lost through respiration.