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.尽管学界普遍认为凋落物质量可通过分解过程调控土壤有机碳（SOC）的形成，但目前仍不清楚，究竟是分解过程本身还是微生物碳分配决定了植物源碳的归趋。本研究选取高寒草甸生态系统中优势植物功能群的6种13C标记凋落物基质，在长期培养实验中追踪了凋落物源碳在微生物呼吸、微生物生物量及微生物残体中的分配与转化路径。凋落物质量对微生物碳利用效率（CUE′）及微生物源碳积累具有显著调控作用，但对本土SOC矿化过程的影响却十分微弱。快速分解的杂类草凋落物并未实现最高的微生物残体积累；相反，豆科植物凋落物的微生物CUE′、残体积累量及碳积累效率均为最高。微生物CUE′是微生物碳积累的核心决定因素，而真菌残体则主导了总微生物残体的变异。本研究结果揭示了凋落物分解与碳固持之间的解耦关系，并证实决定植物源碳是以持久性微生物源碳形式留存，还是通过呼吸作用流失的关键因素是微生物CUE′，而非分解速率。