Productivity-driven decoupling of microbial carbon use efficiency and respiration across global soils

Despite extensive research on soil microbial carbon (C) use efficiency (CUE), its linkage to actual soil C storage remains ambiguous. A key uncertainty is that CUE estimates from short-term labeling incubations assume a linear negative relationship with respiration rates, overlooking nonlinear interactions and long-term microbial acclimation. Here, we use a stoichiometry-based approach to estimate CUE (CUE_ST), which links soil resource availability to microbial demand and captures microbial adaptability under resource constraints. We synthesized 1094 paired observations of CUE_ST and heterotrophic respiration rate (<i>R</i>_h) across natural ecosystems and found a nonlinear relationship between them governed by ecosystem productivity. In low-productivity arid and cold regions, CUE_ST declined with increasing <i>R</i>_h, whereas in productive tropical and temperate regions, CUE_ST stabilized at a low level (0.27 ± 0.11) as <i>R</i>_h exceeded 340 ± 10.8 grams of C per square meter per year. This shift reflects microbial trade-offs between C assimilation and stoichiometric homeostasis, revealing a decoupling of microbial growth from respiration that limits the capacity of productive ecosystems to store additional soil C.