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

Despite extensive research on soil microbial carbon use efficiency (CUE),
its linkage to actual soil organic C dynamics remains ambiguous. A key
uncertainty is that CUE estimates derived from short-term labeling
incubations assume a linear negative relationship with pulse respiration
rates, thereby overlooking widespread nonlinear interactions and long-term
microbial acclimation in natural ecosystems. Here, we synthesized 1,094
paired observations of CUE and average annual heterotrophic respiration
rate (Rh) across global natural ecosystems, and found a nonlinear
relationship between them governed by ecosystem productivity. In
low-productive arid and cold regions, CUE declined with increasing Rh. In
contrast, in high-productivity tropical and temperate regions, CUE
stabilized at a low level (0.27±0.11) as Rh exceeded a threshold of
340±10.8 g C m-2 y-1. This shift reflects a fundamental microbial
trade-off between C assimilation and the maintenance of stoichiometric
homeostasis.