Temperature and Substrate C:N Drive Microbial Carbon Use Efficiency and 13 C Discrimination.

Rates of soil organic carbon flows through microbial biomass and into CO₂ generally increase with temperature, though this can depend on multiple features including substrate C:N.  Studies suggest microbial C use efficiency (CUE) declines with temperature, but CUE is only rarely if ever measured, and it remains unclear how substrate C:N may interact with temperature to influence CUE.  Processes generating CO₂ impart isotopic fingerprints on multiple compounds, useful for discerning mechanisms driving observed patterns, but δ¹³C-CO₂ is used to infer CUE and SOC dynamics without an understanding of the factors governing isotopic fractionation.  Using a simplified system, we quantified C flow through bacteria offered substrates with varying C:N and measured δ¹³C of microbial biomass and respired CO₂ across >13°C.  CUE declined ~1% per °C increase at C:N=10; contrasting with stoichiometric theory, CUE declines with warming were greatest when substrate C:N was lowest.  At C:N=10, fractionation between substrate, biomass, and CO₂ suggests the relative dominance of different metabolic pathways generating CO₂ changed with temperature, but the effect varied with substrate C:N.  We demonstrate that rising temperature induces a large, variable effect on δ¹³C-CO₂, and a smoothly increasing difference between δ¹³C of CO₂ and biomass.  Accounting for these isotopic effects is critical for interpreting δ¹³C of microbial and ecosystem respiration.  We present proof-of-concept that CUE can decline with rising temperature, that mineralization of organic C into CO₂ by heterotrophic microbes can induce isotopic fractionation, and that temperature can predict the magnitude of biomass-CO₂ fractionation in some substrate landscapes.