Microbial carbon use efficiency is governed by site-level soil and climate factors and does not explain soil carbon gains under diversified crop rotations

Soil microbial carbon use efficiency (CUE) is an important parameter governing the formation of soil organic carbon (C). We hypothesized that increased CUE could explain higher soil C with diversified cropping rotations, which do not necessarily have higher C inputs or reduced soil disturbance, i.e. traditional drivers of soil C. Across five long-term experiments, crop rotational diversity increased microbial biomass C but did not affect microbial CUE or its components. CUE was marginally higher when the crop grown the year prior was a legume vs. a grain, though soil C did not differ. Site-level soil and climate factors governed CUE far more than management.. Higher soil C in more diverse rotations thus likely result from mechanisms other than increasing microbial CUE.