Phosphorus limitation rather than temperature governs microbial carbon use efficiency in cropland soils

Soil microbial carbon use efficiency (CUE), an integrated metric defining carbon allocation between microbial growth and respiration, plays a central role in regulating the first steps of carbon accumulation or depletion in soil. The environmental dependment of microbial CUE remain uncertain due to the paucity of large-scale data, particularly in agricultural ecosystems. Here, we conducted a large-scale survey of microbial CUE across Chinese croplands (spanning ~29º of latitude) and combined it with metagenomics to elucidate the most important factors governing microbial CUE.There was significant spatial heterogeneity of microbial CUE (ranged from 0.19 to 0.60), with an average of 0.31. Challenging the theoretical speculation of inverse temperature-efficiency relationship, microbial CUE in croplands is independent with temperature. However, the spatial variability of microbial CUE is primarily governed by phosphorus limitation and secondarily by microbial community composition. In phosphorus limitation soils, soil microbiomes allocate more carbon in gene expression and enzymatic investment to phosphorus-acquisition at the expense of growth efficiency. Our large-scale study provides the first solid evidence that microbial phosphorus limitation governs the microbial CUE in cropland soils, which is critical for policy-making for soil carbon sequestration in cropland by microbial approach.