Microbial carbon use efficiency and soil organic carbon: Which is the determinant?

Microorganisms play a critical role in mediating the soil organic carbon (SOC) turnover processes. Briefly, there are two pathways involved in the role of microorganisms in the processes. First, biosynthesis in microbial metabolism causes the accumulation of microbial necromass that favors SOC formation (e.g., the entombing effect [EE] pathway). Second, extracellular enzyme released from microorganisms boosts SOC decomposition (e.g., the decomposing effect [DE] pathway). These two critical but contrasting pathways spark important questions: which pathway dominates the role of microorganisms in SOC turnover and do microorganisms promote or inhibit SOC storage? Clarifying these issues is critical to both predicting SOC feedback to changing climate and enhancing SOC sequestration.

Microbial carbon use efficiency (CUE), defined as the ratio of the amount of C employed in new biomass relative to the amount of C that has been consumed, is an integrative metric that can capture the SOC turnover processes. According to the definition, high CUE is expected to increase microbial biomass, which potentially enhances both SOC formation and decomposition. With the evidence of an observed positive correlation between SOC and microbial CUE from 132 measurements, Tao et al. recently proposed that the EE pathway dominates the role of microorganisms in SOC turnover. Additionally, they reported a positive CUE-SOC relationship that emerged from a microbial model with 57,267 globally distributed vertical SOC profiles and suggested that microbial CUE is a determinant promoting global SOC storage.

Here, we provide new insight into the role of microorganisms in SOC turnover processes and suggest that the DE dominates the role of microorganisms in SOC turnover and that microbial CUE inhibits SOC storage.