Microbial metabolic efficiency functions as a mediator to regulate rhizosphere priming effects.

Microbial metabolic efficiency (MME), a key physiological property that indicates the allocation of carbon (C) to microbial growth, is surely one potential pathway involved in the regulation of priming effect within soil systems. However, the function and mechanism concerning the regulation of the rhizosphere priming effects (RPE) by MME in plant-soil systems remain unclear. In this study, we performed a pot experiment that included two soil types (paddy soil and lou soil), two plant species (sorghum [Sorghum bicolor (L.) Moench] and maize [Zea mays L.]) and three stages of growth (big trumpet, blooming and mature stage) to investigate the MME mechanism of RPE. Both positive (up to 76% at the big trumpet stage) and negative (down to -11% at the mature stage) RPE were observed. A shift in related enzyme activities and microbial biomass indicated that the 'microbial activation' and 'microbial nitrogen (N) mining' hypotheses functioned together at first. The 'preferential substrate utilization' hypothesis then functioned at the latter two stages. After that, according to a correlation analysis method, the MME was introduced to regulate the RPE: the availability of soil C and N and the microbial biomass jointly shaped the microbial C: N imbalance (MIC:N), and the microbes then regulated their MME based on the MIC:N, thus, regulating the RPE. Specifically, the lower MME induced by a higher MIC:N was responsible for a greater RPE at the big trumpet stage across all the planted treatments, while a higher MME induced by a lower MIC:N was responsible for the lower or negative RPE at the blooming and mature stages. Overall, these findings demonstrate that the MME shaped by MIC:N functions as a mediator to regulate the RPE in planted soil.