The effect of microbial inoculant origin on the rhizosphere bacterial community composition and plant growth-promotion

Microbial inoculation has been proposed as a potential approach for rhizosphere engineering. However, it is still unclear to what extent successful plant growth-promoting effects are driven by the origin of the microbial inocula and which taxa are responsible for the plant-beneficial effects. We conducted a microbial transplant experiment by using different microbial inocula (and nutrient controls) isolated from forest, soybean and tomato field soils and determined their effects on tomato plant biomass and nutrient assimilation in sterilized tomato soil. Rhizosphere bacterial communities were compared at the end of the experiment and correlative and machine learning analyses used to identify potential keystone taxa associated with the plant growth-promotion. Microbial inoculants had a clear positive effect on plant growth compared to control nutrient inoculants. Specifically, positive effects on the plant biomass were significantly associated with microbial inoculants from the forest and soybean field soils, while microbial inoculants from the forest and tomato field soils had clear positive effects on the plant nutrient assimilation. Soil nutrients alone had relatively minor effects on rhizosphere bacterial communities. However, the origin of microbial inoculants had clear effects on the structure of bacterial community structure with tomato and soybean inoculants having positive effects on the diversity and abundance of bacterial communities, respectively. Specifically, Streptomyces, Luteimonas and Enterobacter were identified as the potential keystone genera affecting plant growth. The origin of soil microbiome inoculant can predictably influence plant growth and nutrient assimilation and that these effects are associated with certain key bacterial genera.