Succession of the Resident Soil Microbial Community in Response to Periodic Inoculations

To maintain the beneficial effects of microbial inoculants on plant and soil, repeated inoculation represents a promising option. Until now, the impacts of one-off inoculation on the native microbiome have been explored, but it remains unclear how long and to what extent the periodic inoculations would affect the succession of the resident microbiome in bulk soil. Here we examined the dynamic responses of plant growth, soil functions and resident bacterial community in the bulk soil to periodic inoculations of phosphate-solubilizing and N2-fixing bacteria alone or in combination. Compared to single-strain inoculation, co-inoculation better stimulated plant growth and soil nutrients. However, the benefits from inoculants did not increase with repeated inoculations and were not maintained after transplanting to a different site. In response to microbial inoculants, three patterns of shifts in bacterial composition were observed - fold increased, fold decreased, and resilience. The periodic inoculations impacted the succession course of resident bacterial communities in bulk soil, mainly driven by changes in soil pH and nitrate, resulting in the development of three main cluster types throughout the investigation. The single and mixed inoculants transiently modulated the variation in the resident community in association with soil pH and C/N, but finally the community established and showed resilience to following inoculations. Consequently, the necessity of repeated inoculations should be reconsidered, and while the different microbial inoculants showed distinct impacts on resident microbiome succession, communities ultimately exhibited resilience.IMPORTANCEIntroducing beneficial microbes to the plant-soil system is an environmentally friendly approach to improve crop yield and soil environment. Numerous studies have attempted to reveal the impacts of inoculation on rhizosphere microbiome. However, little is known about the effectiveness of periodic inoculations on soil functioning. In addition, the impact persistence of repeated inoculations on the native community remains unclear. Here, we track the succession traits of resident microbiome in the bulk soil across a growing season and identify the taxa clusters that diversely respond to periodic inoculation. Crucially, we compare the development of resident community composition with and without inoculation, thus providing new insight into understanding the interactions between resident microbes and intruders. Finally, we conclude that initial inoculation plays a more important role in influencing the whole system, and the native microbial community exhibits traits of resilience, but no resistance, to the subsequent inoculations.