Objective Biofertilizers containing composite strains are generally more effective to improve the colonization of beneficial bacteria in crop rhizosphere. We identified 26 strains in maize rhizosphere with different functions in our laboratory and constructed four composite strains. In this research, we investigated their effects in assembling beneficial genus in rhizosphere soil and growth-promotion on crops.

Methods The 26 strains belong to four functional groups: phosphate solubilizing bacteria, potassium solubilizing bacteria, autochthonous nitrogen-fixing bacteria and combined nitrogen-fixing bacteria. The composite microbial agents (SynComs) were constructed by combining one strain from each of the four functioanl groups. C1 contains the four strains with the highest secretion of iron carriers; C2 contains the four strains with the highest secretion of indole acetic acid (IAA); C3 contains the four strains with the strongest growth promotion; C4 contains all the strains in C1 and C2. Maize pot experiment was conducted using the four SynComs under sterilized soil (MCK, MC1, MC2, MC3, MC4), and unsterilized soil (WCK, WC1, WC2, WC3, and WC4). Maize seeds with primary radicle length of 1−2 cm were soaked in the suspension of SymComs for 1 h, and then planted in sterilized and unsterilized soil. After 9 days, the seedlings were harvested for the measurement of plant height, fresh weight, root length, number of root tips and number of forks. The same maize seeds were planted in sterilized and unsterilized soils, and watered with the composite strain suspension every 5 days. And the maize was harvested after 15 days growth for collecting the rhizosphere soil for metagenomics sequencing.

Results The SynComs C4 exhibited the best integrative ability among the four SynComs. Compared with the corresponding CK, C4 significantly promoted plant height, aboveground fresh weight, root fresh weight, root length, number of root tips and number of forks of maize seedlings by 28.97%, 46.00%, 59.99%, 61.99%, 62.27%, and 46.70% in sterilized group, and by 27.49%, 25.86%, 50.13%, 40.11%, 62.26% and 82.55% in unsterilized group, respectively. SynComs C4 increased the abundance of beneficial bacterial genera such as Pseudomonas, Klebsiella, Duganella, Cellvibrio, Pantoea, and Pseudoxanthomonas. Genera Enterobacter and Klebsiella in maize rhizosphere water with SynComs C4 had significantly higher annotated abundance than the control. SynComs C4 increased the abundance of functional genes related to phosphorus and nitrogen transport, IAA synthesis, iron carrier transport and carbon cycling in the rhizosphere soil, microbial traceability analyses of some of the functional genes with increased abundance showed a positive correlation between genus abundance and functional gene abundance for most beneficial microorganisms, while the members of SynComs C4 (Klebsiella) were annotated in the gene sets of ugqB, phnW.

Conclusions SynComs C4 changed the rhizosphere soil bacterial community composition, increased the abundance of beneficial bacterial genera and enriched growth-promoting related functional genes, and possessed the potential for the development of microbial composite fertilizers.