Maize growth promotion by recurrent inoculation is modulated by the timing of application

National audienceMicrobial inoculants have raised a growing interest over the past decade but their efficiency is still unpredictable in field conditions due to the inconstant survival of the inoculated strains, probably due to biotic interactions with the resident microbial community. This study aims at testing (i) different patterns of recurrent inoculations likely to enhance the survival of a PGPR strain Pseudomonas fluorescens, (ii) the effect on various plants traits and (iii) the impact of these inoculation patterns on the resident bacterial community. To this end, five patterns of recurrent inoculation were applied on maize plants under greenhouse cultivation in three soils. Prior sowing and post sowing inoculations were considered with two doses of applications (10 and 106 cfu/g of soil). Recurrent inoculation at high dose led to higher abundance of P. fluorescens and a better persistence in the bulk soil but no persistent colonisation of the rhizosphere was measured at the end of the experiment (90 days after sowing). Recurrent inoculations before sowing further enhanced the plant growth and moderately affected the bacterial resident community while recurrent inoculation after sowing had a greater impact on the community and a lesser effect on plant growth, highlighting the importance of inoculation timing in plant growth promotion. The overall effect of P. fluorescens inoculations was only minor on the bacterial community (a maximum of 45 impacted OTUs), affecting more strongly the low-abundant OTUs