The effects of different fertilization conditions on bacterial plant growth promoting traits: guidelines for directed bacterial prospection and testing

Aims In this work, the effects of fertilization on diazotroph diversity and plant growth promoting traits were analyzed. An interaction model was then suggested and tested in field. Methods One hundred and ninety bacterial strains were isolated from rhizospheric soil and roots of rice cropped in three different fertilization conditions. Phosphate solubilization, indolic compound (IC) and siderophore production, and nitrogen fixation abilities of the isolates were screened and compared. Some isolates were selected for in vivo plant growth promotion in greenhouse and field experiments. Results We found that fertilization had a limited effect on diversity but had a major effect on phosphate solubilization and IC production abilities. We demonstrated that plants select bacteria that present good phosphate solubilization ability for intimate root association in lieu of good IC production under nutrient-poor conditions and select good IC producers in lieu of good phosphate solubilizers under nutrient-moderate conditions. In nutrient-rich conditions, this selection preference seems to be deactivated. In the field trial, good phosphate solubilizers only contributed effectively to plant growth at nutrient-poor conditions and good IC producers only contributed to plant growth at nutrient-moderate conditions. Conclusions Fertilization affects the PGP traits of the diazotrophic community. These findings may be used for directed PGPR prospection and anticipated PGPR candidate selection.