Effect of calcium carbonate, sand, and organic matter levels on mortality of five species of Azospirillum in natural and artificial bulk soils

Abstract

Five bacterial strains, one from each of the five known species of the plant growth-promoting bacteria (PGPB) Azospirillum (A. brasilense, A. lipoferum, A. amazonense, A. halopraeference, and A. irakense) were inoculated into two natural, semiarid soils (terra rosa and loessial sandy) from Israel, and two artificial soils constructed to simulate the native soils. Within 60 days, the populations of all five Azospirillum species declined significantly in a linear fashion, in both the native soils and in the homologous artificial soils. Increased levels of CaCO3, and fine and rough sand, had significant detrimental effects on the survival of the five Azospirillum species, whereas increased organic matter content improved survival. In contrast, when the bacterial strains were incubated in the rhizosphere of tomato seedlings grown in the artificial soils, manipulation of these soil variables had only a marginal effect on bacterial survival; all Azospirillum species survived well in the tomato rhizosphere under conditions that are otherwise detrimental. This study indicates that most cells of the strains of five known species of Azospirillum died out linearly over time in two semiarid soils, and that only the major soil components affected Azospirillum survival in soil. Because mortality was similar in native soils and in artificial homologous soils, artificial soils can be used to study the soil behavior of Azospirillum.