Possible determinants of rhizosphere competence of bacteria

Abstract

Because extensive colonization of the rhizosphere is important to obtain the benefits of bacterial inoculation, a study was conducted to determine possible traits that are important for rhizosphere competence of bacteria. The final population sizes of 19 bacteria introduced into the rhizosphere of soybeans grown in nonsterile soil differed markedly. The ability to colonize the rhizosphere was not correlated with the capacity of low or high densities of these bacteria to adhere to the roots. The growth rates of the 19 bacteria in liquid culture or of eight of the bacteria in sterile rhizosphere soil were also not correlated with the extent of growth in the nonsterile rhizosphere. Tests with small inocula of three bacteria with different rates of growth in the sterile rhizosphere showed no relationship between those growth rates and the extent of colonization of the nonsterile rhizosphere by these organisms. The extent of colonization of the nonsterile rhizosphere by test bacteria was not correlated with their densities in the rhizosphere containing large numbers of protozoa. Additions of a readily-available carbon source to soil receiving a small number of Bradyrhizobium japonicum cells provided no evidence that the concentration of available carbon limited the colonizing ability of this bacterium. With five of seven test bacteria, the maximum population density reached in the rhizosphere depended on the size of the initial population in soil. The results suggest that the ability of bacteria to survive in large numbers in soil is a major determinant of their success in subsequent colonization of the rhizosphere.