Inoculation and competitiveness of a Bradyrhizobium sp. strain in soils containing indigenous Rhizobia

BONFIGLIO, C., BOGINO, P. AND GIORDANO, W.

Cape Town, South Africa

Congreso; 15th International Congress on Nitrogen Fixation.; 2007

The successful nodulation of legumes by a particular rhizobial strain is determined by the competitive ability of that strain against a mixture of other native and inoculant rhizobia.  Selected Bradyrhizobium sp strains inoculated on peanut seeds often fail to occupy a significant proportion of nodules when a competitor rhizobial population is already established in soil. Such failure may result either from a genetic or physiological advantage of the adapted soil population over the introduced inoculant, or from a positional advantage, i.e., the soil population occupies the soil profile where the roots will penetrate, whereas the inoculant remains concentrated around the seeds.  We studied competition among native rhizobia and a Bradyrhizobium sp strain inoculant.  The seed-inoculated strain formed fewer nodules (9%) on peanuts than did bradyrhizobia inoculated in-furrow (78%).  The rest of the nodules were formed by indigenous strains or both strains (inoculated and indigenous), indicating the positional advantage of soil populations or bradyrhizobia inoculated in-furrow for nodulation.  We subsequently assessed the contribution of this positional effect using a laboratory model in which a rhizobial population is stabilized in sterile vermiculite. Our results demonstrate the importance of bradyrhizobial distribution, and/or movement into soil, for strain competitiveness and nodulation in soil-grown peanuts.