Effects of root colonizing bacteria on nodulation of soybean roots by Bradyrhizobium japonicum

Eighteen strains of soybean root colonizing soil bacteria were tested for interference with nodulation of soybeans (Glycine max (L.) Merrill) grown in a field soil – perlite mix or in a soil-less planting medium. Seventeen of the strains were identified as Pseudomonas fluorescens or Pseudomonas putida and one as Aeromonas hydrophila. All strains colonized soybean roots at levels of log 3.9 to 5.7 cfu/g root. Although nine strains increased significantly the weights of nodules formed by Bradyrhizobium japonicum 110 on soybeans grown in the soil–perlite mix, numbers of nodules increased in only three treatments. Significant increases in nodule numbers were not observed when the soil bacteria were tested with B. japonicum 118, and only two treatments increased nodule weights when compared with the controls. One of the 18 treatments increased and 1 decreased significantly the dry weights of shoots of plants inoculated with B. japonicum 110 and grown in the soil-less mix. Only one treatment reduced the dry weights of roots. Several strains stimulated increases in the dry weights of shoots and roots of plants inoculated with B. japonicum 118, but these effects were not correlated with changes in nodule numbers or weights. The results suggest that root colonizing bacteria generally do not interfere with the ability of B. japonicum to form nodules in soybean roots and that certain strains may actually enhance nodulation and plant growth. The relevance of these results for the development of improved Bradyrhizobium inoculant products by the addition of selected plant growth promoting rhizobacteria is discussed.