Comparison of rhizosphere microorganisms of genetically related nodulating and non-nodulating soybean lines.

Abstract

The microorganisms in the rhizosphere of a non-nodulating (nn) soybean strain, which has been shown to exude a nodulation-inhibitory factor(s), were compared with the rhizosphere population of a near-isogenic, normally nodulating (NN) sister soybean strain. Field and greenhouse plots were established and differential microbial counts in the rhizospheres of both strains were obtained at approximately weekly intervals during the growing season. Results of three such experiments show greatly increased numbers of actinomycetes, rhizobia, and total microorganisms in the rhizospheres of the nn plants during approximately the first 40 days after emergence. These differences gradually lessened until, as the plants reached maturity, the nn rhizospheres contained considerably lower total microbial and actinomycete populations. At intervals, microorganisms which grew on Bhat's agar were classified according to Lochhead's nutritional grouping. The nn rhizosphere contained fewer microbes able to grow on basal medium and more microbes which required the various enriched media for growth. This indicates that in addition to having numerical differences, nutritionally different groups of microbes are found in the two rhizospheres. Rhizobia were found in the roots of both soybeans but were present in higher numbers in the roots of the nodulating strain.