Morphological and biochemical characterization of soybean nodulating rhizobia indigenous to Zambia

Soybean [Glycine max (L.) Merrill] is known for symbiotic nitrogen fixation by rhizobia
present in the soil with which it establishes an efficient symbiosis. In Zambia, current
rhizobial inoculants used in soybeans production are based on non-indigenous strains;
this creates a need to isolate local strains that can be used for the development of local
inoculants for soybeans in Zambian soils. This study reports the isolation and
characterization of rhizobial isolates from virgin and cultivated soils of the three agroecological
regions of Zambia. The study was conducted in the greenhouse during the
2013/2014 season. Rhizobia were isolated using the Trap Method; a promiscuous
soybean genotype (magoye) was grown for eight weeks in replicates of three arranged in
Completely Randomised Design. Morphological and biochemical markers were used to
characterize. A total of 61 isolates were isolated on Yeast Extract Mannitol (YEM) agar
medium. The isolates were circular, irregular and punctuate in form with convex
elevation; entire, undulate and lobate colony margins; with cream, white, yellow,
transparent and pink coloured. All isolates produced mucous, were gram negative and
rod shaped, a characteristic of rhizobial cells. None of the isolates could tolerate
extremes of pH (4 and 9) in growth medium but grew well at pH 6.8. All isolates utilized
glucose as a source of carbon. Based on the Bromothymol Blue (BTB) assay, 59 isolates
were fast growing while two isolates from cultivated soils of region II were slow
growing. The fast growing 59 isolates showed an acidic reaction changing the medium
from green to yellow, while the others showed an alkaline reaction. Based on results, the
59 fast-growers could be Ensifer fredii or/and Rhizobium tropici rather than
Bradyrhizobium. However, further tests to confirm these findings using ketolactose,
genetic characterization and inclusion of reference strains, are still needed and are being
recommended here.