Identifying elite rhizobia for commercial soybean (glycine max) inoculants

Abstract

Existence of highly effective rhizobia in African soils is under exploited since commercial inoculants still contain exotic cultures from United States of America. Bio-prospecting was conducted in Kenya to identify elite strains of rhizobia capable of effectively nodulating and fixing large amounts of nitrogen with commonly grown soybean varieties. One hundred isolates were recovered from nodules of wild and cultivated legume hosts growing in different agro-ecological zones, namely coastal sand dunes and mangrove swamps, the uplands and Rift Valley highlands, to the Afro-montane zone of Mount Elgon and the Lake Victoria Basin, covering about 1045 km transect. These isolates were authenticated and tested for effectiveness on soybean (Glycine max) var. SB 19 in sterile vermiculite, and the twenty-four most promising isolates screened in potted soil to assess their competitive abilities using two contrasting varieties of soybean ("promiscuously nodulating" SB 19 and specific Safari). The six best performing isolates were evaluated under field conditions, comparing them to Bradyrhizobium japonicum strain USDA110, an industry standard. Test isolates were classified into five categories, non-infective (20%), ineffective (26%), partly effective (26%), effective (17%) and highly effective (11%) based on their performance relative to controls and commercial inoculants. The indigenous rhizobia that outperformed USDA110 were considered highly effective. In potted soil, all 24 native rhizobia isolates nodulated promiscuous soybean (SB19) but only 46% of them nodulated specific soybean (Safari). In the field experiment; at xiii Nyabeda in west Kenya, NAK 128 performed best on both promiscuous and specific soybean varieties, significantly (P<0.05) outperforming USDA110 by 29% and 24%, respectively. At another site in Butula, NAK 84 emerged as the most promising isolate on both promiscuous and specific soybean varieties and outperforming USDA 110 by 9% and 6%, respectively. The two best isolates from this investigation, NAK 84 and 128 outperformed the treatment receiving 78 kg N ha-1, require further characterization and field testing but clearly have commercial potential.