Relationship between Indigenous Bradyrhizobia Population and the Degree of Nodulation and Nitrogen Fixation among Promiscuous Soybean (Glycine max (L) Merill) Lines in Five Ghanaian Soils

Abstract

In the light of conflicting reports, the earlier expectation that the promiscuous Tropical Glycine cross (TGx)  varieties developed by the International Institute of Tropical Agriculture (IITA) in Ibadan would relieve farmers in Africa of the need to inoculate their soybean crops for optimum nitrogen fixation and yield has become questionable and calls for more verification. Soil must contain enough compatible bradyrhizobia population for optimum nodulation and nitrogen fixation of promiscuous soybean lines.The presence of indigenous bradyrhizobia nodulating promiscuous soybean was determined using the Most Probable Number (MPN) technique. Soil series such as Chichiwere contained 6.0 x 10³ cells/g soil as against the minimum required level of 40x10¹ in Bekwai soils indicating that the soils contained enough bradyrhizobia for nodulation and nitrogen fixation. Greenhouse experiments were carried out at the University of Ghana-Legon to determine the effect of indigenous bradyrhizobia on nodulation, nodule dry weight %N and total N in five soils with local  names Aveime (a Chromic cambisol)  Hatso ( a Gleic cambisol), Adenta (a Ferric acrisol), Chichiwere (a Dystric fluvisol), and Bekwai (a Ferric acrisol) in which  nodulation, nodule dry weight, growth, nitrogen accumulation and nitrogen fixing abilities of three nodulating uninoculated soybean cultivars, comprising three promiscuous soybean lines bred in International Institute of Tropical Agriculture (IITA)-Ibadan, Nigeria such as Anidaso (TGx 813-6D), TGx 1448-2E and TGx 1903-8F soybean were examined using a non-nodulating soybean  variety as the reference crop. Nitrogen fixed by the indigenous bradyrhizobia strains was measured by total nitrogen difference method between the fixing crop and the reference one. Chichiwere soil had the highest nodule number, nodule dry weight, shoot dry weight, total N accumulation as well as the highest N fixed recording an average of 66.8mgN/ plant equivalent to 86.4%N in the plant. TGx 1903-8F, the best symbiotic soybean cultivar in this study fixed as high as 91.0mgN/ plant indicating that even though more bradyrhizobia were found in Chichiwere soil series most were not compatible for nodule infection and nitrogen fixation. The nitrogen fixed by these soybean cultivars in descending order were as follows TGx 1903-8F< Anidaso< TGx 1448-2E.

Keywords: Anidaso, Bradyrhizobium japonicum,  nitrogen fixation,  non-nodulating soybean, promiscuous soybean.