Effect of Bradyrhizobium japonicum Population on Nodulation, Nitrogen Fixation and Dry Matter Yield on Soybean (Glycine max L Merrill) in Some Ghanaian Soils

In as much as the noted ability of the promiscuous TGx soybean varieties developed by the International Instituteof Tropical Agriculture in Ibadan to nodulate profusely with indigenous bradyrhizobia confers advantages wherelack of fertilizers and suitable inoculants act as disincentives to growing the crop, the ability of these nodules tofix nitrogen should be even more important. Selection for promiscuous nodulators in different soils musttherefore go hand in hand with their nitrogen fixing abilities. The symbiotic performance of promiscuoussoybeans depends upon the population size and survival of bradyrhizobia on the field. A pot experiment wasconducted in the greenhouse within the Geography Department in University of Ghana-Legon using soilscollected from ten farmer’s field located in one Agro Ecological Zone (Coastal savanna) to determine nodulation,nitrogen fixation and dry matter yield by indigenous bradyrhizobia in five Ghanaian soils using threepromiscuous soybean varieties and one American type, Bragg, a non promiscuous variety. Nodulation by thethree promiscuous soybean varieties (Bengbie [TGx 536-2D], TGx 1835-10E, and TGx 1830-20E) rangedbetween 21 to 25 and the non-promiscuous variety (Bragg) recorded 13. Hake soil series which contains thehighest bradyrhizobia cell count (6.0 x 103 cellgsoil-1 also produced the highest number of nodules 22. Hakeagain produced the highest nodule dry weight, (176.0mg), shoot dry weight (3.85g), Total nitrogen (N)(106.0mgN plant-1), Total N fixed (88.9mgN plant-1) and %N fixed (69%). TGx 1830-20E also maintain itssuperiority in nodule dry weight (220.0mg plant-1) production, Total N (105.3mgN plant-1), Total N fixed(88.5mgN plant-1) and % N fixed (83.43). Bragg performed poorly in terms of nodule dry weight (67.0mg plant-1), Total N (83.8mg plant-1), Total N fixed (67.0mg plant-1) and %N fixed (78.05). The Most Probable Number(MPN) counts on sand grown soybean indicated 6000, 5300, 4700, 3600 and 3200 native B. japonicum cells g-1in Hake, Goi, Simpa, Agawtaw and Prampram soils respectively. The nodulation, N content and dry matter yieldresults showed that Ghanaian soils contain effective bradyrhizobia capable of nodulating soybean and N2fixation rendering inoculation unnecessary.Keywords: Agrecological zone. Effective bradyrhizobia. Nitrogen fixation. Nodulation Promiscuous soybea

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

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