In the early 1980s, it was assumed that most tropical countries did not have the facilities and personnel required for inoculum production, its storage, and distribution. They depended on the importation of the crop’s final product (Pulver et al., 1982). Soybean breeders at the International Institute of Tropical Agriculture (IITA), Nigeria developed promiscuous soybean genotypes in order to take advantage of high yielding American and promiscuous Asian soybean varieties that were introduced to the country. These genotypes, known as Tropical Glycine cross (TGx) nodulate effectively with Bradyrhizobium spp. populations that are indigenous to African soils (Abaidoo et al., 2007). Okereke et al. (2000) reported that promiscuous soybean may also need to be inoculated with exotic Bradyrhizobia depending on the effectiveness and population of the indigenous bradyrhizobia in the locality.

Furthermore, in order to ensure optimum productivity by these promiscuous genotypes, it is essential to assess the relative effectiveness of the indigenous rhizobia in the soil. The number of indigenous Bradyrhizobia as well as the soil available nitrogen has been cited as some of the main factors that have a significant effect on symbiotic relationships (Keyser and Li, 1992; Maingi et al, 2006). Sanginga et al. (1999) found that the need for inoculation of promiscuous soybean depended on the effectiveness of indigenous Bradyrhizobia population. An increase in nodulation and N₂ fixation of promiscuous soybean cultivars using commercial Bradyrhizobium inoculant was reported by Okereke and Eaglesham (1993) and Yusuf et al. (2012), while Wasike et al. (2009) reported nodulation of promiscuous soybean varieties with indigenous rhizobia in Western and Eastern Kenya. Most of the studies conducted in Nigeria are limited in terms of number and location. They are few studies mostly conducted on-station.

This study was conducted to assess the response of promiscuous soybean to inoculation in soils of the moist Savanna of Nigeria and also to examine the relative effect of the indigenous rhizobia population on soybean biomass. The present investigation attempts to expand on the existing studies on the response of promiscuous soybean to inoculation in these agro-ecologies.

Soil analysis

Particle size distribution was determined by the hydrometer method, as described by Gee and Bauder (1986). The soil texture was determined using the USDA soil textural triangle. Soil pH was measured in both water and 0.01 M CaCl₂ solution using a 1:2.5 soil to solution ratio (Hendershot et al., 1993).

Organic carbon was determined by the Walkey-Black (chromic acid) method as described by Nelson and Sommers (1982). Total nitrogen was determined by micro-Kjeldahl digestion method (Bremner and Mulvaney, 1982). Available phosphorus was determined by the Bray 1 method (Olson and Sommers, 1982). Exchangeable bases were extracted with 1N ammonium acetate at pH 7.0 buffer (Chapman, 1965). Exchangeable K and Na was determined using flame photometer while exchangeable Ca and Mg was determined using Atomic Adsorption Spectrophotometer (AAS) (Jackson, 1958). The soil CEC was determined by 1N ammonium acetate saturation method (Anderson and Ingram, 1993) (Table 1 and Figure 1).

At eight weeks after planting, the plants were harvested and the following parameters were assessed: nodule number, nodule fresh weight, shoot dry weight and root dry weight. The data used for the soil analysis were subjected to descriptive statistics. The total dry matter yield (TDMY) was used to calculate the relative effectiveness index which gives an indication of the relative efficiency (RE) and inoculant response (IR) for each of the agro-ecological zones.

Relative Efficiency (RE) = 100a/b  

Inoculant Response (IR) = 100 (c-a)/a

Where:

a = dry matter weight in control treatment

b = dry matter weight in plus N treatment

c = dry matter weight of inoculated treatment

Relative efficiency greater than 100% implies that the indigenous population of rhizobia was superior to the application of mineral N. The strains isolated from these location(s) have very high potential for use as inoculant. Hence, it is not surprising that very few such cases occurred. Notable among such locations are BCH 2 (102.9%), GRK1 (R111.1%), GRK 4 (104.35%), GRK 5 (105.3%), WRA 1 (101.6%) in the Sudan Savanna. In the Northern Guinea Savanna, there is IGB 2 (101.4%), SOB 4 (114.7%).

In the Southern Guinea Savanna, there is KOL (128.6%). Locations with IR greater than 100% show that inoculation with effective rhizobia will improve soybean biomass. The higher inoculant response observed in the Sudan Savanna could be as a result of high competitiveness of the introduced strains. The presence of large population density of compatible rhizobia does not preclude the possibility that responses to inoculation can be obtained if competitive and highly effective strains are introduced (Giller, 2001).

Similarly, the population of indigenous rhizobia could have been very low in the Sudan Savanna. In a study conducted by Aliyu et al. (2013) in Shanono which lies in the Sudan Savanna, it was reported that the native rhizobia population was 1.10 x 10² which was quite low, suggesting that the soils have little or no N fixing ability for soybean. Hence, introduction of a commercial inoculant would have resulted in high response. Kumaga and Etu-Bonde (2000) demonstrated that nodulation and N₂ fixation of promiscuous soybean may be increased by inoculation with effective Bradyrhizobia.

The negative response observed in certain locations could be as a result of the competitiveness of the indigenous rhizobia that were more efficient than the introduced strain. It was reported by Meade et al. (1985) that successful competition for nodule sites by native rhizobia is one reason for the failure to achieve a response to inoculation with elite rhizobial strains.

In the Northern Guinea Savanna, SOB 4 interestingly had the highest RE and the lowest IR. This suggests that the population of the indigenous rhizobia may have been highly competitive for nodule occupancy and highly effective in their nitrogen fixing ability. Therefore, there will be no need for inoculation or mineral N application. It is generally argued that indigenous populations are highly adapted to their local soil environments and may form more effective symbioses than commercial inoculants isolated from a distant and un-related soil environment (Gandee et al., 1999).

 Similarly, this AEZ has a long history of soybean cultivation and inoculant use (Sanginga et al., 1995, 2003; Okogun et al., 2004; Yusuf et al., 2012). The previously introduced strains might have replaced the indigenous population, thus response to inoculation may be very minimal. This situation can be circumvented if superior strains are present in inoculants to be used in the zone.

The results show variation in the response to inoculation of rhizobia with respect to plant biomass in the various agro-ecological zones. The results showed better response to inoculation in the Sudan Savanna compared to the Northern and Southern Guinea savannas. This is not surprising as response to inoculation is usually lower in areas where soybean has been extensively grown. Of all the three agro-ecological zones, Sudan Savanna will benefit more from inoculation. Also, locations with high relative efficiency would produce promising strains if explored further. While the results show response to inoculation and high relative efficiency of indigenous strains, further field studies need to be carried out in order to improve their use as inoculants. 

The authors have not declared any conflict of interests.

Special thanks to the N2 Africa Project “Putting Nitrogen Fixation to work for small holder farmers in Africa” for providing the grant to conduct this trial.