Evaluation of Bradyrhizobium Japonicum Strains on the Growth of Bambara Groundnut

Bambara groundnut (Vigna subterranea), a Sub-Saharan African native legume, is regarded as a climate-tolerant crop that can grow well in arid conditions and low-quality soil. As a staple crop, it plays a critical role in its nutritional richness that offers superior protein quality, carbohydrate, and essential amino acids. Nonetheless, similar to other legumes, Bambara groundnut heavily depends on the potential to establish a symbiotic association with nitrogen-fixing bacteria, mainly Bradyrhizobium japonicum, for increasing growth and yield.

The efficiency of this symbiotic association is, nevertheless, affected by a number of factors such as the presence of compatible rhizobial strains in the soil and proper nutrients. Whereas B. japonicum is crucial for nitrogen fixation, the use of nitrogen fertilizers has been a routine practice to enhance crop production. This article discusses the contribution of Bradyrhizobium japonicum strains and nitrogen fertilizers to improve the agronomic performance of Bambara groundnut. In this review, we will examine how various B. japonicum strains, when supplemented with the application of nitrogen fertilizer, affect major agronomic characteristics like growth, yield, and nutrient uptake.

Bradyrhizobium japonicum: The Key to Nitrogen Fixation

Bradyrhizobium japonicum is a Gram-negative bacterium known popularly for its involvement in the biological nitrogen fixation in legumes. Fixation of nitrogen is a process where nitrogen from the atmosphere (N₂) is reduced to ammonia (NH₃) with the help of nitrogenase enzymes. This conversion is essential for legumes since it enables them to obtain the nitrogen that they need for development, particularly in nitrogen-deficient soils or conditions where synthetic fertilizers are not available or can be expensive.

For leguminous crops such as Bambara groundnut, the bacterium enters a symbiotic relationship with the plant root system. The bacterium infects roots of the plant and develops specialized structures called nodules. Within these nodules, the bacteria are able to fix atmospheric nitrogen, which is used by the plant to grow and develop. In the case of Bambara groundnut, the nitrogen fixation efficacy depends much on the compatibility of the B. japonicum strain with the host plant, the environment, and the soil condition.

Though nitrogen fixation occurs naturally, it may not always be effective in every type of soil, especially if the soil does not contain the appropriate strains of B. japonicum optimized for Bambara groundnut. This is where the application of high-performing inoculants of B. japonicum can prove to be highly beneficial to improve crop yields.

Bambara Groundnut: Agronomic Significance

Bambara groundnut is a drought-tolerant and hardy legume that withstands poor growing conditions, and as such, it is an important crop for food security in water-short regions. Besides its nutritional value, Bambara groundnut is also a soil improver through nitrogen fixation from the atmosphere, thus enhancing the general health of the soil for the next crop.

Nonetheless, the yield potential of Bambara groundnut can be constrained by low or inefficient nitrogen fixation. On soils where good rhizobial strains are not naturally found, or where the indigenous population of rhizobia is not well suited to the plant, supplementary nitrogen is usually required to satisfy the nutritional needs of the plant. Under such conditions, nitrogen fertilizers are normally used to provide sufficient nitrogen supply for optimal growth and yield. The challenge, however, lies in balancing nitrogen fertilization and biological nitrogen fixation to prevent over-reliance on synthetic fertilizers, which can have environmental implications.

Bradyrhizobium japonicum Strains and Their Impact on Bambara Groundnut

The symbiotic association between Bambara groundnut and Bradyrhizobium japonicum is of great reliance on the strain of B. japonicum utilized for inoculation. All rhizobial strains are not equally efficacious in enhancing nitrogen fixation, and compatibility between the strain and the plant genotype is important in determining the effectiveness of inoculation.

There are various studies that have proven that inoculation with certain strains of B. japonicum greatly improves the growth and yield of Bambara groundnut. For instance, studies have established that strains such as RACA6, USDA110, and 8905 are most effective in nodule formation and nitrogen fixation in Bambara groundnut. Such strains have been linked to increased yields in pods, greater seed weight, and improved general plant health compared to uninoculated controls.

Inoculation with such strains not only enhances nitrogen fixation but also other agronomic characters including plant height, root growth, and biomass production. Greater nitrogen availability allows the plant to set more pods and seeds, thereby leading to increased yields. Additionally, B. japonicum strains enable Bambara groundnut plants to deal with soil nutrient deficiencies through enhanced nutrient acquisition, particularly phosphorus, which is usually a limiting factor in tropical soils.

The Role of Nitrogen Fertilizers in Bambara Groundnut Production

Although Bradyrhizobium japonicum strains greatly improve nitrogen fixation, nitrogen fertilizers are widely applied as a supplement to promote growth, particularly in soils where rhizobial populations are non-effective or lacking. Nitrogen fertilizers, usually urea, ammonium nitrate, or ammonium sulfate, offer an available supply of nitrogen to plants.

But the use of nitrogen fertilizers has to be carefully controlled. Overuse can produce a number of problems, among them the inhibition of rhizobial activity. Excessive nitrogen can prevent nodule formation and lower the plant's dependence on biological nitrogen fixation. This can result in wasteful use of fertilizers and adverse effects on soil health from leaching of nutrients and is referred to as "nitrogen-induced immaturity.".

In addition, synthetic nitrogen fertilizers pose environmental issues, like nitrous oxide emission (a strong greenhouse gas) and water body pollution through runoff. As such, nitrogen fertilizer application combined with inoculation of B. japonicum strains presents a more sustainable strategy. This holistic strategy not only increases the availability of nitrogen but also preserves the ecological balance through lowering the demand for high fertilizer inputs.

Comparative Assessment: Bradyrhizobium japonicum Inoculation and Nitrogen Fertilizers

A number of studies compared the impacts of inoculating Bambara groundnut with various strains of B. japonicum in comparison with the use of nitrogen fertilizers. The results of all these studies uniformly indicate that inoculation with individual rhizobial strains enhances the yield and agronomic productivity of Bambara groundnut in a way that is either similar to or even superior to the application of nitrogen fertilizers, particularly in the long run.

For example, a trial on Bambara groundnut using soils with limited nitrogen showed that inoculation with B. japonicum strains resulted in increased pod and seed yield than for plots provided with only nitrogen fertilizer. Moreover, the plants inoculated with the rhizobial strains also exhibited superior resistance to pests and diseases owing to the greater uptake of nutrients and general plant health.

Conversely, nitrogen fertilizer application only did not have accompanying increases in yield, especially in those soils with already sufficient nitrogen content. The uninoculated plants that were treated with nitrogen fertilizer manifested poorer nodulation and plant condition. Additionally, the financial expense and environmental cost of synthetic fertilizers further provide rationale for trusting in B. japonicum inoculation as being more sustainable in nature.

Synergistic Effects of Inoculation and Fertilizer Application

While B. japonicum inoculation by itself can enhance the agronomic quality of Bambara groundnut to a considerable extent, there is also evidence of synergistic effects occurring as a result of a combination of rhizobial inoculation and controlled use of nitrogen fertilizer. In marginally nitrogen-deficient soils, moderate use of nitrogen fertilizer in combination with inoculation can maximize both biological nitrogen fixation and nitrogen supply.

This integrated system should enable Bambara groundnut plants to attain their potential, particularly under variable soil fertility conditions. The technology needs, however, to be optimized to avoid inhibiting the indigenous nitrogen fixation process while sustaining sufficient nutrient provision for the plants.

The assessment of Bradyrhizobium japonicum strains and the use of nitrogen fertilizer indicates that rhizobial inoculation is an important parameter in the improvement of the agronomic performance of Bambara groundnut. Inoculation with appropriate B. japonicum strains enhances nitrogen fixation, resulting in higher growth, improved pod yields, and better overall plant growth. Conversely, nitrogen fertilizers, when applied sparingly, can complement the nitrogen needs of Bambara groundnut without inhibiting the desirable benefits of rhizobial inoculation.

The combination of rhizobial inoculation and controlled fertilizer application is a sustainable and effective means of increasing Bambara groundnut productivity, especially in low rhizobial population soils. Research should also aim to determine the most efficient strains of B. japonicum for diverse Bambara groundnut genotypes and environmental circumstances, and the optimal nitrogen fertilizer application rates to reduce environmental load while maximizing crop yields.

By implementing these practices, farmers can make a considerable improvement in Bambara groundnut yield, enhancing food security and sustainable agriculture, especially in drought- and soil-degradation-prone areas.