Effects of Bradyrhizobium japonicum Strains on the Growth of Soybeans

Soybeans (Glycine max) are one of the most important legume crops grown worldwide, serving as a key source of protein, oil, and livestock feed. A critical component of soybean productivity is nitrogen availability, as the crop requires substantial nitrogen throughout its growth cycle. While chemical fertilizers are commonly used to meet this need, there is growing interest in more sustainable methods, particularly biological nitrogen fixation (BNF) through microbial inoculants like Bradyrhizobium japonicum.

Lets explores the combined impact of Bradyrhizobium japonicum strains and inorganic nitrogen fertilizer on soybean performance, examining factors such as nodulation, vegetative growth, nutrient uptake, and final yield. Understanding this synergy is essential for both sustainable farming and achieving optimal crop output.

Understanding Bradyrhizobium japonicum

Bradyrhizobium japonicum is a slow-growing, soil-dwelling bacterium known for its ability to fix atmospheric nitrogen in symbiotic association with soybean roots. After successful root colonization and nodulation, it converts atmospheric nitrogen (N₂) into ammonia, which plants can use to synthesize amino acids and proteins.

The efficiency of nitrogen fixation varies depending on the strain of Bradyrhizobium japonicum, environmental conditions, and host plant compatibility. As a result, inoculant manufacturers and agronomists emphasize strain selection for different agro-climatic zones and soil types.

Nitrogen Requirements of Soybeans

Soybeans are nitrogen-intensive crops. A well-grown soybean crop may require over 300 kg of nitrogen per hectare during its lifecycle. While B. japonicum can fix a significant portion of this requirement (up to 70% or more), certain conditions, such as poor soil fertility or low native rhizobial populations, may limit the effectiveness of nitrogen fixation alone.

In these cases, inorganic nitrogen fertilizers are applied to supplement the crop’s early-stage nitrogen demand before effective nodulation is established. However, improper use of nitrogen fertilizer can suppress nodule formation and BNF activity. Therefore, understanding the interaction between inoculation and nitrogen supplementation is crucial.

Research Insights: Inoculant Strains and Nitrogen Levels

Several field studies have analyzed the effects of different B. japonicum strains and varying doses of nitrogen fertilizer. The results consistently indicate that combining effective inoculation with low to moderate levels of nitrogen enhances soybean performance better than using either method alone.

1. Influence of Strain Selection

Different B. japonicum strains vary in their ability to form nodules, fix nitrogen, and compete with native soil microbes. Strains like USDA110 and CB1809 have shown strong performance across diverse environments.

Key observations:

• Strain performance is linked to adaptability to local soil conditions.

• Some strains show early nodulation, which supports rapid vegetative growth.

• Yield improvements of up to 30-40% have been recorded with superior strains compared to uninoculated controls.

  
  

2. Effect of Nitrogen Fertilizer on Nodulation

Excessive nitrogen application can suppress biological nitrogen fixation by making the plant less reliant on symbiosis. However, modest amounts of nitrogen—commonly referred to as “starter nitrogen”—have been shown to support initial plant growth and help bridge the nitrogen gap before nodulation becomes effective.

Findings include:

• Starter nitrogen at 20–30 kg N/ha improves early biomass accumulation.

• Higher doses (>50 kg N/ha) tend to reduce the number of active nodules.

• Synchronizing nitrogen application with the plant’s early vegetative phase is key to avoiding suppression of BNF.

  
  

3. Yield Impact of Combined Application

Treatments involving both B. japonicum inoculation and moderate nitrogen application have demonstrated the most favorable outcomes in terms of yield and nutrient efficiency.

Treatment	Nodulation	Plant Height	Pods per Plant	Yield Increase
Control (no inoculant, no N)	Low	Low	Few	Baseline
Inoculant only	High	Medium	More	+20–30%
Nitrogen only	None	High	Moderate	+10–15%
Inoculant + Starter N	High	High	Most	+35–50%

This shows that a balanced approach offers the highest yield without compromising soil health or long-term productivity.

Field Parameters Affected

Nodulation

Inoculated soybean roots display a higher nodule count, nodule dry weight, and increased acetylene reduction activity—an indirect measure of nitrogenase enzyme function. This directly correlates with nitrogen supply and chlorophyll levels in the plant.

Vegetative Growth

Increased plant height, larger leaf area index, and thicker stems have been observed in treatments with both inoculant and starter nitrogen.

Grain Yield and Protein Content

Yields from integrated treatments are higher due to improved nutrient uptake and efficient seed filling. Moreover, inoculated plants often produce seeds with higher protein content due to enhanced nitrogen availability during pod development.

Best Practices for Farmers

1. Use Recommended Strains :Always choose Bradyrhizobium japonicum strains tested and approved for your region and soil type. Local strains often outperform exotic varieties.

2. Inoculant Quality Matters :Check the expiration date and ensure the inoculant contains a viable count of at least 10⁸–10⁹ cells per gram. Poor-quality inoculants can result in ineffective nodulation.

3. Starter Nitrogen Strategy :Apply 20–30 kg/ha of nitrogen at sowing. Avoid applying nitrogen during the flowering or pod-filling stages, as it can suppress nitrogen fixation.

4. Avoid Incompatibility :Do not mix inoculants directly with chemical fertilizers, fungicides, or pesticides. Apply them separately to prevent bacterial death.

5. Soil pH and Drainage :Ensure that the soil pH is in the 6.0–7.0 range and that drainage is adequate. Poor soil conditions can reduce both nodulation and nutrient absorption.

   
   

Environmental and Economic Considerations

From an environmental standpoint, relying more on biological nitrogen fixation helps reduce greenhouse gas emissions associated with the production and overuse of synthetic nitrogen fertilizers. It also lowers the risk of nitrate leaching and soil degradation.

Economically, farmers benefit from reduced input costs, improved soil fertility, and higher yields. In systems where inoculants are properly applied, the return on investment (ROI) is often significantly higher compared to conventional nitrogen-only fertilization.

Conclusion

The combined use of effective Bradyrhizobium japonicum strains and judicious amounts of inorganic nitrogen fertilizer presents an optimal strategy for enhancing soybean growth and yield. While nitrogen fertilizers offer quick benefits, their overuse can harm biological systems and soil sustainability. In contrast, microbial inoculants provide long-term advantages by improving soil health and supporting eco-friendly agriculture.

As agriculture moves toward more sustainable and efficient practices, integrating biological inoculants with smart nutrient management strategies will play a vital role in global food production systems.