For smallholder farmers in regions like sub-Saharan Africa (SSA), ensuring food security remains a significant challenge, often exacerbated by declining soil fertility and the high cost of chemical fertilizers. A sustainable and affordable alternative lies in biological nitrogen fixationNitrogen is a crucial nutrient for plant growth, but plants can’t directly absorb it from the air. Nitrogen fixation is a process where certain bacteria convert atmospheric nitrogen into a form that plants can use. Biochar can provide a home for these nitrogen-fixing bacteria, enhancing More (BNF), a process where soil microorganisms, primarily rhizobia, convert atmospheric nitrogen into a form plants can use. To maximize BNF, these beneficial bacteria are introduced to crops via inoculants. Traditionally, peat has been the carrier material for commercial inoculants due to its effectiveness. However, peat is a non-renewable resource, and its mining raises environmental concerns, including carbon loss and water quality issues. The high cost and lack of local peat deposits also make inoculant production expensive in many SSA countries. This pressing need for low-cost, sustainable carrier materials led to the research published in Agrosystems, Geosciences & Environment by Winnefred Mensah, Azumah Ayamah, Nana Ewusi-Mensah, Jacob Ulzen, and Ophelia Osei Ulzen. Their study investigated various agricultural waste biochars—coconut coir (CC), coconut shell (CS), rice husk (RH), and sugarcane bagasse (SB)—as potential alternatives to peat for producing rhizobium inoculants.

The team monitored the survival rates of Bradyrhizobium strains in these biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More carriers over 24 weeks and evaluated their impact on cowpea (Vigna unguiculata (L.) Walp.) under field conditions. Biochar’s Edge in Bacterial Longevity: SB and CC Maintain Viable Cells for up to 20 Weeks. A key factor in the effectiveness of an inoculant is the survival rate of the rhizobia cells within the carrier material. The study meticulously tracked the viable cell count of three Bradyrhizobium strains (KNUST 1002, KNUST 1006, and BR 3267) in each biochar carrier and compared them to peat over 24 weeks. While peat consistently maintained the highest viable cell count throughout the entire 24-week period for all strains, some biochar carriers demonstrated impressive longevity.

Sugarcane bagasse (SB) and coconut coir (CC) biochars proved most effective at sustaining Bradyrhizobium survival for extended periods. SB biochar maintained viable cells for weeks for one strain, starting from a high initial count. For another strain, SB also sustained viable cells for an extended period, albeit with a decline from its initial count. CC biochar, while slightly less effective than SB in overall longevity for some strains, still promoted strain survival for weeks. In contrast, coconut shell (CS) and rice husk (RH) biochars supported strain survival for a shorter duration, with viable cell counts dropping to zero after some weeks for all tested strains. This difference in survival rates among biochar types is likely influenced by their varied physical and chemical properties, such as water-holding capacity (WHC) and initial pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More. For example, the favorable WHC of CC, described as a sponge-like matrix, contributes to a slow release of moisture, which is crucial for maintaining higher survival of Bradyrhizobium. Similarly, the optimal pH and organic carbon content of SB contributed to its ability to sustain bacterial populations longer.

Beyond bacterial survival in the carrier, the ultimate test for an inoculant is its ability to enhance plant growth and yield under field conditions. The study evaluated the effectiveness of the inoculated biochar carriers on cowpea nodule dry weight and grain yield. For nodule dry weight, a significant interaction was observed between the carrier type and the Bradyrhizobium strain. Generally, both biochar- and peat-based inoculants resulted in higher nodule dry weight compared to the uninoculated control. Notably, one Bradyrhizobium strain, when combined with coconut coir (CC) and sugarcane bagasse (SB) biochars, significantly increased the nodule dry weight of cowpea compared to the uninoculated control. This positive effect on nodulation is consistent with previous research indicating that inoculation can significantly increase nodulation, especially in soils with low indigenous rhizobia populations, like the study site which had only a very low number of rhizobia cells per gram of soil.

While the treatments did not show a statistically significant effect on overall cowpea grain yield, a promising trend emerged. The highest grain yield was recorded for CC, which was greater than the grain yield achieved with the standard peat-based inoculant using the same strain. Although this difference was not statistically significant, it highlights the potential of CC biochar as a superior carrier material for improving cowpea productivity. The lack of statistically significant yield differences across all treatments, including the control, suggests that other factors such as low soil fertility (particularly N and P) or strong competition from native rhizobia might have influenced the overall yield outcomes. Despite this, the biochar-based inoculants achieved a substantial portion of the potential yield of the Padi-Tuya cowpea cultivar compared to the uninoculated control.

This study offers valuable insights into the potential of agricultural waste-derived biochar as a sustainable and cost-effective alternative to peat in rhizobium inoculants. The findings strongly suggest that coconut coir biochar, in particular, holds significant promise for enhancing Bradyrhizobium survival and boosting cowpea grain yields. The ability of CC and SB to extend rhizobia survival for up to 16 and 20 weeks, respectively , is crucial for developing inoculants with longer shelf lives, a key challenge in commercial inoculant production.

For the future, the authors recommend long-term and multi-location studies to further validate the effectiveness of these biochar-based carriers under diverse field conditions. This research has direct implications for inoculant producers in SSA, demonstrating that agricultural waste materials, which often pose environmental nuisances, can be transformed into valuable products, thereby reducing inoculant production costs and promoting sustainable agricultural practices. This shift could significantly contribute to improving food security and reducing reliance on expensive chemical fertilizers for smallholder farmers in the region.

Source: Mensah, W., Ayamah, A., Ewusi-Mensah, N., Ulzen, J., & Ulzen, O. O. (2025). Potential of biochar-based inoculant in enhancing rhizobia survival and grain yield of cowpea (Vigna unguiculata (L.) Walp.). Agrosystems, Geosciences & Environment, 8(1), e70161.