Potential of biochar-based inoculant in enhancing rhizobia survival and grain yield of cowpea (Vigna unguiculata (L.) Walp.)

The need for low-cost carrier materials is paramount for rhizobium inoculants production, especially in sub-Saharan Africa. This research studied different feedstocks biochar as potential carriers for inoculant production. Coconut coir (CC), coconut shell (CS), rice husk (RH), and sugarcane bagasse (SB) biochars were used as carriers, and their performance was compared to that of standard industrial peat (control). The biochar and peat carriers were inoculated with two elite Bradyrhizobium strains such as KNUST 1002 and KNUST 1006 and the reference strain BR 3267 (where BR is Bradyrhizobium). The survival rate of the rhizobia was monitored by determining the number of cells, pH, and moisture content for 24 weeks. In addition, the effectiveness of different inoculated carriers was tested in cowpea (Vigna unguiculata (L.) Walp) under field conditions via a randomized complete block design with four replications. CC and SB enhanced Bradyrhizobium survival for up to 16 and 20 weeks, respectively, whereas CS and RH promoted strain survival for up to 8 weeks. The viable cell count was highest in the peat-based inoculant during the 24-week period. BR 3267 in CC and SB significantly (p < 0.001) increased the nodule dry weight of cowpea compared to uninoculated control. The highest grain yield (1774 kg ha⁻¹) was recorded for CC, which was 16% greater than the grain yield of the peat-based inoculant (1524 kg ha⁻¹), although this difference was not significant. CC biochar is a potential carrier for inoculant production in Ghana.