Efficacy and competitiveness of indigenous bradyrhizobia strains on soybean (Glycine max (l.) merr.) productivity in Malawi

Abstract

Soybean is an important cash crop with a growing demand in Malawi. Its ability to fix nitrogen has been exploited through the use of rhizobia inoculants to supplement indigenous rhizobia and improve soil fertility. Previous research has however shown instances whereby inoculation has had no yield response which suggests a need to fill the knowledge gap on the nitrogen fixation efficiency of indigenous rhizobia. The main objective of this study was to contribute to increased soybean yields through the use of indigenous rhizobia strains which are efficient in nitrogen fixation. 170 isolates of indigenous rhizobia were obtained from the 4 agro-ecological zones of Malawi and grouped into 19 presumptive species based on a binary matrix of tolerance to acid and salt stressed environments and antibiotic resistance patterns. The Shannon’s species diversity index analysis yielded a low index of 2.48 with a high evenness of 0.8 showing very few evenly distributed species of rhizobia, 90.5% of which were less stress-resilient than the reference strain USDA110. The top 5 most resilient strains were evaluated for BNF potential on 3 common soybean varieties. All the strains were found to perform either at par or better than control treatments strain in Shoot Dry Weight (P <0.001). The top 3 most effective strain-variety combinations were evaluated for their symbiotic efficiency in three soil types. Significant differences were found in the combinations in different soil types (P<0.001) with MAL_120 to Tikolore interaction having the highest symbiotic efficiency (87.71%). All other interactions had less efficiency than the reference interaction (USDA 110-Makwacha). These results led to the recommendation of MAL_120 and MAL_117 for field tests and genetic fingerprinting to ascertain its genotypic uniqueness. The overall conclusion was that the abundance of indigenous rhizobia in Malawi’s soils which affect the success of introduced strains. Differences in symbiotic efficiency of variety-strain combinations based on soil mineralogy show that recommendation of inoculation should be complimented by strain-variety compatibility tests.