Single-Strain Inoculation of Bacillus subtilis and Rhizobium phaseoli Affects Nitrogen Acquisition of an Improved Mungbean Cultivar

Plant growth-promoting rhizobacteria (PGPR) increase plant root growth, potentially improving soil nitrogen (N) uptake, and productivity. Legumes, for instance mungbean, could also benefit from a rise in potential infection sites for nodulation, thereby increasing rates of biological N₂ fixation (BNF). Consequently, the objectives of this study were (i) to assess whether PGPR had an effect on mungbean root biomass and if that was linked to N accumulation and productivity; (ii) to identify whether multi-strain inoculation showed greater efficacy in increasing N accumulation and overall productivity than single-strain inoculation; (iii) to test whether N acquisition was based on BNF rather than on soil N uptake. Field trials were conducted in two seasons at the University of Agriculture, Faisalabad with mungbean cultivar NM11 and multi-strain inoculation consisting of Rhizobium phaseoli, Bacillus subtilis, and Pseudomonas fluorescens. The strains were tested additionally in the second season as single-strain inoculation. Multi-strain and inoculation with P. fluorescens alone had no effect on root biomass, total plant-N, BNF or soil N uptake. Inoculation with B. subtilis, however, resulted in significantly increased root dry matter (+ 211 kg ha^− 1), total dry matter (+ 1.7 t ha^− 1), and total plant-N (+ 36 kg ha^− 1). Only inoculation with R. phaseoli enhanced BNF (+ 24%). Yield was not affected by any inoculation. The results suggested that total plant-N was based on soil N uptake rather than on BNF and demonstrated that only single strains affected total N accumulation, pointing to antagonistic mechanisms of the strains in a mixed inoculum.