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

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AbstractPlant 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.