Wild bradyrhizobia that occur in the Burdekin region of Queensland are as effective as commercial inoculum for mungbean (Vigna radiata (L.)) and black gram (Vigna mungo (L.)) in fixing nitrogen and dry matter production

Abstract

Anecdotal reports have suggested wild bradyrhizobia (Bradyrhizobium spp.) were as effective as commercial inoculum for mungbean (Vigna radiata (L.)) grown in the Burdekin region of far north Queensland, Australia. To test this hypothesis, we sampled the diversity of bradyrhizobium strains at two field sites in the Burdekin region of north Queensland, and from plants grown in inoculated or uninoculated soil collected from one field site in Millmerran, southern Queensland. We then compared two wild bradyrhizobium with the commercial strain for their ability to fix nitrogen and promote biomass production on plants in a glasshouse experiment. Ten mungbean nodules were collected from each of four plants, from each of two sites in the Burdekin region. One site had been treated with commercial bradyrhizobium inoculum strain CB1015, the other had never been treated with commercial inoculum. Protein analysis of the nodules using matrix assisted desorption ionization time of flight mass spectrometry (MALDI-TOF MS) showed a total of eight different strains of bradyrhizobia, distinctly different to CB1015, were present in the nodules of these plants. Of these eight strains, two strains dominated, one at each of the two collection sites. On the other hand, mungbean plants grown in soils from Millmerran, on the Darling Downs, hosted only CB1015 where inoculated, and did not nodulate when not inoculated. The two dominant wild bradyrhizobia were cultured from nodules from these field collected plants, and used in a controlled glasshouse experiment. In the glasshouse experiment, two mungbean cultivars and one black gram (Vigna mungo (L.)) cultivar were: (i) not inoculated and not supplied with nitrogen, (ii) not inoculated and supplied with nitrogen, (iii) inoculated with the commercial bradyrhizobia strain CB1015, (iv) inoculated with the dominant wild bradyrhizobia strain from Site 1, or (v) inoculated with the dominant wild bradyrhizobia strain from Site 2. Cultivars inoculated with either of the two dominant wild bradyrhizobia strains were similar in biomass and fixed a similar amount of nitrogen, to those inoculated with CB1015. There was no significant difference between the three inoculated treatments for shoot biomass nor nodule biomass per plant. Nodule number did not differ significantly from CB1015 for either of the wild bradyrizobia treatments. For root biomass per plant, the two mungbean cultivars did not differ between the three inoculated treatments, but the black gram had significantly less root biomass compared to the CB1015 treatment for one of the two wild bradyrhizobia treatments. For nitrogen fixed per plant, none of the cultivars showed significant difference between inoculated treatments. Our findings are consistent with the hypothesis that in some parts of Queensland some wild bradyrhozobia may be as effective as commercial inoculum for mungbean and black gram, potentially abrogating the need for artificial inoculation, and perhaps offering new options for the future development of better adapted commercial inoculum.