We evaluated symbiotic (rhizobia) and non-symbiotic efficiency and the diversity of native communities of plant growth-promoting rhizobacteria (PGPR) in soil samples from an area in recovery after iron mining (RA) and from adjacent phytophysiognomies (Ironstone Outcrops, Neotropical Savanna, Atlantic Forest) in the Quadrilátero Ferrífero region, MG, Brazil, in two climate seasons. Greenhouse experiments were conducted with siratro (Macroptilium atropurpureum) as a trap plant. The SPAD index in siratro was more effective in detecting the difference between the symbiotic efficiencies of the communities of PGPR than shoot dry matter, with readings of the SPAD index greater in RA. Genetic diversity, of 327 strains evaluated by partial sequencing of the 16S rRNA gene, showed 29 genera in the dry season and 19 in the rainy season, among these genera some recognized as nodulating and endophytes, and others potentially endophytes. Genera belonging to the phyla Acidobacteria, Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria were found. The nodule-forming genera Bradyrhizobium and Rhizobium and the endophytic Paenibacillus were those of highest occurrence in both seasons and in all phytophysiognomies. The greater symbiotic efficiency and the high genetic diversity of the rhizobacterial community found in RA indicate that soil rehabilitation is effectively in process. As most of native legume species form symbiosis with Rhizobium and mainly Bradyrhizobium, these bacterial communities are an important source of potential inoculant strains, as well as, a relevant feature for the successful establishment of diverse legumes for revegetation purposes.
Lima bean (Phaseolus lunatus L.) is the second most important species of the Phaseolus genus worldwide. In Brazil, it is grown primarily in the semiarid Northeast region. Although lima bean establishes symbiosis with N 2 -fixing bacteria (rhizobia), there is not yet a recommended inoculant. The aim of this study was to phenotypically and genotypically characterize rhizobia strains isolated from root nodules of lima bean collected in a Northeast region, and to evaluate their efficiency in supplying N nutrition to the crop under axenic conditions and in a Humic Rhodic Hapludox with and without correction of acidity. The symbiotic efficiencies were evaluated in a greenhouse under axenic conditions and, afterward, in pots with nonautoclaved soil with and without liming. Treatments consisted of the strains under evaluation, controls without inoculation (with low or high concentration or without mineral N), and a strain efficient for the crop. In all, 66 strains were isolated from the root nodules with predominance of strains belonging to the Bradyrhizobium genus. Liming brought about increases in lima bean development as it interacted with some strains. Six strains of the Bradyrhizobium genus, belonging to groups of type strains that include B. elkanii and B. japonicum, stood out for their biotechnological potential as inoculants for lima bean, and they should be tested in the field with the aim of validating their recommendation as inoculants for that crop.
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