Cowpea (Vigna unguiculata) is an important grain-producing legume that can forego nitrogen fertilization by establishing an efficient symbiosis with nitrogen-fixing bacteria. Although inoculating strains have already been selected for this species, little is known about the genotypic and symbiotic diversity of native rhizobia. Recently, Bradyrhizobium has been shown to be the genus most frequently trapped by cowpea in agricultural soils of the Amazon region. We investigated the genetic and symbiotic diversity of 148 bacterial strains with different phenotypic and cultural properties isolated from the nodules of the trap species cowpea, which was inoculated with samples from soils under agroforestry systems from the western Amazon. Sixty non-nodulating strains indicated a high frequency of endophytic strains in the nodules. The 88 authenticated strains had varying symbiotic efficiency. The SPAD (Soil Plant Analysis Development) index (indirect measurement of chlorophyll content) was more efficient at evaluating the contribution of symbiotic N2-fixation than shoot dry matter under axenic conditions. Cowpea-nodulating bacteria exhibited a high level of genetic diversity, with 68 genotypes identified by BOX-PCR. Sequencing of the 16S rRNA gene showed a predominance of the genus Bradyrhizobium, which accounted for 70 % of all strains sequenced. Other genera identified were Rhizobium, Ochrobactrum, Paenibacillus, Bosea, Bacillus, Enterobacter, and Stenotrophomonas. These results support the promiscuity of cowpea and demonstrate the high genetic and symbiotic diversity of rhizobia in soils under agroforestry systems, with some strains exhibiting potential for use as inoculants. The predominance of Bradyrhizobium in land uses with different plant communities and soil characteristics reflects the adaptation of this genus to the Amazon region.

Biological nitrogen fixation, performed by diazotrophic bacteria, plays an important role in the maintenance of agricultural systems, as it contributes with significant amounts of the nitrogen (N) needed for plant growth, totally or partially exempting the use of industrial N fertilisers. Twenty-five bacterial strains isolated from nodules of Leucaena leucocephala and Phaseolus vulgaris trap plants were studied. These nodules were formed after inoculation with suspensions of soil samples collected close to the root system of Sesbania virgata. In previous studies, these bacteria were identified as Cupriavidus necator. This study aimed to evaluate the ability of these strains to fix N2 in the free-living state and to use carbon (C) sources; their resistance to antibiotics; growth in media with different pH values and salt concentrations and symbiotic efficiency with L. leucocephala and P. vulgaris. In each test, these strains were compared to C. taiwanensis LMG 19424T. Although a high variability regarding antibiotic resistance, salt tolerance and use of C sources were observed among the 25 C. necator strains, a large group behaved similar regarding salt tolerance (20 strains) and antibiotic resistance (22 strains). C. necator strains behaved in a different way of LMG 19424T. Only one of the 25 strains studied, UFLA02-69, was not able to establish symbiosis with its trap species, P. vulgaris. Only the strains LMG19424T and UFLA01-672 were efficient in symbiosis with L. leucocephala. The ability to use C sources, grow in different pHs and salt concentrations and resistance to several antibiotics, may grant high saprophytic competence and greater competitivity to these strains in relation to the native Leguminosae-nodulating bacterial populations, suggesting potential use in inoculant strain selection studies for legumes cultivated in soils with a wide range of pH and salt concentrations.