Brazilian Semiarid Rhizobial Biodiversity As a Source of Desmanthus Inoculant Strains.

The legume-rhizobia symbiosis is currently growing again in importance due to the raising demands of both reduction in nitrogen fertilizer usage and increased yield. Tropical legumes are known to generally nodulate with highly diverse bacterial symbionts, thus new strains must be continuously evaluated as to their potential for inoculant production. Desmanthus is a forage legume, and recent studies have indicated that it has potential for use in tropical semi-arid pastures in the Brazilian Northeast, but there are no currently recommended rhizobial strains for inoculant production for this genus, while Brazilian federal law demands that only federally-mandated strains may be used for this. Thus, this research aims to evaluate rhizobial biodiversity to evaluate strains with potential for inoculant production. Soil samples were collected in all physiographic regions of Pernambuco State´s semiarid, and separated into high and low soil fertility groups. Soil samples were inoculated in cowpea (Vigna unguiculata) seeds, nodules were obtained and bacterial strains were isolated following traditional techniques. 182 isolates were obtained, 54 from the high and 128 from the low fertility soil groups. The latter were more diverse than the former, likely due to higher inter-group soil characteristics variation. While 20 of 54 high-soil-fertility isolates did not nodulate, only 6 of 128 low-soil-fertility ones did not. Only three strains were not fast growers, and 32% of the strains acidified the growth media, 40% did not change its pH and 28% alkalized it. 142 morphological groups were found with an overall low similarity of only 30%. This high diversity indicates potential for strain selection.