Diversity of rhizobia isolated from an agricultural soil in Argentina based on carbon utilization and effects of herbicides on growth

Abstract

Seventy-six rhizobial isolates belonging to four different genera were obtained from root nodules of several legumes (Vicia sativa, Vicia faba, Medicago sativa, Melilotus sp., Glycine max and Lotus corniculatus). The action of five commonly used herbicides (2,4-dichlorophenoxyacetic acid [2,4-D], glyphosate [GF], dicamba, atrazine and metsulfuron-methyl), on the growth of rhizobial strains was assessed. Subsequently, GF and 2,4-D were tested in a minimum broth as C and energy sources for twenty tolerant strains. The ability of these strains to metabolize different carbon sources was studied in order to detect further differences among them. Tolerance of the bacteria to agrochemicals varied; 2,4­-D and GF in solid medium inhibited and diminished growth, respectively, in slow-growing rhizobial strains. Among slow-growing strains we detected Bradyrhizobium sp. SJ140 that grew well in broth+GF as the sole C and energy source. No strain was found which could use 2,4-D as sole C source. The twenty strains studied exhibited different patterns of C sources utilization. Cluster analysis revealed three groups, corresponding to four genera of rhizobia: Rhizobium (group I), Sinorhizobium (group II) and Mesorhizobium-Bradyrhizobium (group III). On the basis of the results obtained on responses to herbicides and C sources utilization by the isolates investigated, it was possible to differentiate them at the level of strains. These results evidenced a considerable diversity in rhizobial populations that has not been previously described for Argentinean soils, and suggested a physiological potential to use natural and xenobiotic C sources.