Effect of Soil Salinity on Survival and Performance of Alfalfa Rhizobia

The effect of salt stress on the growth and survival of four introduced Sinorhizobium meliloti strains in the rhizosphere of alfalfa Medicago sativa L. was studied. Response to inoculation with these strains and the competition among them and against native rhizobia were investigated in two salt affected soils with different salt concentrations of 10 and 36 dS m-1. Results showed that none of the introduced rhizobial strains was affected by the soil salinity up to 36 dS m-1 as they were able to grow and survive in the rhizosphere of alfalfa plants. However, nodule formation by both native and introduced rhizobial strains was negatively affected by salt concentration. This indicates that the first step in nodule formation is extremely sensitive to salinity, likely due to the effect of salinity on the root infection sites but not on the survival of rhizobia. Acetylene reduction of nodulated roots and nitrogen accumulation were adversely affected by salt concentration in the soil. This may show the negative effect of salt stress on plant growth and subsequently nodule function. Patterns of rhizosphere colonization by the introduced rhizobial strains declined markedly with increased salt concentration of the soil. Data showed that the introduced rhizobial strains could grow, survive and fix nitrogen at salt concentrations inhibitory to the growth of alfalfa. Therefore, we recommend that the future studies on the effect of salinity on symbiotic N2 fixation should focus more on aspects related to the symbiosis rather than the study of each separate partner