Heavy metal effects on soil populations and heavy metal tolerance of Rhizobium meliloti,nodulation, and growth of alfalfa

Effects of heavy metals on rhizobia and the symbiotic association with leguminous hosts are currently unclear. To investigate this problem, we examined Rhizobium meliloti (microsymbiont) and alfalfa (Medicago sativa) (macrosymbiont) collected from soils contaminated with varying concentrations of heavy metals (varying distances from a Zn smelter operating 90 yr.). Soil populations of R. meliloti were not correlated with metal concentrations in soil. The lowest rhizobial population was found in the soil with the highest extractable metal concentrations, but the highest populations were found in soil which was moderately contaminated. A greenhouse study in which alfalfa was grown in the same soils showed no significant trend for nodulation or nitrogenase activity of roots. Highest nodule number and nitrogenase activity were observed in those soils which had the lowest population of R. meliloti. When the heavy metal Minimum Inhibitory Concentration (MIC) of individual isolates was examined, no correlation was found between the MIC and soil metal concentration (total, or water or 0.01 M Ca(NO₃)₂ extractable).These results indicate that even in highly contaminated soils, metal activity was not high enough to exert an antagonistic influence on the soil rhizobial population or the symbiotic association between alfalfa and R. meliloti.