Soil pollution with heavy metals is an important environmental matter with negative impact on agriculture and human health. This work was investigated to evaluate the effects of different concentrations of three heavy metals (Cd, Mn and Zn) on plant growth, nodulation, N uptake, some hydrolases enzymes and total fungal and bacterial population by two leguminous crops grown on sandy and clay soils. Results showed that the determined growth parameters (plant height, fresh and dry weight, nodulation, and N-uptake) markedly reduced with elevating heavy metal concentration in the soil. In general, among the lowest concentration (LC10) values, Cd was the most effective metal to cause 10% reduction in each growth parameter. At 250 mg kg-1 soil, asparaginase, glutaminase, and arginase activities were inhibited by heavy metals. The percentage of enzyme inhibition increased by metal contamination, while less reduction was obtained under rhizobium inoculation. Total counts of bacteria and fungi were greater in soils inoculated than non-inoculated with rhizobium. Using plant growth promoter microbes such as rhizobium caused enhancement in the plant growth. Overall, the toxic effects of the three studied heavy metals on two leguminous plants and on the soil microbial activities can be arranged as follows: Cd > Zn> Mn.
Keywords: Heavy metals;Nodulation;Soybean;Egyptian clover;enzyme activity;microbial population