Effects of Concentration of Selected Heavy Metals on Nodulation and Nitrogen Nutrition of Leguminous Crops.

Heavy metal concentrations in the environment and their roles in plant nutrition and productivity have received much attention in recent years. This work was carried out to study the effects of six heavy metals (Cd, Co, Cr, Cu, Ni, and Pb) at concentrations ranging from 0 to as high as 4.8 mmol k g^-1 soil on dry matter yield, nodulation, and N uptake by broad bean (Vicia faba, Giza 3), Egyptian clover (Trifolium alexandrium, Giza 6) and soybean (Glycine max, Giza 35) grown under greenhouse conditions in two Egyptian soils for 50 days. Results showed that all the three plant parameters measured decreased significantly with increasing heavy metal concentration.  Plots of the natural log of each parameter against metal concentration were linear. From the slopes of those lines, the concentration of each heavy metal required to achieve 50% reduction (R₅₀) of those parameter was calculated. The results showed that those values varied among the soils, crops, and the crop parameters studied. In general, the lowest metal concentrations for R₅₀ were for Cd²⁺ and Pb²⁺. The dry matter and N yields of the Egyptian clover required greater metal concentration than the other two crops for R₅₀. Analysis of variance (ANOVA) showed that the concentration and type of heavy metal significantly (p < 0.001) affected dry matter yield, N uptake, and the number of nodules. The potential effects of heavy metal concentrations on plant growth and development will be presented.

Keywords: Heavy metals; Nodulation; N₂-fixation; Broad bean; Soybean; Egyptian clover.