The influence of humic acids on the accumulation of lead (Pb) and cadmium (Cd) in tobacco leaves grown in different soils

Four types of soil (red, yellow, cinnamon, and paddy soil) from China were used to conduct experiments in pots to study the influence of humic acid (humic acid (HA) and fulvic acid (FA)) on the accumulation of lead (Pb) and cadmium (Cd) in tobacco leaves. In non-polluted soils, the tobacco leaf uptake of Cd was dramatically influenced by the total soil metal concentration and soil properties, with no relationship observed for Pb. However, in soils that were polluted with 500 mg kg-1 of Pb or10 mg kg-1 of Cd, negative relationships were observed between the soil pH and the metal concentration in the tobacco leaves. When the soil was polluted with Pb, the application of 4 g kg-1 humic acids (HA or FA) decreased the leaf Pb concentration by 17.78%-48.32% in the acidic red and paddy soils but increased the concentration by 11.69%-37.54% in the alkaline cinnamon soil. A 8.74%32.84% decrease in the leaf Cd concentration occurred in the acidic soils due to 4 g kg-1 HA, while there was a 4.45%-8.13% decrease and a 14.20%-46.37% increase in leaf Cd concentration in the yellow and cinnamon soils when HA and FA were applied, respectively. This finding suggests that the effects of humic acids on metal availability to plants in soil were pH-dependent, with inhibitory and stimulatory effects on acidic and alkaline soils, respectively. Therefore, humic acids (HA and FA) could be used to reduce Pb and Cd accumulation in plants growing in polluted acidic soil but not in alkaline soils.