Effects of humic acids on the accumulation in tobacco leaves of lead (Pb) and cadmium (Cd) found in different soils
Lead (Pb) and cadmium (Cd) are two harmful heavy metals in tobacco leaf that mainly come from soil. To explore a method to reduce lead and cadmium in tobacco leaf, pot cultivation with unpolluted and polluted soil was executed with four main tobacco growing soils (red soil, yellow soil, cinnamon soil and paddy soil) in China. The accumulation of Pb and Cd in tobacco leaf was measured and the effects of humic and fulvic acids were discussed. The results showed that in unpolluted soils, the Pb concentration of tobacco leaf in different soils was similar despite its great difference in soil while the Cd concentration was significantly different in different soils. The higher soil Cd level and lower soil pH showed positive effects on leaf Cd accumulation. This result suggested that under unpolluted conditions, Pb absorbed by tobacco leaf was mainly related to soil properties while the absorbance of Cd was related to both soil properties and its level in the soil. In polluted soils, Pb and Cd concentrations in tobacco leaf grown in red and paddy soils were significantly higher than that in yellow and cinnamon soils, and pH became the main factor affecting their level in tobacco leaf. Under polluted conditions, Pb and Cd concentrations of tobacco leaf were decreased 17.78%~19.44% and 8.74%~13.58% in red soil and 30.22%~48.32% and 11.03%~32.84% in paddy soil, but increased 11.69%~37.54% in cinnamon soil with both humic and fulvic acid treatments except for a slight decrease in leaf Cd concentration with humic acid treatments in cinnamon soil. However, no obvious tendency was observed in yellow soil. These results suggest that humic and fulvic acid showed different effects on Pb and Cd absorption of tobacco leaf in different soils, the former mainly showed a decreased effect especially in acid soils, and the latter showed restrain and promotion effects in acid and alkaline soils, respectively. The two acids could be used as soil additives to reduce Pb and Cd concentration in tobacco leaf in polluted red and paddy soils.