Abstract:

It is valuable to compare the effects of different amounts of tobacco stalk biochar on Cd bioactivity in acidic soil, which may contribute to the safe planting of polluted farmland, and provide theoretical support for the resource utilization of a large amount of tobacco stalk waste as well. In this study, a series of tobacco stalk biochar (0, 5, 15 and 30 g/kg, denoted as B0, B5, B15 and B30, respectively) were applied to typical Cd-polluted acidic soil collected from the local region, and then C. coronarium and C. endivia were planted continuously in pots. The results showed that the biochar amendment significantly improved the soil physiochemical characters. All the parameters such as pH, TOC, CEC, available P and K in the biochar-amended soil were significantly higher than those of the control soil (P<0.05). Biochar addition promoted the transition from acid-soluble (Cd_acid) to the reducible Cd fraction (Cd_red) in the soil. The proportion of Cd_acid in the total was decreased from 64.03%-71.91% to 52.94%-56.51%, which indicated the large reduction of Cd bioavailability. Cd content in both above- and below-ground tissues of C. coronarium and C. endivia were significantly decreased by 53.47%-91.71% and 56.32%-87.95%, respectively. The biological cumulative factor (BCF) values for both crops were also significantly decreased by 54.47%-89.54%. The transfer factor (TF) values of C. coronarium in all treatments were highly similarity, no significant difference was observed. However, TF of C. endivia increased by 2.94%-86.76% compared to those in control. In addition, the larger rate of biochar amendment induced the greater changes in those parameters. The crop of C. coronarium is more sensitive to biochar addition than C. endivia. With the addition of 30 g/kg biochar (B30), Cd content in edible parts of both C. coronarium and C. endivia were decreased to 0.11 mg/kg and 0.20 mg/kg respectively, which were lower than the national limit for food. It was concluded that adding suitable rate of tobacco stalk biochar (5-15 g/kg) could not only improve soil properties like pH, organic C and CEC, but also promote the transformation of acid-extractable Cd to reducible Cd, and reduce the Cd bioavailability. Biochar addition reduced the Cd content in both C. coronarium and C. endivia, and strongly enhanced vegetable biomass production. C. coronarium was more sensitive to the biochar addition than that of C. endivia.

Key words: acidic soil, Cd, passivation, tobacco stalk, biochar, C. coronarium, C. endivia.