Effects of exogenous sulfur on growth and Cd uptake in Chinese cabbage (Brassica campestris spp. pekinensis) in Cd-contaminated soil.

Soil pollution with heavy metals has many adverse effects on ecosystem health as well as food security. A pot experiment was performed to investigate the effects of different valence states of exogenous sulfur (S) on the uptake of cadmium (Cd) in Chinese cabbage in Cd-contaminated soil. The results showed that S significantly promoted plant growth in Chinese cabbage, with the following order of magnitude for the different S treatments: sodium sulfite (Na2SO3) > sodium sulfate (Na2SO4) > powdered sulfur (S0). Additionally, enzyme activity and the content of reductive substances in the leaves markedly increased, while malondialdehyde content significantly decreased; hence, S observably enhanced the ability of Chinese cabbage to tolerate Cd stress. S0 significantly reduced soil pH, thus increasing the mobility and bioavailability of Cd in the soil, while Na2SO3 increased soil pH, and Na2SO4 had no effect on soil pH. The acid-soluble and oxidizable fractions of Cd in soil increased with the S0 treatment. The applied Na2SO3 and Na2SO4 both increased the residual fraction of Cd in the soil, but they reduced the amount of the acid-extractable, reducible, and oxidizable Cd. The results showed that compared with S0, the Na2SO3 and Na2SO4 treatments decreased the acid-extractable Cd concentrations by 6.3 and 4%, respectively, in the most contaminated soil. In conclusion, the influence of S on the bioavailability and speciation of Cd varied not only with the soil Cd content but also with the application rate and S valence state.