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 (S-0). 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. S-0 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 S-0 treatment. The applied Na-2 SO3 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 S-0, 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.