Characteristics of Enzyme Activities during Phytoremediation of Cd-Contaminated Soil

In order to study the effects of exogenous Cd on the soil enzyme activities of three herbs, a pot experiment was conducted to study the changes of soil urease, protease, catalase and phosphatase activities in different growth periods of Solanum nigrum L., Phytolacca acinose Roxb. , and Bidens pilosa L. under different concentrations of Cd stress. The results showed that the content and proportion of each form of Cd were different in different periods. Compared with the control, the activities of urease, protease and catalase in the soil of three herbs decreased under 5 mg/kg and 10 mg/kg Cd stress, while the phosphatase activities increased first and then decreased. The activities of urease, protease, catalase and phosphatase were 4.24–6.84, 2.17–5.83, 2.09–2.79 and 34.57–37.25 mg/g, respectively, and the recovery degrees were 50.81–66.41%, 32.10–90.54%, 46.97–69.28% and 54.78–56.69%, respectively. After 60 days of remediation, the activities of urease, protease, catalase and phosphatase were 6.05–8.55, 2.83–9.89, 3.32–4.48 and 37.62–41.15 mg/g, respectively, and the recovery degrees were 70.19–84.57%, 41.86–161.34%, 72.35–140.44% and 58.38–63.20%, respectively. Soil enzyme activities were affected by Cd solution stress, which could be improved to a certain extent by plant self–healing, and different grass species recovered to varying degrees under various Cd solution stresses. Different soil enzymes displayed different responses to Cd stress, the inhibition of urease and phosphatase activities was temporary, and the effect of Cd concentration on soil phosphatase activity was close, and it could stimulate the activities of soil protease and catalase, and the higher the concentration of Cd solution, the greater the degree of stimulation. Principal component analysis shows that, after 60 days of repair, the best repair effect plants were Solanum nigrum L. under 5 mg/kg Cd stress and Phytolacca acinosa Roxb. under 10 mg/kg Cd stress.