Objective  The paper aims to clarify the effect of biochar and microbial fertilizer on the remediation of cadmium (Cd) pollution and to explore the suitable remediation techniques in calcareous soil.

  Method  The pot experiment was conducted to study the effects of 3% biochar (B3) and 1.5% microbial fertilizer ( M1.5 ) on pH value, Cd form distribution, enzyme activities and Cd contents in various organs of cotton in calcareous soil with different added Cd concentrations ( 0, 1, 2, 4 mg kg⁻¹ ).

  Result  The results showed that both biochar and microbial fertilizer could significantly increase soil pH, but with the extension of incubation time, the pH of soil treated with amendments showed a downward trend. The application of biochar and bacterial fertilizer could reduce the proportion of exchangeable Cd in soil and increase the proportion of residual Cd in soil. Compared with the control, the contents of exchangeable Cd in biochar and microbial fertilizer treatments decreased by 18.42% - 48.46% and 15.21% - 50.19%, respectively. The addition of biochar and microbial fertilizer significantly increased soil enzyme activities, and the maximum increase rates of sucrase, catalase, urease and alkaline phosphatase were 89.1%, 140.1%, 39.7% and 38.1%, respectively. The microbial fertilizer treatment was generally superior to biochar treatment. The applications of biochar and microbial fertilizer reduced the Cd content in each organ of the plant, in which the maximum reduction of Cd content in biochar treatment was 34.0%, and that of microbial fertilizer treatment was 39.5%. Correlation and principal component analysis showed that exchangeable Cd was significantly negatively correlated with soil enzyme activities ( P < 0.05 ), and positively correlated with Cd contents in roots, stems and leaves of plants (P < 0.05 ).

  Conclusion  Biochar and microbial manure as soil heavy metal passivators can alleviate the effect of Cd on soil enzyme activities, reduce the absorption of Cd by cotton and affect the form distribution of Cd. Overall, 1.5% bacterial fertilizer is better than 3% biochar in remediation of Cd pollution in calcareous soil.