Role of compost biochar amendment on the (im)mobilization of cadmium and zinc for Chinese cabbage (Brassica rapa L.) from contaminated soil

Purpose This study aims to assess the effect of amendment of an alkaline Zn, Cd-contaminated soil with compost of wheat straw biochar (CB) (4, 8, 12, and 18%) and sludge and different dosages of nitrogen. In addition, this study aimed to substantially mitigate the bioavailability of Zn and Cd for Chinese cabbage (Brassica rapa L.) from smelter-contaminated soils. Materials and methods This study was based on Chinese cabbage (B. rapa L.) growth for phytoremediation of soil toxic metals (TMs) (Zn and Cd) in different dosages of CB nitrogen (50, 150, 250, and 300 kg ha(-1)) and soil (T1) alone as well as different nitrogen dosages (50-T18, 150-T19, 250-T20, and 300-T21 kg ha(-1)) of chemical fertilizer (CF). The total and bioavailability of Zn and Cd were measured in shoot and root dryweight biomass, and soil in initial and after harvesting of Chinese cabbage. The chlorophyll content, pH, electrical conductivity, cation exchange capacity, and dissolved organic carbon content were measured to understand their role for plant growth and bioavailability of Zn and Cd. The phytoremediation of Zn and Cd was estimated by diethylene triamine pentaacetic acid (DTPA)-extractable method using smelter-contaminated soils amended with four different dosages of biochar. Results and discussion The CB application considerably reduced the DTPA-extractable Zn from contaminated soils, while in comparison to the control and CF treatment evidently mobilized the Zn in soil by approximately 4.17%. The maximum solubility of Zn of 23.46 and 26.78% was obtained with the control and CF-applied treatments in soils, respectively. The DTPA-extractable Cd concentration became elevated with increasing CB dosage, and a significantly higher Cd concentration was recorded in T6 compared to the control and chemical fertilizer-applied treatments. In the cases of higher dosages of CB, the Zn translocation in plants from soil was evidently reduced, but a lower CB amendment increased Zn levels by 5.15% in the shoot and 4.78% in the root, respectively. The changes in the soil EC, pH, and CEC by the amendment of CB could be the main reasons for the (im)mobilization of TMs in contaminated soil. Conclusions Finally, the results confirmed that the CB amendment, especially with 300 kg ha(-1) nitrogen, could reduce maximum bioavailability of Zn and Cd and the ecological risk of contaminated soil with TMs.