Effect of garden waste biochar on the bioavailability of heavy metals and growth of Brassica juncea (L.) in a multi-contaminated soil

Metal-contaminated soils are considered a global concern due to their adverse effects on ecosystem vitality. This study was conducted to assess the efficiency of garden waste biochar (GB), as a modern remediation tool of soils contaminated with heavy metals (HMs), compared with the other Paulownia biochar (PB) and bamboo biochar (BB), on the growth of Brassica juncea (L.). The experiments and analyses were carried out at the Institute of Agricultural Resources and Environment, Guangdong Academy of Agricultural Sciences (Guangzhou, China), during the year 2018. The studied contaminated soil sample was amended with 0, 2%, 4%, and 6% (w/w) of PB, BB, GB(400), and GB(600) biochar. The experiment was designed in a completely randomized block design with 3 replications for each treatment. Garden waste biochar significantly duplicated the soil organic matter and EC 1.43-2.21 and 1.33-1.51 times, respectively, enhanced soil pH, and improved plant growth. It was more efficient to improve soil properties than PB and BB. DTPA-extractable metals showed the highest reduction at 6% of GB(400), which was 29.56, 46.04, and 59.98% for Pb, Cd, and Cu, respectively, while it was 48.29% for Zn at GB(600) over the control. Moreover, GB treatments increased the nitrogen (N) and phosphorus (P) contents of the plant shoots. Besides, HM contents of soil and plant tissues were significantly reduced after biochar applications and the highest reduction was recorded using 6% of GB. The results obviously pointed out that the metal transfer coefficient (TC) of shoots was decreased compared with the control after biochar applications, and GB showed the optimum response in this regard. The huge amounts of garden waste, after transforming them to biochar (GB) which has been considered more effective in safe crop production, can be used to reduce the availability of toxic metals in the soil and keep them under safe limits, especially with large-scale vegetable crops in China. It will clean the environment and improve resource sustainability. Therefore, GB is recommended to be used in soil HM remediation on a large scale in polluted areas.