Biochar reduces Cd accumulation in Brassica rapa var. chinensis: Role of particle size

Biochar, which is produced via biomass pyrolysis, has been widely applied to remediate polluted soils. However, the remediation efficiency and mechanism of biochar with different particle sizes (DPS-BC) are still unclear. Here we conduct a pot experiment to explore the effects of DPS-BC in reducing Cd contents in pak choi (Brassica rapa var. chinensis), a common vegetable in China. Biochar from tobacco stem was screened to 2.00- 4.00 (ST1), 0.85- 2.00 (ST2), 0.42- 0.85 (ST3), and < 0.42 (ST4) mm, which were applied to the Cd-polluted soil at a rate of 2.5%. The soil properties and DTPA-extractable Cd (DTPA-Cd) contents were analyzed after 1-month incubation. After cultivation of pak choi for 2 months, plant fresh weight, Cd contents in leaves, and the relative expression of Cd transporter genes were investigated. The results showed that the application of DPS-BC increased soil pH, electrical conductivity, and organic matter, available P, and available K contents, while the DTPA-Cd contents were reduced by 43.2- 52.5%. Under ST3 (0.42- 0.85 mm biochar) treatment, the leaf Cd contents were lowest and reduced by 48.3% compared with the control, which could be attributed to the decreased soil Cd bioaccessibility, down-regulated Cd transporter gene expression, and increased crop fresh weight. The structural equation model showed the "dilution effect" and "passivation effect" were both the key mechanisms responsible for the lower Cd accumulation in plant leaves. These results indicate that biochar is promising for reducing Cd content in pak choi and the remediation efficiency could be influenced by the size fraction of BC.