Biochar decreases Cd mobility and rice (Oryza sativa L.) uptake by affecting soil iron and sulfur cycling

Biochar has been used as an amendment in Cd-contaminated soils. However, the mechanisms of which biochar reduce Cd mobility and rice (Oryza sativa L.) Cd uptake by modifying the iron and sulfur cycling in soil has rarely been addressed in the literature. A pot experiment has been carried out with two Cd-contaminated paddy soils (FG and DBS) from South China. Rice straw biochar (RSB) and rape straw biochar (RASB) pyrolyzed at 450 °C were applied at 0, 0.5, and 1% (w/w), respectively. The results showed that biochar amendment at a rate of 1% reduced grain Cd concentrations by 29.3-35.2%. Furthermore, biochar significantly reduced the Cd concentration of root, while the decline of Cd concentration by RASB treatment was higher than by RSB treatment. Root Cd in RASB_0.5 was significantly reduced by 56.4-51.8% compared to than that in RSB_0.5 at the maturing stage. Biochar reduced soil acid-soluble Cd by 15.9-25.3% with the increase of pH at the maturing stage in FG soil, and 30.1-59.3% by promoting soil into more reductive condition at the heading stage in DBS soil with higher contents of Fe and S. In addition, biochar impeded Cd transport from soil to rice roots by increasing the formation of iron plaque at the flooding stage. Owing to the influence of RASB₁, DCB-Cd concentration increased significantly, with 99.7% at the heading stage in FG soil and 237.9% at the tillering stage in DBS soil, respectively. Furthermore, RASB with a higher sulfur concentration had a more positive effect on Cd immobilization and iron plaque formation compared to RSB. As a conclusion, this study suggested that biochar might be able to promote the Cd immobilization by affecting the cycling of iron and sulfur in soil.