Enhancement of soil DOC and electron transport by biochar strengthens Cd immobilization via iron oxides transformations in waterlogged soils.

Cadmium (Cd) is typically influenced by soil iron oxides. Soil initial characteristics and biochar applications strongly influence iron transformation and Cd immobilization. In this experiment, 12 different Cd-contaminated paddy soils were subjected to anaerobic incubation with CK (control) and BC (2 % biochar). The results demonstrated that soil pH and Eh are crucial factors affecting Cd immobilization. Fe(II) concentration peaked on the 10th day and then decreased, and amorphous iron oxides (Feo) gradually increased. Compared to CK, biochar increased Fe(II) by 27.29 % on 10th day and Feo by 17.77 % on the 60th day. The contributions of Fe(II) and Feo were increased by 81.69 % and 317.35 % to Cd potential availability, and by 17.42 % and 172.43 % to Cd immobilization index, respectively. Compared with that on the 10th day in CK, biochar increased the Fe/Mn oxide-Cd by 11.81 % on average on the 60th day, and by an average of 48.70 % in the soil groups with a high proportion of initial crystalline iron oxides (Fec). Both the increases in amorphous iron oxides and Fe/Mn oxide-Cd during the 10th-60th days were positively correlated (p < 0.05) with the proportions of Fec in initial soils. Biochar increased soil dissolved organic carbon (DOC) and electron transfer capacity (Transe-) by an average of 8.38 %-34.83 % and 44.58 %-45.73 %. The contributions of DOC to Cd potential availability and immobilization index increased by 55.71 % and 26.41 %, respectively. These results suggested that biochar significantly enhanced Cd fixation by iron oxides in the initial soil with a higher Fec. Biochar might strengthen iron transformation and Cd immobilization by enhancing DOC and electron transport during waterlogged conditions.