Effects of organic and inorganic amendments on cadmium fraction in the submersion process of contaminated paddy soil

The toxic accumulation of cadmium (Cd) in rice paddies is of concern. In this study, nine amendments were investigated for their effect on Cd-polluted paddy soil, including four organic (biochar (BI) and organic fertilizers with low (TL), medium (TM), and high (TH) C:N ratios), two inorganic (AXP and lime (LI)), and three organic–inorganic combined materials (BXP, AYF, and BYF). Soil underwent a 14-day unsubmerged period, followed by 14-day submerged incubation. The results showed that exchangeable Cd concentrations significantly decreased by 13.36% with AXP and 5.45% with BXP on day 7, by 6.67% with LI from day 14–28, and by 22.47% with BXP and 20.26% with BYF on day 28 (P < 0.05) Both TL and TM markedly increased the Fe-Mn oxide-bound and organic matter-bound Cd by 52.87% and 129%, respectively. Carbonate-bound Cd significantly decreased with all amendments on day 7 (except BXP, AYF, and LI) and with organic fertilizers after submersion (P < 0.05). Inorganic materials and BI significantly increased residual Cd on day 21. In general, amendments that worked better under submerged conditions may promote the translation of Cd. TM was more suitable under unsubmerged conditions for the safe Cd utilization, whereas LI could be used as a candidate amendment during the submerged period to decrease the ecological risk of soil Cd. The combination of organic and inorganic amendments, in a combination of powder and particle form, is beneficial for ensuring soil remediation, long-term cultivation of farmland fertility in paddy fields, and reducing ecological risks.