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The effectiveness and longevity of amendments will influence the reduction in cadmium (Cd) bioavailability by in situ remediation techniques. Different amendments, including red mud (RM), corn straw (CS), rape straw (RS), and their combinations with zinc (Zn) fertilizer, were evaluated based on a long-term field experiment. It was found that all amendments decreased the concentration of available-Cd (EDTA-Cd and Exch-Cd) in soil and also reduced the Cd content (%) in plants. The amendments were still effective after eight years, although the effect did decline over time. The reduction of plant Cd content was 83.5% for red mud plus rape straw (RMRS), 65.3% for red mud plus corn straw (RMCS), 50.9% for RS, 54.0% for RM, and 37.3% for CS in the first few years. The reduction in plant Cd content was still 17.2% for RMRS after eight years, and RMRS was found to be the most effective treatment for decreasing Cd bioavailability. This study also explained that why the reduction in plant Cd content declined over time is the change of available Cd content in soil over time, which is important for guiding agricultural practice. It was concluded that RM, CS, RS, and their combinations with Zn fertilizer as effective amendments can have a profound and lasting positive impact on Cd-contaminated soils.