One-time calcium carbonate versus annual lime application for cadmium immobilization in a contaminated paddy field: A...

Cadmium (Cd) contamination in paddy fields poses a serious threat to food security. However, the comparative remediation efficiency and microbial community responses of one-time calcium carbonate versus annual lime application remain unclear. A four-year field experiment (2021-2024) was conducted in Zhuzhou, China, using four treatments: no fertilizer (NF), chemical fertilizer (CF), annual lime application (L, 3000 kg hm^-2 yr^-1), and one-time carbonate application (CC, 15 000 kg hm^-2 applied in 2021). Soil properties, Cd fractions, rice yield, brown rice Cd, and bacterial communities were analyzed. The CC treatment increased soil pH to 7.75, while the L treatment maintained pH at 6.97. The two amendments significantly reduced exchangeable Cd content. The four-year average brown rice Cd was 0.07 mg kg^-1 under CC and 0.13 mg kg^-1 under L, compared with 0.87 mg kg^-1 under NF and 1.02 mg kg^-1 under CF. However, late rice yield under CC (5.76 t hm^-2) was lower than under L (6.43 t hm^-2), and exchangeable Ca under CC (18.1 cmol kg^-1) was significantly higher than under L (13.9 cmol kg^-1). At year 4, these amendments decreased the abundance of Pseudomonadota and Acidobacteriota while increasing Chloroflexota and Bacillota. Additionally, the CC treatment significantly enriched Thermodesulfobacteriota (15.4%). Annual lime application maintained higher yield while reducing Cd, supporting safe rice production; one-time CC achieved the strongest Cd immobilization over the four-year period, though at the cost of yield and nutrient balance. These findings provide data support for optimizing passivation remediation strategies in Cd-contaminated paddy fields.