Objectives This study investigates the effects of co-application of calcium carbonate and oligosaccharides on improving acidic soils in South China and on enhancing rice-rice-lettuce rotation yields, aiming to provide theoretical support for the sustainable remediation of regional desilicated, aluminum (Al)-enriched acidic soils.

Methods This study employed pot experiments within an annual rice-rice-lettuce water-land rotation cropping system, with six treatments: no fertilization (T1), conventional fertilization (T2), T2+ CaO₂ (T3), T2 + CaCO₃ (T4), T2+ CaCO₃ + bagasse (T5), T2 + CaCO₃+seaweed oligosaccharide (T6). Crop yields were measured at harvest, and soil samples were collected to assess physicochemical properties and microbial community indicators.

Results  Compared with the T3 treatment, the early rice yields of the T5 and T6 treatments increased by 13.04% and 12.61%, respectively; the late rice yields increased by 7.36% and 6.59%, respectively. Compared with the T2 treatment, the lettuce yields of the T5 and T6 treatments increased by 46.98% and 52.25%, respectively. All four soil amendment treatments (T3, T4, T5, and T6) significantly reduced exchangeable acidity and increased pH. Among these, the T5 treatment raised pH by 0.92, 0.71, and 0.52 units at the harvest stages of the rice-rice-lettuce rotation, respectively. The T6 treatment increased pH by 1.39, 0.96, and 0.64 units, respectively. At the lettuce harvest stage, the T5 and T6 treatments reduced soil exchangeable acidity by 65.48% and 85.48%, respectively. The T6 treatment also exhibited a higher cation exchange capacity. Compared with the T4 treatment alone, the T5 treatment increased the organic-bound Al and humic acid-bound Al by 12.85% and 9.35%, respectively, while reducing the inorganic-adsorbed Al by 18.95%. Compared with the T2 treatment, the T5 treatment showed a significant increase of 4.76% in organic carbon; compared with the T1 treatment, the T4, T5, and T6 treatments showed increases in readily oxidizable organic carbon of 17.21%, 18.09%, and 20.15%, respectively; compared with the T4 treatment, the T3 and T5 treatments showed increases in microbial organic carbon of 33.13% and 19.00%, respectively. The T5 treatment significantly increased soil urease activity by 9.01%, while T6 treatment significantly increased urease and sucrase activity by 8.55% and 285.99%, respectively. The four amendment treatments (T3, T4, T5, and T6) significantly increased soil catalase activity by 7.44%, 9.06%, 9.60%, and 11.87%, respectively. Microbial community analysis revealed that all amended treatments significantly increased bacterial abundance compared with the T2 treatment, while the T5 treatment showed the greatest increase in fungal abundance. Both the T5 and T6 treatments increased the relative abundance of beneficial microorganisms, including the bacteria Bacillus, Conexibacter, and Anaeromyxobacter and the fungus Cladosporium, to varying degrees.

Conclusions  Co-application of CaCO₃ with either bagasse or alginate oligosaccharide can increase crop yields, effectively prevent soil re-acidification and raise soil pH, increase soil organic carbon content, enhance soil enzyme activity, and optimize microbial community composition. Bagasse has effects comparable to those of alginate oligosaccharide in raising soil pH, reducing exchangeable H⁺ content, and increasing total organic carbon content, and outperforms alginate oligosaccharide in regulating Al forms to alleviate Al toxicity, improving cation exchange capacity, and enhancing urease activity and microbial abundance. Therefore, the combined application of CaCO₃ and bagasse is not only an excellent acidic soil amendment but also a low-cost and green disposal measure for bagasse