Supplemental carbonate enhances MnCO₃-mediated mitigation of Cd accumulation in wheat plants

Cadmium (Cd) accumulation in wheat remains a major food-safety concern in Cd-contaminated weakly alkaline croplands. Under these conditions, carbonate chemistry may substantially influence Mn-mediated Cd immobilization, yet its role in regulating Cd transformation and soil-to-wheat transfer remains elusive. Here, pot experiments were conducted to investigate the coupled effects of MnCO₃ and supplemental carbonate on Cd migration, transformation, and accumulation in a soil-wheat system. The results showed that MnCO₃ reduced wheat grain Cd content by 40.1%, and supplemental carbonate further enhanced this reduction to 51.0%-57.2%, while simultaneously enhancing grain biomass and the plant antioxidant defense. Mechanistically, supplemental carbonate was associated with lower soil Cd availability by transforming labile Cd into carbonate-bound Cd, increasing carbonate-bound Cd by 2.9%-13.5% while decreasing DTPA- and TCLP-extractable Cd by 3.8%-9.5% and 39.8%-66.0%, respectively. Moreover, MnCO₃ increased Mn availability, strengthened Mn-Cd antagonism, and reduced intracellular Cd accumulation in wheat, while also shifting soil Mn toward more stable fractions associated with Cd retention. Rhizosphere analysis further demonstrated that MnCO₃ altered microbial community composition and predicted functional profiles associated with metal transport/resistance and elemental cycling. Overall, MnCO₃ amendment combined with supplemental carbonate reduced wheat Cd accumulation through coordinated changes in soil Cd immobilization and plant Mn-Cd interactions. The findings provide new mechanistic insights into supplemental carbonate-enhanced Cd immobilization and offer a theoretical basis for developing efficient strategies to mitigate Cd exposure risks in wheat production on weakly alkaline croplands.