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.