Optimizing Manganese Sulfate Application Timing with Thiol-Modified Attapulgite Reduces Cadmium Transfer to the Grain of Wheat (Triticum aestivum L.) in Alkaline Soil

Thiol-modified attapulgite (TM) combined with manganese sulfate (MnSO4) can reduce cadmium (Cd) availability in alkaline soils, but the influence of MnSO4 application timing on Cd transfer to wheat grain remains unclear. In this study, a soil incubation experiment and two wheat pot experiments were conducted to evaluate soil Cd immobilization, organ-specific Cd distribution, and the effects of applying MnSO4 before sowing, at the jointing stage, or at the grain-filling stage. Low- and high-dose MnSO4 treatments were applied alone or with TM, and the combined treatments were defined as TM+LS and TM+HS. Compared with the untreated control, TM+LS and TM+HS decreased dissolved Cd from 1.80 μg L−1 to 0.35 and 0.20 μg L−1, respectively, and reduced grain Cd from 0.19 mg kg−1 to 0.08 and 0.06 mg kg−1. These reductions were associated with decreased DTPA-extractable Cd, enhanced Cd adsorption, lower Cd desorption, and restricted Cd transfer from roots to shoots and from glumes to grains. Jointing-stage MnSO4 application produced the lowest grain Cd concentration and glume-to-grain transport coefficient. These findings indicate that optimizing MnSO4 application timing can improve TM-assisted Cd immobilization and wheat grain safety in alkaline Cd-contaminated soil.