Cadmium partitioning and maize accumulation under coal gangue and cattle manure amendments in two contrasting soils

Assessing Cd behavior in agricultural soils requires consideration of total soil Cd, its operational partitioning, and crop accumulation. A 122-day greenhouse pot experiment evaluated coal gangue, air-dried cattle manure, and their mixtures in two contrasting composite soils: an acidic lower-Cd soil (LS) and a neutral-to-alkaline higher-Cd soil (HS). After cultivation, total soil Cd averaged 0.648 and 1.825 mg kg^-1 in LS and HS, respectively, whereas operationally labile Cd concentrations were similar (0.195 and 0.207 mg kg^-1). Residual Cd accounted for 19.97% of fraction-sum Cd in LS and 57.50% in HS, while mean root-associated, stem, and leaf Cd concentrations were consistently higher in LS. Among grain-bearing amended pots, mixed coal gangue-manure treatments had lower grain Cd concentrations than manure-only treatments in both soils. Mean grain Cd mass decreased from 8.94 to 2.49 μg pot^-1 in LS and from 6.92 to 1.50 μg pot^-1 in HS for manure-only and mixed treatments, respectively. Grain dry mass was comparable between these treatment groups in LS and slightly lower under mixed treatments in HS, indicating that the lower grain Cd was not attributable solely to biomass dilution. Ten of 18 manure-only grain samples, but none of the 29 mixed-treatment samples, exceeded the 0.10 mg kg^-1 reference threshold. Because the unamended planted controls produced no grain at harvest, grain-based comparisons were restricted to grain-bearing amended pots. The contrasting soil and plant responses demonstrate that total soil Cd alone does not adequately represent crop Cd behavior. Integrating soil Cd partitioning with grain Cd concentration and Cd mass provides a more informative basis for evaluating amendment responses in soil-maize systems.