Influence of tropical grasses on cadmium speciation in the soil solution of Cd-rich soils

Cadmium (Cd) availability in soil solution is controlled by pH, solution ligands, and ionic competition, all influenced by plant species. This study evaluated Cd chemical speciation in the soil solution of a Cd-contaminated Oxisol from a previous experiment involving grass cultivation. The soil was artificially enriched with Cd using Cd(NO3)2·4H2O, reaching 7.5 mg Cd kg−1. Samples were irrigated to field capacity, and the resulting soil solution, representing a model system, was collected for chemical analysis. Cd speciation was estimated using Visual MINTEQ 4.0. Results showed Cd predominantly as free Cd2+, followed by fulvic acid–bound species (FACd(aq) and FA2Cd+(aq)) and chloride complexes (CdCl+). High-biomass grasses significantly influenced Cd concentration and speciation. The highest Cd2+ levels were observed in soils cultivated with Andropogon gayanus cv. Planaltina and Brachiaria brizantha cv. Piatã. In Brachiaria-cultivated soils, Cd occurred mainly in organic-complexed forms, whereas inorganic species were more abundant under A. gayanus cv. Planaltina and Sorghum sudanense. Variations in soil properties, particularly pH and ion availability, modulated Cd speciation and plant uptake, influencing biomass production. Overall, Brachiaria species reduced Cd availability through organic complexation, while A. gayanus cv. Planaltina and S. sudanense increased Cd mobility, indicating lower suitability for Cd-contaminated soil management.