Molecular Insights into Soil Amendments Regulating Selenium and Cadmium Bioavailability in a Soil-Rapeseed System: Synergistic DOM Transformation and Microbial Metabolic Reprogramming

Different soil amendments differentially affect selenium (Se) and cadmium (Cd) bioavailability in soils where Se and Cd naturally co-occur, and the underlying regulatory mechanisms remain unclear. This study examined how calcium superphosphate (P), biochar (BC), and organic manure (OM) regulate Se/Cd migration and transformation in a seleniferous soil-rapeseed system. P significantly reduced Se accumulation with little effect on Cd; BC enhanced Se uptake but reduced Cd; and OM reduced both. All amendments decreased microbial-derived humic-like components (C1) and increased fulvic-like, terrestrial humic-like, and highly humified humic acid components. OM induced the greatest enrichment of Actinobacteriota, followed by BC and P. Metabolomics revealed that P activated lipid metabolism and plant growth pathways; BC enhanced amino acid metabolism, promoting the transformation of DOM-bound Se; OM stimulated microbial metabolism, reducing C1 and enhancing Actinobacteriota (particularly Promicromonospora )-mediated Se/Cd immobilization. These findings demonstrate that amendments regulate Se/Cd bioavailability through DOM–microbe–metabolite interactions. Amendment selection should be based on the Se/Cd bioavailability in the target soil.