Dissolved organic matter induces selenium and cadmium immobilization in natural selenium–cadmium-rich paddy soils

Cadmium (Cd) commonly coexists with selenium (Se) in the natural Se-rich soils of China, severely limiting their agricultural utilization. Dissolved organic matter (DOM) extracted from agricultural residues is considered a potential soil amendment capable of mobilizing Se and immobilizing Cd. This study investigates the mechanism by which DOM from different agricultural residues affects Se and Cd dynamics in natural Se-Cd-rich soils. Two-season pot experiments were performed to examine Se/Cd transformation and bioavailability under different residue-derived DOM treatments: chicken, ox, and earthworm manures, rice straw, and straw-manure mixtures. Our findings revealed that DOM application increased soil pH (+4.85 %), organic matter content (+33.47 %), and cation exchange capacity (+61.97 %). The 2 % chicken manure-derived DOM treatment achieved optimal results, increasing grain-Se content by 46.14 % while reducing the grain-Cd levels by 47.22 %. Structural equation modeling indicated that the increased soluble Se fraction (+54.81 %) and decreased oxidizable Cd fraction (-20.97 %) primarily influenced these results. We conclude that DOM mobilizes Se and immobilizes Cd in natural Se-Cd-rich paddy soils, with chicken manure-derived DOM demonstrating superior efficacy.